A cavity length insensitive nonlinear frequency converter

Abstract

The application discloses a cavity length insensitive nonlinear frequency converter, comprising: a mode matching lens group composed of a first thin lens and a second thin lens, which ensures the imaging beam waist position and size to be consistent with the intrinsic spatial mode of a six-mirror ring cavity; output laser of a single frequency light source, which forms a PDH cavity locking system with an EOM and a servo locking system, so as to realize amplification of the fundamental frequency light in the six-mirror ring cavity; when the intensity of the fundamental frequency light in the nonlinear optical crystal reaches the nonlinear frequency conversion laser threshold, nonlinear frequency conversion laser is generated; different types of nonlinear frequency conversion laser output are realized according to different coatings of the six-mirror ring cavity and the nonlinear optical crystal. By adjusting the length of the delay line corresponding to the two parallel light paths of the six-mirror ring cavity, the optimal resonant cavity mode matching effect and the adjustment of the free spectral range are realized; on the basis of realizing the resonance of the fundamental frequency light, the six-mirror ring cavity can realize external cavity resonance nonlinear frequency conversion and multi-wavelength resonance.

Description

Technical Field

[0001] This invention relates to the field of lasers, and more particularly to a cavity length-insensitive nonlinear frequency converter. Background Technology

[0002] Nonlinear optical frequency conversion technology can realize laser output from the ultraviolet to the mid-infrared band, which has significant practical applications. Pulsed lasers, due to their high peak power (typically exceeding megawatts), can achieve high energy conversion efficiency when interacting with nonlinear crystals. Continuous-wave lasers, however, cannot achieve high conversion efficiency through single-pumping because their power density is much lower than that of pulsed lasers. To achieve high-efficiency continuous-wave single-frequency nonlinear frequency conversion, external resonant cavity enhancement technology is typically employed. When the frequency of the fundamental light matches the eigenfrequency of the resonant cavity, the power of the fundamental light is amplified by tens to hundreds of times within the resonant cavity.

[0003] Under the premise of maintaining frequency matching, the main factor limiting the conversion efficiency of nonlinear frequency lasers is the degree of matching between the spatial mode of the fundamental frequency light and the intrinsic spatial mode of the resonant cavity. Since the quality factor of the laser beam output from stimulated emission in actual lasers is greater than 1, the mode matching position and beam waist size calculated using Gaussian beam propagation theory deviate from the actual situation. Therefore, mode matching cannot be further optimized under the condition of fixed resonant cavity parameters and frequency matching.

[0004] Under the condition of enhanced fundamental frequency resonance, the methods for generating high-efficiency laser radiation vary slightly depending on the different nonlinear effects produced by the fundamental frequency light and the nonlinear medium. For nonlinear frequency conversion (frequency harmonics, sum-frequency conversion) with large frequency shifts, while ensuring the resonant cavity operates at a high Q value for the fundamental frequency light, the characteristics of different transmittances of the coating for the fundamental frequency light and the nonlinear frequency-converted laser are utilized to allow it to be output from the resonant cavity, achieving the maximum laser conversion efficiency. The first-order Stokes light output from stimulated Raman scattering with a large Raman frequency shift can also be separated from the fundamental frequency light through different optical coatings on the resonant cavity mirrors. However, for lasers with frequency shifts much smaller than the optical frequency, such as stimulated Brillouin scattering, high-efficiency output can only be obtained when the fundamental frequency light and the Brillouin laser resonate simultaneously. Furthermore, cascaded stimulated Brillouin scattering and stimulated Raman scattering laser outputs require the resonant cavity to resonate simultaneously with multiple orders of Stokes light to generate higher-order laser outputs. Meeting the requirement of simultaneous resonance of multiple wavelengths necessitates manual tuning of the resonant cavity.

