Polarization-stable high-energy nanosecond laser and method of operation

Abstract

The application belongs to the technical field of laser. A polarization stable high-energy sub-nanosecond laser and a working method are provided. The polarization stable microchip oscillator implements differential temperature regulation on the built-in microchip crystal in horizontal and vertical orthogonal directions, adjusts the competition relationship between the heat distribution in the crystal and the orthogonal polarization state in the cavity, and outputs stable sub-nanosecond seed pulses with a rotatable polarization of ninety degrees. The end-pumped pre-amplification unit improves the seed pulse energy and suppresses self-excitation and parasitic oscillation. The side-pumped main amplification unit completes the step-by-step energy improvement of the fundamental frequency pulse by means of a two-stage side-pumped double-pass amplifier. The frequency conversion unit carries out frequency doubling and sum frequency conversion on the high-energy fundamental frequency laser, and outputs high-energy sub-nanosecond ultraviolet laser. The application can improve the polarization performance of the microchip light source and is suitable for precision machining and nonlinear optical related applications.

Description

Technical Field

[0001] This invention relates to the field of laser technology, and in particular to a polarization-stable high-energy sub-nanosecond laser and its operating method. Background Technology

[0002] The statements in this section merely provide background information related to the present invention and do not necessarily constitute prior art.

[0003] Subnanosecond pulsed solid-state lasers, with their short pulse width, high peak power, and compact structure, have significant application value in precision machining, laser ranging, lidar, nonlinear frequency conversion, and scientific experiments. Microchip passively Q-switched lasers based on the bonding of a gain medium and a saturable absorber represent a crucial technological approach for achieving subnanosecond pulse output. These lasers can be used directly as compact pulsed light sources or as seed sources in high-energy solid-state laser systems, working in conjunction with subsequent amplification and frequency conversion units. Their output polarization state directly affects the energy extraction efficiency and operational stability of the subsequent system.

[0004] The output polarization direction and degree of polarization of existing microchip passively Q-switched lasers are easily affected by factors such as crystal orientation, pump conditions, and thermal effects, leading to phenomena such as polarization drift and polarization jumps. This makes them unsuitable for applications requiring polarization-dependent amplification and frequency conversion. Conventional polarization improvement schemes often rely on specific crystal tangencies, polarization pumping, or preset polarization selection structures, which have limitations such as insufficient flexibility in polarization direction control and high complexity in fabrication and assembly, restricting the integrated application of this type of laser in high-stability, high-energy laser systems. Summary of the Invention

[0005] To address the issues of insufficient polarization stability and limited flexibility in polarization direction control in existing sub-nanosecond microchip lasers, which hinder their integration into high-stability, high-energy amplification and frequency conversion systems, this invention provides a polarization-stable high-energy sub-nanosecond laser and its operating method. By implementing differentiated temperature control along two orthogonal directions (horizontal and vertical) of the microchip crystal to adjust the competition between orthogonal polarization states within the cavity, and combining this with a cascaded optical path architecture of end-pump pre-amplification, multi-stage side-pump main amplification, and frequency conversion, polarization-stable and direction-switchable sub-nanosecond pulse output is achieved, improving the operational stability of high-energy fundamental frequency laser and ultraviolet laser output.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a polarization-stable high-energy sub-nanosecond laser.

[0007] A polarization-stabilized high-energy sub-nanosecond laser includes a polarization-stabilized microchip oscillator, and an end-pump pre-amplification unit, a side-pump main amplification unit, and a frequency conversion unit arranged sequentially along the optical path. The polarization-stabilized microchip oscillator incorporates a microchip crystal to generate polarization-stabilized sub-nanosecond seed pulses, which serve as the front-end seed source for subsequent amplification and frequency conversion links. By implementing differentiated temperature control on the microchip crystal along two orthogonal directions (horizontal and vertical), the internal heat distribution, thermal stress, and polarization-related characteristics of the microchip crystal are adjusted, thereby changing the competitive relationship between the two orthogonal polarization states within the cavity. This results in a polarization-stabilized sub-nanosecond seed pulse output. By altering the temperature relationship between the two orthogonal directions, a polarization output with a 90-degree rotation in polarization direction and stable polarization output can be obtained. The end-pump pre-amplification unit is used to perform end-pump single-pass pre-amplification on the sub-nanosecond seed pulse, thereby increasing the seed pulse energy and reducing the risk of self-excited oscillation and parasitic oscillation during subsequent high-gain amplification. The side-pumped main amplification unit includes at least two stages of side-pumped dual-pass amplifiers arranged sequentially along the optical path, which are used to progressively boost the energy of the fundamental frequency pulse after end-face pumping pre-amplification to obtain high-energy fundamental frequency laser output. The frequency conversion unit is used to perform frequency doubling and sum-frequency conversion on the high-energy fundamental frequency laser to obtain high-energy sub-nanosecond ultraviolet laser output.

[0008] In one implementation of the first aspect of the present invention, the polarization-stabilized microchip oscillator includes a first semiconductor laser pump source, a first pump light shaping system, an Nd:YAG / Cr:YAG microchip crystal, and a temperature control structure thermally coupled to the Nd:YAG / Cr:YAG microchip crystal. The first semiconductor laser pump source is used to provide the pump energy required for laser oscillation to the Nd:YAG / Cr:YAG microcrystal wafers. The first pump light shaping system is located between the first semiconductor laser pump source and the Nd:YAG / Cr:YAG microcrystal, and is used to shape and couple the pump light output from the first semiconductor laser pump source. Nd:YAG / Cr:YAG microcrystals are microcrystals used to construct passively Q-switched microcrystal laser oscillation cavities to generate sub-nanosecond seed pulses; The temperature control structure is used to regulate the temperature of Nd:YAG / Cr:YAG microcrystals along two orthogonal directions, horizontal and vertical.

[0009] As a further limitation of the first aspect of the present invention, the Nd:YAG / Cr:YAG microcrystal is a bonded composite microcrystal, wherein Nd:YAG is used as a gain medium, Cr:YAG is used as a saturable absorber, and the front and rear surfaces of the Nd:YAG / Cr:YAG microcrystal are provided with a coating structure to form a monolithic microcrystal laser oscillation cavity.

[0010] As a further definition of the first aspect of the present invention, the temperature control structure includes an upper TEC, a right TEC, a lower TEC, a left TEC, an upper thermistor, a right thermistor, a lower thermistor, a left thermistor, a copper plating mounting base, and a temperature control board. The upper TEC, right TEC, lower TEC, and left TEC are distributed along the four directions of the Nd:YAG / Cr:YAG microcrystal, which is used to differentiate the temperature of the Nd:YAG / Cr:YAG microcrystal along two orthogonal directions. The upper thermistor, right thermistor, lower thermistor, and left thermistor are used to detect temperature information in the corresponding direction in real time. Copper-plated mounting bases are used to support and fix Nd:YAG / Cr:YAG microcrystal wafers; The temperature control board is used to provide feedback control to the corresponding TEC based on the deviation between the detected temperature and the set temperature.

[0011] As a further limitation of the first aspect of the present invention, the right TEC and the left TEC correspond to the horizontal direction of the Nd:YAG / Cr:YAG microcrystal, and the upper TEC and the lower TEC correspond to the vertical direction of the Nd:YAG / Cr:YAG microcrystal. The temperatures on both sides of the same orthogonal direction are kept consistent, while the temperatures between the horizontal and vertical orthogonal directions are set differently, so that an asymmetric temperature distribution is formed inside the Nd:YAG / Cr:YAG microcrystal, which in turn generates an equivalent thermal stress difference between the two orthogonal directions. Under the action of the elasto-optic effect, the equivalent thermal stress difference causes the equivalent refractive index difference in two orthogonal directions, which in turn produces a polarization-dependent phase delay. The polarization-dependent phase delay alters the phase delay, eigenaxis direction, and non-degeneracy of the two orthogonal polarization eigenstates within the cavity. Combined with the difference in polarization-dependent gain and loss within the cavity, this changes the equivalent net gain and competition relationship between the two orthogonal polarization states. When the horizontal temperature is higher than the vertical temperature, the output seed pulse is stabilized in the first stable polarization direction; when the vertical temperature is higher than the horizontal temperature, the output seed pulse is stabilized in the second stable polarization direction, which is orthogonal to the first stable polarization direction. When the temperature relationship between the horizontal and vertical directions changes, the output polarization direction switches between two orthogonal stable polarization directions.

[0012] In one implementation of the first aspect of the present invention, the end-face pump pre-amplification unit includes a first half-wave plate, a first spatial optical isolator, a second half-wave plate, a first plano-convex lens, a first dichroic mirror, a second pump light shaping system, a second semiconductor laser pump source, and an Nd:YAG gain crystal. The first half-wave plate, the first spatial optical isolator, and the second half-wave plate constitute an optical isolation system, which is used to block the back-propagating light and parasitic reflected light in the subsequent amplification link from entering the front-end optical path, and to match and adjust the polarization state of the seed pulse. The first plano-convex lens is used for focusing and spot matching of the sub-nanosecond seed pulse; The pump light output from the second semiconductor laser pump source is shaped by the second pump light shaping system and then coupled into the Nd:YAG gain crystal by the first dichroic mirror to perform end-face pumping on the Nd:YAG gain crystal. The Nd:YAG gain crystal is used for single-pass pre-amplification of the input sub-nanosecond seed pulse.

[0013] In one implementation of the first aspect of the present invention, the side-pumped main amplification unit includes a first-stage side-pumped double-pass amplifier and a second-stage side-pumped double-pass amplifier cascaded sequentially along the optical path. The first-stage side-pumped dual-pass amplifier, along the seed light propagation direction, includes, in sequence, a third half-wave plate, a second optical isolator, a fourth half-wave plate, a first plano-concave lens, a second plano-convex lens, a first thin-film polarizer, a first quarter-wave plate, a first Nd:YAG side-pump module, a third plano-convex lens, a first vacuum tube, a fourth plano-convex lens, a first Faraday rotator, and a first 0° high-reflection mirror. The second-stage side-pumped dual-pass amplifier, along the seed light propagation direction, includes, in sequence, a fifth half-wave plate, a third optical isolator, a sixth half-wave plate, a second plano-concave lens, a fifth plano-convex lens, a second thin-film polarizer, a second quarter-wave plate, a second Nd:YAG side-pump module, a sixth plano-convex lens, a second vacuum tube, a seventh plano-convex lens, a second Faraday rotator, and a second 0° high-reflection mirror.

[0014] As a further limitation of the first aspect of the present invention, the first plano-concave lens and the second plano-convex lens are used to adjust the aperture, collimate and spatially shape the pulse light input to the first-stage side-pumped dual-pass amplifier so that the size and propagation state of the incident beam match the effective light transmission area of ​​the first Nd:YAG side-pump module. The second plano-concave lens and the fifth plano-convex lens are used to adjust the aperture, collimate and spatially shape the pulse light input to the second-stage side-pumped dual-pass amplifier so that the size and propagation state of the incident beam match the effective light transmission area of ​​the second Nd:YAG side-pump module. The first and second quarter-wave plates are used to coordinate the polarization evolution in the dual-path optical system and reduce the nonlinear accumulation effect of high peak power laser in the gain medium and related optical components. The third plano-convex lens, the first vacuum tube, and the fourth plano-convex lens together constitute the first-stage relay transmission and vacuum space filtering structure. The sixth plano-convex lens, the second vacuum tube, and the seventh plano-convex lens together constitute the second-stage relay transmission and vacuum space filtering structure. The first and second Faraday rotators are used to compensate for the thermally induced depolarization effect generated under high pump power amplification conditions. The first 0° high-reflection mirror and the second 0° high-reflection mirror are used to fold the amplified fundamental frequency light back to the corresponding gain module to complete the dual-pass amplification. The first and second thin-film polarizers are used to separate and export the amplified return pulse from the original input optical path.

