Optical system for carrying out nonlinear optical processes and methods for operating an optical system

Abstract

The invention relates to an optical system (1) for carrying out nonlinear optical processes in at least one laser signal, comprising a crystal (2) and an auxiliary source (3), wherein the crystal (2) is a nonlinear optical crystal such that, when the at least one laser signal is coupled in the form of a pump beam, the pump beam interacts nonlinearly in the crystal (2), wherein the auxiliary source (3) is configured to emit auxiliary illumination that disrupts the energy levels of quasiparticles generated by coupling the pump beam into the crystal (2), and the auxiliary source (3) is optically connected to the crystal (2) so that the auxiliary illumination is coupled into a region of the crystal (2) together with the pump beam. In this way, an optical system is provided with which photorefraction in nonlinear optical crystals is reduced.

Description

The present invention relates to an optical system for carrying out nonlinear optical processes in at least one laser signal, comprising a crystal, wherein the crystal is a nonlinear optical crystal, such that when the at least one laser signal is coupled in the form of a pump beam, the pump beam interacts nonlinearly in the crystal. Nonlinear optical effects, particularly those of the second and third order, as well as the related electro-optic effect (Pockels effect), form the basis of numerous applications in modern optics and quantum optics. Typical applications include laser systems and tunable light sources such as optical parametric oscillators, single-photon sources, quantum light filters, frequency converters, and electro-optical modulators for high-speed signal processing. Due to the inherently low efficiency of nonlinear processes, all these applications require high optical field intensities to achieve a sufficiently strong interaction with the crystal medium. The increasing development of photonic systems for cryogenic environments, particularly in the fields of quantum communication and quantum information processing, places additional demands on the performance of nonlinear optical components. Applications such as quantum interconnects, which link quantum computers of different technologies and platforms via optical networks, or superconducting detection systems, require solutions that operate reliably at extremely low temperatures. In such applications, efficient frequency conversion is often necessary to couple different operating frequencies. Due to optical losses in transmission links, it is advantageous for the frequency conversion to take place directly in the cryogenic environment. However, nonlinear optical crystals encounter fundamental limitations in this regard. Besides the risk of irreversible damage due to thermal effects at high optical power densities, reversible but power-limiting effects such as photorefraction pose a significant problem. This arises from quasiparticles within the crystal material. For example, free charge carriers in the form of electrons and holes can be pumped out of their bound states across a band gap using a pump beam. Free charge carriers can also originate from the excitation of defects, such as foreign atoms. These free charge carriers cause local changes in the crystal structure and lead to its deformation. The combination of charge carriers and local lattice distortion is called a polaron. The polaron is a quasiparticle that causes local changes in the refractive index and absorption of the nonlinear optical crystal.This, in turn, leads to the photorefraction mentioned above. The resulting optical losses and phase mismatches, e.g., due to altered refractive indices, lead to a reduction in efficiency or a change in the signal wavelengths of the nonlinear processes and can even contribute to irreversible material damage in the long term. Despite advances, such as material doping with magnesium-doped lithium niobate, reliable protection against photorefraction has not yet been achieved, especially at low temperatures. From DE 10 300 080 A1 a method for desensitizing a crystal with nonlinear optical properties, in particular a lithium niobate or lithium tantalate crystal, against damage caused by intense light exposure is known, wherein the damage is caused by light-induced changes in the refractive indices and wherein the dark conductivity of the crystal is increased by doping with extrinsic ions. Previously used countermeasures, such as locally heating the crystals to reduce the lifetime of free charge carriers, are only of limited practical use in cryogenic operation. The additional heat input consumes valuable resources from the already limited thermal budget of cryogenic systems and can lead to adverse side effects such as thermomechanical stresses, pyroelectric stress build-up, component deformation, or damage to adjacent, temperature-sensitive components. These solutions also prove inadequate in other sensitive application areas such as aerospace, where a strict energy budget must be adhered to. Starting from this premise, the