[0005] In traditional four-mirror ring cavity structures, adjusting the cavity length alters the beam reflection angle, leading to increased beam loss, lower resonance enhancement, and a higher threshold for nonlinear effects. To compensate for the spatial dispersion of the fundamental frequency light caused by the cavity length change, both mirrors constituting the resonant cavity must be adjusted simultaneously, altering the position of the fundamental frequency light entering the cavity-locked loop. Therefore, traditional four-mirror ring cavities cannot achieve multi-wavelength resonance tuning under frequency matching conditions. These issues limit the application of spatial optical nonlinear frequency-converting lasers primarily to fundamental frequency resonance. Currently, no spatial optical external cavity structure can simultaneously achieve multi-wavelength resonance by adjusting the cavity length under fundamental frequency resonance conditions. Summary of the Invention

[0006] This invention provides a cavity length-insensitive nonlinear frequency converter. It utilizes a six-mirror ring cavity to achieve adjustment of the resonant cavity mode and free spectral range. When applied to an external cavity resonant nonlinear frequency conversion laser, the angle of the reflected light from the fundamental frequency light does not shift. Therefore, the free spectral range of the resonant cavity can be adjusted under frequency-locked conditions to achieve simultaneous resonance enhancement of multiple wavelengths. Its two parallel optical paths overcome the technical drawback of reflected light shift during the adjustment process of traditional four-mirror ring cavities. Under the condition that the pump light mode-matching lens is determined, the resonant cavity mode can be adjusted by adjusting the length of the resonant cavity delay line to obtain the optimal mode-matching effect, as detailed below:

[0007] A cavity length-insensitive nonlinear frequency converter, the nonlinear frequency converter comprising: a single-frequency light source, a first thin lens, a second thin lens, an EOM, an optical isolator, a half-wave plate, a first 45-degree reflector, a second 45-degree reflector, a first 60-degree reflector, a concave reflector, a nonlinear optical crystal, a PZT-driven concave reflector, a second 60-degree reflector, a movable platform, and a servo locking system;

[0008] The first 45-degree reflector, the second 45-degree reflector, the first 60-degree reflector, the concave reflector, the nonlinear optical crystal, the PZT-driven concave reflector, and the second 60-degree reflector constitute a six-mirror annular cavity.

[0009] The first 60-degree reflector, the concave reflector, the nonlinear optical crystal, the PZT-driven concave reflector, and the second 60-degree reflector are mounted on a movable platform to form a delay line structure. The part mounted on the movable platform can be translated relative to the first 45-degree reflector and the second 45-degree reflector.

[0010] The first and second thin lenses constitute a pattern-matching lens group; the EOM and the servo locking system constitute a PDH locking cavity system.

[0011] Once the fundamental frequency light is frequency-locked under the control of the servo-locking system, it is amplified in the resonant cavity. When the intensity of the fundamental frequency light inside the nonlinear optical crystal reaches the nonlinear frequency conversion laser threshold, nonlinear frequency conversion laser is generated. Depending on the cavity mirror coating of the six-mirror ring cavity and the different nonlinear optical crystals, different types of nonlinear frequency conversion laser outputs are achieved.

[0012] Specifically, by adjusting the length of the delay lines L5+L6 of the six-mirror annular cavity, the free spectral range and beam waist size of the resonant cavity can be adjusted. The laser beam with a beam waist of w0 emitted by the single-frequency light source, after passing through the mode-matching lens group, an optical isolator, and a half-wave plate, images a beam waist position and size that are consistent with the intrinsic spatial mode w1 of the six-mirror annular cavity.

[0013] The EOM modulates the single-frequency light source to generate side frequencies on both sides of its frequency ω0. At the position of the first 45-degree reflector, part of the light enters the six-mirror annular cavity, and part of the light is reflected into the servo locking system. The servo locking system demodulates the reflected signal and then controls the PZT to drive the concave reflector to achieve resonance of the six-mirror annular cavity structure with the fundamental frequency light. During multi-wavelength resonance, the cavity length is tuned to make the cavity length meet the multi-wavelength resonance condition.

[0014] The nonlinear frequency converter can be used to realize different types of nonlinear optical frequency conversion laser output, and can be divided into two cases according to the different nonlinear frequency shift amounts:

[0015] Fundamental optical frequency ω0 and nonlinear frequency conversion laser frequency ω s When the difference is much smaller than the fundamental frequency, the components constituting the six-mirror annular cavity are coated with a high-reflectivity film for both the fundamental frequency and nonlinear frequency-converted laser light, while the cavity length is tuned to ensure multi-wavelength resonance; when the fundamental frequency ω0 and the nonlinear frequency-converted laser frequency ω s When the difference is comparable to the fundamental frequency, the first 45-degree mirror, the second 45-degree mirror, the first 60-degree mirror, the nonlinear optical crystal, the PZT-driven concave mirror, and the second 60-degree mirror constituting the six-mirror ring cavity element are coated with a high reflectivity film for fundamental frequency light and nonlinear frequency conversion laser, and the concave mirror is coated with a high reflectivity film for fundamental frequency light and a nonlinear frequency conversion laser antireflection film, in order to achieve high-efficiency output of nonlinear frequency conversion laser.

