A kind of corner cube long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation

Abstract

The application discloses a kind of pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed loop compensation, including pump source, first polarizer, at least two groups of folding unit, pyramidal prism, fifth 45° full reflection mirror and sixth 45° full reflection mirror;Wherein, the pyramidal prism and at least two groups of folding unit jointly constitute folding ring resonator;First half-wave plate, second polarizer, quarter-wave plate, gain medium, third half-wave plate, acousto-optic modulator and second half-wave plate are sequentially arranged in the folding ring resonator;Compared with prior art, the physical structure cooperation of spatial position-reflection order is used in the application, the accumulated phase difference is compensated, and it is guaranteed that the light beam after multiple folding still maintains linear polarization output;Using acousto-optic modulator and wave plate with special rotation angle, the diffraction loss is reduced to minimum, and high loss is formed to reverse beam inhibition loss, so that the energy utilization rate and output efficiency of single-longitudinal-mode laser are greatly improved.

Description

Technical Field

[0001] This invention relates to the field of solid-state lasers, and more specifically to a pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation. Background Technology

[0002] Single-mode lasers possess significant advantages such as a single output frequency, long coherence length, narrow linewidth, and high beam quality, making them widely used in cutting-edge fields such as lidar, Raman spectroscopy, holographic interferometry, precision machining, and space optical communication. Particularly in coherent Doppler wind radar and differential absorption radar, high-power, narrow-linewidth single-mode lasers in the 2-micron band are indispensable core light sources. Higher laser output power results in longer radar range; narrower laser linewidth leads to longer coherence length and higher detection accuracy. Therefore, simultaneously increasing the output power and narrowing the linewidth of 2-micron single-mode lasers has become a key research direction in this field.

[0003] However, existing single-mode laser technology suffers from the following bottlenecks:

[0004] First, single-longitudinal-mode lasers obtained from traditional standing-wave cavities (such as plano-concave cavities) typically achieve 2-micrometer single-longitudinal-mode laser output using microcavity methods, torsional mode methods, or intracavity FP etalon methods. These methods are limited by extremely short cavity lengths, resulting not only in extremely low laser output power (typically in the hundreds of milliwatts range), but also in the limitation of narrowing the linewidth, which is constrained by the cavity length, making it difficult to achieve breakthroughs in narrower linewidths.

[0005] Secondly, traditional ring cavities (such as four-mirror, six-mirror, or bipyramidal ring cavities) typically rely on the Faraday rotator effect to achieve unidirectional beam movement within the cavity and eliminate spatial hole burning effects. If the linewidth is narrowed by multiplying the cavity length, the laser's threshold power will also increase dramatically; however, increasing the pump power is limited by the low optical damage threshold of the intracavity Faraday rotator, ultimately making it impossible to simultaneously achieve high power and narrow linewidth. Furthermore, simply increasing the physical size of a traditional ring cavity makes the laser bulky and highly susceptible to mechanical vibrations and airflow disturbances, leading to deterioration in frequency stability and beam quality.

[0006] Thirdly, to address the issues of large volume and susceptibility to misalignment in traditional ring cavities, researchers have attempted to construct folded cavities using cornerstone prisms with self-collimating properties. However, when the beam undergoes total internal reflection within the cornerstone prism, an inherent phase delay is inevitably introduced, leading to severe depolarization. If the cavity length is extended by increasing the number of mirrors to force the beam to repeatedly pass through the cornerstone prism, this depolarization phase difference accumulates, causing the oscillating light within the cavity to become an uncontrollable, chaotic polarization state. This has fatal consequences: no polarization-sensitive devices can be used within the cavity, and it is impossible to create a large diffraction loss difference through polarization differences to achieve unidirectional operation. Therefore, how to completely solve the depolarization accumulation effect from a physical structure perspective while realizing a single-cornerstone, multi-folded, ultra-long cavity has become a critical bottleneck problem in this field. Summary of the Invention

[0007] The purpose of this invention is to address the problems in the prior art, such as the limited effective optical path of the ring cavity, the low damage threshold of unidirectional operating devices, the insufficient stability of the single longitudinal mode, and the difficulty in balancing high power and narrow linewidth. To address these shortcomings, a pyramidal long cavity acousto-optic induced single longitudinal mode laser based on polarization state closed-loop compensation is proposed.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A pyramidal long cavity acousto-optic induced single longitudinal mode laser based on polarization state closed-loop compensation includes a pump source, a first polarizer, at least two sets of folding units, a pyramidal prism, a fifth 45° total reflection mirror, and a sixth 45° total reflection mirror;

[0010] The cornerstone prism, the fifth 45° total reflection mirror, the sixth 45° total reflection mirror, and at least two sets of folding units together constitute a folded ring resonant cavity. A first half-wave plate, a second polarizer, a quarter-wave plate, a gain medium, a third half-wave plate, an acousto-optic modulator, and a second half-wave plate are sequentially arranged within the folded ring resonant cavity. The sixth 45° total reflection mirror is located on the other side of the second half-wave plate, and the fifth 45° total reflection mirror is perpendicular to and above the sixth 45° total reflection mirror.

