DE102004053137A1 - Multispectral laser with multiple gain elements - Google Patents

Abstract

A simple and compact laser resonator is presented, with which it is possible to operate an array of individual gain elements, preferably semiconductor lasers, with a sequence of wavelengths, which can then simply be spectrally superimposed outside the resonator. The arrangement avoids high power densities on optical components within the resonator by dispensing with resonator-internal superposition of the individual beams. DOLLAR A The common laser resonator is very compact and ideally consists of only three elements: the bearing bar (1), a cylindrical collimating lens (3, 25), which is slightly tilted about the optical axis to radiate the individual beams at different angles, and a feedback grating (5, 7) in Littrow arrangement (see Fig.). The useful light (4), an array of beams of different wavelengths, z. B. can be easily decoupled via the zeroth diffraction order of the grid. By means of a further lens (11) or grating and a dispersive element (12) matching thereto, the individual beams can be made practically completely collinear (13). DOLLAR A Instead of tilting the collimator (3), a grating with spatially varying grating period can be used, eg. As a specially designed (volume Bragg) grid, or a grid with spatially varying surface normal, z. B. a twisted conventional grid. DOLLAR A decoupling of the useful light (4) can over the zeroth diffraction order of the grating ...

Description

International patent Classification IPC (proposal)

• H01S05 / 40, H01S05 / 14, H01S05 / 0687, H01S01 / 082, H01S01 / 05, H01S01 / 23

technical Environment of the invention

The The invention relates to increasing the power density of Lasers, in particular semiconductor lasers, by different beams wavelength superimposed by means of spectral multiplexing become.

Background and stand of the technique

It is known (eg WO 03/055018) that very compact external resonators can drastically improve the beam quality of high-power diode lasers at high average powers. Nevertheless, if even higher powers are desired, arrays - regular arrays of multiple elements - such lasers must be used, which usually lowers the beam quality, or in other words the ability to focus the beam on small foci. The achievable power density remains practically constant. To overcome this problem, Daneu et.al. (Opt. Lett., Vol. 25, No.6, pp. 405-407) and Sanchez-Rubio ( US 6,192,062 ) spectral multiplexing proposed. Based on this, there were other patent submissions (eg WO 03/036766, WO 20/02091077). These all suggest that a common external resonator be constructed in which the array of gain elements is collimated or imaged onto a common spot on a dispersive medium, usually a grating, to overlap the beams. This grating reflects light in the direction of a feedback element (mirror, phase-conjugate mirror, etc.), which thus terminates the resonators. The light extraction is done - with some variations between the applications - via the feedback element. However, this is unfavorable in practice, since all the power is conducted through a relatively small spot on a grid, which usually has only a low damage threshold.

Together Not only is this submissions a spatial overlap in the feedback branch exists but also fans the dispersive element respectively in the plane of the array. Hereinafter It is shown that other arrangements are more advantageous.

problem and principle solution

The urgent problem is laser arrays with a preferably linear Sequence of different wavelengths with a simple structure in order to easily achieve a spectral overlay. in the The state of the art is the radiation of the entire array collimated to virtually a single spot on a dispersive medium. The desired Power densities then lead easy to destroy on this component.

The basic idea of this submission are arrangements in which the feedback beams are not (necessarily) overlap on optical elements, what the power densities reduced. An overlay The rays outside the resonator allow more robust arrangements.

reached advantages

main benefit The invention is the reduced power density in the feedback load the laser arrangement.

The presented arrangements can for the most part Part on the otherwise necessary conversion lens along the Dispense with the direction of the dispersion, which otherwise overlaps the rays. This will make the feedback simpler, more stable and more compact.

In Many of the forms proposed here reflect the grid the light directly back into the gain material due to Bragg or Littrow conditions. This will even make other lenses and mirrors of the otherwise necessary feedback element according to the state the technology superfluous.

If the invention is applied to lasers that themselves have some Extension along have the direction of the array, in particular broad-band laser, so finds a much more precise Wavelength selection instead, because in the prior art, a multi-wavelength operation, by design, also within a single laser occurs.

An overlay the individual beams outside of the (common) resonator can the power density in the feedback branch and on the optical components further reduce.

