DEVICE FOR GENERATING LASER RADIATION

Description

[0001] The present invention relates to a device for generating laser radiation. In particular, the present invention relates to an efficient device for generating laser radiation by means of frequency doubling. State of the art

[0002] Using so-called nonlinear materials, whose electrical polarization reacts nonlinearly to an external electric field, it is possible to convert light to a different wavelength (nonlinear frequency conversion). Examples of such processes are frequency doubling (second harmonic generation, SHG) and spontaneous parametric down conversion (SPDC). Various implementation methods exist, but they are similar in their basic principle and the properties described here. Since nonlinear media are generally crystalline, only crystals will be discussed in the following. It is possible to use the crystal as a bulk material, whereby the light from the source is shaped by optical elements to create a specific beam profile or caustic within the crystal.Furthermore, the crystal can also contain waveguide structures that guide the incident and / or generated light (ribbed waveguide structure, channel waveguide, etc.). For this to work, the incident light must be coupled into this waveguide structure. Combinations of both are also possible, i.e., waveguides in one dimension and bulk material in the other (planar waveguide structure).

[0003] In order for the nonlinear processes to run as efficiently as possible, the so-called phase matching between the involved light rays must be achieved.

[0004] There are two widely used methods for this. One is to exploit the birefringent properties of the crystals, which requires precise alignment of the light rays and the crystal's crystallographic axes. Due to the crystal's birefringent property, the refractive index for the incident and generated light changes depending on the angle of the light rays to the crystallographic axes.

[0005] Another method is quasi-phase matching. In this process, the nonlinear crystal is periodically polarized, resulting in a layer sequence with alternating orientations of electrical polarization along the direction of the incident light. The period of this polarization is chosen to achieve phase matching for the desired nonlinear process. The optimal polarization depends on the type of process, the wavelength of the incident light, and the refractive indices of the crystal for both the wavelength of the incident light and the wavelength of the light to be generated. For example, the polarization periodicity can be chosen to enable efficient frequency doubling for precisely one wavelength. A slightly different periodicity is required for a different wavelength.

[0006] Both methods require precise adjustment of the wavelength of the incident light and meticulous control of all crystal parameters that influence phase matching. Since the refractive index of the crystal has a significant impact on phase matching, and this in turn depends on the crystal temperature, the crystal temperature must be actively controlled. For complete systems consisting of a laser source and a crystal, both the laser source and the crystal must be individually controlled by feedback loops.

[0007] Particularly in semiconductor lasers with integrated wavelength stabilization (e.g., distributed Bragg reflector (DBR)), the emission wavelength changes when the output power or temperature is varied. These lasers can exhibit abrupt changes in the emission wavelength (so-called mode shifts). The wavelength shift during a mode shift is inversely related to the laser's resonator length. In the case of semiconductor lasers with typical resonator lengths of less than one centimeter, the wavelength shift can be as much as several tens of picometers, which negatively impacts the nonlinear process. Therefore, the disadvantage arises that the use of such lasers necessitates complex control technology to individually adjust the operating parameters of the laser and / or the crystal at each operating point.The main problem is the fact that for nonlinear processes, the laser source and the crystal are two separate assemblies that must be controlled individually.

[0008] Furthermore, the use of periodically polarized crystals presents additional disadvantages due to the periodic polarization. Small changes in the refractive index occur at the boundaries of the individual polarization layers, resulting in a small portion of the light being reflected at each of these domain boundaries. Due to the periodic arrangement of the polarization, the nonlinear crystal also behaves like a Bragg grating, exhibiting typical resonance maxima at specific wavelengths. The location of these resonances depends on the periodicity of the polarization and the refractive index at the respective wavelength. Thus, the resonances are influenced by the choice of periodic polarization; however, a distinction must be made between the wavelength for which the periodic polarization has been optimized for an efficient nonlinear process and the wavelength at which a resonance occurs.Since both have different dependencies, there are polarization periods that cause resonance and the efficient nonlinear process to occur at the same wavelength. Conversely, they can also differ significantly if the polarization period is varied only slightly. Because refractive indices, in particular, vary with temperature, and resonance and the nonlinear process scale differently with the refractive index, the temperature can be adjusted, within a certain range, to determine whether both effects occur at the same wavelength.

