A nonlinear cascade laser amplifier based on single crystal fiber

Abstract

The present invention discloses a single-crystal fiber nonlinear cascade laser amplifier, which relates to the field of laser technology. The amplifier comprises a seed source, a pre-chirp management module, a single-crystal fiber cascade module, and a pulse compression module arranged along an optical path. A pump source is also arranged between the pre-chirp management module and the single-crystal fiber cascade module, emitting pump light into the optical path between the pre-chirp management module and the single-crystal fiber cascade module. The single-crystal fiber cascade module comprises a plurality of cylindrical single-crystal fibers connected in series. The present invention utilizes a hybrid cascade of different types of single-crystal fibers to control the nonlinear phase shift and self-focusing length during the amplification process, thereby achieving high average / peak power, short-cycle pulse amplification and generation.

Description

Technical Field

[0001] The present invention relates to the field of laser technology, and more particularly to a nonlinear cascade laser amplifier based on single crystal optical fiber. Background Art

[0002] Ultrafast lasers with high average (~W) / peak power (~GW), especially pulses with few optical cycles (2-5 optical cycles), currently hold enormous potential for application in fundamental research, large-scale scientific facilities, micromachining of non-metallic materials, precision medicine, and national defense. Since 1965, when the first passively mode-locked ruby ​​laser produced picosecond pulses, researchers have continuously pursued these two key objectives, exceeding the limits of short pulse width and high average power, conducting extensive experimental research.

[0003] Based on the differences in ultrafast laser generation technology, methods for generating high average / peak power, few-cycle pulses can be divided into two categories: direct generation (mode-locking technology) and indirect generation (amplification technology). The first category is based on mode-locked oscillators using activated ion-doped laser materials, mainly including solid-state mode-locked lasers, fiber mode-locked lasers, and disk mode-locked lasers. Their structure mainly consists of a pump source, a gain medium, a resonant cavity, and a mode-locking element. This type of laser is often limited by the material properties of the gain medium itself, such as spectral bandwidth, damage threshold, nonlinear effects, material dispersion, and other factors, making it difficult to achieve few-optical-cycle pulse output. Among them, with the development of mode-locking technology and mode-locking elements, solid-state mode-locked lasers can now achieve shorter pulse output (1μm~17fs, 2μm~37fs). However, for solid-state mode-locked lasers to obtain pulses with few optical cycles, a high-quality factor laser resonator is often required, which means that the laser output coupling is low, seriously limiting the average power increase (<1W); fiber mode-locked lasers exhibit very strong nonlinear effects (such as stimulated Brillouin scattering) when femtosecond lasers are transmitted in micron-level optical fiber cores, which causes problems such as pulse splitting, limiting their high energy and high peak power improvements; in order to reduce thermal effects, disk mode-locked lasers use very thin gain media. The weak nonlinear effect makes it difficult to generate narrow pulses, and due to the self-phase modulation caused by the optical nonlinear effect of air, if the dispersion compensation is not sufficient, it will lead to Q-switching or Q-switched mode-locking and other phenomena, making it difficult to obtain stable mode-locked output. At present, the ultrafast laser pulse widths generated by this type of oscillator are mostly in the hundreds of femtoseconds.

