WO2006058381A1

WO2006058381A1 - External cavity raman laser

Info

Publication number: WO2006058381A1
Application number: PCT/AU2005/001816
Authority: WO
Inventors: Richard Paul Mildren; Helen Margaret Pask; James Austin Piper
Original assignee: Macquarie University
Current assignee: Macquarie University
Priority date: 2004-12-01
Filing date: 2005-12-01
Publication date: 2006-06-08
Anticipated expiration: 2007-06-01

Abstract

The invention provides a wavelength converter (10) for generating at least one wavelength of output laser light (40) from'a pump laser beam (35). The wavelength converter comprises a resonator cavity (15) comprising at least a first reflector (20) and a second reflector (25), and a Raman-active medium (30) located in the resonator cavity (15) and an output coupler (25). , The Raman-active medium (30) is capable of shifting the wavelength of the pump laser beam (35 to generate Raman-shifted laser light, and the first and second reflectors (20, 25) are adapted and disposed so as to reflect said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium (30) so as to provide medium to high gain Raman-shifted laser light (40) having at least one selected wavelength.

Description


  External cayity Raman laser
Technical Field
The present invention relates to laser systems for generating output at a variety of wavelengths. Background of the invention
Lasers are commonly needed in the wavelength ranges of 55fJ-65Gnm, 350-450nm and 270350nm and other wavelength ranges often inaccessible by common solid-state lasers. Applications include skin treatments (vascular treatment, acne treatment and photo rejuvenation), atmospheric sensing and environmental sensing, It is often important to have a laser with high average power and sometimes useful io have more than one wavelength in the output beam. Currently used systems in such wavelength ranges include dye lasers or gas lasers (copper vapour or ion lasers).

   These have the well known disadvantages.
There is therefore a need to generate output in the wavelength ranges 550-650nm, 350450nm and 270-350nm at multiwatt powers and for a versatile system for altering the output wavelength. Such sources are needed in medical treatment applications in which' ultiple photosens<'>rtizers can be targeted using a single device. Other applications may include use in visual displays, remote sensing applications and biomedicine,
Object of the Invention It is the object of the present invention to overcome or substantially ameliorate at least one of the above disadvantages.

   It is a further object to at least partially satisfy the above need.
Summary of the Invention in a first aspect of the invention there is provided a wavelength converter for generating at least one wavelength of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate at least one wavelength of Ramanshifted laser light; and . c) an output coupler for outputting at least one wavelength of Raman-shifted laser light produced by the Rama[pi]-activ[theta] medium from the resonator cavity.
There is also provided a wavelength converter for generating at least one wavelength of output laser light from a pump laser beam, comprising:

   a) a resonator cavit comprising at least a first reflector and a second reflector; b) a Raman-active medium. located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least one selected wavelength; and c) an output coupler for outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength.
The wavelength converter may be configured such that the pump laser beam is at least partially mode matched with a Raman-shifted laser beam in the Raman-active medium.

   The curvature and/or transmission characteristics of at least one of the reflectors, and/or the locations of the first and second reflectors and the Raman-active medium may be such that the wavelength converter is capable of providing the medium to high gain Raman-shifted laser light. The medium to high gain may be such that the power conversion efficiency of the pump beam to the Raman shifted light having at least one selected wavelength may be in the range 20 to 90%. Thus for a suitable power of pump laser beam the output power of the wavelength converter may be greater than about 3Q0mW, 350mW or 4G0mW, or equal to or greater than about 400m , SGOmW, 750m , 1W, 1.25W,, 1.5W, 2W, 3W, 4W, 5W, 6W, 7W, 8W, 9W, 10W, 11W, 12W, 13W, 14W, 15W, 16Wr17W, 18W, 19W, 20W, 22W, 23W, 24W, 25 or 30W.

   The output power may be between 400mW and 35W, 400mW and 25W, 400m and 20W, 400mW and 15W 40QmW and 10 , 400m and 8W, 400mW and 5W. The wavelength converter may be configured such that there is at least partial spatial and temporal overlap between the spot size of the pump laser beam and the spot size of the Raman-shifted laser beam in the Raman-active medium. The mode matching, or the overlap, may be sufficient that the wavelength converter has an efficiency of at least 20%, or at least 25, 30, 40, 50 or 60%, where the efficiency is defined as the output power divided by the input power of the wavelength converter.

   The mode matching, or the overlap of the pump laser beam spot size and the Raman shifted laser light spot size, may be at least 50%, or at least about 60, 70, 80 or 90% or may be between 50% and about 100%, 60 and about 100%, 70 and about 100%, 80 and about 100%, 50% and about 90%, 60 and about 90%, 70 and about 90%, or 80 and about 90%. The mode matching may be sufficient that the output power of the wavelength converter is greater than about 400mW, or greater than about 500, 750mW, 1 , 1.25 or 1 ,5W.
The wavelength converter may additionally comprise a focusing lens for focusing the pump laser beam on or in the Raman-active medium. The focusing lens may be located outside the resonator cavity or inside the resonator cavity.

   At least one parameter selected from the power of the focusing lens (if present), the position of the focusing lens (if present), the curvature of the first reflector, the curvature of the second reflector, the curvature of the output coupler and the distance of the Raman-active medium from the output coupler may be such that, in use, the pump laser beam and a Raman-shifted laser beam are at least partially mode matched in the Raman-active medium, or may be such as to provide the medium to high gain.
The resonator cavity may be a short resonator cavity. It may be sufficiently short that a pump laser beam pulse can overlap with a Raman-shifted laser beam pulse within the Raman-active medium.

   At least one parameter selected from the position of the Raman-active medium in the resonator cavity and the distance between the Raman-active medium ard the output coupler may be such that a Raman-shifted laser beam pulse reflected from the output coupler is capable of interacting in the Raman-active medium with a pump laser beam pulse. The at least one parameter may be such that a laser beam pulse reflected from the output coupler ree[pi]ters the Raman-active medium while either a part of said laser beam pulse or another laser beam pulse is present in the Raman-active medium. The at least one parameter may be such as to allow spatial and temporal overlap in the Raman-active medium between a laser beam pulse and a laser beam pulse reflected by the output coupler.

   The laser beam pulse and the laser beam pulse reflected from the output coupler may be the same pulse or may be different pulses, The laser beam pulse reflected from the output coupler may seed the Raman-active medium. The cavity may be such that a<>>reflected pump laser beam pulse from the output coupler transits the Raman-active medium while the pump laser beam pulse is present therein. The cavity may be such that the feedback from the output coupler achieves a positive effect on build-up and evolution of a Stokes fields. The shortest distance from the Raman-active medium to the output coupler may be such that a pump laser beam pulse reflected from the output coupler reenters the Raman-active medium while at least a part of the pump laser beam pulse is present therein. Thus spatial and temporal overlap may occur between the forward and backward reflected pump laser beam pulse.

   The Raman-active medium may be capable of shifting the wavelength of the pump laser to generate two or more wavelengths of Raman-shifted laser light (Stokes wavelengths). The output coupler may be capable of selectively outputting a selected wavelength or group of wavelengths from the two or more wavelengths of Raman-shifted laser light. A selector may be provided to select the selected wavelength or group of wavelengths to be outputted. The selector may be a variable selector, whereby the selected wavelength or group of wavelengths may be varied, or it may be- a fixed selector, whereby the selected wavelength or group of wavelengths is not capable of being varied, An example of a fixed selector is a selectively transmissive coating on the output coupler. An example of a variable selector is a polarization selector. Both a fixed and a variable selector may be provided.

   More than one fixed selector may be provided. For example the wavelength converter may have more than one output coupler, and the selectively transmissive coatings thereon may selectively transmit different wavelengths or different groups of wavelengths. The wavelength of Raman-shifted laser light outputted by the output coupler (the "output laser beam") is different from the wavelength of the pump laser beam. The pump laser beam may have a wavelength between about 250 and 700nm, It may be a visible laser beam, and may have a wavelength between about 400 and 700[pi]m or between about 500 and [beta]OOnm, or may be a UV laser beam, and may have a wavelength between about 250 and 400nm. In an example, the pump laser beam is green and has a wavelength of 532 nm.

   The output laser beam may be a visible laser beam, and may have a wavelength between about 400 and 700nm, or between about 500 and 700nm, or may be a UV laser beam, and may have a wavelength between about 250 and 400nm. The first reflector may be highly transmissive for the wavelength of the pump laser beam, and may be highly reflective for the wavelength of the Raman-shifted laser light. The second reflector may be highly reflective for the wavelength of the pump laser beam, and may be at least partially transmissive for the Ramanshifted laser light having at least one selected wavelength.

   The output coupler may comprise the second reflector, or there may be a separate output coupler, In the case that there is a separate output coupler, none of the reflectors may be transmissive towards the wavelength of output laser iight, or one or more reflectors may be transmissive towards the wavelength of output laser light. The output coupler may comprise a polarizing beam splitter. The Raman-active medium may be solid, and may be a crystal. It may have high thermal conductivity, and may have a low thermooptic coefficient dn/dT. The Raman-active medium may be birefringent, and may for example be diamond, KGW, KYW, YVO4 or G V[theta] - The first reflector may comprise a dichroic mirror, and the cavity may have a third reflector. The third reflector may be a focusing mirror.

   The wavelength converter may further comprise a non-linear medium, for example a sum frequency, a frequency doubler or difference frequency generator, or it may have no non-linear medium apart from the Raman-active medium.
In an embodiment there is provided a wavelength converter for generating at least one wavelength of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate at least one wavelength of Ramanshifted laser light; and c) an output coupler for outputting at least one wavelength of Raman-shifted laser light produced by the Raman-active medium from the resonator cavity;

   wherein the wavelength converter does not comprise a non-linear medium except for the Raman-active medium.
In one embodiment there is provided a wavelength converter for generating at least one wavelength of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said' Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least one selected wavelength;

   and c) an output coupler for outputting from the resonator cavity said medium to high gain Raman-ehifted laser light having at least one selected wavelength; wherein the shortest distance from the Raman-active medium to the output coupler is such that a pump laser beam pulse reflected from the output coupler reenters the Raman-active medium while at least a part of the pump laser beam pulse is present therein.
In an embodiment there is provided a wavelength converter for generating at least one wavelength of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector;

   b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least one selected wavelength; and c) an output coupler for outputting from the resonator cavity said medium to high gain
Raman-shifted laser light having at least one selected wavelength; wherein the wavelength converter does not comprise a non-linear medium except for the Raman-active medium.

   The wavelength converter may additionally comprise a wavelength selector, for selecting a single wavelength of output laser light by either deselecting all but one of the wavelengths resonating in the cavity or by selecting one of the wavelengths.
In one embodiment there is provided a wavelength converter for generating at least two wavelengths of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector;

   b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said
Raman-shifted laser light having at least two selected wavelength a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least one selected wavelength; and c) an output coupler for outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least two selected wavelength
In another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second reflector;

   b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least two selected wavelengths a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least two selected wavelengths; c) a seeding device to seed the Raman-active medium with a seed beam in order to cause it to produce predominantly a single wavelength of Raman-shifted laser light with a
Raman-shifted wavelength corresponding to the wavelength of the seed beam;

   and d) an output coupler for outputting from the resonator cavity said mediium to high gain Raman-shifted laser light having at least two selected wavelength.
In one embodiment there is provided a wavelength converter for generating at least two wavelengths of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate a plurality wavelengths of Raman-shifted laser light; and c) an output coupler for outputting at least two of the plurality of avelengths of
Raman-shifted laser light produced by the Raman-active medium.

   In another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a Raman^active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate at least two wavelengths of Ramanshifted laser tight; c) a seeding device to seed the Raman-active medium with a seed beam in order to cause it to produce predominantly a single wavelength of Raman-shifted laser light with a Raman-shifted wavelength corresponding to the wavelength of the seed beam; and d) an output coupler for outputting the Raman-shifted wavelength of laser light.
The seeding device may comprise a seed beam generator, such as a diode laser or some other suitable device.

   The wavelength of the seed beam may be such that it is capable of seeding the Raman-active medium in order to cause it to provide primarily a single selected wavelength of Raman-shifted laser light, said Raman-shifted wavelength being the same as the wavelength of the seed beam. The seed beam may be polarized so that the Raman-active medium is caused to produce predominantly a single polarization of laser light.
In another embodiment of the invention the wavelength converter comprises: a) resonator cavity comprising at least a first reflector and a second reflector; b) a birefringent Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate at least two wavelengths of Raman-shifted laser light; c) a selector for selecting a single wavelength of the at least two wavelengths of Raman-shifted laser light;

   and d} an output coupler for outputting the single wavelength of laser light. In another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a birefringent Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least two selected wavelengths a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least two selected wavelengths; c) a selector for selecting a single wavelength of the at least two wavelengths of
Raman-shifted laser light;

   and d) an output coupler for outputting the single wavelength of laser light
The birefringent Raman-actfve medium may be capable of generating two polarizations of Raman-shifted laser light spatially separated from each other, each having a different Stokes, wavelength. The selector may be capable of selecting the single wavelength of Raman-shifted laser light by realigning one of the reflectors and or the Raman-active medium.
The orientation of the Raman-active medium, and.

   the curvature, orientation and position of at least one of the reflectors may be such tha only one of the Stokes wavelengths emanating from the Raman-active medium is capable of efficiently resonating within the cavity, due to the effects of birefringence on different polarizations.
The selector may comprise an adjustor to adjust the orientation of the Raman-active medium and/or of at least one of the reflectors so that only the selected wavelength is directed in such a way that allows it to resonate within the cavity, The adjustor may comprise one or more orientation adjusters, for example one or more motors or piezoelectric devices.
The Raman-active medium may comprise a separate Raman-active element and birefringent element, whereby the Raman-active element is capable of generating Raman-shifted wavelengths of laser iighl,

   and the birefringent element is capable of separating the Raman-shifted wavelengths according to their polarization, In another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a birefringent Raman-active medium located in the resonator cavity.and capable of shifting the wavelength of the pump laser beam to generate at least two .wavelengths of laser light of different polarizations; c) a polarization selector for selecting one of the polarizations of laser light; and d) an output coupler for outputting a wavelength of laser light produced'<,>by the Ramanactive medium, said wavelength of laser light having the selected polarization.

