A hybrid high-power and high-energy femtosecond chirped pulse amplification system and method

Abstract

In order to solve the problems existing in the existing high-power and high-energy femtosecond fiber amplification system, such as poor beam quality, low output energy, and high system integration complexity under the condition of high-power femtosecond output, the present invention provides a hybrid high-power and high-energy femtosecond chirped pulse amplification system and method. The amplification system and method of the present invention use a silicate glass fiber amplifier to form a pre-amplification system, cascade a single-crystal fiber with a single-pass structure as the main amplifier, abandon the use of an isolator based on a Faraday rotator, prevent the distortion of the beam quality caused by the thermal effect of the Faraday rotator crystal, self-assemble an isolator, and adopt a simple combination of independent devices of a polarization beam splitter prism and a quarter-wave plate, eliminating the thermal lens effect of the Faraday rotator and improving the beam quality. By using as few lenses as possible and a single-pass amplification structure, the optical path is simple, stable and reliable; by using aperture spatial filtering, a high beam quality output under a high power of more than 100 W is obtained.

Description

Technical Field

[0001] The present invention belongs to the field of laser technology, and particularly relates to a hybrid high-power and high-energy femtosecond chirped pulse amplification system and method. Background Art

[0002] Due to the advantages of high integration, easy maintenance, good heat dissipation, good beam quality, high amplification gain, etc., fiber lasers have attracted much attention in the field of lasers. As a new means of precision machining, high-power and high-energy femtosecond lasers have shown great application prospects in the aerospace field because of their small thermal effect during machining, high surface integrity after machining, and no machining defects such as microcracks and recast layers. However, affected by nonlinear effects, thermal distortion, amplified spontaneous emission, gain narrowing and other effects, there are still great challenges for fiber femtosecond amplification systems with an output power greater than 100W and an output energy greater than 100 μJ.

[0003] In terms of high-power and high-energy amplification, in order to obtain a high-gain amplified output, generally a longer gain fiber is required to obtain a high-power amplified output. However, as the amplification energy increases, when the fiber length is too large, the nonlinear accumulation of the amplification system will be strengthened, resulting in pulse distortion after compression, an increase in the pulse pedestal, and a reduction in the peak power of the system. At the same time, as the amplification power increases, thermal effects, thermal depolarization, peak power damage and average power damage of devices have become important factors affecting the output stability of high-power amplification systems.

[0004] For fiber-solid hybrid amplification systems, due to the small single-pass gain of traditional crystal rod-packaged solid-state laser amplifiers and the large crystal diameter, during high-power pumping, the thermal effect is serious, affecting the stability of the system. At the same time, thermal distortion will also deteriorate the beam quality. Due to the low amplification gain, generally traditional solid-state amplifiers will adopt multi-pass and cascaded methods to achieve an output of hundreds of watts, resulting in a complex structure. During the cascaded amplification process, the absorption and re-amplification of signal laser by the crystal will also cause a more serious gain narrowing effect, which is not conducive to obtaining a narrow pulse output after final compression. If the gain narrowing is serious, the system cannot guarantee the output of a femtosecond-level pulse width.

[0005] In existing high-power and high-energy femtosecond fiber amplification systems, for the main amplification, by using large-mode-field silica fibers, due to the limitation of the fiber mode field diameter, it is difficult to achieve high-energy output. And in order to obtain a high-gain amplified output, a longer fiber is required, which results in strong nonlinear accumulation in the system, a large output pulse pedestal, and a large pulse width. Summary of the Invention

[0006] The object of the present invention is to solve the problems existing in the existing high-power and high-energy femtosecond fiber amplification system, namely, poor beam quality, low output energy, and high system integration complexity under the condition of high-power femtosecond output. Therefore, a hybrid high-power and high-energy femtosecond chirped pulse amplification system and method are provided. A silicate glass fiber amplifier is used to form a pre-amplification system, and a single-crystal fiber with a single-pass structure is cascaded as a main amplifier to achieve high-power and high-energy femtosecond output.

[0007] To achieve the above object, the technical solution adopted by the present invention is as follows:

[0008] A hybrid high-power and high-energy femtosecond chirped pulse amplification system, characterized in that it includes an all-fiber pre-amplification based on a silicate glass fiber amplifier and a two-stage amplification unit connected thereto;

[0009] The two-stage amplification unit includes a first lens, a first self-assembled isolation component, a single-crystal fiber, and a first dichroic mirror sequentially arranged on the output optical path of the all-fiber pre-amplification;

[0010] The two-stage amplification unit further includes a pump LD and a third lens, a second lens, and a second dichroic mirror sequentially arranged on the output optical path of the pump LD;

[0011] The output light of the all-fiber pre-amplification is collimated by the first lens and then incident on the first self-assembled isolation component. After the polarization state is adjusted by the first self-assembled isolation component, it is incident on the single-crystal fiber; the pump light output by the pump LD passes through the third lens and the second lens for pump coupling in sequence, is focused and then incident on the second dichroic mirror and the first dichroic mirror for pumping, and then incident on the single-crystal fiber to pump the single-crystal fiber, amplify the signal light, and reflect the amplified signal light through the first dichroic mirror for output;

[0012] On the optical path of the signal light reflected by the first dichroic mirror, a fourth lens, a second self-assembled isolation component, a fourth high-reflection mirror, a compressor component, and a lifting mirror are sequentially arranged; the compressor component includes a first grating and a second grating;

[0013] The signal light reflected by the first dichroic mirror is incident on the fourth lens for collimation. After collimation, it is incident on the second self-assembled isolation component for polarization state adjustment. The adjusted polarized light is incident on the first grating and the second grating. After being diffracted by the first grating and the second grating, it is incident on the lifting mirror to reduce the light height, and then reflected to the second grating and the first grating. After diffraction and transmission, it is incident on the fourth high-reflection mirror for reflection to obtain the amplified and compressed femtosecond pulse output;

[0014] The first self-assembled isolation component includes a first half-wave plate, a first polarization beam splitter prism, and a first quarter-wave plate sequentially arranged along the optical path; the second self-assembled isolation component includes a second quarter-wave plate, a second polarization beam splitter prism, and a second half-wave plate sequentially arranged along the optical path.