object of the present invention is to provide an optical system with which photorefraction in nonlinear optical crystals is reduced. This task is solved by the subject matter of the independent claim. Further details can be found in the dependent claims. According to the invention, an optical system for carrying out nonlinear optical processes in at least one laser signal is provided, comprising a crystal and an auxiliary source, wherein the crystal is a nonlinear optical crystal, such that when the at least one laser signal is coupled in the form of a pump beam, the pump beam interacts nonlinearly in the crystal, wherein the auxiliary source is configured to emit an auxiliary illumination that breaks up energy levels of quasiparticles generated by coupling the pump beam into the crystal, and the auxiliary source is optically connected to the crystal, so that the auxiliary illumination is coupled into a region of the crystal together with the pump beam. According to the invention, the implementation of nonlinear optical processes relates to the targeted generation or utilization of higher-order optical effects in an optical medium, wherein the intensity of the incident light causes a nonlinear dependence in the material response. A laser signal can be understood as a coherent light signal generated by stimulated emission in a laser source and exhibiting a defined wavelength and beam quality. The laser signal can be in pulsed form or as a continuous laser beam. According to the invention, the optical system comprises a crystal and an auxiliary source. In this context, the crystal serves as the medium for the nonlinear optical interaction, in which coupled electromagnetic waves interact with one another. An auxiliary source can be understood as a radiation source that operates independently of a primary pump source used for the nonlinear process and emits the pump beam, and that generates a separate light field. The crystal is designed to be a nonlinear optical crystal, such that when at least one laser signal is coupled in as a pump beam, the pump beam interacts nonlinearly within the crystal. A nonlinear optical crystal can be understood as a crystal made of a material that exhibits a nonlinear response to an applied electromagnetic field. Coupling the laser signal as a pump beam means that the pump beam is directed into the volume of the crystal, resulting in an interaction between the electromagnetic field of the pump beam and the crystal material. This creates the basis for generating nonlinear optical effects. In this way, for example, frequency conversion can be achieved, allowing different optical communication channels to be coupled or quantum states to be transmitted efficiently. According to the invention, the auxiliary source is further configured to emit auxiliary illumination that disrupts the energy levels of quasiparticles generated by coupling the pump beam into the crystal. Auxiliary illumination can be understood as an additional electromagnetic radiation field generated by the auxiliary source and directed towards the crystal. Quasiparticles, such as polarons, are generated in the crystal by the interaction between the electromagnetic field of the pump beam and the crystal's lattice structure. The disruption of the energy levels of these quasiparticles means that the auxiliary illumination provides energetic excitation, reducing the lifetime and / or stability of these states.This reduces the effect of quasiparticles on the optical properties of the crystal, decreases the number and / or lifetime of quasiparticles, and diminishes photorefractive effects within the crystal. In particular, this enables the crystal to operate stably even at low temperatures without the need for thermal measures to reduce photorefraction. In this way, the efficiency of the nonlinear optical process can be maintained even at cryogenic operating temperatures, facilitating the integration of the optical system into modern quantum technology platforms. Finally, the auxiliary light source is optically connected to the crystal, so that the auxiliary illumination is coupled into a region of the crystal along with the pump beam. The optical connection means that the auxiliary illumination is aligned and guided so that it reaches the same region of the crystal as the pump beam. This optical connection allows the auxiliary illumination emitted by the auxiliary light source to be transmitted. This can be achieved, for example, by transmission in a gas mixture such as air or in a vacuum, or by fiber optic connections suitable for transmitting light of an appropriate optical bandwidth. Alternatively, the respective components, such as the auxiliary illumination and the crystal, can be arranged side by side in such a way that they are directly adjacent to each other. The auxiliary illumination thus acts on the region within the crystal where the quasiparticles excited by the pump beam are generated. This ensures that the effect of the auxiliary illumination is efficient and locally confined to the relevant regions within the crystal. This further improves the effectiveness