[0015] In one implementation of the first aspect of the present invention, the frequency conversion unit includes a seventh half-wave plate, an eighth plano-convex lens, a third vacuum tube, a ninth plano-convex lens, a frequency doubling unit, a sum-frequency unit, a second dichroic mirror, and a third dichroic mirror. The seventh half-wave plate is used to adjust the polarization direction of the fundamental frequency laser entering the frequency conversion unit, so that the polarization direction of the fundamental frequency laser is adapted to the phase-matching polarization direction of the nonlinear frequency conversion crystal in the frequency doubling unit and the sum-frequency unit; The eighth plano-convex lens, the third vacuum tube, and the ninth plano-convex lens together constitute the beam transmission and shaping section before frequency conversion, which is used for relay imaging, spatial filtering, and beam shaping of the fundamental frequency laser after main amplification. The frequency doubling unit is used to convert the fundamental frequency laser into a frequency-doubled laser; The sum-frequency unit is used to perform a sum-frequency transformation between the fundamental frequency laser and the frequency-doubled laser to obtain ultraviolet laser output; The second and third dichroic mirrors are used to separate and extract laser light of different wavelengths after frequency conversion.

[0016] Secondly, the present invention provides a method for operating a polarization-stabilized high-energy sub-nanosecond laser.

[0017] A method for operating a polarization-stabilized high-energy sub-nanosecond laser, employing the polarization-stabilized high-energy sub-nanosecond laser of the first aspect of this invention, includes the following steps: Differential temperature control is applied to the microchip crystal in the polarization-stabilized microchip oscillator along two orthogonal directions, namely horizontal and vertical, to form a controlled asymmetric thermal field inside the microchip crystal, thereby adjusting the competitive relationship between the two orthogonal polarization states in the cavity and generating polarization-stabilized sub-nanosecond seed pulses. By changing the temperature relationship between the two orthogonal directions, horizontal and vertical, the output polarization direction is rotated by 90 degrees while maintaining a stable output. By using a semiconductor laser pump source to provide end-face pump energy to a microcrystalline wafer, the gain medium absorbs the energy to form population inversion, and combined with the passive Q-switching mechanism of a saturable absorber, a fundamental frequency sub-nanosecond seed pulse is generated. A polarization-stable sub-nanosecond seed pulse is injected into the end-face pump pre-amplification unit, and single-pass pre-amplification is performed under synchronous end-face pumping. The fundamental frequency pulse, which is pre-amplified by end face pumping, is injected into the side-pumped main amplification unit and then undergoes two stages of side-pumped dual-pass amplification process to increase the energy step by step. The high-energy fundamental frequency laser output from the side-pumped main amplification unit is injected into the frequency conversion unit, and frequency doubling and sum-frequency conversion are performed sequentially to obtain a stable high-energy sub-nanosecond ultraviolet laser output.

[0018] Compared with the prior art, the beneficial effects of the present invention are: This invention employs differentiated temperature control in horizontal and vertical orthogonal directions within a microchip oscillator to adjust the internal thermal distribution, thermal stress, and polarization-related characteristics of the microchip crystal. This alters the competitive relationship of orthogonal polarization states within the cavity, achieving stable linearly polarized seed pulse output without relying on specific crystal tangentiality or a preset polarization selection structure. Furthermore, the polarization direction can be switched by 90 degrees by adjusting the temperature relationship between the two orthogonal directions, enhancing the laser's adaptability to different subsequent optical paths and application scenarios. A stable seed source with front-end polarization, combined with an end-pumped pre-amplification structure, increases the seed pulse energy while suppressing self-oscillations and parasitic oscillations during high-gain amplification, providing a stable input foundation for subsequent main amplification stages. The cascaded architecture of two-stage side-pumped dual-pass amplification can progressively increase the fundamental frequency pulse energy, maintaining beam quality and polarization consistency while obtaining high-energy fundamental frequency output. After a stable high-energy fundamental frequency source is connected to the frequency conversion unit, the conversion efficiency of the frequency doubling and sum-frequency processes remains relatively stable, reducing energy fluctuations in the ultraviolet output. This invention integrates optical path design with polarization modulation, multi-stage amplification and frequency conversion, improving the polarization stability and application adaptability of microchip lasers in high-energy laser system integration.

[0019] Advantages of additional aspects of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 A schematic diagram of the overall structure of a polarization-stabilized high-energy sub-nanosecond laser provided as an exemplary embodiment of the present invention; Figure 2 A schematic diagram of a bidirectional differential temperature control structure for microcrystalline wafers provided as an exemplary embodiment of the present invention; Figure 3 A schematic diagram illustrating how the output polarization direction is controlled by changing the temperature relationship between two orthogonal directions (horizontal and vertical) in an exemplary embodiment of the present invention is provided. Figure 4 A polarization switching and stable output test result diagram provided for an exemplary embodiment of the present invention; The components include: 1. First semiconductor laser pump source; 2. First pump light shaping system; 3. Nd:YAG / Cr:YAG microcrystal wafer; 4. Temperature control structure; 4-1. Upper TEC; 4-2. Right TEC; 4-3. Lower TEC; 4-4. Left TEC; 4-5. Upper thermistor; 4-6. Right thermistor; 4-7. Lower thermistor; 4-8. Left thermistor; 4-9. Copper plating mounting base; 5. First half-wave plate; 6. First spatial optical isolator; 7. Second half-wave plate; 8. Waveplate; 9. First 45° reflector; 10. Second 45° reflector; 11. First plano-convex lens; 12. Nd:YAG gain crystal; 13. First dichroic mirror; 14. Second pump light shaping system; 15. Second semiconductor laser pump source; 16. Third 45° reflector; 17. Third half-wave plate; 18. Second optical isolator; 19. Fourth half-wave plate; 20. First plano-concave lens; 21. Second plano-convex lens; 22. Fifth 45° reflector; 23. ... 24. First quarter-wave plate; 25. First Nd:YAG side-pump module; 26. Third plano-convex lens; 27. First vacuum tube; 28. Fourth plano-convex lens; 29. ​​First Faraday rotator; 30. First 0° high-reflection mirror; 31. Sixth 45° reflector; 32. Fifth half-wave plate; 33. Third optical isolator; 34. Sixth half-wave plate; 35. Second plano-concave lens; 36. Fifth plano-convex lens; 37. Seventh 45° reflector; 38. Eighth 45° reflector; 39. 40. Second thin-film polarizer; 41. Second quarter-wave plate; 42. Second Nd:YAG side pump module; 43. Sixth plano-convex lens; 44. Second vacuum tube; 45. Seventh plano-convex lens; 46. Second Faraday rotator; 47. Second 0° high-reflection mirror; 48. Ninth 45° reflecting mirror; 49. Seventh half-wave plate; 50. Eighth plano-convex lens; 51. Third vacuum tube; 52. Ninth plano-convex lens; 53. Frequency doubling unit; 54. Sum-frequency unit; 55. Second dichroic mirror; 56. Third dichroic mirror. Detailed Implementation

[0022] Embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. While some embodiments of the invention are shown in the drawings, it should be understood that the invention can be implemented in various forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided to provide a more thorough and complete understanding of the invention. It should be understood that the drawings and embodiments of the present invention are for illustrative purposes only and are not intended to limit the scope of protection of the invention. In the description of the embodiments of the present invention, the terms "including" and similar terms should be understood as open-ended, i.e., including but not limited to. The term "based on" should be understood as at least partially based on. The term "an embodiment" or "this embodiment" should be understood as at least one embodiment. Without conflict, embodiments and features in the embodiments of the present invention can be combined with each other.

[0023] like Figure 1 As shown, this embodiment provides a polarization-stabilized high-energy sub-nanosecond laser, including a polarization-stabilized microchip oscillator, and an end-pump pre-amplification unit, a side-pump main amplification unit, and a frequency conversion unit arranged sequentially along the optical path. The polarization-stabilized microchip oscillator incorporates an Nd:YAG / Cr:YAG microchip crystal 3 to generate a polarization-stabilized sub-nanosecond seed pulse, which serves as the front-end seed source for subsequent amplification and frequency conversion links. The end-pump pre-amplification unit performs end-pump single-pass pre-amplification on the sub-nanosecond seed pulse, increasing its energy and reducing the risk of self-excited oscillation and parasitic oscillation during subsequent high-gain amplification. The side-pump main amplification unit includes a first-stage side-pump double-pass amplifier and a second-stage side-pump double-pass amplifier cascaded sequentially along the optical path, used to progressively increase the energy of the fundamental frequency pulse after end-pump pre-amplification to obtain a high-energy fundamental frequency laser output. The frequency conversion unit performs frequency doubling and sum-frequency conversion on the high-energy fundamental frequency laser to obtain a high-energy sub-nanosecond ultraviolet laser output.

[0024] The overall optical path architecture embodies the MOPA (Modular Optical Amplifier) ​​structure concept, which combines a master oscillator and a power amplifier. The front-end microchip oscillator generates high-quality, polarization-stable seed pulses, while the subsequent multi-stage amplification chain progressively increases the pulse energy to a high level. Finally, efficient nonlinear frequency conversion achieves high-energy output of 355nm ultraviolet laser. This architecture organically integrates polarization control, energy amplification, and frequency conversion into a single system. The functional units work collaboratively through precisely designed optical path connections and polarization matching. The overall system is characterized by its compact structure, high integration, and strong engineering adaptability, enabling stable output of high-energy sub-nanosecond ultraviolet laser while maintaining good beam quality and polarization purity.

[0025] The core components of the polarization-stabilized microchip oscillator include a first semiconductor laser pump source 1, a first pump beam shaping system 2, an Nd:YAG / Cr:YAG microchip crystal 3, and a temperature-controlled structure 4 thermally coupled to the Nd:YAG / Cr:YAG microchip crystal 3. The first semiconductor laser pump source 1 is preferably an 808nm fiber-coupled quasi-continuous-wave semiconductor laser. Its output pump light is collimated, focused, and its spot size adjusted and coupled by the first pump beam shaping system 2 to achieve a suitable spot size, which is then incident on the gain region of the Nd:YAG / Cr:YAG microchip crystal 3. This improves the spatial matching relationship between the pump light and the microchip crystal, which is beneficial for optimizing the output energy and pulse width of the sub-nanosecond seed pulse. The collimation and focusing functions of the first pump beam shaping system 2 ensure that the pump beam is incident on the gain region of the microchip crystal with a controlled divergence angle and spot size, making the pump light energy highly concentrated in the gain region. This improves the pump absorption efficiency and population inversion density, laying the foundation for high-efficiency passive Q-switched oscillation. The first semiconductor laser pump source 1 can achieve a maximum peak output power in the range of 100W, and the pump pulse width can be selected in the range of 230μs. The actual operating peak power can be adjusted according to the absorption characteristics and oscillation stability of the Nd:YAG / Cr:YAG microcrystal 3. The selection of the pump pulse width needs to take into account both population inversion accumulation efficiency and thermal effect control, achieving a balance between sufficient population inversion accumulation time and minimum thermal load to obtain optimal seed pulse output characteristics.

[0026] Nd:YAG / Cr:YAG microcrystal 3 is a bonded composite microcrystal, in which Nd:YAG is used as the gain medium and Cr:YAG is used as the saturable absorber. The working principle of Nd:YAG as the gain medium is based on a four-level laser system: Nd³ + Ions transition from the ground state to the pump absorption band under 808 nm pump light excitation, and then undergo rapid non-radiative relaxation to the upper laser level. 4 (F3 / 2) Accumulates population inversion in the upper laser energy level. When the intracavity light intensity exceeds the oscillation threshold, 1064nm laser photons are generated through stimulated emission, achieving gain amplification. The working principle of Cr:YAG as a saturable absorber is based on the difference between its ground-state absorption cross-section and excited-state absorption cross-section: under low light intensity conditions, Cr... 4+ The ions, in their ground state, exhibit strong ground-state absorption of the 1064nm laser, resulting in high intracavity loss and keeping the oscillating cavity in a high-threshold state. When the intracavity light intensity increases to near the saturation light intensity, Cr... 4+When ions transition from the ground state to the excited state, the number of ground-state particles decreases, ground-state absorption rapidly declines, cavity losses drop sharply, the quality factor increases rapidly, and the stored energy in the cavity is released in an extremely short time, forming a sub-nanosecond pulse with high peak power. The bonded composite structure of Nd:YAG and Cr:YAG enables the two functional materials to achieve tight spatial coupling within the same crystal, eliminating the losses caused by the spatial gap between them in discrete component schemes. This is beneficial for reducing the oscillation threshold and increasing the pulse energy.