[0009] In Fig. 1 The reflection spectra for two different periodically polarized lithium niobate crystals at a temperature of 25°C are shown. In both cases, these are finned waveguide crystals, meaning that the light is guided through the crystal in a finned waveguide. The crystal made of Fig. 1a) is optimized for SHG at a wavelength of 1122 nm. However, the Bragg resonance in the observed wavelength range is at 1100 nm. In the case of the crystal made of Fig. 1b In this case, the crystal is for SHG at 1070 nm, while here a Bragg resonance occurs only a few nanometers lower at 1065 nm. The intense Bragg resonances, which reflect almost 10% of the incident light at this wavelength, can significantly disrupt the operation of the light source, especially if the resonance is close to the wavelength for nonlinear frequency conversion, i.e., close to the emission wavelength of the light source. However, the observed noise background outside the Bragg resonances, with reflectivities of 0.001% to 0.1%, can also pose a problem for the stable operation of a laser as a light source.

[0010] Semiconductor lasers, in particular, can be highly sensitive to back reflections, resulting in abrupt changes in output power and spectral characteristics. Since frequency conversion typically requires the wavelength to remain stable within a very narrow range (wavelength acceptance) and is also strongly dependent on the incident power, it is essential for reliable operation that the laser is affected as little as possible by back reflections.

[0011] The fundamental explanation for the disrupted operation of lasers under the influence of back reflections lies in the competition between different resonators. These are, firstly, the resonator that is part of the laser itself (internal resonator), and secondly, the resonator formed by the laser and the nonlinear medium (external resonator). Here, too, the main cause is that the laser and the crystal are two separate assemblies, and in the case of the laser, they react to external influences.

[0012] To solve or at least mitigate this problem, various conventional approaches are known.

[0013] An optical isolator between the laser source and the nonlinear medium can attenuate back reflections by several orders of magnitude, resulting in significant optical losses for the external resonator (reducing its quality factor) and minimizing or eliminating disturbances to laser operation. However, using an optical isolator increases the cost and complexity of the setup. Furthermore, with regard to microintegration, miniaturized optical isolators are not available for all wavelengths and are not suitable for all power classes. Additionally, optical isolators always introduce some optical losses due to light absorption.

[0014] As a second approach, the laser can be optimized by increasing the quality factor of the internal resonator, so that it maintains stable laser operation even with back reflections up to a certain level. For semiconductor lasers, for example, the front-facet reflectivity can be increased. However, a disadvantage of this is that the maximum output power of the laser is usually reduced. Depending on the laser type, such optimizations may not be possible due to design limitations.

[0015] The third approach is to modify the crystal so that back reflections are reduced or at least prevented from reaching the laser. This essentially corresponds to a reduction in the quality factor of the external resonator. The periodic polarization can be implemented such that the domain boundaries of the polarization are no longer perpendicular to the incident beam. While back reflections can still occur, the majority of the power is reflected back at an angle, preventing it from reaching the laser source. Disadvantages include the increased complexity of creating the periodic polarization in the crystal and a potentially reduced efficiency of the nonlinear process. Furthermore, a small portion of the light will always still be reflected back towards the laser source.

[0016] The third approach, already known in the art, deflects reflections from the crystal's entrance and exit facets away from the light source. Despite antireflective coatings, the residual reflectivity of the facets is typically in the range of 0.1%. The facets are designed such that the light rays have an angle of incidence of a few degrees to the facet.

[0017] Conventional frequency doubling devices using nonlinear crystals therefore either result in optical losses or require increased manufacturing effort. Furthermore, more complex control systems are needed to individually manage the operating parameters of the laser and the crystal.