[0011] The pump source emits pump light of 1.94μm, which is reflected by the first polarizer and the second polarizer, and then incident on the gain medium through a quarter-wave plate to produce horizontally polarized light in the a direction (clockwise) and the b direction (counterclockwise).

[0012] The horizontally polarized light propagating along the a direction becomes vertically polarized light after passing through the third half-wave plate. It then enters the acousto-optic modulator with applied radio frequency power for diffraction loss, preventing the vertically polarized light in the a direction from continuing to oscillate in the cavity, thus completely turning it off and achieving high-loss diffraction turn-off in the a direction.

[0013] Horizontally polarized light propagating along direction b passes through a quarter-wave plate and is incident on the second polarizer. After passing through, it enters the first half-wave plate. By rotating the first half-wave plate, the beam is rotated into linearly polarized light with a specific azimuth angle, and then incident on the first region of the cornerstone prism. After exiting the first region in a reflection sequence, it completes one cornerstone fold. It is then reflected by the folding unit corresponding to the first region. The beam enters the second region of the cornerstone prism, exits the second region in a reflection sequence, and completes a second cornerstone fold. It is then reflected by the folding unit corresponding to the second region. The beam enters the first region of the cornerstone prism, exits the first region in a reflection sequence, and completes a third cornerstone fold, so that the beam cancels the depolarization effect after three folds. The beam in the first region... The second region and its corresponding folding unit undergo N round-trip folding and reflection, where N is a positive integer. After the final number of folds is completed, the output linearly polarized beam is reflected by the fifth and sixth 45° total reflection mirrors and then incident on the second half-wave plate. By rotating the second half-wave plate, the beam propagating along the b direction is made to pass through the acousto-optic modulator with low loss transmittance, eliminating the spatial hole burning effect and realizing single-longitudinal-mode operation. The beam passes sequentially through the third half-wave plate, the gain medium, the quarter-wave plate, and the second polarizer. The second polarizer transmits the horizontally polarized light component of the beam, allowing it to continue oscillating within the folded ring resonant cavity. The vertically polarized light component of the beam is reflected out of the cavity and passes through the first polarizer, serving as a high-power, narrow-linewidth single-longitudinal-mode laser output.

[0014] As a further preferred embodiment of the present invention, the folding unit is mapped to two alternating regions of the corner cube prism according to the grouping mapping relationship, that is, the first region and the second region are alternately set. The light beam is alternately reflected between the corner cube prism and the folding unit to achieve an ultra-long optical path. The folding unit includes two 45° total reflection mirrors set relatively perpendicularly. Each group of folding units is used to redirect the light beam emitted from one region of the corner cube prism to another symmetrical region.

[0015] As a further preferred embodiment of the invention, the rotation angle of the second half-wave plate is configured such that the linearly polarized light of the final outgoing beam is rotated to the low diffraction loss polarization direction of the acousto-optic modulator, depending on the parity of the final folding number N.

[0016] As a further preferred embodiment of the present invention, the acousto-optic modulator is tilted, and the tilt angle is the Bragg angle of the acousto-optic modulator at the corresponding driving frequency, with a tilt angle of 0.1° to 5°.

[0017] As a further preferred embodiment of the present invention, the specific azimuth angle of the beam passing through the first half-wave plate is calculated based on the Jones matrix and the incident order of the first region of the cornerstone prism.

[0018] The rotation angle of the first half-wave plate is configured such that the azimuth angle of the incident ray polarized light matches the intrinsic polarization state of the Jones matrix in the first region of the current corner bevel prism.

[0019] As a further preferred embodiment of the present invention, the operating frequency of the acousto-optic modulator is 40.68MHz and the applied radio frequency power is approximately 50W.

[0020] As a further preferred embodiment of the present invention, the gain medium includes anisotropic crystal or isotropic crystal; the anisotropic crystal is Ho:YLF crystal, and the isotropic crystal is Ho:YAG crystal or Nd:YAG crystal.

[0021] As a further preferred embodiment of the present invention, when the gain medium is an isotropic crystal, the third half-wave plate is replaced with a quarter-wave plate.