The fact that the superposition of the individual beams takes place outside the laser resonator makes further simplifications and cost reductions possible, since the requirements for quality and size of the components are lower here as within. In particular, the misalignment problems described in WO03 / 036766 on page 22, lines 11 below are so completely eliminated.

The unmissable Fact that at least the last optical component of the the superimposed Radiation is hit, the total power density is experienced also defused, because an overlay has more flexibility outside the resonator in terms of beam expansion, length, stability, losses, Accuracy, etc.

Detailed Description of the invention

The The invention extends known concepts of external resonators, in particular for wide-band semiconductor lasers, on arrays. She ensures that the individual emitters of the array with fixed wavelength gradations work. Compared with the state of the art, these can be Build resonators simpler and more compact. In particular, based this invention is not on the overlay all individual rays on the surface of optical elements within the feedback link of the laser array, thereby lowering the resulting power density and get the elements further away from their damage threshold.

in the The following describes how to use arrays of gain material can be combined with a multispectral laser. The array of gain material is only by his ability characterized by responding to feedback with stimulated emission. It may be arrangements with and without individual resonators. Thus, no distinction is made here between external to resonance forced (English: "seeding") arrangements, the sometimes referred to as a regenerative amplifier, and Such arrangements in which the resonators only by the externally added feedback Completely become. If in the following at all A distinction is made, then in the first case of "lasers" or "laser arrays" talked and in the second of "half-open Lasers "to clear to make that they still unilaterally lack feedback.

outgoing from the publication WO 03/055018 as a basis for single semiconductor lasers in an external resonator, you can see that a complete one tunable high-power laser already from only three optical Components can be manufactured: from the semiconductor with highly reflective facet, a cylindrical lens as a collimator and a grid in Littrow arrangement. It is obvious that a tuning is possible by that either the grating period is changed and / or the angle of incidence (Littrow angle) of the radiation on the grid. Therefore, deals this invention with how both variants with the lowest possible Effort for a complete one Realize array of gain material so that every single one Emit lasers at different wavelengths.

In the simplest case, the individual emitters receive their feedback from a grating whose period varies spatially or from a grating having non-planar or twisted lattice planes and / or a similar surface. The grating may be a conventional scribed or holographically structured grating or a volume element, eg a volume Bragg grating or holographic element. Details are in 1 shown. element 1 represents the Gainarray, which preferably back with a highly reflective mirror 20 and on the front, ie in the direction of the element 5 , with an antireflective layer 21 is provided. In (1 a) is shown a plan view in which all the emitters 1 each parallel to each other beams 2 send out. This is common but not necessary as long as the feedback takes account of any non-parallelism. Due to the fact that semiconductor lasers are usually very astigmatic, a cylindrical lens can 3 improve the efficiency of the feedback. This is easier in (1b) , a side view, to recognize. An element 19 can optionally be added at various points to reduce or avoid crosstalk between the channels. These are typically an array of apertures, one per laser, or an array of photoconductive tubes or the like. The feedback element 5 , in (1c) and (1d) shown enlarged, has a varying grid line spacing or grating period or angle of incidence, so that the different emitters experience different wavelengths as optimal for their respective resonators. Depending on the polarization of the lasers and the diffraction efficiency of the grating, an additional λ / 2 phase plate in the feedback branch may be necessary, which in these figures is implicit in the feedback arrangement 5 included is assumed. Depending on the application element 5 be a (tilted) grid, so that by global tilting a tuning of the entire emission can be achieved. Alternatively, a layer structure may be used in which dielectric layers lead to Bragg reflections according to the physical laws. This element can then be a volume Bragg grating. A tuning of the entire emission is then possible by placing the grid in the plane of (1 a) is shifted transversely to the light propagation.