   In another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second! reflector; b) a birefringent Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pum laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least two selected wavelengths a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least two selected wavelengths; c) a polarization selector for selecting one of the polarizations of laser light; and d) an output coupler for outputting a wavelength of laser light produced by the Raman-active medium, said wavelength of laser light having the selected polarization.

   The Raman-active medium may be capable of producing different Stokes wavelengths in response to different polarizations of incident laser light. The Stokes wavelengths may have the same polarization or they may have different polarizations. The polarization selector may be a mechanically rotatable selector or it may be a Faraday rotator or an electro-optic rotator whereby selecting the polarization is accomplished electronically. The polarization selector may be one or two plates of glass at Brewster's angle and/or a cube or other polarizer. Some polarization discrimination may also be introduced through the use of mirrors at non^nor al incidence. Cooling means may be provided to cool the Raman-active medium. The cooling means may be a heat sink. The heat sink may be water cooled or air cooled.

   The Raman-active medium may be a solid.The Raman-active medium may have a straight through geometry. The ramn-active medium may have a non zig zag geometry.
The polarization selector may be transmissive for both polarizations of laser light in such a way that the transmitted intensity of a selected polarization is greater than the transmitted intensity of a deselected polarization.

   In this way, the efficiency of frequency shifting by the Raman-active medium Is greater for the selected polarization, leading to a more efficient conversion of pump power to the selected polarization.
In another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a birefringent Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate at least two wavelengths of
Raman-shifted laser light; , c) a wavelength tunable element for selecting a single wavelength of.

   Raman-shifted laser light from the at least two wavelengths; and d) an output coupler for outputting the single wavelength of Raman-shifted laser light selected by the wavelength tunable element in another embodiment of the invention the wavelength converter comprises: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a birefringent Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least two selected wavelength a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least two selected wavelengths;

   c) a wavelength tunable element for selecting a single wavelength of Raman-shifted laser light from the at least two wavelengths; and d) an output coupler for outputting the single wavelength of Raman-shifted laser light selected by the wavelength tunable element. The wavelength tunable element may be for example an optical filter, a prism, a grating, an etalon, an interference filter or some other element for selecting the wavelength of laser light to be outputted from the resonator cavity. The output coupler may be selectively transmissive for particular wavelength ranges. For example the output coupler may only transmit the two second Stokes wavelengths, and may reflect the pump and first Stokes wavelengths.

   The wavelength tunable element, or some other element, may then be used to select between the two second Stokes wavelengths. :
In a second aspect there is provided a laser system for producing at least one wavelength of output laser light from a pump laser beam, comprising; a) a laser beam generator capable of generating a pump laser beam selected from the group consisting of a visible laser beam and a UV laser beam; b) a resonator cavity comprising at least two reflectors; c) a Raman-active medium located in the resonator cavity capable 'of shifting the wavelength of the pump laser beam to generate at least one wavelength of Ramanshifted laser light;

   and d) an output coupler for outputting at least one wavelength of Raman-shifted laser light produced by the Raman-active medium.
There is also provided a laser system for producing at least one wavelength of output laser light from a pump laser beam, comprising: a) a laser beam generator capable of generating a pump laser beam selected from the group consisting of a visible laser beam and a UV laser beam; b) a resonator cavity comprising at least a first reflector and a second reflector;

   c) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide a medium to high gain Raman-shifted laser light having at least one selected wavelength; and d) an output coupler for outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength.
The laser system may comprise at least one lens for controlling the pump beam size in the Raman-active medium, The Raman-active medium may be capable of shifting the wavelength of the pump laser to generate two or more wavelengths of Raman-shifted laser light.

   The pump laser beam may be polarized or may be unpolarized. The pump laser beam may be a high quality laser beam. It may be a highly coherent laser beam. It may be a polarized laser beam. The pump laser beam may be a visible laser beam, and may have a wavelength between about 400<:>and 700nm, or between about 500 and 600[pi]m, or may be a UV laser beam, and may have a wavelength between about 250 and 400nm. The pump laser beam may be for example a green laser beam. The at least one wavelength of Raman-shifted laser light outputted by the output coupler may be a visible laser beam, and may have a wavelength between about 400 and 700nm, or between about 500 and . 700[pi]m, or may be a UV laser beam, and may have a wavelength between about 250 and 400nm, or may be a second harmonic of any of these. The pump laser beam may be a pulsed laser beam.

   The laser beam generator may be a solid state laser. It may be for example an NdYLF, NdYAG, NdYALO and NdYV04 laser.
The resonator cavity, the Raman-active medium and the output coupler may each be the same as the corresponding element of the wavelength converter of the first aspect of the invention. The laser system may additionally comprise a wavelength selector for selecting a single wavelength of Raman-shifted laser light for output by the output coupler by either deselecting all but one of the wavelengths resonating in the cavity or by selecting one of the wavelengths. The wavelength selector may be the same as the wavelength selector of the first aspect of the invention.

   The laser system may have a non-linear medium, for example a sum frequency or difference frequency generator, or it may have no non-linear medium.
There is also provided a laser system for producing at least one wavelength of output laser light from a pump laser beam, comprising: a) a laser beam generator capable of generating a pump laser beam selected from the " group consisting of a visible laser beam and a UV laser beam; and b) a wavelength converter according to the first aspect of the invention.
In a third aspect of the invention there is provided a method of generating at least one wavelength of output laser light from a pump laser beam, comprising:

   a) . passing the pump laser beam into a Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least one wavelength of Raman-shifted laser light is generated; and b) outputting at least one wavelength of Raman-shifted laser light produced by the Ramanactive medium from the resonator cavity.There is also provided a method of generating at least one wavelength of output laser light from a pump laser beam, comprising; a) passing the pump laser beam into a wavelength converter according to claim 1 ; b) passing the pump laser beam through the Raman-active medium so as to generate Raman-shifted laser light;

   c) reflecting said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide a medium to high gain Ramanshifted laser light having at least one selected wavelength; and d) outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength.
The method may additionally comprise the step of selecting a single wavelength of Ramanshifted laser light produced by the Raman-active medium for output from the resonator cavity.
In an embodiment there is provided a method of generating at least two wavelengths of output laser light from a pump laser beam, comprising;

   a) passing the pump laser beam into a Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable' of shifting the wavelength of the pump laser beam such that a plurality of wavelengths of Raman-shifted laser light are generated; and b) outputting at least two of the plurality of wavelengths of Raman-shifted laser light produced by the Raman-active medium from the resonator cavity.
In another embodiment the method comprises: a) passing the pump laser beam into a Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Raman-shifted laser light are generated;

   b) seeding the Raman-active medium with a seed beam such that the Raman-active medium produces predominantly a single wavelength of Raman-shifted laser light with a Raman-shifted wavelength corresponding to the wavelength of the seed beam; and c) outputting the single wavelength of laser light from the resonator cavity.

   In an embodiment there is provided a method of generating at least two wavelengths of output laser light from a pump laser beam, comprising: a) passing the pump laser beam into a wavelength converter according to claim 1 ; b) passing the pump laser beam through the Raman-active medium so .as to generate Raman-shifted laser light; c) reflecting said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide a medium to high gain Ramanshifted laser light haying at least one selected wavelength; and d) outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength
In another embodiment the method comprises:

   a) passing the pump laser beam into a Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least two .wavelengths of Raman-shifted laser light are generated; b) seeding the Raman-active medium with a seed beam such that the.Raman-active medium produces predominantly a single wavelength of Raman-shifted laser light with a Raman-shifted wavelength corresponding to the wavelength of the seed beam; and c) outputting the single wavelength of laser light from the resonator cavity.

   The step of seeding may comprise seeding the Raman-acilve medium with a polarized seed beam such that the Raman-active medium produces predominantly a single polarization of Ramanshifted laser light.
In another embodiment the method comprises: a) passing the pump laser beam into a birefringent Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Ramanshifted laser light are generated; b) selecting a single wavelength from the at least two wavelengths;

   and c) outputting the single wavelength of Raman-shifted laser light produced by the Ramanactive medium from the resonator cavity.
The birefringent Raman-active medium may be capable of producing two polarizations of Raman-shifted laser light spatially separated from each other, each having a different Stokes wavelength, and the step of selecting may comprise realigning at least one of the reflectors and/or the Raman-active medium.

   In another embodiment the method comprises: a) passing the pump laser beam into a birefringent Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Ramanshifted laser light of different polarization are generated; b) selecting a single polarization of Raman-shifted laser light; and c) outputting the single polarization of Raman-shifted laser light produced by the Rama[pi]actlve medium from the resonator cavity.<'>
The step of selecting a single polarization may be by means of a polarization selector. The single polarization of Raman-shifted laser light may have a single wavelength.

   In another embodiment the method comprises: a) passing the pump laser beam into a birefringent Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Ramanshifted laser light are generated; b) selecting a single Raman-shifted wavelength from the at least two wavelengths using a wavelength tunable element; and c) outputting the single wavelength of Raman-shifted laser light produced by the Ramanactive medium from the resonator cavity. The wavelength tunable element may be for example an optical filter, a prism, a grating, an etalon, an interference filter or some other element for selecting the wavelength of laser light to be outputted from the resonator cavity.

   The step of selecting may comprise tuning the wavelength tunable element in order to select a desired wavelength of Raman-shifted laser light.
In a fourth aspect of the invention there is provided a method of producing at least one wavelength of output laser light, comprising: a) generating a pump laser beam selected from the group consisting of a visible laser beam and a UV laser beam; b) passing the pump laser beam into a Raman-active medium located in a resonator cavity comprising at least two reflectors, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least one wavelength of Raman-shifted laser light is generated;

   and c) outputting at least one wavelength of Raman-shifted laser light produced by the Ramanactive medium from the laser cavity.
There is also provided a method of producing at least one wavelength of output laser light, comprising: a) generating a pump laser beam selected from the group consisting of a visible laser beam and a UV laser beam; b) passing the pump laser beam into a wavelength converter according to claim 1; c) passing the pump laser beam through the Raman-active medium so as to generate Raman-shifted laser I d) reflecting said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide a medium to high gain Raman-shifted laser light having at least one selected wavelength;

   and e) outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength The method may additionally comprise the step of selecting a single wavelength of Ramanshifted laser light produced by the Raman-active medium for output from the laser cavity.
In a fifth aspect of the invention there is provided a method of using a laser system, or a wavelength converter, according to the invention for treating, detecting or diagnosing a selected area on or in a subject requiring such diagnosis or treatment, comprising illuminating the selected area with the output laser beam from the laser system of the invention.

   The selected area may be illuminated with a laser beam having a wavelength and for a time and at a power level, which is appropriate and effective for the diagnosis or therapeutically effective for the treatmen The subject may be a mammal or vertebrate or other animal or insect, or fish. The method of the invention may find particular application in treating the eyes and skin of a mammal or vertebrate. The laser system, or a wavelength converter, may be solid-state.
In a sixth aspect of the invention there is provided a laser system, or a wavelength converter, according to the invention when used for treating, detecting or diagnosing a selected area requiring such diagnosis or treatment on or in a subject.

   The laser system, or a wavelength converter, may be solid-state.
Brief Description of the Drawings
A preferred form of the present invention will now be described by way of example with reference to the accompanying drawings wherein: Figure 1 is a graph showing the output spectrum of an external cavity KGW Raman laser pumped at 532nm (labelled L) with pump polarization aligned with the Nm axis (solid line) and the Ng axis
(dashed line), in which S' and S" are the two highest gain Raman modes of KGW (901cm-1 and
76[delta]cm-1 respectively);
Figure 2 Is a diagrammatic representation of a wavelength converter according to the invention, having an intracavity beamsplitting polarizer;
Figure 3 shows a diagram of a wavelength converter according to the present invention;
Figure 4 shows a diagram of another design of wavelength converter according to the present invention;

  
Figure 5 shows the spontaneous Raman-spectrum of KGW; Figure 6 is a graph of the output coupler transmission characteristics for Example 1; Figure 7 is a graph showing the output performance in Example 1 ; and Figure 8 is a graph showing the output spectral composition for Example 1. Detailed Description of the Preferred Embodiments The invention provides a method to generate output in the UV or visible wavelength range, for example in the range of about 250 to 700[pi]m, or about 250 to 400, or about 400 to 700, or about 250 to 300, or about 350 to 410, or about 550 to 660, or about 550 to 650 nm at multiwatt average powers. The invention may also provide a versatile system for altering the output wavelength.

   The invention provides a wavelength converter capable of providing high output power.- It may provide high conversion efficiency.
In one aspect, the invention comprises a solid-state Raman laser configured to generate multi-watt output power at a selected single wavelength or at multiple wavelengths simultaneously. It may for example, be capable of converting a pump laser beam at 532nm wavelength (green) to . an output laser beam at 580nm (yellow) or 650nm (red) or both simultaneously, and may be capable of switchably outputting one or other of these.

