Double-crystal regeneration amplification system

Abstract

The invention relates to the technical field of ultrafast laser, and discloses a bicrystal regeneration amplification system. Comprising an optical resonant cavity, two Yb: KGW crystals are adopted as gain media in the optical resonant cavity, seed pulses sequentially penetrate through the two Yb: KGW crystals in each circulation when oscillating back and forth in the optical resonant cavity, energy is extracted from gains jointly provided by the two Yb: KGW crystals, and injected pumping energy and generated thermal loads are dispersed on the two Yb: KGW crystals; by adjusting the physical cavity length of the optical resonant cavity, the time for the optical pulse to go back and forth for a circle in the cavity is longer than the starting time required by the Pockels cell in the cavity from pressurization to establishment of stable quarter-wave voltage before the seed pulse returns to the Pockels cell for the first time; two concave reflectors are arranged in the optical resonant cavity, and light beam distortion caused by the thermal lens effect of the Yb: KGW crystal is compensated under high-power pumping.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of ultrafast laser technology, and particularly relates to a double-crystal regenerative amplification system. BACKGROUND

[0002] In the field of ultrafast laser technology, regenerative amplification lasers are widely used in industrial precision machining, medical surgery and other scenarios due to their ability to achieve high-energy pulse output. Although traditional single-crystal regenerative amplification technology improves the signal-to-noise ratio through an acoustic-optic mode selection module, the single-gain-crystal structure has a fundamental defect: the energy storage and energy extraction capacity of a single crystal is limited, and the output pulse energy is usually limited to below the millijoule level. Moreover, under high-power pumping, thermal effects are significant, which can easily cause thermal lens effects, leading to instability of the resonant cavity, degradation of beam quality, and even damage to the crystal. Although the traveling wave amplification scheme can improve the energy, it requires multiple stages of amplification, resulting in a complex system, a large volume, high difficulty in mode matching, and large optical loss, making it difficult to balance high energy and beam quality. SUMMARY

[0003] The present application provides a double-crystal regenerative amplification system, which aims to solve the problem in the prior art that there is no design for achieving distributed gain and heat load dispersion through a double-crystal series configuration, and there is no scheme for optimizing the cavity length for the Pockels cell response time to ensure timing matching.

[0004] The present application provides a double-crystal regenerative amplification system, which comprises: An optical resonant cavity, two Yb:KGW crystals are used as gain media in the optical resonant cavity, and the two Yb:KGW crystals are placed in series. When the seed pulse oscillates back and forth in the optical resonant cavity, it passes through the two Yb:KGW crystals in each cycle, extracts energy from the gain provided by the two Yb:KGW crystals, and the injected pump energy and generated heat load are dispersed on the two Yb:KGW crystals; By adjusting the physical cavity length of the optical resonant cavity, the time for the optical pulse to make one round trip in the cavity is greater than the startup time required for the Pockels cell to establish a stable quarter-wave voltage from pressurization, so that a stable regenerative amplification process is realized before the seed pulse returns to the Pockels cell for the first time. Two concave mirrors are arranged in the optical resonant cavity, which compensate for the beam distortion caused by the thermal lens effect of the Yb:KGW crystal under high-power pumping.

[0005] In some embodiments, the seed pulse passes through the gain medium twice in one cavity cycle, and the two Yb:KGW crystals are placed in series to multiply the gain energy in the cavity, thereby improving the extraction efficiency of single-pulse energy.

[0006] In some embodiments, the injected pump energy and its generated heat load are naturally distributed to two Yb:KGW crystals, each of which bears a reduced pump energy and heat load compared to a single Yb:KGW crystal.

[0007] In some embodiments, the physical cavity length of the optical resonator is set to 2.2 meters, ensuring that the time for a light pulse to make one round trip in the cavity is greater than the start-up time required for the Pockels cell to go from pressurized to establish a steady quarter-wave voltage, which includes the rise time, fall time, and temporal jitter.

[0008] In some embodiments, the radii of curvature of the two concave mirrors are optimally matched with the incident seed light mode, ensuring that the spot size is always optimal within the Yb:KGW crystals.

[0009] In some embodiments, the radii of curvature of the two concave mirrors are 1000 mm.

[0010] In some embodiments, the concave mirrors with a radius of curvature of 1000 mm provide a corresponding dynamic adjustment range for the resonator, which can compensate for beam distortion caused by the thermal lens effect of the Yb:KGW crystals under high-power pumping.

[0011] In some embodiments, each mirror in the optical resonator is fixed on an optical platform through a mirror holder. After the optical path is aligned, the two Yb:KGW crystals are inserted into the cavity, ensuring that the spot size matches the crystal mode. The pump source is connected to a collimating mirror through an optical fiber, and the collimating mirror and focusing mirror are fixed near the crystal through a support, ensuring that the pump light is focused on the center of the crystal.

[0012] In some embodiments, it further includes an input coupling system, corresponding input mirrors and PBSs are installed in the seed light path, the seed pulse is injected with p-polarization, and after passing through the PBS, it enters the resonator.

[0013] In some embodiments, it further includes an output coupling system, corresponding output mirrors and PBSs cooperate when the Pockels cell switches, the laser polarization changes, and is coupled out through the PBS; a water cooling system is started to stabilize the Yb:KGW crystal temperature at 24°C; the pump source is started, the pump power is set to 100W, and it operates at a repetition rate of 1 kHz, each pulse undergoes the process of seed injection, regenerative amplification, and output switching.

[0014] The application makes the seed pulse pass through the gain medium twice in a single cycle by the series configuration of the double Yb:KGW crystals, stores the gain energy in the cavity, breaks through the bottleneck of energy extraction of single crystal, disperses the pump energy and heat load to the two crystals, improves the upper limit of total energy storage and improves the heat management, realizes the high pulse energy (breaks through the millijoule level) and the stable output of high average power. By accurately designing the 2.2-meter cavity length, the light pulse round-trip time is ensured to be greater than the voltage establishment time of the Pockels cell, the energy fluctuation or pulse leakage caused by the timing mismatch is avoided, and the stability and reliability of the regenerative amplification process are ensured.

