A nonlinear frequency conversion device and conversion method

Abstract

The application relates to the technical field of nonlinear optics, in particular to a nonlinear frequency conversion device and a conversion method. The nonlinear frequency conversion device comprises an input mirror and an output mirror; a frequency doubling unit comprising a frequency doubling crystal and a spatial light modulator; fundamental light is transmitted through the input mirror into a frequency doubling resonant cavity, the fundamental light forms a conversion light beam through the frequency doubling unit, the conversion light beam resonates in the frequency doubling resonant cavity, the conversion light beam comprises unconverted fundamental light and frequency doubled light, and the frequency doubled light satisfying a preset wavelength condition is transmitted through the output mirror and output; the frequency doubling crystal is used for realizing conversion from the fundamental light to the frequency doubled light; and the spatial light modulator is used for dynamically adjusting the wavefront phase of the frequency doubled light output by the frequency doubling crystal, so as to suppress the beam separation displacement of the frequency doubled light caused by the walk-off effect, and then correct the elliptical distortion of the frequency doubled light spot. The application is at least beneficial to obtaining a higher-quality frequency doubled light beam.

Description

Technical Field

[0001] This invention belongs to the field of nonlinear optics technology, and particularly relates to a nonlinear frequency conversion device and conversion method. Background Technology

[0002] Laser frequency doubling technology is a relatively mature and commonly used frequency conversion technology that can generate lasers of new wavelengths, making it one of the important methods for expanding the laser wavelength range. Frequency doubling refers to doubling the frequency of incident laser light (fundamental frequency light) using a nonlinear crystal, thereby obtaining a laser output with half the wavelength, enabling the output of lasers of different wavelengths from infrared to ultraviolet. The specific principle is to utilize the second-order nonlinear polarization effect of the nonlinear crystal to superimpose the energies of two fundamental frequency photons, converting them into frequency-doubled photons.

[0003] During laser frequency doubling, the walk-off effect reduces the interaction length between the fundamental frequency light and the frequency-doubled light, which seriously affects the frequency doubling quality and conversion efficiency. Therefore, reducing the walk-off effect is crucial during laser frequency doubling. Common methods to compensate for the walk-off effect include adjusting the temperature of the crystal or the wavelength of the laser so that the phase matching condition is satisfied when the beam propagates along a specific principal axis of the crystal. However, this method requires high precision in temperature control and is not easy to achieve dynamic compensation. Summary of the Invention

[0004] In view of this, the present invention aims to provide a nonlinear frequency conversion device and conversion method, which is at least advantageous for obtaining higher quality frequency-doubled beams.

[0005] To achieve the above objectives, the technical solution created by this invention is implemented as follows: This invention provides a nonlinear frequency conversion device, comprising: an input mirror and an output mirror, which serve as two reflecting ends of a frequency-doubling resonant cavity; a frequency doubling unit located within the frequency-doubling resonant cavity, comprising a frequency-doubling crystal and a spatial light modulator arranged sequentially along the beam transmission path; wherein, fundamental frequency light is transmitted through the input mirror into the frequency-doubling resonant cavity, the fundamental frequency light passes through the frequency-doubling unit to form a converted beam, the converted beam resonates within the frequency-doubling resonant cavity, the converted beam includes unconverted fundamental frequency light and frequency-doubled light, and the frequency-doubled light satisfying a preset wavelength condition is transmitted through the output mirror and output; the frequency-doubling crystal is used to realize the conversion from fundamental frequency light to frequency-doubled light, and the spatial light modulator is used to dynamically adjust the wavefront phase of the frequency-doubled light output by the frequency-doubling crystal to suppress the beam separation displacement of the frequency-doubled light caused by the walk-off effect, thereby correcting the elliptical distortion of the frequency-doubled light spot.

[0006] Furthermore, the nonlinear frequency conversion device includes multiple frequency doubling units, which are arranged sequentially on the beam transmission path within the frequency doubling resonant cavity. The converted beam output from the previous frequency doubling unit is incident on the next frequency doubling unit and forms the next converted beam.

[0007] Furthermore, the spatial light modulator is a reflective liquid crystal spatial light modulator.

[0008] Furthermore, the reflective liquid crystal spatial light modulator includes a silicon substrate, a control electrode, a reflective layer, a liquid crystal layer, an alignment film, a transparent electrode, and a cover glass, which are stacked sequentially.