   Such wavelengths are useful for example in treatment of tattoos.
Novel features of the invention include:
1) Use of high thermal conductivity, low dn/dT (i.e. small refractive index dependence on temperature), medium to high gain materials such as diamond, KGd(WO*)2 (KGW),- and K (W04)2(KWY), and possible YVo4and GdVO4, as the Raman material. The thermal conductivity may be greater than about 2W/m/K, and may be greater than about 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 120, 140, 160, 180 or 200W/m/K.

   The thermal conductivity may be between about 2 and 5000W/m/K, 2 and 2500 W/m/K, 2 and 1000 W/m/K. 2 and 500 W/m/K, 2 and 200W/m/K, or between about 2 and 100, 2 and 50, 2 and 25, 5 and 200, 10 and 200, 0 and 200, 50 and 200, 100 and 200, 5 and 100, 20 and 100, 50 and 100 or 10 and 50W/m/K, and may be about 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 160, 190 or 200W/m/K. The thermal conductivity of diamond at room temperature is reportedly 630W/m/K at room temperature and is 2400W/m/K at 77<a>K. Isotopically pure diamond is reportedly up to 5000 W/m/K. The absolute value of dn/dT may be less than about 10-5K-<1>, or less than about.5*10-<6>, 2*10-<6>, 10-<6>, 5<*>10-<7>, 2<*>10<-7>or 10-<7>[kappa]<-1>. it may be between 0 and 10-5K<-1>, or between 0 and 5*10<-6>, 0 and 2*10<6>,0 and 10<6>,0 and 5*10-<7>,0 and 2*10<

  -7>,0 and 10-<7>, 10<-7>and 10<-5>, 10-<6>and 10-<5>or 10<-7>and 10<-7>K-<1>, and may be about 10-<5>, *10<-6>, 2*10<6>, 10*, 5*10-<7>, 2*10<-7>or 10-7 K<-[iota].>
2) The output coupler transmission characteristics, crystal length and the instantaneous power density of the pump mode in the Raman crystal may be selected using an established method in order to provide efficient output on a selected single wavelength or multiple wavelengths simultaneously as needed by applications. When generating multiple wavelength output, the ratio of output power at each wavelength may be engineered using the same method.

   This may be accomplished by control of the pump power density, the length of the Raman-active medium and of the transmission characteristics of the output coupler.
3) A method to achieve good pump laser pump and Raman mode overlap at high power when a large thermal lens is generated in the Raman-active medium. This may be achieved by appropriate choice of curvatures of reflectors, lens powers and the positions of the lenses and reflectors. The overlap may be measured at the output end of the Raman-active medium or at some other position, or may be an integrated value over the entire Raman-active medium. The overlap may be between about 20 and 100%, or between 30 and 100, 40 and 100, 50 and 100, 60 and 100, 70 and 100, 80 and 100, 90 and 100 or 95 and 100%, and may be about 20, 30, 40, 50, 60, 70, 80, 90, 95, 96, 97, 98, 99 or 100%.

   The pump laser beam and the Raman shifted light may overlap in the Raman-active medium over as long a length of the Raman-active medium as is practically possible.
4) A method for accessing a large range of fixed wavelengths, selectable by the user or in the stage of manufacturing.
5) Use of the two Raman shifts in KGW or KYW to access an increased number of wavelengths.
6) A method to ensure output wavelengths are generated with high spectral purity and at high average power. The high spectral purity may be greater than about 20%, or greater than about 30, 40, 50, 60, 70, 80, 85, 90, 95, 96, 97, 98 or 99% on a power basis, and may be about 20, 30, 40, 50, 60, 70, 80, 85, 90, 95, 96, 97, 99, 99 or 100% The invention encompasses pumped external cavity lasers, and UV-pumped external resonators (i.e. pumped by the 2nd, 3rd and 4th harmonics of Nd<3>* lasers, e.g.

   Nd:YAG I064nm, 532 nm, 355 nm, and 266nm). The output wavelength Is selectable in each case, for example by using rotating the bi-modal Raman-active media with respect to the pump polarization, or by . exchanging the output coupler. The invention may also be applied to coupled-cavity (three-mirror) resonators.
Good design of external cavity Raman lasers commonly aims to match the spot-size of the pump beam with that of the Raman beam. Good overlap is important for efficient conversion from the pump to Raman output. At high Raman output powers, the increased power deposited in the Raman-active medium may give rise to a thermal lens, which alters the spot-size of the Raman beam in the Raman-active medium, As a result, the overlap (i.e. mode matching) may be reduced and the efficiency may fall.
The pump laser beam may be provided by a high power laser beam generator.

   The laser beam generator may be a flash lamp pumped laser or a diode pumped laser. It may be a UV, visible or IR laser. It may be for example a green laser. It may have a power greater than about 1 , or greater than about 2, 3, , 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50 W, or between about 1 and 50W, or between about 5 and 50, 10 and 50, 25 and 50, 1 and 25, 1 and 10, 5 and 25 or 10 and 25W, and may have a power about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45 or 50W. The flash lamp pumped laser may have a frequency between about 10 and 100Hz, or between about 20 and 100, 50 and 100, 10 and 50, 10 and 25, 25 and 75, 0 and 50 or 50 and 80Hz, and may have a frequency about 10, 0, 30, 40, 50, 60, 70, 80, 90 or 100Hz.

   The diode pumped laser may have a frequency between about 2 and 100kHz, or between about 2 and 50, 2 and 20, 2 and 10, 5 and 100, 10 and 100, 20 and 100, 50 and 100, 5 and 50, 5 and 20 or 5 and 15kHz, and may have a frequency about 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 0, 5, 30, 35,.40, 6, 50, 55, 60, 65, 70, 75, 80, 85, SO, 95 or 100kHz. The input power to the laser beam generator may be between about 1 and 100W, or between about 1 and 50, 1 and 20, 1 and 10, 1 and 5, 5 and 100, 10 and 100, 50 and 100, 5 and 50, 5 and 20, 5 and 35, 10 and 35, 20 and 35, 25 and 35, 10 and 40 or 20 and 40W, and may be about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90 or 100W. The pump beam of the present invention may be a pulsed beam, or it may be converted using a Q-switch into a pulsed beam.

   The repetition rate of the pulse beam may be between about 0,5kHz and 50kHz, and may be between about 0.5kHz and 10kHz or about 0,5kHz and 1 Hz or about 1 and 50kHz or about 1 and 10kHz or about 1 and 5kHz or about 5 and 50kHz or about 10 and 50kHz or about 20 and 50kHz or about 1 and 15kHz or about 15 and 50kHz or about 10 and 30kHz or about 5 and 10kHz or about 5 and 15kHz or about 5 and 20kHz or about 5 and 25kHz or about 7.5 and 10kHz or about 7.5 and 15kHz or about 7.5 and 20kHz or about 7.5 and 25kHz or about 7.5 and 30kHz or about 10 and 15kHz or about 1 and 20kHz or about 10 and 25kHz, or about 5 and 35kHz, or about 10 and 35kHz, or about 15 and 35kHz or about 15 and 20kHz, and may be about 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9, 10,11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 25, 30, 35, 40, 5 or 50kHz.

   It may be greater than 50kHz, for example between about 50 and about 500kHz, or between about 50 and 250, 50 and 100, 100 and 500, 250 and 500, 100 and 400 or 100 and 300kHz, for example about 50, 60, 60, 00, 90, 100, 150, 200, 250, 300, 350, 400, 450 or 500kHz. The pulse duration of the pulsed beam may be in the range of about 0.5 to 300ns, or about 0.5 to 100[pi]s or about 1 to 300[pi]s, or about 50 to 300ns, or about 100 to 300ns or about 1 to 100ns, or about 1 to 50ns, or about 1 to 20ns or about 1 to 10ns or about 5 to 80ns or about 5 to 75[pi]s or about 5 to 50[pi]s or about 5 to 30ns or about 5 to 25ns or about 5 to 15ns or about 5 to 10ns or about 10 to 50ns or about 10 to 75ns or about 20 to 75ns or about 5 to 100ns or about 10 to 100ns or about 20 to 100ns or about 50 to 100ns or about 5 to 50ns or about 10 to 50ns, and may be about 0,5, 1, , 3, 4, 5, 6, 7, 8, 9, 10, 11,

   12, 13, 14, 15, 16, 17, 8, 19, 20, 2, 25, 28, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 120, 140, 160, 180, 200, 220, 240, 60,.280 or 300ns. Alternatively it may have a pulse duration of less than 0.5 s, for example between about 0.1 and about 0.5ns, or between 0.1 and 0.4, 0.1 and 0.3, 0.1 and 0.2, 0.2 and 0.5, 0.3 and 0.5, 0.4 and 0.5 or 0.2 and 0,4, for example about 0.1 , 0.2, 0.3, 0.4 or 0.5nm:

   It may have a pulse duration up to 1ms or more, for example between about 100ns and 10ms, or between about 1 microsecond and 10ms, 10 microseconds and 10ms, 100 microseconds and 10ms, 1 and 10ms, 100ns and 100 microseconds, 100ns and 10 microseconds, 100ns and 1 microseconds, 100ns and 1 microsecond, 1 microsecond and 1 ms, 1 and 100 microseconds, 1 and 10 microseconds, 10 and 100 microseconds or 10 and lOOOmicroseconds, for example about 100, 200, 300, '400, 500, 600, 700, 800 or 900ns, about 1, 2, 3, , 5, 6, 7, 8, 9, 10, 50, 100, 200, 300, 400, 500, 600, 700, 800 or 900 microseconds, or about 1, 2, 3, , 5, 6, , 8, 9 or 10ms, or may be greater than 10ms. The beam may be a continuous wave (CW) beam rather than a pulsed beam. The beam may comprise a single pulse or a sequence of pulses.

   The pulsed pump laser beam may have a, peak power of less than the damage threshold of the Raman-active medium where peak power of a pulse is determined from pulse e[pi]ergy(joules)/pulse width (seconds).. The damage threshold of the Ramanactive medium is dependent on the nature of the Raman-medium. The pump laser beam (pulsed or continuous) may have a high average power of greater than about 800mW.

   The pump laser beam (pulsed or continuous) may have an average power, of between about 0.8 and 50W, about 0.8 and 40W. about 0.8 and 30W, about 0.8 and 25W, about 0.8 and 15W, 0.8 and 10W, 0.8 and 9W, 0.8 and 8W, 0.8 and 7W, 0.8 and 6W, 0.8 and 5W, 0.8 and 4W, 0.8 and 3.5W, about 0.1 and 20W, or between about 0.1 and 10, 0.1 and 5, 0.1 and 1, about 1 and 50W, about 1 and 40W, about 1 and 30W, about 1 and 25W, about 1 and 15W about 1.2 and 20W, about 2 and 50W, about 2 and 40W, about 2 and 30W, about 2 and 25W, about 2 and 15W about 2 and 20W, about 3 and 50W, about 3 and 40W, about 3 and 30W, about 3 and 25W, about 3 and 15W about 3 and 20W, 3 and 10W, 3 and 8W, 3 and 6W, 3 and 5W, 3 and 4W, or between about 1.2 and 10, 1.2 and 5, 1.5 and 2, 2 and 3, 2 and 4, 2 and 5, 2 and 6, 2 and 7, and 8, 2 and 9, 3 and 3.5, 3 and 4, 3

   and 5, 3 and 6, 3 and 7, 3 and 8, 3 and 9, 1 and 20, 5 and 20, 0 and 20, 1 and 5, 1 and 3, 1 and 2, 2 and 10, 3 and 10, 5 and 10, 2 and 5 or 2 and 3W. The average power may be about 0.1, 0,5, 1, 1.5, 2, 2.2, 2.4, 2.6, 2.8, 3, 3.2, 3.4, 3.6, 3,8, 4, 4.5, 5, 5,5, 6, 6.5, 7, 7,5, 8, 8.5, 9, 9.5, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or20W. The conversion efficiency of the Raman-active medium, or of the wavelength converter, may be between about 1 and 90%, for example, and may be between about 10 and j90, 20 and 90, 30 and 90, 10 and 30, 1 and 50, 1 and 30, 30 and 80, 30 and 70, 30 and 60, 30 a[pi]d<'>50, 40 and 90, 50 and 90, 60 and 90, 70 and 90, 40 and 80 or 50 and 70, and may be about 1, 5, 1 , 20, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80 or 90%. The output power of the laser system, or of the wavelength converter, of the invention may

   be between about 0.1 and 12W or higher, for example, or may be greater than 12W, or may be between about 0.1 and 10, 0.5 and 10, 0.5;and 5, 0.5 and 2, 1 and 12, 1 and 9, 5 and 9, 1 and 5, 1 and 3 or 1 and 2W, and may be about 0.1, 0.5, 1, 1.1, 1.2, 1,3, 1.4, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9, 9!5, 10, 10.5, 11, 1 T.5 or 12W.

   The output power may be concentrated in a single wavelength of output laser light or may be divided between two or more wavelengths of output laser light.
It should be noted that Raman-shifted wavelengths are known as Stokes wavelengths, and that wavelengths produced by shifting an input wavelength to a Raman-active medium by an integral multiple of the Raman shift are known as the higher order Stokes wavelengths.
In the wavelength converter, or the laser system, of the present Invention, there may be one or more collimating lenses and one or more focusing lenses, for collimating and/br focusing the pump beam, and, if present, each lens may be either intracavity or e[kappa]tracavity. The focusing lens may be located between the collimating lens and the Raman-active medium. The focusing lens may focus the pump beam on the Raman-active medium.