[0015] The dynamic stable resonant cavity is formed by two concave mirrors with a curvature radius of 1000mm, which realizes the best matching of the seed light mode to maximize the energy extraction, and compensates the light beam distortion caused by the thermal lens effect through dynamic adjustment range, and maintains the excellent beam quality under high power.

[0016] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0018] Fig. 1 is a structural schematic block diagram of a double-crystal regenerative amplification system provided by an embodiment of the present application; Fig. 2 is a light path schematic diagram of a double-crystal regenerative amplification system provided by an embodiment of the present application.

[0019] It should be understood that the above general description and the following detailed description are only exemplary and explanatory, and cannot limit the present application. DETAILED DESCRIPTION

[0020] The technical solutions in the embodiments of the present application will be described clearly and completely in combination with the drawings in the embodiments of the present application. Obviously, the described embodiments are some embodiments of the present application, not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor are within the scope of protection of the present application.

[0021] The flowchart shown in the drawings is only an example description, not necessarily including all contents and operations / steps, and not necessarily executed in the described order. For example, some operations / steps can be decomposed, combined or partially combined, so that the actual execution order may be changed according to the actual situation.

[0022] It should be understood that, in order to facilitate the clear description of the technical solutions of the embodiments of the present application, in the embodiments of the present application, the terms "first", "second", etc. are used to distinguish the same items or similar items with basically the same functions and effects. Those skilled in the art can understand that the terms "first", "second", etc. do not limit the quantity and execution order, and the terms "first", "second", etc. also do not necessarily mean different.

[0023] It should be understood that the terms used in the present application specification are only for the purpose of describing specific embodiments and do not intend to limit the present application. As used in the present application specification and the appended claims, the singular forms "a", "an" and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0024] It should also be understood that the term "and / or" used in the present application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes these combinations.

[0025] Some embodiments of the present application will be described in detail below with reference to the accompanying drawings. The following embodiments and features in the embodiments can be combined with each other without conflict.

[0026] In the field of ultrafast laser technology, regenerative amplifiers are widely used in industrial precision machining, medical surgery and other scenarios due to their ability to achieve high-energy pulse output. Although traditional single-crystal regenerative amplification technology improves the signal-to-noise ratio through acoustic-optic single-mode selection modules, there is a fundamental flaw in the single-gain crystal structure: the energy storage and energy extraction capability of a single crystal is limited, and the output pulse energy is usually limited to below the millijoule level. Moreover, under high-power pumping, thermal effects are significant, which can lead to thermal lens effects, causing the resonant cavity to become unstable, the beam quality to deteriorate, and even the crystal to be damaged. Although the traveling wave amplification scheme can improve the energy, it requires multiple stages of amplification, resulting in a complex system, a large volume, high difficulty in mode matching, and large optical loss, making it difficult to balance high energy and beam quality.

[0027] In the prior art, there is no design for achieving distributed gain and heat load dispersion through a double-crystal series configuration, nor is there a scheme for optimizing the cavity length for Pockels box response time to ensure timing matching, and there is also a lack of technical inspiration for dynamically compensating thermal lens effects using a concave mirror with a specific curvature radius. The present application breaks through the energy bottleneck and thermal management problems of traditional single-crystal regenerative amplification through the synergistic innovation of double-crystal distributed gain, cavity length timing matching design, and dynamically stabilizing the resonant cavity, providing a new technical path for the field of high-power ultrafast lasers.

[0028] Please refer to Figs. 1-2Embodiments of the present application provide a double-crystal regenerative amplification system, which comprises an optical resonant cavity, two Yb:KGW crystals are used as gain medium in the optical resonant cavity, the two Yb:KGW crystals are placed in series, a seed pulse passes through the two Yb:KGW crystals in turn in each cycle when oscillating back and forth in the optical resonant cavity, the energy is extracted from the gain provided by the two Yb:KGW crystals, and the injected pump energy and generated heat load are dispersed on the two Yb:KGW crystals; the physical cavity length of the optical resonant cavity is adjusted, so that the time of the optical pulse in one round trip in the cavity is greater than the start-up time required for the Pockels cell in the cavity to be pressurized to establish a stable quarter-wave voltage, so that the stable regenerative amplification process is realized before the seed pulse returns to the Pockels cell for the first time; two concave mirrors with a curvature radius of 1000 mm are arranged in the optical resonant cavity, and the two concave mirrors constitute a dynamically stable resonant cavity.

[0029] Specifically, two Yb:KGW crystals are used as gain medium in the optical resonant cavity, and the two crystals are placed in series. The Yb:KGW crystal has a wide absorption spectrum (suitable for ultra-short pulse amplification), a high laser damage threshold and excellent thermal performance, and can withstand high-power pumping.

[0030] The seed pulse passes through the two crystals in turn in each cycle in the cavity, and the energy can be extracted from the gain provided by the two crystals, so that the gain energy storage in the cavity is doubled, and the extraction efficiency of single-pulse energy is greatly improved, breaking the bottleneck of limited gain of single-crystal regenerative amplifier. At the same time, the injected pump energy and the generated heat load are dispersed on the two crystals, effectively improving the heat management and improving the total energy storage and energy extraction limit of the system.

[0031] The physical cavity length of the optical resonant cavity is accurately designed to be 2.2 meters. The cavity length is determined by the response time of the electro-optical switch of the Pockels cell in the cavity, to ensure that the time of the optical pulse in one round trip in the cavity is greater than the start-up time (including rising edge, falling edge and time jitter) required for the Pockels cell to be pressurized to establish a stable quarter-wave voltage.

[0032] It can be ensured that the voltage is stably loaded before the seed pulse returns to the Pockels cell for the first time, so that the "injection" and "throwing out" of the pulse can be accurately controlled, a stable and reliable regenerative amplification process is realized, and the problems of unstable energy or pulse leakage caused by time sequence mismatch are effectively avoided.

[0033] Two concave mirrors with a curvature radius of 1000 mm are arranged in the optical resonant cavity, and the two concave mirrors constitute a dynamically stable resonant cavity.