[0015] Furthermore, the first grating and the second grating are arranged in parallel.

[0016] Furthermore, it further includes a first high reflector disposed between the first half-wave plate and the first polarization beam splitter prism, a second high reflector between the first quarter-wave plate and the single crystal fiber, and a third high reflector on the reflection optical path of the first dichroic mirror, for deflecting the optical path;

[0017] The first high reflector and the second high reflector are arranged perpendicular to each other.

[0018] Furthermore, a water-cooled aperture is provided at the incident end of the single crystal fiber, for absorbing the pump light that is not completely absorbed by the single crystal fiber and is transmitted;

[0019] A small hole aperture for optimizing the beam quality of the pulsed output is provided on the reflection optical path of the fourth high reflector.

[0020] Furthermore, the linear distance between the first grating and the second grating is 1.8 m, the compression efficiency is 80%, and the maximum output power after compression is 110 W;

[0021] The size of the first grating is 41 mm * 30 mm * 6.36 mm, and the size of the second grating is 130 mm * 30 mm * 6.35 mm, which can avoid light cutting;

[0022] The single crystal fiber is a Yb:YAG crystal with a length of 40 mm and a diameter of 1 mm, and is encapsulated with a water-cooled structure. The optical path distance between the single crystal fiber and the all-fiber pre-amplification is 540 mm, and the signal spot size in the crystal remains at 400 μm;

[0023] The pump LD is a fiber-coupled output semiconductor laser, with an output wavelength of 940 nm, a core diameter of 135 μm, a numerical aperture of 0.22, and a maximum output power of 200 W.

[0024] Furthermore, the focal length of the second lens is 100 mm, for ensuring that the LD pump laser converges and enters the single crystal fiber, and the focus is located 5 mm inside the single crystal fiber;

[0025] The focal length of the third lens is 40 mm, for approximately collimating the LD laser output by the pump LD;

[0026] Both the first dichroic mirror and the second dichroic mirror have high transmittance for 940 nm, with a transmittance greater than 96%, and high reflectance for 1030 nm, with a reflectance greater than 99.5%.

[0027] Furthermore, the first lens is installed on a two-dimensional lens adjustment mount, and by finely adjusting the two-dimensional lens adjustment mount, it is ensured that the light intensity fed back to the all-fiber pre-amplification during the amplification process is minimized.

[0028] The present invention also provides a hybrid high-power and high-energy femtosecond chirped pulse amplification method, based on the above-mentioned hybrid high-power and high-energy femtosecond chirped pulse amplification system, which is characterized by including the following steps:

[0029] 1), Start the all-fiber pre-amplification, adjust the position of the first lens, use a CCD detector to measure that the spot sizes of the incident and output laser beams of the single-crystal fiber meet the amplification requirements, fixedly install the first lens at the position where the spot size meets the amplification requirements, and adjust so that the low-power laser beam signal light output by the all-fiber pre-amplification passes through the center of the first lens;

[0030] 2), Gradually increase the pump current in the all-fiber pre-amplification, measure the amplification power of the output laser at different powers, and finely adjust the first lens so that the feedback signal light at the monitoring end of the photoelectric detection module in the all-fiber pre-amplification is the weakest;

[0031] 3), Rotate the first half-wave plate so that the power of the laser beam passing through the first polarization beam splitter prism is the largest;

[0032] 4), Add an auxiliary polarization beam splitter prism behind the first quarter-wave plate, rotate the first quarter-wave plate so that the transmitted light and reflected light powers of the auxiliary polarization beam splitter prism are equal, fix the first quarter-wave plate and remove the auxiliary polarization beam splitter prism;

[0033] 5), Adjust the direction of the incident signal light so that the signal light is coaxially incident on the single-crystal fiber, and use a CCD detector to measure to ensure that the spot passing through the single-crystal fiber has a single-mode energy distribution;

[0034] 6), Turn on the pump LD, inject the initial low-power pump light into the pump end of the single-crystal fiber, adjust the second lens so that the power of the signal light reflected by the first dichroic mirror is optimized to the maximum; gradually increase the pump power so that the power of the signal light output by the single-crystal fiber reaches more than 130 W;

[0035] 7), Rotate the second quarter-wave plate so that the output power after the second polarization beam splitter prism is the largest, and rotate the second half-wave plate so that the polarization injection into the first grating is the best, achieving the highest diffraction efficiency;

[0036] 8), Adjust the angle of the first grating so that the diffraction angle is the best, achieving the highest diffraction efficiency, use a CCD detector to measure the spot of the compressed signal light, and optimize the angle of the second grating so that the spot reaches the best roundness;

[0037] 9), Use a lifting mirror to reduce the optical height of the signal light, reflect it to the second grating and the first grating, and after diffraction and transmission, it is incident on the fourth high-reflection mirror, and the amplified and compressed femtosecond pulse is derived.