It is not necessary for the decoupled light 4 as in 1 through the feedback element 5 is extracted. In 5 four different variants are shown. Sub-figure (a) corresponds to the direct extension of WO 03/055018 to arrays, using a V-shaped "off-axis" emission of the gain material. This arrangement is particularly advantageous when the semiconductor laser is substructured, that is, for example, is a so-called stripe array with a preferred off-axis emission, since this preferred direction implicitly as a direction selector 19 acts. This arrangement uses the facet 21 as feedforward to the wavelength-selective element, as feedback in the semiconductor and for coupling out the light. The entire laser resonator for the array consists of only three elements: the semiconductor as an active element with an applied reflective coating, a collimating lens and a feedback element. Here, the overlay optics are not counted, since those are outside the resonator and thus have no effect on the laser action. Alternatively, the light could also be through the backside semi-permeable facet 22 be decoupled, as in (5b) shown. Or the light can be like in (5c) through the feedback element 7 be extracted, which then must be partially permeable. Of course, a decoupling via an explicit output coupler 8th conceivable as in (5d) ,

In 6 Some arrangements are shown, as the individual coupled out beams 4 to practically completely collinear rays 13 overlay.

In (6a) becomes an array of dichroic mirrors 10 used as edge filters: the decoupled beams 4 have monotonically increasing or decreasing wavelengths and are gradually united. The illustrated variant shows a gradual overlay, but other variants, eg tree structures are equally possible. In (6b) acts a prism 12 as an essential angle-dispersive element to the rays with or without further optical auxiliary elements 14 to a ray 13 to overlay. This is an optical element 11 , here a lens, needed to "collect" the light. In case of (6c) it concerns both the collector 11 around a grid, as well as the dispersive element 12 , In case of (6d) is the collector 11 a lens and the dispersive element 12 a grid. Qualified persons can easily find further arrangements which have the same functionality.

Regardless of the ease of use of a grating with varying grating period, its availability can be problematic. That is why in 2 presented an important alternative. As mentioned above, the wavelength of the individual emitters for a given grating period depends on the angle of incidence on the grating. Spatial variability can then be achieved by twisting the grid. However, a much simpler variant is the cylindrical lens 3 weak to rotate around the propagation direction of the light. This is called a pseudo-isometric view in (2a) shown as well as a frontal view in (2 B) , as a side view in (2c) and as a top view in (2d) , In particular in (2c) it becomes clear that the vertical position of the collimation lens 3 the vertical propagation direction of the light 2 influences and thus indirectly the angle of incidence on the grid 5 certainly. Continuous variation of the vertical position of the collimating lens along the grating accordingly results in a continuous variation of the Littrow wavelength. This continuous variation of position is most easily achieved by tilting the collimator somewhat with respect to the direction of the gain array, or by rotating the direction of light propagation. The important parameter in this case is the relative height of the laser emitter and the associated collimation element. Therefore, an array of multiple lenses at different heights instead of a single long lens is equally suitable (see 3 ) or an array of individual lasers at different heights before the collimator or a combination of the different variants.

Mathematically, the arrangement according to 2 and 3 also be formulated by means of local coordinate systems. The gain array can be characterized by its transformation vector (x), which transforms the individual elements into each other; second, the light is emitted in the direction (z) to the feedback element; and third, there is a direction (y) that is perpendicular to (x) and (z). It is worth mentioning that (x) and (z) do not necessarily have to be perpendicular to one another, which happens in particular in off-axis arrangements in which the direction of the feedback is not perpendicular to the surface normal of the gain elements. Accordingly, the collimation element can be characterized by its axis of symmetry (x '), its optical axis (z') and the axis which is perpendicular to (x ') and (z'). Here, the axis (x ') is defined as the direction of the invariable cross section in the case of a single cylindrical lens (corresponding to FIG 2 ) or also as the direction of the transformation vector in the case of several lenses ( 3 ). The direction (y ') is the direction along which the collimation takes place. The feedback element also has local coordinates: the direction of light incidence (z ''), an axis (x '') around which the dispersion occurs and a direction (y '') perpendicular to (x '') and (z '' ). For lattices the direction (x '') is parallel to the grid lines, for prisms it is along the indifferent coordinate, ie along the roof. When feedback element is later occasionally on the "dispersion axis" or on the "direction of Dispersi In the first case, the direction (x '') is meant, in the second case the direction (y ''). According to the prior art, as homogeneous an operation as possible is achieved when the axes (x), ( x ') and (y'') (attention: not (x'')!) are parallel.) In addition, in many cases the directions (y) and (y') should also be parallel in order to avoid optical distortions In this invention, it is expressly proposed to firstly align the axis (x '') instead of (y '') approximately parallel to (x) and (x ') and secondly at least one of the axes (x), (x'). ) or (x '') slightly tilting about the corresponding axis (z), (z ') or (z'').