[0034] The radius of curvature of the concave mirror is carefully calculated to achieve the best match with the incident seed light mode, ensuring that the spot size is always optimal inside the crystal, thereby maximizing energy extraction and avoiding optical damage.

[0035] Under high-power pumping, the gain crystal will produce thermal lensing effects, and the radius of curvature of the concave mirror provides sufficient dynamic adjustment range for the resonator, effectively compensating for beam distortion caused by thermal lensing, maintaining long-term stability of the cavity during operation, and ensuring excellent beam quality of the output laser.

[0036] Each mirror in the optical resonator is fixed on the optical platform through a mirror holder. After the optical path is aligned, crystal A and crystal B are inserted into the cavity, respectively, to ensure that the spot size matches the crystal mode. The pump source is connected to the collimating mirror through an optical fiber, and the collimating mirror and focusing mirror are fixed near the crystal through a bracket to ensure that the pump light can be focused on the center of the crystal. The input mirror of the input coupling system and the polarizing beam splitter (PBS) are installed in the seed light path, and the seed pulse is injected in a p-polarized state. After passing through the PBS, it enters the resonator. The output mirror of the output coupling system cooperates with the PBS. When the Pockels cell switches, the polarization of the laser changes, and the laser is coupled out through the PBS.

[0037] Start the water cooling system and stabilize the crystal temperature at 24°C to ensure that the crystal is at the optimal operating temperature. Start the pump source and set the pump power to 100W to provide energy for the system. Start the seed laser source to generate seed pulses. The seed pulses are injected into the resonator through the input mirror and PBS. At this time, the Pockels cell does not apply voltage, and the laser propagates in the cavity in a linearly polarized state, starting to circulate in the cavity. The Pockels cell applies voltage after the seed pulse enters, causing the laser polarization to rotate 90°. The laser propagates in the cavity multiple times, and each time it passes through crystal A and crystal B, it is amplified. The pump light continues to be injected, maintaining the population inversion, so that the laser energy continues to accumulate. When the laser energy reaches the saturation state, the Pockels cell removes the voltage, and the laser polarization returns to its original state. The laser is coupled out through the output mirror and PBS, completing the amplification and output process of a pulse. The system operates at a repetition rate of 1 kHz, and each pulse follows the above process, achieving high-frequency and stable laser output.

[0038] Compared with traditional single-crystal regenerative amplification technology, the double-crystal series configuration realizes distributed gain and heat load dispersion, solves the problems of limited energy storage and energy extraction capacity of single crystals, and significant thermal effects, enabling output pulse energy to break through the millijoule level limit and be suitable for high-energy application scenarios.

[0039] By optimizing the cavity length for the Pockels cell response time, the timing matching is ensured, and the stability of the regenerative amplification process is improved. By using a concave mirror with a specific curvature radius to dynamically compensate for thermal lens effects, the long-term stability of the resonant cavity and the excellent beam quality of the output laser are maintained, overcoming the complexity of the traveling wave amplification system and the high difficulty of mode matching.

[0040] The system can be widely applied to precision machining (such as high-precision machining of superhard materials and semiconductor materials), strong field physics research, and attosecond pulse generation, etc. high-end fields that require high energy, high stability, and high beam quality lasers.

[0041] In some embodiments, the seed pulse passes through the gain medium twice in one cavity circulation, and the gain energy storage in the doubling cavity is increased by placing two Yb:KGW crystals in series, thereby improving the extraction efficiency of single pulse energy.

[0042] By placing two Yb:KGW crystals in series, the seed pulse passes through the gain medium twice in one cavity circulation, the gain energy storage in the doubling cavity is increased, and the extraction efficiency of single pulse energy is improved.

[0043] In the optical resonant cavity, two Yb:KGW crystals are arranged in series along the light path direction (i.e. the pulse propagation path sequentially passes through the crystal A and the crystal B as shown in Fig. 2 Each time the seed pulse completes one cavity round trip, it passes through the gain medium twice, and each time it extracts energy from the energy storage of the two crystals, achieving gain stacking and significantly improving energy amplification efficiency.

[0044] In some embodiments, the injected pump energy and the resulting heat load are naturally distributed to the two Yb:KGW crystals, and each Yb:KGW crystal bears a reduced pump energy and heat load compared to a single Yb:KGW crystal.

[0045] The pump energy and the resulting heat load are dispersed to the two Yb:KGW crystals, reducing the pump energy and heat load of each crystal.

[0046] The pump light emitted by the pump source is uniformly distributed to the two crystals through an optical system (such as a collimating mirror, a focusing mirror), so that each crystal bears about 50% of the total power. The heat load is independently or cooperatively dissipated by the water cooling system of the crystals (such as each crystal is equipped with an independent temperature control module), avoiding the occurrence of thermal lens effect or damage due to overheating of the single crystal.

[0047] In some embodiments, the physical cavity length of the optical resonant cavity is set to 2.2 meters, ensuring that the time for the optical pulse to complete one round trip in the cavity is greater than the startup time required for the Pockels cell to establish a stable quarter-wave voltage from pressurization, including the rising edge, the falling edge, and the time jitter.

[0048] The physical cavity length of the optical resonator is set to 2.2 meters to ensure that the round-trip time of the light pulse is greater than the start-up time (including rising edge, falling edge, and jitter) of the Pockels cell to establish a stable quarter-wave voltage. DETAILED DESCRIPTION According to the electro-optical switching parameters (such as start-up time Δt) of the Pockels cell, the cavity length L needs to satisfy: L = 0.5 * c * Δt, where c is the speed of light. In actual design, L = 2.2 meters is taken, so that the round-trip time of the light pulse (about 14.7 ns) is greater than the start-up time of the Pockels cell (such as 10 ns), ensuring that when the pulse returns to the Pockels cell for the first time, the voltage has been stably loaded, avoiding energy fluctuations or pulse leakage caused by timing mismatch.

[0050] In some embodiments, the radii of curvature of the two concave mirrors are optimally matched with the incident seed light mode, ensuring that the spot size is always in the optimal state inside the Yb:KGW crystal.