[0018] US 2008 / 317072 A1, EP 2 738 600 A1, JP H06 194708 A,

[0019] QING-YANG XU ET AL: "Theoretical Analysis of Intra-Cavity Second-Harmonic Generation of Semiconductor Lasers by a Periodically Poled Nonlinear Crystal Waveguide", IEEE JOURNAL OF QUANTUM ELECTRONICS, Bd. 47, Nr. 4, 1. April 2011 (2011-04-01), Seiten 462-470, XP011478511,

[0020] E. U. RAFAILOV ET AL: "Efficient frequency doubling of a pulsed laser diode by use of a periodically poled KTP waveguide crystal with Bragg gratings", OPTICS LETTERS, Bd. 26, Nr. 24, 15. Dezember 2001 (2001-12-15), Seite 1961, XP055664065, und

[0021] SHINOZAKI K ET AL: "SELF-QUASI-PHASE-MATCHED SECONDHARMONIC GENERATION IN THE PROTON-EXCHANGED LINB03 OPTICAL WAVEGUIDE WITH PERIODICALLY DOMAIN-INVERTED REGIONS", APPLIED PHYSICS LETTERS, Vol. 59, No. 5, July 29, 1991 (1991-07-29), pages 510-512, XP000233660 discloses conventional devices for frequency doubling using nonlinear crystals. A device according to the preamble of claim 1 is known from EP 1 586 940 A1. Disclosure of the invention

[0022] It is therefore an object of the present invention to provide a device for generating laser radiation using a nonlinear crystal which overcomes the aforementioned disadvantages.

[0023] These problems are solved according to the invention by the features of independent claim 1. Advantageous embodiments of the invention are contained in the respective dependent claims.

[0024] According to one aspect of the present invention, the device for generating laser radiation comprises an optical amplifier with an active zone, wherein the optical amplifier has a front facet and a back facet, between which the active zone extends; and a resonator with a first resonator element and a second resonator element, between which the optical amplifier extends, wherein the first resonator element is arranged on a side of the active zone facing away from the front facet and the second resonator element is arranged on a side of the active zone facing the front facet, the second resonator element comprising a nonlinear crystal with periodic polarization. The periodic polarization is formed from a plurality of individual periodically arranged polarization layers, and the reflectivity of the nonlinear crystal is caused by the boundaries of the polarization layers.The optical amplifier is designed as an electrically pumped semiconductor optical amplifier, with the active zone configured to emit radiation of a first wavelength. The nonlinear crystal is configured to convert radiation of the first wavelength into radiation of a second wavelength by means of nonlinear frequency conversion. The device further includes a temperature control loop, which is configured to actively regulate the temperature of the nonlinear crystal so that the temperature of the nonlinear crystal is always adjusted to ensure optimal frequency conversion, i.e., that the frequency-converted radiation has maximum power. The ratio of the crystal's reflectivity for the first wavelength to the front facet's reflectivity for the first wavelength is greater than or equal to 10.The optical amplifier and the crystal are aligned with each other in such a way that the radiation emitted by the optical amplifier is coupled into an entrance facet of the crystal, the boundaries of the periodically arranged polarizing layers of the crystal being perpendicular to the radiation coupled into the crystal, and the nonlinear crystal not comprising any beam-guiding elements.

[0025] In conventional devices, the resonator around the active medium is always formed independently of the crystal. In the case of periodically polarized crystals, attempts are always made to reduce the quality factor of the external resonator or to increase that of the internal resonator (i.e., the optical amplifier). The idea of ​​the present invention is to selectively utilize the crystal as an external resonator mirror. For this purpose, the existing back reflections generated by a periodically polarized crystal are used, in particular spectrally selective back reflections, so that the periodically polarized crystal serves as a resonator mirror.

[0026] The resonator around the amplifier medium (optical amplifier) ​​required for laser operation is formed by the crystal itself on the output coupling side of the laser radiation. In other words, according to one embodiment, the optical amplifier only reaches the laser threshold through the back reflection of the crystal's periodic polarization.

[0027] While a large part of the optical power (e.g. for frequency doubling) is propagated through the crystal and can be used for nonlinear processes, at the same time a portion of the radiation is reflected back to the optical amplifier at the domain boundaries of the periodic polarization, thus ensuring laser operation.