[0009] Furthermore, the nonlinear frequency conversion device also includes detection components corresponding to the frequency doubling units. The detection components include a beam splitter and a wavefront sensor. The beam splitter is located in the beam transmission optical path on the side of the corresponding frequency doubling unit away from the input mirror. The beam incident on the beam splitter is split into a first beam and a second beam. The first beam is a converted beam, and the second beam is incident on the wavefront sensor. The wavefront sensor detects the second beam in real time to form a detection signal. The spatial light modulator of the corresponding frequency doubling unit dynamically adjusts the wavefront phase of the frequency-doubled light output by the frequency doubling crystal based on the detection signal.

[0010] Furthermore, the nonlinear frequency conversion device also includes a host computer, which receives the detection signal output by the wavefront sensor, generates a reverse compensation phase function based on the detection signal, and sends it to the corresponding spatial light modulator. The corresponding spatial light modulator dynamically adjusts the wavefront phase of the frequency-doubled light output by the frequency-doubled crystal based on the reverse compensation phase function so that the frequency-doubled light spot meets the preset requirements.

[0011] Furthermore, the host computer generates the reverse compensation phase function based on the detection signal, including: determining the walk-off angle based on the detection signal. Radial deformation of the current light spot relative to the ideal light spot and the spatial distribution function of the light field Based on the walk-off angle Radial deformation of the current light spot relative to the ideal light spot and the spatial distribution function of the light field Obtain the reverse compensation phase function , ,in, This is the phase conversion coefficient.

[0012] Furthermore, the frequency doubling crystal is mounted on a semiconductor refrigeration chip, which is mounted on a water-cooled plate.

[0013] Furthermore, the frequency doubling crystal is an LBO crystal.

[0014] Another aspect of this invention provides a nonlinear frequency conversion method, comprising: providing the aforementioned nonlinear frequency conversion device, the nonlinear frequency conversion device comprising: an input mirror and an output mirror constituting two reflecting ends of a frequency doubling resonant cavity, and a frequency doubling unit including a frequency doubling crystal and a spatial light modulator; fundamental frequency light is transmitted through the input mirror into the frequency doubling resonant cavity, the fundamental frequency light passes through the frequency doubling unit to form a converted beam, the converted beam resonates within the frequency doubling resonant cavity, the converted beam includes unconverted fundamental frequency light and frequency doubling light, the frequency doubling light satisfying a preset wavelength condition is transmitted through the output mirror and output, the frequency doubling crystal realizes the conversion of fundamental frequency light to frequency doubling light, and the spatial light modulator dynamically adjusts the wavefront phase of the frequency doubling light output by the frequency doubling crystal to suppress the beam separation displacement of the frequency doubling light caused by the walk-off effect, thereby correcting the elliptical distortion of the frequency doubling light spot.

[0015] Compared with existing technologies, the present invention can achieve the following beneficial effects: Addressing the problem of beam separation caused by walk-off effect, leading to elliptical distortion of the frequency-doubled beam spot, reduced interaction length between the frequency-doubled and fundamental light, impact on frequency doubling quality, and reduced conversion efficiency, the present invention provides a nonlinear frequency conversion device and method. It employs a spatial light modulator to modulate the wavefront phase of the frequency-doubled light output from the frequency-doubled crystal, applying a reverse compensation phase to the frequency-doubled light. This dynamically corrects the elliptical distortion of the frequency-doubled beam spot caused by the walk-off effect, thereby restoring the spot of the converted beam to an ideal circular state. This extends the effective nonlinear interaction length, improves the second harmonic conversion efficiency, and yields a higher quality frequency-doubled beam. Attached Figure Description

[0016] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments and descriptions of the invention are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 A schematic diagram of the nonlinear frequency conversion device described in the embodiment of the present invention; Figure 2 This is a schematic diagram of the structure of the reflective liquid crystal spatial light modulator described in an embodiment of the present invention.

[0017] Explanation of reference numerals in the attached figures: 1. Input mirror; 2. Output mirror; 3. Frequency doubling crystal; 4. Spatial light modulator; 10. Silicon substrate; 11. Control electrode; 12. Reflective layer; 13. Liquid crystal layer; 14. Alignment film; 15. Transparent electrode; 16. Cover glass; 5. Beam splitter; 6. Wavefront sensor; 7. Host computer. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and do not constitute a limitation thereof.