[0027] The Nd:YAG / Cr:YAG microcrystal 3 comprises Nd:YAG and Cr:YAG segments along the optical path. The crystal cross-section can be 4mm × 4mm, with the Nd:YAG segment length being 5.5mm and the Cr:YAG segment length being 2.5mm. The Nd:YAG doping concentration can be 1 at.%, and the initial transmittance T0 of Cr:YAG can be 15%. The selection of the Nd:YAG doping concentration requires a balance between pump absorption efficiency and thermal effects: a higher doping concentration can achieve efficient pump absorption within a shorter crystal length, but it also increases the heat load and concentration quenching effect; a lower doping concentration requires a longer crystal or higher pump power to achieve sufficient population inversion. The initial transmittance T0 of Cr:YAG determines the initial loss of the saturable absorber, thus affecting the oscillation threshold and pulse energy: a lower T0 corresponds to higher initial loss, requiring more population inversion energy storage to trigger the Q-switching process, which is beneficial for obtaining higher pulse energy, but also increases the oscillation threshold; a higher T0 lowers the threshold, but the pulse energy decreases accordingly. The above parameters can be adjusted according to the target output energy, pulse width and system stability requirements to achieve optimal seed pulse characteristics.

[0028] The front and rear surfaces of the Nd:YAG / Cr:YAG microcrystal 3 are coated to form a monolithic microchip laser oscillator cavity: the Nd:YAG end face is coated with a high transmittance for 808nm pump light and a high reflectance for 1064nm laser light. This end face acts as a total reflection mirror of the oscillator cavity, enabling the 1064nm laser to travel back and forth efficiently within the cavity, while allowing the 808nm pump light to be incident efficiently. The Cr:YAG end face is coated with a high transmittance for 808nm pump light and a partial transmittance for 1064nm laser light, where the output transmittance of the 1064nm laser light can be 40%±2%. This end face acts as the output coupling mirror of the oscillator cavity, determining the output coupling efficiency of the oscillator cavity, thus forming a monolithic passively Q-switched microchip laser oscillator cavity. The selection of the output coupling ratio directly affects the pulse energy and peak power: a higher output coupling ratio is beneficial for triggering the Q-switching process at a lower intracavity light intensity, but the pulse energy may be lower; a lower output coupling ratio is beneficial for accumulating more intracavity energy, but may lead to excessive intracavity light intensity and optical damage. The output transmittance of 40%±2% strikes a good balance between output coupling efficiency and intracavity damage risk, which is beneficial for obtaining sub-nanosecond pulse output with high peak power.

[0029] During laser generation, the 808nm quasi-continuous pump light is coupled into the Nd:YAG gain medium via the first pump light shaping system 2. It is absorbed by Nd:YAG, resulting in population inversion, and the pump energy is gradually stored in the gain medium in the form of population inversion. Because the Cr:YAG saturable absorber has high absorption loss under low light intensity conditions, the cavity loss is large in the early stages of laser oscillation, and the oscillation cavity is in a high threshold state, with energy continuously accumulating in the Nd:YAG gain medium. When the cavity light intensity increases with stimulated emission to near the saturation threshold of Cr:YAG, Cr:YAG is rapidly bleached, its absorption loss is significantly reduced, the cavity quality factor rapidly improves, and the stored energy is released in a concentrated manner within a short time, thus forming a 1064nm sub-nanosecond seed pulse. Because the Nd:YAG / Cr:YAG microcrystal 3 adopts a monolithic short-cavity structure with a short cavity length and short photon round-trip time, it is beneficial to shorten the passive Q-switching pulse width. The short-cavity structure is also beneficial to improve the longitudinal mode spacing of the oscillation cavity, reduce the number of longitudinal modes in the cavity, improve the temporal coherence of the output pulse, and facilitate the satisfaction of the phase matching condition in the subsequent nonlinear frequency conversion process. Combining the above microcrystal parameters, pumping conditions and cavity membrane parameters, this embodiment can obtain a 1064nm seed pulse output in the range of hundreds of picoseconds to sub-nanoseconds.

[0030] like Figure 2As shown, the temperature control structure 4 includes a thermoelectric cooler (TEC), a thermistor, a copper plating mounting base 4-9, and a temperature control board arranged around the Nd:YAG / Cr:YAG microcrystal 3 in four directions: top, bottom, left, and right. Specifically, the upper TEC 4-1 is the upper thermoelectric cooler, the right TEC 4-2 is the right thermoelectric cooler, the lower TEC 4-3 is the lower thermoelectric cooler, and the left TEC 4-4 is the left thermoelectric cooler. The upper thermistor 4-5, the right thermistor 4-6, the lower thermistor 4-7, and the left thermistor 4-8 are used to detect the temperature information in the corresponding directions in real time. The copper plating mounting base 4-9 is used to support and fix the Nd:YAG / Cr:YAG microcrystal 3, and at the same time, efficiently transfers the temperature regulation effect of the thermoelectric cooler in each direction to the microcrystal. In the actual structure, copper plating mounting bases 4-9 are respectively arranged in the four directions of the microcrystal to support, clamp, or couple the thermoelectric cooler and microcrystal in the corresponding directions, ensuring that a good thermal contact interface is formed between the TEC and the microcrystal in each direction.

[0031] It should be noted that, in order to simplify the illustrations, Figure 2 The copper plating mounting bases 4-9 are shown only in their entirety in the image. Figure 2 The diagram does not depict temperature control boards for each direction individually. In the actual structure, the thermoelectric cooler and thermistor corresponding to each direction can be connected to their respective independent temperature control boards, which independently control the temperature in their respective directions. The copper mounting bases 4-9 are made of high thermal conductivity copper to ensure good thermal contact between the TEC and the microcrystal, allowing the heating or cooling effect of the TEC to be quickly and evenly transferred to the corresponding side of the microcrystal, achieving precise temperature control. The temperature information detected by each thermistor is fed back to the corresponding temperature control board. The temperature control board then controls the corresponding TEC based on the deviation between the detected temperature and the set temperature, achieving differentiated temperature adjustment of the Nd:YAG / Cr:YAG microcrystal 3 along the horizontal and vertical orthogonal directions. The temperature control board employs a closed-loop feedback control algorithm, which can compensate in real time for temperature fluctuations in the external environment and temperature changes in the crystal caused by pump heat effects, maintaining the set temperature difference in the orthogonal directions and ensuring the long-term stability of the polarization control effect.

[0032] like Figure 2As shown, TEC4-2 on the right and TEC4-4 on the left correspond to the horizontal direction (i.e., left-right direction) of the Nd:YAG / Cr:YAG microcrystal 3, while TEC4-1 on the upper side and TEC4-3 on the lower side correspond to the vertical direction (i.e., up-down direction) of the Nd:YAG / Cr:YAG microcrystal 3. The temperatures on both sides of the same orthogonal direction are consistent, meaning the left and right sides are approximately the same temperature, and the upper and lower sides are approximately the same temperature. However, the temperature settings differ between the horizontal and vertical orthogonal directions, thus constructing a controlled asymmetric thermal field within the microcrystal. To quantitatively describe the temperature relationship in the orthogonal directions, the equivalent temperature T_H in the horizontal direction and the equivalent temperature T_V in the vertical direction can be defined as follows: (1); (2); Where T_L, T_R, T_U, and T_D represent the temperatures at the left, right, top, and bottom positions of the Nd:YAG / Cr:YAG microcrystal 3, respectively. The equivalent temperature difference between the two orthogonal directions can be expressed as: (3); Here, ΔT represents the difference between the equivalent temperature in the horizontal direction and the equivalent temperature in the vertical direction. The magnitude and sign of ΔT determine the intensity and orientation of the asymmetric thermal field. By adjusting the magnitude and sign of ΔT, a controlled asymmetric thermal field can be precisely constructed inside the Nd:YAG / Cr:YAG microcrystal 3, thereby regulating the internal heat distribution, thermal stress, and polarization-related characteristics of the crystal. The aforementioned temperature difference is achieved through independent closed-loop control by temperature control boards in corresponding directions: the thermoelectric cooler (TEC) and thermistor corresponding to each direction are connected to the corresponding temperature control board, and each temperature control board independently provides feedback control of the temperature in its corresponding direction, ensuring that the temperatures in the horizontal and vertical directions can be set independently and accurately without interference, thus achieving precise programmable control of ΔT.

[0033] The physical effect chain generated by the asymmetric thermal field inside the microcrystal is as follows: When the boundary temperatures in the horizontal and vertical directions are different, an asymmetric temperature distribution is formed inside the microcrystal. This steady-state temperature distribution can be described by the heat conduction equation: (4); Where T represents the internal temperature distribution of the microcrystal, k is the equivalent thermal conductivity, and Q is the equivalent volumetric heat source caused by pump absorption, etc. Due to the different boundary temperatures in the horizontal and vertical directions, an asymmetric temperature distribution is formed inside the microcrystal, further causing an equivalent thermal stress difference in the two orthogonal directions. Under the action of the photoelastic effect, the equivalent thermal stress difference will cause an asymmetric change in the refractive index in the two orthogonal directions, forming a thermally induced birefringence effect. The equivalent thermal stress difference in the two orthogonal directions can be expressed as: (5); in, The equivalent thermal stress difference is between two orthogonal directions. This represents the equivalent thermal stress component in the horizontal direction. Let be the equivalent thermal stress component in the vertical direction. Under the action of the elasto-optic effect, the equivalent refractive index difference (i.e., thermally induced birefringence) caused by the equivalent thermal stress difference in two orthogonal directions can be expressed as: (6); in, The equivalent refractive index difference is between two orthogonal directions. and These are the refractive indices in the horizontal and vertical directions, respectively. The equivalent elastic-optical coefficient reflects the material's ability to produce birefringence under thermal stress. The resulting polarization-dependent phase delay can be expressed as: (7); in, This represents the phase delay difference accumulated by the two orthogonal polarization components along the direction of light propagation. The wavelength of the laser. Let be the direction of light propagation, and the integral is performed along the light propagation path within the crystal. The polarization-dependent phase delay alters the phase delay, eigenaxis direction, and non-degeneracy of the two orthogonal polarization eigenstates within the cavity, causing differences in the propagation characteristics of the two orthogonal polarization states within the cavity. Combining the differences in polarization-dependent gain and loss within the cavity, the equivalent net gain of the two orthogonal polarization states can be expressed as: (8); (9); in, , These are the equivalent net gains for the two orthogonal polarization states, respectively. , These are the equivalent gains for two orthogonal polarization states, , These represent the equivalent losses for two orthogonal polarization states. Differential temperature control can be achieved by changing... Furthermore, regulation and The relative magnitudes of M1 and M2 control the competition between the two orthogonal polarization states within the cavity: when M1 > M2, the first orthogonal polarization state is more likely to oscillate preferentially; when M2 > M1, the other orthogonal polarization state is more likely to oscillate preferentially. Therefore, by adjusting the temperature relationship between the horizontal and vertical orthogonal directions, the output polarization direction can be stabilized at one of the two mutually orthogonal polarization directions, thus achieving polarization-stable sub-nanosecond seed pulse output from the microchip laser.

[0034] like Figure 3 As shown, in this embodiment, when the horizontal temperature is higher than the vertical temperature (i.e., the equivalent temperature in the horizontal direction is greater than the equivalent temperature in the vertical direction), the output polarization direction can be stabilized at the first stable polarization direction. When the vertical temperature is higher than the horizontal temperature (i.e., the equivalent temperature in the vertical direction is greater than the equivalent temperature in the horizontal direction), the output polarization direction can be stabilized at the second stable polarization direction, and the second stable polarization direction is orthogonal to the first stable polarization direction. By changing the temperature relationship between the two orthogonal directions (horizontal and vertical), the internal thermal field distribution and polarization competition relationship of the Nd:YAG / Cr:YAG microcrystal 3 can be switched, thereby achieving the switching of the output polarization direction between the two orthogonal stable polarization directions and maintaining a stable polarization output. The physical essence of this polarization switching mechanism is that changing the temperature relationship in the orthogonal directions is equivalent to changing the orientation of the asymmetric thermal field, thereby changing the direction of the fast and slow axes of thermally induced birefringence, making the polarization state with a smaller net gain become a polarization state with a larger net gain, thus triggering polarization state switching. Since the entire switching process is controlled by the temperature control board, the switching timing and speed can be adjusted through software parameters, resulting in good controllability and repeatability.

