DE102008058002A1 - Wavelength stabilized laser has laser medium, which emits primary radiation, and folded external resonator having three resonator mirrors, one of which is designed as folding mirror - Google Patents

Abstract

The wavelength stabilized laser (1) has a laser medium (2), which emits a primary radiation (P), and a folded external resonator (3) having three resonator mirrors (31,32,33), one of which is designed as a folding mirror. The folding mirror has an inner surface (4) facing the resonator and an outer surface (5) facing away from the resonator. Two detector elements are provided with the folding mirror to transmit a partial radiation of the primary radiation to the outer surface without reflection. An independent claim is included for a wavelength stabilization method for a laser.

Description

It becomes a wavelength stabilized Laser, in particular a wavelength-stabilized Semiconductor laser and a method for wavelength stabilization of a laser, in particular a semiconductor laser specified.

A to be solved The object is a laser, in particular a semiconductor laser indicate that stable emitted radiation at a certain wavelength. Another to be solved The object is a method for wavelength stabilization of a laser, Specify in particular a semiconductor laser.

At least an embodiment of the wavelength stabilized Lasers this is a wavelength-stabilized Semiconductor lasers.

At least an embodiment of the wavelength stabilized Lasers this includes a laser medium, in particular exactly one laser medium. By means of the laser medium is a primary radiation during operation of the laser generated. The laser medium is for example by a semiconductor material, a crystalline or glassy solid or by a gas or a liquid given. The laser medium can be optically and / or electrically pumped be, or even chemical energy to generate the primary radiation implement.

At least an embodiment of the wavelength stabilized Lasers, in particular semiconductor laser, this has at least one Semiconductor laser chip, in particular exactly one semiconductor laser chip on, which represents the laser medium. The semiconductor laser chip is adapted to emit the primary radiation during operation. Of the Semiconductor laser chip may be optically or electrically pumped. Semiconductor laser chip means that an active generator intended for generating radiation Area of the chip is formed with a semiconductor material or out at least one such material.

At least an embodiment of the wavelength stabilized Laser has this a folded, external resonator. pleated means that a light path within the resonator is not one single, straight line follows. The light path thus has at least two, in particular exactly two sections on which do not coincide. External can mean that the resonator is about as a semiconductor laser chip designed laser medium is not limited to the semiconductor laser chip itself, the light path runs then in the resonator thus also outside of the semiconductor chip.

Under The light path of the radiation is in each case the course of the radiation along a beam axis or an optical axis designated.

At least an embodiment of the wavelength stabilized Lasers, the folded external resonator at least three, in particular exactly three resonator mirrors. The resonator mirrors are designed to the primary radiation that emitted by the laser medium during operation, at least in part reflect. The resonator mirrors are for example by Bragg mirrors and / or dielectric mirrors. A reflectivity of the resonator mirror in terms of exceeds the primary radiation preferably 98%, in particular 99.8%, particularly preferably 99.95%.

At least an embodiment of the wavelength stabilized Lasers is one, in particular exactly one of the resonator mirrors designed as a folding mirror. In particular, if the resonator not designed as a ring resonator, but as a linear resonator is, the folding mirror is preferably between two as End mirror designed resonator mirrors. Beyond the folding mirror the course of the primary radiation within the resonator deflected so that the course of the primary radiation deviates from a straight line. In other words, by the folding mirror formed the partial paths of the light path in the resonator.

At least an embodiment of the wavelength stabilized Lasers, the folding mirror has an inner surface facing the resonator and an outer surface facing away from the resonator. The inner surface presents preferably that area of the folding mirror, at which the primary radiation is deflected in the resonator becomes. The inner surface can thus be a mirror surface be. The folding mirror preferably has a surface on the inner surface reflectivity in terms of the primary radiation of at least 98%, in particular of at least 99.8%. The outer surface of the Folding mirror represents an area the folding mirror, which faces away from the inner surface.

At least an embodiment of the wavelength stabilized Lasers this includes at least two detector elements. Both Detector elements can be individual, separate components, which are configured for example as photodiodes. Likewise is it is possible that the detector elements through individual pixels, for example a semiconductor chip or on a CCD camera are formed.

In accordance with at least one embodiment of the wavelength-stabilized laser, the folding mirror is set up to transmit a first partial radiation of the primary radiation without reflection on the inner surface and without reflection on the outer surface. In other words, it permeates you small portion of the primary radiation, preferably less than 0.5%, in particular less than 0.1%, the inner surface of the folding mirror, passes through the folding mirror and leaves the folding mirror on the outer surface, without being reflected on the outer surface. The first partial radiation is thus coupled out of the resonator at the folding mirror.