of reducing photorefractive effects without the auxiliary illumination affecting other, unaffected areas of the crystal, thereby contributing to the high energy efficiency of the optical system. According to a further development of the invention, the auxiliary illumination is coupled into the crystal collinearly with the pump beam. Collinear coupling can be understood as an alignment of the auxiliary illumination in a common direction with the pump beam, such that both beams travel along an optical axis and penetrate the same region of the crystal. An advantage of this design is that the auxiliary illumination is coupled precisely into the region or volume of the crystal in which the pump beam also acts. This ensures that the auxiliary illumination directly affects the quasiparticles generated in the crystal by the pump beam, thus enabling targeted and effective manipulation of these quasiparticles. This supports the reduction of photorefraction in the crystal and contributes to improving the efficiency of the nonlinear optical process. According to a further embodiment of the invention, the auxiliary light source is configured to emit auxiliary illumination that illuminates at least one crystal surface corresponding to a cross-sectional area of ​​the pump jet on the crystal surface. A cross-sectional area of ​​the pump jet on the crystal surface can be understood as the area of ​​the crystal upon which the pump jet strikes the crystal upon entering it. An advantage of this embodiment is that the auxiliary illumination is directed specifically at the entry surface of the pump jet. This ensures that targeted influencing of the quasiparticles occurs as early as the point where the pump jet enters the crystal, thus reducing the formation or accumulation of quasiparticles at this critical location. In this way, the formation of photorefractive effects is reduced in the early stages, thereby increasing efficiency. In a further development of the invention, the auxiliary light source is designed to emit auxiliary illumination that illuminates the crystal over a larger area. This illumination can be understood as the auxiliary illumination spreading over a larger area of ​​the crystal, so that not just a local area, but a significant portion or even the entire crystal surface is irradiated. An advantage of this design is that the auxiliary illumination acts not only locally, but over a large area on quasiparticles forming within the crystal, thereby also influencing quasiparticles that have moved away from their point of origin due to internal fields or diffusion processes. This allows for a comprehensive reduction in photorefraction, which further increases the operational reliability of the crystal at high optical power. In a further development of the invention, the auxiliary light source is configured to emit the auxiliary illumination at a wavelength corresponding to a band gap of the crystal. A band gap of the crystal can be understood as the energy range between the valence band and the conduction band of the crystal material in which no electronic states exist. A wavelength corresponding to the band gap describes a wavelength at which electrons can be excited from the valence band to the conduction band. It is assumed that the energy transferable by the auxiliary light source is also sufficient to excite the electrons across the band gap. An advantage of this embodiment is that, by specifically selecting the wavelength of the auxiliary light source, free charge carriers can be generated as secondary particles in the crystal, which in turn are suitable for resolving the quasiparticles generated by the pump beam.This allows for targeted manipulation of the quasiparticles, which supports the reduction of photorefractive effects and improves the stability of the nonlinear optical process. According to the invention, the auxiliary illumination can therefore be configured to break up the energy levels of the quasiparticles directly or via a secondary particle cascade. One embodiment of the invention provides that the optical system has a pump source, wherein the pump source is configured to emit a pump beam that breaks up the energy levels of the quasiparticles; furthermore, the optical system also has a beam guidance optic downstream of the crystal, wherein the crystal is configured to couple out at least one signal beam and one idler beam from the nonlinear interaction of the pump beam in the crystal, and the beam guidance optic returns the idler beam to the crystal in the form of auxiliary illumination, so that the auxiliary source is formed with the crystal and the beam guidance optic. The pump source can be understood as a radiation source that generates the laser signal, which is coupled into the crystal as a pump beam. The pump beam serves to excite nonlinear optical effects within the crystal. If the requirements for the nonlinear process in the crystal and for the energy level breakup of the quasiparticles coincide, a portion of the nonlinearly processed pump beam can be used as auxiliary illumination. A beam guiding optic can be understood