   The size of the focused beam on the
Raman-active medium may be given by:
 <EMI ID=20.1> 
where Dmin is the focal spot size (i.e. the size of the focused beam), f is the focal length of the focusing lens, [lambda] is the wavelength of the pump laser beam and D is the beam diameter of the unfocussed beam, The pump laser beam is passed to an intracavity Raman-active medium which is capable of generating at least one, optionally at least two, wavelengths of Raman-shifted laser beams which may also differ from each other in either polarization or location or some other property. The system may be fitted with a wavelength selector which either promotes resonance of a selected wavelength of laser beam or discourages resonance within the cavity of all but one of the Raman-shifted laser beams.

   This may be by means of a polarizer, or by means of motors which orient either a mirror or the Raman-active crystal or both so that only one wavelength is capable of efficiently resonating, or may be by some other method such as seeding. The selected wavelength may be directly outputted from the laser cavity. The outputting may by means of an output coupler, which may be for example an output reflector or a polarizing beam splitter. The output reflector may be an output coupler, for decoupling and outputting an output beam from the cavity.
The pump laser beam of. the present invention may be generated by a diode-pumped laser,
 <EMI ID=20.1> 
 
and may be generated by a solid-state laser system.
In a resonator cavity according to the invention, there may potentially be a plurality of wavelengths of laser light resonating.

   Thus there may be one or more of a fundamental, or pump, wavelength (i.e. the wavelength of the pump laser beam), a first Stokes wavelength, a second Stokes wavelength and higher order Stokes wavelengths, Further, in cases in which the Ramanactive mediu has two or more Raman shifts, there may be a first and a second Stokes wavelengths from each of the two or more Raman shifted beams generated by the Raman-active medium, Therefore the present invention provides means to selectively output a wide variety of wavelengths from the cavity.
For example, if the Raman-active medium is KGW, which has Raman shifts of 768cnv<1>and 901 cm-<1>, then Table 1 shows the wavelengths that may be available from different wavelengths of pump laser beam.

   Table 1 pump wavelength Raman shift (cnr Raman-shifted wavelengths available (1<st>to 4<Ih>Stokes (nm) wavelengths in nm)
532 768 555, 579, 606, 636
901 559, 588, 621, 658
355 768 365, 375, 387, 398
 <EMI ID=21.1> 
901 367, 379, 393, 407
The power of the output laser beam from the laser system may be dependent on the wavelength of the pump laser beam, and the system may have means (such as a wavelength controller) for altering the wavelength of the pump laser beam in order to after the power of the output laser beam, as well as to alter the wavelengths of output laser light.
In constructing a wavelength converter or a laser system according to the present invention, it is crucial that components are correctly positioned in order to achieve acceptable conversion efficiency to output laser power. Materials
The materials used for the Raman-active medium are well known in the art.

   Examples of suitable Raman-active media Include KGW (potassium gadolinium tungstate), KYW (potassium yttrium tungstate) barium nitrate, lithium iodate, barium tungstate, lead tungstate calcium tungstate, gadolinium vanadate and yttrium vanadate. Each of the Raman-active media produces at least one characteristic Raman shift (to generate at least one characteristic Stokes wavelength). Commonly higher order Stokes wavelengths are available, up to at least the 4<lh>Stokes wavelength. If different pump wavelengths are provided, a wide range of output wavelengths will be accessible. 
 <EMI ID=22.2> 

Barium nitrate [Ba(N03)2] is often the chosen Raman medium due to its excellent gain coefficient for nanosecond pumps. The highest conversion efficiencies have been .obtained using millijoule pump lasers (flashlamp pumped) that operate at low pulse rates (¯30rrlz).

   Potassium gadolinium tungstate (KGW, KGd(W04)2) may be used as an alternative to Ba(NO3)2for Raman conversion of nanosecond laser pulses, KGd(WO4)2has excellent physical properties and has advantages over Ba(NO3)2for high average power due to its relatively low hermo-optic coefficient, low thermal resistivity and natural birefringence, Some properties of common Raman active materials are shown below: For KGW, it is possible to select a desired Raman shift:
- E /Nm: 901 cm<-1>(i.e. pump beam polarization parallel to the Nm axis of KGW.)
- E/Ng: 768cm<- 1>and 901cm<-1>
Table 2 shows the Raman shifts for a range of Raman-active media, and Table 3 shows the Raman shifts and corresponding Stokes wavelengths for several Raman-active media. Table 2.

   Raman shifts for selected Raman-active media
Raman-active Raman shift (cm-<1>) Crystal
CaCO31085
NaNO31066
Ba(NO3)21046
YV04890
Gd O4882
KDP 915
NaBrO3795
 <EMI ID=22.1> 
LilO3822 770
BaWO* 926
PbW04901
CaW04908
2nW04907
CdW04890
KY(W04)2765 905
KGd(W[theta]4)2768 901
NaY(W0a)2914
NaBi(WO*)2910
 <EMI ID=23.1> 
NaBi(Mo[theta] )2 877
Table 3 Raman shifts and corresponding Stokes wavelengths for selected Raman-active media from a pump wavelength of 1064nm.
crystal Raman shift 1<st>Stokes (run) 2<[pi]cl>Stokes (nm) 3"<1>Stokes (nm) (cm-<1>)
KGW 768 1158 1272 1410
KGW 901 1176 1320 1500
PbW04911 1177 1316 1494
Ba(NOa). 1048 1198 1369 1599
 <EMI ID=23.2> 
LilOs 745 1156 1264 1396
Location of elements
It is important for the efficient operation of the laser system described herein that the component parts of the system be located correctly.

   In particular, the Raman-activemedium should be located at a position in the cavity where the diameter of the beam to be wavelength converted is sufficiently small to achieve acceptable conversion efficiency. Thermal lensing arises from the inelastic nature of the stimulated Raman scattering processes. Thus for every scattering event, a small amount of the fundamental photon is deposited as heat in the Raman-active medium. This leads to a non-uniform temperature profile across the Raman-active medium. Commonly therefractive index of a Raman-active medium decreases with an increase in temperature, and consequently the Raman-active medium acts as a concave (diverging) lens. The wavelength converter or laser system of the present invention may be operated under conditions in which thermal lensing arises. The thermal lens may impact on the stability characteristics of the laser system.

   The Raman-active medium may have a negative thermal lensing effect. The thermal lensing effect of the components of the laser system may change with a change in power of the pump laser beam. The power of the thermal fens in the Raman-active medium is primarily dependant upon the power of the Raman-shifted beam, the size of the Raman-shifted beam inside the Raman-active medium, the wavelength of the Raman-shifted beam and the Raman shift. In order to adjust the cavity for the effect of the changing thermal lens of ihe Raman-active medium as it heats, the lens, or at least one of the lenses, and or one or more of the reflectors, may be moved. Other means to adjust may use the curvature of the reflectors (which may be mirrors) and/or the length of the resonator cavity.

   In general however the length of the resonator cavity should be kept as short as possible, Due to thermal lensing within the resonator cavity, in addition to curvature of the cavity reflectors and natural diffraction, the beam width of a laser beam will vary along the length of the cavity as a result of heating effects. Since the heating of components of the system is due to passage of a laser beam through those elements, the optimum location of the components will vary both with time during warm-up of the system and with the power of the laser system.

   A laser system may be designed for a particular output power, and will be designed to operate at peak efficiency after reaching normal operating temperature.
The present inventors have discovered that the resonator stability problems associated with operation of Raman solid-state lasers can be solved by designing a solid-state wavelength converter or laser system taking into account the thermal lensing power of the Raman-active medium.
With frequency conversion by SRS (stimulated Raman scattering) heat is generated inside the Raman-active medium leading to significant lensing effects and a focal length fn. These effects arise from the inelastic nature of the nonlinear process and for every scattering event, a small fraction of the photon energy (7.9% in the case of LilOa) is deposited as heat in the Ramanactive medium.

   The degree of heating increases with the power generated at the Stokes wavelengths, more specifically for every first or second Stokes photon generated inside the laser cavity, a small but fixed amount of heat is deposited inside the medium. The resulting temperature distribution which is affected by the thermal conductivity of the crystal and the size of the laser beam inside the resonator cavity causes a variation of refractive index across the medium. The thermal lens in the Raman-actfve medium, far example KGW, depends on the intracavity power density at the first-Stokes wavelength and any higher order Stokes wavelength. For KGW, the thermo-optlc coefficient (dn dT) is reportedly in the range of about -1.0 to -5.5 K at a wavelength of 1 micron.

   This means that light passing through the Raman-active medium is caused to diverge as though passing through a conventional lens with focal length "-fH". The size of the negative thermal lens in KGW may be as short as -10cm.
The thermal lens in the Raman-active medium impacts substantially on the mode overlap between the pump laser beam and the cavity laser beam in the resonator in a<'>dynamic way.
Suitably the position of the Raman-actfve medium in the cavity and or reflector (mirror) curvatures is such that the laser is capable of stable operation over a sufficiently-wide range of combinations for
FR including the special case where fpHnfinite (so that laser action can be initiated).
Suitably a curvature of at least one of the reflectors and/or the position<;

  >of the Ramanactive medium relative to the cavity configuration are such that the focal length of the Raman-active medium at the desired Raman laser output power range are maintained within an efficient operating region for mode matching. In preferred embodiments this can be achieved by optimising the cavity configuration as a function of the focal length by in addition to positioning Raman-active medium within the cavity and/or selecting a curvature of at least one of the reflectors, optimising one of more of: a separation between one or more of the reflectors and the Raman-active medium; a position of one or more of the reflectors; positions of the one or more lenses; and transmission characteristics of the output coupler.
Mode matching may also be affected by control of the temperature of the Raman-active medium.

   Additional effects such as gain focusing and self-focusing of the Raman and/or pump laser beams may affect the resonator stability but these are considered to be of lesser importance than the effects already discussed. There may be >50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, or 98% overlap between the modes of the pump and Raman laser beams.

   There may be - 50%, 53%, 55%, 58%, 60%, 63%, 65%, 68%, 70%, 73%, 75%, 78%, 80%, 83%, 85%, 88%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, 99%, or 100% overlap between the modes of the pump and Raman laser beams.
In preferred embodiments the laser is also optimized for given pump powers for optimum mode sizes in the Raman gain medium and optimum laser output power so as to obtain efficient conversion through stimulated Raman scattering (SRS) in the Raman-actfve medium whilst maintaining cavity stability and avoiding optical damage of the Raman-active medium. The optimum spot size and power density in the Raman-active medium may be a compromise between maximizing the conversion efficiency and avoiding optical damage.

   The cavity is suitably optimized so that the relative mode size of the pump beam and the laser beam in the Raman-active medium is such so as to provide efficient stable output. Suitably optical conversion efficiencies of greater than 30%, 35%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, more preferably greater than 50% are obtainable at<'>output powers up to 3W or greater. Conversion efficiencies may be up to 70%, 65%, 60%, 55%, 52% or 51%.
In order for the laser system to operate with suitable optimal efficiency the key design parameters (I.e. mirror curvatures, cavity length, positioning of the Raman-active medium) are suitably chosen so that the resonator mode size in the Raman-active medium is near-optimum at a desired operating point. One can denote the beam size (radius) in the Raman-active medium as COR.

   In cases where the laser beam is not circular, it is commonly elliptical, and the beam size may be considered along the long and short axes of the ellipse. The beam size is taken to be the distance from the beam axis to the point where the intensity of the beam falls to (e2) of the intensity of the beam axis. The beam size may vary along the length of the Raman-active medium. The beam size may be taken as the average beam size within the Raman-active medium (commonly used) or as the minimum beam size therein. The optimum value for [omega]n varies from crystal to crystal because (I) different Raman-active crystals have different Raman gains and different thresholds for optical damage. If [omega]R is too large, then the conversion efficiency of the SRS process will be lower than optimum.

   If [omega]R is too small, then (I) the optical power density in the Raman-active medium can approach the threshold for optical damage in that crystal; (ii) the thermal lens associated with the Raman-active medium may become more aberrated, resulting in increased resonator losses (due to diffraction), and (iii) higher order Stokes wavelengths may be generated, thereby affecting the spectral purity of the output laser beam. Typical values for COR are in the range of about 50 microns to 1 cm, and may be in the range 50 microns to 1mm, 50 to 500 microns, 50 to 100 microns, 100 microns to 1cm, 1mm to 1cm, 5mm to 1cm, 100 microns to 1mm, 500 microns to 1mm or 1mm to 5mm, and may be about 50, 100, 150, 200, 250, 300, 350, 400, 450, 500, 600, 700, 800 or 900 microns, or about 1, , 3, 4, 5, 6, 7, 8 or 9mm or 1cm.

   Since relatively large beam diameters are possible, in certain circumstances the pump beam may not be focused using a lens.
Suitably in a Raman-active medium such as a KGW, KYW, diamond or vanadate crystal the spot size am is optimized for stable operation and efficient conversion such that con is similar to or smaller than the pump laser beam size [omega]p.
The ratio of the mode size of the Raman-shifted laser beam in the Raman-active medium to the mode size of the pump laser beam in the Raman-active medium may be between about 0.5 and 2, or between about 0,75 and 1.5, 0,8 and 1.25 or 0.9 and 1.1, and may be about 0.5, 0.6, 0.7,
0.8, 0,9, 1, 1.1, 1.2, 1.3, 1.4, 1.5, 1,6, 1.7, 1.8, 1,

  9 or2.
In preferred embodiments the thermal lens focal lengths for the Raman-active medium at the laser output powers are determined and the position of the Raman-active medium in the resonator cavity is selected to ensure that during operation of the laser the resonator is stable.
Suitably the thermal lenses for the Raman-active medium may be calculated and then confirmed by cavity stability measurement Alternatively Ihe thermal lens may be determined by standard measurement techniques such as lateral shearing i[pi]terf[beta]rometry measurements which can also provide information on any aberrations. A suitable interferornetric technique is described in .
Revermann, H. . Pask, J.L. Blows, T, Omatsu 'Thermal lensing measurements in an intracavity
Lil[theta]3 Laser", ASSL Conference Proceedings February 2000; in J.L. Blows, J.M.