[0051] The radii of curvature of the two concave mirrors are matched with the seed light mode, ensuring that the spot size in the crystal is optimal.

[0052] According to the Gaussian beam transmission theory, the beam waist radius and divergence angle of the seed light are calculated, and the concave mirror with a radius of curvature R is selected to make the spot size w(z) in the crystal satisfy: w(z) = (λz / πw0) 0.5 (where λ is the wavelength of the laser, w0 is the beam waist radius, and z is the transmission distance). Adjust the position of the mirror to make the spot uniformly distributed in the crystal, avoiding local power density too high leading to damage.

[0053] In some embodiments, the radii of curvature of the two concave mirrors are 1000 mm.

[0054] The concave mirror with a radius of curvature R = 1000 mm is selected and installed on both sides of the resonator to form a specific cavity type (such as a flat-flat cavity or a flat-concave cavity) with the crystal position. The mirror spacing and angle are optimized by optical design software to make the light beam form a stable oscillation mode in the cavity, while satisfying the matching of the spot size and the crystal aperture.

[0055] In some embodiments, the concave mirror with a radius of curvature of 1000 mm provides a corresponding dynamic adjustment range for the resonator, which can compensate for the beam distortion caused by the thermal lens effect of the Yb:KGW crystal under high-power pumping.

[0056] When the crystal generates a thermal lens effect due to pumping, its equivalent focal length fth will change the stability parameters (such as G parameter) of the resonator. The mirror with a radius of curvature of 1000 mm dynamically offsets the phase distortion caused by the thermal lens by adjusting the wavefront curvature of the light beam in the cavity, maintaining G1*G2 ≈ 0.5 (stable cavity condition), ensuring stable beam quality.

[0057] In some embodiments, the mirrors in the optical resonator are fixed on the optical platform by a high-precision mirror holder, and the coaxiality of the optical path is adjusted by a laser collimator.

[0058] The mirrors in the resonator are fixed on the optical platform by a high-precision mirror holder, and the coaxiality of the optical path is adjusted by a laser collimator.

[0059] The two Yb:KGW crystals are inserted into the cavity by a clamp, and the position of the clamp is adjusted to make the center of the light spot coincide with the axis of the crystal. The pump source (such as a fiber laser) is transmitted to the collimating mirror through the optical fiber, and focused to the center of the crystal through the focusing mirror, to ensure that the pump light and the seed light are coaxial in the crystal.

[0060] In some embodiments, it further includes an input coupling system, and a corresponding input mirror and PBS are installed in the seed light path. The seed pulse is injected with p-polarization, and enters the resonator through the PBS.

[0061] The input coupling system realizes the p-polarization injection of the seed pulse through the input mirror and the PBS. The pulse generated by the seed laser source is incident on the polarizing beam splitter (PBS) through the input mirror (highly transparent to the laser wavelength), and the polarization state of the seed light is adjusted to be p-polarization (parallel to the incident plane). After passing through the PBS, it enters the resonator. The PBS also isolates the reflected light in the cavity to avoid interference of the reverse light with the seed source.

[0062] In some embodiments, it further includes an output coupling system, and a corresponding output mirror and PBS cooperate to change the polarization of the laser when the Pockels cell switches, and the laser is coupled out through the PBS; a water cooling system is started to stabilize the temperature of the Yb:KGW crystal at 24°C; the pump source is started, and the pump power is set to 100W. The system operates at a repetition frequency of 1kHz, and each pulse undergoes the processes of seed injection, regenerative amplification and output switching.

[0063] When the Pockels cell applies a voltage, the polarization of the laser rotates by 90° to become s-polarization, which is reflected by the PBS to the output mirror (partially transmissive) for coupling out; when the voltage is removed, the p-polarized light returns to the cavity through the PBS, completing the energy extraction.

[0064] A water cooling system is started to stabilize the temperature of the crystal at 24°C by circulating cooling water, to suppress the thermal birefringence and thermal lens effect.

[0065] The pump power is set to 100W, and the system operates at a repetition frequency of 1kHz. Each pulse period sequentially performs the processes of "seed injection → intracavity amplification → polarization switching and output", to realize stable high-frequency output.

[0066] In some embodiments, in a birefringent regenerative amplifier, the seed pulse is captured by an optical switch and oscillates back and forth within the resonant cavity. In this process, the pulse passes through two crystals in succession in each cycle, continuously extracting energy from the combined gain provided by the two crystals. This layout constitutes a distributed gain module, enabling the pulse energy to grow exponentially within the cavity, with the total gain determined by the combined energy storage of the two crystals. At the same time, the injected pump energy and the resulting heat load are naturally distributed between the two crystals, which not only significantly improves the total energy storage and energy extraction limit of the system, but also effectively improves the heat management, enabling stable output of high pulse energy and high average power while maintaining excellent beam quality and ultra-short pulse characteristics.

[0067] Single-crystal regenerative amplification technology, such as devices, improves signal-to-noise ratio through acousto-optic single-mode modules and electro-optic chopping systems, but its inherent single-gain crystal structure still has significant drawbacks. First, the energy storage and energy extraction capability of a single crystal is limited, resulting in output pulse energy typically limited to below the millijoule (mJ) level, making it difficult to meet the needs of high-energy applications, and at high peak power, the beam quality is prone to decline and the crystal is prone to damage due to thermal effects. The traveling wave amplification scheme can improve energy, but its system is complex and bulky, and the complex mode matching requirements between multiple amplification stages not only introduce additional optical loss and alignment difficulty, but also can lead to beam quality degradation.

[0068] In the application of regenerative amplification lasers, traditional single-crystal regenerative amplification resonant cavities are prone to significant thermal effects under high-power pumping conditions, causing changes in thermal lens effects, leading to instability of the resonant cavity, limiting the energy, stability, and beam quality of the laser output, making it difficult to meet the needs of high-energy, high-stability, and high-beam-quality lasers in precision machining, strong-field physics research, and attosecond pulse generation. The patent proposal aims to provide a birefringent regenerative amplification resonant cavity structure to solve the problems of poor thermal stability and limited energy output of traditional single-crystal resonant cavities under high-power pumping, and to improve the overall performance of regenerative amplification lasers to adapt to more high-end application scenarios.

