Joule-level high-energy picosecond pulse laser amplifier with kilohertz repetition frequency

Abstract

The joule-level high-energy picosecond pulse laser amplifier with the KHz repetition frequency can effectively avoid heat accumulation in a crystal, compensate a thermal lens effect caused by excessive heat, reduce the influence of the lath crystal on the quality of a light beam to the minimum in a light path of the multi-pass laser amplifier, realize uniform absorption of an optical pump and balanced distribution of internal heat, and improve the optical efficiency of the multi-pass laser amplifier. And efficient amplification of the signal light is realized. The joule-level high-energy picosecond pulse laser amplifier with the kilohertz repetition frequency comprises a picosecond pulse fiber laser (1), a solid regenerative amplifier (2), a solid multi-pass amplifier (3) and a composite slab amplifier (4).

Description

Technical Field

[0001] The invention relates to the technical field of lasers, and in particular to a Joule-level high-energy picosecond pulse laser amplifier with a kilohertz repetition frequency. Background Art

[0002] In the past 20 years, diode-pumped high-energy pulsed lasers with a duration of tens of picoseconds and a repetition rate of kilohertz have been widely used in the fields of industry, biomedicine, and scientific research due to their high pulse energy, high average power, and high peak power. In industrial processing, high-energy picosecond pulsed lasers can be used for micro-processing of most metal and non-metal materials; in biomedicine, laser medicine has basically developed into a complete and relatively independent discipline, and its diagnosis and treatment scope covers clinical multi-disciplines such as internal medicine, surgery, dentistry, and dermatology; in ultrafast science, high-energy pulsed lasers are special light fields with ultrafast time domain characteristics and ultra-high pulse energy characteristics, which can create unprecedented extreme physical conditions such as ultrafast time, ultra-high intensity field, ultra-high temperature, and ultra-high pressure. Its development directly drives the research of physics, chemistry, biology, materials, and information science into the field of microscopic ultrafast processes, and is one of the important tools for frontier basic scientific research to expand human cognition.

[0003] Obtaining picosecond pulse lasers with higher pulse energy, higher average power, and higher beam quality is an extremely challenging task. The key points are high-efficiency gain extraction, uniform pumping technology to avoid thermal lenses and thermally induced stress birefringence, and high-quality new thermal management processes. MOPA (Master Oscillator Power-Amplifier) ​​technology is an effective solution for obtaining high-energy picosecond pulse lasers, which mainly include fiber laser amplifiers and solid-state laser amplifiers. Fiber laser amplifiers have the characteristics of good heat dissipation characteristics, compact structure, and stable performance. However, in ultrashort pulse lasers, they are very easy to introduce nonlinear effects such as stimulated Raman scattering and self-phase modulation, which limits their high energy acquisition. Solid-state laser amplifiers can obtain a larger gain mode field while avoiding the influence of nonlinear effects on the output laser. However, in order to obtain high average power output, more pump energy needs to be injected into the gain medium. The generation of excessive heat will lead to thermal lensing and thermally induced stress birefringence, which will lead to a sharp weakening of the laser amplification effect, deterioration of the output beam quality, and severe thermal depolarization. What's worse, the deformation caused by excessive thermal stress will cause the crystal to break and damage.

[0004] Efficiently removing excess heat from the gain medium is a key issue that needs to be addressed in high-energy lasers. A common solution is to increase the surface-to-volume ratio of the gain medium to enhance the heat dissipation structure to avoid thermal distortion, such as slab laser amplifiers and thin-film laser amplifiers. However, for slab laser amplifiers, how to obtain laser output with joule-level pulse energy at a kilohertz repetition rate while ensuring that the beam is distortion-free, the key technology lies in solving problems such as high-efficiency distributed heat dissipation, efficient absorption of pump light, extraction and amplification of high-gain picosecond pulse energy, beam quality fidelity control, and comprehensive control of complex lasers. Summary of the invention

[0005] In order to overcome the defects of the prior art, the technical problem to be solved by the present invention is to provide a Joule-level high-energy picosecond pulse laser amplifier with a kilohertz repetition rate, which can effectively avoid heat accumulation inside the crystal, compensate for the thermal lens effect caused by excessive heat, minimize the influence of the slab crystal on the beam quality in the optical path of the multi-pass laser amplifier, achieve uniform absorption of the optical pump and balanced distribution of internal heat, and realize efficient amplification of the signal light.