At least an embodiment of the wavelength stabilized Lasers is the folding mirror set up to a second partial radiation the primary radiation after reflection both on the outer surface as also on the inner surface of the folding mirror. The proportion of the second partial radiation at the primary radiation is preferably less than 0.5%, in particular less than 0.1%.

With In other words, the second partial radiation is emitted via the folding mirror decoupled from the resonator. That is, the second partial radiation happens once the inner surface of the folding mirror, passes, running in the folding mirror, to Outer surface and is reflected on the outer surface so that the second partial radiation remains in the folding mirror. After reflection on the outer surface runs the second partial radiation towards the inner surface and is on the inner surface back in the direction reflected to the outer surface. The second partial radiation then passes the outer surface of the following Folding mirror.

At least an embodiment of the wavelength stabilized Lasers run the first partial radiation and the second partial radiation after leaving the folding mirror in the same direction. That means, that, immediately after leaving the folding mirror, an angle between the first partial radiation and the second partial radiation less than 10 °, preferably less than 3 °. In particular, within the manufacturing tolerances of the folding mirror, the first partial radiation and the second partial radiation directly after leaving the folding mirror aligned parallel to each other.

Of the Angle between the first partial radiation and the second partial radiation after leaving the folding mirror, according to at least one embodiment of the laser by an angle between the inner surface and the outer surface of the Defined folding mirror. Are inner surface and outer surface, within the manufacturing tolerances, parallel to each other, so runs also the first partial radiation parallel to the second partial radiation.

At least an embodiment of the wavelength stabilized Lasers are the first partial radiation and the second partial radiation each directed at least partially on the detector elements. With In other words, a proportion of the first partial radiation meets at least one of the detector elements. The same applies with respect to second partial radiation.

In at least one embodiment of the wavelength-stabilized Laser, in particular semiconductor laser, this includes at least a laser medium, which in particular may be at least one semiconductor chip can. The laser medium is set up to receive a primary radiation to emit. Furthermore, the laser has a folded, external Resonator with at least three resonator mirrors. One of the resonator mirrors is designed as a folding mirror and has an inner surface facing the resonator and an outer surface facing away from the resonator. Further includes the laser has at least two detector elements. The folding mirror is arranged to a first partial radiation of the primary radiation without reflection on the outer surface of the Folding mirror and a second partial radiation of the primary radiation after reflection both on the outer surface as also on the inner surface of the folding mirror. The first and the second Partial radiation can in particular be transmitted so that they are in the same Direction. The first and the second partial radiation are respectively at least partially directed to the detector elements.

One Such lasers, in particular semiconductor lasers, can be timed stable wavelength having the pump radiation.

At least an embodiment of the wavelength stabilized Laser is a distance between the first partial radiation and the second partial radiation between inclusive 100 μm and 1 mm, in particular between 150 μm and 700 μm inclusive. At a distance is hereby the distance of the partial beams from each other after transmission through the folding mirror, before passing through, for example, another optical element, understood. First and second partial radiation do not run parallel to each other, so the distance is right after leaving the first and second partial radiation from the folding mirror to determine. The distance relates in particular to the distance between beam axes of the first and second partial radiation. For example the distance is the distance that the beam axes of the first and second partial radiation to each other when penetrating the outer surface of the Have folding mirror.

At least an embodiment of the wavelength stabilized Lasers, the folding mirror is designed plane-parallel. That is, in the context The manufacturing tolerances are outer surface and inner surface parallel arranged to each other. The tolerance for parallelism is preferred less than 0.5 °, in particular less than 0.1 °.

At least an embodiment of the wavelength stabilized Laser, the folding mirror has a thickness of between 100 μm and 2 mm inclusive, especially between 150 μm and 800 μm inclusive. The Thickness here refers to an extension perpendicular to the inner surface of the Folding mirror.

At least an embodiment of the wavelength stabilized Lasers is both the inner surface as well also the outer surface of the Folding mirror, in the context of manufacturing tolerances, just designed.

At least an embodiment of the wavelength stabilized Lasers become the first partial radiation and the second partial radiation spatially superimposed after leaving the folding mirror, so that a Interference pattern with respect to first and second partial radiation is formed. The detector elements are hereby set up in each case a subregion of the interference pattern to detect. The interference pattern can, for example, on a projection surface in the optical Far field or on a projection surface in an image plane be formed of an optical element. A radiation sensitive area The detector elements then each takes a portion of the screen or the image plane. As a result, the detector elements can be different spatial Areas of the interference pattern on the screen or detect the image plane. A measure of the detector elements is in particular, the respective intensity of the partial radiations striking the detector elements.

At least an embodiment of the wavelength stabilized Lasers are the detector elements adjacent in the projection plane or arranged on the image plane. For example, the Detector elements directly to each other. The spatial distance between the photosensitive surfaces the detector elements in the projection plane or imaging plane, that is to say the extent of a light-insensitive region is preferably less than 200 μm, in particular less than 100 microns.