   Dawes and T.
Omatsu, 'Thermal lensing measurement in line-focus end-pumped [pi]eodymium yttrium aluminium garnet using holographic lateral shearing [iota]nterferometry", J. Applied Physics, Vol. 83, No. 6, March 1998; and in H.M Pask, J.L, Blows, J.A. Piper, M. Revermann, T. Omatsu, Thermal lensing in a barium nitrate Raman laser", ASSL Conference Proceedings February 2001.
Suitably at least the position of the Raman-active medium in the cavity is selected such that the thermal lens power for the Raman active medium falls within a stable operating region of a stability plot. A stability plot of a simple two mirror cavity is a plot of the parameters gi on the y-axis and g2on the x-axis of a graph.

   These parameters can be represented by the equations: g1= 1-LiR1(1) g2= 1-LR2(2) wherein L is the distance between the two mirrors, Ri is the radius of curvature of' one of the two mirrors and R2 is the radius of curvature of the other.
It has been determined that, for a resonator cavity to be stable,
0 <= g1x g2<= 1 (3)
If either one of gi and g2is negative and the other one is positive, their product is negative and the resonator cavity will be unstable.

   If both are positive or if both are negative and if their product is less than 1 , then the resonator cavity will be stable.
In order to ensure that the cavity remains stable for large temperature gradients in the
Raman-active medium, the length of the resonator cavity and the position of the Raman-active medium relative to the mirrors defining the resonator cavity are selected such that the laser modes do not expand to the extent that the radiation suffers large losses.

   Thus the position of the Raman active medium relative to the position of the at least two reflectors, the length of .the cavity, the curvature of at least one of the reflectors of the cavity, as well as the combination of the focal lengths of the thermal lenses formed in the Raman active medium during operation of the laser may . be such that the laser resonator (cavity), remains optically stable when the current to the pump laser is increased from zero to a desired operating laser power. The desired operating power may be such that the output power is greater than 1W. It is preferable that the resonator cavity be as short as possible, consistent with other considerations described herein.
A suitable stability plot for a twomirror resonator can be determined as follows. The ray transfer matrix (M) is calculated for a transit of the optical resonator.

   The eleme<'>nts of this matrix able an equivalent (two-mirror) resonator to be defined with equivalent g
 <EMI ID=28.3> 
parameters g[iota]"= A, g[iota] = D and L" = B. The optical system in the resonator cavity may be described by an ABCD matrix which is the product of one or more ABCD matrices, each of which corresponds to an optical element through which light passes. The ABCD law enables one to calculate the change in a Gaussian laser beam as the beam passes through a particular element. The determinant of the matrix M should be unity for a stable arrangement of the resonator cavity, i.e. AD-BC=1. The stability regime for the resonator cavity is where the cavity laser beam obeys the inequality
 <EMI ID=28.2> 
The predominant mode of the cavity laser beam may be a Gaussian beam.

   A Gaussian beam is one in which the cross-sectional power profile of the beam has a Gaussian distribution. The q parameter of a Gaussian laser beam at a particular position in a resonator needs to satisfy the ABCD law: q=(Aq+B)/(Cq+D), The solutions to this are given by:
 <EMI ID=28.3> 
able an equivalent (two-mirror) resonator to be defined with equivalent g
 <EMI ID=28.2> 

 <EMI ID=28.1> 

The allowed solution should have a negative imaginary component. The q parameter incorporates the mode size and the beam curvature, and is described in detail in the B.EA Saleh and M.C.
Teich, Fundamentals of Photonics, John Wiley and Sons, New York, 1991, the contents of which are incorporated herein by cross-reference.

   The mode size of the cavity laser beam may be determined along the resonator cavity from the q parameter.
In particular, for a system having a lens,of focal length f (i.e. refractive power 1/f) located a distance1from a first mirror having radius of curvature R[iota]:and a distance d[sum] from a second mirror having radius of curvature R2, the elements of the matrix M are:1
 <EMI ID=28.1> 
1
 <EMI ID=28.4> 
 
 <EMI ID=29.1> 
Texts describing this method are N. Hodgson and A. Weber, "Optical Resonators",
Springer-Verlag London Limited, 1997 and W. Koechner, "Solid-state Laser Engineering", SpringerV[beta]rlag, 1992.
The dynamic nature of the Raman laser resonator as the power of the laser pump is increased can be simulated by calculating g1and g2- for the thermal lens in the Raman-active crystal. When plotted on a stability plot, a curve can be defined.

   In a well-designed resonator, this curve will lie in a stable region of th stability plot (i.e. 0 <=1g2'<= 1) from the point where laser action is initiated to the point corresponding to the desired operating power,
In an embodiment a computer model is used to determine suitable cavity configurations for a particular power regime. In such an embodiment the thermal lensing power for a variety of Raman media crystals can be measured over a wide parameter space ol Raman laser output powers and mode sizes and thermally modeled. A standard resonator design program using 2irror configurations to more complex folded resonators can then be used to determine the pump and Raman laser mode sizes as a function of pump power enabling stable resonators to be designed with good mode overlap to produce output powers in specified regions from mWs to multiwatt outputs.

   The output power may be varied by varying the frequency of the pump laser beam.
The power of the laser beam in the Raman-active medium should however be below its damage threshold. The damage threshold will depend, inter alia, on the nature of the Raman-active medium. The pulse frequency of the pump laser beam should be chosen such that the system is stable and so that the damage thresholds of the elements are not exceeded due to excessive peak power loads.

   The frequency may be between about 1Hz and 50kHz, and may be between about 1 Hz and 10kHz or about 1 Hz and 1 kHz or about 1 and 100Hz or about 1 and 10Hz or about 100Hz and 50kHz or about 1 and 50kHz or about 10 and 50kHz or about 20 and 50kHz or about 1 and 15kHz or about 15 and 50kHz or about 10 and 30kHz or about 5 and 10kHz or about 5 and 15kHz or about 5 and 20kHz or about 5 and 25kHz or about 7.5 and 10kHz or about 7.5 and 15kHz or about 7.5 and 20kHz or about 7.5 and 25kHz or about 7.5 and 30kHz or about 10 and 15kHz or about 10 and 20kHz or about 10 and 25kHz, and may be about 1, 5, 10, 0, 30, 40, 50, 100, 200, 300, 400, 500, 600, 700, B00 or 900Hz or about 1, 2, 3, , 5, 6:7/8, 9, 10, 15, 0, 25, 30, 35, 40, 45 or 50kHz.

   The pulse duration of the pump laser beam may be in the range of about 1 to 100ns, or about 1 to 50ns, or about 1 to 20ns or about 1 to 10ns or about 5 to 80ns or about 5 to 75ns or about 10 to 50ns or about 10 to 75ns or about 20 to 75ns or about 5 to 100ns or about 10 to 100ns or about 20 to 100ns or about 50 to 100ns or about 5 to 50ns, or about 10 to 50ns, and may be about 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 25, 30, 5, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 or 100ns. The pulse energy, pulse duration and pump beam waist should be such that the pump beam does not exceed the damage threshold of the Raman-active medium.
The laser beam generator may be a solid state laser. It may comprise a laser material for generating the pump laser beam for the Raman-active medium of the present invention.

   The laser material of the laser beam generator suitably generates laser beams at a fundamental wavelength (1064nm for Nd:YAG) when stimulated by pump light of an appropriate wavelength, and the fundamental laser beam then propagates inside the laser resonator. Suitably the laser material is formed by one of the following crystals; Nd:YAGrNd:YLF, Nd:glass, Ti-sapphire, Erbium:glass, Ruby, Erbium:YAG, Erbium:YAB, Nd:YAI03, Yb:YAI03, Nd:SFAP, Yb:YAG, Yb;YAB, Cobalt:MgF2, Yb:YVO[Lambda], Nd:YAB, Nd:YV6 , NdiYALO, Yb.[Upsilon]LF, Nd:YCOB, Nd:GdCOB, Yb:YCOB, Yb:GdCOB or other suitable laser material. Each of these media may operate at its fundamental wavelength and/or at one or more higher harmonics. The laser beam may be in the TEMoo mode or higher order mode. The laser material may be broadband AR-coated for the 1-1.2 micron region to minimize resonator losses.

   Optionally the laser material is wavelength tunable and capable of generating high power output which can be mode-locked. The Raman-active medium suitably enables the fundamental (pump) radiation to be converted to first (or higher) Stokes wavelength through the nonlinear process Stimulated Raman Scattering (SRS). Depending on application, the Raman-active medium suitably converts the fundamental (pump) wavelength to the first Stokes wavelength, to the second Stokes wavelength or to a higher Stokes wavelength. The Raman-active medium may be broadband AR-coated for the 11.2 micron region to minimize resonator losses.

   The Raman-active medium is suitably chosen on the basis of high transmission at the fundamental and Stokes wavelengths, useful Raman shift, fairly high Raman cross-section, high damage threshold and availability in lengths exceeding 1cm and chosen such that the Raman gain is adequate. The Raman-active medium may be a crystal, and may be a single crystal. The length of the crystal may be between 0,5 and 15cm long, and may be 1 -12cm long, MOcm long, 1-7cm long 2-6cm long or 3-5cm long or 0.2, 0.25, 0.5, 0.75, 1 , 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.6, 6, 6.5, 7, 7.5, 8, 8.5, 9, or 99.5, 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5 or 15cm long. A typical dimension of the crystal is 0.5<*>0,5*y cm where y is crystal length and may be in the range 0.2-15cm or 0.5-l5cm, 0.5-12cm, 0.75-l0cm, 1-7cm, 2-6cm, 3.7cm.

   Crystal lengths that are too short may provide low conversion efficiency.- Crystal lengths that are too long require excessively long focal length lenses to provide the requited long beam waist. They may also lead to loss of spectral purity due to generation of higher order Stokes wavelengths, and also loss of gain properties and wavelength selection achievable within the pump pulse duration. Alternatively a longer path through the Raman-active medium can be achieved using a multipass of zigzag geometry such as described in Byer patent US .5673231. Suitably the Raman-active medium is a single crystal of KGW, KYW. diamond or vanadate crystals or other suitable Raman active material such as KDP (potass)um dihydrogen phosphate), KD*P (deuterated), lithium [pi]iobate, and various tungstates (KYW, CaW04), molybdate crystals, lithium iodate or barium nitrate.

   Other suitable Raman active crystals are described in the [Phi]RC Handbook of Laser or the text "Quantum Electronics" by Pa[pi]tell and Puthoff. KGW, Lil03and Ba(NOa). are preferred. KGW is a biaxial crystal with a high damage threshold, and is capable of providing Raman shifts of 768 and 901cm-<1>. Ba( 03)2 is an isotropic crystal with a high gain coefficient (11 cm/GW with 1064nm pump) leading to low threshold operation and can provide a Raman shift of 1048.6cm-<1>. KGW has a high damage threshold of about lOGWcm . KGW, Ba(NO_.)2and LilOa all have good slope efficiencies (determined by the ratio of Stokes to fundamental photon energies) with optical to optical conversion efficiencies of 70-80% being reported for all three.

   The laser system is preferably operated such that optical damage of the Raman active medium is avoided.
The following laser material/extra-cavity Raman-active medium combinations are examples that may be used, for example: Nd:YAG/Lil03, Nd:YAG/Ba(NO.)2, d:YAG KGW, NdGdV04/LH03, NdGdVO4Ba(NO3)2, NdGdVO KGW, NdYVO4LilO3, N YVO4NO3, NdYVOVKGW and Nd:YLF/CaW04.
Preferably the resonator cavity comprises at least two reflectors which can be two mirrors at least one of which is preferably curved to provide a stable output laser beam (the other mirror may be flat). The curved reflector may be concave. Other suitable reflectors that fea[pi] be used in the present invention include prisms or gratings.

   More preferably at least two curved mirrors are used, although it is possible to use more than two mirrors, different sets of mirroifs reflecting the propagating laser beam and the propagating Raman-shifted beam such as in a bow-tie resonator. Another mirror may be present such as in a dichrotc cavity. Suitable reflectors defining the resonator cavity are well known in the art and can be coated to enable operation at lower Raman thresholds for the first Stokes order thereby helping to suppress higher-order Stokes generation and self-focusing if desired. The mirrors may also be coated to have high transmission at the output wavelengths of interest. Reflectors can be provided with special dielectric 'coating for any desired frequency.

   The mirrors can provide for the laser output to be coupled out of, the cavity such as by use of a broadband dichroic mirror transmissive at the frequency of the output beam but suitably highly reflective at higher frequencies so as to cause build-up of the power intensities of the beams in the cavity. The generation of higher Stokes orders may be reduced if desired by introducing high resonator losses at frequencies lower than that of the output beam: These losses may be introduced by using input coupling and output coupling mirrors that are highly transmitting at the lower frequencies or by introducing an element into the resonator that absorbs at the lower frequencies. Alternatively a polarization beam splitter can be used to outcouple the laser output.

   The radi[upsilon]s of curvature and separation between the reflectors (cavity length) and transmission characteristics of the outcoupling mirror are suitably chosen to provide cavity stability for a sufficiently wide range of fa. The radius of curvature of the reflectors are appropriately selected on the basis of the Raman-active medium used (for some Raman-active crystals +ve effective lens powers of the reflector are desirable and for others -ve effective lens powers of the reflectors are desirable). The output mirror may be chosen (to optimize the first Stokes output) to be 10 to 90% reflective at the Raman wavelength with the other mirror being greater than 99% reflective at the Raman wavelengths.