[0019] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other.

[0020] In the description of this invention, it should be understood that the terms "center," "longitudinal," "lateral," "length," "width," "thickness," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are only for the convenience of describing this invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation on this invention. Furthermore, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, features defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0021] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art will understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0022] The invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0023] refer to Figure 1This invention provides a nonlinear frequency conversion device, comprising: an input mirror 1 and an output mirror 2, which serve as two reflecting ends of a frequency-doubling resonant cavity; a frequency doubling unit located within the frequency-doubling resonant cavity, comprising a frequency-doubling crystal 3 and a spatial light modulator 4 arranged sequentially along the beam transmission path. In some embodiments, the spatial light modulator 4 is located on the beam transmission path of the frequency-doubling crystal 3 away from the input mirror 1; wherein, fundamental frequency light is transmitted through the input mirror 1 into the frequency-doubling resonant cavity, and the fundamental frequency light passes through the frequency-doubling unit to form a converted beam, which resonates within the frequency-doubling resonant cavity, and the converted beam includes unconverted fundamental frequency light and frequency-doubled light, and the frequency-doubled light satisfying a preset wavelength condition is transmitted through the output mirror 2 and output; the frequency-doubling crystal 3 is used to realize the conversion from fundamental frequency light to frequency-doubled light, and the spatial light modulator 4 is used to dynamically adjust the wavefront phase of the frequency-doubled light output by the frequency-doubling crystal 3 to suppress the beam separation displacement of the frequency-doubled light caused by the walk-off effect, thereby correcting the elliptical distortion of the frequency-doubled light spot.

[0024] It should be noted that "fundamental frequency light" and "frequency doubled light" are both relative to the current-level nonlinear frequency conversion performed by the corresponding frequency doubled crystal. For any frequency doubled crystal, the light incident on the frequency doubled crystal is the fundamental frequency light, and the light generated after frequency conversion by the frequency doubled crystal is the frequency doubled light. It can be understood that some of the fundamental frequency light incident on the frequency doubled crystal may not undergo frequency conversion after passing through the frequency doubled crystal. Therefore, the converted beam output from the frequency doubled crystal includes both the unconverted fundamental frequency light and the frequency doubled light that has undergone frequency conversion.

[0025] In some cases, the end face of input mirror 1 facing the frequency doubling resonant cavity is a total reflection end face.

[0026] Furthermore, the nonlinear frequency conversion device includes multiple frequency doubling units, which are arranged sequentially along the beam transmission path within the frequency doubling resonant cavity. The converted beam output from the previous frequency doubling unit is incident on the next frequency doubling unit to form the next converted beam. The converted beam generated by the previous frequency doubling unit includes frequency-doubled light and unconverted fundamental frequency light. The frequency-doubled light from the previous frequency doubling unit serves as the fundamental frequency light for the next frequency doubling unit. The next frequency doubling unit does not perform frequency conversion on the unconverted fundamental frequency light in the converted beam; therefore, the unconverted fundamental frequency light in the converted beam continues to propagate along the optical path. The converted beam generated by the previous frequency doubling unit, after passing through the next frequency doubling unit, generates the next converted beam, which also includes frequency-doubled light and unconverted fundamental frequency light. If the frequency of the fundamental frequency light incident on the previous frequency doubling unit is ω1, then the frequency of the frequency-doubled light in the converted beam generated by the previous frequency doubling unit is ω2, where ω2 = 2 × ω1. The frequency of the unconverted fundamental frequency light in the converted beam generated by the previous frequency doubling unit is ω1. The converted beam generated by the previous frequency doubling unit includes... If there are light with frequency ω1 and light with frequency ω2, then although the light input to the next frequency doubling unit includes light with frequency ω1 and light with frequency ω2, the light with frequency ω2 is the fundamental frequency light input to the next frequency doubling unit. The converted beam output by the next frequency doubling unit includes light with frequency ω1, light with frequency ω2, and light with frequency ω3. Among them, the light with frequency ω1 is the unconverted fundamental frequency light with frequency ω1 output by the previous frequency doubling unit, the light with frequency ω2 is the unconverted fundamental frequency light output by the next frequency doubling unit, and the light with frequency ω3 is formed by frequency conversion of the frequency doubling light with frequency ω2 output by the previous frequency doubling unit. ω3 = 2 × ω2. Therefore, for the next frequency doubling unit, the frequency doubling light in its output converted beam refers to the light with frequency ω3 formed by frequency conversion.