At least an embodiment of the wavelength stabilized Lasers is on the outside surface of the Folding mirror a coating applied in a spectral range the primary radiation has a wavelength-dependent reflectivity. In this case, one of the detector elements is preferably configured to to detect the first partial radiation, and the other detector element is adapted to detect the second partial radiation. The first and the second partial radiation are therefore different Directed detector elements.

At least an embodiment of the wavelength stabilized Laser changes in a spectral range with a width of 20 nm, the one Central wavelength the primary radiation includes, the reflectivity the coating of the outer surface of the Folding mirrors by at least 30 percentage points, in particular by at least 50 percentage points, preferably by at least 80 percentage points. In other words, for example, at the high-frequency edge of this Spectral range the reflectivity the coating the primary radiation very low, for example less than 5%, and low frequency Edge of this spectral range very high, for example greater than 95%. Such a coating can be achieved by a dielectric layer sequence and / or be formed by a Bragg mirror.

At least an embodiment of the wavelength stabilized Lasers is a reflectivity the coating in the spectral range of the primary radiation approximately wavelength independent and lies between inclusive 20% and 80%, in particular between 40% and 65% inclusive, preferred by 50%. Wavelength independent mean that the reflectivity in a wavelength range from +/- 10 nm around the central wavelength less than 10 percentage points, in particular less than 3 percentage points changes At least an embodiment of the wavelength-stabilized Laser is a spectral width of the primary radiation, in particular FWHM, less than 1 nm, preferably less than 0.5 nm.

At least an embodiment of the wavelength stabilized Lasers go through neither the primary radiation, nor the first Partial radiation, nor the second partial radiation an optical element, which serves to a spectral splitting of said radiation. Such an optical element can by a prism, an optical Be formed grid or an optical gradient plate. With others In words, none of the aforementioned radiations becomes spectral subcomponents split that spatially separated from each other and due to the spatial separation via separate Detector elements can be detected. The laser thus includes in particular no arrangement that corresponds to a spectrometer.

According to at least one embodiment of the wavelength-stabilized laser, this comprises at least one control unit. The control unit is designed to regulate a wavelength of the primary radiation as a function of at least one signal of the detector elements. In other words, the signal that the detector elements deliver depends on the wavelength of the primary radiation. About this signal of the detector elements can then Wel lenlängen the primary radiation, at least in a spectral sub-range, are determined. This makes it possible that the control unit regulates to a specific, predetermined wavelength of the primary radiation.

At least an embodiment of the wavelength stabilized Lasers is the control unit designed to be a spatial Position of at least one of the resonator mirrors, in particular from exactly one of the resonator mirrors, to regulate. Preferably regulates the control unit the position of that resonator, the from the laser medium, in particular from the semiconductor laser chip furthest is removed and / or the greatest optical distance, along the light path to the laser medium has. The regulation takes place furthermore preferably in a direction parallel to the beam axis the primary radiation inside the resonator. In other words, the control unit regulates an optical length of Resonator, without significantly affecting the beam position of the primary radiation.

At least an embodiment of the wavelength stabilized Lasers, the resonator at least one optical crystal, to an at least partial frequency conversion of the primary radiation in a secondary radiation is set up. about the optical crystal is, for example, a frequency doubling the primary radiation possible. As well can use a four-wave mixing the primary radiation for example, at least partially converted into secondary radiation, their wavelength is larger as the wavelength the primary radiation.

At least an embodiment of the wavelength stabilized Lasers is the optical crystal with lithium niobate, short LN, with Potassium titanyl phosphate, KTP for short, with rubidium titanyl arsenide, RTA for short, with potassium titanyl-Arsendid, short KTA, with rubidium titanyl phosphate, short RTP, and / or with Potassium niobate, short KN, designed or consists of one of these Materials. The optical crystal can be periodically poled, to achieve a quasi-phase adaptation. A geometric length of the Optical crystal is preferably between 0.5 inclusive mm and 5.0 mm.

At least an embodiment of the wavelength stabilized Lasers, in particular semiconductor laser, the wavelength of the primary radiation in the near-infrared spectral range. In particular, the wavelength of the Primary radiation between 800 nm and 1500 nm, preferably between 1030 nm and 1070 nm. The wavelength the secondary radiation is preferably half the wavelength the primary radiation and is especially in the visible spectral range, especially in the green Spectral range around 525 nm.

At least an embodiment of the wavelength stabilized Lasers, the control unit regulates a temperature of the optical crystal. About the Temperature of the crystal is also the optical length of the crystal and thus also the optical length the resonator adjustable. About here is a tuning of the wavelength the primary radiation, and thus also optimizing, for example, a doubling efficiency in the optical crystal, possible.