   (i) Suitably the transmission characteristics, radius of curvatures and separation of the reflectors are tailored to achieve efficient operation of the laser system or wavelength converter. Suitably the curvature of the reflectors and cavity length are optimized to obtain the desired mode overlap. Reflectors and output couplers The transmission properties of the dielectric coatings on the cavity reflectors may be optimized to suit the output wavelength(s) of the laser system or wavelength converter. The output coupler of the cavity may be a reflector, and may be selectively transmissive for the desired output wavelength(s) of the cavity.

   It may be about 50% transmissive for the desired output wavelength(s), e.g. between about 30 and 70%, 30 and 50, 50 and 70, 40 and 60 or 45 and 55% transmissive, and may be about 30, 35, 40, 45, 50, 55, 60, 65 or 70% transmissive for the desired output wavelengths. It may be reflective, e.g. highly reflective, for wavelengths that are not selected for outputting from the cavity. It may have a reflectivity of at least about 90%, or at least about 91 , 2, 93, 94, 95, 96, 97, 98, 99, 99.5 or 99.9% for wavelengths that are not selected for outputting from the cavity.

   If the cavity has more than one output coupler, each output coupler may be selectively transmissive for a different wavelength or group of wavelengths, Thus for example when the system comprises a non-linear medium for converting the frequency of the laser beam outputting from the Raman-active medium, the reflector may be transmissive for the converted frequency and reflective for all other frequencies generated in the cavity. Each of the reflectors may be a low loss reflector. Each relector may have a loss value of less than about 2%, or less than about 1.5, 1.2, 1, 0.9, 0.8, 0.7, 0:6, 0.5, 0.4 or 0.3%, (being a percentage of the power of the light incident on a reflector) with respect to the wavelengths resonating in the cavity, or with respect to wavelengths
. that reflect from the reflector.

   Each reflector may be a low loss curved reflector SUG[Pi] that the loss value of the reflector is less than 2%. The loss value may be between 0.3 and 2%, 0.5 and 2%, 0.8 and 2%, 1 and 2%, 0.3 and 1.5%, 0,5 and 1.5%, 0.8 and 1.5%, 1 and 1.5%, 0,3 and 1%, 0.5 and 1 %, 0.6 and 1 %, or 0.9 and 1 % per reflection from each rellector. Each of the reflectors may have a loss value of less than about 2%, or less than about 1.5, 1.2. 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4 or 0.3%, with respect to each reflection of the Raman light having at least one selected wavelength resonating in the cavity such that the Raman light having at least one selected wavelength makes multiple passes through the Raman-active crystal.

   The cavity may be a stable cavity such that the cavity has a loss value of less than about 2%, or less than about 1.5, 1.2, 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4 or 0.3%, with respect to each pass of the Raman light having at least one selected wavelength in the cavity. The loss value of the Raman light having at least one selected wavelength may be between 0.3 and 2%, 0.5 and 2%, 0.8 and 2%, 1 and 2%, 0.3 and 1.5%, 0.5 and 1.5%, 0.8 and 1.5%, 1 and 1.5%, 0.3 and 1%, 0.5 and 1 %, 0.6 and 1%, or 0.9 and 1% with respect to each pass of the Raman light having at least one selected wavelength in the cavity. Wavelength selection
The present invention envisages a variety of methods in which to select the predominant Raman-shifted frequency that will resonate within the resonator cavity.

   These methods include: Seeding; Seeding may be used when the Raman-active crystal has a spontaneous Raman spectrum which includes 2 or more sufficiently strong peaks, corresponding to two or more Raman shifts. A Raman-active medium that is capable of producing more than one Stokes wavelength may be seeded by irradiating it with a seed beam of the desired wavelength, said wavelength being one of the Stokes wavelengths of the Raman-active medium. This causes the Raman-active medium to convert most or all of the photons reaching it from the laser material to the wavelength of the seed beam. Therefore the wavelength of the output laser light beam may be selected by selecting an appropriate wavelength of seed beam.

   For example, to produce a yellow output laser light beam at 555nm from a system comprising laser beam generator capable of producing a pump laser beam at 532nm, a seed beam at 565nm applied to the KGW crystal would cause it to generate a Ra anshifted laser beam at 555nm. The seed laser may be a low power diode laser or it may be an LED or it may be some other type of seed laser. Low powered diode lasers are readily available at the desired wavelengths. The power of the. seed beam should be sufficient to cause one Raman . transition to reach threshold and significantly deplete the fundamental field in order to prevent the other Raman transition from reaching threshold.

   The seed power may be between i[mu]W and10mW, or between lO[mu]W and 1mW or between lOO[mu]W and 500[mu] W, and may be about 1, 2, 5, 10, 20, 50, 100, 200, 300, 400, 500, 600, 00, 800 or 900[mu] W or about 1 , 2, 3, 4, 5, 6, 7, 8, 9 or 10mW, or it may be below i[mu]W or it may be above 10mW, The angle of incidence of the seed beam is not critical, although higher seed powers may be required if the seed power is injected off-axis. The seeding may involve seeding with a polarized beam. This causes the Raman-active medium to convert most or all of the photons reaching it from the laser material to the polarization of the seed beam. Therefore the polarization of the output laser light beam may be selected by selecting an appropriate polarization of seed beam.

   Since a particular polarization is associated with a particular set of wavelengths, this may be used to select a particular range of wavelengths Of output laser light.
Birefringence: A birefringent Raman-active crystal may produce different Stokes, wavelengths which are shifted spatially relative to each other. If the reflector at the output end of the laser system is curved, that reflector may be oriented so that only one Stokes wavelength is capable of resonating within the resonator cavity. By altering the orientation of that reflector and/or of the Raman-active crystal, a particular wavelength of output laser light may be selected. In the case where the fundamental beam is unpolarised, a preferred method may be to leave the Raman-active crystal fixed and move the mirrors to choose the particular Stokes frequency.

   In the case where the fundamental beam is polarised, it may be preferred to rotate the Raman-active crystal and adjust the mirror In order to optimise the resonance of the desired wavelength of laser light. Polarization selection: A Raman-active crystal may be capable of producing a different Stokes wavelength in response to different polarizations of incident laser light. If the incident laser light is unpolarised, such a crystal would produce two separate wavelengths of output laser light, each polarised orthogonally to each other. However the pump laser beam itself may be polarised. In this case only one first Stokes wavelength is produced corresponding to one of the two orthogonally polarised Stokes shifts, and consequently.

   The polarizer, or polarization selector, which polarises the light resonating within the resonator cavity, may be a mechanically rotatable polariser, or<'>if may be a Faraday rotator or an electro-optic rotator whereby selecting the polarization is accomplished electronically. By rotating the polarization of the polariser, a wavelength, or set of wavelengths, of output laser light from the laser system may be selected. The polariser may be located within the pump laser beam generator, or between the laser beam generator and the resonator cavity, within the resonator cavity, or after the resonator cavity.
Direct wavelength selection: A wavelength selector for directly selecting the wavelength to be outputted may be incorporated in the cavity of the invention.

   The wavelength selector may be for example an optical filter, a prism, a grating, an etalon, an interference filter or some other element 
 <EMI ID=35.1> 
for selecting the wavelength of laser light to be outputted from the resonator cavity. In this case, the selected wavelength will be outputted from the cavity, and those wavelengths not selected will be suppressed or continue to resonate within the cavity until, through the various wavelength conversion process occurring in the cavity, they are converted to the selected wavelength and outputted from the cavity. The output coupler may comprise one of the reflectors of the cavity, and may be selectively transmissive for particular wavelength ranges. For example the output coupler may only transmit the two second Stokes wavelengths, and may reflect the fundamental and first Stokes wavelengths.

   The wavelength tunable element, or some other element, may then be used to select between the two second Stokes wavelengths. In this manner, the output coupler may be a component of the selector which is used to select the wavelength to be outputted from the cavity. Wavelength Switching in External Cavity Raman lasers
The methods described below to switch between selected wavelengths are based on a change of the angle [theta] between the pump polarization direction (which is typically plane polarized) and a polarization axis of the Raman-active medium. The output spectrum of an external cavity Raman laser such that described in the present disclosure Is determined in part by the Stokes shift of the Raman medium and the output coupler transmission characteristics.

   (The output spectrum is of course also affected by the pump beam frequency.) For materials such as KGW and KYW, the dominant Raman mode is different for different polarizations, i.e. for different pump laser beam polarizations relative to the crystatlo-optic axes of the Raman-active medium, and the Raman gain on each Raman mode varies continuously, depending on the alignment of the pump polarization relative to the optical axes. Thus the dominant Raman-mode may vary with the angle [theta]. Fig. 1 shows how the output spectrum varies for the example of an output coupler that is near-optimum for generating second Stokes output.
The theory for how the output spectrum depends on the output coupler transmission characteristics is well developed.

   The decay in the pump field l_ and the growth in the Stokes fields Isn are described by the following equations:
(17)

 <EMI ID=35.2> 
and so on for 7s3, /s4etc., Here the loss coefficients [alpha]snare principally determined by the output coupler transmission values at each Stokes wavelength [lambda]sn. [omega]s[pi]are the angular frequencies (=2[pi]c/[lambda] where c is the speed of light) of the radiation fields.
The efficiency of generation of selected Stokes order Sn depends on the loss factor at the next highest order [alpha]s[pi]ti. For example, in order to achieve efficient and spectrally pure output at [lambda]sn, it is desirable to ensure that the output coupler transmission for the next highest order is high so that energy loss due to further cascading is zero or minimized. a) Case V.

   Bireffm[alpha]en Raman media with two high-gain Raman modes
According to the above theory, the output spectrum may be tailored by appropriately designing the losses in the Raman laser at each of the Stokes wavelengths. If the dominant Stokes mode is switched by varying [theta], then the output spectrum switches to one largely dominated by the output coupling values of the new spectrum of Stokes orders. In principle, switching the output wavelength from S'nto S"mmay be achieved for any combination of n and by carefully designing the output coupler transmission spectrum.

   For example, the following table lists the approximate output coupler transmission values for achieving switching from S<M>2 (yellow 579nm output for the 532nm pumped KGW Raman laser) and S' (red 658nm for the 532[pi]m pumped KGW Raman laser).
Stokes Order Wavelength Output Coupling (%) (nm)
L (fundamental) 532 0 (i.e. double pass pump)
S"1 555 0
S'1 559 0
S"2 579 50
S'2 588 0
S"3 606 >50%
S'3 621 0
S"4 636 Anv value
S'4 658 50%
S"5 669 Anv value
 <EMI ID=36.1> 
S'6 700 >50%
In the table, the non-zero output coupling values are estimates based on empirical evidence. For 0% values, in practice the value is <0.5%. Note that for E parallel to Ng in KGW .(where E is the pump beam polarization and Ng is a crystallographic axis of the KGW), the gain of the 901cm-[iota] Raman mode is less than that of the 768cm-[iota] dominant mode but is still significant.

   As a result, the Raman laser may generate output at 579nm and 658nm simultaneously for the conditions of the above table. Thus in the present example, S'1 and/or S"1 (depending on angle [theta]) resonates within the cavity and is not outputted, as the output coupling for these wavelengths is 0. If the polarization of the pump laser beam is fully aligned with the polarization axis of the Raman active medium corresponding to S'1, then only S'1 will be generated. If the polarization of the pump laser beam is fully aligned with the polarization axis of the Raman active medium corresponding to' S"1 , then only S"1 will be generated. At intermediate angles, both S' and S" will be produced, with the ratio dependent on the angle [theta]. If S*1 is present, it can generate S"2 on interacting with the Raman active medium, and this may be outputted through the output coupler.

   If S'1 is present, it can generate S'2. However the output coupler, in this example, can not transmit S'2, nor S'3, and thus this polarization cascades to S'4, which may be outputted by the output coupler. Thus switching between output of S"2 and S'4 may be achieved by varying the angle [theta].
The same principal may be used to switch between multiwavelength output, (S'j/.") and a single wavelength (S"m), or between multiwavelength output amongst the two sets of Stokes lines
(S^.and S4m)b) Case 2:

   Intracavity Polarizer and isotropic Raman media In this case, a beamsplitting polarizer is placed in the resonator so that the two orthogonal polarizations in the resonator impinge on different output couplers, This is illustrated in Fig.2.
The orthogonal components of the pump and the Stokes fields, which are assumed to have similar polarization states, are thus defined by separate resonators. By varying the angle [theta] between the pump and the polarizer axes, the output spectrum may be determined by the selected output coupler or by both output couplers simultaneously. With reference to Fig. 2, pump beam 200 (from a pump laser, not shown for simplicity) is polarized. It enters cavrty 205 through input coupler 210, and passes into Raman active medium 220.

   The resulting cavity laser beam passes into beamsplitting polarizer 230, which is capable of directing different polarizations of the cavity laser beam to different output couplers: one polarization may be directed to output coupler 240, to exit as output beam 250, and another polarization may be directed to output coupler 260 to exit as output beam 270. By rotating pump beam 200 or cavity 205, different ratios of output beams 250 and 270 may be achieved.
In selecting a polarizer, the contrast ratio, maximum transmission value and wavelength dependence are important considerations. Rochon, Wollaston, Gla[pi] Laser, beamsplitter cubes and birefringent crystals are all relevant types of polarizers. The outputs corresponding to each output coupler may exit the Raman laser at different angles or on different beam paths, depending on which type of polarizer is used.