[0016] The beneficial effects of the technical solution provided by this invention are:

[0017] 1. Based on a six-mirror ring cavity, this nonlinear frequency converter utilizes the unique delay line structure of the six-mirror ring cavity to achieve adjustment of spatial mode and free spectral range while maintaining resonance of the resonant cavity;

[0018] 2. Due to the unique structure of the six-mirror ring cavity, the position of the spatial light entering the servo-locking system will not change when the resonant cavity mode is adjusted, which can ensure that the resonant cavity optimizes the conversion efficiency of the nonlinear frequency conversion laser under the condition of fundamental frequency light resonance.

[0019] 3. Using the structure proposed in this invention, the resonant cavity can achieve multi-wavelength resonance based on the fundamental frequency optical resonance, and high-order lasers can be generated by nonlinear frequency conversion. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a cavity length-insensitive nonlinear frequency converter.

[0021] Figure 2 This is a schematic diagram of the six-mirror annular cavity structure;

[0022] Figure 3 This is a schematic diagram of multi-wavelength resonance.

[0023] Figure 4 A schematic diagram illustrating the mode matching between the injected laser and the resonant cavity;

[0024] Figure 5 This is a schematic diagram showing the variation of the waist size of the meridional plane and sagittal plane with the sum of distances L5 and L6.

[0025] The attached diagram lists the components represented by each number as follows:

[0026] 1: Single-frequency light source; 2: First thin lens;

[0027] 3: Second thin lens; 4: EOM;

[0028] 5: Optical isolator; 6: Half-wave plate;

[0029] 7: First 45-degree reflector; 8: Second 45-degree reflector;

[0030] 9: Movable platform; 10: First 60-degree reflector;

[0031] 11: PZT-driven concave mirror; 12: Nonlinear optical crystal;

[0032] 13: Concave reflector; 14: Second 60-degree reflector;

[0033] 15: Servo locking system; 16: Equivalent resonant cavity. Detailed Implementation

[0034] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below.

[0035] Example 1

[0036] A cavity length-insensitive nonlinear frequency converter, such as Figure 1 As shown, the nonlinear frequency converter includes: a single-frequency light source 1, a first thin lens 2, a second thin lens 3, an EOM 4, an optical isolator 5, a half-wave plate 6, a first 45-degree reflector 7, a second 45-degree reflector 8, a movable platform 9, a first 60-degree reflector 10, a PZT-driven concave reflector 11, a nonlinear optical crystal 12, a concave reflector 13, a second 60-degree reflector 14, and a servo locking system 15.

[0037] The first 45-degree reflector 7, the second 45-degree reflector 8, the first 60-degree reflector 10, the concave reflector 13, the nonlinear optical crystal 12, the PZT-driven concave reflector 11, and the second 60-degree reflector 14 constitute a six-mirror annular cavity. The laser beam direction between the first 45-degree reflector 7 and the second 60-degree reflector 14 is parallel to the laser beam direction between the second 45-degree reflector 8 and the first 60-degree reflector 10. Simultaneously, the first 60-degree reflector 10, the concave reflector 13, the nonlinear optical crystal 12, the PZT-driven concave reflector 11, and the second 60-degree reflector 14 are mounted on a movable platform 9, forming a delay line structure.

[0038] Figure 2 This is a schematic diagram of a six-mirror annular cavity structure. L1 is the distance between the PZT-driven concave mirror 11 and the concave mirror 13; L2 is the distance between the PZT-driven concave mirror 11 and the second 60-degree mirror 14; L3 is the distance between the first 60-degree mirror 10 and the concave mirror 13; L4 is the distance between the first 45-degree mirror 7 and the second 45-degree mirror 8; L5 is the distance between the first 45-degree mirror 7 and the second 60-degree mirror 14; and L6 is the distance between the second 45-degree mirror 8 and the first 60-degree mirror 10. The nonlinear optical crystal 12 has a length of l and a refractive index of n. Therefore, the actual optical length between the PZT-driven concave mirror 11 and the concave mirror 13 is L1 + (n-1)l. The total optical length of the resonant cavity is L = L1 + (n-1)l + L2 + L3 + L4 + L5 + L6.