[0038] Further, in step 1), the power of the laser beam output by the all-fiber pre-amplification is 500 mW;

[0039] The optical path distance between the single-crystal optical fiber and the first lens is 540 mm, and the spots of the incident and output laser of the single-crystal optical fiber are 400 μm;

[0040] In step 6), the initial low power of the pump light output by the pump LD is 1-2 W;

[0041] In step 6), the gradually increasing of the pump power means that the pump power of the pump light output by the pump LD is 150-200 W.

[0042] Further, in step 9), the lifting mirror reduces the beam height by 10 mm.

[0043] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0044] 1. The hybrid high-power and high-energy femtosecond chirped pulse amplification system provided by the present invention uses a silicate glass fiber amplifier to form a pre-amplification system, and cascades a single-crystal optical fiber with a single-pass structure as the main amplifier. The combined amplifier has unique advantages under the high-power output of more than one hundred watts at MHz. The front-end silicate glass fiber amplifier can output high-power ultrashort pulses with high beam quality and a large output spectral width. The main amplification is a simple single-pass amplifier, which is amplified under large-signal injection, with a small amplification multiple and small gain narrowing, which is conducive to the system to achieve ultrashort pulse output. At the same time, the rear-end Yb:YAG single-crystal fiber amplifier has better amplification efficiency than traditional crystal rods, requires low pump power, has a large surface-volume ratio, and good heat dissipation, effectively ensuring the beam quality under high-power output.

[0045] 2. The hybrid high-power and high-energy femtosecond chirped pulse amplification system and method provided by the present invention improve the stability and beam quality of the optical path system. On the one hand, it is the optimization of the thermal effect, mainly the selection of devices, the combination of silicate and single-crystal optical fibers, and the abandonment of the isolator based on the optical rotator to prevent the distortion of the beam quality caused by the thermal effect of the optical rotator crystal. A simple combination of independent devices of a polarization beam splitter prism and a quarter-wave plate is used to form a self-made isolator, eliminating the thermal lens effect of the optical rotator and improving the beam quality. Using as few lenses as possible and a single-pass amplification structure makes the optical path simple, stable and reliable; using aperture spatial filtering to obtain high beam quality (M 2 <1.3) output at a high power of more than 100 W.

[0046] 3. The hybrid high-power and high-energy femtosecond chirped pulse amplification system provided by the present invention has the characteristics of high amplification output power, compact structure, high output beam quality, and small nonlinear accumulation, and is particularly suitable for industrial integration. As an industrial femtosecond laser, it is applied to the field of femtosecond precision microfabrication. Description of the Drawings

[0047] Figure 1 This is the system structure diagram of the all-fiber preamplifier in the embodiment of the present invention;

[0048] Figure 2 This is the output power curve diagram of the all-fiber preamplifier in the embodiment of the present invention;

[0049] Figure 3 This is the output beam quality test diagram of the all-fiber preamplifier in the embodiment of the present invention;

[0050] Figure 4 This is the schematic structural diagram of the hybrid high-power and high-energy femtosecond chirped pulse amplification system in the embodiment of the present invention;

[0051] Figure 5 This is the power stability test diagram of the amplified output by the embodiment of the present invention;

[0052] Figure 6 This is the spectrum test diagram of the amplified output by the embodiment of the present invention;

[0053] Figure 7 This is the output beam quality test diagram of the amplified and compressed output by the embodiment of the present invention;

[0054] Figure 8 This is the pulse width autocorrelation curve diagram of the output by the embodiment of the present invention;

[0055] Reference numerals:

[0056] 1 - All-fiber preamplifier, 2 - First lens, 3 - First half-wave plate, 4 - First high-reflection mirror, 5 - First polarization beam splitter prism, 6 - First quarter-wave plate, 7 - Second high-reflection mirror, 8 - Water-cooled aperture stop, 9 - Single-crystal fiber, 10 - First dichroic mirror, 11 - Second dichroic mirror, 12 - Second lens, 13 - Third lens, 14 - Pumping LD, 15 - Third high-reflection mirror, 16 - Fourth lens, 17 - Second quarter-wave plate, 18 - Second polarization beam splitter prism, 19 - Second half-wave plate, 20 - Small aperture stop, 21 - Fourth high-reflection mirror, 22 - First grating, 23 - Second grating, 24 - Lift mirror. Detailed implementation manners

[0057] To make the objectives, advantages and features of the present invention clearer, the following further elaborates on a hybrid high-power and high-energy femtosecond chirped pulse amplification system and method proposed by the present invention in conjunction with the accompanying drawings and specific embodiments. Those skilled in the art should understand that these embodiments are only used to explain the technical principles of the present invention and are not intended to limit the protection scope of the present invention. In the description of the present invention, it should be noted that the terms "first" and "second" are only used for descriptive purposes and cannot be construed as indicating or implying relative importance.

[0058] A hybrid high-power and high-energy femtosecond chirped pulse amplification system provided by the present invention includes an all-fiber pre-amplifier 1 based on a silicate glass fiber amplifier and a secondary amplification unit connected thereto.