[0022] As a further preferred embodiment of the present invention, the corner cube prism includes three mutually perpendicular reflecting surfaces and a bottom surface. The three solid edges and three imaginary edges of the corner cube prism divide the bottom surface into six regions, which are symmetrically arranged in pairs, including: BAC region, CAB region, ACB region and BCA region; wherein any one of the regions is a first region, and the region symmetrically arranged with respect to that region is a second region.

[0023] As a further preferred embodiment of the present invention, when the light beam enters the cornerstone prism for the kth time...

[0024] If k is odd, the beam enters the first region;

[0025] When k is even, the beam enters the second region;

[0026] The beam's exit polarization azimuth angle alternates between 73.94° and 46.06° or between 16.06° and 43.94°, while maintaining a linear polarization state.

[0027] The present invention proposes a pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation, which has the following advantages compared with the prior art:

[0028] 1. This invention utilizes spatial folding multiplexing technology to realize an ultra-long resonant cavity. By using the self-collimation characteristics of a single pyramidal prism in combination with multiple 45° total reflection mirrors, the cavity length is increased several times within a very small physical volume. Compared with the traditional six-mirror or double pyramidal ring cavity, the resonant cavity is significantly extended within the same laser volume, thereby fundamentally narrowing the linewidth of the single longitudinal mode laser and exhibiting extremely high anti-misalignment stability.

[0029] 2. In terms of physical structure, the present invention has made strict spatial grouping and specific position mapping of the six reflectors, so that they are strictly corresponding to two different reflection sequences inside a single pyramid. Through the alternation of reflection sequences such as CAB / BAC, the polarization state after multiple folds is still controlled linear polarization, thereby avoiding depolarization accumulation and ensuring that the beam is still output with a pure high linearity polarization state after multiple folds.

[0030] 3. This invention achieves closed-loop compensation design through spatial position-reflection sequence, and in conjunction with waveplate angle, ensures that the diffraction loss of the retained unidirectional oscillating light is minimized when passing through the acousto-optic modulator (AOM).

[0031] 4. This invention also abandons the traditional Faraday rotator and uses the huge loss difference of AOM to different polarization states to achieve stable unidirectional operation. That is, it uses a half-wave plate and an acousto-optic modulator to create a high and low loss difference between the forward and reverse beams, so that one direction can pass through with low loss and the other direction is blocked by diffraction loss, thereby eliminating spatial hole burning and realizing single longitudinal mode. Since the damage threshold of AOM is much higher than that of Faraday magneto-optical crystal, this invention can withstand extremely high pump power, thereby breaking the bottleneck of 2-micron single longitudinal mode lasers moving towards high power.

[0032] 5. This invention uses a combination of polarizer and quarter-wave plate to replace the traditional output mirror, achieving continuous and precise control of the laser output coupling rate from 0 to 100%. Under different pump power levels, the optimal transmittance can be found through simple adjustments, which is approximately between 20% and 35%, ensuring that the laser always operates at its highest efficiency. Attached Figure Description

[0033] Figure 1 It is a prism coordinate system;

[0034] Figure 2 This is a schematic diagram of a pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation;

[0035] Figure 3 This is a schematic diagram of a pyramidal ultralong cavity acousto-optic induced single-long-mode laser based on polarization state closed-loop compensation.

[0036] The meanings of the labels in the figure are as follows: 1. Pump source, 2. First polarizer, 3. Second polarizer, 4. Quarter-wave plate, 5. Gain medium, 6. First half-wave plate, 7. Pyramidal prism, 8. First 45° total reflection mirror, 9. Second 45° total reflection mirror, 10. Third 45° total reflection mirror, 11. Fourth 45° total reflection mirror, 12. Fifth 45° total reflection mirror, 13. Sixth 45° total reflection mirror, 14. Second half-wave plate, 15. Acousto-optic modulator, 16. Third half-wave plate, 17. Seventh 45° total reflection mirror, 18. Eighth 45° total reflection mirror. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0038] This invention relates to a pyramidal long-cavity acousto-optic induced single-longitudinal-mode laser based on polarization-state closed-loop compensation. It employs an innovative folded topology using a single pyramidal prism and multiple total reflection mirrors. Without significantly increasing the laser system volume, it utilizes spatial folding multiplexing technology to achieve an ultra-long resonant cavity, increasing the cavity length several times within a very small physical volume. This significantly improves the cavity's Q value and greatly narrows the linewidth of the output single-longitudinal-mode laser. Simultaneously, the self-collimating properties of the pyramidal prism ensure the long cavity's anti-misalignment and frequency stability.