At least an embodiment of the wavelength stabilized Lasers will use the secondary radiation over the Folding mirror coupled out of the resonator. The secondary radiation runs thereby preferably in a different direction than the first and the second partial radiation. So it's the secondary radiation as well as first and second partial radiation then spatially separable. As a result, a background on the partial radiations measuring Detector elements, caused by the secondary radiation, reducible.

At least an embodiment of the wavelength stabilized Laser is an angle of incidence of the primary radiation on the folding mirror between 40 ° and 75 ° inclusive, in particular between 60 ° and 75 ° inclusive.

At least an embodiment of the wavelength stabilized Lasers this includes at least one wavelength-selective element. This wavelength selective Element is designed to match the primary radiation to a wavelength interval with a spectral width of at most 4 nm, in particular from at most 2 nm restrict. The wavelength-selective Element is formed for example by an etalon.

At least an embodiment of the wavelength stabilized Lasers is the wavelength interval, on that the primary radiation through the wavelength selective Element is restricted smaller than a spectral range in which over detection of the first and the second partial radiation at the detector elements, the wavelength of primary radiation especially clearly identifiable. In other words, there is wavelength selective Element a spectral range in which the signal of the detector elements in terms of the wavelength the primary radiation is unique.

At least an embodiment of the wavelength stabilized Lasers is an optical length of the resonator is less than or equal to 100 mm. Preferably, the optical lies Length of the Resonator between inclusive 1 mm and 20 mm, in particular between 3 mm and 12 mm inclusive.

In accordance with at least one embodiment of the wavelength-stabilized laser, the detector elements are integrated on a single semiconductor chip and / or fabricated in a single semiconductor chip.

At least an embodiment of the wavelength stabilized Lasers gives way to an intensity the first partial radiation of an intensity of the second partial radiation at most 75%, especially at most 55%, preferably at most 30%. The intensities the partial radiations are therefore about the same size. This can be structured by one on the folding mirror applied coating can be achieved.

Farther becomes a method for wavelength stabilization a laser, in particular a semiconductor laser specified. Of the Semiconductor laser is preferred according to at least one of the preceding embodiments designed. The method may include one or more of the with the laser, in particular with the semiconductor laser features mentioned exhibit.

In at least one embodiment of the method becomes one of measured values of one of the at least two detector elements detected signal to a control of the optical Length of the Resonator used. In a laser, the optical resonator length L corresponds to one integer multiples of the half wavelength λ / 2 of a radiation in this Trap of primary radiation. So L = n λ / 2, where n is a natural one Number is. The optical resonator length L is thus at the wavelength λ of the radiation correlated. about adjusting the optical resonator length L is thus also the wavelength λ of the radiation, in particular the primary radiation, tunable at least in a certain spectral range.

At least an embodiment of the Procedure, the first and the second partial radiation on the Focused detector elements. This is for example by means of a Lens or by means of a mirror possible.

Some Areas of application in which wavelength-stabilized Lasers, in particular semiconductor lasers, can be used are For example, lighting devices for projection purposes. Furthermore you can the lasers described here in headlamps or light emitters, in the general lighting or in the backlight and / or be used by displays or display devices.

following is a wavelength-stabilized laser described here as well as a method for wavelength stabilization of a laser with reference to the drawing based on embodiments explained in more detail. Same Reference numerals indicate the same elements in the individual figures at. However, there are no scale references shown, but rather individual elements shown in an exaggerated way for a better understanding be.

It demonstrate:

1 and 2 schematic representations of embodiments of lasers described herein,

3 to 6 schematic representations of patterns formed by the first and the second partial radiation by superposition of exemplary embodiments of lasers described here,

7 a schematic representation of a dependence of a signal of the detector elements on the wavelength of the primary radiation,

8th a schematic representation of a course of the radiation at a folding mirror of an embodiment of a laser described herein, and

9 a schematic representation of a reflectivity of a coating of a folding mirror (A) and a signal from detector elements (B) as a function of a wavelength of a primary radiation in an embodiment of a laser described herein.

In the 1A and 1B is an embodiment of a wavelength stabilized laser 1 shown. The laser 1 is a semiconductor laser in this embodiment and has a resonator 3 on. In the resonator 3 it is not a so-called ring resonator.

A semiconductor laser chip 2 , which serves as a laser medium and is adapted to generate a pump radiation P, simultaneously forms a resonator mirror 31 of the resonator 3 out. The semiconductor laser chip 2 includes, for example, an unselected active layer in which the primary radiation P is generated. On one side of the semiconductor laser chip 2 that is another resonator mirror 32 is remote, for example, not shown, with respect to the primary radiation P highly reflective layer or layer sequence attached.