[0028] Behind the optical amplifier is also a rear-facing resonator mirror (resonator element). To efficiently utilize the back reflection of the crystal, an amplifier is provided according to one embodiment in which the reflectivity of the facet of the optical amplifier located between the amplifier and the crystal (front facet) is set in a corresponding ratio to the reflectivity of the crystal by the periodic polarization at the operating wavelength of the amplifier. The ratio of the reflectivity of the crystal (with respect to the operating wavelength of the amplifier) ​​to the reflectivity of the front facet of the amplifier (also with respect to the operating wavelength of the amplifier) ​​is equal to or greater than 10, and more preferably greater than 100.

[0029] By using the crystal as an external resonator, the crystal is responsible for both wavelength selection and the nonlinear process within the overall system. It is sufficient to actively control only the crystal's temperature; that is, the temperature must be adjusted by a control loop to ensure optimal frequency conversion, meaning the frequency-converted radiation has its maximum power. The optical amplifier may only require passive temperature control to dissipate the heat generated, which does not necessitate a complex control loop. Because the external resonator is longer than the amplifier's internal resonator, wavelength shifts due to mode transitions are significantly reduced, thus eliminating any issues. Furthermore, optical isolators between the amplifier and the crystal can be omitted.Furthermore, it is advantageously unnecessary to increase the reflectivity of the amplifier's front facet to suppress any back reflections. According to one embodiment, an anti-reflective coating (or other anti-reflective devices) can be used on the crystal's entrance facet.

[0030] According to the invention, the optical amplifier is designed as an electrically pumped semiconductor optical amplifier. The active zone is configured to emit radiation of a first wavelength (the amplifier's operating wavelength). Furthermore, according to one embodiment, the resonator is designed to increase the intensity of the radiation of the first wavelength within the resonator beyond the laser threshold, so that laser radiation of the operating wavelength is converted (frequency doubling) within the crystal and can be coupled out for further use via an exit facet of the crystal. The crystal therefore acts, on the one hand, to form a resonator for generating laser radiation using the electrically pumped amplifier, and on the other hand, to generate secondary laser radiation by frequency conversion of the primary laser radiation generated by the amplifier and resonator (crystal).

[0031] According to the invention, the ratio of the reflectivity of the crystal due to the periodic polarization for the first wavelength to the reflectivity of the front facet of the amplifier for the first wavelength is greater than 10, preferably greater than 100, and even more preferably greater than 500. According to an embodiment of the invention, the reflectivity of the front facet of the amplifier for the first wavelength is less than 0.001 (i.e., less than 0.1% or less than 10⁻³), more preferably less than 10⁻⁴, even more preferably less than 10⁻⁵, and even more preferably less than 10⁻⁶.

[0032] According to the invention, the nonlinear crystal is configured to convert the radiation (of the first wavelength) generated by the amplifier into radiation of a second wavelength by means of nonlinear frequency conversion. The crystal is configured to subject the radiation coupled in via the entrance facet to a nonlinear conversion process. Preferably, the first wavelength is twice the second wavelength. It is preferred that the crystal is matched to the wavelength of the amplifier for frequency doubling. In other words, it is preferred that the crystal has its maximum conversion efficiency at the first wavelength. As an alternative to frequency doubling, other nonlinear conversion processes, e.g., SPDC, are also possible, for which the crystal and amplifier are matched with respect to their respective maximum conversion efficiencies.

[0033] Preferably, the nonlinear crystal with periodic polarization is designed such that the difference between the wavelength for which the crystal has maximum reflectivity and the wavelength for which the crystal has maximum conversion efficiency is less than 30 nm, more preferably less than 25 nm, even more preferably less than 20 nm, even more preferably less than 15 nm, even more preferably less than 10 nm and even more preferably less than 5 nm.

[0034] Preferably, the periodic polarity is a homogeneous periodic polarity. Here, homogeneous means that the periodicity of the polarity is constant over the entire length of the periodic polarity.