as an optical element used to guide, focus, or deflect light beams. The crystal is designed to couple out at least one signal beam and one idler beam from the nonlinear interaction of the pump beam within the crystal. A signal beam and an idler beam can be understood as the two output beams generated within the crystal through a parametric nonlinear interaction, where energy and momentum conservation are satisfied. An advantage of this design is that the beam guidance optics selectively capture the idler beam generated in the crystal and make it available for further use. The beam guidance optics are designed as an arrangement consisting of a beam splitter and mirrors. Depending on the configuration of the nonlinear process, the signal and idler beams can differ in various properties. The beam splitter is designed accordingly to separate the signal and idler beams from each other based on these properties.If the signal and idler beams differ in their polarization, the beam splitter is a polarization beam splitter. If the signal and idler beams differ in wavelength, the beam splitter is a dichroic mirror. According to a further embodiment of the invention, the beam guidance optics redirect the idler beam back into the crystal as auxiliary illumination, so that the auxiliary source is formed by the crystal and the beam guidance optics. Redirecting the idler beam back into the crystal means that the idler beam generated during the nonlinear process is specifically used as auxiliary illumination. The auxiliary illumination can be coupled into the crystal in the opposite direction to the propagation direction of the pump beam using a retroreflector. A combination of a mirror and a retroreflector allows the idler beam to be coupled into the crystal as auxiliary illumination in the propagation direction of the pump beam. An advantage of this design is that existing radiation is efficiently reused, thus eliminating the need for a separate auxiliary source. This improves the energy efficiency of the optical system and enables a compact design.According to a further embodiment of the invention, the crystal comprises either lithium niobate or lithium tantalate. Lithium niobate can be understood as a nonlinear optical crystal made of lithium, niobium, and oxygen, exhibiting strong electro-optical coupling. Lithium tantalate can be understood as a nonlinear optical crystal made of lithium, tantalum, and oxygen with comparable properties. An advantage of this embodiment is that both materials, due to their ferroelectric and pyroelectric properties, enable high efficiency for nonlinear optical processes. They also exhibit pronounced nonlinear effects and a high sensitivity to photorefraction.By selecting such a crystal, the effect of the auxiliary lighting can be specifically directed towards the quasiparticles in the form of polarons that arise in the crystal material, thereby supporting a reduction of photorefractive effects. According to a further embodiment of the invention, the auxiliary light source is configured to emit the auxiliary illumination with a wavelength between 280 and 600 nm, preferably between 450 and 600 nm. A wavelength of approximately 280 nm corresponds to the band gap of the crystal in the case of lithium tantalate. Wavelengths in the range between 300 and 350 nm correspond to the band gap of the crystal in the case of lithium niobate, depending on the doping. Through absorption of this radiation in the range between 280 and 450 nm, secondary particles in the form of free charge carriers are generated in the crystals. A wavelength between 450 and 600 nm is the range in which the energy levels of the polarons are directly disrupted.One advantage of this design is that the selected wavelength allows for either the targeted initiation of secondary processes or the direct excitation of polarons. This ensures that the auxiliary illumination effectively contributes to reducing photorefraction by decreasing the lifetime or concentration of polarons in the crystal, either through secondary particles or direct excitation. In a further development of the invention, it is intended that the crystal has a waveguide, and the auxiliary light source is optically connected to the waveguide, so that the auxiliary illumination is coupled into the waveguide together with the pump beam. A waveguide can be understood as a structured region within the crystal that directs light along a defined path, with the light being guided by total internal reflection or by differences in refractive index. The optical connection between the auxiliary light source and the waveguide means that the auxiliary illumination is directly coupled into the waveguide, so that both beams traverse the same optical path. An advantage of this embodiment is that the auxiliary illumination is focused precisely on the area in which the pump beam is guided through the waveguide.This ensures that the auxiliary illumination efficiently acts on the quasiparticles generated along the waveguide, reducing the photorefractive effects within the waveguide and supporting the efficiency of the nonlinear