[0006] The technical solution of the present invention is: the kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier comprises: a picosecond pulse fiber laser (1), a solid regenerative amplifier (2), a solid multi-pass amplifier (3) and a composite slab amplifier (4);

[0007] The picosecond pulse fiber laser comprises a SESAM mode-locked picosecond fiber oscillator (11), a fiber laser preamplifier (12), a single-mode fiber stretcher (13) and a fiber laser main amplifier (14); the fiber laser oscillator uses the SESAM mode-locked technology to generate a picosecond pulse laser with a repetition frequency of tens of megahertz; the fiber laser preamplifier is an ytterbium-doped single-mode fiber, which pre-amplifies the picosecond pulse laser of tens of megahertz generated by the SESAM mode-locked picosecond fiber oscillator; the single-mode fiber stretcher is a kilometer-long single-mode passive fiber, which stretches the laser pulse width to hundreds of picoseconds; the fiber laser main amplifier is an ytterbium-doped double-clad fiber, which further amplifies the laser pulse;

[0008] The solid-state regenerative amplifier amplifies the laser signal obtained by the picosecond pulse fiber laser to a repetition frequency of kilohertz and a pulse energy of millijoule level through a regenerative resonant cavity and a Pockels cell modulator;

[0009] The solid multi-pass amplifier includes multiple end-pumped solid laser amplifiers and beam shaping devices; the end-pumped solid laser amplifiers increase the pulse energy to the ten millijoules level; the beam shaping device collimates and expands the laser spot into a vertical thin strip;

[0010] The composite slab amplifier includes a gain crystal and several quarter-wave plates, thin-film polarizers, and reflectors. By controlling the polarization state and the incident angle, the picosecond pulse laser of tens of millijoules obtained by the solid multi-pass amplifier passes through the composite slab crystal multiple times, amplifying the kilohertz picosecond pulse laser of the solid multi-pass amplifier to the joule level.

[0011] The beneficial technical effects of the present invention are as follows:

[0012] 1. The present invention uses for the first time a method combining fiber laser, regenerative amplifier, multi-pass amplifier and composite slab to achieve picosecond pulse laser with joule-level pulse energy at a kilohertz repetition frequency. A high-stability picosecond pulse laser is obtained through a fiber laser oscillator, and the high-repetition-frequency small-signal pulse laser output by the fiber laser is amplified by a regenerative amplifier. The signal light is further amplified by an end-pumped solid multi-pass amplifier, and enters the slab amplifier after collimation and beam expansion, and the tiny signal light pulse energy is amplified step by step to tens of millijoules, achieving high-efficiency gain while maintaining the overall high heat dissipation efficiency and high beam quality output of the system. For the first time, a composite slab amplifier with a special structure is used, taking into account uniform pumping, fast and balanced heat dissipation, efficient gain extraction and high-fidelity beam quality, so as to obtain high-energy picosecond pulse laser output at the kilohertz repetition frequency.

[0013] 2. In the present invention, the composite slab amplifier is bonded by multiple YAG / Nd:YAG crystals, ytterbium-doped Nd:YAG crystals with lower thermal conductivity in contact with the microchannel water-cooled heat sink, and YAG crystals without rare earth ion doping with higher thermal conductivity, forming a Nd:YAG / YAG / Nd:YAG sandwich structure in the main part of the slab. The large outer doped area is directly in contact with the water-cooled heat sink, which is more conducive to heat dissipation. At the same time, it is easier to accurately control the temperature of the Nd:YAG doped area at 25°C, which is the temperature at which Nd:YAG efficiently absorbs pump light. At the same time, as the input and output ends of the pump light and signal light, YAG crystals are also bonded at both ends of the slab. The bonding structure of the composite slab can quickly and evenly dissipate heat, effectively avoid heat accumulation inside the crystal, compensate for the temperature gradient inside the slab, reduce crystal thermal distortion, and significantly improve the gain effect of the laser amplifier.