   For birefringent crystal polarizers, Rochon or Wollaston polarizers, the ordinary and extraordinary beams may emerge separaled by a small angle or a small distance: in these cases it may be a single output-coupler optic may be used having two regions on the optic with different coatings. The output coupler may curved or flat. c) Case 3: Intracavity polarizer and birefringent Raman media
This option combines options a) and b) above. This may provide a solution to a potential problem in a) that the coupling transmission spectrum may not be practically achievable (especially with adequate damage threshold), in order to provide this solution, the axis of the polarizer and one of the optical axes of the Raman crystal should be aligned. A 90 degree change, in [theta] therefore simultaneously switches the dominant Raman mode and the output coupler defining the Raman resonator.

   As a result, design output coupler transmission characteristics for each output coupler are less stringent than for the case of single output coupler in a). A possible further advantage of this scheme compared to a) is that the switching of output wavelength also changes the exit beam path from the Raman laser.
In the most general case, the angle between the axes of the polarizer and Raman crystal optical axis may be arbitrary. It may then be possible to select, by varying [theta], either a mixture of two Raman modes, or simultaneous output through the two output couplers, or continuous mixture of both. For these intermediate settings, the gain will be less than maximum gain, and the energy is essentially split between two resonators, and thus Raman conversion will be less efficient compared to the more specific case discussed above.

   With regard to options to a) and c);
The number of Raman modes of the material may be greater than two so that the above arguments may be generalized for S', S", S'", etc.
For angles such that the gain of two Raman modes are similar (e.g. for E parallel to Ng in KGW), it is possible to generate Stokes wavelengths which are a cascade involving both Raman modes.
 <EMI ID=38.1> 
<">Anamorphic lensing
For birefringent Raman media like KGW and KYW, the thermal conductivity and thermo-optic coefficients are dependent on the direction of heat flow and the polarization direction of the incident beam.

   For example, for propagation along the Np axis of KGW, the thermal conductivity and.dn/dT values are:
 <EMI ID=38.1> 
<"> Thermal conductivity dn dT(x10-6 K-i)
(W/m*/K)
E parallel to 2.6 -0.8
Nm
E parallel to 3.4 -5.5
 <EMI ID=39.1> 
Nfl
The temperature profile is thus not rotation&lly symmetric and the refractive index is dependent on the temperature profile and the beam polarization, and as a result, the thermal lens is anamo[phi]hic. At high average power, the anamorphic lens becomes significant and alters the spatial beam characteristics of the pump and Stokes beams in the resonator. In order to optimise conversion efficiency at high powers, anamorphic resonator optics such as cylindrical concave mirrors, are needed to ensure good overlap between the pump and Stokes beams.

   The mirror design may be achieved by calculating waist size of the resonator modes for both the two preferred directions of the anamo[phi]hic lensing and ensuring good overlap with the pump mode size. For materials such as KGW, the anlsotropic thermal and optical properties' ay cause an anamorphic lens of strength in one plane (e.g. the Ng direction) that is up to 5 times stronger the orthogonal plane (e.g. the Nm direction). As the lens is negative [iota] KGW, the lenses have the effect of expanding the mode size in the Raman crystal by a larger amount in the plane having the stronger lensing. The mode size in one plane may therefore increase by up to 5 times.

   In order to maintain good overlap between the pump and Raman modes at high power, it is therefore important to select non-spherical (e.g. cylindrical or ellipsoidal) end mirrors (reflectors) with lens strengths in each plane which provide an overall spot-size that matches the pump spotsize. It may be an advantage to alter the pump spot to match that of the Raman mode. However, the change in pump mode area may also affect the Raman gain. The end resonator mirrors may have a lens strength in one plane that is 5 times the strength in the other.

   A focusing lens may be included in the cavity to compensate for the anamo[phi]hic nature of the Raman-active medium.
For example example, for a representative wavelength converter according to the present invention in which KGW is used as the Raman-active medium, when Raman output powers at 588[pi]m exceed about 2W, the lens in the KGW may become significant. The lens in the Ng direction may in that case be about 5 times stronger than the along Nm, Therefore to maintain conversion efficiency, special ellipsoidal mirrors with curvature in the Nm plane around 175mm and in the Ng plane around 125mm may be used. Alternatively, the pump spot size may be increased in the Ng direction by a factor 2-3.

   Cavity length
Case A; The pump pulse duration is comparable to the transit time of the light through the Raman crystal
For pump pulses which are of comparable duration io the transit time of the Raman crystal, it is important that the reflected beam from the output coupler transits the crystal while the pump field is present for the feedback from the output coupler to achieve a positive affect on the build-up and evolution of the Stokes fields, (e.g. for modelocked Nd:

  YAG pulses the pulse duration is ¯2Q0ps, which corresponds to the time required to pass through 3cm of KGW.)
Assuming single pass pumping of the Raman medium (which would occur if the output coupler was transmissive at the pump wavelength), then it is important that feedback from the output coupler through the Raman medium is within the duration of the pump pulse, in order for the output coupler to affect the build-up and evolution of Stokes fields, i.e.
Raman active medium - output coupler distance < pump pulse duration * speed of light 12
If this criterion is not satisfied, the output coupler mirror has no effect on the Raman laser other than to attenuate the output.

   The distance between the Raman medium and the input coupler mirror may also important due to the presence backwards Stokes generation.
For double pass pumping, the distance between the Raman crystal and the output coupler is not important from the point of view of temporal overlap (i.e. the pump and Stokes pulses overlap).
However, the effects of wavelength dispersion, which act to temporally displace different wavelengths, may become important if the optical path is long and dispersive media are present in the resonator. The effect of a backwards generated Raman beam may be controlled depending on whether the Raman medium is at one end or at the midpoint of the resonator,
Case B: The pump pulse duration is long compared to the transit time of the light through the
Raman crystal.

   The cavity length should be as short as practicable so thai the number of round-trips of the
Stokes radiation during the pump pulse duration Is maximized. Conveniently the number of round trips during the pump pulse duration may be greater than about 5, or greater than about 10, 15, 20,
26, 30, 35, 0 or 50. It may be between about 5 and 50, or between about 5 and 30, 5 and 20, 5 and 10, 10 and 50, 20 and 50, 10 and 30 or 10 and 20, for example about 5, 10, 15, 20, 25, 30, 35, 40, 45 or 50. In doing so, the wavelength converter provides suflicient gain for efficient wavelength conversion. In an example, if the pulse time is 10[pi]s and the cavity is 6cm long, ihe number of round-trips during the pulse duration would be about 15-20.

   By appealing to the above equations for Stokes generation, it may be shown that this assists in: * reducing the threshold for the Raman laser, increasing the Raman laser conversion efficiency, and
enhancing the degree of control on the output spectrum (since the effect of [alpha] is increased). The cavity length may be as short as Is practicable. This may have the benefit of controlling thermal lensing effects. The cavity length may be about 5mm longer than the Raman active medium (for example between about 1 and about 10mm longer, or between about 1 and 5, 5 and 10, 2 and 8, 3 and 7 or 4 and 6mm, for example about 1 , 2, 3, , 5,6 , 7, 8, 9 or 10mm longer).

   The Raman active medium may be for example about 50mm long, but may be any convenient length, for example , between about 1 and about 100mm, or between about 1 and 20, 1 and 50, 5 and 20, 5 and 50, 0 and 100, 20 and 80, 20 and 60, 20 and 40, 40 and 100, 60 and 100, B0 and 100, 0' nd 80, 40 and 60 or 45 and 55mm, for example about 1 , 2, 3, 4, 5, 6, 7, 8, 9, 10, 15, 20, 30, 40, 50, 60, 70, 80, 90 or 100mm. Thus the cavity may be between about 25 and about 105mm long, or some other convenient length. In order to use short lengths of Raman-active medium, a Raman-active medium of high Raman gain coefficient is necessary in order to achieve the necessary gain to obtain high conversion efficiency. Thus for example, diamond has a high Raman gain coefficient, and may be used efficiently in lengths as short as 0.5cm.

   In this case the cavity may be as short as about 0.5cm (If the reflectors, which may be mirrors, are disposed on the faces of the Raman-active medium). Thus a practical guide is that the cavity may be at least about 0.6cm long, or at least about 1 , 1.2, 1.4, 1.6, 1.8, 2, .5, 3, 3.5, 4, 4.5, 5, 6, 7, 8, 9 or 10cm long.
In one example, the Raman-active medium is located at the midpoint of the resonator, and the resonator length is equal to the pump period (i.e. the distance between successive pulses of the pump laser beam), that is, the resonator length is equivalent to the length over which light travels during an inte[phi]ulse period of pump pulses.

   In this case, good overlap may be obtained not only for the backwards propagating Stokes-shifted laser beam pulse and a subsequent pump laser beam pulse, but also for the same pulse propagating through the Raman-active medium after a complete round trip through the cavity and a second subsequent pulse. This may be called a "sync-pumped" Raman laser. Such a setup may be used with pump lasers that have high pulse rates, such as modelocked lasers.<'>Use of the laser
The invention includes a method of using laser light for treating, detecting or diagnosing a selected area requiring such diagnosis or treatment on or in a subject comprising illuminating the selected area with the output laser beam of the invention. The invention may also comprise use of an aim beam in order to aim the output laser beam towards the selected area.

   The aim beam may have a wavelength in the visible range, Accordingly, the laser system may also comprise a source of the aim beam, which may be a diode laser, an LED or some other suitable source. A mirror, which may be a dichroic mirror, may also be provided in order to direct the aim. beam In the same direction as the output laser beam. The selected area may be illuminated with a laser beam having a wavelength for a time and at a power level which is appropriate and effective for the diagnosis or therapeutically effective for the treatment. The subject may be a mammal or vertebrate or other animal or insect, or fish. The subject may be a mammal or vertebrate which is a bovine, human, ovine, equine, caprine, leporine, feline or canine vertebrate. Advantageously the vertebrate is a bovine, human, ovine, equine, caprine, leporine, domestic fowl, feline or canine vertebrate.

   The method of the invention finds particular application in treating the eyes and skin of a mammal or vertebrate. A yellow/green laser beam produced by the system or method of the invention has the advantage of having selectable wavelengths of 532 and 579nm which are particularly advantageous in treating, detecting or diagnosing certain disorders especially certain disorders in ophthalmology and dermatology.
The invention includes a method of using laser light for displaying laser light on a selected area comprising illuminating the selected area with the output laser beam of the invention .

   Other applications for the laser system, or the wavelength converter, ,of the present invention include remote sensing of biological materials such as proteins, for example in the range 280-300nm or remote sensing of atmospheric species, for example hydroxyl at 308nm.
The laser of the present invention may be capable of selectively outputting a single wavelength or of selectively outputting more than one wavelength simultaneously. In the latter case, when applied for example to treatment of tattoos, more than one pigment of the tattoo may be targeted at the same time. For example a green pump laser beam may be converted using the cavity described herein into yellow or red or a combination of yellow and red (579 amd 621 nm), and one or more of these options may be selectable within the same device.

   Similarly the device may be switchable between 606 and 588nm and a combination of the two. Example 1
Fig. 3 shows a diagram of a wavelength converter according to the present invention. In Fig. 3, wavelength converter 10 comprises resonator cavity 15 comprising first reflector 20, with focal length 15cm and second reflector 25, with focal length 20cm, and is 60mm long. First reflector 20 Is highly transmitting at 532nm and is highly reflecting at wavelengths, above 550nm. Lens 27 is provided for focusing the pump beam on Raman-active medium 30. Lens 27 is located 100mm from the centrepoint of Raman-active medium 30. Raman-active medium 30 is a 5cm long KGW crystal located in the resonator cavity equidistant between reflectors 20 and 25, and is capable of shifting the wavelength of pump laser beam 35 (having wavelength 532nm) to generate the various Stokes wavelengths.

   Second reflector 25 is highly reflecting at 532[pi]m and 559nm,< and is partially transmitting at 588[pi]m and highly transmitting at 621 nm. It acts as and [omicron]utput.coupler for outputting the desired wavelengths of laser light produced by Raman-active medium 30 as output laser beam 40. Thus wavelength converter 10 is pumped by a 532 nm pulsed pump laser beam 35, which passes through reflector 20 to Raman-active medium 30. Upon stimulation by beam 35, Ramanactive medium 30 produces Raman-shifted wavelengths at 559, 588 and 621 nm. These resonate within cavity 10, and second reflector 25 allows wavelengths 588 and 621nm to exit the cavity as output laser beam 40. The converter and generated 1-2W of output power at 588nm.

   An input pump laser power of 2.3W generated an output power of 1.35W (59% efficiency) divided 60% 588nm a[pi]d 40% 621nm.
Details of the resonator used in Example 1 are summarized below: Pump Laser;
Air-cooled diode pumped
2.5W output power
Pulse rate 5-10kHz
Pulse duration 8-10ns
Beam quality 2=<1.5 Input Dichroic Mirror:
15cm adius-of-curvature - HT pump (92%T)
HR Stokes (>95% for [lambda]>555[pi]m) KGW Crystal (undoped):
5mm x 5mm x 50mm
Propagation along b-axis Output Coupler:
20cm radius of curvature
The transmission characteristics of the output coupler are shown in Fig. 6.

   Details of output performance characteristics are detailed below:
Polarisation Output Power Conversion Efficiency Slope Efficiency
E/Nm 1.35 58% 78%
E/ Ng 1.17W 50% 67%
 <EMI ID=43.1> 

This is shown graphically in Fig.7 Spectral Composition: The spectral purity and the output Stokes order are functions of the input power density and the resonator spectral characteristics. Single or multi-line output may be selected via the output coupler spectral characteristics. The spectral composition is shown graphically in Fig.8. Output power and efficiency:
Maximum slope efficiency were obtained for input powers <0.6 W.