[0035] The experimental results of this embodiment show that as the absolute value of the temperature difference |ΔT| between the two orthogonal directions (horizontal and vertical) gradually increases, the output polarization fluctuation is gradually suppressed when |ΔT| exceeds approximately 6°C. Under the representative temperature difference condition of ΔT = ±10°C, stable linearly polarized output can be obtained regardless of whether the horizontal temperature is higher than the vertical temperature or vice versa. When the temperature difference between the two orthogonal directions is small, the output laser may exhibit switching or jumping phenomena between the two polarization directions. This is because the temperature difference is insufficient for the net gain advantage of a certain polarization state to exceed the influence of intracavity random perturbation, leading to instability in the competition between the two polarization states. Therefore, by setting a sufficient temperature difference between the orthogonal directions, the preferred oscillation effect of a polarization state can be effectively enhanced, making its net gain advantage exceed the intracavity perturbation level, thereby improving the stability of the output polarization state.

[0036] like Figure 4As shown, in the polarization switching and stable output test, the test time span was 0 to 10 hours, and the vertical axis range of the single-pulse energy during the test was between 520 and 600 μJ. Before and after the temperature control relationship switching, the output light was in the first stable polarization direction output region (horizontal temperature is higher than vertical temperature, i.e., ΔT > 0) and the second stable polarization direction output region (vertical temperature is higher than horizontal temperature, i.e., ΔT < 0), respectively, and the two stable polarization directions were orthogonal to each other. The output energy remained stable at approximately 568 μJ throughout the entire 0 to 10 hour test period, with minimal energy fluctuation in the vertical axis, always remaining within the stable range of 520 to 600 μJ. The test results show that this embodiment can achieve controllable switching of the output polarization direction by changing the temperature relationship between the two orthogonal directions, and maintain stable polarization output after switching. The output energy remained highly consistent throughout the test process, and no energy fluctuations caused by polarization switching occurred. This test result verifies the effectiveness of the differentiated temperature control mechanism in polarization stability and polarization switching controllability: the output energy remains consistent before and after polarization switching, indicating that both orthogonal polarization states can efficiently extract and store energy from the gain medium under the corresponding temperature control conditions, and the two polarization output states are symmetrical; the energy fluctuation is minimal during the stable output test lasting up to 10 hours, indicating that the independent closed-loop feedback control of the temperature control system can effectively suppress the influence of external disturbances on the polarization state, ensuring the long-term reliability of stable polarization output.

[0037] Furthermore, the sub-nanosecond seed pulse output from the polarization-stabilized microchip oscillator passes through a front-end optical isolation and polarization matching section before entering the end-face pump pre-amplification unit. This section includes a first half-wave plate 5, a first spatial optical isolator 6, and a second half-wave plate 7. The first half-wave plate 5, the first spatial optical isolator 6, and the second half-wave plate 7 constitute an optical isolation system used to block the back-propagating light and parasitic reflected light in the subsequent amplification link from entering the front-end microchip oscillator, preventing the back light from adversely affecting the oscillation stability of the microchip oscillator. At the same time, it matches and adjusts the polarization state of the seed pulse before and after entering the amplification link to ensure that the polarization requirements of the subsequent dual-pass amplification optical path are met. The first spatial optical isolator 6 achieves unidirectional optical transmission based on the Faraday effect: the forward-propagating light undergoes a specific polarization state transformation through the combination of a Faraday rotator and a waveplate, and can pass smoothly through the polarization analysis element of the isolator; the back-propagating light undergoes the same polarization state transformation, and its polarization state does not match the polarization analysis element at the incident end of the isolator, thus being effectively blocked to prevent the back light from entering the front-end optical path. The first half-wave plate 5 is located on the incident side of the first spatial optical isolator 6. It is used to rotate the polarization state of the seed pulse to an angle that matches the transmission polarization direction of the first spatial optical isolator 6, ensuring that the seed pulse can pass through the isolator with the lowest loss. The second half-wave plate 7 is located on the output side of the first spatial optical isolator 6. It is used to further adjust the polarization state of the pulse after transmission through the optical isolator to meet the polarization requirements of the Nd:YAG gain crystal 11 in the end-pump pre-amplification unit for the incident light, and also to provide the optimal incident polarization state for the subsequent amplification optical paths.

[0038] The specific optical path structure of the end-pump pre-amplification unit is as follows: The sub-nanosecond seed pulse, after being processed by the first half-wave plate 5, the first spatial optical isolator 6, and the second half-wave plate 7, is sequentially folded by the first 45° reflector 8 and the second 45° reflector 9, then focused and matched by the first plano-convex lens 10, and finally enters the Nd:YAG gain crystal 11. The optical path folding function of the first 45° reflector 8 and the second 45° reflector 9 is used to rationally arrange the optical elements in a limited space, achieving a compact optical path arrangement. At the same time, the two folds ensure that the optical path direction is consistent with the overall system optical path direction, facilitating the docking of subsequent optical paths. The first plano-convex lens 10 is used to focus the sub-nanosecond seed pulse, improving its spatial coupling with the Nd:YAG gain crystal 11, so that the seed pulse spot size and the gain region of the Nd:YAG gain crystal 11 achieve optimal spatial mode matching, thereby improving the extraction efficiency of the end-pump pre-amplification. The physical significance of spot matching is that when the seed pulse spot completely overlaps with the gain region, the seed pulse can fully extract the population inversion energy stored in the gain region, achieving the highest single-pass gain. If the spot is too small, it can only extract the energy stored in the center of the gain region, and the energy stored in the periphery is wasted. If the spot is too large, the edge of the seed pulse does not overlap with the gain region, resulting in a decrease in extraction efficiency.

[0039] The 808nm pump light output from the second semiconductor laser pump source 14 is shaped by the second pump light shaping system 13 and then coupled into the Nd:YAG gain crystal 11 by the first dichroic mirror 12, performing end-face pumping on the Nd:YAG gain crystal 11. The first dichroic mirror 12 has high reflectivity for the 808nm pump light and high transmittance for the 1064nm signal light, achieving coaxial coupling between the pump light and the signal light at the crystal's incident end. This allows the pump light to enter the gain crystal in the same direction as the signal light, achieving co-directional end-face pumping. This co-directional end-face pumping method ensures a high degree of overlap between the absorption region of the pump light and the gain extraction region of the signal light, which is beneficial for improving gain extraction efficiency and reducing spatial gain inhomogeneity. The second pump light shaping system 13 shapes the pump light to match the pump spot with the gain region of the Nd:YAG gain crystal 11, thereby improving the pump absorption efficiency and establishing a high-density population inversion within the gain crystal, providing sufficient gain reserve for efficient single-pass pre-amplification of the seed pulse.

[0040] The second semiconductor laser pump source 14 is also preferably an 808nm fiber-coupled quasi-continuous-wave semiconductor laser, providing synchronous end-face pump energy to the Nd:YAG gain crystal 11. Population inversion is established within the gain crystal, and the input sub-nanosecond seed pulse is pre-amplified in a single pass, increasing the seed pulse energy, improving the input signal level of the subsequent main amplification unit, and reducing the risk of self-oscillation and parasitic oscillation during high-gain amplification. Synchronous pump timing control ensures that sufficient population inversion has been established by the time the seed pulse reaches the gain crystal, enabling the seed pulse to efficiently extract and store energy from the gain crystal, achieving high-gain single-pass pre-amplification. The third 45° reflector 15 is arranged at the output end of the end-face pump pre-amplification unit, deflecting the pre-amplified pulse optical path and guiding it into the side-pump main amplification unit, thus achieving optical path connection between the functional units.

[0041] The side-pumped main amplification unit comprises a first-stage side-pumped double-pass amplifier and a second-stage side-pumped double-pass amplifier cascaded along the optical path. These amplifiers progressively boost the energy of the pre-amplified fundamental frequency pulse after end-pumping, ultimately obtaining a high-energy fundamental frequency laser output. Both stages of the side-pumped double-pass amplifiers employ the same double-pass amplification configuration, utilizing a large-aperture Nd:YAG crystal rod as the gain medium. A high-density population inversion is established through side-pumping, and the double-pass amplification structure fully extracts the stored energy from the crystal, achieving highly efficient pulse energy boosting.

[0042] Side-pumping offers several advantages over end-pumping: First, the pump light enters from the side of the crystal, resulting in a uniformly distributed pump area along the crystal's length. This facilitates the establishment of a uniform population inversion distribution in large-diameter crystal rods, reducing beam quality degradation caused by gain inhomogeneities. Second, side-pumping spatially separates the pump and signal paths, allowing for independent adjustment of the pump power and enabling the use of multiple pump modules for multi-directional uniform pumping of the same gain crystal, further improving pump uniformity and total pump power. The dual-pass amplification structure allows the signal light to pass through the gain medium twice, achieving higher energy extraction with the same gain medium length compared to single-pass amplification. This improves the utilization efficiency of the gain medium and allows for high-energy output within a compact system structure.

[0043] The first-stage side-pumped double-pass amplifier, along the seed light propagation direction, sequentially includes a third half-wave plate 16, a second optical isolator 17, a fourth half-wave plate 18, a first plano-concave lens 19, a second plano-convex lens 20, a fourth 45° reflector 21, a fifth 45° reflector 22, a first thin-film polarizer 23, a first quarter-wave plate 24, a first Nd:YAG side-pump module 25, a third plano-convex lens 26, a first vacuum tube 27, a fourth plano-convex lens 28, a first Faraday rotator 29, and a first 0° high-reflection mirror 30. The third half-wave plate 16, the second optical isolator 17, and the fourth half-wave plate 18 constitute the polarization control and reverse optical isolation section at the front end of the first-stage side-pumped double-pass amplifier. The third half-wave plate 16 and the fourth half-wave plate 18 are located on the incident and exit sides of the second optical isolator 17, respectively, and are used to adjust the polarization state of the laser pulse before and after entering the isolator to meet the polarization requirements of the subsequent double-pass amplification optical path. The third half-wave plate 16 rotates the polarization state of the incident pulse to an angle that matches the transmission polarization direction of the second optical isolator 17, ensuring that the pulse passes through the isolator with minimal loss. The fourth half-wave plate 18 adjusts the polarization state of the pulse transmitted through the isolator to a polarization direction that meets the transmission requirements of the first thin-film polarizer 23, enabling the pulse to pass through the thin-film polarizer with high transmittance and enter the dual-pass amplification optical path. The second optical isolator 17 is used to suppress the back-propagating light in the subsequent link from returning to the previous amplification optical path, thereby improving the working stability of the previous amplification and reducing the adverse effects of the returned light on the front-end optical components, preventing high-energy back light from causing damage to the front-end optical components or interfering with the normal operation of the front-end optical path.

[0044] The first plano-concave lens 19 and the second plano-convex lens 20 constitute the beam shaping system of the first-stage side-pumped dual-pass amplification front end. Their function is to expand, collimate, and spatially shape the pre-amplified pulse light after end-pumping, ensuring that the size and propagation state of the incident beam match the effective light-passing area of ​​the first Nd:YAG side-pumping module 25, thereby improving the spatial coupling effect of the side-pumped amplification. The first plano-concave lens 19 diverges the incident beam, and the second plano-convex lens 20 collimates the diverging beam. The beam expansion and collimation system formed by these two lenses expands the beam aperture to a size matching the effective light-passing aperture of the first Nd:YAG side-pumping module 25. By expanding the incident beam aperture and optimizing its spatial matching state in the gain medium, the energy extraction efficiency of the pump gain region can be improved, while reducing the optical power density of high-energy pulses in the crystal and related optical components, thereby reducing the risk of optical damage. The increased beam aperture allows the pulse energy to be distributed over a larger cross-sectional area, reducing the optical power density while maintaining the same total energy. This keeps the optical power density below the damage threshold of the optical components, ensuring the long-term reliability of the system under high-energy operating conditions. The fourth 45° reflector 21 and the fifth 45° reflector 22 are used to deflect the optical path of the expanded and collimated beam, allowing for the rational arrangement of the spatial positions of subsequent optical components and achieving a compact optical path layout.