[0004] The second category utilizes amplification techniques to generate short-cycle pulses with high average / peak power. These techniques primarily include chirped pulse amplification (OPCPA) based on optical parametric processes, chirped pulse amplification (CPA) combined with nonlinear post-compression, and fiber-based nonlinear amplification. OPCPA combines optical parametric amplification (OPA) and chirped pulse amplification (CPA). OPA utilizes the second-order nonlinear effects of parametric crystals. This amplification process eliminates heat accumulation and, through noncollinear phase matching, maintains broadband laser amplification, supporting the generation of periodic mid-infrared pulses. However, due to its short pulse width, the output pulse energy is limited by the damage threshold of the optical components and nonlinear effects, making it difficult to further improve amplification efficiency. OPCPA, on the other hand, utilizes optical parametric processes in nonlinear crystals to amplify chirped pulses. This amplification has no gain bandwidth limitations and requires no energy storage within the crystal, significantly mitigating the limitations of OPA technology. However, due to the simultaneous participation of both signal and idler light in the amplification process, efficiency is low. Furthermore, at high repetition rates (>100 kHz), thermal effects are severe, making it difficult to increase average power. Furthermore, this system requires synchronous pumping with a high-energy picosecond laser, which inevitably increases complexity and is extremely costly. CPA amplification combined with nonlinear post-compression technology primarily utilizes bulk materials and fiber-based materials for amplification. Fiber-based CPA systems can achieve high average power due to their high gain and robust heat dissipation capabilities, but are limited by severe nonlinear effects and a low laser damage threshold, resulting in low pulse energy and peak power. In particular, the fabrication of high-quality mid-infrared microstructured gain fibers and highly nonlinear microstructured fibers is complex and requires extremely high process requirements, with only a few groups worldwide able to provide them. Bulk-based CPA systems, however, struggle to amplify pulse energies to very high levels and compress pulse widths to very short lengths due to gain narrowing and severe thermal effects caused by the material's bandwidth. Furthermore, the amplified prepulses can severely affect the interaction between the main pulse and the material in practical applications. While fiber-based nonlinear amplification can directly generate broadband, short-cycle pulses, the fiber length is significantly greater than the self-focusing length, leading to pulse splitting and even fiber damage at high peak powers, making it difficult to achieve both high average and high peak power compatible amplification.

[0005] Therefore, how to achieve the compatibility of peak power, high average power and high peak power of short-cycle pulses at high repetition frequency in the laser and simplify the laser structure while ensuring the output effect is an urgent problem that technicians in this field need to solve. Summary of the Invention

[0006] In view of this, the present invention provides a single-crystal fiber nonlinear cascade laser amplifier, which involves a few-cycle, high-power pulse laser. It proposes an amplification technology based on single-crystal fiber nonlinear cascade to solve the above-mentioned problems. By hybrid cascading different types of single-crystal fibers, the nonlinear phase shift and self-focusing length in the amplification process are regulated, thereby achieving high average / peak power few-cycle pulse amplification generation.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A single-crystal fiber nonlinear cascade laser amplifier comprises a seed source, a pre-chirp management module, a single-crystal fiber cascade module, and a pulse compression module arranged along an optical path; a pump source is further arranged between the pre-chirp management module and the single-crystal fiber cascade module, and the pump source emits pump light into the optical path between the pre-chirp management module and the single-crystal fiber cascade module; the single-crystal fiber cascade module comprises a plurality of cylindrical single-crystal fibers connected in series.

[0009] Preferably, the seed source is a femtosecond laser with adjustable wavelength and repetition rate, including a solid laser, a fiber laser or a laser based on frequency conversion technology.

[0010] Preferably, the pre-chirp management module uses a chirped mirror pair or ZnSe material with positive or negative group delay dispersion to achieve dispersion control and regulate the chirp amount of the seed source pulse.

[0011] Preferably, the pump source adopts a continuous laser, including a semiconductor laser or a fiber laser.

[0012] Preferably, the doping ions of the single crystal optical fiber include Nd 3+ 、Yb 3+ 、Er 3+ 、Tm 3+ 、Ho 3+ The matrix materials of single-crystal optical fibers include garnet and aluminate. The spectral bandwidth and output power achieved by the single-crystal optical fiber cascade module are determined by the three factors of nonlinear accumulation, self-focusing length, and dispersion management.

[0013] The pulse compression module preferably uses a chirped mirror assembly, including several chirped mirror pairs, to provide precise control and compensation for second-order and higher-order dispersion, achieving high-quality, short-cycle pulse compression. The number of chirped mirror pairs is determined based on the actual dispersion requirements.

[0014] Preferably, the chirped mirror pair includes two offset and parallel chirped mirrors. The input light is irradiated by the second chirped mirror and then reflected to the first chirped mirror. After multiple reflections between the two chirped mirrors, it is emitted from the first chirped mirror along the transmission path of the input light.