optical process. According to a further embodiment of the invention, the auxiliary illumination emitted by the auxiliary light source is either monochromatic or broadband. Monochromatic auxiliary illumination can be understood as narrowband light with only a few wavelengths in the nanometer range, as is typically produced by a laser. Broadband auxiliary illumination can be understood as light with a broad spectrum of wavelengths, as is produced, for example, by an intense LED. An advantage of this embodiment is that the effective range of the auxiliary illumination can be flexibly adapted to the requirements of the crystal material by selecting the type of radiation.By using monochromatic auxiliary illumination, a specific energy state change of the quasiparticles can be targeted, whereas broadband auxiliary illumination allows for a broader range of energy levels to be addressed. This supports the reduction of photorefraction in different operating scenarios. A further embodiment of the invention provides that the auxiliary light source emits either pulsed or continuous auxiliary illumination. Pulsed auxiliary illumination can be understood as a periodic sequence of light pulses, while continuous auxiliary illumination can be understood as an uninterrupted emission of light. An advantage of this embodiment is that the dynamics of the interaction with the quasiparticles can be influenced by the operating mode of the auxiliary illumination. By using pulsed auxiliary illumination, it is achieved that these quasiparticles are selectively excited immediately after their formation, whereas continuous operation results in permanent excitation, which prevents the formation of stable quasiparticle states. According to a further development of the invention, the auxiliary source is configured to emit auxiliary illumination that breaks up the energy levels of polarons generated by coupling the pump beam into the crystal. Polarons can be understood as quasiparticles resulting from the interaction between a charge carrier and the local distortion of the crystal lattice, which is generated by optically pumping the crystal upon coupling of the pump beam. Breaking up the energy levels means that the polarons generated by coupling the pump beam are excited to higher energy states by the auxiliary illumination, thereby reducing their effect on the optical properties of the crystal. This reduces the number and / or lifetime of the polarons and diminishes the photorefractive effects in the crystal.In particular, this makes it possible to operate the crystal stably even at low temperatures without the need for thermal measures to reduce photorefraction. According to a further embodiment of the invention, the optical system includes a filter, the filter being optically connected downstream of the crystal and designed to block the auxiliary illumination emanating from the crystal. A filter can be understood as an optical element that selectively transmits or blocks electromagnetic radiation depending on its frequency, wavelength, or polarization. The downstream connection means that the filter is arranged in the beam path behind the crystal. An advantage of this embodiment is that the auxiliary illumination is selectively removed after passing through the crystal, thereby preventing undesirable influences of the auxiliary illumination on downstream optical components or measuring systems. This improves the purity of the subsequently processed optical signals and reduces the susceptibility of the overall system to interference. In a further development of the invention, the optical system is provided to include a control unit. This control unit is connected to the auxiliary source for signal transmission and is configured to activate the auxiliary source immediately upon the pump jet striking the crystal, thus coupling the auxiliary illumination into the crystal along with the pump jet. A control unit can be understood as an electronic system responsible for controlling the operation of the auxiliary source and providing signals to activate or deactivate the auxiliary illumination. Signal transmission means that the control unit is electrically or optically connected to the auxiliary source to transmit control commands. An advantage of this embodiment is that the auxiliary source is activated precisely at the correct time, ensuring that the auxiliary illumination enters the crystal synchronously with the pump jet.This achieves a targeted temporal overlap between the auxiliary illumination and the quasiparticles generated in the crystal by the pump beam, thereby improving the reduction of photorefractive effects. According to a further embodiment of the invention, the control unit is also connected to the pump source for signal transmission. An advantage of this embodiment is that the control unit receives information about the operating state of the pump source, allowing the activation of the auxiliary light source to be directly coordinated with the operation of the pump source. This improves the timing between the pump jet and the auxiliary lighting. According to a further