[0069] Meanwhile, regenerative amplification technology improves many shortcomings of traveling wave amplification, such as: in traveling wave amplification, laser pulses pass through the gain medium a single or limited number of times, with the characteristics of simple structure and low heat accumulation, but the gain is limited, making it difficult to achieve high energy output. Regenerative amplification technology injects seed pulses into a resonant cavity excited by pumping, and controls the pulse to circulate and amplify in the cavity through fast switching of electro-optic crystals, effectively obtaining gain and achieving exponential growth of energy.

[0070] In some embodiments, to achieve high-energy ultrafast laser output, please refer to Figs. 1-2The application provides a regenerative amplification laser based on double Yb:KGW crystals, and the core technical scheme is as follows: 1. Double gain crystal configuration: two Yb:KGW crystals are used as gain media in the regenerative amplification resonant cavity of the application. The reason for selecting Yb:KGW is that it has a wide absorption and emission spectrum, and is particularly suitable for amplifying ultra-short pulses. At the same time, it has a high laser damage threshold and excellent thermal performance, and can withstand high-power pumping. The two crystals are placed in series to double the gain energy storage in the entire cavity, so that the seed pulse can pass through the gain medium twice in one cavity circulation, thereby greatly improving the extraction efficiency of single pulse energy and overcoming the bottleneck of limited gain of single crystal regenerative amplifier.

[0071] 2. Optimized cavity length design: the physical cavity length of the regenerative amplification cavity is accurately designed to be 2.2 meters. This design is determined based on the response time of the cavity Pockels cell. The cavity length ensures that the time for the optical pulse to return to the cavity once is greater than the startup time required for the Pockels cell to establish a stable quarter-wave voltage (including the rising edge, falling edge and time jitter). This design ensures that the voltage is stably loaded before the seed pulse returns to the Pockels cell for the first time, so that the "injection" and "ejection" of the pulse can be accurately controlled, a stable and reliable regenerative amplification process is realized, and energy instability or pulse leakage caused by timing mismatch is effectively avoided.

[0072] 3. Resonant cavity dynamic stability design: two concave mirrors with a curvature radius of 1000 mm are arranged in the regenerative amplification resonant cavity. The core function of the two concave mirrors is to form a dynamically stable resonant cavity. The curvature radius is carefully calculated to achieve the best match with the incident seed light mode, ensuring that the spot size is always in the optimal state in the crystal, thereby maximizing energy extraction and avoiding optical damage. Under high-power pumping, the gain crystal will produce significant thermal lensing effect. The concave mirror with a curvature radius of 1000 mm provides sufficient dynamic adjustment range for the resonant cavity, effectively compensates for the beam distortion caused by thermal lensing, maintains the long-term stability of the cavity during operation, and ensures the excellent beam quality of the output laser.

[0073] 1. The problem of poor thermal stability and limited energy output under high-power pumping is solved.

[0074] 2. It performs better in high peak power and ultra-short pulse amplification, and has greater energy output potential.

[0075] 3. It can be used for precision machining, such as high-precision machining of super-hard materials and semiconductor materials; and strong-field physics research to provide high-energy laser support for related experiments.

[0076] The connection and cooperation relationship between the parts includes: 1. Each lens in the optical resonant cavity is fixed on the optical platform by the lens frame. After the optical path is aligned, crystal A and crystal B are inserted into the cavity respectively to ensure that the spot size matches the crystal mode.

[0077] 2. The pump source is connected to the collimating lens via optical fiber. The collimating lens and focusing lens are fixed near the crystal by a bracket to ensure that the pump light is focused at the center of the crystal.

[0078] 3. The input mirror and PBS of the input coupling system are installed in the seed optical path. The seed pulse is injected with p-polarization and enters the resonant cavity after passing through the PBS.

[0079] 4. The output mirror of the output coupling system works with the PBS. When the Pockels cell is switched, the laser polarization changes and is output through the PBS.

[0080] The action process or operation steps include: 1. Initialization: Start the water cooling system and stabilize the crystal temperature at 24°C. Start the pump source and set the pump power to 100 W.

[0081] 2. Seed Injection: The seed laser source is activated to generate a seed pulse. The seed pulse is injected into the resonant cavity through the input mirror and PBS. At this time, no voltage is applied to the Pockels cell, and the laser travels back and forth in the cavity with linear polarization.

[0082] 3. Regenerative Amplification: After the seed pulse enters, a voltage is applied to the Pockels cell, causing the laser polarization to rotate by 90°. The laser travels back and forth within the cavity multiple times, being amplified each time it passes through crystal A and crystal B. Pump light is continuously injected to maintain population inversion.

[0083] 4. Output switching: When the laser energy reaches saturation, the voltage of the Pockels cell is removed, the laser polarization is restored, and the output is coupled to the PBS through the output mirror.

[0084] 5. Repetitive operation: The system operates at a repetitive frequency of 1 kHz, and each pulse goes through the above process.

[0085] In some embodiments, to address the limitation of traditional fixed-curvature concave mirrors in dynamically tracking changes in thermal lensing effects, this embodiment employs a deep learning-driven deformable mirror (DM) to replace the original fixed concave mirror in the resonant cavity. By sensing the thermal lensing state of the dual crystals in real time, the curvature of the mirror is intelligently adjusted to achieve precise compensation for the thermal lensing effect. This approach overcomes the performance limitations of fixed optical elements, enabling the resonant cavity to maintain stable mode matching and beam quality over a wide pump power range.

[0086] Two fixed-curvature (1000 mm) concave mirrors in the original resonator are replaced by MEMS variable-curvature mirrors (e.g. DM97-15 from Thorlabs), whose curvature can be continuously adjusted in the range of 500 mm ~ 2000 mm with a response time < 1 ms.