[0059] As Figure 1 shown, the all-fiber pre-amplifier 1 (a high-gain and low-nonlinear all-fiber ultrashort pulse amplifier and amplification method based on silicate glass, CN111200234A) sequentially employs a polarization-maintaining mode-locked seed source. The seed source outputs mode-locked pulses with an output power of 15 mW, a spectral width of 14 nm, and a repetition frequency of 25 MHz. The output end is connected to a three-port single-mode polarization-maintaining fiber-coupled circulator. The second port is connected to a reflective chirped fiber grating. The dispersion amount of the chirped fiber grating is 100 ps / nm, and the pulse width after broadening is a sub-nanosecond pulse width, reducing the peak power and nonlinear accumulation of the subsequent stage amplification. The third port is connected to a polarization-maintaining beam combiner and pumped by a 9W LD. The multimode output fiber of the 9W fiber-coupled LD is connected to the pumping end of the fiber beam combiner. The output end of the combiner is welded to a double-clad ytterbium-doped fiber with a core diameter of 10 μm and a cladding of 125 μm. The fiber length is 1.2 m. This stage achieves an amplified power output of 1W and is sequentially connected to a fiber-coupled isolator and a fiber-coupled acousto-optic modulator to modulate the repetition frequency of the seed source and reduce the repetition frequency to 2 MHz. The optical power after frequency reduction is only about 3 mW.

[0060] Then it enters the next-stage 10 / 125 double-clad amplifier for power boosting. The 3mW signal fiber is connected to the signal end of the beam combiner and pumped by a 9W LD. The multimode output fiber of the 9W fiber-coupled LD is connected to the pumping end of the fiber beam combiner. The output end is welded to a double-clad ytterbium-doped fiber with a core diameter of 10 μm and a cladding of 125 μm. The fiber length is 1 m. The amplified output power provided by this stage is about 600 mW at 2 MHz, and the injected laser of about 600 mW is isolated and output through the isolator. The input and output fibers of the isolator are both double-clad fibers with a core diameter of 10 μm and a cladding of 125 μm. The output end of the isolator is respectively connected to a circulator and a high-power beam combiner, and the remaining end is used as the monitoring end of the feedback light. Among them, the fibers of the three ports of the circulator are all double-clad fibers with a core diameter of 10 μm and a cladding of 125 μm, reducing the splicing loss caused by different fiber core diameters. The high-power beam combiner has two pumping ends, and the maximum single-arm power can withstand 100W. Two high-power narrow-linewidth 976nm multimode LDs are connected to the two pumping ends of the high-power beam combiner by welding. The output end of the combiner is connected to a silicate fiber. The length of the silicate fiber is only 0.2 m, and the mode field diameter is 40 μm. The silicate fiber uses a double-V groove filled with thermally conductive silicone for water cooling for heat dissipation.

[0061] The maximum output of the all-fiber preamplifier 1 is ~100 W. The output power cannot be further increased. After output, compression is performed. Generally, the compression efficiency is only 80%-85%. After compression, the femtosecond output power is basically around 80 W, and high-power femtosecond output above 100 W cannot be achieved.

[0062] As Figure 2 and Figure 3 shown, in the double-arm pumping, two 100-W LDs are used in the two pumping arms of the silicate amplification module. The core diameter of the pumping fiber is 105 μm, and the maximum output power is 100 W. It is injected through the two pumping ends of the beam combiner. At a repetition rate of 2 MHz, when the injected signal power is ~600 mW and the pumping power is 162 W, the amplified output power reaches 100.5 W. The beam quality of the output laser is measured. The beam quality M x 2 = 1.035, M x 2 = 1.051, and the beam quality is close to the diffraction limit. The slope efficiency of this amplifier is very high, reaching 70%, and it is an ideal high-gain and low-nonlinear amplifier for achieving high-power output.

[0063] As Figure 4 shown, the two-stage amplification unit includes a first lens 2, a first half-wave plate 3, a first high-reflection mirror 4, a first polarization beam splitter prism 5, a first quarter-wave plate 6, a second high-reflection mirror 7, a water-cooled aperture 8, and a single-crystal fiber 9 arranged in sequence on the output optical path of the all-fiber preamplifier 1;

[0064] A pumping LD 14 is provided at the rear end of the single-crystal fiber 9; on the output optical path of the pumping LD 14, a third lens 13, a second lens 12, a second dichroic mirror 11, and a first dichroic mirror 10 are arranged in sequence;

[0065] On the optical path of the signal light output from the single-crystal fiber 9, a fourth lens 16, a second quarter-wave plate 17, a second polarization beam splitter prism 18, a second half-wave plate 19, a first grating 22, a second grating 23, and a boosting mirror 24 are arranged in sequence;

[0066] A fourth high-reflection mirror 21 is provided on the reflection optical path of the boosting mirror 24 for reducing the light height, and a small-aperture diaphragm 20 for optimizing the beam quality is provided on the reflection output optical path of the fourth high-reflection mirror 21.

[0067] After the output of the all-fiber preamplifier 1, it is collimated by the first lens 2 with a diameter of 50.8 mm. The first lens 2 is installed on a two-dimensional lens adjustment mount and can be adjusted up, down, left, and right. The output light is perpendicularly incident on the first lens 2 to reduce the influence of the first lens 2 on the beam quality. Through fine adjustment of the two-dimensional lens adjustment mount, it is ensured that the light intensity of the light fed back into the pre-amplification gain module in the all-fiber preamplifier 1 is minimized during the amplification process. The spot of the transmitted light is approximately collimated and slightly convergent. The spot size is ~400 μm at 540 mm behind the lens, meeting the injection requirements of the single-crystal fiber 9 for the signal. The transmitted light is incident on the single-crystal fiber 9 after the optical path is folded by the first highly reflective mirror 4 and the second highly reflective mirror 7 arranged vertically.