[0027] The frequency-doubled light that meets the preset wavelength conditions can be the frequency-doubled light generated after passing through all stages of frequency doubling. For example, if a nonlinear frequency conversion device includes two frequency doubling units, then the frequency-doubled light that meets the preset wavelength conditions can be the fourth-harmonic light.

[0028] In some embodiments, the nonlinear frequency conversion device includes two frequency doubling units. It is understood that the specific number of frequency doubling units within the frequency doubling resonant cavity can be set according to frequency doubling efficiency, compensation accuracy, cavity loss, and optical path stability. The foregoing embodiments are merely examples and do not constitute a limitation on the number of frequency doubling units.

[0029] The working principle of a nonlinear frequency conversion device, using a nonlinear frequency conversion device comprising two frequency doubling units as an example, is explained below. When the nonlinear frequency conversion device is working, the fundamental frequency light enters the frequency doubling resonant cavity through the input mirror 1. It is first converted into frequency-doubled light by the frequency doubling crystal 3 of the first frequency doubling unit. However, the converted beam output by the frequency doubling crystal 3 contains not only the converted frequency-doubled light but also some unconverted fundamental frequency light. During the nonlinear frequency conversion process, due to the walk-off effect, the frequency-doubled light will undergo spatial separation, causing the output frequency-doubled light spot to deviate from the ideal circular distribution. The spatial light modulator 4 of the first frequency doubling unit will perform precise dynamic compensation on the frequency-doubled light in the converted beam. After spatial light modulation... The beam separation phenomenon of the frequency-doubled light in the converted beam after compensation by device 4 is weakened, disappears, or basically disappears. The compensated converted beam enters the second frequency-doubled unit and forms a converted beam. The converted beam then passes through the spatial light modulator of the second frequency-doubled unit to complete secondary compensation. Finally, high-quality frequency-doubled light that meets the preset conditions is output from the output mirror 2. The converted beam that does not meet the preset conditions is reflected back to the frequency-doubled resonant cavity through the output mirror 2 and passes through two frequency-doubled units again to achieve frequency doubling and compensation. This cycle repeats, so that the elliptical distortion of the frequency-doubled light caused by the walk-off effect is continuously corrected during the resonance process of the beam in the frequency-doubled resonant cavity, so as to ensure that the beam quality and frequency doubling efficiency of the output frequency-doubled light are stable and meet the standards.

[0030] Furthermore, the spatial light modulator 4 is a reflective liquid crystal spatial light modulator.

[0031] refer to Figure 2Furthermore, the reflective liquid crystal spatial light modulator includes a silicon substrate 10, a control electrode 11, a reflective layer 12, a liquid crystal layer 13, an alignment film 14, a transparent electrode 15, and a cover glass 16 stacked sequentially. The cover glass 16 provides physical protection and structural support; the transparent electrode 15 applies a common bias voltage, and the transparent electrode 15 and the control electrode 11 form a vertical electric field, generating a potential difference between the liquid crystal layer 13 and the control electrode 11; the alignment film 14 constrains the initial tilt angle and azimuth angle of the liquid crystal molecules, and, in conjunction with the electric field, deflects the liquid crystal molecules. When the applied voltage changes, the liquid crystal molecules tilt, thereby changing the refractive index; the liquid crystal layer 13 includes liquid crystal molecules, which are anisotropic, and the tilt angle of the liquid crystal molecules varies with the magnitude of the applied electric field. The equivalent refractive index changes, and after the incident light passes through the liquid crystal layer 13, the phase of the light is modulated. The entire liquid crystal layer 13 is divided into many independent pixels, and each pixel is individually controlled by the lower control electrode 11, realizing independent adjustment of the incident light phase pixel by pixel in space. After the incident light passes down through the liquid crystal layer 13 and reaches the reflective layer 12, it is reflected back. The light passes through the liquid crystal layer 13 again, and the phase modulation amount is doubled, which is beneficial to improving the modulation efficiency. This is also the key difference between the reflective liquid crystal spatial light modulator and the transmissive spatial light modulator. The control electrode 11 includes multiple electrode units, which control multiple independent pixels.