[0014] 3. In the present invention, the YAG crystals at both ends are composed of four curved cut corners. The use of curved concave cut corners at both ends for the first time can effectively compensate for the thermal lens effect of the crystal end face in the x-axis direction. Under high-energy pumping conditions and long-distance multi-pass laser amplification and transmission, the introduction of spherical aberration to cause beam quality distortion is avoided, and efficient laser energy extraction and amplification and beam quality fidelity are achieved.

[0015] 4. In the present invention, the Nd:YAG crystals of the two large outer layers are gradient doped with neodymium ions, and the doping concentration decreases from the center to the two ends. The pump light is transmitted in a zigzag shape inside the composite slab crystal. The light intensity at the light inlets at both ends of the slab is higher, and the crystal doping concentration is lower, which can reduce the absorption of the pump light; while the pump light intensity in the center of the slab is lower, and the crystal doping concentration is higher, which can increase the absorption of the pump light. This design can balance the absorption of the pump light intensity at different positions of the doping area under ultra-high pump power, thereby achieving uniform absorption of the pump light inside the large-sized composite slab, and effectively achieving uniform distribution of the gain range inside the composite slab crystal and balanced heat distribution.

[0016] 5. In the present invention, the composite slab amplifier innovatively realizes a 6-pass amplifier by controlling the laser polarization state and the incident angle. The signal light is transmitted in a zigzag shape inside the composite slab, and each transmission and amplification path of the signal light inside the composite slab is axially symmetrical with the vertical section of the slab as the center. The signal light of the first 4 passes is incident and emitted in a symmetrically identical path and at a larger angle by controlling the polarization state. This can make the signal light evenly distributed inside the composite slab, while making it reflect and pass through the gain zone more times with smaller energy in the first 4 passes. Due to the uniform distribution of the pump light and the efficient signal light beam filling factor design, the signal light can achieve efficient gain extraction after passing through the composite slab multiple times. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 A schematic diagram of the structure of a joule-level high-energy picosecond pulse laser amplifier with a kilohertz repetition rate provided in an example of the present invention.

[0018] Figure 2 A structural block diagram of a picosecond pulse fiber laser of a kilohertz repetition rate Joule-level high-energy picosecond pulse laser amplifier provided in an example of the present invention.

[0019] Figure 3 A solid-state regenerative amplifier structure block diagram of a kilohertz repetition rate Joule-level high-energy picosecond pulse laser amplifier provided in an example of the present invention.

[0020] Figure 4 A solid multi-pass amplifier structure block diagram of a joule-level high-energy picosecond pulse laser amplifier with a kilohertz repetition rate provided by an example of the present invention.

[0021] Figure 5 A three-dimensional perspective view and a partially enlarged schematic diagram of a composite slab amplifier of a kilohertz repetition rate Joule-level high-energy picosecond pulse laser amplifier provided as an example of the present invention.

[0022] Figure 6A top view of the pump light and crystal heat sink of a composite slab amplifier of a kilohertz repetition rate Joule-level high-energy picosecond pulse laser amplifier provided in an example of the present invention. DETAILED DESCRIPTION

[0023] In order to enable those skilled in the art to better understand the scheme of the present invention, the technical scheme in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work should fall within the scope of protection of the present invention.

[0024] It should be noted that the term "comprises" and any variations in the specification and claims of the present invention and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, apparatus, product or equipment that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or equipment.

[0025] like Figure 1 As shown, this kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier comprises: a picosecond pulse fiber laser 1, a solid regenerative amplifier 2, a solid multi-pass amplifier 3 and a composite slab amplifier 4;

[0026] The picosecond pulse fiber laser includes a SESAM mode-locked picosecond fiber oscillator 11, a fiber laser preamplifier 12, a single-mode fiber stretcher 13 and a fiber laser main amplifier 14; the fiber laser oscillator uses the SESAM mode-locked technology to generate a picosecond pulse laser with a repetition frequency of tens of megahertz; the fiber laser preamplifier is an ytterbium-doped single-mode fiber, which pre-amplifies the picosecond pulse laser of tens of megahertz generated by the SESAM mode-locked picosecond fiber oscillator; the single-mode fiber stretcher is a single-mode passive fiber with a length of kilometers, which stretches the laser pulse width to hundreds of picoseconds; the fiber laser main amplifier is an ytterbium-doped double-clad fiber, which further amplifies the laser pulse;