[0035] Preferably, the periodic polarization extends over the entire length of the crystal.

[0036] The reflectivity of the crystal is caused by the domain boundaries. The individual domain boundaries have a constant reflectivity along the length of the crystal and also across the cross-sectional area of ​​the crystal (at least the cross-sectional area occupied by the guided light).

[0037] With regard to the wavelength difference, the following three cases can be distinguished.

[0038] In the first case (explained above), the crystal is designed for a specific wavelength at a specific temperature, so that the reflection maximum and the wavelength for maximum conversion efficiency are identical.

[0039] In the second case, the wavelengths are slightly different. In this case, changing the temperature or other factors can bring both wavelengths into alignment. However, this common wavelength shifts slightly, which is taken into account when designing the setup. Furthermore, this method is highly dependent on the properties of the crystal and is generally only suitable for small wavelength deviations (preferably up to 5 nm). This is because the wavelength changes slowly with temperature, and one quickly reaches temperatures that are not practical to implement, for example, when a difference of 15 nm needs to be compensated for – depending on the crystal and amplifier.

[0040] In the third case, the wavelength difference is even greater, so temperature control (alone) is also not useful.

[0041] However, it is still possible to use the crystal as a resonator mirror.

[0042] Firstly, the wavelength-selective property of the sharp resonance maximum is not used, but rather the more homogeneous noise background at approximately 0.1% reflectivity (see Figure 1a However, an additional wavelength-selective element must be introduced into the resonator so that the laser operation through this element takes place at the wavelength for the effective nonlinear process and not at the resonance wavelength.

[0043] According to one embodiment, no optical insulators or optical filters are arranged between the front facet of the optical amplifier and the entrance facet of the crystal.

[0044] According to the invention, the nonlinear crystal has no beam-guiding elements, such as a waveguide, wherein the radiation emitted by the optical amplifier (and coupled into the crystal for frequency conversion) propagates perpendicularly along the domain boundaries of the crystal's periodic polarity. In contrast to an embodiment with a crystal containing a waveguide, the reflectivity of the periodic polarity is lower in crystals without beam-guiding elements, which is why the domain boundaries are arranged perpendicular to the radiation.

[0045] The facets of the crystal are preferably inclined to the main direction of radiation within the crystal, since otherwise the facets would form an additional resonator, which in turn would interfere with laser operation. The angle between the facets of the crystal and the radiation propagating within the crystal is preferably between 1° and 10°, more preferably between 2° and 5° (a full circle of 360°).

[0046] Preferably, the amplifier has a ribbed waveguide. According to one embodiment, the crystal is made of lithium niobate.

[0047] According to one embodiment, the rear resonator mirror can be formed by integrating a surface grating into the end (facing away from the amplifier) ​​or rear part of the finned waveguide. This is particularly preferred when the difference between the wavelength at which the crystal has maximum reflectivity and the wavelength at which the crystal has maximum conversion efficiency is greater than 5 nm, more preferably greater than 10 nm, and even more preferably greater than 15 nm.

[0048] Alternatively, the rear resonator mirror can be formed by the back facet of the optical amplifier. This is particularly preferred when the difference between the wavelength at which the crystal has maximum reflectivity and the wavelength at which the crystal has maximum conversion efficiency is less than 10 nm, more preferably less than 5 nm, and even more preferably less than 3 nm. Brief description of the drawings

[0049] The invention is explained below using exemplary embodiments with reference to the accompanying drawing. The drawing shows: Fig. 1a and Fig. 1b: Reflection spectra of two periodically polarized finned waveguide crystals; Fig. 2: A schematic sectional view of a device according to the invention for stabilizing an optical amplifier using reflections from a periodically polarized crystal for frequency conversion; Fig. 3a: A schematic top view of a device according to a first embodiment, which is not part of the invention; Fig. 3b: A schematic side view (sectional view) of a device according to the first embodiment; Fig. 3c: A schematic side view (sectional view) of a device according to an embodiment of the invention; and Fig. 4: A sectional view of the [unclear] in the Figures 3a and 3b semiconductor amplifier shown. Detailed description of the drawings

[0050] Figs. 3a and 3bshow a device according to a first embodiment, which is not part of the invention, in top view and lateral section view, in which a periodically polarized crystal with ribbed waveguide is used as a resonator mirror for a semiconductor amplifier with ribbed waveguide structure.