[0039] A cornerstone prism consists of three mutually perpendicular reflecting surfaces and a base. Viewed from the base, the three solid edges and three imaginary edges divide it into six regions: ABC, ACB, CAB, CBA, BCA, and BAC. Figure 1 Incident light enters the corner prism from any of the BAC, CAB, ACB, or BCA regions and undergoes three total internal reflections on three mutually perpendicular reflecting surfaces. It then exits from a region symmetrical to the incident region with respect to the center of the corner prism's vertex (e.g., if the incident light enters the CAB region, the beam will exit from the BAC region). The exiting light remains parallel to the incident light and propagates in the opposite direction. If linearly polarized light with a specific polarization azimuth angle is incident, the exiting light is still linearly polarized, only with a different polarization azimuth angle than the incident light. Therefore, the corner prism possesses a back-reflection self-collimation characteristic. This invention utilizes this characteristic and combines it with an external mirror to spatially redirect the beam, forming a compact, long-path closed loop. Compared to simply relying on multiple ordinary mirrors to fold the optical path, this structure is more advantageous in maintaining cavity stability and reducing assembly sensitivity.

[0040] Example 1: This example is applicable not only to the 2-micron band but also to the 1-micron band. A pyramidal long-cavity acousto-optic induced single-longitudinal-mode laser based on polarization-state closed-loop compensation includes a pump source, a first polarizer, at least two sets of folding units, a pyramidal prism, a fifth 45° total reflection mirror, and a sixth 45° total reflection mirror. The pyramidal prism, the fifth 45° total reflection mirror, the sixth 45° total reflection mirror, and the at least two sets of folding units together constitute a folded ring resonant cavity. Within the folded ring resonant cavity, a first half-wave plate, a second polarizer, a quarter-wave plate, a gain medium, a third half-wave plate, an acousto-optic modulator, and a second half-wave plate are sequentially arranged. The sixth 45° total reflection mirror is located on the other side of the second half-wave plate, and the fifth 45° total reflection mirror is perpendicular to and above the sixth 45° total reflection mirror. The acousto-optic modulator is tilted at an angle equal to the Bragg angle of the acousto-optic modulator at the corresponding driving frequency, specifically ranging from approximately 0.1° to 5°.

[0041] The corner cube prism comprises three mutually perpendicular reflecting surfaces and a bottom surface. The three solid edges and three imaginary edges of the corner cube prism divide the bottom surface into six symmetrically arranged regions. One of the following regions—BAC, CAB, ACB, and BCA—is selected as the first region, and the region symmetrically arranged with it is designated as the second region. The incident surface of the corner cube prism is coated with a 1.9-2.1 μm high-transmittance film.

[0042] The folding units are mapped to two alternating regions of the cornerstone prism, namely the first region and the second region, according to a grouping mapping relationship. When the cornerstone surface element is infinitely large, an unlimited number of folding units can be added for reflection to achieve an ultra-long optical path. The folding unit includes two 45° total reflection mirrors arranged relatively perpendicularly. Each group of folding units is used to redirect the light beam emitted from one region of the cornerstone prism to another symmetrical region. All total reflection mirror surfaces are coated with a 1.9-2.1μm high-transmittance film.

[0043] The rotation angle of the second half-wave plate 14 is configured such that the linearly polarized light of the final outgoing beam is rotated to the low diffraction loss polarization direction of the acousto-optic modulator, depending on the parity of the final folding number N.

[0044] The pump source emits pump light of 1.94μm, which is reflected by the first polarizer 2 and the second polarizer 3, and then incident on the gain medium 5 through a quarter-wave plate to produce horizontally polarized light in the a direction (clockwise) and the b direction (counterclockwise).

[0045] The horizontally polarized light propagating along the a direction becomes vertically polarized light after passing through the third half-wave plate. It then enters the acousto-optic modulator where radio frequency power is applied for diffraction loss, preventing the vertically polarized light in the a direction from continuing to oscillate in the cavity. This results in complete shutdown, achieving high-loss diffraction shutdown in the a direction.