[0032] This invention introduces a reflective liquid crystal spatial light modulator into the laser frequency doubling process. By dynamically modulating the wavefront phase of the frequency-doubled light, it compensates in real time for the spot distortion and beam separation caused by the walk-off effect, extends the interaction length between the fundamental frequency light and the frequency-doubled light, thereby improving the conversion efficiency of the second harmonic and obtaining a higher quality frequency-doubled beam. This provides an active, advanced, efficient and stable frequency conversion scheme.

[0033] Furthermore, the nonlinear frequency conversion device also includes detection components corresponding to the frequency doubling units. The detection components include a beam splitter 5 and a wavefront sensor 6. The beam splitter 5 is located in the beam transmission optical path on the side of the corresponding frequency doubling unit away from the input mirror 1. The beam incident on the beam splitter 5 is split into a first beam and a second beam. The first beam is a converted beam, and the second beam is incident on the wavefront sensor 6. The wavefront sensor 6 detects the second beam in real time to form a detection signal. The spatial light modulator 4 of the corresponding frequency doubling unit dynamically adjusts the wavefront phase of the frequency doubling light output by the frequency doubling crystal 3 based on the detection signal.

[0034] Among them, 1% of the beam of the incident beam splitter 5 is the second beam.

[0035] Furthermore, the nonlinear frequency conversion device also includes a host computer 7, which is used to receive the detection signal output by the wavefront sensor 6, generate a reverse compensation phase function based on the detection signal and send it to the corresponding spatial light modulator 4. The corresponding spatial light modulator 4 dynamically adjusts the wavefront phase of the frequency-doubled light output by the frequency-doubled crystal 3 based on the reverse compensation phase function so that the frequency-doubled light spot meets the preset requirements.

[0036] The frequency-doubled light spot must meet the preset requirements, meaning the roundness of the frequency-doubled light spot must be no less than 0.95.

[0037] Taking a nonlinear frequency conversion device with two frequency doubling units as an example, the process of light being transmitted from the input mirror 1 to the output mirror 2 will pass through two spatial light modulators 4 in the frequency doubling resonant cavity. Each time it passes through the spatial light modulator 4, the host computer 7 updates the reverse compensation phase function according to the detection signal obtained by the corresponding wavefront sensor 6 in real time, so as to ensure that the device can continuously and dynamically suppress the walk-off effect.

[0038] Understandably, if the circularity of the frequency-doubled light spot after compensation by the spatial light modulator 4 meets the preset requirements, the compensation is complete. If the circularity of the frequency-doubled light spot after compensation does not meet the preset requirements, that is, the light spot still has elliptical distortion or beam separation, the host computer 7 will recalculate the reverse compensation phase function based on the detection information obtained by the wavefront sensor 6 and reload it to the spatial light modulator 4 for further compensation. Through such feedback iteration, the compensation effect can be continuously optimized to ensure effective compensation of the elliptical distortion and beam separation of the light spot caused by the walk-off effect.

[0039] Furthermore, the host computer 7 generates a reverse compensation phase function based on the detection signal, including: determining the walk-off angle based on the detection signal. Radial deformation of the current light spot relative to the ideal light spot and the spatial distribution function of the light field Based on the walk-off angle Radial deformation of the current light spot relative to the ideal light spot and the spatial distribution function of the light field Obtain the reverse compensation phase function , ,in, This is the phase conversion coefficient.

[0040] Because the walk-off effect causes the frequency-doubled light spot to be distorted from an ideal circular distribution to an elliptical distribution, and the overlap between the fundamental light and the frequency-doubled light in the lateral space decreases, the frequency doubling conversion efficiency is reduced and the output beam quality is deteriorated. To solve this problem, this invention uses a wavefront sensor 6 to detect the beam emitted from the frequency-doubled crystal 3 in real time, and obtains the radial deformation, walk-off angle and optical field spatial distribution function. The host computer 7 calculates the reverse compensation phase function based on the radial deformation, walk-off angle and optical field spatial distribution function. The reverse compensation phase function is the reverse compensation phase loaded onto the spatial light modulator 4. The phase conversion coefficient is used to convert the walk-off angle and radial deformation into the phase modulation amount that the spatial light modulator 4 can load. This reverse compensation phase is opposite to the direction of the beam offset and wavefront distortion caused by the walk-off effect. Based on the reverse compensation phase function, the distorted elliptical spot of the frequency-doubled light can be subjected to reverse phase modulation, so that its lateral energy distribution is gradually restored to a circular or approximately circular distribution.