[0087] Two fiber Bragg grating (FBG) temperature sensors are attached on the surfaces of the two Yb:KGW crystals to monitor the temperature distribution of the crystals (accuracy ±0.1℃); a high-resolution CCD camera (e.g. Basler acA2500-14uc) is installed at the output of the resonator to capture the spot size and shape inside the crystals in real time (sampling frequency 1 kHz).

[0088] Control unit: FPGA is used as the driver controller of the DM, and a deep learning inference engine (e.g. NVIDIA Jetson Xavier NX) is mounted to realize real-time algorithm processing.

[0089] Under different pump powers (50 W ~ 200 W) and repetition rates (500 Hz ~ 2 kHz), the crystal temperature data (FBG output), spot parameters (spot diameter and ellipticity measured by the CCD), pump power data, and the corresponding optimal DM curvature values (obtained by offline optimization, i.e. the curvature when the spot size and crystal mode matching degree is the highest) are collected to construct a training set containing 100,000 samples.

[0090] Deep learning model: a hybrid model of convolutional neural network (CNN) + long short-term memory network (LSTM) is used: CNN branch: extract the spatial features of the spot image (such as spot edge gradient, symmetry), input is the spot grayscale image (256x256 pixels) captured by the CCD; LSTM branch: process the time series features of temperature and pump power (such as temperature change rate, pump power fluctuation), input is the past 10 periods of FBG temperature data (10x2 dimensions, 1 dimension for each crystal) and pump power data (10x1 dimension); fusion layer: fuse the output features of CNN and LSTM, output the target DM curvature value (1 dimension, range 500 mm ~ 2000 mm) through the fully connected layer. Training and optimization: use Adam optimizer (learning rate 1e-4), take the mean square error (MSE) of the predicted curvature and the offline optimal curvature as the loss function, train the model until the loss converges (MSE < 0.5 mm 2 ).

[0091] The FBG sensor collects the temperature data of the two crystals in real time, the CCD camera synchronously captures the spot image in the crystal, and the data is transmitted to the inference engine through the high-speed bus (USB3.0). The inference engine loads the pre-trained CNN-LSTM model, inputs the temperature time series and the spot image, and outputs the optimal DM curvature value at the current time. The FPGA controller converts the target curvature into the driving voltage of the DM (according to the voltage-curvature calibration curve of the DM), and drives the MEMS mirror to adjust the curvature, with an adjustment time of less than 1ms.

[0092] After adjustment, the CCD camera collects the spot image again, and the model corrects the next curvature prediction through the feedback of the spot parameters, forming a closed-loop control of "perception-decision-adjustment-feedback".

[0093] This embodiment breaks through the static compensation limitation of traditional fixed curvature mirrors by combining deep learning with variable optical elements, and realizes dynamic and accurate compensation of thermal lens effect. Compared with the original invention, it uses deep learning to extract multi-dimensional features (temperature, spot, pump power) of thermal lens effect, solving the problem that traditional methods cannot quantify the nonlinear changes of thermal lens; the real-time adjustment of the variable curvature mirror makes the resonant cavity maintain the beam quality (M 2 ) within 1.1 (1.2~1.3 in the original invention) when the pump power fluctuates (±10%), and the output energy stability is improved.

[0094] In some embodiments, to solve the problem of uneven thermal load caused by traditional double-crystal pump energy fixed allocation, this embodiment uses a reinforcement learning (RL) algorithm to dynamically adjust the pump power ratio of the two Yb:KGW crystals, achieving uniform distribution of thermal load and maximization of energy extraction efficiency. This scheme replaces manual experience with intelligent decision-making, allowing the system to maintain optimal thermal management in a wide operating range.

[0095] The original single pump source is replaced by two independent pump sources (e.g., two IPG Photonics YLR-100-AC fiber pumps), each pump source is focused on the corresponding Yb:KGW crystal (crystal A and crystal B) through an independent fiber collimation system, and the pump power can be continuously adjusted in the range of 0~100W.

[0096] Miniature thermocouple sensors are attached to the surfaces of the two crystals (monitoring the center temperature of the crystals, accuracy ±0.2℃); a pulse energy meter (e.g., Gentec-EO QE80SP) and a beam quality analyzer (e.g., Ophir SP6200) are installed at the output end of the resonant cavity, to monitor the output pulse energy (accuracy ±1%) and beam quality (M 2 , accuracy ±0.05) in real time.

[0097] Agent: Pump power allocation controller, responsible for outputting the pump power ratio of the two crystals (denoted as a, a ∈ [0, 1], the pump power of crystal A is a × P_total, and the pump power of crystal B is (1-a) × P_total, P_total is the total pump power).

[0098] State: Contains 4-dimensional continuous variables: s1: temperature of crystal A (T_A, unit ℃); s2: temperature of crystal B (T_B, unit ℃); s3: output pulse energy (E_out, unit mJ); s4: beam quality (M 2 ). Action: Discrete action set, divide a into 10 intervals (for example, a = 0.1, 0.2, …, 1.0), a total of 10 actions.

[0099] Reward: Design a comprehensive reward to balance thermal uniformity, energy output, and beam quality: R = w1*(1 | T A T B | / Tmax) + w2*(Eout / E target) + w3*(1 M2 / M target); where Tmax is the maximum temperature allowed by the crystal (80℃), Etarget is the target output energy (for example, 20mJ), Mtarget is the target beam quality (1.1); w1 = 0.4, w2 = 0.4, w3 = 0.2 are weight coefficients, emphasizing the priority of thermal uniformity and energy output.

[0100] Deep Q Network (DQN) algorithm is used to solve the instability problem of reinforcement learning through experience replay and target network. Q network uses a 3-layer fully connected neural network (input layer 4 dimensions, hidden layer 128 dimensions × 2, output layer 10 dimensions, corresponding to the Q values of 10 actions), and the activation function is ReLU.

[0101] ε-greedy strategy (ε linearly decays from 0.9 to 0.1, decay step 1e5) is used to balance the exploration of new actions and the use of known optimal actions. Experience replay: Store the state-action-reward-next state (s, a, r, s') of each step in the experience pool (capacity 1e5), and randomly sample 32 experiences to train the Q network each time. Target network update: update the target network (copy the weights of the Q network) every 100 steps to stabilize the training.