[0015] Preferably, the self-focusing length during the nonlinear accumulation and amplification process is regulated by the size of the single crystal fiber and the number of cascade stages in the single crystal fiber cascade module to generate a pulsed laser with a broadband spectrum and high peak power.

[0016] Preferably, the arrangement method of the single crystal optical fiber cascade module is:

[0017] Step 1: Collect the seed light spectrum width of the seed source, set the target spectrum width of the amplified laser, and calculate the spectrum broadening amount based on the seed light spectrum width and the target spectrum width;

[0018] Step 2: Calculate the target B integral value based on the spectral broadening amount, and set a single-crystal fiber in the single-crystal fiber cascade module; the relationship between the spectral broadening amount and the target B integral value B is as follows:

[0019]

[0020] Among them, Δω out and Δω in are the frequency widths of the output and input spectra, respectively;

[0021] Step 3: Collect the laser power and laser focusing radius of the current single crystal fiber output laser, and calculate the self-focusing length Z of the laser in the current single crystal fiber f , expressed as:

[0022]

[0023] Where P is the laser power, ω0 is the laser focusing radius, n0 and n2 are the linear refractive index and nonlinear refractive index of the single crystal fiber, respectively;

[0024] The length of the current-stage single crystal fiber is set to be less than the self-focusing length and greater than the preset minimum length; the minimum length is set to 30 mm;

[0025] Step 4: Collect the frequency domain characteristics, time domain characteristics, and power level of the laser output from the current single crystal fiber. The frequency domain characteristics include the spectral width and central wavelength, and the time domain characteristics include the pulse width. Combined with the volume integral model, the effective mode volume corresponding to different signal light transmission parameters is calculated. Based on the effective mode volume, the axial laser intensity at any position during the laser transmission in the current single crystal fiber is calculated.

[0026] Step 5: Calculate the actual B integral value B′ at different amplification powers based on the B integral formula and the nonlinear refractive index and axial laser intensity of the current-stage single crystal fiber, expressed as:

[0027]

[0028] Where n2 represents the nonlinear refractive index of the single crystal fiber, I(z) represents the axial laser intensity, and λ represents the wavelength of the seed source emission light;

[0029] Step 6: If the actual B integral value is less than the target B integral value, add a level of single crystal fiber and return to step 3 until the actual B integral value is greater than or equal to the target B integral value.

[0030] Preferably, the length of the single crystal fiber (gain medium length) L gain Less than the self-focusing length Z f , expressed as:

[0031]

[0032] Where I0 represents the axial laser intensity at the laser focusing radius ω0.

[0033] Through the above technical solution, it can be seen that compared with the existing technology, the present invention discloses a single-crystal fiber nonlinear cascade laser amplifier. Through the characteristic advantages of the single-crystal fiber cascade structure and the technical characteristics of nonlinear amplification, the design structure is simple, the operation is convenient, the integration is easy, and the industrialization is easy. The single-crystal fiber cascade structure is composed of multiple single-crystal fiber gain modules connected in series to achieve nonlinear amplification, that is, nonlinear phase shift is generated while power amplification is performed, and spectrum broadening is achieved through nonlinear accumulation (SPM effect), thereby breaking through the gain bandwidth limitation and achieving the generation of short-cycle pulses after dispersion management. The specific beneficial effects are as follows:

[0034] 1) It is expected to resolve the conflict between high average and high peak power. Leveraging the SCF's large surface-to-volume ratio, high thermal conductivity, and long gain region, it effectively reduces thermal effects and increases average power. A cascade structure is used to control the nonlinear phase shift and self-focusing length during amplification, minimizing pulse splitting, beam distortion, and optical breakdown caused by self-focusing. Furthermore, laser free-space transmission reduces the SCF's axial light intensity, preventing laser damage and achieving high peak power.