[0039] The free spectral range (FSR) of the six-mirror ring cavity is:

[0040]

[0041] Where c is the speed of light, the part installed on the movable platform 9 can be translated relative to the first 45-degree reflector 7 and the second 45-degree reflector 8, and the distance between L5 and L6 can be changed synchronously, thereby realizing the change of the free spectral range.

[0042] EOM4 and servo locking system 15 constitute a PDH cavity-locking system. The single-frequency laser output from the single-frequency light source 1 undergoes electro-optic phase modulation through EOM4 to obtain a laser signal with a sideband period equal to the modulation frequency. Using the single-frequency laser frequency as a reference source and combined with PZT cavity scanning technology, dispersive spectral lines and error signals are obtained. The cavity length of the six-mirror annular cavity is adjusted by the servo control system 15 to lock the resonant frequency of the resonant cavity to the single-frequency laser frequency. At this time, the optical path of the fundamental frequency light circulating once in the cavity is exactly stable at an integer multiple of its wavelength, ensuring that the fundamental frequency light is always in a resonant enhancement state in the cavity, and the power of the fundamental frequency light in the cavity can be superimposed by tens or hundreds of times.

[0043] In practice, after the fundamental frequency light is frequency locked under the control of the servo locking system 15, the fundamental frequency light is amplified in the resonant cavity. When the intensity of the fundamental frequency light inside the nonlinear optical crystal reaches the nonlinear frequency conversion laser threshold, nonlinear frequency conversion laser is generated.

[0044] Furthermore, depending on the different coatings of the six-mirror ring cavity mirrors and the nonlinear optical crystals, different types of nonlinear frequency-converting laser outputs can be achieved. For example, the first-order Stokes light output of stimulated Raman scattering with a large Raman frequency shift can also be separated from the fundamental frequency light through different optical coatings of the resonant cavity mirrors. However, lasers with frequency shifts much smaller than the optical frequency, such as stimulated Brillouin scattering, can only achieve high-efficiency output under the condition that the fundamental frequency light and the Brillouin laser resonate simultaneously. Furthermore, the cascaded stimulated Brillouin scattering and stimulated Raman scattering laser outputs require the resonant cavity to resonate simultaneously with multiple-order Stokes light to generate higher-order laser outputs. To satisfy the simultaneous resonance of multiple wavelengths, the resonant cavity needs to be manually tuned.

[0045] Figure 3 This is a schematic diagram of multi-wavelength resonance. Assume that multiple wavelengths of light, λ, exist within the six-mirror ring cavity. i (i = 1, 2, 3...), a cavity will form like... Figure 3 The standing wave mode shown only allows constructive interference to occur within the cavity when laser light reaches a specific wavelength. The condition for enhanced resonance in the resonant cavity is that the length of one cycle must be an integer multiple of that wavelength, i.e.:

[0046] L = m i λ i =L1+(n-1)l+L2+L3+L4+L5+L6

[0047] Where m is a positive integer, the cavity length L is changed by manually adjusting the distance between L5 and L6, so that the cavity length L is an integer multiple of the least common multiple of different wavelengths, thereby realizing multi-wavelength resonance.

[0048] Figure 4This is a schematic diagram of the mode matching between the injected laser and the resonant cavity. Here, d represents the relative distance between the first thin lens 2 and the first thin lens 3. The equivalent resonant cavity 16 is a six-mirror ring cavity structure, equivalent to a straight cavity structure in which the fundamental frequency light circulates once. The fundamental frequency light enters from the first 45-degree reflector 7 and circulates once along the direction of first 45-degree reflector 7 → second 45-degree reflector 8 → first 60-degree reflector 10 → concave reflector 13 → PZT-driven concave reflector 11 → second 60-degree reflector 14 → first 45-degree reflector 7. The distances corresponding to the various mirrors in the equivalent resonant cavity 16 are... Figure 2 The six annular cavities in the image are spaced at the same distance. L2 is the distance from the waist of the single-frequency light source 1 to the first thin lens 2, and L7 is the distance from the second thin lens 3 to the first 45-degree plane mirror 7.