embodiment of the invention, a beam block with a light sensor is arranged in the beam path of the pump jet, and the control unit is configured to move the beam block out of the beam path immediately when the auxiliary illumination strikes the crystal. A beam block can be understood as an optical component that temporarily blocks the pump jet. A light sensor can be understood as a detector that recognizes the presence of the pump jet in the beam path. An advantage of this embodiment is that the pump jet only enters the crystal when the auxiliary illumination is available, thus achieving precise synchronization of the two light sources. This improves the effect of the auxiliary illumination on the quasiparticles formed in the crystal. According to a further embodiment of the invention, the control unit is aware of the time at which the pump jet is coupled into the crystal, and the control unit activates the auxiliary light source at this time. An advantage of this embodiment is that the activation of the auxiliary light source can be precisely timed to the operation of the pump jet, thereby achieving simultaneous, premature, or delayed coupling of the auxiliary light into the crystal with the pump jet. According to a further embodiment of the invention, the control unit is configured to activate the auxiliary light source for a predetermined time before the pump jet is coupled into the crystal. An advantage of this embodiment is that the auxiliary illumination is already active before the pump jet strikes the crystal, thereby providing free charge carriers within the crystal even before the quasiparticles are formed, which contribute to reducing photorefraction. According to a further embodiment of the invention, the pump jet is pulsed, and the auxiliary light source is configured to emit the auxiliary illumination at a pulse frequency of the pump jet. The control unit is configured to activate the auxiliary light source with a pulse frequency offset from the pump jet. A pulsed pump jet can be understood as a periodic sequence of light pulses that traverse the crystal. A pulse frequency describes the rate at which the pulses are emitted. An advantage of this embodiment is that by matching the auxiliary illumination to the pulse frequency of the pump jet, a temporal synchronization is achieved that optimizes the effect of the auxiliary illumination on the formation and degradation of quasiparticles in the crystal.The time offset of the auxiliary light source control means that the auxiliary illumination is applied precisely at a time when no pump beam is present in the crystal. Because both beams are not present simultaneously, any unwanted signals generated by the auxiliary beam can be selectively filtered out by the time offset from the pump beam. The auxiliary illumination disrupts the energy levels of the quasiparticles in the crystal during each pause of the pump beam. This reduces photorefractive effects and improves the stability of the nonlinear optical process. In a further development of the invention, the optical system is intended to include a cooling element, wherein the cooling element is thermally connected to the crystal, and the cooling element is configured to cool the crystal to a temperature below 293 K, preferably below 70 K. Particularly preferably, the crystal is cooled to a temperature of 0.7 K. A cooling element can be understood as a device for cooling the crystal. The thermally conductive connection means that heat is efficiently transferred from the crystal to the cooling element. An advantage of this design is that, by selectively lowering the temperature of the crystal, the influence of thermally induced material changes on the nonlinear optical process is minimized.Cooling to temperatures below 293 K, preferably below 70 K, makes it possible to operate the crystal in a cryogenic environment, which extends the lifetime of the crystal and improves its suitability for applications in quantum optics and other high-precision optical systems. The invention further relates to a method for operating an optical system with a nonlinear optical crystal, comprising the following method steps: coupling an auxiliary illumination into the crystal, wherein the auxiliary illumination is designed such that the energy levels of quasiparticles generated by coupling a pump beam into the crystal are broken up, and coupling the pump beam into a region of the crystal illuminated by the auxiliary illumination. According to the invention, the pump jet and auxiliary illumination are coupled into the crystal so that the quasiparticles excited by the pump jet can be reduced by the auxiliary illumination. The region in the crystal illuminated by the auxiliary illumination should correspond at least partially to the region in which the pump jet is coupled into the crystal. According to one embodiment of the invention, the pump jet is coupled into the crystal simultaneously with or after the auxiliary illumination. Alternatively, the auxiliary illumination is coupled into the crystal immediately after the pump beam strikes the crystal. Naturally, the