[0068] Isolation must be carried out between the all-fiber preamplifier 1 and the single-crystal fiber 9. Otherwise, when the backward amplification feedback enters the high-gain all-fiber preamplifier 1, it will damage the amplifier. In existing amplification systems, an isolator composed of two polarization beam splitters with a 45-degree optical rotation crystal in the middle is often directly added. However, in this system, the spot size is relatively small, with a diameter of only 1 - 1.5 mm and a power of nearly 100 W, resulting in a very high average power density. The optical rotation crystal in the isolator itself has a thermal lens effect, which affects the spot size injected into the single-crystal fiber at different powers, causing instability in the system amplification. At the same time, the size of the isolator is limited by the length of the optical rotation crystal, generally about 80 mm. The direct normal incidence has a strong feedback on the all-fiber preamplifier 1, posing a risk of damaging the pre-stage amplifier. If a certain small angle is added, the isolator is relatively long, and it is easy to have a large deviation from the optimal angle of incidence of the polarization beam splitter, resulting in an increase in loss. Therefore, in this embodiment, a self-assembled isolation component is added between the all-fiber preamplifier 1 and the single-crystal fiber.

[0069] The first self-assembled isolation component consists of a first half-wave plate 3, a first polarization beam splitter 5, and a first quarter-wave plate 6. The first half-wave plate 3 is used to adjust the polarization direction of the transmitted light to horizontal polarization. The first polarization beam splitter 5 performs polarization splitting, and the first quarter-wave plate 6 converts the polarization into circular polarization. The specific adjustment method for circular polarization is as follows: An auxiliary adjustment polarization beam splitter is added behind the first quarter-wave plate 6 to make the splitting power on both sides of the polarization beam splitter equal. After fixing the angle and position of the first quarter-wave plate 6, the auxiliary adjustment polarization beam splitter is removed.

[0070] The single-crystal fiber 9 is a Yb:YAG crystal with a length of 40 mm and a diameter of 1 mm, encapsulated with a water-cooling structure. The optical path distance between the single-crystal fiber 9 and the output end face of the all-fiber pre-amplifier 1 is 587 mm, and the signal spot size in the crystal remains at about 400 μm. The output end of the single-crystal fiber 9 is connected to the pump LD14. The pump source is a fiber-coupled output semiconductor laser with an output wavelength of 940 nm, a core diameter of 135 μm, a numerical aperture of 0.22, and a maximum output power of 200 W. The output light of the pump LD14 is sequentially pumped and coupled through the third lens 13 with a focal length of 40 mm and the second lens 12 with a focal length of 100 mm. The third lens 13 first approximately collimates the output LD laser, and the second lens 12 then focuses the pump light, controlling the beam waist size to be ~400 μm, so that it enters about 5 mm into the single-crystal fiber 9. After focusing, it is incident on the second dichroic mirror 11 and the first dichroic mirror 10 arranged at 45° for pumping, and then incident on the single-crystal fiber 9 to pump the single-crystal fiber 9. The amplified signal light is reflected and output by the first dichroic mirror; both the first dichroic mirror 10 and the second dichroic mirror 11 are lenses with high transmittance for 940 nm (transmittance > 96%) and high reflectance for 1030 nm (reflectance > 99.5%).

[0071] When the signal injection is 90 W and the pump power is 147.9 W, the output power of the single-crystal fiber 9 reaches 138.9 W. In the case of high-power output, due to the adoption of a single-pass amplification structure and the slender structure of the single-crystal fiber 9, with good heat dissipation, it has good output power stability, and the output power stability RMS = 0.21% (as Figure 5 shown). Due to the large signal intensity at the front end, the amplification multiple requirement of the single-crystal fiber 9 is low, and the spectral width of the amplified output can still ensure an output of about 10 nm, effectively ensuring the output of the subsequent pulse width.

[0072] The laser beam after beam splitting is reflected by the third high-reflection mirror 15 and then incident on the fourth lens 16 for collimation. After transmission, it is incident on the second quarter-wave plate 17 to convert the transmitted light into horizontally polarized light. After polarization by the second polarization beam splitter prism 18, the depolarization loss during the amplification process is eliminated, and linearly polarized light with a high degree of polarization is obtained. Then it is incident on the second half-wave plate 19 to adjust the polarization direction of the laser beam to the polarization direction with the maximum grating diffraction efficiency.

[0073] The adjusted polarized light is incident on the first grating 22, the second grating 23, and the lifting mirror 24, and finally reflected and output by the fourth high-reflection mirror 21.

[0074] The first grating 22 and the second grating 23 are arranged in parallel, and the linear distance between the two is 1.8 m. The formed grating pair compressor has a compression efficiency of about 80%, and the maximum output power after compression is 110 W. The size of the first grating 22 is 41 mm * 30 mm * 6.36 mm, and the size of the second grating 23 is 130 mm * 30 mm * 6.35 mm, which can avoid beam cutting. To improve the polarization degree, the second polarization beam splitting prism 18 is used for polarization. After the fourth high reflector 21 reflects and outputs, a small hole aperture 20 is used to optimize the beam quality. After polarization optimization, the output power reaches 100 W (as Figure 6 shown), and the beam quality test results are: M x 2 = 1.27; M Y 2 = 1.19 (as Figure 7 shown). After preliminary compression, the pulse width reaches 507 fs (as shown in Figure 8). Through the optimization of the higher-order dispersion of the system, the pulse width is expected to be further compressed. The first grating 22 and the second grating 23 are used to provide dispersion compensation to compress the pulse to femtoseconds. After passing through the second grating 23, the light is incident on the lifting mirror 24, and the light is reflected back by about 10 mm along the original path. The fourth high reflector 21 is used to export the light, and a small hole aperture is used to optimize the beam quality.