[0046] Horizontally polarized light propagating along direction b passes through the second polarizer with low loss. After passing through the quarter-wave plate, it enters the first half-wave plate. By rotating the first half-wave plate, the beam is transformed into linearly polarized light with a specific azimuth angle. The rotation angle of the first half-wave plate is configured to match the azimuth angle of the incident linearly polarized light to the intrinsic polarization state of the Jones matrix in the first region of the corner prism. The specific azimuth angle is calculated based on the Jones matrix and the incident sequence in the first region of the corner prism. The beam is incident on the first region of the corner prism; after exiting the first region in the reflection sequence, it completes one corner fold; it is reflected by the folding unit corresponding to the first region; the beam enters the second region of the corner prism, exits the second region in the reflection sequence, and then... The beam undergoes a second pyramidal fold, followed by reflection through a folding unit corresponding to the second region. The beam enters the first region of the pyramidal prism, exits after reflection in the first region, and completes a third pyramidal fold, thus canceling the depolarization effect after three folds. The beam undergoes N round-trip folding and reflections between the first region, the second region, and the corresponding folding unit, where N is a positive integer, forming a closed-loop polarization compensation. When the beam enters the pyramidal prism for the kth time, if k is odd, the beam enters the first region, for example, region CAB; if k is even, the beam enters the second region, for example, region BAC. The beam's exit polarization azimuth angle alternates between 73.94° and 46.06°, maintaining a linear polarization state.

[0047] After the final number of folds is completed, the output linearly polarized beam is reflected by the fifth and sixth 45° total reflection mirrors and then incident on the second half-wave plate. By rotating the second half-wave plate, the beam propagating along the b-direction passes through the acousto-optic modulator with low loss transmittance, eliminating the spatial hole-burning effect and achieving single-longitudinal-mode operation. The beam then passes sequentially through the third half-wave plate, the gain medium, the quarter-wave plate, and the second polarizer. The second polarizer transmits the horizontally polarized component of the beam, allowing it to continue oscillating within the folded ring resonant cavity. It reflects the vertically polarized component of the beam out of the cavity and through the first polarizer, outputting as a high-power, narrow-linewidth single-longitudinal-mode laser.

[0048] The gain medium includes anisotropic crystals or isotropic crystals; the anisotropic crystal is a Ho:YLF crystal, and the isotropic crystal is a Ho:YAG crystal or a Nd:YAG crystal. When using an isotropic crystal, since the beam may not have fixed linear polarization characteristics within the cavity, the third half-wave plate needs to be replaced with a quarter-wave plate. In this case, the beam in direction a is converted into vertically polarized light after passing through the quarter-wave plate and is then turned off by high loss after entering the acousto-optic modulator; the beam in direction b is combed and output through the quarter-wave plate and the second polarizer, and after passing through the first half-wave plate, the pyramidal folding system, and the second half-wave plate, it is adjusted to the low-loss polarization state of the acousto-optic modulator and passes smoothly.

[0049] Example 2, combined with Figure 2 The present invention relates to a pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation with a three-fold basic configuration.

[0050] The specific structure of the pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation includes: pump source 1, first polarizer 2, second polarizer 3, quarter-wave plate 4, gain medium 5, first half-wave plate 6, pyramidal prism 7, first 45° total reflection mirror 8, second 45° total reflection mirror 9, third 45° total reflection mirror 10, fourth 45° total reflection mirror 11, fifth 45° total reflection mirror 12, sixth 45° total reflection mirror 13, second half-wave plate 14, acousto-optic modulator 15, and third half-wave plate 16.

[0051] The cornerstone prism 7, the first 45° total reflection mirror 8, the second 45° total reflection mirror 9, the third 45° total reflection mirror 10, and the fourth 45° total reflection mirror 11 together constitute a folded ring resonant cavity. Inside the folded ring resonant cavity (between the cornerstone prism 7 and the total reflection mirrors), the first half-wave plate 6, the second polarizer 3, the quarter-wave plate 4, the gain medium 5, the third half-wave plate 16, the acousto-optic modulator 15, and the second half-wave plate 14 are arranged sequentially. The sixth 45° total reflection mirror 13 is located on the other side of the second half-wave plate 14. The fifth 45° total reflection mirror 12 is arranged perpendicularly to the sixth 45° total reflection mirror 13 and is located above the sixth 45° total reflection mirror 13.

[0052] After selecting the incident region, for example, taking the CAB region as the incident region, the first 45° total reflection mirror 8 and the second 45° total reflection mirror 9 are set to correspond to the incident region. The region symmetrically set with respect to the incident region is the exit region, and the third 45° total reflection mirror 10 and the fourth 45° total reflection mirror 11 are set to correspond to the exit region. By combining the two different reflection sequences inside the pyramid, a strict spatial mapping is performed to achieve closed-loop compensation of the polarization state. At the same time, based on the Jones matrix, the specific azimuth angle of the linearly polarized light of the incident pyramid prism 7, which is 73.94°, and the rotation angle of the fast axis of the first half-wave plate 6 relative to the horizontal polarization direction, which is 36.97°, are calculated.