[0045] The first quarter-wave plate 24 is used to convert the linearly polarized light entering the first Nd:YAG side-pumping module 25 into circularly polarized or approximately circularly polarized light, enabling the pulse to be amplified in a circularly polarized manner in the gain medium. This reduces the adverse effects caused by polarization-dependent nonlinearity under high peak power conditions and lowers the risk of small-scale self-focusing and optical damage. Circular polarization amplification distributes energy uniformly across the pulse in all polarization directions, avoiding the gain inhomogeneity caused by the imperfect match between the polarization direction and the anisotropic gain axis of the crystal in linear polarization amplification. This is beneficial for maintaining good beam quality under high power amplification conditions. Furthermore, the self-phase modulation and cross-phase modulation effects induced by circularly polarized light in the nonlinear medium are more uniform than those of linearly polarized light, which helps reduce spectral broadening and beam quality degradation caused by nonlinear phase accumulation, exhibiting significant advantages under high peak power amplification conditions.

[0046] The first Nd:YAG side-pumping module 25 uses a large-aperture Nd:YAG crystal rod as the gain medium, with lateral pump energy provided by an 808nm quasi-continuous-wave semiconductor laser. The Nd:YAG crystal rod in the first Nd:YAG side-pumping module 25 can have an aperture of 7mm, a lateral pump peak power of 5600W, and a pump pulse width of 230μs. Under the control of a synchronous control signal, it maintains timing matching with the input sub-nanosecond pulse to achieve high energy amplification of the input fundamental frequency pulse. Synchronous pump control ensures that the pump pulse establishes the optimal population inversion state when the seed pulse arrives at the gain medium, enabling each seed pulse to efficiently extract stored energy from the gain medium and achieve the highest single-pulse energy output. The selection of the pump pulse width must consider the balance between population inversion accumulation time and thermal effects: excessively long pump pulses will cause excessive heat accumulation in the gain medium, leading to thermal lensing and thermal depolarization, degrading beam quality; excessively short pump pulses will not accumulate enough population inversion, resulting in insufficient gain. The pump pulse width of 230 μs is matched with the repetition frequency of the seed pulse, which controls the thermal effect while ensuring sufficient population inversion.

[0047] The third plano-convex lens 26, the first vacuum tube 27, and the fourth plano-convex lens 28 together constitute the first-stage relay transmission and vacuum space filtering structure. This structure is used to maintain the beam propagation state and spatial quality of high-energy pulses during dual-pass amplification, and to reduce the nonlinear effects and penetration risk of high-peak-power lasers in air. The third plano-convex lens 26 and the fourth plano-convex lens 28 form a 4f relay transmission structure. The focal length design of the 4f structure ensures that the light field distribution at the output end face of the first Nd:YAG side-pump module 25 is precisely imaged onto the first 0° high-reflection mirror 30. This ensures that the returned light precisely overlaps with the light field distribution during its first pass when it re-enters the first Nd:YAG side-pump module 25, achieving efficient dual-pass amplification.

[0048] The first vacuum tube 27 is located at the focal point between the third plano-convex lens 26 and the fourth plano-convex lens 28, and has a spatial filter aperture inside. The 4f relay transmission structure precisely positions the beam focal point inside the first vacuum tube 27. High-peak-power lasers have the highest intensity at the focal point; focusing in air would cause strong nonlinear effects (such as stimulated Brillouin scattering and air breakdown). The first vacuum tube 27 maintains a vacuum environment in the focal region, eliminating the influence of air nonlinear effects and effectively reducing the risk of breakdown. The spatial filter aperture, located at the focal point, utilizes the spatial frequency distribution characteristics of the beam on the focal plane to allow the low spatial frequency fundamental mode beam to pass through while blocking high spatial frequency scattered and parasitic light, thereby improving the spatial quality and purity of the output beam and suppressing the accumulation of small-scale beam distortion.

[0049] The first thin-film polarizer 23, the first Faraday rotator 29, and the first 0° high-reflection mirror 30 are used together to construct the first-stage side-pumped dual-pass amplification and output separation optical path. The first 0° high-reflection mirror 30 is used to fold the fundamental frequency light after single-pass amplification back to the first Nd:YAG side-pumping module 25 to achieve dual-pass amplification; the first Faraday rotator 29 is used to cause the returned light to undergo non-reciprocal polarization rotation relative to the incident light, and helps to compensate for the thermally induced depolarization effect generated under high-power pump amplification conditions; the first thin-film polarizer 23 is used to separate and export the returned pulse after dual-pass amplification from the original input optical path.

[0050] The specific working process is as follows: After the pulse is transmitted through the first thin-film polarizer 23, its linear polarization state is converted to circular polarization by the first quarter-wave plate 24. The first single-pass amplification is completed in the first Nd:YAG side-pump module 25. After transmission through the 4f structure composed of the third plano-convex lens 26, the first vacuum tube 27, and the fourth plano-convex lens 28, it passes through the first Faraday rotator 29 and is reflected by the first 0° high-reflection mirror 30. The second single-pass amplification is completed again by passing through the first Faraday rotator 29, the 4f structure, and the first Nd:YAG side-pump module 25. After passing through the first quarter-wave plate 24, the polarization state rotates 90° to become orthogonal linear polarization. The return pulse is reflected and separated by the first thin-film polarizer 23, completing the dual-pass amplification output. The non-reciprocal characteristic of the first Faraday rotator 29 causes the polarization rotation amounts after the two passes to be superimposed, ensuring that the return pulse is effectively separated at the first thin-film polarizer 23. Simultaneously, it compensates for polarization state disturbances caused by thermal depolarization in the gain crystal, maintaining the polarization purity of the amplified pulse. Thermally induced depolarization is a significant and detrimental effect in high-power solid-state laser amplification. Under high-power pumping, large-aperture gain crystals generate non-uniform thermal distribution, and thermal stress induces birefringence, causing beams passing through different positions to accumulate different polarization states, resulting in deterioration of the output beam's polarization uniformity (depolarization). The first Faraday rotator 29 applies a non-reciprocal polarization rotation to the returning light, allowing the depolarization effect to be mutually compensated during the two passes through the gain crystal. This effectively restores the polarization uniformity of the output beam, ensuring that the polarization purity of the output pulse after double-pass amplification meets the requirements of subsequent frequency conversion.

[0051] The pulsed light, after being amplified by the first-stage side-pumped double-pass amplifier, is separated and extracted by the first thin-film polarizer 23, and then deflected by the sixth 45° reflector 31 before entering the second-stage side-pumped double-pass amplifier. The sixth 45° reflector 31 realizes the spatial deflection of the optical path, guiding the pulsed light output from the first-stage amplifier to the incident end of the second-stage amplifier, thus achieving optical path connection between the two amplifier stages. The second-stage side-pumped double-pass amplifier has essentially the same working principle and optical path configuration as the first-stage side-pumped double-pass amplifier, and is used to further enhance the fundamental frequency pulse energy based on the first-stage main amplification. The second-stage side-pumped dual-pass amplifier, along the seed light propagation direction, sequentially includes a fifth half-wave plate 32, a third optical isolator 33, a sixth half-wave plate 34, a second plano-concave lens 35, a fifth plano-convex lens 36, a seventh 45° reflector 37, an eighth 45° reflector 38, a second thin-film polarizer 39, a second quarter-wave plate 40, a second Nd:YAG side-pump module 41, a sixth plano-convex lens 42, a second vacuum tube 43, a seventh plano-convex lens 44, a second Faraday rotator 45, and a second 0° high-reflection mirror 46.

[0052] Among them, the fifth half-wave plate 32, the third optical isolator 33, and the sixth half-wave plate 34 are used for polarization control and reverse light isolation before the second-stage amplification. Their working principle is the same as that of the third half-wave plate 16, the second optical isolator 17, and the fourth half-wave plate 18 at the front end of the first stage. They are used to block the back-propagating light in the second-stage amplification link from returning to the first-stage amplification optical path, protecting the stable operation of the first-stage amplifier and the front-end optical path. The second plano-concave lens 35 and the fifth plano-convex lens 36 are used to further expand, collimate, and match the aperture of the pulse light after the first-stage side-pumped dual-pass amplification, so that the aperture of the incident beam matches the effective light-passing aperture of the second Nd:YAG side-pumping module 41, in order to adapt to the actual situation that the beam aperture and pulse energy have increased after the first-stage amplification. The seventh 45° reflector 37 and the eighth 45° reflector 38 are used for optical path folding and reasonable arrangement. The optical components of the second-stage amplifier are arranged as follows: the second thin-film polarizer 39, the second Faraday rotator 45, and the second 0° high-reflection mirror 46 are used together to construct the second-stage side-pumped double-pass amplification and output separation optical path, and the working principle is the same as that of the first stage; the second quarter-wave plate 40 is used to amplify the pulse in the second Nd:YAG side-pumped module 41 in a circularly polarized or approximately circularly polarized manner to reduce the nonlinear effects and optical damage risk under high peak power conditions; the sixth plano-convex lens 42, the second vacuum tube 43, and the seventh plano-convex lens 44 together constitute the second-stage 4f relay transmission and vacuum space filtering structure, which is used to maintain the spatial quality of high-energy pulses in the second-stage double-pass amplification process and reduce air nonlinear effects and penetration risk, and the working principle is the same as that of the third plano-convex lens 26, the first vacuum tube 27, and the fourth plano-convex lens 28 of the first stage.

[0053] The second Nd:YAG side-pumping module 41 uses a large-diameter Nd:YAG crystal rod as the gain medium. Its effective aperture is larger than that of the first Nd:YAG side-pumping module 25 to accommodate the larger beam diameter and higher pulse energy after the first-stage amplification. The Nd:YAG crystal rod in the second Nd:YAG side-pumping module 41 can have a diameter of 11mm, a side-pumping peak power of 7000W, and a pump pulse width of 230μs. Under the control of the synchronization control signal, it maintains timing matching with the input sub-nanosecond pulse to achieve further high-energy amplification of the input fundamental frequency pulse. The second-stage amplifier uses a larger-diameter gain crystal rod. On the one hand, this is to match the increased beam diameter after the first-stage amplification, avoiding diffraction effects and beam quality degradation caused by the beam edge being truncated by the gain crystal. On the other hand, it is to store more population inversion energy in a larger gain volume, supporting higher pulse energy extraction. At the same time, by increasing the effective aperture, the optical power density is reduced, thus reducing the risk of optical damage. By increasing the effective aperture during the second-stage side-pumped amplification process, the pulse energy can be increased while the optical power density is reduced, thereby minimizing the risk of optical damage and balancing amplification stability and output beam quality. Through two-stage side-pumped dual-pass main amplification, this embodiment can increase the energy of a 1064nm fundamental frequency laser pulse to the level of several hundred millijoules, achieving stable output of high-energy sub-nanosecond fundamental frequency laser.

[0054] The specific optical path structure of the frequency conversion unit is as follows: The high-energy 1064nm fundamental frequency laser output from the side-pumped main amplification unit is deflected by the ninth 45° reflecting mirror 47 before entering the frequency conversion unit. The frequency conversion unit includes a seventh half-wave plate 48, an eighth plano-convex lens 49, a third vacuum tube 50, a ninth plano-convex lens 51, a frequency doubling unit 52, a summing unit 53, a second dichroic mirror 54, and a third dichroic mirror 55. The seventh half-wave plate 48 is used to adjust the polarization direction of the fundamental frequency laser entering the frequency conversion unit, so that the polarization direction of the fundamental frequency laser matches the phase-matching polarization direction of the nonlinear frequency conversion crystal in the frequency doubling unit 52 and the summing unit 53, thereby improving the frequency doubling and summing efficiency. The frequency doubling and summing processes have strict requirements on the polarization direction of the input light. The polarization direction of the input light must be precisely aligned with the phase-matching polarization direction of the crystal; otherwise, the phase-matching condition will deviate from the optimal value, and the conversion efficiency will decrease significantly. The rotation angle of the seventh half-wave plate 48 can be precisely adjusted according to the phase-matching polarization direction of the nonlinear crystal to ensure that the fundamental frequency laser is incident on the nonlinear crystal in the optimal polarization state. The polarization stability output characteristics of the front-end microchip oscillator are of great value here: because the polarization direction of the seed pulse is highly stable, the polarization purity of the output fundamental laser after multi-stage amplification is also guaranteed, enabling the seventh half-wave plate 48 to precisely adjust the polarization direction to the angle required for phase matching of the nonlinear crystal and maintain stability during long-term operation, which is conducive to maintaining an efficient and stable frequency conversion process.