[0035] 2) A nonlinear cascade approach achieves ultra-broadband spectral support for short-cycle pulse generation without requiring additional nonlinear post-compression. This approach exploits the self-phase modulation (SPM) effect within each SCF stage to induce nonlinear spectral broadening. By cascading the SCFs, the B-integral is accumulated step-by-step rather than continuously, ultimately achieving dispersion-compensated, compressed pulses of at least one optical period without requiring additional nonlinear post-compression.

[0036] 3) Simple and compact structure, easy to operate and popularize. The nonlinear cascade structure previously used continuous laser pumping, with each stage of SCF gain module connected in series. Compared with the OPCPA system and the traditional gain medium CPA combined with nonlinear post-compression scheme, the structure is simpler and easier to operate, facilitating subsequent popularization and application.

[0037] This invention will provide a new research solution for the field of short-cycle pulse generation and amplification technology. The high-performance laser pulses obtained have important application potential in basic research and large scientific facilities, semiconductor and transparent organic material micro-processing, ultra-long-distance reconnaissance and other fields. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0039] Figure 1 A schematic diagram of the structure of a single-crystal fiber nonlinear cascade laser amplifier provided by the present invention;

[0040] Figure 2 Schematic diagram of the chirped mirror combination structure consisting of two groups of chirped mirror pairs provided by the present invention. DETAILED DESCRIPTION

[0041] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. 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 making creative efforts are within the scope of protection of the present invention.

[0042] The embodiment of the present invention discloses a nonlinear cascade laser amplifier based on a single crystal fiber. Figure 1 As shown, it includes a seed source 1, a pre-chirp management module 2, a single-crystal fiber cascade module 4 and a pulse compression module 5 arranged along the optical path. A pump source 3 is also arranged between the pre-chirp management module 2 and the single-crystal fiber cascade module 4 to emit pump light to the optical path between the two; the single-crystal fiber cascade module 4 includes a plurality of cylindrical single-crystal fibers connected in series.

[0043] Furthermore, the seed source 1 is a commercial or homemade femtosecond laser with adjustable wavelength and repetition rate, which may include solid lasers, fiber lasers, or lasers based on frequency conversion technology, and its output wavelength corresponds to the emission wavelength of the single crystal fiber.

[0044] Furthermore, the pre-chirp management module 2 utilizes optical elements or materials with positive or color group delay dispersion to achieve dispersion control and regulate the chirp amount; the optical elements may be chirped mirror pairs, and the materials may be ZnSe materials.

[0045] Furthermore, the pump source 3 is a continuous laser, including a semiconductor laser and a fiber laser.

[0046] Furthermore, the doping ions of the single crystal optical fiber include Nd 3+ 、Yb 3+ 、Er 3+ 、Tm 3+ 、Ho 3+ The matrix materials of single-crystal optical fibers include garnet and aluminate. The spectral bandwidth and output power achieved by the single-crystal optical fiber cascade module are determined by the three factors of nonlinear accumulation, self-focusing length, and dispersion management.

[0047] Furthermore, the pulse compression module 5 uses a chirped mirror combination to compensate for the second-order and high-order dispersion of the amplified pulse laser, and controls the amount of dispersion introduced by changing the number of reflections on the chirped mirror combination, thereby achieving the generation of short-cycle pulses; the chirped mirror combination includes at least one pair of chirped mirror pairs, and the number of chirped mirrors can be set according to actual needs. The chirped mirror combination consisting of two groups of chirped mirror pairs is as follows: Figure 2 shown.

[0048] Furthermore, the chirped mirror pair includes two offset and parallel chirped mirrors. The input light is irradiated by the second chirped mirror and then reflected to the first chirped mirror. After multiple reflections between the two chirped mirrors, it is emitted by the first chirped mirror along the transmission path of the input light.

[0049] On the other hand, in a specific embodiment, a single-crystal fiber nonlinear cascade laser amplifier is a 2-micron laser based on Ho:SCF single-crystal fiber nonlinear cascade amplification. The single-crystal fiber cascade module 4 utilizes Ho-doped single-crystal fiber (Ho:SCF). Both light-passing end faces of the Ho:SCF are coated with an antireflection coating with a transmittance of ≥99% for pump light in the 1940 nm band. The absorption length of the Ho:SCF is controlled based on the self-focusing length of the amplification process and is typically between 30 and 50 mm.