[0075] Generally speaking, this system can verify the output of femtoseconds above 100 W, and has good beam quality under high-power conditions, which can be better than 1.3. From a safety perspective, at a MHz repetition rate, it has a stable output capacity of more than 100 W with high beam quality and stability. From the perspective of high energy, at a repetition rate of hundreds of kHz, verified at 200 kHz, this system also has a high-energy femtosecond output capacity of more than 30 W / 150 μJ.

[0076] Based on the above patents, this embodiment conducts cascaded coupling of the system. The amplification system is used as a pre-amplification system, and then a single-crystal fiber amplifier is added. A feedback suppression adjustment method, device optimization under high power, and system structure optimization are developed to achieve high beam quality and high-stability femtosecond output above 100 W.

[0077] A method for obtaining a high-power and high-beam-quality femtosecond pulse output by using the above hybrid high-power and high-energy femtosecond chirped pulse amplification system specifically includes the following steps:

[0078] 1. Turn on the all-fiber preamplifier 1 with a power output of 500 mW, allowing the laser to pass through the middle of the first lens 2 (f = 50.8 mm). Move the first lens 2 back and forth to collimate and slightly converge the output laser beam, so that the spot size at the position where the laser beam enters the end face of the single-crystal fiber 9 is approximately 400 μm, meeting the spot requirements during amplification. In the embodiment, the spot size is ensured to be 400 μm at a distance of 587 mm from the output end face of the all-fiber preamplifier 1.

[0079] 2. Gradually increase the pump current of the all-fiber preamplifier 1 until the output power reaches 20 W. Optimize the up, down, left, and right positions of the first lens 2 with an adjustment amount not exceeding one turn of the thread (about 100 μm) to make the feedback light of the all-fiber preamplifier the weakest. Then increase the silicate pump until the output power reaches ~90 W, and measure the feedback light intensity P0. In the embodiment, when the output power of the all-fiber preamplifier 1 is 90 W, the feedback light intensity is 1.8 mW. After the measurement, adjust the output power of the all-fiber preamplifier 1 back to 500 mW.

[0080] 3. Move the first half-wave plate 3 to maximize the power of the laser beam passing through the first polarization beam splitter prism 5.

[0081] 4. Add an auxiliary polarization beam splitter prism after the first quarter-wave plate 6. Rotate the first quarter-wave plate 6 to make the transmitted light and reflected light powers of the incident light to the auxiliary polarization beam splitter prism equal. Fix the first quarter-wave plate 6 and remove the auxiliary polarization beam splitter prism.

[0082] 5. The position of the single-crystal fiber 9 is adjusted in coordination with the fine-tuning of the directions of the first high-reflection mirror 4 and the second high-reflection mirror 7. The signal light end face of the single-crystal fiber 9 is 587 mm away from the output end face of the all-fiber preamplifier 1. Adjust this pair of mirror combinations to maximize the power passing through the single-crystal fiber 9 and ensure that the output laser energy distribution pattern is basically the same as the pattern before passing through the single crystal.

[0083] 6. Increase the pre-amplification pump to make the power output reach about 10 W. Turn on the pump LD14 and inject 1 - 2 W of pump light into the pump end of the single-crystal fiber 9.

[0084] In this embodiment, the distance from the output end of the pump LD14 to the third lens 13 is 40 mm, the distance between the third lens 13 and the second lens 12 is 68 mm, the distance from the second lens 12 to the single-crystal fiber is about 76 mm, and the pump spot size at this position is about 400 μm.

[0085] When the output power of the all-fiber preamplifier 1 reaches about 10 W and 1 - 2 W of pump light is injected into the pump end of the crystal, adjust the up, down, left, and right of the 100 mm lens to optimize the amplified light reflected by the dichroic mirror to the maximum.

[0086] 7. Adjust the signal light intensity to 90 W for injection, gradually increase the pump power, test the amplified output power, and measure the feedback light intensity of the all-fiber preamplifier 1. If the feedback light intensity increases approximately linearly and the increased signal light is no more than 30 mW. If the feedback intensity is greater than 30 mW, the pair of reflecting mirrors for single-crystal amplification coupling injection is readjusted until the feedback intensity meets the requirements.

[0087] In this embodiment, when the pump light of the single-crystal fiber 9 reaches 147.9 W, the output power of the single-crystal fiber 9 can reach 138.9 W. (If the feedback is strong, the all-fiber preamplifier 1 is easily damaged. This part is the feedback suppression adjustment of the all-fiber preamplifier 1 during the single-crystal fiber amplification process).