In the semiconductor laser chip 2 generated primary radiation P extends in a direction perpendicular to the resonator mirror 31 towards a folding mirror 33 , the resonator 3 belongs. The folding mirror 33 has one inside the resonator 3 facing, with respect to the primary radiation P reflective acting inner surface 4 and one the Resona gate 3 facing away, the inner surface 4 opposite outer surface 5 on. On the inner surface 4 the folding mirror 33 there is a reflection of the semiconductor laser chip 2 emitted primary radiation P in the direction of the resonator mirror 32 , The primary radiation P is in 1A symbolized by a dotted line.

Between the semiconductor laser chip 2 , and thus the resonator mirror 31 , and the folding mirror 33 there is a wavelength-selective element 10 which can be designed as an etalon. Through the wavelength-selective element 10 the wavelength of the primary radiation P is limited. This happens because the wavelength-selective element 10 For example, only for certain wavelengths in a free spectral range is permeable. For other wavelengths, the losses are inside the resonator 3 then so big that at these wavelengths no laser action can arise. A possible wavelength of the primary radiation P is thus on the free spectral range of the wavelength-selective element 10 limited.

Between the folding mirror 33 and the resonator mirror 32 there is an optical crystal 9 , The optical crystal 9 is adapted to at least partially convert the primary radiation P into a secondary radiation S, symbolized by an arrow line. The secondary radiation S has in particular half the wavelength of the primary radiation P. For example, the optical crystal 9 a periodically poled lithium niobate crystal, PPLN for short.

The secondary radiation S, which is in the optical crystal 9 is generated, runs between the folding mirror 33 and the resonator mirror 32 essentially in the same way as the primary radiation P. towards the resonator mirror 32 ongoing secondary radiation S is from the resonator mirror 32 back towards the folding mirror 33 reflected. The resonator mirror 32 acts highly reflective both for the primary radiation P and for the secondary radiation S.

To increase the stability of the resonator 3 can the resonator mirror 32 to be focused. For this purpose, an area of the resonator mirror 32 in which the primary radiation P and the secondary radiation S are reflected, has a radius of curvature which is in particular greater than an optical length of the resonator 3 ,

The secondary radiation S passes through the inner surface 4 in the folding mirror 33 , goes through the folding mirror 33 and occurs on the outside surface 5 from the folding mirror 33 out. Both interior surface 4 as well as outer surface 5 are preferably provided with a respect to the secondary radiation S antireflective acting, not shown coating, so that the secondary radiation S efficiently from the resonator 3 can be decoupled.

The semiconductor laser 1 according to 1A . 1B So it's designed to be inside the resonator 3 the primary radiation P, which preferably has wavelengths in the infrared spectral range to convert into a visible secondary radiation S. To enable efficient frequency conversion, the resonator is 3 with regard to the primary radiation P designed as low loss. That is, the resonator mirrors 31 . 32 and the folding mirror 33 have the highest possible reflectivity with respect to the primary radiation P. The material of the folding mirror 33 is preferably transparent with respect to the wavelengths of the secondary radiation S and the primary radiation P. Are the resonator mirrors 31 . 32 and the folding mirror 33 For example, designed as a dielectric mirror, so reflectivities with respect to the primary radiation P in the range of about 99.8% can be realized. A reflectivity of exactly 100% can not be achieved. In other words, a small part of the primary radiation P passes through the coating not shown on the inner surface 4 in the folding mirror 33 ,

This is in 1B illustrated in more detail. To simplify the illustration, the primary radiation P and the secondary radiation S in 1B not drawn. Instead of the primary radiation P and the secondary radiation S is an optical axis O within the resonator 3 drawn as a dash-dot line. The optical axis O corresponds to the beam axes of the primary radiation P and the secondary radiation S.

On the inner surface 4 the folding mirror 33 has the optical axis O to a Lot to the inner surface 4 an incident angle θ which is approximately 67.5 °. In a direction parallel to the perpendicular, ie in a direction perpendicular to the inner surface 4 , indicates the folding mirror 33 a thickness t of about 200 microns. The inner surface 4 and the outer surface 5 are, in the context of manufacturing tolerances, just designed and oriented parallel to each other. On the outer surface 5 the folding mirror 33 is a coating 7 applied, which acts with respect to the primary radiation P partially reflecting. The reflectance of the coating 7 with respect to the primary radiation P is about 50%.