[0027] The solid-state regenerative amplifier amplifies the laser signal obtained by the picosecond pulse fiber laser to a repetition frequency of kilohertz and a pulse energy of millijoule level through a regenerative resonant cavity and a Pockels cell modulator;

[0028] The solid multi-pass amplifier includes multiple end-pumped solid laser amplifiers and beam shaping devices; the end-pumped solid laser amplifiers increase the pulse energy to the ten millijoules level; the beam shaping device collimates and expands the laser spot into a vertical thin strip;

[0029] The composite slab amplifier includes a gain crystal and several quarter-wave plates, thin-film polarizers, and reflectors. By controlling the polarization state and the incident angle, the picosecond pulse laser of tens of millijoules obtained by the solid multi-pass amplifier passes through the composite slab crystal multiple times, amplifying the kilohertz picosecond pulse laser of the solid multi-pass amplifier to the joule level.

[0030] The beneficial technical effects of the present invention are as follows:

[0031] 1. The present invention uses for the first time a method combining fiber laser, regenerative amplifier, multi-pass amplifier and composite slab to achieve picosecond pulse laser with joule-level pulse energy at a kilohertz repetition frequency. A high-stability picosecond pulse laser is obtained through a fiber laser oscillator, and the high-repetition-frequency small-signal pulse laser output by the fiber laser is amplified by a regenerative amplifier. The signal light is further amplified by an end-pumped solid multi-pass amplifier, and enters the slab amplifier after collimation and beam expansion, and the tiny signal light pulse energy is amplified step by step to tens of millijoules, achieving high-efficiency gain while maintaining the overall high heat dissipation efficiency and high beam quality output of the system. For the first time, a composite slab amplifier with a special structure is used, taking into account uniform pumping, fast and balanced heat dissipation, efficient gain extraction and high-fidelity beam quality, so as to obtain high-energy picosecond pulse laser output at the kilohertz repetition frequency.

[0032] 2. In the present invention, the composite slab amplifier is bonded by multiple YAG / Nd:YAG crystals, ytterbium-doped Nd:YAG crystals with lower thermal conductivity in contact with the microchannel water-cooled heat sink, and YAG crystals without rare earth ion doping with higher thermal conductivity, forming a Nd:YAG / YAG / Nd:YAG sandwich structure in the main part of the slab. The large outer doped area is directly in contact with the water-cooled heat sink, which is more conducive to heat dissipation. At the same time, it is easier to accurately control the temperature of the Nd:YAG doped area at 25°C, which is the temperature at which Nd:YAG efficiently absorbs pump light. At the same time, as the input and output ends of the pump light and signal light, YAG crystals are also bonded at both ends of the slab. The bonding structure of the composite slab can quickly and evenly dissipate heat, effectively avoid heat accumulation inside the crystal, compensate for the temperature gradient inside the slab, reduce crystal thermal distortion, and significantly improve the gain effect of the laser amplifier.

[0033] 3. In the present invention, the YAG crystals at both ends are composed of four curved cut corners. The use of curved concave cut corners at both ends for the first time can effectively compensate for the thermal lens effect of the crystal end face in the x-axis direction. Under high-energy pumping conditions and long-distance multi-pass laser amplification and transmission, the introduction of spherical aberration to cause beam quality distortion is avoided, and efficient laser energy extraction and amplification and beam quality fidelity are achieved.

[0034] 4. In the present invention, the Nd:YAG crystals of the two large outer layers are gradient doped with neodymium ions, and the doping concentration decreases from the center to the two ends. The pump light is transmitted in a zigzag shape inside the composite slab crystal. The light intensity at the light inlets at both ends of the slab is higher, and the crystal doping concentration is lower, which can reduce the absorption of the pump light; while the pump light intensity in the center of the slab is lower, and the crystal doping concentration is higher, which can increase the absorption of the pump light. This design can balance the absorption of the pump light intensity at different positions of the doping area under ultra-high pump power, thereby achieving uniform absorption of the pump light inside the large-sized composite slab, and effectively achieving uniform distribution of the gain range inside the composite slab crystal and balanced heat distribution.