[0053] Pump source 1 emits pump light of 1.94μm, which is reflected by first polarizer 2 and second polarizer 3, and then incident on Ho:YLF crystal through quarter-wave plate 4 to produce horizontally polarized light in the a direction (clockwise) and b direction (counterclockwise).

[0054] The horizontally polarized light propagating along the a direction becomes vertically polarized light after passing through the third half-wave plate 16. It then enters the acousto-optic modulator 15, which operates at a frequency of 40.68 MHz and applies an RF power of 50 W, to undergo diffraction loss. Since the acousto-optic modulator 15 has high diffraction loss for vertically polarized light, the beam in the a direction is strongly diffracted and completely lost here, and cannot continue to oscillate in the cavity, thus being completely turned off.

[0055] Horizontally polarized light propagating along direction b passes through a quarter-wave plate 4 and is incident on a second polarizer 3, which has high transmittance for 2.05μm horizontally polarized light. After transmission, it enters a first half-wave plate 6. By adjusting the rotation angle of the first half-wave plate 6, i.e., rotating its fast axis by 36.97° relative to the horizontal polarization direction, the beam is rotated into linearly polarized light with a specific azimuth angle. The 73.94° linearly polarized light is incident on the CAB region of the corner bevel prism 7. After being reflected sequentially out of the CAB region, its linearly polarized azimuth angle evolves to 46.0° due to the polarization phase modulation of the corner bevel. The beam folds once, at 6°. After reflection by the first 45° total reflection mirror 8 and the second 45° total reflection mirror 9, linearly polarized light at 46.06° is incident on the BAC region of the corner prism 7. After reflection from the BAC region, the linearly polarized azimuth angle returns to 73.94°, completing the second corner fold. After reflection by the third 45° total reflection mirror 10 and the fourth 45° total reflection mirror 11, linearly polarized light at 73.94° is incident on the CAB region of the corner prism 7. After reflection from the CAB region, the azimuth angle returns to 46.06°, completing the third corner fold. Due to the precise alternating mapping between the first to fourth reflecting mirrors and the specific regions of the corner prism (BAC / CAB), the depolarization effect is canceled after the three folds, and the output light remains linearly polarized. The linearly polarized beam, output at 46.06°, is then reflected by the fifth 45° total reflection mirror 12 and the sixth 45° total reflection mirror 13, reaching the second half-wave plate 14. By rotating the second half-wave plate 14, the polarization state of the beam propagating along the b direction is adjusted to a polarization state that allows it to pass through the acousto-optic modulator 15 with optimal transmittance (minimal diffraction loss). Due to the significant difference in diffraction loss between the acousto-optic crystal and the beams in the acousto-optic directions, only the oscillating light operating stably in the b direction is ultimately retained within the cavity, thus eliminating the spatial hole-burning effect and achieving single-mode operation. The beam then sequentially passes through the third half-wave plate 16, the gain medium 5, the quarter-wave plate 4, and the second polarizer 3.

[0056] The second polarizer 3 also serves as the laser output coupler. The unidirectional b-direction oscillating light, after passing through the Ho:YLF crystal, is modulated into elliptically polarized light by the quarter-wave plate 4. When this elliptically polarized light reaches the second polarizer 3, its horizontal polarization component continues to oscillate through the second polarizer 3, while its vertical polarization component is reflected out of the cavity by the second polarizer 3 and finally passes through the first polarizer 2 (which has high transmittance for vertically polarized light) as the final output of the single-longitudinal-mode laser. By adjusting the rotation angle of the quarter-wave plate 4, the output coupling rate can be continuously changed to obtain the maximum output power.

[0057] Example 3, combined with Figure 3 This invention features an ultra-long optical path extension configuration.

[0058] This invention achieves an ultra-long optical path extension configuration by increasing the surface size of the cornerstone prism 7 and the number of mirrors, while simultaneously breaking through the limit of linewidth narrowing. Strictly adhering to the core mechanism of this invention—spatial mapping and sequence compensation—regardless of the number of optical path folds, all mirrors are strictly grouped and mapped in pairs to two alternating regions (such as the ACB region and the BCA region) of the cornerstone prism 7. This ensures that even after the beam undergoes numerous alternations of a specific reflection sequence inside the cornerstone, its accumulated depolarization phase difference can still be perfectly closed-loop canceled out, guaranteeing that the beam reaching the acousto-optic modulator 15 remains linearly polarized, thereby maintaining extremely low diffraction loss.