quasiparticles only arise through the nonlinear interaction of the pump beam within the crystal. This means that a certain predetermined time elapses between the pump beam striking the crystal surface and the loss of brightness in the pump beam. This predetermined time is typically several milliseconds to seconds. In this context, a further embodiment of the invention provides that the auxiliary illumination is coupled in parallel with the pump beam after the predetermined time. This reduces the energy consumption of the inventive method and avoids undesirable interactions of the auxiliary beam with the pump beam or the generated signal / idler beams.At the same time, requirements for the control unit regarding temporal accuracy and reaction speed are reduced, which means that fewer resources need to be allocated in the form of complex controls. According to a further development of the invention, it is provided that the pump jet and the auxiliary lighting are pulsed, the pump jet and the auxiliary lighting each have an identical pulse frequency, and the pump jet and the auxiliary lighting are coupled into the crystal offset from each other by the pulse frequency. The invention is explained in more detail below with reference to the accompanying drawings and exemplary embodiments. However, it is important to note that these exemplary embodiments do not limit the invention but merely represent different configurations. The features shown can be implemented individually or in combination with other features described in the patent claims. In the drawings, Fig. 1 schematically shows an optical system for carrying out nonlinear optical processes according to one embodiment of the invention, and Fig. 2 schematically shows an optical system for carrying out nonlinear optical processes according to another embodiment of the invention. Figure 1 shows an optical system 1 for carrying out nonlinear optical processes according to an embodiment of the invention. The optical system 1 comprises a nonlinear optical crystal 2 made of lithium niobate. A pump source 4 emits a first laser signal and a second laser signal. Both laser signals are coupled into the crystal 2 in the form of a first pump beam and a second pump beam and interact nonlinearly within the crystal 2. The optical system 1 also includes a cooling element 9, which is thermally connected to the crystal 2. The cooling element 9 is thermally connected to the crystal 2 by enclosing it and is designed to cool the crystal 2 to a temperature of 7 K. The optical system 1 includes an auxiliary light source 3 configured to emit broadband auxiliary illumination with a wavelength of 450–600 nm, thus breaking up the energy levels of polarons generated in the crystal 2. The auxiliary light source 3 is an LED that emits the auxiliary illumination continuously. The auxiliary lighting is coupled into crystal 2 via two mirrors 11 parallel to the pump beams, so that the auxiliary lighting and the two pump beams traverse the same region of crystal 2. The auxiliary lighting illuminates a crystal surface corresponding to a cross-sectional area of ​​the pump beams on the crystal surface. The optical system 1 also includes a filter 7, which is optically connected downstream of the crystal 2. The filter 7 is designed to be opaque to the frequency of the auxiliary illumination, so that the auxiliary illumination emanating from the crystal 2 can be filtered out. The optical system 1 comprises a control unit 8, which is connected to the auxiliary source 3 for signal transmission. The control unit 8 is configured to actuate the auxiliary source 3 immediately upon the pump jet striking the crystal 2, so that the auxiliary illumination is coupled into the crystal 2 by the pump jet. Finally, Fig. 2 shows an optical system 1 according to a further embodiment of the invention. The optical system 1 also comprises a nonlinear optical crystal 2 made of lithium niobate, wherein the crystal 2 includes a waveguide 6 which serves to guide light. A laser signal in the form of a pump beam is coupled into the waveguide 6, the pump beam serving to excite nonlinear optical processes in the crystal 2. The pump beam is emitted by a pump source 4, which is configured such that the pump beam breaks up energy levels of polarons generated in the crystal 2. The crystal 2 is configured to couple out at least one signal beam and one idler beam from the nonlinear interaction of the pump beam. A beam guiding optic 5 is optically connected downstream of the crystal 2 and is designed to receive the idler beam coupled out of the crystal 2 and return it to the crystal 2 as auxiliary illumination. The auxiliary source 3 is thus formed by the combination of crystal 2 and beam guiding optic 5. The idler beam is coupled back into the crystal 2 as auxiliary illumination. The beam guiding optic 5 comprises a beam splitter 10, a mirror 11, and a retroreflector 12 downstream of the mirror 11 to ensure that the idler beam is returned as auxiliary illumination to the waveguide 6 of the crystal 2. The idler beam and the signal beam are separated from each other in the beam splitter 10. The idler beam is reflected in the beam splitter 10 towards a mirror 11, which is optically connected to the retroreflector 12. Finally, the idler beam is coupled into the waveguide via the retroreflector 12. Reference symbol list 1 Optical system 2 Crystal 3 Auxiliary source 4 Pump source 5 Beam guidance optics 6 Waveguide 7 Filter 8 Control unit 9 Cooling element 10 Beam splitter 11 Mirror 12 Retroreflector