[0035] 5. In the present invention, the composite slab amplifier innovatively realizes a 6-pass amplifier by controlling the laser polarization state and the incident angle. The signal light is transmitted in a zigzag shape inside the composite slab, and each transmission and amplification path of the signal light inside the composite slab is axially symmetrical with the vertical section of the slab as the center. The signal light of the first 4 passes is incident and emitted in a symmetrically identical path and at a larger angle by controlling the polarization state. This can make the signal light evenly distributed inside the composite slab, while making it reflect and pass through the gain zone more times with smaller energy in the first 4 passes. Due to the uniform distribution of the pump light and the efficient signal light beam filling factor design, the signal light can achieve efficient gain extraction after passing through the composite slab multiple times.

[0036] Preferably, the gain crystal of the composite slab amplifier is a composite of multiple YAG crystals and Nd:YAG crystals, and the contact surfaces between the crystals are tightly connected through a bonding process; the two large outer layers of the composite slab are Nd:YAG crystals 42, and the central interlayer part is a first YAG crystal 43, thereby forming a Nd:YAG / YAG / Nd:YAG sandwich structure, the large outer layers are directly in contact with the microchannel water-cooled heat sink 45, and the two ends of the composite slab are second YAG crystals 41, which serve as the input and output ends of the pump light and the signal light.

[0037] Preferably, the Nd:YAG crystal has a thickness of 250-300 μm and is directly welded to the microchannel water-cooled heat sink 45 , and the cooling water temperature is kept constant at 25° C.

[0038] Preferably, the second YAG crystal is composed of four curved cut corners 44, and after two angles of about 45° are cut at each end of the composite strip, a curved depression is ground out using a crystal ball milling process, and the radius of curvature is 1800-2000 mm.

[0039] Preferably, the Nd:YAG crystal is gradient doped with neodymium ions, the doping concentration decreases from the center to both ends, and the doping concentration gradient ranges from 0.3at% to 0.1at%.

[0040] Preferably, the quarter wave plate, thin film polarizer, reflector and gain crystal of the composite slab amplifier constitute a 6-pass laser amplifier, which is realized by laser polarization state control and incident angle control. The laser polarization state control is realized by the thin film polarizer and the quarter wave plate, and the reflector controls the incident angle at 20 to 30° to meet the total reflection condition inside the composite slab, so that the signal light is transmitted in a zigzag shape inside the composite slab amplifier.

[0041] Preferably, the SESAM mode-locked picosecond fiber oscillator is a SESAM mode-locked picosecond fiber oscillator, the single-mode fiber preamplifier is a fiber laser preamplifier, and the fiber laser main amplifier is a double-clad fiber main amplifier. The SESAM mode-locked picosecond fiber oscillator uses ytterbium-doped single-mode fiber as a working material and uses the SESAM mode-locking technology of a linear cavity to obtain a stable small-signal pulse laser with a duration of ten picoseconds at a repetition frequency of tens of megahertz. This signal light is initially amplified in the fiber laser preamplifier and then enters the single-mode fiber stretcher, and the pulse duration is stretched to hundreds of picoseconds. The stretched signal light enters the double-clad fiber main amplifier connected thereto and passes through the fiber coupler output device to obtain a pulse laser with an average power of hundreds of milliwatts and a pulse duration of hundreds of picoseconds at a repetition frequency of tens of megahertz.

[0042] Preferably, the solid regenerative amplifier comprises: a fiber laser seed injection 21, an optical isolator 22 and a regenerative amplifier resonant cavity 23; the fiber laser seed injection injects the signal light of the previous stage into the regenerative amplifier resonant cavity at a specific angle and position, the optical isolator avoids the backward return light of the regenerative amplifier resonant cavity, and the signal light with a megahertz repetition frequency repeatedly passes through the Nd:YAG crystal in the regenerative amplifier resonant cavity to obtain a huge small signal gain, and the polarization state and output timing of the laser in the cavity are controlled by the Pockels cell and the quarter-wave plate in the regenerative amplifier resonant cavity, and the mode-locked picosecond pulse laser is selected for amplification.