[0059] Pump source 1 emits pump light of 1.94μm, which is reflected by first polarizer 2 and second polarizer 3, and then incident on Ho:YLF crystal through quarter-wave plate 4 to generate horizontally polarized oscillating light in the a direction (clockwise) and b direction (counterclockwise).

[0060] The beam in direction a is strongly diffracted and completely lost by the acousto-optic modulator 15, and can no longer oscillate within the cavity.

[0061] After undergoing three pyramidal folds, the horizontally polarized oscillating light propagating in direction b is reflected by the seventh 45° total reflection mirror 17 and the eighth 45° total reflection mirror 18. The 16.06° linearly polarized light is incident on the ACB region of the pyramidal prism 7, and after being reflected sequentially through the BCA region, the linear polarization azimuth angle is restored to 43.94°, completing four pyramidal folds. Subsequently, the linearly polarized beam output at 43.94° is reflected by the fifth 45° total reflection mirror 12 and the sixth 45° total reflection mirror 13, reaching the second half-wave plate 14. The beam then sequentially passes through the acousto-optic modulator 15, the third half-wave plate 16, the gain medium 5, the quarter-wave plate 4, and the second polarizer 3. The horizontally polarized component of the elliptically polarized light is highly transparent and remains in the ultra-long cavity to continue oscillating with a high Q value; the vertically polarized component is reflected out of the cavity by the second polarizer 3 and finally passes through the first polarizer 2 (which is highly transparent to vertically polarized light), becoming the high-power, narrow-linewidth single-longitudinal-mode laser output.

[0062] By increasing the number of mirrors, the optical path length is multiplied. This embodiment achieves an extremely high Q value and an extremely narrow linewidth output that is far narrower than conventional cavity lengths, while maintaining extremely low intracavity loss and maximum energy transmittance. The operator only needs to adjust the rotation angle of the quarter-wave plate 4 to precisely change the proportion of the vertically polarized light component at any extremely high pump power, thereby achieving the optimal equivalent output transmittance and ultimately obtaining an ideal single-mode laser output with high power, extremely narrow linewidth, and highly stable frequency.

[0063] Example 4: By using a 25.4 mm aperture prism and 4 sets (8) of folding mirrors, a multi-path folding optical path can be constructed, extending the equivalent optical cavity length to approximately 5 meters. Due to the significant increase in the total cavity length, the relative length fluctuation caused by cavity mechanical vibration is effectively reduced, thereby narrowing the actual linewidth of the single-longitudinal-mode laser to the range of 10 kHz - 50 kHz.

[0064] Example 5: When facing the application requirements of higher spectral purity, if a 50.8 mm corner prism with a larger light-transmitting aperture is used, and 10 sets of reflective mirrors are used, the equivalent cavity length can be further extended to more than 15 meters without avoiding long-distance beam diffraction loss, thereby narrowing the laser linewidth to below 1 kHz and achieving ultra-narrow linewidth output.

[0065] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the above embodiments do not limit the present invention in any way, and all technical solutions obtained by equivalent substitution or equivalent transformation fall within the protection scope of the present invention.

Claims

1. A polarization state closed-loop compensated corner-cube long-cavity acousto-optic induced single-longitudinal-mode laser, characterized in that, It includes a pump source (1), a first polarizer (2), at least two sets of folding units, a corner cube prism (7), a fifth 45° total reflection mirror (12) and a sixth 45° total reflection mirror (13); The corner prism (7), the fifth 45° total reflection mirror (12), and the sixth 45° total reflection mirror (13), along with at least two sets of folding units, together constitute a folded ring resonant cavity. The folded ring resonant cavity is sequentially provided with a first half-wave plate (6), a second polarizer (3), a quarter-wave plate (4), a gain medium (5), a third half-wave plate (16), an acousto-optic modulator (15), and a second half-wave plate (14). The sixth 45° total reflection mirror (13) is located on the other side of the second half-wave plate (14). The fifth 45° total reflection mirror (12) is perpendicular to the sixth 45° total reflection mirror (13) and is located above the sixth 45° total reflection mirror (13). Pump source (1) emits pump light of 1.94μm, which is reflected by the first polarizer (2) and the second polarizer (3), and then incident on the gain medium (5) through the quarter-wave plate (4) to generate horizontally polarized light in the a and b directions; The horizontally polarized light propagating along the a direction becomes vertically polarized light after passing through the third half-wave plate (16). It then enters the acousto-optic modulator (15) with applied radio frequency power for diffraction loss, so that the vertically polarized light in the a direction cannot continue to oscillate in the cavity, thus achieving high-loss diffraction cutoff in the a direction. Horizontally polarized light propagating along direction b passes through the second polarizer (3) via a quarter-wave plate (4) and then enters the first half-wave plate (6). By rotating the first half-wave plate (6), the beam is rotated into linearly polarized light with an azimuth angle and enters the first region of the corner prism (7). After exiting the first region in a reflection sequence, it completes one corner fold. It is then reflected by the folding unit corresponding to the first region. The beam enters the second region of the corner prism (7), exits the second region in a reflection sequence, and completes a second corner fold. It is then reflected by the folding unit corresponding to the second region. The beam enters the first region of the corner prism (7), exits the first region in a reflection sequence, and completes a third corner fold, thus canceling the depolarization effect after the beam has undergone three folds. The beam passes through the first region, the second region, and the corresponding region. The folding units are folded and reflected N times back and forth, where N is a positive integer, to form a polarization state closed-loop compensation. After N folds are completed, the output linearly polarized beam is reflected by the fifth 45° total reflection mirror (12) and the sixth 45° total reflection mirror (13) and then incident on the second half-wave plate (14). By rotating the second half-wave plate (14), the beam transmitted along the b direction is made to pass through the acousto-optic modulator (15) with low loss transmittance, eliminating the spatial hole burning effect and realizing single longitudinal mode operation. The beam passes through the third half-wave plate (16), the gain medium (5), the quarter-wave plate (4), and the second polarizer (3) in sequence. The second polarizer (3) transmits the horizontal polarized light component of the beam, so that it continues to oscillate in the folded ring resonant cavity. The vertical polarized light component of the beam is reflected out of the cavity and passes through the first polarizer (2) as a high-power, narrow-linewidth single longitudinal mode laser output.