[0055] The eighth plano-convex lens 49, the third vacuum tube 50, and the ninth plano-convex lens 51 together constitute the beam transmission and shaping section before frequency conversion. This section performs relay imaging, spatial filtering, and beam shaping on the fundamental frequency laser after main amplification, adjusting the beam intensity distribution entering the frequency doubling unit 52 and the sum-frequency unit 53 to meet the requirements of efficient frequency doubling and sum-frequency conversion. This also improves frequency conversion efficiency while reducing the risk of damage to the nonlinear crystal and related optical components. The eighth plano-convex lens 49 and the ninth plano-convex lens 51 form a 4f relay imaging structure, accurately imaging the light field distribution at the output of the side-pumped main amplification unit onto the nonlinear crystal. This ensures that the beam entering the nonlinear crystal has good spatial uniformity and a regular beam profile, which is beneficial for uniformly satisfying the phase matching condition across the entire beam cross-section, thus improving overall conversion efficiency. The third vacuum tube 50 is located at the focal point between the eighth plano-convex lens 49 and the ninth plano-convex lens 51. It reduces the nonlinear effects and penetration risk of high peak power lasers in air. Its internal spatial filtering aperture also removes scattered light, improving the purity of the beam entering the nonlinear crystal and reducing interference from scattered light on the frequency conversion process.

[0056] The frequency doubling unit 52 converts the 1064nm fundamental frequency laser into a 532nm frequency-doubled laser, and the sum-frequency unit 53 performs a sum-frequency transformation between the 1064nm fundamental frequency laser and the 532nm frequency-doubled laser to obtain a 355nm ultraviolet laser output. Both the frequency doubling unit 52 and the sum-frequency unit 53 include a nonlinear frequency conversion crystal, an angle adjustment structure, and a high-precision temperature control structure. The angle adjustment structure precisely adjusts the phase matching angle of the nonlinear frequency conversion crystal, while the high-precision temperature control structure precisely controls the temperature of the nonlinear frequency conversion crystal to further optimize the frequency conversion efficiency and ensure the stability of the output laser. The angle adjustment structure typically employs a high-precision rotary stage, capable of adjusting the phase matching angle of the crystal with arcsecond-level precision, ensuring that the phase matching condition is precisely met and achieving the highest conversion efficiency. The high-precision temperature control structure maintains the temperature of the nonlinear crystal at a set value through a precision temperature control element, compensating for crystal temperature drift caused by changes in ambient temperature and laser thermal effects, preventing deviations in the phase matching condition due to temperature changes, and ensuring the stability of the conversion efficiency during long-term operation.

[0057] The physical mechanism of frequency doubling is based on the second-order nonlinear polarization effect: in a nonlinear crystal with a non-centrosymmetric structure, a strong laser field drives bound electrons in the crystal to generate a nonlinear polarization response. The second-order component of the nonlinear polarization radiates at a frequency twice that of the fundamental frequency, producing frequency-doubled photons. Frequency doubling requires a phase-matching condition, meaning the phase mismatch between the fundamental and frequency-doubled light must be as small as possible to ensure that the frequency-doubled radiation generated by the nonlinear polarization coherently superimposes over the length of the crystal, achieving efficient energy conversion. The physical mechanism of sum-frequency doubling is similar to that of frequency doubling, but involves two different frequencies of input light (1064nm fundamental light and 532nm frequency-doubled light) generating 355nm ultraviolet photons with a frequency equal to the sum of the two frequencies through second-order nonlinear polarization. Sum-frequency doubling also requires a phase-matching condition, and the 1064nm fundamental light and 532nm frequency-doubled light must precisely overlap in time and space to maximize sum-frequency conversion efficiency. By precisely adjusting the crystal angle and temperature, optimal phase matching conditions can be maintained over a wide power range, achieving efficient and stable doubling and sum-frequency conversion.

[0058] The second dichroic mirror 54 and the third dichroic mirror 55 are used to separate and extract laser light of different wavelengths after frequency conversion. The surfaces of the second dichroic mirror 54 and the third dichroic mirror 55 are coated with a beam-splitting coating, exhibiting high transmission characteristics for 1064nm and 532nm lasers and high reflectivity for 355nm ultraviolet lasers, thereby achieving effective beam splitting output of the target 355nm ultraviolet laser. Through the cascaded beam splitting of the two dichroic mirrors, the 355nm ultraviolet laser in the frequency-converted mixed beam can be fully separated from the residual 1064nm fundamental frequency light and 532nm frequency-doubled light, obtaining a high-purity 355nm ultraviolet laser output. The residual 1064nm and 532nm laser light, after being transmitted by the dichroic mirrors, can be recovered or absorbed by a beam blocker to prevent them from entering subsequent optical paths and causing interference. Through the above-described frequency doubling, summing, and beam splitting extraction processes, this embodiment can obtain a 355nm sub-nanosecond ultraviolet laser output at the level of hundreds of millijoules.

[0059] At the operational level, the complete operational process of the polarization-stabilized high-energy sub-nanosecond laser in this embodiment is as follows: First, bidirectional differential temperature control is applied to the Nd:YAG / Cr:YAG microcrystal 3 in the polarization-stabilized microcrystal oscillator: the temperature of the microcrystal is adjusted in two orthogonal directions, horizontal and vertical, by the temperature control structure 4, so that the temperature on both sides (left and right or top and bottom) in the same direction is basically the same, while a set temperature difference is formed between the two orthogonal directions, thereby constructing a controlled asymmetric thermal field inside the microcrystal. Based on the real-time temperature information detected by the upper thermistor 4-5, right thermistor 4-6, lower thermistor 4-7, and left thermistor 4-8, the temperature control board performs independent closed-loop feedback control on the upper TEC4-1, right TEC4-2, lower TEC4-3, and left TEC4-4 respectively, precisely maintaining the set temperature difference in the orthogonal directions and ensuring the stable construction of the asymmetric thermal field. This asymmetric thermal field further modulates the heat distribution, thermal stress, and polarization-related characteristics within the microchip crystal through thermally induced birefringence, altering the competitive relationship between the two orthogonal polarization states within the cavity. This results in one polarization state having a higher equivalent net gain than the other, causing the higher-gain polarization state to oscillate preferentially and maintain a stable output. When the horizontal temperature is higher than the vertical temperature, the seed pulse output by the microchip oscillator can be stabilized in the first polarization direction; when the vertical temperature is higher than the horizontal temperature, the seed pulse output by the microchip oscillator can be stabilized in the second polarization direction, and the second stable polarization direction is orthogonal to the first stable polarization direction. By changing the temperature relationship between the two orthogonal directions (horizontal and vertical), the output polarization direction can be switched between the two orthogonal stable polarization directions while maintaining a stable polarization output, providing a high-quality seed pulse with controllable polarization direction for subsequent amplification and frequency conversion processes.

[0060] Subsequently, the first semiconductor laser pump source 1 provides end-face pump energy to the Nd:YAG / Cr:YAG microcrystal 3. After being shaped and spot-matched by the first pump light shaping system 2, the pump light is coupled into the Nd:YAG gain medium, causing the Nd:YAG to absorb the pump energy and form population inversion. Under the passive Q-switching of the Cr:YAG saturable absorber, the stored energy in the cavity is released in a short time, thereby generating a polarization-stable sub-nanosecond seed pulse. By adjusting the pump spot size, pump energy, and microcrystal parameters, the output energy, pulse width, and stability of the seed pulse can be optimized to ensure that the seed pulse characteristics meet the requirements of subsequent multi-stage amplification and frequency conversion. Further, the polarization-stable sub-nanosecond seed pulse is injected into the end-face pump pre-amplification unit. The seed pulse is isolated and polarization matched by an optical isolation system consisting of a first half-wave plate 5, a first spatial optical isolator 6, and a second half-wave plate 7. After being deflected by a first 45° reflector 8 and a second 45° reflector 9, and then spot-matched by a first plano-convex lens 10, it enters an Nd:YAG gain crystal 11. Under the synchronous end-face pumping provided by the second semiconductor laser pump source 14, it undergoes single-pass pre-amplification to improve the seed pulse energy and the input signal level of the subsequent main amplification unit, and to reduce the risk of unstable effects such as self-excited oscillation and parasitic oscillation during high-gain amplification.

[0061] Subsequently, the pre-amplified fundamental frequency pulse from the end-pump is deflected by a third 45° reflector and injected into the side-pump main amplification unit. The fundamental frequency pulse is then sequentially amplified by the first-stage side-pump dual-pass amplifier and the second-stage side-pump dual-pass amplifier. In the first-stage side-pumped dual-pass amplification process, the pulse undergoes polarization control and isolation processing via the third half-wave plate 16, the second optical isolator 17, and the fourth half-wave plate 18. It is then expanded and collimated by the first plano-concave lens 19 and the second plano-convex lens 20. After being refracted by the fourth 45° reflector 21 and the fifth 45° reflector 22, it is transmitted through the first thin-film polarizer 23 into the dual-pass amplification optical path. After being converted to circular polarization by the first quarter-wave plate 24, it completes the first single-pass amplification in the first Nd:YAG side-pumped module 25. After transmission through the 4f relay transmission structure composed of the third plano-convex lens 26, the first vacuum tube 27, and the fourth plano-convex lens 28, it is reflected back by the first 0° high-reflection mirror 30 via the first Faraday rotator 29. It then undergoes a second single-pass amplification via the 4f structure and the first Nd:YAG side-pumped module 25. After polarization rotation by the first quarter-wave plate 24, the returned pulse is reflected and separated by the first thin-film polarizer 23, completing the first-stage dual-pass amplification. After being deflected by the sixth 45° mirror 31, the laser enters the second-stage side-pumped double-pass amplifier. Following the same double-pass amplification configuration as the first stage, the second-stage energy amplification is completed. Polarization control and isolation are achieved through the fifth half-wave plate 32, the third optical isolator 33, and the sixth half-wave plate 34. Beam expansion and collimation are performed by the second plano-concave lens 35 and the fifth plano-convex lens 36. Deflected by the seventh 45° mirror 37 and the eighth 45° mirror 38, the second-stage double-pass amplification is completed under the synergistic action of the second thin-film polarizer 39, the second quarter-wave plate 40, the second Nd:YAG side-pump module 41, the sixth plano-convex lens 42, the second vacuum tube 43, the seventh plano-convex lens 44, the second Faraday rotator 45, and the second 0° high-reflection mirror 46. The output pulse is then separated and exported by the second thin-film polarizer 39. After two stages of side-pumped double-pass amplification, a 1064nm fundamental frequency laser output in the hundreds of millijoules range can be obtained.

[0062] Finally, the high-energy 1064nm fundamental frequency laser output from the side-pumped main amplification unit is refracted by the ninth 45° reflector 47 and injected into the frequency conversion unit. After the pulsed light is polarized by the seventh half-wave plate 48, it undergoes relay imaging, spatial filtering, and beam shaping in the beam transmission and shaping section composed of the eighth plano-convex lens 49, the third vacuum tube 50, and the ninth plano-convex lens 51. Subsequently, it enters the frequency doubling unit 52 and the summing unit 53 in sequence. First, a portion of the 1064nm fundamental frequency laser is frequency-doubled to generate 532nm laser, and then the remaining 1064nm fundamental frequency laser is summed with the 532nm laser to obtain 355nm ultraviolet laser output. After being split and guided by the second dichroic mirror 54 and the third dichroic mirror 55, a high-purity 355nm target ultraviolet laser output is achieved.