   Reasons for the decrease in slope-efficiency for input powers >0.6W include thermal lens formation in the Raman medium or mode changes in the pump beam.
This example has demonstrated:
An all-solid-state Q-switched 532 nm pumped KGd(W04)2 Raman laser generating 1.35 W Stokes output at 58% efficiency.  The conversion efficiency is the highest reported for nanosecond pump lasers and approaches the quantum limit (90%).
Efficient output at multiwatt output powers, which is evidence for the excellent thermal properties of KGd(W04)2.
Example 1 The laser system for this experiment is shown in Fig. 4. Pump laser 50 was a diode-pumped, coplanar folded Nd:YAG slab laser repetitively Q-switched at 5kHz with pulse duration 10ns. Laser 50 was extra-cavity frequency doubled. The resulting pump beam 55 was directed towards cavity 60 by means o1 reflector 65.

   The polarization of pump beam 55 was oriented to the vertical using alf-waveplate 70 and focused into KGd(W0-[iota])2 crystal 80 (hereinafter referred to as KGW crystal 80) using 100mm focal length lens 75. KGW crystal 80 was supplied by EKSMA, cut for laser propagation along the b-axis, had dimensions 5 mm x 5 mm x 50mm and was AR<'>coated for 532600 nm. Raman resonator cavity 60 was a 3 mirror cavity folded by plane dichroic input mirror 85 placed at ¯60[deg.] to the incident s-polarized pump beam. This angle was chosen for maximum reflection for the Stokes orders (R>98%) and high transmission at 532 nm (T-90%). The other mirrors of cavity 60 were reflector 90 and output reflector 95. Best overlap between the pump waist and the resonator mode was achieved for high reflector and output coupler mirrors 90 and 95 of concave radii-of-curvature ¯200mm spaced as closely as practicable.

   A variety of output couplers 95 were used, having transmission values at each Stokes wavelength as listed in Table 4 below. The Stokes and pump powers were measured using power meter (Ophir). The pump powers and resulting calculated efficiencies are based on the measured pump power transmitted by input mirror 85 and incident on Raman crystal 80, A small amount of residual 1064nm fundamental output was produced by pump module 50 (typically <25mW depending on the chosen output coupler), however, this was removed from all the output power measurements. The spectral content of the Stokes output 100 was measured using a fibre-coupled spectrometer (Spectra-Array, Ocean Optics).

   Transmission values of the output couplers 95 were measured using a spectrophotometer (Cary5, Varian). 
Table 4
1<st>Stokes 2<nd>Stokes 3<rd>Stokes 4<th>Stokes
Target Wavelength 555nm 559nm 579nm 589nm 606nm 622nm 636nm 658nm
Pump polarization direction c-axis a-axis c-axis a-axis c-axis a-axis c-axis a-axis
Maximum Raman Power (mW) 314 386 245 396 155 215 89 192
Overall Efficiency (%) 30 36 25 39 17 ! ,22 8 18
 <EMI ID=46.1> 
Threshold (mW) 550 440 475 360 320 430 410 420 Approx.

   Spectral Content of Output 50%@555 60%@559 >99%@579 >90%@589 65%@606 >90%@622 70%@636 80%@658
50%@559 40%@589 <10%@622 15%@579 <10%@658 23%@669 20%<@>622 Output Coupling (%) 3%@555 30%@559 40%@579 30%@559 80%@588
Slope efficiency (%) 68 64 47_
10%@583 5%@641
7%@612 2%@606
7%@636
70%@589 50%@606 1%@622 61%@669 2%@622
75%@622 1%@579 80%@658 0.4%@606 72%@658 2%@583 17%@636 38%@612 41%@641 90%@636
 <EMI ID=46.2> 
62 28 40 13 30 
Figure 5 shows the spontaneous Raman-spectrum of KGW, which illustrates the different wavelengths which may be obtained at different orientations of the KQW relative to a pump laser beam.

Claims

Claims:

1. A wavelength converter for generating at least one wavelength of output laser light from a pump laser beam, comprising: a) a resonator cavity comprising at least a first reflector and a second reflector; b) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light, said first and second reflectors being adapted and disposed so as to reflect sai flaman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide medium to high gain Raman-shifted laser light having at least one selected wavelength; and c) an output coupler for outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength.

2. The wavelength converter of claim 1 wherein the gain is sufficiently high as to generate output laser light with a power of at least about 400W. 3. The wavelength converter of claim 1 wherein the pump laser beam is mode matched with the Raman-sniffed laser light.

4. The wavelength converter of claim 1 wherein at least one of the first and second reflectors is adapted and disposed so as to focus the Raman-shifted laser light in the Raman-active medium. 5. The wavelength converter of claim 1 wherein at least one of the first and second reflectors is adapted and disposed to reflect the Raman-shifted laser light so as to achieve temporal and spatial overlap between the Raman-shifted laser light and the pump laser beam within the

Raman-active medium.

6. The wavelength converter of claim 1 additionally comprising a focusing lens for focusing the pump laser beam on or in the Raman-active medium.

7. The wavelength converter of claim 1 wherein at least one parameter selected from the position of the Raman-active medium in the resonator cavity and the distance between the Ramanactive medium and the output coupler is such that a laser beam pulse reflected from the output coupler reenters the Raman-active medium while either a part of said laser beam pulse or another laser beam pulse is present in the Raman-active medium.

8. The wavelength converter of claim 1 wherein the Raman-active medium is capable of shifting the wavelength of the pump laser beam to generate Raman-shifted laser light having two or more wavelengths.

9. The wavelength converter of claim 1 wherein the Raman-active medium is selected from ihe group consisting of diamond, KGW (KGd(WQ4)2), KYW (KY(W04)2), YV04, GdV04, Ba(N03)2, CaW04, BaW04and LiIO3.

10. The wavelength converter of claim 1 wherein the output coupler comprises the second reflector. 11. The wavelength converter of claim 1 where the Raman-active medium is birefringent.

1 . The wavelength converter of claim 11. wherein the Raman-active medium comprises a separate Raman-active element and birefringent element, whereby the Raman-active element is capable of generating Raman-shifted wavelengths of laser light, and the birefringent element is capable of separating the Raman-shifted wavelengths according to their polarization.. 13.

The wavelength converter of claim 11 comprising a rotator for rotating a polarization of the pump laser beam relative to the Raman-active medium, the output coupler being capable of selectively outputting a predetermined set of wavelengths, whereby, when the polarization of the pump laser beam is aligned with a first polarization axis of the Raman-active medium a first output wavelength or group of wavelengths is outputted from the cavity, and when the polarization of the pump laser beam is aligned with a second polarization axis of the Raman-active medium a second output wavelength or group of wavelengths is outputted from the cavity.

14. The wavelength converter of claim 11 wherein at least one of the first and second reflectors comprises anamorphic resonator optics to ensure good overlap in the, Raman-active medium between the pump laser beam and the Raman-shifted laser light. 15: The wavelength converter of claim 1 further comprising a non-linear medium.

16. The wavelength converter of claim 15 wherein the non-linear medium is selected from the group consisting of a sum frequency generator, a frequency doubler and a difference frequency generator.

17. The wavelength converter of claim 1 additionally comprising a wavelength selector for selecting a single wavelength of output laser light by either deselecting all but one of the wavelengths resonating in the cavity or by selecting one of the wavelengths.

18. The wavelength converter of claim 17 wherein the wavelength selector comprises a seeding device to seed the Raman-active medium with a seed beam in order to cause it to produce predominantly a single wavelength of Raman-shifted laser light with a Raman-shifted wavelength corresponding to the wavelength of the seed beam.

19. The wavelength converter of claim 17 wherein the Raman-active medium is birefringent and is capable of generating two polarizations of Raman-shifted laser light spatially separated from each other, each having a different Stokes wavelength, said wavelength selector being capable of selecting the single wavelength of Raman-shifted laser light by realigning one of the reflectors and/or the Raman-active medium.

20. The wavelength converter of claim 17 wherein the Raman-active medium is birefringent and is capable of shifting the wavelength of the pump laser beam to generate at least two wavelengths of Raman-shifted laser light, and the cavity comprises a wavelength tunable element for selecting a single wavelength of Raman-shifted laser light from the at least two wavelengths.

21. The wavelength converter of claim 1 additionally comprising: f) a polarizing beam splitter located within the cavity, g) a rotator for rotating a polarization of the pump laser beam relative to the Raman active medium; and h) a second output coupler; wherein the output coupler comprises a first output coupler and wherein the polarizing beam splitter is capable of allowing a first polarization of the Raman-shifted laser light to pass to:the first output coupler and a second polarization of the Raman-shifted laser light to pass to the second output coupler.

22. The wavelength converter of claim 21 wherein the first output coupler and the second output coupler are capable of selectively transmitting Raman-shifted laser light having desired output wavelengths.

23. The wavelength converter of claim 1 additionally comprising: d) a beam splitter located within the cavity, e) a rotator for rotating a polarization of the pump laser beam relative to the Raman-active medium, and f) a second output coupler, wherein the output coupler comprises a first output coupler and wherein the beam splitter is capable allowing a first portion of the Raman-shifted light to pass to the first output coupler and a second portion of the Raman-shifted light to pass to the second output coupler, whereby the first and second output couplers are capable of selectively transmitting Raman-shifted laser light having desired output wavelengths.

24. A laser system for producing at least one wavelength of output laser light,, comprising: a) a laser beam generator capable of generating a pump laser beam selected from the group consisting of a visible laser beam and a UV laser beam; b) a resonator cavity comprising at least a first reflector and a second reflector; c) a Raman-active medium located in the resonator cavity and capable of shifting the wavelength of the pump laser beam to generate Rama[pi]-sh'rfted laser light, said first and second reflectors being adapted and disposed so as to reflect said Raman-shifted laser llight having at least one selected wavelength a plurality of times through said Raman medium so as to provide a medium to high gain Raman-shifted laser light having at least one selected wavelength;

and d) an output coupler for outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength.

25. The laser system of claim 24 wherein the gain is sufficiently high as to generate output laser light with a power of at least about 400W.

26. The laser system of claim 24 wherein the pump laser beam has a wavelength between about 250 and 700[pi]m. 27. A method of generating at least one wavelength of output laser light from a pump laser beam, comprising: e) passing the pump laser beam into a wavelength converter according to claim 1 ; f) passing the pump laser beam through the Raman-active medium so as to generate Raman-shifted laser light; g) reflecting said Raman-shifted laser light having at least one selected wavelength a plurality of times through said Raman medium so as to provide a medium to high gain Ramanshifted laser light having at least one selected wavelength;

and h) outputting from the resonator cavity said medium to high gain Raman-shifted laser light having at least one selected wavelength . 28, The method of claim 27 wherein the gain is sufficiently high as to generate output laser light with a power of at least about 400W.

29. The method of claim 27 comprising mode matching the pump laser beam and the Raman-shifted laser light.

30. The method of claim 27 additionally comprising the step of selecting a single wavelength or group of wavelengths of Raman-shifted laser light produced by the Raman-active medium for output from the resonator cavity,

31. The method of claim 27 comprising seeding the Raman-active medium with a seed beam such that the Raman-active medium produces predominantly a single wavelength of Ramanshifted laser light with a Raman-shifted wavelength corresponding to the wavelength of the seed beam.

32. The method of claim 27 wherein the Raman-active medium is birefringent and is capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Raman-shifted laser light are generated which are spatially separated from each other, said method comprising realigning at least one component selected from the group consisting of the reflectors and the Raman-active medium so as to selectively output one of said at least two wavelengths of Raman-shifted laser light.

33. The method of claim 27 wherein the Raman-active medium is capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Raman-shifted laser light of different polarization are generated, said method comprising selecting a single .polarization of Raman-shifted laser light so as to selectively output said single polarization of Raman-shifted laser light.

34. The method of claim 27 wherein the Raman-active medium is birefringent, said Raman-active medium being capable of shifting the wavelength of the pump laser beam such that at least two wavelengths of Raman-shied laser light are generated said method comprising the step of selecting one or more Raman-shifted wavelengths from the at least two wavelengths using a wavelength tunable element and outputting the selected one or more wavelengths of Ramanshifted laser light from the resonator cavity.

35. The method of claim 27 wherein the Raman-active medium is birefringent, wherein the cavity comprises a rotator for rotating a polarization of the pump laser beam reiatlve.to the Ramanactive medium, and wherein the output coupler is capable of selectively outputting a:predetermined set of wavelengths, said process comprising rotating the polarization of the pump laser beam relative to the Raman-active medium such that a desired output wavelength or group of wavelengths is outputted from the cavity, 36.

The method of claim 27 wherein the wavelength converter additionally comprises: d) a polarizing beam splitter located within the cavity, e) a rotator for rotating a polarization of the pump laser beam relative to the Raman-active medium; and f) a second output coupler; said process comprising rotating a polarization of the pump laser beam relative to the Ramanactive medium such that the polarizing beam splitter directs a first polarization of the Raman-shifted laser light to the first output coupler or a second polarization of the Raman-shifted laser light to the second output coupler or both.

37. The method of claim 27 additionally comprising generating the pump laser beam using a laser beam generator, said pump laser beam being selected from the group consisting of a visible laser beam and a UV laser beam.

38. A method for detecting or diagnosing a selected area on or in a subject requiring such diagnosis or treatment, comprising illuminating the selected area with an output laser beam from a wavelength converter according to claim 1.