[0102] When the system starts, the total pump power P total=100W, and the initial pump ratio a=0.5 (50W for each crystal). State collection: the thermocouple sensor collects the temperature of the two crystals (T A, T B), and the energy meter and beam quality analyzer collect the output parameters (E out, M 2), to form the current state s. Action decision: the DQN agent outputs the action a with the maximum Q value (i.e., the optimal pump ratio a) according to the current state s.

[0103] The pump controller allocates the total pump power to the two crystals according to a (for example, when a=0.6, crystal A gets 60W and crystal B gets 40W), and the adjustment time is <10ms. The reward R is calculated according to the adjusted state s' (temperature, energy, and beam quality of the next pulse period), and the experience (s, a, R, s') is stored in the experience pool. Every 32 experiences are sampled, and the weights of the Q network are updated using the gradient descent method to optimize the objective function.

[0104] The dynamic allocation of pump energy is realized through reinforcement learning, breaking through the static limitations of traditional fixed allocation and solving the problem of uneven thermal load of the two crystals. Compared with the original invention, the innovation lies in: using the trial-and-error-learning mechanism of reinforcement learning to automatically explore the optimal pump ratio without human intervention; when the total pump power is 100W, the temperature difference between the two crystals is reduced from 8℃ in the original invention to below 3℃, and the energy extraction efficiency is improved by 15% (the output energy is increased from 15mJ to 17.25mJ).

[0105] In some embodiments, to overcome the defect that the traditional fixed cavity length cannot adapt to the response time change of the Pockels cell, a PID neural network (PIDNN) is used to control the cavity length, which intelligently adjusts the physical length of the resonant cavity by real-time sensing the start-up time of the Pockels cell, to ensure that the round-trip time of the light pulse is always greater than the stable time of the Pockels cell. This scheme solves the timing mismatch problem of the fixed cavity length and improves the stability of regenerative amplification.

[0106] The cavity length adjustment mechanism installs a piezoelectric translator (PZT) (for example, P-611.3S of Physik Instrumente) at the bottom of the output mirror (original fixed mirror) of the resonant cavity, with a stroke of 100μm, a resolution of 0.1nm, and a response time of <1ms, for fine adjustment of the cavity length (cavity length change amount AL=2xAx, Ax is the moving distance of the PZT, because the light passes through the translator twice).

[0107] The state sensing module monitors the start-up time of the Pockels cell (i.e. the time from pressurization to the establishment of a stable quarter-wave voltage, including the rising edge, falling edge and jitter, denoted as tpk) by installing a high-speed oscilloscope (such as the MSO54 of Tektronix) at the voltage input end of the Pockels cell; and monitors the repetition frequency f of the seed pulse by installing a repetition frequency counter (such as the 53132A of Agilent) at the seed pulse input end.

[0108] Control target: Ensure that the time tround of the optical pulse in the cavity for one round trip is greater than tpk+Δt, where Δt is the timing redundancy (10 ns is taken to avoid the influence of jitter). The relationship between the round-trip time and the cavity length is: tround =2L / c (L is the cavity length, and c is the speed of light), so the target cavity length is: Ltarget=c*(tpk+Δt) / 2.

[0109] PID neural network structure: PIDNN is an intelligent controller that combines traditional PID control and neural networks, and its structure includes an input layer, a hidden layer and an output layer: the input layer has 3 nodes corresponding to the error signal (e=Ltarget Lcurrent, Lcurrent is the current cavity length), the error change rate (de / dt) and the error integral (∫edt); the hidden layer has 6 nodes, which use a Sigmoid activation function and are responsible for nonlinear mapping; and the output layer has 1 node corresponding to the movement amount Δx of the PZT (ΔL=2Δx), which uses a linear activation function.

[0110] The back propagation (BP) algorithm is used to train the model, with the mean square error (MSE) of the cavity length error as the loss function, and the training data coming from the variation scenarios of the Pockels cell start-up time (such as temperature variation ±10℃ and voltage fluctuation ±5%), and the tpk, f and corresponding Ltarget, Lcurrent data are collected to train the model to loss convergence (MSE<0.1μm 2 ).

[0111] The oscilloscope monitors the start-up time tpk of the Pockels cell in real time (the sampling frequency is 1 kHz), and the repetition frequency counter monitors the repetition frequency f of the seed pulse, and the data is transmitted to the controller.

[0112] The target cavity length is calculated according to tpk, and the target cavity Ltarget=c*(tpk+10ns) / 2 (c=3×10 8 m / s).

[0113] The error is calculated by acquiring the current cavity length Lcurrent through the cavity length sensor (a laser interferometer installed on the PZT translation stage with an accuracy of 0.1 nm), and calculating the error e=Ltarget Lcurrent, rate of error change de / dt, error integral ∫edt.

[0114] The PIDNN controller inputs an error signal and outputs the moving amount Δx of the PZT, which drives the translation stage to adjust the cavity length, and the adjustment time is <1ms.

[0115] Closed-loop feedback: after adjustment, the laser interferometer measures the cavity length Lcurrent again, corrects the error signal, and forms a closed-loop control.

[0116] The adaptive adjustment of the cavity length is realized through the PID neural network, which breaks through the time sequence limitation of the traditional fixed cavity length and solves the time sequence mismatch problem caused by the response time change of the Pockels cell. Compared with the original invention, the nonlinear adaptive ability of the PIDNN is utilized to realize the precise adjustment of the cavity length and ensure that the round-trip time is always greater than the start-up time of the Pockels cell; when the response time of the Pockels cell changes due to temperature change (from 20℃ to 30℃, the start-up time increases from 100ns to 120ns), the cavity length is automatically adjusted from 2.2m to 2.206m (Ltarget=3×10 8 ×(120ns+10ns) / 2=2.206m), avoiding the energy fluctuation caused by time sequence mismatch.