[0043] Preferably, the solid multi-pass amplifier comprises: a first-stage Nd:YVO4 two-way solid amplifier 31, a second-stage Nd:YVO4 two-way solid amplifier 32 and a beam collimating expander 33; the pulse energy of the signal light from the solid regenerative amplifier is increased to tens of millijoules after passing through the first-stage Nd:YVO4 two-way solid amplifier and the second-stage Nd:YVO4 two-way solid amplifier, and enters the beam collimating expander for beam transformation. After collimation and expansion, the light spot is transformed into a vertical thin strip parallel light spot, and the spot size is consistent with the cross-section of the cut angles of the two end faces of the slat.

[0044] The present invention is described in more detail below.

[0045] like Figure 1As shown, the present invention includes a picosecond pulse fiber laser 1, a solid regenerative amplifier 2, a solid multi-pass amplifier 3 and a composite slab amplifier 4. The picosecond pulse laser generated by the SESAM mode-locked fiber laser oscillator is amplified by the fiber laser, amplified by the solid regenerative amplifier, amplified by the solid multi-pass amplifier and amplified by the composite slab, and finally outputs a high-energy picosecond pulse laser of the Joule level at a kilohertz repetition frequency. The picosecond pulse laser of the vertical stripe light spot with a kilohertz repetition frequency output by the multi-pass amplifier in the figure is in a horizontal linear polarization state. After passing through the first thin film polarizer 47, it enters the composite slab crystal for the first time, and is transmitted along a zigzag optical path after multiple total reflections, and is emitted from the other end of the slab at the same angle. This is the first-pass amplification. Afterwards, the laser returns to the original path through the first reflector 49, passes through the first quarter-wave plate 48 twice, and the laser is converted into a vertical linear polarization state. This is the second-pass amplification. The vertical linear polarized signal light, after being reflected by the first thin-film polarizer 47 and the second thin-film polarizer 50, enters from the other side of the composite slab crystal end face, and is also transmitted in a zigzag shape. The transmission light path is axially symmetrical with the first-pass light path about the center of the vertical section of the slab. This is the third-pass amplification. Similarly, the output light is converted into a horizontal linear polarization state after passing through the second quarter-wave plate 51 and the second reflector 52 twice and returns to the slab along the original path. This is the fourth-pass amplification. Furthermore, the horizontal linear polarized light is emitted from the slab and passes through the second thin-film polarizer 50, and enters the composite slab at a slightly larger angle by the third reflector 53 and the fourth reflector 54. This is the fifth-pass amplification. The laser emitted from the other end passes through the fifth reflector 55 and the sixth reflector 56, and is incident on the other side of this end face at the same angle as when it entered the fifth pass. This is the sixth-pass amplification. Picosecond pulse signal lasers with tens of millijoules are gradually amplified after passing through the composite slab crystal six times, achieving high-gain extraction, and ultimately obtaining high-energy picosecond pulse laser output at the joule level at a kilohertz repetition rate.

[0046] like Figure 2 As shown, the picosecond pulse fiber laser 1 in the present invention includes a picosecond fiber oscillator 11, a single-mode fiber preamplifier 12, a single-mode fiber stretcher 13 and a double-clad fiber main amplifier 14. The SESAM mode-locked picosecond fiber oscillator uses ytterbium-doped single-mode fiber as a working material and uses the SESAM mode-locked technology of a linear cavity to obtain a stable small signal pulse laser with a duration of tens of picoseconds at a repetition frequency of tens of megahertz. This signal light is initially amplified in the single-mode fiber preamplifier and then enters the single-mode fiber stretcher, and the pulse duration is stretched to hundreds of picoseconds. Furthermore, the stretched signal light enters the double-clad fiber main amplifier connected thereto and passes through the fiber coupler output device to obtain a pulse laser with an average power of hundreds of milliwatts and a pulse duration of hundreds of picoseconds at a repetition frequency of tens of megahertz.

[0047] like Figure 3As shown, the solid regenerative amplifier in the present invention includes a fiber laser seed injection 21, an optical isolator 22 and a regenerative amplifier resonant cavity 23. The fiber laser seed injection 21 is used to inject the signal light of the previous stage into the regenerative amplifier resonant cavity 23 at a suitable angle and position. The optical isolator can effectively prevent the backward return light of the regenerative amplifier resonant cavity from interfering with or damaging the fiber laser of the previous stage. After the signal light with a megahertz repetition frequency is injected into the regenerative amplifier resonant cavity, it repeatedly passes through the Nd:YAG crystal in the cavity to obtain a huge small signal gain. In the regenerative amplifier resonant cavity, the polarization state and output timing of the laser in the cavity are controlled by the Pockels cell and the quarter-wave plate, and the mode-locked picosecond pulse laser is amplified by a single selection, and finally a picosecond pulse laser of several millijoules can be obtained at a repetition frequency of kilohertz.