[0051] The periodically polarized crystal 5 exhibits the same periodic polarity as the crystal whose reflection spectrum in Fig. 1 b)As shown, the domains are arranged at an angle of θ = 92° to the waveguide 10. The aim is to use the reflection spectrum of crystal 5 additionally as a spectral filter and thus to define the emission wavelength of the laser operation at 1065 nm. By varying the temperature of the crystal and the different scaling of the Bragg resonances and the phase matching for frequency doubling, the emission wavelength of the laser can finally be adjusted so that the light is optimally converted to frequency-doubled radiation (SHG) as it passes through crystal 5.

[0052] The semiconductor amplifier 1 was processed on gallium arsenide (GaAs) using metal-organic vapor phase epitaxy. The amplifier 1 has a length of W1 = 4 mm and a width of the finned waveguide 3 of W2 = 4 µm. In the vertical direction, a layered waveguide with a thickness of W3 = 4.8 µm is implemented. The rear facet 2 is mirrored and thus forms the rear resonator mirror (21 in Fig. 2 ) The front facet 4 has an anti-reflective coating and a reflectivity of less than 0.01% for the operating wavelength of 1065 nm.

[0053] The periodically polarized crystal 5 is made of lithium niobate and is commercially available, for example, from HC Photonics Corp. Crystal 5 has a length of W4 = 10 mm, while the period of the periodic polarization W5 is approximately 6.6 µm. The domains of the periodic polarization are arranged at an angle of θ = 92° to the waveguide. The finned waveguide 10 has a width W6 of approximately 6 µm and a height W7 = 4 µm. Although the finned waveguides 3 and 10 of amplifier 1 and crystal 5 have slightly different dimensions, the fundamental modes guided in them (i.e., at approximately 1065 nm) are highly similar. Therefore, two aspherical lenses 8 and 9 with a focal length of 4 mm are used for optical coupling of both components. These are each positioned so that the distance W8 to the front facet 4 of the amplifier 1 or the distance W9 to the entrance facet 6 of the crystal 5 corresponds to the effective focal length of the lenses 8 and 9.The distance between the two lenses 8, 9 can be up to several meters, as long as it does not approach the Rayleigh length of the laser beam to be coupled. This is usually greater than 1 m when using lenses with a 4 mm focal length.

[0054] The Crystal 11 of the in Fig. 3c The illustrated embodiment of the invention, in contrast to crystal 5, has no beam-guiding elements, and the coupled radiation therefore propagates freely through the crystal. A lens 12, which differs from lens 9, is used for coupling.

[0055] The vertical layer structure of the semiconductor amplifier 1 is in Fig. 4The epitaxial layer sequence is deposited on a GaAs substrate 14. An InGaAs triple quantum trench forms the active zone 17, which is asymmetrically embedded in a waveguide. The waveguide consists of an n-doped AlGaAs waveguide core 16 with a thickness of 4000 nm and a p-doped AlGaAs waveguide core 18 with a thickness of 800 nm. This results in the total thickness of 4.8 µm, which is shown in Fig. 3b are designated as W3. The waveguide is surrounded by an n-doped cladding layer 15 with a thickness of 500 nm and a p-doped cladding layer 19 with a thickness of 500 nm.

[0056] The ribbed waveguide 3, which is produced by etching, extends a further 800 nm above the cladding layer 19 with a height W11. Electrical contact is finally ensured by the p-contact 20 and the n-contact 13.