[0041] Understandably, in actual operation, the wavefront sensor 6 continuously detects the beam output by the frequency doubling crystal 3 to form a detection signal that includes information on the beam shape and wavefront change. The host computer 7 continuously updates the reverse compensation phase function based on the detection signal obtained in real time and loads it onto the corresponding spatial light modulator 4 to achieve compensation.

[0042] Furthermore, the frequency doubling crystal 3 is mounted on a thermoelectric cooler, which in turn is mounted on a water-cooled plate. The thermoelectric cooler is used for active and precise temperature control of the frequency doubling crystal 3, while the water-cooled plate is responsible for dissipating the heat generated by the thermoelectric cooler during operation. By employing thermoelectric cooler temperature control and water cooling, the temperature of the frequency doubling crystal 3 is kept at its optimal operating temperature, ensuring stable crystal operation and thus outputting high-quality frequency-doubled light.

[0043] Furthermore, the frequency doubling crystal 3 is an LBO crystal. The optimal operating temperature for an LBO crystal is 27.28℃.

[0044] In another aspect, the present invention provides a nonlinear frequency conversion method. The nonlinear frequency conversion method realizes nonlinear frequency conversion based on the aforementioned nonlinear frequency conversion device. It should be noted that other identical or corresponding parts can be referred to the aforementioned embodiments, and will not be repeated here.

[0045] The nonlinear frequency conversion method includes: providing the aforementioned nonlinear frequency conversion device, which includes: an input mirror 1 and an output mirror 2 constituting two reflecting ends of a frequency doubling resonant cavity, and a frequency doubling unit including a frequency doubling crystal 3 and a spatial light modulator 4; the fundamental frequency light is transmitted through the input mirror 1 into the frequency doubling resonant cavity, and the fundamental frequency light passes through the frequency doubling unit to form a converted beam, which resonates in the frequency doubling resonant cavity, and the converted beam includes unconverted fundamental frequency light and frequency doubling light, and the frequency doubling light that meets the preset wavelength condition is transmitted through the output mirror 2 and output, the frequency doubling crystal 3 realizes the conversion of fundamental frequency light to frequency doubling light, and the spatial light modulator 4 dynamically adjusts the wavefront phase of the frequency doubling light output by the frequency doubling crystal 3 to suppress the separation displacement of the frequency doubling light beam caused by the walk-off effect, thereby correcting the elliptical distortion of the frequency doubling light spot.

[0046] This invention ensures that the light beam receives timely compensation and correction each time it passes through the frequency doubling unit during the circulation process within the frequency doubling resonant cavity, thereby continuously maintaining the beam quality and guaranteeing a stable output of high-quality frequency-doubled light.

[0047] It should be further explained that the walk-off effect refers to the phenomenon during laser frequency doubling where, due to the different polarization states of the fundamental frequency light and the generated frequency-doubled light propagating in the anisotropic frequency doubling crystal 3, the energy flow direction and wave vector direction are inconsistent, resulting in a fixed walk-off angle. This, in turn, leads to spatial separation and wavefront distortion of the frequency-doubled light. It is a key parameter for measuring the strength of the walk-off effect, and can directly reflect the degree of deviation between the propagation directions of the fundamental frequency light and the frequency harmonic light. The walk-off angle formula is as follows: walk-off angle of slow light (with a higher refractive index and slower propagation speed) as follows: ; The walk-off angle of fast light (with a lower refractive index and a faster propagation speed) as follows: ; in, This represents the frequency of the i-th optical wave participating in the nonlinear frequency transformation. Let X be the X-axis component of the unit vector of the electric field intensity direction of slow light in the principal axis coordinate system. Let Y be the Y-axis component of the unit vector of the electric field intensity direction of slow light in the principal axis coordinate system. Let Z be the Z-axis component of the unit vector of the electric field intensity direction of slow light in the principal axis coordinate system. Let X be the X-axis component of the unit vector of the electric displacement direction of slow light in the principal axis coordinate system. Let Y be the Y-axis component of the unit vector of the electric displacement direction of slow light in the principal axis coordinate system. Let Z be the Z-axis component of the unit vector of the electric displacement direction of slow light in the principal axis coordinate system. Let X be the X-axis component of the unit vector of electric field intensity direction of fast light in the principal axis coordinate system. Let Y be the Y-axis component of the unit vector of electric field intensity direction of fast light in the principal axis coordinate system. Let Z be the Z-axis component of the unit vector of electric field intensity direction of fast light in the principal axis coordinate system. Let X be the X-axis component of the unit vector of electric displacement direction of fast light in the principal axis coordinate system. Let Y be the Y-axis component of the unit vector of electric displacement direction of fast light in the principal axis coordinate system. Let Z be the Z-axis component of the unit vector of electric displacement direction of fast light in the principal axis coordinate system.