In 1B are also beam paths of a first partial radiation 11 and a second partial radiation 22 illustrated. The first 11 and the second partial radiation 22 are formed by a part of the primary radiation P which, from the semiconductor laser chip 2 Coming here, the inner surface 4 the folding mirror 33 passes. This radiation passes within the folding mirror 33 towards the outer surface 5 , At the Be stratification 7 the outer surface 5 becomes part of the folding mirror 3 extending radiation transmitted without further reflection. This radiation component forms the first partial radiation 11 , Because the coating 7 has a reflectivity of about 50%, is another part of the folding mirror 33 extending radiation back toward the inner surface 4 reflected. Because the inner surface 4 With respect to the wavelength of the primary radiation has a reflectivity of just under 100%, this is in the folding mirror 33 running radiation almost unattenuated back toward the outer surface 5 steered and passes, at least partially, the outer surface 5 , This, the outer surface 5 after reflection both on the outer surface 5 as well as on the inner surface 4 Passing radiation forms the second partial radiation 22 ,

Because the inner surface 4 and the outer surface 5 are parallel to each other, also run first partial radiation 11 and second partial radiation 22 parallel to each other, in one direction towards two in 1A . 1B not drawn detector elements 6 , A distance d between the first 11 and second partial radiation 22 is about 150 microns. First 11 and second partial radiation 22 extend in a direction parallel to the portion of the optical axis O, which is located between the semiconductor laser chip 2 and the folding mirror 33 located.

From the primary radiation P, which is in the direction of the resonator mirror 32 away to the folding mirror 33 runs, results in a further partial radiation 44 , analogous to the first 11 and second partial radiation 22 arises. This additional partial radiation 44 runs parallel to the out of the resonator 3 decoupled secondary radiation S.

To the intensity of partial radiations 11 . 22 to match each other, the coating can 7 on the outside surface 5 the folding mirror 33 optionally be applied only in places or at different locations have a different degree of reflection with respect to the wavelength of the primary radiation P.

In 2 is the further course of the first partial radiation 11 and the second partial radiation 22 shown. About an imaging optics 12 , formed by a condenser lens, become the front of the imaging optics 12 parallel to each other first 11 and second partial radiations 22 on the two detector elements 6a . 6b displayed. In other words, first partial radiation 11 and second partial radiation 22 each other in a plane of the detector elements 6a . 6b superimposed.

Since the secondary radiation S in the direction of the further partial radiation 44 and not in the direction of first 11 and second partial radiation 22 is a background signal on the detector elements 6a . 6b , caused by the secondary radiation S, suppressed. In addition, hamper the detector elements 6a . 6b not the course of secondary radiation S. Alternatively, it is also possible that the additional partial radiation 44 acting as the first and the second partial radiation and the detector elements 6a . 6b in the direction of the resonator 3 decoupled secondary radiation S are located.

Because the first partial radiation 11 and the second partial radiation 22 from the through the inner surface 4 transmitted primary radiation P are divided and have a stable phase relationship to each other, resulting in an interference pattern from the superposition of the first 11 and second partial radiation 22 , Depending on the intensity of the on the detector elements 6a . 6b meeting partial radiations 11 . 22 becomes a signal from the detector elements 6a . 6b to a control unit 8th forwarded. Depending on this signal, the control unit controls 8th a position of the resonator mirror 32 in a direction parallel to the optical axis O and / or a temperature of the optical crystal 9 , Thus, via the control unit 8th the optical length of the resonator 3 and thus also set the wavelength of the primary radiation P.

The efficiency of frequency conversion in the optical crystal 9 is strongly dependent on the wavelength of the primary radiation to be converted P. Therefore, small changes in the wavelength of the primary radiation P lead to significant fluctuations in the intensity of the secondary radiation S. To ensure a stable output power with respect to the secondary radiation S, therefore, the wavelength of the primary radiation P is stable over time to keep. Fluctuations in the wavelength of the primary radiation P can be caused by thermo-mechanical or thermo-optical effects and in particular lead to sudden spectral jumps, a so-called mode hopping.

In the 3 to 7 is an exemplary way in which 8th and 9 illustrates another exemplary possibility about the detector elements 6a . 6b to obtain a dependent of the wavelength of the primary radiation P signal.

In 3A is projected onto a projection surface a beam with a Gaussian intensity profile in the lateral direction. In the projection, the beam profile appears as approximately circular with a rotationally symmetric intensity distribution. In 3B is the interference pattern, which results from the superposition of two mutually phase-stable partial beams, to see on a projection screen in the optical far field. In the lateral direction, light and dark areas of the strip-shaped interference pattern can be clearly seen alternately.

In 4 are simulations of the far field of the superposition of the first partial radiation 11 and second partial radiation 22 for different thicknesses t of the folding mirror 33 to see, for example, for a semiconductor laser 1 according to 2 , In 4A the thickness t of the folding mirror is 700 μm, in 4B 500 μm and in 4C 200 μm. The smaller the thickness t of the folding mirror 33 is, the smaller is the distance d between the first partial radiation 11 and second partial radiation 22 , and so on, adjacent light or dark stripes of the intensity pattern are distant from each other.