[0088] 8. After the single-crystal fiber amplification output, the output light needs to be collimated. To test the collimation of the laser, the amplified high-power laser needs to be attenuated. In the case of low-power output (about 1 W), the laser reflected by the third high reflector 15 first passes through the fourth lens 16, the second quarter-wave plate 17, the second polarization beam splitter prism 18, the second half-wave plate 19, and the auxiliary polarization beam splitter prism in sequence;

[0089] 9. Rotate the second quarter-wave plate 17 to convert the light into horizontal linearly polarized light. The judgment method is that the power transmitted through the second polarization beam splitter prism 18 reaches the maximum. Rotate the optical axis angle of the second half-wave plate 19 to make most of the signal light reflected by the auxiliary polarization beam splitter prism and the transmitted light the weakest.

[0090] Add the entire amplification pump to make the maximum amplification power output. Due to attenuation, the size of the light spot can be directly collected by the CCD. We test the size of a light spot in the near field (0.2 m) and the far field (2 m) behind the polarization beam splitter prism. According to the change in the size of the tested light spot, the divergence angle is calculated. The divergence angle is controlled within 1 mrad. If it is too large, reduce the power to 1 W output, adjust the front and rear positions of the 250 mm lens, repeat the above operations until the collimation meets the requirements, fix the second half-wave plate 19 and remove the auxiliary polarization beam splitter prism.

[0091] 10. After the incident light enters the grating, it is necessary to adjust the angle of the first grating 22 and the angle of the second half-wave plate 19 in front of the grating to make the first grating 22 satisfy the incident optimal polarization direction and diffraction angle. The judgment criterion is that the diffracted light is the strongest. The grating ruling of the first grating 22 is 1600 line / mm, and the corresponding optimal diffraction angle is 55.6°. On this basis, fine-tune the angle to make the diffracted light the strongest. Rotate the second half-wave plate 19 to test the strongest diffracted light.

[0092] After diffraction, the dispersion amount of the compressed pulse should be satisfied. The straight-line distance between the grating pairs is 1.8 m. The second grating 23 should be parallel to the first grating 22 in theory. After the elevation mirror 24 reduces the light height, the light returns along the original path, and the laser is reflected and output through the fourth high reflector 21. The parallelism between the second grating 23 and the first grating 22 can be judged by the roundness of the spot output by reflection. If the spot is elliptical, optimize the angle of the second grating 23 to make the spot reach the best roundness.

[0093] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present invention.

Claims

1. A hybrid high-power and high-energy femtosecond chirped pulse amplification system, characterized in that: it includes an all-fiber pre-amplification (1) based on a silicate glass fiber amplifier and a secondary amplification unit connected thereto; the secondary amplification unit includes a first lens (2), a first self-assembled isolation component, a single-crystal fiber (9), and a first dichroic mirror (10) sequentially arranged on the output optical path of the all-fiber pre-amplification (1); the secondary amplification unit further includes a pump LD (14) and a third lens (13), a second lens (12), and a second dichroic mirror (11) sequentially arranged on the output optical path of the pump LD (14); the output light of the all-fiber pre-amplification (1) is collimated by the first lens (2) and then incident on the first self-assembled isolation component. After the polarization state is adjusted by the first self-assembled isolation component, it is incident on the single-crystal fiber (9); the pump light output by the pump LD (14) is sequentially pumped and coupled through the third lens (13) and the second lens (12), focused and then incident on the second dichroic mirror (11) and the first dichroic mirror (10) for pumping, and then incident on the single-crystal fiber (9) to pump the single-crystal fiber (9), amplify the signal light, and reflect and output the amplified signal light through the first dichroic mirror (10); on the optical path of the signal light reflected by the first dichroic mirror (10), a fourth lens (16), a second self-assembled isolation component, a fourth high-reflection mirror (21), a compressor component, and a lifting mirror (24) are sequentially arranged; the compressor component includes a first grating (22) and a second grating (23); the signal light reflected by the first dichroic mirror (10) is incident on the fourth lens (16) for collimation. After collimation, it is incident on the second self-assembled isolation component for polarization state adjustment. The adjusted polarized light is incident on the first grating (22) and the second grating (23). After diffraction by the first grating (22) and the second grating (23), it is incident on the lifting mirror (24) to reduce the light height, and then reflected to the second grating (23) and the first grating (22). After diffraction and transmission, it is incident on the fourth high-reflection mirror (21) for reflection to obtain an amplified and compressed femtosecond pulse output; the first self-assembled isolation component includes a first half-wave plate (3), a first polarization beam splitter prism (5), and a first quarter-wave plate (6) sequentially arranged along the optical path; the second self-assembled isolation component includes a second quarter-wave plate (17), a second polarization beam splitter prism (18), and a second half-wave plate (19) sequentially arranged along the optical path.

2. The hybrid high-power and high-energy femtosecond chirped pulse amplification system according to claim 1, characterized in that: the first grating (22) and the second grating (23) are arranged in parallel.

3. The hybrid high-power and high-energy femtosecond chirped pulse amplification system according to claim 2, characterized in that: it further includes a first high-reflection mirror (4) arranged between the first half-wave plate (3) and the first polarization beam splitter prism (5), a second high-reflection mirror (7) between the first quarter-wave plate (6) and the single-crystal fiber (9), and a third high-reflection mirror (15) on the reflected optical path of the first dichroic mirror (10) for deflecting the optical path; the first high-reflection mirror (4) and the second high-reflection mirror (7) are arranged perpendicular to each other.

4. The hybrid high-power large-energy femtosecond chirped pulse amplification system according to any one of claims 1-3, characterized in that: a water-cooled aperture (8) is provided at the incident end of the single-crystal optical fiber (9) for absorbing the pump light transmitted without being completely absorbed by the single-crystal optical fiber (9); a small-aperture diaphragm (20) for optimizing the beam quality of the pulsed output light is provided on the reflection optical path of the fourth high-reflection mirror (21).