[0048] ,in, The unit vector representing the electric displacement direction of slow light; ,in, The unit vector representing the electric displacement direction of fast light; It is the angle between the unit vector of electric displacement and the plane containing the wave vector k and the crystal z-axis. Represents the polar angle (the angle between the wave vector k and the crystal z-axis). This represents the azimuth angle of the projection of the wave vector k onto the XY plane relative to the X-axis. yes , and The function, The angle between the optical axis and the Z-axis of a biaxial crystal is the angle between the optical axis and the Z-axis. The optical axis of a biaxial crystal is the direction of propagation in the crystal where birefringence splitting does not occur. and The following relationship must be satisfied: ; ; , Let P be the unit vector of the electric field intensity direction of the slow light, and let P be the normalization factor of the electric field direction vector of the slow light. ; ; Let be the unit vector representing the direction of the electric field intensity of fast light. Normalization factor representing the direction vector of the fast electric field; ; in, Let be the principal refractive index of the crystal along the optical principal axis X-axis. Let be the principal refractive index of the crystal along the optical principal axis Y-axis. Let be the principal refractive index of the crystal along the optical principal axis Z-axis. .

[0049] It is precisely because of the departure angle The presence of this will cause the frequency-doubled light to undergo spatial separation. The walk-off displacement d of the frequency-doubled light (the difference between the major axis and minor axis of the elliptical light spot) can be expressed as: ; The walk-off displacement d directly determines the compensation amplitude of the spatial light modulator 4. When d increases, the spatial light modulator 4 needs to generate a larger gradient of reverse compensation phase to counteract the beam separation of the frequency-doubled light and ensure stable frequency doubling efficiency. .

[0050] In summary, the nonlinear frequency conversion device and method provided by this invention, by allowing the light beam to oscillate in the frequency doubling resonant cavity and then oscillate back and forth through the frequency doubling crystal 3 and the spatial light modulator 4, continuously optimizes the beam quality of the frequency-doubled light and improves the frequency doubling efficiency by leveraging the superposition effect of multiple frequency doublings and dynamic compensation, thus ensuring high stability and high quality laser output. This solves the current problems of poor beam quality and low frequency doubling efficiency caused by the walk-off effect in frequency-doubled light.

[0051] It should be understood that the various forms of processes shown above can be used to reorder, add, or delete steps. For example, the steps described in this invention disclosure can be executed in parallel, sequentially, or in different orders, as long as the desired result of the technical solution disclosed in this invention can be achieved, and this is not limited herein.

[0052] The specific embodiments described above do not constitute a limitation on the scope of protection of this invention. Those skilled in the art should understand that various modifications, combinations, sub-combinations, and substitutions can be made according to design requirements and other factors. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this invention should be included within the scope of protection of this invention.

Claims

1. A nonlinear frequency conversion device, characterized in that, include: An input mirror and an output mirror, which serve as two reflecting ends of a frequency doubling resonant cavity, respectively; A frequency doubling unit, located within the frequency doubling resonant cavity, and comprising a frequency doubling crystal and a spatial light modulator arranged sequentially along the beam transmission path; The fundamental frequency light is transmitted through the input mirror into the frequency doubling resonant cavity. The fundamental frequency light passes through the frequency doubling unit to form a converted beam. The converted beam resonates in the frequency doubling resonant cavity. The converted beam includes unconverted fundamental frequency light and frequency doubling light. The frequency doubling light that meets the preset wavelength condition is transmitted through the output mirror and output. The frequency doubling crystal is used to convert the fundamental frequency light into frequency doubling light, and the spatial light modulator is used to dynamically adjust the wavefront phase of the frequency doubling light output by the frequency doubling crystal in order to suppress the beam separation displacement caused by the walk-off effect, thereby correcting the elliptical distortion of the frequency doubling light spot.