The interference pattern results from the fact that the first partial radiation 11 and second partial radiation 22 after leaving the folding mirror 33 have a phase difference Δφ to each other, which depends in particular on a central wavelength λ _{0 of} the primary radiation P. Assuming that a refractive index of the environment of the folding mirror 33 is approximately equal to 1, as in the case of air, the phase difference Δφ can be given by the following formula, where n is the optical refractive index of the folding mirror 33 corresponds to:

In the 5A to 5F the interference pattern is shown in the far field for different values of the central wavelength λ ₀ . According to 5A with the central wavelength λ ₀ = 1050.0 nm, the interference pattern has a central stripe with high brightness, which shows stripes with significantly reduced brightness on both sides. About the 5B and 5C finally results in 5D the interference pattern at the wavelength λ ₀ = 1051.5 nm, in which two central, approximately equal stripes are adjacent. about 5E towards the 5F at the central wavelength λ ₀ = 1052.5 nm, the interference pattern is again similar to that in FIG 5A , The interference pattern is thus dependent on the central wavelength λ ₀ .

In 6 is illustrated how the detector elements 6a . 6b Detect different, adjacent areas of the interference pattern. Photosensitive surfaces of the detector elements 6a . 6b are symbolized by a frame. The detector elements 6a . 6b measure a power Pa and Pb of the radiation emitted on the photosensitive surfaces of the detector elements 6a . 6b falls. The signal A can be calculated from the measured powers Pa, Pb. The signal A is defined, for example, as follows:

The signal A, which depends on the central wavelength λ _{0 of} the primary radiation P approximately according to the interference patterns according to 5 results in is 7 applied. There is a clear, periodic dependence of the signal A from the central wavelength λ ₀ recognizable. A periodicity or period length of the wavelength dependence of the signal A is approximately 2.5 nm. That is, in a spectral range which corresponds approximately to half a period length of the signal A, there is a one-to-one correspondence between the central wavelength λ ₀ and the size of the signal A. , If a nominal value of the central wavelength λ _{0 is,} for example, 1051.75 nm, then in the spectral range between approximately 1051.0 nm and 1052.3 nm the signal A and the control unit can be used 8th be clearly controlled to the central wavelength λ ₀ . For example, the fact that the primary radiation P assumes wavelengths outside this spectral range can be achieved via the wavelength-selective element 10 be prevented.

In summary, therefore, the fact that the inner surface 4 the folding mirror 33 has no 100 percent reflectivity with respect to the primary radiation P, a part of the primary radiation P in the folding mirror 33 , In particular through the coating 7 it is possible to have two approximately equal in intensity first 11 and second partial radiations 22 get in the same direction and can interfere with each other. About the interference pattern and the signal A, the wavelength of the primary radiation P can be determined and thus readjusted without an additional portion of the primary radiation P from the resonator 3 would have to be disconnected.

In 8th are the folding mirror 33 , the detector elements 6a . 6b as well as the course of the first 11 and the second partial radiation 22 illustrated. Another possibility for determining the central wavelength λ _{0 of} the primary radiation P is that the coating 7 has a dependent of the central wavelength λ ₀ reflectivity R, cf. 9A , The coating 7 in this case preferably covers the entire outer surface 5 or at least those areas of the outer surface 5 about which the first 11 and the second partial radiation 22 the folding mirror 33 leave.

Indicates the folding mirror 33 a thickness t of about 700 microns, so is the distance d between the first partial radiation 11 and second partial radiation 22 at a value of 67.5 ° for the angle of incidence θ about 450 μm. It follows that also the detector elements 6a . 6b have a small distance from each other and can be mounted on a single chip or made in a single chip. This lowers the cost of the detector elements 6a . 6b and simplifies their adjustment. To suppress stray radiation of Se Kundärwellenlänge S may be in front of the detector elements 6a . 6b a not shown filter, which acts only with respect to the primary radiation P transmissive.

A reflectivity R4 of the inner surface 4 with respect to the pump radiation P is approximately equal to 1. A proportion P 'of the primary radiation P, due to the incomplete reflection of the inner surface 4 in the folding mirror 33 gets into the first partial radiation 11 and the second partial radiation 22 divided up. The proportion P 'has a radiation power P0. The power P1 of the first partial radiation 11 can then be specified as: Pa = P0 * (1-R)

The power P2 of the second partial radiation 22 then results in: Pb = P0 * R * R4 * (1-R)

The signal A results, analogous to 7 , from the quotient of the difference and the sum of the on the detector elements 6a . 6b achievements of the first partial radiation 11 and the second partial radiation 22 :

Using the above-mentioned terms for the powers Pa, Pb, the result is:

The approximation R4 = 1 then yields:

In the vicinity of an operating point, corresponding to the nominal value of the central wavelength λ ₀ , the derivative of the signal A according to the wavelength λ results as follows:

Around one operating point, therefore, the change in the signal A is proportional to the negative of the change in the reflectivity R, in each case as a function of the wavelength λ. This is for the change of the reflectivity R according to 9A resulting signal A is in 9B as a function of the central wavelength λ ₀ . Over the spectral range between approximately 1040 nm and approximately 1060 nm, a one-to-one correspondence of the signal A to a central wavelength λ _{0 is} given.