5. The hybrid high-power large-energy femtosecond chirped pulse amplification system according to claim 4, characterized in that: the linear distance between the first grating (22) and the second grating (23) is 1.8 m, the compression efficiency is 80%, and the output power after compression is at most 110 W; the size of the first grating (22) is 41 mm * 30 mm * 6.36 mm, and the size of the second grating (23) is 130 mm * 30 mm * 6.35 mm, which can avoid light cutting; the single-crystal optical fiber (9) is a Yb:YAG crystal with a length of 40 mm and a diameter of 1 mm, and is encapsulated by a water-cooled structure. The optical path distance between the single-crystal optical fiber (9) and the all-fiber pre-amplification (1) is 540 mm, and the signal spot size in the crystal remains at 400 μm; the pump LD (14) is a fiber-coupled output semiconductor laser with an output wavelength of 940 nm, a core diameter of 135 μm, a numerical aperture of 0.22, and a maximum output power of 200 W.

6. The hybrid high-power large-energy femtosecond chirped pulse amplification system according to claim 5, characterized in that: the focal length of the second lens (12) is 100 mm, which is used to ensure that the LD pump laser converges and enters the single-crystal optical fiber (9), and the focal point is located 5 mm inside the single-crystal optical fiber (9); the focal length of the third lens (13) is 40 mm, which is used to approximately collimate the LD laser output by the pump LD (14); both the first dichroic mirror (10) and the second dichroic mirror (11) have high transmittance for 940 nm, with a transmittance greater than 96%, and high reflectance for 1030 nm, with a reflectance greater than 99.5%.

7. The hybrid high-power large-energy femtosecond chirped pulse amplification system according to claim 4, characterized in that: the first lens (2) is installed on a two-dimensional lens adjustment bracket, and by finely adjusting the two-dimensional lens adjustment bracket, the light intensity fed back to the all-fiber pre-amplification (1) during the amplification process is minimized.

8. A hybrid high-power large-energy femtosecond chirped pulse amplification method, based on the hybrid high-power large-energy femtosecond chirped pulse amplification system according to any one of claims 1-7, characterized in that, it includes the following steps: 1), start the all-fiber pre-amplification (1), adjust the position of the first lens (2), use a CCD detector to measure that the spot sizes of the incident and output lasers of the single-crystal optical fiber (9) meet the amplification requirements, fixedly install the first lens (2) at the position where the spot size meets the amplification requirements, and adjust so that the low-power laser beam signal light output by the all-fiber pre-amplification (1) is transmitted through the center of the first lens (2); 2), gradually increase the pump current in the all-fiber pre-amplifier (1), measure the amplification power of the output laser at different powers, and finely adjust the first lens (2) to make the feedback signal light at the monitoring end of the optoelectronic detection module in the all-fiber pre-amplifier (1) the weakest; 3), rotate the first half-wave plate (3) to make the power of the laser beam passing through the first polarization beam splitter prism (5) the largest; 4), add an auxiliary polarization beam splitter prism behind the first quarter-wave plate (6), rotate the first quarter-wave plate (6) to make the transmitted light and reflected light powers of the auxiliary polarization beam splitter prism equal, fix the first quarter-wave plate (6) and remove the auxiliary polarization beam splitter prism; 5), adjust the direction of the incident signal light so that the signal light is coaxially incident on the single-crystal fiber (9), and use a CCD detector to measure to ensure that the light spot passing through the single-crystal fiber (9) has a single-mode energy distribution; 6), turn on the pump LD (14) to inject the initial low-power pump light into the pump end of the single-crystal fiber (9), adjust the second lens (12) to optimize the power of the signal light reflected by the first dichroic mirror (10) to the maximum; gradually increase the pump power so that the power of the signal light output by the single-crystal fiber (9) reaches more than 130 W; 7), rotate the second quarter-wave plate (17) to make the output power after the second polarization beam splitter prism (18) the largest, and rotate the second half-wave plate (19) to make the polarization injection into the first grating (22) the best, achieving the highest diffraction efficiency; 8), adjust the angle of the first grating (22) to make the diffraction angle the best, achieving the highest diffraction efficiency, use a CCD detector to measure the compressed signal light spot, and optimize the angle of the second grating (23) to make the light spot reach the best roundness; 9), use the lifting mirror (24) to reduce the optical height of the signal light, reflect it to the second grating (23) and the first grating (22), and after diffraction and transmission, it is incident on the fourth high-reflection mirror (21), and the amplified and compressed femtosecond pulse is derived.

9. The hybrid high-power large-energy femtosecond chirped pulse amplification method according to claim 8, characterized in that: in step 1), the power of the laser beam output by the all-fiber pre-amplifier (1) is 500 mW; the optical path distance between the single-crystal fiber (9) and the first lens (2) is 540 mm, and the light spots of the incident and output lasers of the single-crystal fiber (9) are 400 μm; in step 6), the initial low power of the pump light output by the pump LD (14) is 1 - 2 W; in step 6), the gradually increasing of the pump power means that the pump power output by the pump LD (14) is 150 - 200 W.

10. The hybrid high-power large-energy femtosecond chirped pulse amplification method according to claim 9, characterized in that: in step 9), the lifting mirror (24) reduces the height of the light beam by 10 mm.

Images