Various embodiments which enable wavelength-selective feedback without all the beams being superimposed on an optical element are disclosed in US Pat 4 shown. Common to all four variants is that they are a visor 16 own, all the rays 2 have to happen. Behind the aperture are all rays 2 "Reflected back in." So there are two optical arrangements necessary: a collection optics 15 in front of the aperture and a re-imagining look 17 or 18 behind it. At least one of the two must be wavelength-selective. Preferably, this consists of a grid or prism. Behind the panel this is preferably operated in Littrow arrangement. Since all individual beams have different wavelengths in laser mode, the collector is one lens ( 4a and 4b ) also a dispersive element can be used ( 4c and 4d ).

More preferably design

In particular, the embodiment with a tilted collimating lens and gratings in a Littrow arrangement is advantageous for equidistant wavelength spacings between the individual emitters (cf. 7 ). Moreover, it is very compact. In the case of semiconductor lasers, the gain array is typically in the plane of epitaxy, the so-called "slow axis" the direction perpendicular thereto is usually referred to as "fast-axis". It should therefore be emphasized that the arrangement proposed here, in contrast to the prior art, benefits from a direction of dispersion perpendicular to the extent of the array in that the grid lines lie substantially parallel to the slow axis.

In particular in off-axis arrangements, different lengths of the resonators can occur in the arrangements proposed here. However, these can be reduced or eliminated by the proposed arrangements due to the fact that the Littrow grating is also inclined, as is (2d) can be removed. This leads to (partial) compensation if the collimating lens is tilted correctly, since a beam inclined to the plane of the epitaxy strikes an inclined grating at different distances.

When the tilted collimating lens configuration is used, the individual beams are not usually radiated in one plane, but are mutually lost. Therefore, the external beam combiner must 11 superimpose the individual rays in a plane that is also inclined. That should not cause any problems.

Of the Term "array" does not have to necessarily on a single component or a monolithic Refer to integration. A preferred embodiment may be arrange any laser or gain material so that its forward and feedback channels along one Line are located. This is especially easy in the case of fiber lasers or fiber-coupled lasers, but also feasible with semiconductor lasers, optionally edge or surface emitting, or solid-state lasers in bar, bar or disk geometry.

There in most of the proposed embodiments, the gain materials do not completely fill the aperture of the array, often corresponding Gaps in the spectrum of combined light. Therefore, a preferred Design the polarization coupling of more than one Arrangement in which all arrays are set so that they respectively the spectral gaps fill the other.

A very essential for the Efficient operation of optically pumped assemblies is usually a high power density. This is especially true for optical pumped lasers. If the pumping process in higher than linear power more efficient is such. in upconversion processes, this is especially true Dimensions. Since such processes often have a relatively broad absorption band have is the spectral width of a multiplexed pump laser immaterial. The main advantage is the higher performance at unchanged Beam quality. When the optimal absorption band of the excited state in upconversion processes a slightly different wavelength has as the ground state, so lets even a laser with two by means of the proposed invention or more precisely set wavelengths produce.

1

laser array or array of laser gain regions with one-sided feedback

2

array from rays within the resonator before or after collimation or

Feedback elements

3

collimating optics

4

array the decoupled beams

5

Feedback element

6

Gain element the radiation "off-axis" (ie not parallel to its surface normal)

emits and at the same time acts as a coupling-out element

7

Feedback element, which is combined with the decoupling element

8th

outcoupling

9

Feedback element (s) of the laser array, which also act as a decoupling element

10

array from dichroic mirrors

11

collimating lens or grid

12

Winkeldispersives Element (prism, grid, etalon, etc.)

13

Overlapping decoupled beams that are substantially collinear

14

collimation

15

optical Element to overlap the rays in a common point (Prism,

grid, Lens, etc.)