[0063] In the above-described working method, differentiated temperature control is the core mechanism for achieving stable polarization output. The temperature control structure 4 applies a controlled asymmetric thermal field to the Nd:YAG / Cr:YAG microcrystal 3, utilizing the thermally induced birefringence effect to alter the phase delay and competition relationship of the polarization eigenstates within the cavity. This allows a specific polarization state to gain a net gain advantage within the cavity, thereby achieving stable single-polarization oscillation output. This mechanism does not rely on a fixed crystal tangential relationship or precise alignment of the pump polarization direction. Instead, it achieves flexible control of the polarization state through programmable temperature control, exhibiting outstanding advantages such as compact structure, high integration, and good engineering adaptability.

[0064] Compared with existing technologies that achieve polarization control through specific crystal orientation, polarization pumping, or polarization structure, this embodiment has significant advantages: The scheme using a specific tangential crystal material (e.g., replacing conventional

[100] tangential Cr:YAG with

[110] tangential Cr:YAG, or replacing conventional

[111] tangential Nd:YAG with

[100] tangential Nd:YAG, and jointly optimizing the crystal orientation combination of Nd:YAG and Cr:YAG) primarily relies on a pre-defined crystal orientation relationship for polarization control. Essentially, it represents a fixed preference for a specific polarization state. The polarization direction is essentially determined after crystal processing and assembly, making flexible adjustment difficult. Furthermore, the scheme also affects the crystal orientation. High precision is required in terms of bulk tangential alignment, orientation consistency, and assembly accuracy. While polarization pumping can improve output polarization characteristics to some extent, its effectiveness typically depends on the pump polarization state, the matching relationship between the pump direction and the crystal axis, and the stability of the system collimation conditions. Instability in transition regions may also occur between different pump polarization angles, and it is susceptible to factors such as thermal depolarization. Using polarization-type structures such as Brewster angle interfaces usually requires specialized design and fabrication of the internal interface configuration of the bonded crystal or the microchip oscillator cavity configuration. Although this enhances the polarization selectivity within the cavity, it increases the complexity of fabrication and assembly, and imposes certain limitations on the miniaturization and engineering implementation of the device. This embodiment achieves stable polarization output within a microchip laser structure without relying on a fixed crystal orientation, and allows for controllable switching of the output polarization direction by changing the temperature relationship of the orthogonal directions. Compared to existing technologies that typically only provide a single fixed polarization direction and are difficult to flexibly control, this embodiment not only achieves stable polarization output but also enables controllable switching of the polarization direction, thereby improving the laser's adaptability to subsequent amplification, frequency conversion, and different application scenarios. This embodiment employs an amplification architecture combining a front-end master oscillator and a subsequent power amplifier. End-pump pre-amplification effectively enhances the seed pulse energy and reduces the adverse effects of parasitic oscillations and self-excited oscillations on system stability under high-gain conditions. Two-aperture side-pumped dual-pass master amplification, combined with 4f relay transmission imaging, vacuum tube spatial filtering, and Faraday rotator depolarization compensation, achieves stepwise stable extraction of pulse energy while mitigating the effects of high-power pump thermal depolarization, reducing air penetration, and suppressing small-scale self-focusing and other nonlinear effects. This allows for a stable boost of the 1064nm fundamental frequency pulse energy to the hundreds of millijoules while maintaining good spatial beam quality. Thanks to the excellent polarization stability and output consistency of the front-end microchip seed source, this embodiment improves the stability of input conditions during subsequent nonlinear frequency conversion, thereby maintaining stable efficiency in the doubling and summing processes and reducing fluctuations in ultraviolet output energy.Ultimately, this embodiment achieves stable sub-nanosecond ultraviolet laser output with pulse energies in the hundreds of millijoules range, which can meet the requirements of high-energy precision 355nm micromachining, nonlinear optical detection, and high-energy physics applications for high-stability ultraviolet laser sources.

[0065] In terms of specific application scenarios, the polarization-stabilized high-energy sub-nanosecond 355nm ultraviolet laser provided in this embodiment demonstrates significant technical advantages and broad application prospects in the following fields.

[0066] In high-energy precision micromachining applications, 355nm ultraviolet lasers, with their high photon energy and short wavelength, match the absorption characteristics of various materials (including metals, semiconductors, ceramics, glass, and organic materials). This enables cold or near-cold material removal processes, significantly reducing the heat-affected zone (HAZ) and improving processing accuracy and surface quality. The sub-nanosecond pulse width results in extremely high peak power for each pulse, facilitating efficient photon absorption and material removal on the material surface. Simultaneously, the pulse width is shorter than the thermal relaxation time of most materials, effectively suppressing the expansion of the HAZ caused by thermal diffusion, achieving high-precision, minimized HAZ processing. The hundreds of millijoules of pulse energy provided in this embodiment, combined with high repetition rate, can achieve a high material removal rate while maintaining high processing accuracy, meeting the demands of high-end manufacturing fields such as semiconductor wafer dicing, precision glass drilling, flexible electronic substrate microstructure processing, printed circuit board precision drilling, and solar cell scribing for high-precision, high-efficiency ultraviolet laser processing. The stable polarization output characteristic ensures the uniformity of energy distribution of the processing spot in all directions, avoiding inconsistencies in processing morphology caused by polarization drift. This is beneficial for improving the consistency and repeatability of batch processing, and is of great significance in large-scale industrial production. The controllable switching function of polarization direction also provides flexible laser source configuration capabilities for polarization-related laser processing processes (such as using polarization direction to control the orientation of surface microstructures), expanding the process dimensions of laser processing.

[0067] In nonlinear optical detection and spectral analysis applications, 355nm sub-nanosecond high-energy pulsed lasers can serve as efficient pump sources, driving various nonlinear optical processes, including stimulated Raman scattering, optical parametric oscillations, and third harmonic generation, providing high-quality driving light for the generation of wide-tunable coherent light sources. In stimulated Raman scattering spectroscopy applications, the high photon energy of the 355nm pump light significantly increases the Raman scattering cross-section compared to near-infrared pumping, which is beneficial for improving Raman signal intensity and detection sensitivity. The high peak power of the sub-nanosecond pulse can effectively drive the stimulated Raman scattering process, achieving high-conversion-efficiency coherent Raman source output. The polarization-stable output characteristics ensure the polarization purity of the excitation light in Raman scattering polarization analysis experiments, providing a high-quality light source for polarization-resolved Raman spectroscopy measurements. This facilitates the differentiation of molecular vibrational modes with different symmetries through polarization Raman spectroscopy, obtaining richer molecular structure information. In optical parametric oscillation (OPO) applications, 355nm high-energy pulses can pump various nonlinear crystals (such as BBO and LBO) to generate wide-tunable coherent radiation covering the visible to near-infrared bands. This provides a tunable laser source for applications such as molecular vibration spectroscopy, lidar, and optical frequency standards. Its polarization-stable input characteristics are beneficial for improving the phase-matching stability and output power stability of the OPO process.

[0068] In laser-induced breakdown spectroscopy (LASPS) applications, the high photon energy and peak power of 355nm ultraviolet lasers combine to generate stable laser-induced plasmas on the surfaces of various materials (including metals, minerals, biological tissues, and soil samples), exciting the characteristic radiation lines of each element and enabling rapid, non-contact, in-situ elemental analysis. The sub-nanosecond pulse width facilitates the generation of high-temperature, high-density plasma plumes, improving the signal-to-background ratio of characteristic spectral lines. Pulse energy in the hundreds of millijoules range ensures reliable triggering of the breakdown process on various material surfaces, adapting to differences in breakdown thresholds among different materials. High repetition rate supports statistical averaging of multiple excitations, effectively reducing signal fluctuations caused by plasma fluctuations in single measurements and improving the accuracy and repeatability of elemental analysis. The polarization-stable output characteristics are crucial in polarization-resolved LASPS measurements, enabling the extraction of additional material information using the polarization characteristics of plasma radiation, thus improving elemental identification accuracy and quantitative analysis capabilities. This technology has broad application prospects in geological and mineral exploration, environmental pollution monitoring, cultural relic identification, food safety testing, and industrial online quality inspection.

[0069] In high-energy physics experiments, polarization-stable high-energy sub-nanosecond ultraviolet lasers can be used for photoelectron generation in photocathode electron guns, providing high-brightness, high-polarization initial electron beams for electron accelerators. Photocathode electron guns utilize the laser photoelectric effect to generate electrons from the surface of the photocathode material; the polarization characteristics of the laser directly affect the polarization of the generated electron beam. The high-polarization-purity, high-stability 355nm ultraviolet laser provided in this embodiment can generate high-quantum-efficiency photoelectron emission on the photocathode material, and the controllable switching of polarization direction allows for flexible switching of electron beams with different polarization directions, meeting the needs of high-energy physics experiments for electron beams with different polarization directions. The sub-nanosecond pulse timing structure matches the operating timing of the accelerator's radio frequency cavity, which is beneficial for generating electron beam clusters with precise and controllable timing structure, providing a high-quality initial electron source for high-brightness electron beam physics experiments. Pulse energies in the hundreds of millijoules range, combined with a high repetition rate, can generate sufficient photon flux on the photocathode, meeting the photon flux requirements for high-brightness electron beam generation and supporting the long-term stable generation and transport of high-brightness electron beams.

[0070] In lidar and atmospheric remote sensing applications, 355nm high-energy sub-nanosecond pulsed lasers can serve as the emission source for atmospheric sounding lidar, used for detecting the vertical distribution of atmospheric aerosols, clouds, ozone, and other atmospheric components. The 355nm band lies within the atmospheric transmission window, with a moderate atmospheric scattering cross-section, enabling both sufficient detection range and measurable backscattered signals. The sub-nanosecond pulse width corresponds to a decimeter-level distance resolution, meeting the requirements for high-precision atmospheric vertical distribution detection. Pulse energy in the hundreds of millijoules range ensures sufficient lidar detection range, enabling vertical detection in the troposphere and even the stratosphere. The polarization-stable output characteristics are crucial in polarization lidar applications; pure polarized emitted light is a prerequisite for high-precision depolarization ratio measurement. Depolarization ratio information can be used to distinguish between spherical and non-spherical particles (such as cloud ice crystals and water droplets), improving the accuracy of atmospheric component identification. This has significant application value in climate change research, weather forecasting, and aviation safety.

[0071] In scientific research and cutting-edge exploration applications, the polarization-stable high-energy sub-nanosecond 355nm ultraviolet laser provided in this embodiment can serve as an excitation source for various cutting-edge experimental studies. In plasma physics research, high-energy ultraviolet laser pulses can generate high-temperature, high-density plasmas in gaseous or solid targets, providing a driving light source for laboratory simulations of inertial confinement fusion-related physical processes, studying the fundamental physical laws of laser-plasma interactions, and providing basic data for fusion energy research. In quantum optics and quantum information research, polarization-stable single-mode ultraviolet lasers can be used to drive coherent manipulation of specific quantum systems. The controllable switching function of the polarization direction provides flexible excitation source configuration capabilities for quantum state preparation and measurement experiments, which is beneficial for achieving precise control of the polarization state of the excitation light in experiments such as quantum entanglement generation, quantum gate manipulation, and quantum state tomography. In materials science research, sub-nanosecond ultraviolet laser pulses can be used to study the ultrafast dynamics of materials. Through a pump-probe experimental scheme, ultraviolet laser pulses are used as pump light to excite the non-equilibrium state of the material, and other probe beams are used to track the dynamic process of the material recovering from the non-equilibrium state to the equilibrium state. This reveals the basic physical processes of the material, such as phonon dynamics, carrier relaxation, and phase transition dynamics, and provides an experimental basis for the design and optimization of novel functional materials.