[0049] Furthermore, the six-mirror annular cavity adopts a symmetrical structure, with two waist positions for the intrinsic space mode, located at the center of L1 and the center of L4, respectively. When light is incident on the meridional and sagittal planes of the PZT-driven concave mirrors 11 and 13, reflections occur at different angles. The focal length f of the light on the meridional and sagittal planes of the concave mirrors... T and f S They are represented as follows: and (R is the radius of curvature of the concave mirror, and 15° is the angle between the incident beam axis and the concave mirror axis). Using the ABCD matrix, the square of the beam waist at the center of L1 can be obtained. for:

[0050]

[0051] Where λ is the resonant wavelength, f i The focal length f of the meridional plane T Or the focal length f of the sagittal plane S L0 = L2 + L3 + L4 + L5 + L6. The stability condition of the resonant cavity is that the square of the optical waist is non-negative.

[0052] The focal length of the first thin lens 2 is f1, and the focal length of the second thin lens 3 is f2. The equivalent focal length f of the two is:

[0053]

[0054] The beam waist of the laser emitted by the single-frequency light source 1 is w0, and the distance from the beam waist to the first thin lens 2 is d. in The image is formed by a coupling lens group consisting of a first thin lens 2 and a second thin lens 3, and the distance between the image position and the second thin lens 3 is d. out The beam waist of the image is w′0, and satisfies:

[0055]

[0056]

[0057] To ensure a good match between the fundamental frequency light mode and the resonant cavity, the beam waist of the fundamental frequency light needs to be focused at the L1 center position of the resonant cavity, i.e., d. out =L8+L3+L4+L6+L1 / 2, by adjusting the relative distance d, the equivalent focal length f is changed, thereby determining the imaging position and the size of the imaging beam waist w′0, achieving a good match between the fundamental frequency light mode and the beam waist w1 of the intrinsic space mode of the six-mirror ring cavity. However, since the laser beam quality factor of the stimulated emission output of an actual laser is greater than 1, the included angles of the mirrors in the ring cavity will also introduce certain aberrations. Therefore, the mode matching position and beam waist size calculated using the Gaussian beam propagation theory deviate from the actual situation. At this time, further optimization of mode matching can be achieved by adjusting the movable platform 9, changing the delay line length of the resonant cavity, and thus adjusting the cavity length of the six-mirror ring cavity, thereby obtaining the optimal mode matching effect.

[0058] In summary, this invention proposes a cavity length-insensitive nonlinear frequency converter. By adjusting the corresponding delay lines of the two parallel optical paths of the resonant cavity, it overcomes the technical shortcomings of reflected light offset in the adjustment process of the traditional four-mirror ring cavity, and realizes the adjustment of the resonant cavity mode and free spectral range. At the same time, the six-mirror ring cavity can realize the external cavity resonant nonlinear frequency conversion on the basis of realizing the fundamental frequency optical resonance, and realize multi-wavelength resonance.

[0059] Example 2

[0060] This second embodiment is a diamond Brillouin nonlinear frequency converter. The following description uses specific device parameters as an example for further details:

[0061] The single-frequency light source 1 is a single-frequency laser with a wavelength of 1064nm and a linewidth of <1MHz. The nonlinear optical crystal 12 is made of diamond as a Brillouin medium, with a length of 5mm and a refractive index of 2.392. The first 45-degree reflector 7 serves as the input coupling mirror and is coated with a high-reflectivity film with a reflectivity R = 96%@1064nm. The second 45-degree reflector 8, the first 10-degree reflector 10, and the second 60-degree reflector 14 are all coated with high-reflectivity films with a reflectivity R > 99.98%@1064nm. The PZT-driven concave reflector 11 and concave reflector 13 both have a radius of curvature of 100mm and are both coated with high-reflectivity films with a reflectivity R > 99.98%@1064nm. The initial cavity length of the six-mirror annular cavity is approximately 821.96 mm, corresponding to a free spectral range, i.e., a longitudinal mode spacing of 364.98 MHz. The corresponding distances between the mirrors are: L1 = 100 mm, L2 = L3 = 130 mm, L4 = 115 mm, and L5 = L6 = 165 mm.