16

all Rays common aperture

17

Dispersive element, which together with Apertur ( 16 ) acts as a wavelength filter

18

additional optical elements that complete the optical feedback

19

directional filter against crosstalk, advantageous an array of apertures or

light transmitting roar

20

reflective Coating on the feedback element opposite side of the

laser arrays

21

antireflective Coating on the feedback element facing side of the

laser arrays

22

Teilreflektive Coating on the feedback element opposite side of the

laser arrays

23

grid with varying grating period

24

grid with a crooked Surface, whereby the angle of incidence varies, that for rays

in a common level

25

Cylindrical lens, which is (slightly) tilted about the optical axis

26

array from cylindrical lenses

1 :

laser array with common feedback element, whose optimally reflected wavelength varies spatially continuously. In the feedback element it is advantageous to a grid or a volume Bragg grating with spatial varying grating period. (a) top view, (b) side view, (c) schematic Representation of two different forms of a varying Grating period; (d) pseudo-isometric representation of a grating with a crooked ("twisted") surface.

2 :

Laser array in which the individual feedback wavelength is realized by tilting a cylindrical collimating lens about the direction of light propagation. As a result, gradually varying angles of incidence occur at the grating. This in turn results in a gradually varying wavelength. (An optional directional filter ( 19 ) is omitted for the sake of clarity.) (a) Pseudo-isometric view, (b) frontal view, (c) side view, (d) top view.

3 :

Laser array, in which the individual feedback wavelength is realized in that a plurality of, advantageously cylindrical, lenses of an array are mounted at different heights with respect to the active region of the semiconductor laser. As a result, gradually varying angles of incidence occur at the grating. This in turn results in a gradually varying wavelength. (An optional directional filter ( 19 ) is omitted for the sake of clarity.) (a) Pseudo-isometric view, (b) frontal view, (c) side view, (d) top view.

4 :

exemplary self-imaging wavelength-selective Arrangement of a feedback, consisting of an aperture, a beam-overlapping optics and a retro-reflective optics. (a) collimation by means of a lens, Retroreflex by means of a grid in Littrow arrangement, (b) collimation by means of a lens, Retroreflex by means of a grid in Litman-Metcalf arrangement, (c) collimation by means of a grid, retroreflex by means of a curved mirror, a lens-mirror combination, a retroreflective mirror ("cat's eye") or a combination thereof, (d) both beam overlay as well as retroreflector by means of bars.

5 :

different Arrangements to decouple the light from the laser: (a) by means of Beam splitter effect of the individual laser, (b) by decoupling elements the individual laser, (c) by the common feedback element, (D) by a separate decoupling element.

6 :

different Arrangements to make the light of the coupled-out beams into an im to rearrange the essential collinear ray: (a) by dichroic Elements, (b) by means of a collimating lens and a prism, (c) by means of two gratings, (d) by means of a collimating lens and a grid.

7 :

Top view and side view of a preferred arrangement. The laser array 1 emits rays 2 , When the cylinder lens 3 slightly inclined, hit the rays 2 the grid 5 at different angles, so that the Littrow wavelength is different for each emitter. The decoupling can be done via the zeroth diffraction order of the feedback element and a superposition of the beams 4 to a common beam 13 is by means of a collimation 11 and a (dispersive) prism 12 possible.

Claims (18)