[0117] The present application realizes the twice through of the seed pulse in the gain medium in a single cycle through the series configuration of the double Yb:KGW crystals, doubles the energy storage in the cavity, breaks through the bottleneck of single crystal energy extraction; the pump energy and thermal load are dispersed to the two crystals, which improves the upper limit of total energy storage and improves the heat management, realizes the stable output of high pulse energy (breaks through the millijoule level) and high average power. By accurately designing the 2.2m cavity length, the round-trip time of the light pulse is ensured to be greater than the voltage build-up time of the Pockels cell, avoiding the energy fluctuation or pulse leakage caused by time sequence mismatch, and ensuring the stability and reliability of the regenerative amplification process.

[0118] The dynamic stable resonant cavity is composed of two concave mirrors with a curvature radius of 1000mm, which not only realizes the best matching of the seed light mode to maximize the energy extraction, but also compensates the beam distortion caused by thermal lens effect through dynamic adjustment range, maintaining excellent beam quality under high power.

[0119] It will be understood that the terms used herein are for the purpose of describing particular embodiments and are not intended to limit the application. It will be understood that when an element or layer is referred to as being "on" or "connected to" another element or layer, it can be directly on or connected to the other element or layer or intervening elements or layers can be present. In contrast, when an element is referred to as being "directly on" or "directly connected to" another element or layer, there are no intervening elements or layers present. It will also be understood that, although the terms first, second, third, etc. can be used herein to describe various elements, components, regions, layers and / or sections, these elements, components, regions, layers and / or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present application.

[0120] Spatially relative terms, such as "beneath", "below", "lower", "under", "above", "upper" and the like, can be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is turned over, elements described as "below" or "beneath" other elements or features would then be oriented "above" the other elements or features. Thus, the exemplary term "below" can encompass both an orientation of above and below. The device can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly.

[0121] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprises" and / or "comprising", when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and / or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and / or groups thereof. As used herein the term "and / or" includes any and all combinations of one or more of the associated listed items.

[0122] It will be understood that the terms "and / or", as used herein, refers to and encompasses any and all possible combinations of one or more of the associated listed items. It will be understood that the terms "and / or", as used herein, refers to and encompasses any and all possible combinations of one or more of the associated listed items.

[0123] The above description is provided as an enabling teaching of the application and is not intended to limit its scope in any way. Any modification of the application in keeping with the spirit thereof that is apparent to those skilled in the art is to be considered within the scope of the application as defined by the appended claims.

Claims

1. A dual-crystal regenerative amplification system, characterized in that, include: An optical resonant cavity is provided, in which two Yb:KGW crystals are used as gain media. The two Yb:KGW crystals are placed in series. When the seed pulse oscillates back and forth in the optical resonant cavity, it passes through the two Yb:KGW crystals in succession in each cycle, extracting energy from the gain provided by the two Yb:KGW crystals. The injected pump energy and the generated heat load are distributed on the two Yb:KGW crystals. By adjusting the physical cavity length of the optical resonant cavity, the time for the light pulse to travel back and forth within the cavity is made greater than the start-up time required for the Pockel cell within the cavity to establish a stable quarter-wave voltage from the application of pressure, so as to achieve a stable regeneration amplification process before the seed pulse first returns to the Pockel cell; Two concave mirrors are installed inside the optical resonant cavity to compensate for beam distortion caused by the thermal lensing effect of the Yb:KGW crystal under high-power pumping.

2. The dual-crystal regeneration amplification system according to claim 1, characterized in that, The seed pulse passes through the gain medium twice in one cavity cycle, and the cavity gain is multiplied by two Yb:KGW crystals connected in series to store energy, thereby improving the extraction efficiency of single pulse energy.

3. The dual-crystal regeneration amplification system according to claim 1, characterized in that, The injected pump energy and the resulting heat load are naturally distributed to the two Yb:KGW crystals, with each Yb:KGW crystal bearing less pump energy and heat load compared to a single Yb:KGW crystal.

4. The dual-crystal regeneration amplification system according to claim 1, characterized in that, The physical cavity length of the optical resonant cavity is set to 2.2 meters to ensure that the time for the light pulse to travel back and forth once in the cavity is greater than the start-up time required for the Pockel cell to establish a stable quarter-wave voltage from the application of pressure. The start-up time includes the rising edge, falling edge, and timing jitter.

5. The dual-crystal regenerative amplification system according to claim 1, characterized in that, The radii of curvature of the two concave mirrors are optimally matched with the incident seed light mode, ensuring that the spot size is always in the optimal state within the Yb:KGW crystal.

6. The dual-crystal regenerative amplification system according to claim 5, characterized in that, The two concave mirrors have a radius of curvature of 1000 mm.

7. The dual-crystal regenerative amplification system according to claim 5, characterized in that, A concave mirror with a radius of curvature of 1000 mm provides a corresponding dynamic adjustment range for the resonant cavity, which can compensate for beam distortion caused by the thermal lensing effect of Yb:KGW crystal under high-power pumping.

8. The dual-crystal regenerative amplification system according to claim 1, characterized in that, Each lens in the optical resonant cavity is fixed to the optical platform by a lens frame. After the optical path is aligned, two Yb:KGW crystals are inserted into the cavity to ensure that the spot size matches the crystal mode. The pump source is connected to the collimating lens via an optical fiber. The collimating lens and the focusing lens are fixed near the crystal by a bracket to ensure that the pump light is focused at the center of the crystal.

9. The dual-crystal regenerative amplification system according to claim 1, characterized in that, Also includes: The input coupling system has an input mirror and a PBS installed in the seed optical path. The seed pulse is injected with p-polarization and enters the resonant cavity after passing through the PBS.

10. The dual-crystal regenerative amplification system according to claim 1, characterized in that, Also includes: The output coupling system, with the corresponding output mirror working in conjunction with the PBS, allows the laser polarization to change when the Pockels cell is switched, and the laser is output through the PBS coupling. Start the water cooling system to stabilize the temperature of the Yb:KGW crystal at 24°C; Start the pump source, set the pump power to 100W, and run at a repetition frequency of 1kHz. Each pulse goes through the process of seed injection, regeneration amplification, and output switching.

Images