[0062] Figure 5This is a schematic diagram showing the variation of the waist size of the meridional and sagittal planes with the sum of distances L5 and L6. When L5 = L6 = 165 mm, the waist size w of the meridional plane... T and sagittal plane w s The sum of the L5 and L6 distances is approximately equal, both being 34.7401 μm, and the waist sizes of both the meridional and sagittal planes are inversely proportional to the sum of the L5 and L6 distances. Under the condition of resonant cavity stability, the sum of the L5 and L6 distances can be tuned between 0 and 560 mm.

[0063] The pump power P required for the nonlinear frequency converter to reach the Brillouin threshold th for:

[0064]

[0065] Among them, g s L is the Brillouin gain coefficient. loss For passive loss within the cavity, l eff Where L is the effective length of the gain medium, and γ is the mode matching factor. By reducing the delay line length L5+L6, the beam waist size of the meridional and sagittal planes can be reduced to some extent, thus lowering the threshold power. When the 1064nm fundamental frequency light inside the diamond reaches the Brillouin threshold, a 1064.12nm Stokes light can be generated. By optimizing the cavity length, dual resonance of the fundamental frequency and the Stokes field frequency can be supported.

[0066] Example 3

[0067] This embodiment 3 is a diamond Raman nonlinear frequency converter, which has the same structure as the diamond Brillouin nonlinear frequency converter in embodiment 2. The difference lies in the coating of the six-mirror annular cavity and the different threshold values, as detailed below:

[0068] The nonlinear optical crystal 12 is made of diamond as the Raman medium, with a length of 5 mm and a refractive index of 2.392. The first 45-degree reflector 7 serves as the input coupling mirror and is coated with a high-reflectivity film with reflectivity R = 96.5%@1064nm and R > 99.9%@1240nm. The second 45-degree reflector 8, the first 10-degree reflector 10, and the second 60-degree reflector 14 are all coated with high-reflectivity films with reflectivity R > 99.9%@1064nm and 1240nm. The PZT-driven concave reflector 11 has a radius of curvature of 100 mm and is coated with a high-reflectivity film with reflectivity R > 99.9%@1064nm and 1240nm. The concave reflector 13 has a radius of curvature of 100 mm and is coated with a high-reflectivity film with reflectivity R > 99.9%@1064nm and an anti-reflection film with transmittance T = 0.5%@1240nm.

[0069] The pump power P required for the nonlinear frequency converter to reach the Raman threshold th for:

[0070]

[0071] Where α is the Raman crystal absorption coefficient, l is the length of Raman crystal 12, g is the Raman gain coefficient, and T is the transmittance of PZT-driven concave mirror 11 at 1240 nm. By reducing the delay line length L5+L6, the beam waist size of the meridional and sagittal planes can be reduced to a certain extent, thus lowering the threshold power. When the fundamental frequency light of 1064 nm inside the diamond reaches the Raman threshold, 1240 nm Stokes light can be generated. With the increase of the fundamental frequency light power, the generation of second-order Stokes light may be excited. Antireflective coatings with a second-order Raman wavelength of 1485 nm can be deposited on PZT-driven concave mirror 11 and concave mirror 13 to effectively suppress the generation of second-order Stokes light within a certain range. By optimizing the distance between cavity lengths L5 and L6, the fundamental frequency light and Raman light can achieve double resonance within the resonant cavity.

[0072] Unless otherwise specified, the model numbers of the various devices in this embodiment of the invention are not limited, and any device that can perform the above functions is acceptable.

[0073] Those skilled in the art will understand that the accompanying drawings are merely schematic diagrams of a preferred embodiment, and the sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments.