2. The nonlinear frequency conversion device according to claim 1, characterized in that, The nonlinear frequency conversion device includes multiple frequency doubling units, which are arranged sequentially on the beam transmission path within the frequency doubling resonant cavity. The converted beam output from the previous frequency doubling unit is incident on the next frequency doubling unit and forms the next converted beam.

3. The nonlinear frequency conversion device according to claim 1, characterized in that, The spatial light modulator is a reflective liquid crystal spatial light modulator.

4. The nonlinear frequency conversion device according to claim 3, characterized in that, The reflective liquid crystal spatial light modulator comprises a silicon substrate, a control electrode, a reflective layer, a liquid crystal layer, an alignment film, a transparent electrode, and a cover glass, which are stacked sequentially.

5. The nonlinear frequency conversion device according to claim 1, characterized in that, The nonlinear frequency conversion device further includes detection components corresponding to the frequency doubling units. The detection components include a beam splitter and a wavefront sensor. The beam splitter is located in the beam transmission optical path on the side of the corresponding frequency doubling unit away from the input mirror. The beam incident on the beam splitter is split into a first beam and a second beam. The first beam is a converted beam. The second beam is incident on the wavefront sensor. The wavefront sensor detects the second beam in real time to form a detection signal. The spatial light modulator of the corresponding frequency doubling unit dynamically adjusts the wavefront phase of the frequency-doubled light output by the frequency doubling crystal based on the detection signal.

6. The nonlinear frequency conversion device according to claim 5, characterized in that, The nonlinear frequency conversion device further includes a host computer, which is used to receive the detection signal output by the wavefront sensor, generate a reverse compensation phase function based on the detection signal, and send it to the corresponding spatial light modulator. The corresponding spatial light modulator dynamically adjusts the wavefront phase of the frequency-doubled light output by the frequency-doubled crystal based on the reverse compensation phase function so that the frequency-doubled light spot meets the preset requirements.

7. The nonlinear frequency conversion device according to claim 6, characterized in that, The host computer generates a reverse compensation phase function based on the detection signal, including: The walk-off angle is determined based on the detection signal. Radial deformation of the current light spot relative to the ideal light spot and the spatial distribution function of the light field ; Based on the walk-off angle Radial deformation of the current light spot relative to the ideal light spot and the spatial distribution function of the light field Obtain the reverse compensation phase function , ,in, This is the phase conversion coefficient.

8. The nonlinear frequency conversion device according to claim 1, characterized in that, The frequency doubling crystal is disposed on a semiconductor refrigeration chip, which is disposed on a water-cooled plate.

9. The nonlinear frequency conversion device according to claim 1, characterized in that, The frequency doubling crystal is an LBO crystal.

10. A nonlinear frequency conversion method, characterized in that, include: A nonlinear frequency conversion device according to any one of claims 1 to 9 is provided, the nonlinear frequency conversion device comprising: an input mirror and an output mirror constituting two reflecting ends of a frequency doubling resonant cavity, and a frequency doubling unit including a frequency doubling crystal and a spatial light modulator; The fundamental frequency light is transmitted through the input mirror into the frequency doubling resonant cavity. The fundamental frequency light passes through the frequency doubling unit to form a converted beam. The converted beam resonates in the frequency doubling resonant cavity. The converted beam includes unconverted fundamental frequency light and frequency doubling light. The frequency doubling light that meets the preset wavelength condition is transmitted through the output mirror and output. The frequency doubling crystal realizes the conversion of fundamental frequency light to frequency doubling light. The spatial light modulator dynamically adjusts the wavefront phase of the frequency doubling light output by the frequency doubling crystal to suppress the separation displacement of the frequency doubling light beam caused by the walk-off effect, thereby correcting the elliptical distortion of the frequency doubling light spot.

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