[0048] like Figure 4 As shown, the solid multi-pass amplifier in the present invention includes a first-stage Nd:YVO4 two-way solid amplifier, a second-stage Nd:YVO4 two-way solid amplifier and a beam collimator. The picosecond pulse laser with a repetition frequency of several millijoules output by the front stage has a pulse energy of several millijoules after passing through the first-stage Nd:YVO4 two-way solid amplifier, and a pulse energy of several tens of millijoules after passing through the second-stage Nd:YVO4 two-way solid amplifier. This signal light enters the beam collimator and expander for beam transformation. After collimation and expansion, the light spot is transformed into a vertical thin strip parallel light spot, and the light spot size is consistent with the cross section of the cut angle of the two end faces of the slat.

[0049] In order to more intuitively demonstrate the composite slab crystal structure in the composite slab amplifier of the present invention, its three-dimensional perspective view and partial enlarged schematic diagram are shown as follows: Figure 5 As shown, Figure 1 and Figure 6 The composite slab crystal in the figure is a top view. It is composed of YAG crystals at both ends of the composite slab crystal, two large outer layers of gradient-doped Nd:YAG crystals, and a YAG crystal in the center sandwich part. It can maintain uniform and rapid heat dissipation while efficiently extracting signal light gain. The four curved cut corners at both ends can effectively compensate for the thermal lens effect of the high-energy composite slab laser amplifier, avoid signal beam distortion, and effectively improve pump efficiency and amplification efficiency.

[0050] like Figure 6 As shown in the figure, the microchannel water-cooled heat sink of the composite slab crystal is directly in contact with the large surface of the two outer layers of gradient-doped Nd:YAG crystals by welding. The pump light enters from the end faces in four directions, which can provide uniform and efficient high-energy pump light for the composite slab crystal. The design of microchannel water cooling and its direct large-surface contact with the doped area are more conducive to the temperature control and heat dissipation of the composite slab crystal.

[0051] The above description is only a preferred embodiment of the present invention and does not limit the present invention in any form. Any simple modification, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the protection scope of the technical solution of the present invention.

Claims

1. A kilohertz repetition rate Joule-level high energy picosecond pulse laser amplifier, characterized by: It includes: Picosecond pulse fiber laser (1), solid regenerative amplifier (2), solid multi-pass amplifier (3) and composite slab amplifier (4); The picosecond pulse fiber laser comprises a SESAM mode-locked picosecond fiber oscillator (11), a fiber laser preamplifier (12), a single-mode fiber stretcher (13) and a fiber laser main amplifier (14); the fiber laser oscillator uses the SESAM mode-locked technology to generate a picosecond pulse laser with a repetition frequency of tens of megahertz; the fiber laser preamplifier is an ytterbium-doped single-mode fiber, which preamplifies the picosecond pulse laser with a repetition frequency of tens of megahertz generated by the SESAM mode-locked picosecond fiber oscillator; The single-mode fiber stretcher is a kilometer-long single-mode passive optical fiber that stretches the laser pulse width to hundreds of picoseconds. The fiber laser main amplifier is an ytterbium-doped double-clad optical fiber that further amplifies the laser pulse. The solid-state regenerative amplifier amplifies the laser signal obtained by the picosecond pulse fiber laser to a repetition frequency of kilohertz and a pulse energy of millijoule level through a regenerative resonant cavity and a Pockels cell modulator; The solid multi-pass amplifier includes multiple end-pumped solid laser amplifiers and beam shaping devices; the end-pumped solid laser amplifiers increase the pulse energy to the ten millijoules level; the beam shaping device collimates and expands the laser spot into a vertical thin strip; The composite slab amplifier includes a gain crystal and several quarter-wave plates, thin-film polarizers, and reflectors. By controlling the polarization state and the incident angle, the picosecond pulse laser of tens of millijoules obtained by the solid multi-pass amplifier passes through the composite slab crystal multiple times, amplifying the kilohertz picosecond pulse laser of the solid multi-pass amplifier to the joule level.

2. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 1, characterized in that: The gain crystal of the composite slab amplifier is a composite of multiple YAG crystals and Nd:YAG crystals, and the contact surfaces between the crystals are tightly connected through a bonding process; the two large surface outer layers of the composite slab are Nd:YAG crystals (42), and the central sandwich part is a first YAG crystal (43), thereby forming a Nd:YAG / YAG / Nd:YAG sandwich structure; the large surface outer layers are directly in contact with a microchannel water-cooled heat sink (45), and the two ends of the composite slab are second YAG crystals (41) serving as input and output ends of pump light and signal light.

3. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 2, characterized in that: The Nd:YAG crystal has a thickness of 250-300 μm and is directly welded to a microchannel water-cooled heat sink (45), and the cooling water temperature is kept constant at 25° C.

4. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 3, characterized in that: The second YAG crystal is composed of four curved cut corners (44), and after two angles of about 45 degrees are cut at each end of the composite strip, a curved depression is ground using a crystal ball milling process, and the curvature radius is 1800-2000mm.

5. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 4, characterized in that: The Nd:YAG crystal is gradient doped with neodymium ions, the doping concentration decreases from the center to both ends, and the doping concentration gradient ranges from 0.3at% to 0.1at%.

6. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 5, characterized in that: The quarter wave plate, thin film polarizer, reflector and gain crystal of the composite slab amplifier constitute a 6-pass laser amplifier, which is realized by laser polarization state control and incident angle control. The laser polarization state control is realized by the thin film polarizer and the quarter wave plate. The reflector controls the incident angle at 20 to 30 degrees to meet the total reflection condition inside the composite slab, so that the signal light is transmitted in a zigzag shape inside the composite slab amplifier.

7. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 6, characterized in that: The single-mode fiber preamplifier is a fiber laser preamplifier, the fiber laser main amplifier is a double-clad fiber main amplifier, and the SESAM mode-locked picosecond fiber oscillator uses ytterbium-doped single-mode fiber as the working material and uses the SESAM mode-locked technology of the linear cavity to obtain a stable small-signal pulse laser with a duration of ten picoseconds at a repetition frequency of tens of megahertz. This signal light is initially amplified in the fiber laser preamplifier and then enters the single-mode fiber stretcher. The pulse duration is stretched to hundreds of picoseconds. The stretched signal light enters the double-clad fiber main amplifier connected to it and passes through the fiber coupler output device to obtain a pulse laser with an average power of hundreds of milliwatts and a pulse duration of hundreds of picoseconds at a repetition frequency of tens of megahertz.

8. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 7, characterized in that: The solid regenerative amplifier comprises: a fiber laser seed injection (21), an optical isolator (22) and a regenerative amplification resonant cavity (23); the fiber laser seed injection injects the signal light of the previous stage into the regenerative amplification resonant cavity at a specific angle and position, the optical isolator prevents the backward return light of the regenerative amplification resonant cavity, the signal light with a megahertz repetition frequency repeatedly passes through the Nd:YAG crystal in the regenerative amplification resonant cavity, thereby obtaining a huge small signal gain, the Pockels cell and the quarter-wave plate in the regenerative amplification resonant cavity control the polarization state and output timing of the laser in the cavity, and selects and amplifies the mode-locked picosecond pulse laser.

9. The kilohertz repetition frequency Joule-level high-energy picosecond pulse laser amplifier according to claim 8, characterized in that: The solid multi-pass amplifier comprises: a first-stage Nd:YVO4 double-pass solid amplifier (31), a second-stage Nd:YVO4 double-pass solid amplifier (32) and a beam collimating expander (33); after the signal light from the solid regenerative amplifier passes through the first-stage Nd:YVO4 double-pass solid amplifier and the second-stage Nd:YVO4 double-pass solid amplifier, the pulse energy is increased to a level of tens of millijoules, and then enters the beam collimating expander for beam conversion; after the collimating expander, the light spot is converted into a vertical thin strip parallel light spot, and the light spot size is consistent with the cross section of the cut angle of the two end faces of the strip.

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