The The invention described herein is not by the description the embodiment limited. Rather, the invention encompasses every new feature as well as every combination of features, in particular any combination of features in the claims includes, even if this feature or this combination itself not explicitly in the patent claims or embodiments is specified.

Claims (15)

Wavelength-stabilized laser ( 1 ) with - at least one laser medium ( 2 ) emitting a primary radiation (P), - a folded external resonator ( 3 ), which has at least three resonator mirrors ( 31 . 32 . 33 ), one of which as a folding mirror ( 33 ), wherein the folding mirror ( 33 ) a resonator ( 3 ) facing inner surface ( 4 ) and one from the resonator ( 33 ) facing away from outer surface ( 5 ), and - at least two detector elements ( 6 ), where - the folding mirror ( 33 ) is adapted to a first partial radiation ( 11 ) of the primary radiation (P) without reflection on the outer surface ( 5 ) and a second partial radiation ( 22 ) of the primary radiation (P) after reflection both on the outer surface ( 5 ) as well as on the inner surface ( 4 ), and - the first partial radiation ( 11 ) and the second partial radiation ( 22 ) each at least partially on the detector elements ( 6 ) are directed. Wavelength-stabilized laser ( 1 ) according to claim 1, wherein a distance (d) between the first partial radiation ( 11 ) and the second partial radiation ( 22 ) is between 100 μm and 1 mm inclusive. Wavelength-stabilized laser ( 1 ) according to claim 1 or 2, wherein the folding mirror ( 33 ) has a thickness (t) of between 100 μm and 2 mm inclusive. Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, in which the first partial radiation ( 11 ) and from the second partial radiation ( 22 ) an interference pattern is formed, wherein the detector elements ( 6 ) are adapted to detect respective subregions of the interference pattern. Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, in which on the outer surface ( 5 ) of the folding mirror ( 33 ) a coating ( 7 ), which has a wavelength-dependent reflectivity in a spectral range of the primary radiation (P), one of the detector elements ( 6a ) is adapted to the first partial radiation ( 11 ) and the other of the detector elements ( 6b ) to directed, the second partial radiation ( 22 ) to detect. Wavelength-stabilized laser ( 1 ) according to the preceding claim, in which the reflectivity of the coating ( 7 ) with respect to the primary radiation (P) between 20% and 80% inclusive. Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, in which neither the primary radiation (P) nor the first partial radiation ( 11 ) nor the second partial radiation ( 22 ) pass through an optical gradient plate, a prism or a grid. Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, comprising at least one control unit ( 8th ), which is set up in response to at least one signal of the detector elements ( 6 ) to regulate a wavelength of the primary radiation (P). Wavelength-stabilized laser ( 1 ) according to the preceding claim, in which the control unit ( 8th ) is adapted to a spatial position of at least one of the resonator mirrors ( 31 . 32 ). Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, comprising at least one optical crystal ( 9 ) in the resonator ( 3 ), which is set up for an at least partial frequency conversion of the primary radiation (P) into a secondary radiation (S), the wavelength of the primary radiation (P) being in the near-infrared spectral range. Wavelength-stabilized laser ( 1 ) according to the preceding claim and according to claim 8 or 9, in which the control unit ( 7 ) is adapted to a temperature of the optical crystal ( 9 ). Wavelength-stabilized laser ( 1 ) according to any one of the preceding claims, wherein the secondary radiation (S) via the folding mirror ( 33 ) is coupled out of the resonator and in which the secondary radiation (S) in a different direction than the first ( 11 ) and the second partial radiation ( 22 ). Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, in which an angle of incidence (θ) of the primary radiation (P) on the folding mirror ( 33 ) between 40 ° and 75 ° inclusive. Wavelength-stabilized laser ( 1 ) according to one of the preceding claims, comprising at least one wavelength-selective element ( 10 ) arranged to restrict the primary radiation (P) to a wavelength interval having a spectral width of at most 4 nm. Method for wavelength stabilization of a laser ( 1 ) according to one of the preceding claims, wherein one of measured values of the at least two detector elements ( 6 ) determined signal to a regulation of an optical length of the resonator ( 3 ) is used.