[0074] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A cavity length-insensitive nonlinear frequency converter, characterized in that, The nonlinear frequency converter includes: a single-frequency light source, a first thin lens, a second thin lens, an EOM, an optical isolator, a half-wave plate, a first 45-degree reflector, a second 45-degree reflector, a first 60-degree reflector, a concave reflector, a nonlinear optical crystal, a PZT-driven concave reflector, a second 60-degree reflector, a movable platform, and a servo locking system. The first 45-degree reflector, the second 45-degree reflector, the first 60-degree reflector, the concave reflector, the nonlinear optical crystal, the PZT-driven concave reflector, and the second 60-degree reflector constitute a six-mirror annular cavity. The first 60-degree reflector, the concave reflector, the nonlinear optical crystal, the PZT-driven concave reflector, and the second 60-degree reflector are mounted on a movable platform to form a delay line structure. The first and second thin lenses constitute a pattern-matching lens group; the EOM and the servo locking system constitute a PDH locking cavity system. After the fundamental frequency light is frequency locked under the control of the servo locking system, the fundamental frequency light is amplified in the resonant cavity. When the intensity of the fundamental frequency light inside the nonlinear optical crystal reaches the nonlinear frequency conversion laser threshold, nonlinear frequency conversion laser is generated. Depending on the cavity mirror coating of the six-mirror ring cavity and the different nonlinear optical crystals, different types of nonlinear frequency conversion laser outputs are achieved.

2. The cavity length-insensitive nonlinear frequency converter according to claim 1, characterized in that, By adjusting the length of the delay lines L5+L6 of the six-mirror annular cavity, the free spectral range and beam waist size of the resonant cavity can be adjusted.

3. The cavity length-insensitive nonlinear frequency converter according to claim 1, characterized in that, The laser beam with a beam waist of w0 emitted by the single-frequency light source passes through the mode-matching lens group, and after passing through the optical isolator and half-wave plate, the beam waist position and size of the image are consistent with the intrinsic space mode w1 of the six-mirror annular cavity.

4. The cavity length-insensitive nonlinear frequency converter according to claim 1, characterized in that, The EOM modulates the single-frequency light source to generate side frequencies on both sides of its frequency ω0. At the position of the first 45-degree reflector, part of the light enters the six-mirror annular cavity, and part of the light is reflected into the servo locking system. The servo locking system demodulates the reflected signal and then controls the PZT to drive the concave reflector to achieve resonance of the six-mirror annular cavity structure with the fundamental frequency light. During multi-wavelength resonance, the cavity length is tuned to make the cavity length meet the multi-wavelength resonance condition.

5. A cavity length-insensitive nonlinear frequency converter according to claim 4, characterized in that, The nonlinear frequency converter is used to realize different types of nonlinear optical frequency conversion laser output, which can be divided into two cases according to the different nonlinear frequency shift amounts: Fundamental optical frequency ω0 and nonlinear frequency conversion laser frequency ω s When the difference is much smaller than the fundamental frequency, the components constituting the six-mirror annular cavity are coated with a high-reflectivity film for both the fundamental frequency and nonlinear frequency-converted laser light, while the cavity length is tuned to ensure multi-wavelength resonance; when the fundamental frequency ω0 and the nonlinear frequency-converted laser frequency ω s When the difference is comparable to the fundamental frequency, the first 45-degree mirror, the second 45-degree mirror, the first 60-degree mirror, the nonlinear optical crystal, the PZT-driven concave mirror, and the second 60-degree mirror constituting the six-mirror ring cavity element are coated with a high reflectivity film for fundamental frequency light and nonlinear frequency conversion laser, and the concave mirror is coated with a high reflectivity film for fundamental frequency light and a nonlinear frequency conversion laser antireflection film, in order to achieve high-efficiency output of nonlinear frequency conversion laser.

6. A cavity length-insensitive nonlinear frequency converter according to claim 2, characterized in that, The free spectral range (FSR) of the six-mirror annular cavity is: Where c is the speed of light, L is the cavity length of the six-mirror annular cavity, n is the refractive index of the nonlinear crystal, l is the length of the nonlinear crystal, L1 is the distance between the PZT-driven concave mirror and the concave mirror, L2 is the distance between the PZT-driven concave mirror and the second 60-degree mirror, L3 is the distance between the first 60-degree mirror and the concave mirror, L4 is the distance between the first 45-degree mirror and the second 45-degree mirror, L5 is the distance between the first 45-degree mirror and the second 60-degree mirror, and L6 is the distance between the second 45-degree mirror and the first 60-degree mirror.

7. A cavity length-insensitive nonlinear frequency converter according to claim 6, characterized in that, The square of the waist for: Where λ is the resonant wavelength, f i The focal length f of the meridional plane T Or the focal length f of the sagittal plane S L0 = L2 + L3 + L4 + L5 + L6.

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