[0057] Since the Bragg resonances of crystal 5 are sometimes several tens of nm away from the optimal wavelength for frequency doubling, it is not always possible to use the Bragg resonances as a wavelength-selective element and simultaneously achieve optimal conditions for frequency conversion. Nevertheless, the non-zero reflectivity of at least approximately 0.01% of the crystal can be used to reach the laser threshold through periodic polarization. The front facet 4 of the amplifier must be highly anti-reflective and exhibit a reflectivity of 10⁻⁶ or less. Here, crystal 5 again acts as a front resonator mirror 22, but without wavelength selection. To still determine the emission wavelength, the rear resonator mirror 21 must be designed as a wavelength-selective element. One possible embodiment is the integration of a surface grating directly into the rear part of the finned waveguide 3. Reference symbol list

[0058] 1 Optical amplifier 2 Rear facet 3 Ribbed waveguide 4 Front facet 5 Optical crystal (periodically polarized) 6 Entrance facet 7 Exit facet 8 Lens 9 Lens 10 Ribbed waveguide (crystal) 11 Optical crystal (periodically polarized without beam-guiding elements) 12 Lens 13 n-contact 14 Substrate 15 n-conducting cladding layer 16 n-conducting core layer 17 Active zone 18 p-conducting core layer 19 p-conducting cladding layer 20 p-contact 21 Rear resonator mirror (first resonator element) 22 Front resonator mirror (second resonator element)

Claims

1. Device for generating laser radiation, comprising: a) an optical amplifier (1) with an active zone (17); b) wherein the optical amplifier (1) has a front facet (4) and a rear facet (2) between which the active zone (17) extends; and c) a resonator comprising a first resonator element (21) and a second resonator element (22), between which the optical amplifier (1) extends, wherein the first resonator element (21) is arranged on a side of the active zone (17) facing away from the front facet (4) and the second resonator element (22) is arranged on a side of the active zone (17) facing the front facet (4), wherein the second resonator element (22) comprises a nonlinear crystal (5, 11) with a periodic polarization (W5), wherein the periodic polarization (W5) is formed from a plurality of individual periodically arranged polarization layers, and wherein the reflectivity of the nonlinear crystal (5, 11) is caused by the boundaries of the polarization layers, wherein the optical amplifier (1) is configured as an electrically pumped optical semiconductor amplifier (1), wherein the active zone (16) is configured to emit radiation of a first wavelength, wherein the optical amplifier (1) and the crystal (5) are aligned with each other such that the radiation emitted by the optical amplifier (1) is coupled into an entrance facet (6) of the crystal (5), wherein the boundaries of the periodically arranged polarizing layers of the crystal (11) are perpendicular to the radiation coupled into the crystal (11), and wherein the nonlinear crystal (5, 11) is configured to convert radiation of the first wavelength into radiation of a second wavelength by means of nonlinear frequency conversion, characterized in that the ratio of the reflectivity of the crystal (5) for the first wavelength to the reflectivity of the front facet (4) for the first wavelength is greater than or equal to 10, the nonlinear crystal (11) does not include any beam-guiding elements, and the device further comprises a temperature control loop, the temperature control loop being configured to actively control the temperature of the nonlinear crystal (5, 11) so that the temperature of the nonlinear crystal (5, 11) is adapted so that optimal frequency conversion always takes place, so that the frequency-converted radiation of the second wavelength is at its maximum power.

2. The device according to claim 1, wherein the ratio of the reflectivity of the crystal (5) for the first wavelength to the reflectivity of the front facet (4) for the first wavelength is greater than or equal to 100.

3. The device according to any one of the preceding claims, wherein the reflectivity of the front facet (4) for the first wavelength is less than 0.001.

4. The device according to any one of the preceding claims, wherein the first wavelength is twice the second wavelength.

5. The device according to any one of the preceding claims, wherein no optical insulators and / or optical filters are arranged between the front facet (4) of the optical amplifier (1) and an entrance facet (6) of the crystal (5, 11).

6. The device according to any one of the preceding claims, wherein the periodic polarization (W5) is a homogeneous periodic polarization.

7. The device according to any one of the preceding claims, wherein the periodic polarization (W5) extends over the entire length of the crystal (5, 11).