2. The polarization state closed-loop compensated corner-cube-long-cavity acousto-optic induced single-longitudinal-mode laser according to claim 1, wherein, The folding unit is mapped to two alternating regions of the corner cube prism (7) according to the grouping mapping relationship, that is, the first region and the second region are alternately set. The light beam is alternately reflected between the corner cube prism and the folding unit to achieve an ultra-long optical path. The folding unit includes two 45° total reflection mirrors set relatively perpendicularly.

3. The polarization state closed-loop compensated corner-cube-long-cavity acousto-optic induced single-longitudinal-mode laser of claim 1, wherein, The rotation angle of the second half-wave plate (14) is configured such that the linearly polarized light of the final outgoing beam is rotated to the low diffraction loss polarization direction of the acousto-optic modulator, depending on the parity of the final fold number N.

4. The polarization state closed-loop compensated corner-cube-long-cavity acousto-optic induced single-longitudinal-mode laser of claim 1, wherein, The acousto-optic modulator (15) is tilted, and the tilt angle is the Bragg angle of the acousto-optic modulator at the corresponding driving frequency, with a tilt angle of 0.1°~5°.

5. A pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation according to claim 1, characterized in that, The azimuth angle of the beam passing through the first half-wave plate (6) is calculated based on the Jones matrix and the incident order of the first region of the corner cube prism (7); The rotation angle of the first half-wave plate (6) is configured such that the azimuth angle of the incident ray polarized light matches the intrinsic polarization state of the Jones matrix in the first region of the current corner bevel prism.

6. A pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation according to claim 1, characterized in that, The operating frequency of the acousto-optic modulator (15) is 40.68MHz, and the applied radio frequency power is about 50W.

7. A pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation according to claim 1, characterized in that, The gain medium (5) includes anisotropic crystals or isotropic crystals; the anisotropic crystal is a Ho:YLF crystal, and the isotropic crystal is a Ho:YAG crystal or a Nd:YAG crystal.

8. A pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation according to claim 7, characterized in that, When the gain medium (5) is an isotropic crystal, the third half-wave plate (16) is replaced with a quarter-wave plate (4).

9. A pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation according to claim 1, characterized in that, The corner cube prism (7) includes three mutually perpendicular reflecting surfaces and a bottom surface. The three solid edges and three virtual edges of the corner cube prism (7) divide the bottom surface into six regions, which are symmetrically arranged in pairs, including: BAC region, CAB region, ACB region and BCA region; wherein, any region is selected as the first region, and the region symmetrically arranged with respect to the first region is the second region.

10. A pyramidal long cavity acousto-optic induced single-longitudinal-mode laser based on polarization state closed-loop compensation according to claim 9, characterized in that, When the beam enters the cornerstone prism (7) for the kth time, If k is odd, the beam enters the first region; When k is even, the beam enters the second region; The beam's exit polarization azimuth angle alternates between 73.94° and 46.06° or between 16.06° and 43.94°, while maintaining a linear polarization state.

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