Laser source consisting of a. an array from gain ranges, i. those with stimulated emission to external feedback can react, ii. each having at least one feedforward channel, iii. each at least one feedback channel have, advantageously identical to the feedforward channel, iv. the each have at least one decoupling channel, more advantageous identical to the feedforward channel, b. at least one spatially extended wavelength-selective (common) feedback arrangement, i. which receives input signals from the feedforward channels and whose corresponding reflections lead to the corresponding feedback channels, ii. which couples a different wavelength to each gain range iii. that of the different feedforward channels at different Jobs are taken, c. an optical connection between the gain array and the feedback array, d. an optical arrangement that the decoupled light to a in the superimposed on the essential collinear beam. Laser source according to claim 1, in which the different gain elements sections of one or more spatial are extended elements. Laser source according to claim 1 or 2, in which the different gain elements either • complete lasers are, i. have separate resonators that are different on the Wavelengths of Feedback element be brought (English: "locking") or • incomplete lasers ("semi-open lasers"), i. without complete resonators, which are only by means of the common feedback element to resonators. Laser source according to claim 1 in the Gainarray • one Array off - possible fiber-coupled - edged or surface emitting Semiconductor lasers or fiber lasers or solid-state lasers in bar or bar or disc geometry, • or from an extended Laser material is made, preferably a doped solid or semiconductor or organic material, • preferably an antireflective coating on the feedback element facing Side and / or a highly reflective coating on the opposite Own page. Laser source according to claim 1, in which the wavelength-selective Arrangement a combination of one or more of the following Elements is: grid, Bragg grating, Volume Bragg grating, fiber Bragg grating, etalon, prism, holographic element, diffractive optical element, microstructured element, arrangement Dielectric layers, chirped mirror (dielectric mirror with varying layer thicknesses), wave plates, interference filters, Loyot filter, regular optical elements such as free propagation or lenses or mirrors. Laser source according to claim 1, in which the wavelength-selective Arrangement of a grid or Bragg grating with spatially varying Grid period and / or with curved Lattice levels exists. Laser source according to claim 1 or 5 in which the translation vector of the gain elements and the axis, around which the wavelength-selective Splits arrangement, are substantially parallel. Laser source according to claim 1, in which the optical connection between the Gainarray and the feedback arrangement a. consists of one or more of the following elements: free propagation, cylindrical collimation, spherical Collimation, aspherical Collimation, telescope, direct coupling (English: "butt-coupling"), diffractive optics, holographic Element, etc., b. advantageously direction-selective filters, Elements or arrangements against crosstalk of the array elements, e.g. Optical fibers, tubes, diaphragms, Apertures etc., c. advantageous from common optical elements for all array elements exists and / or in turn has Arraystruktur. Laser source according to claim 1, in which the light of different gain ranges, the feedback element meets at different angles. Laser source according to claim 1 or 9, in which a. the elements of the array along one direction (x) offset from each other, b. the collimation element consists of a cylindrical lens or an array of cylindrical lenses whose symmetry axis or translation vector extends along a direction (x '), c. the wavelength selective array, preferably a grating or prism array, is dispersed about an axis (x '') (for a grating it is parallel to the grating lines), d. the axes (x), (x ') and (x'') are not all mutually parallel to each other. Laser source according to claim 1 or 9, in which the height between the elements of the gain array and the corresponding one Part of - advantageous cylindrical - collimation optics, measured along the splitting direction of the feedback element, for all elements is different, which is advantageously realized by a Choice or combination of the following arrangements: • the symmetry axis at least one Kollimationselements is against the direction of Translation vector of Gainarrays tilted, • the collimation optics consists of several elements, located at different heights along the Direction of dispersion, • the gainarray consists of several elements, located at different heights along the Direction of dispersion. Laser source according to claim 1, in which the feedback consists of an arrangement, a. in a first optical Arrangement superimposed on the individual rays in a common spot, advantageous consisting of prisms, lenses and / or grids, b. the one has common aperture in this spot, c. a second optical arrangement, each individual beam through the aperture back reflected in itself, d. in the at least one of the two Arrangements of sub-item (a.) Or (c.) Dispersive and / or wavelength-selective is. Laser source according to claim 1, in which the decoupling over the feedback element that happens to be beneficial only part of the incident light back to the gain elements reflected. Laser source according to claim 1, in which the overlay through a combination of grids, lenses, prisms, free propagation, Mirroring, dielectric filters, edge filters, dichroic Elements, etalons, etc. on a prism or grid more appropriate Angle dispersion happens. Laser source according to previous Claims, in the a. the gain elements one or more preferred directions, preferably outside the facet normal, have what is preferably happens by substructuring the gain elements, b. the feedback preferably longitudinally such a preferred direction happens c. the decoupling preferably longitudinally another such preferred direction happens. Laser source according to previous Claims, operated in the at least two such laser polarization coupled are, preferably, to the spectral profile of the Nutzlichts to smooth. Application of a spectrally multiplexed laser to optical pumps of other arrangements, • in the optical arrangement preferably a laser, • preferably by the different ones wavelength to more than one absorption band of the material to be optically pumped are adapted • preferably to optimize upconversion processes. Any combination of claims 1 to 17.