Claims

Optical system (1) for carrying out nonlinear optical processes in at least one laser signal, comprising a crystal (2) and an auxiliary source (3), wherein the crystal (2) is a nonlinear optical crystal, such that when the at least one laser signal is coupled in the form of a pump beam, the pump beam interacts nonlinearly in the crystal (2), wherein the auxiliary source (3) is configured to emit an auxiliary illumination that breaks up energy levels of quasiparticles generated by coupling the pump beam into the crystal (2), and the auxiliary source (3) is optically connected to the crystal (2), such that the auxiliary illumination is coupled into a region of the crystal (2) together with the pump beam. Optical system (1) according to claim 1, wherein the auxiliary illumination is coupled into the crystal (2) collinearly with the pump jet. Optical system (1) according to claim 1 or 2, wherein the auxiliary source (3) is configured to emit an auxiliary illumination which illuminates at least one crystal surface corresponding to a cross-sectional area of ​​the pump jet on the crystal surface. Optical system (1) according to one of the preceding claims, wherein the auxiliary source (3) is configured to emit the auxiliary illumination with a wavelength corresponding to a band gap of the crystal (2). Optical system (1) according to one of the preceding claims, comprising a pump source (4), wherein the pump source (4) is configured to emit a pump beam that breaks up the energy levels of the quasiparticles, a beam guidance optic (5) optically connected downstream of the crystal (2), wherein the crystal (2) is configured to couple out at least one signal beam and one idler beam from the nonlinear interaction of the pump beam in the crystal (2), and the beam guidance optic (5) returns the idler beam to the crystal (2) in the form of the auxiliary illumination, so that the auxiliary source (3) is formed with the crystal (2) and the beam guidance optic (5). Optical system (1) according to one of the preceding claims, wherein the crystal (2) comprises a lithium niobate or a lithium tantalate. Optical system (1) according to the preceding claim, wherein the auxiliary source (3) is configured to emit the auxiliary illumination with a wavelength between 280 - 600 nm, preferably a wavelength between 450 - 600 nm. Optical system (1) according to one of the preceding claims, wherein the crystal (2) has a waveguide (6) and the auxiliary source (3) is optically connected to the waveguide (6) so that the auxiliary illumination is coupled into the waveguide (6) together with the pump jet. Optical system (1) according to one of the preceding claims, wherein the auxiliary illumination emitted by the auxiliary source (3) is monochromatic or broadband. Optical system (1) according to one of the preceding claims, wherein the auxiliary source (3) emits a pulsed or a continuous auxiliary illumination. Optical system (1) according to one of the preceding claims, wherein the auxiliary source (3) is configured to emit an auxiliary illumination that breaks up energy levels of polarons generated by coupling the pump jet into the crystal (2). Optical system (1) according to one of the preceding claims with a control unit (8), wherein the control unit (8) is connected to the auxiliary source (3) for signal transmission, and the control unit (8) is configured to actuate the auxiliary source (3) directly upon the impact of the pump jet on the crystal (2), so that the auxiliary illumination is coupled into the crystal (2) with the pump jet. Optical system (1) according to the preceding claim, wherein the pump jet is pulsed and the auxiliary source (3) is configured to emit the auxiliary illumination with a pulse frequency of the pump jet, and the control unit (8) is configured to actuate the auxiliary source (3) offset by the pulse frequency relative to the pump jet. Method for operating an optical system (1) with a nonlinear optical crystal (2), comprising the following process steps: coupling an auxiliary illumination into the crystal (2), wherein the auxiliary illumination is designed such that energy levels of quasiparticles generated by coupling a pump beam into the crystal (2) are broken up, and coupling the pump beam into a region of the crystal (2) illuminated by the auxiliary illumination. Method according to the preceding claim, wherein the pump jet and the auxiliary illumination are pulsed, the pump jet and the auxiliary illumination each have an identical pulse frequency, and the pump jet and the auxiliary illumination are coupled into the crystal (2) offset from each other by the pulse frequency.

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