[0072] It is understood that the size, doping concentration, initial transmittance of Cr:YAG, and end-face coating parameters of the Nd:YAG / Cr:YAG microcrystal 3 in the above embodiments are not limited to the specific values ​​of this embodiment. They can be adjusted accordingly based on the target output energy, pulse width, and system stability requirements, and can still achieve polarization-stable sub-nanosecond seed pulse output. It is understood that the temperature control structure 4 in the above embodiments is not limited to the specific implementation of a four-directional TEC thermistor. Other temperature control schemes capable of differentially controlling the temperature of the microcrystal along two orthogonal directions (horizontal and vertical) can also be used, as long as a controlled asymmetric thermal field can be constructed within the microcrystal to achieve polarization-stable output. It is understood that the crystal rod aperture, pump peak power, and other parameters of the first-stage side-pumped dual-pass amplifier and the second-stage side-pumped dual-pass amplifier in the above embodiments are not limited to the specific values ​​of this embodiment and can be adjusted accordingly based on the target output energy and beam quality requirements. It is understood that the nonlinear frequency conversion crystals in the frequency doubling unit 52 and the sum-frequency unit 53 in the above embodiments are not limited to a specific type. Suitable nonlinear crystals can be selected based on conversion efficiency requirements, damage thresholds, and engineering implementation conditions, as long as they can achieve efficient frequency conversion from 1064nm to 355nm. Where there is no conflict, the above technical features can be combined to achieve the optimal system configuration for different application scenarios and experimental needs.

[0073] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A polarization-stabilized high-energy sub-nanosecond laser, characterized in that, It includes a polarization-stabilized microchip oscillator, and an end-pumped pre-amplifier unit, a side-pumped main amplifier unit, and a frequency conversion unit arranged sequentially along the optical path; The polarization-stabilized microchip oscillator incorporates a microchip crystal to generate polarization-stabilized sub-nanosecond seed pulses, which serve as the front-end seed source for subsequent amplification and frequency conversion links. By implementing differentiated temperature control on the microchip crystal along two orthogonal directions (horizontal and vertical), the internal heat distribution, thermal stress, and polarization-related characteristics of the microchip crystal are adjusted, thereby changing the competitive relationship between the two orthogonal polarization states within the cavity. This results in a polarization-stabilized sub-nanosecond seed pulse output. By altering the temperature relationship between the two orthogonal directions, a polarization output with a 90-degree rotation in polarization direction and stable polarization output can be obtained. The end-face pump pre-amplification unit is used to perform end-face pump single-pass pre-amplification of sub-nanosecond seed pulses; The side-pumped main amplification unit includes at least two stages of side-pumped dual-pass amplifiers arranged sequentially along the optical path, which are used to progressively boost the energy of the fundamental frequency pulse after end-face pumping pre-amplification to obtain high-energy fundamental frequency laser output. The frequency conversion unit is used to perform frequency doubling and sum-frequency conversion on the high-energy fundamental frequency laser to obtain high-energy sub-nanosecond ultraviolet laser output.

2. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 1, characterized in that, The polarization-stabilized microchip oscillator includes a first semiconductor laser pump source, a first pump light shaping system, an Nd:YAG / Cr:YAG microchip crystal, and a temperature control structure thermally coupled to the Nd:YAG / Cr:YAG microchip crystal. The first semiconductor laser pump source is used to provide the pump energy required for laser oscillation to the Nd:YAG / Cr:YAG microcrystal wafers. The first pump light shaping system is located between the first semiconductor laser pump source and the Nd:YAG / Cr:YAG microcrystal, and is used to shape and couple the pump light output from the first semiconductor laser pump source. Nd:YAG / Cr:YAG microcrystals are microcrystals used to construct passively Q-switched microcrystal laser oscillation cavities to generate sub-nanosecond seed pulses; The temperature control structure is used to regulate the temperature of Nd:YAG / Cr:YAG microcrystals along two orthogonal directions, horizontal and vertical.

3. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 2, characterized in that, The Nd:YAG / Cr:YAG microcrystal is a bonded composite microcrystal in which Nd:YAG is used as a gain medium and Cr:YAG is used as a saturable absorber. The front and back surfaces of the Nd:YAG / Cr:YAG microcrystal are coated with a film structure to form a monolithic microcrystal laser oscillation cavity.

4. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 2, characterized in that, The temperature control structure includes an upper TEC, a right TEC, a lower TEC, a left TEC, an upper thermistor, a right thermistor, a lower thermistor, a left thermistor, a copper plating mounting base, and a temperature control board; The upper TEC, right TEC, lower TEC, and left TEC are distributed along the four directions of the Nd:YAG / Cr:YAG microcrystal, which is used to differentiate the temperature of the Nd:YAG / Cr:YAG microcrystal along two orthogonal directions. The upper thermistor, right thermistor, lower thermistor, and left thermistor are used to detect temperature information in the corresponding direction in real time. Copper-plated mounting bases are used to support and fix Nd:YAG / Cr:YAG microcrystal wafers; The temperature control board is used to provide feedback control to the corresponding TEC based on the deviation between the detected temperature and the set temperature.

5. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 4, characterized in that, The right-side TEC and left-side TEC correspond to the horizontal direction of the Nd:YAG / Cr:YAG microcrystals, while the upper-side TEC and lower-side TEC correspond to the vertical direction of the Nd:YAG / Cr:YAG microcrystals. The temperatures on both sides of the same orthogonal direction are kept consistent, while the temperatures between the horizontal and vertical orthogonal directions are set differently, so that an asymmetric temperature distribution is formed inside the Nd:YAG / Cr:YAG microcrystal, which in turn generates an equivalent thermal stress difference between the two orthogonal directions. Under the action of the elasto-optic effect, the equivalent thermal stress difference causes the equivalent refractive index difference in two orthogonal directions, which in turn produces a polarization-dependent phase delay. The polarization-dependent phase delay alters the phase delay, eigenaxis direction, and non-degeneracy of the two orthogonal polarization eigenstates within the cavity. Combined with the difference in polarization-dependent gain and loss within the cavity, this changes the equivalent net gain and competition relationship between the two orthogonal polarization states. When the horizontal temperature is higher than the vertical temperature, the output seed pulse is stabilized in the first stable polarization direction; when the vertical temperature is higher than the horizontal temperature, the output seed pulse is stabilized in the second stable polarization direction, which is orthogonal to the first stable polarization direction. When the temperature relationship between the horizontal and vertical directions changes, the output polarization direction switches between two orthogonal stable polarization directions.

6. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 1, characterized in that, The end-pump pre-amplification unit includes a first half-wave plate, a first spatial optical isolator, a second half-wave plate, a first plano-convex lens, a first dichroic mirror, a second pump light shaping system, a second semiconductor laser pump source, and an Nd:YAG gain crystal. The first half-wave plate, the first spatial optical isolator, and the second half-wave plate constitute an optical isolation system, which is used to block the back-propagating light and parasitic reflected light in the subsequent amplification link from entering the front-end optical path, and to match and adjust the polarization state of the seed pulse. The first plano-convex lens is used for focusing and spot matching of the sub-nanosecond seed pulse; The pump light output from the second semiconductor laser pump source is shaped by the second pump light shaping system and then coupled into the Nd:YAG gain crystal by the first dichroic mirror to perform end-face pumping on the Nd:YAG gain crystal. The Nd:YAG gain crystal is used for single-pass pre-amplification of the input sub-nanosecond seed pulse.

7. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 1, characterized in that, The side-pumped main amplification unit includes a first-stage side-pumped dual-pass amplifier and a second-stage side-pumped dual-pass amplifier cascaded along the optical path. The first-stage side-pumped dual-pass amplifier, along the seed light propagation direction, includes, in sequence, a third half-wave plate, a second optical isolator, a fourth half-wave plate, a first plano-concave lens, a second plano-convex lens, a first thin-film polarizer, a first quarter-wave plate, a first Nd:YAG side-pump module, a third plano-convex lens, a first vacuum tube, a fourth plano-convex lens, a first Faraday rotator, and a first 0° high-reflection mirror. The second-stage side-pumped dual-pass amplifier, along the seed light propagation direction, includes, in sequence, a fifth half-wave plate, a third optical isolator, a sixth half-wave plate, a second plano-concave lens, a fifth plano-convex lens, a second thin-film polarizer, a second quarter-wave plate, a second Nd:YAG side-pump module, a sixth plano-convex lens, a second vacuum tube, a seventh plano-convex lens, a second Faraday rotator, and a second 0° high-reflection mirror.

8. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 7, characterized in that, The first plano-concave lens and the second plano-convex lens are used to adjust the aperture, collimate and spatially shape the pulse light input to the first-stage side-pumped dual-pass amplifier so that the size and propagation state of the incident beam match the effective light transmission area of ​​the first Nd:YAG side-pump module. The second plano-concave lens and the fifth plano-convex lens are used to adjust the aperture, collimate and spatially shape the pulse light input to the second-stage side-pumped dual-pass amplifier so that the size and propagation state of the incident beam match the effective light transmission area of ​​the second Nd:YAG side-pump module. The first and second quarter-wave plates are used to coordinate the polarization evolution in the dual-path optical system and reduce the nonlinear accumulation effect of high peak power laser in the gain medium and related optical components. The third plano-convex lens, the first vacuum tube, and the fourth plano-convex lens together constitute the first-stage relay transmission and vacuum space filtering structure. The sixth plano-convex lens, the second vacuum tube, and the seventh plano-convex lens together constitute the second-stage relay transmission and vacuum space filtering structure. The first and second Faraday rotators are used to compensate for the thermally induced depolarization effect generated under high pump power amplification conditions. The first 0° high-reflection mirror and the second 0° high-reflection mirror are used to fold the amplified fundamental frequency light back to the corresponding gain module to complete the dual-pass amplification. The first and second thin-film polarizers are used to separate and export the amplified return pulse from the original input optical path.

9. The polarization-stabilized high-energy sub-nanosecond laser as described in claim 1, characterized in that, The frequency conversion unit includes a seventh half-wave plate, an eighth plano-convex lens, a third vacuum tube, a ninth plano-convex lens, a frequency doubling unit, a frequency summing unit, a second dichroic mirror, and a third dichroic mirror; The seventh half-wave plate is used to adjust the polarization direction of the fundamental frequency laser entering the frequency conversion unit, so that the polarization direction of the fundamental frequency laser is adapted to the phase-matching polarization direction of the nonlinear frequency conversion crystal in the frequency doubling unit and the sum-frequency unit; The eighth plano-convex lens, the third vacuum tube, and the ninth plano-convex lens together constitute the beam transmission and shaping section before frequency conversion, which is used for relay imaging, spatial filtering, and beam shaping of the fundamental frequency laser after main amplification. The frequency doubling unit is used to convert the fundamental frequency laser into a frequency-doubled laser; The sum-frequency unit is used to perform a sum-frequency transformation between the fundamental frequency laser and the frequency-doubled laser to obtain ultraviolet laser output; The second and third dichroic mirrors are used to separate and extract laser light of different wavelengths after frequency conversion.

10. A method for operating a polarization-stabilized high-energy sub-nanosecond laser, characterized in that, Using a polarization-stabilized high-energy sub-nanosecond laser as described in any one of claims 1 to 9, the process includes the following steps: Differential temperature control is applied to the microchip crystal in the polarization-stabilized microchip oscillator along two orthogonal directions, namely horizontal and vertical, to form a controlled asymmetric thermal field inside the microchip crystal, thereby adjusting the competitive relationship between the two orthogonal polarization states in the cavity and generating polarization-stabilized sub-nanosecond seed pulses. By changing the temperature relationship between the two orthogonal directions, horizontal and vertical, the output polarization direction is rotated by 90 degrees while maintaining a stable output. By using a semiconductor laser pump source to provide end-face pump energy to a microcrystalline wafer, the gain medium absorbs the energy to form population inversion, and combined with the passive Q-switching mechanism of a saturable absorber, a fundamental frequency sub-nanosecond seed pulse is generated. A polarization-stable sub-nanosecond seed pulse is injected into the end-face pump pre-amplification unit, and single-pass pre-amplification is performed under synchronous end-face pumping. The fundamental frequency pulse, which is pre-amplified by end face pumping, is injected into the side-pumped main amplification unit and then undergoes two stages of side-pumped dual-pass amplification process to increase the energy step by step. The high-energy fundamental frequency laser output from the side-pumped main amplification unit is injected into the frequency conversion unit, and frequency doubling and sum-frequency conversion are performed sequentially to obtain a stable high-energy sub-nanosecond ultraviolet laser output.

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