[0050] The pump source 3 is a thulium-doped all-fiber laser with an output wavelength of 1940 nm, coupled to a pigtail output, and its pumping mode is end-face pumping.

[0051] Seed source 1 is a 2μm solid-state mode-locked laser based on rare earth ion doping, in which wavelength tuning is achieved through a birefringent filter, outputting <100fs pulses close to the Fourier transform limit, and a pulse picker is used for pulse frequency selection.

[0052] The pre-chirp management module 2 uses ZnSe polycrystalline with different lengths (30-60mm) and double-ended Brewster angle cutting to provide positive dispersion (GVD ~ 240fs 2 / mm), chirped mirror combination (GDD~-1000fs 2 / bounce) provides negative dispersion to pre-chirp the seed source (including zero chirp), regulates the dispersion of the seed source before amplification, and stretches the pulse to 0.3-1ps.

[0053] Pulse compression module 5 uses a chirped mirror assembly to provide negative dispersion for pulse compression. The chirped mirror assembly includes at least one set of chirped mirrors, which measure the quality of the compressed pulse to optimize the pre-chirp management module and dispersion compensation. Pulse compression module 5 regulates the dispersion of the amplified seed source. Because the seed source amplified by the single-crystal fiber cascade module introduces positive dispersion, resulting in wider pulses, chirped mirrors are required to provide negative dispersion for pulse width compression.

[0054] Single-crystal fiber cascade module 4 employs a four-stage nonlinear cascade. Assuming the average power after amplification through the fourth-stage single-crystal fiber (SCF) is 100W, the repetition rate is 1MHz, and the spectral half-width is approximately 50nm, the spot radius reaching the fourth-stage SCF is ω0 = 350μm. Using the self-focusing length formula, the self-focusing length is calculated to be approximately 55mm. Therefore, the length of the fourth-stage SCF must be controlled within 55mm to ensure that beam distortion, pulse splitting, or optical breakdown are not caused during the amplification process. Furthermore, using the B-integral formula, the corresponding B-integral value for this stage is approximately π. Based on the spectral broadening factor, it is known that when the B-integral is greater than 2.5, the broadening factor and the B-integral value are linearly related. To effectively guarantee beam transmission parameters, the dimensions of the first three SCF stages, as well as the focusing parameters of the pump light and laser, are adjusted so that the B-integral value within the SCF is less than π. Using the spectral broadening factor formula, the fourth-stage broadening reaches three times the input spectrum, with a half-peak width Δλ of ~150nm, sufficient to support 50fs pulse generation. The total four-stage nonlinear accumulation is approximately 3.5π. If the final stage uses an undoped aluminate SCF with high nonlinearity (n2) to enhance the SPM effect, the corresponding B-integral value will be even larger. The total phase shift of the four-stage nonlinear cascade is far greater than 3.5π, and the spectrum can be broadened to cover 1850nm-2400nm, with a Fourier transform limit width of ~16fs, and three-cycle pulses can be generated.

[0055] On the other hand, the specific process of determining the arrangement of single crystal optical fibers at each level in the single crystal optical fiber cascade module is as follows:

[0056] S1: The pre-chirp management module 2 stretches the seed source 1 pulse to the picosecond level, collects the seed light spectrum width of the seed source, sets the target spectrum width of the amplified laser, and calculates the spectrum stretching amount based on the seed light spectrum width and the target spectrum width;

[0057] S2: Calculate the target B integral value according to the spectrum broadening amount, and set a single crystal fiber in the single crystal fiber cascade module;

[0058] The amount of spectral broadening can be calculated as:

[0059]

[0060] where Δω out and Δω in are the frequency widths of the output and input spectra, respectively, and B is the calculated B integral;

[0061] S3: Measure the laser power and laser focusing radius of the single-stage single crystal fiber output laser, and calculate the self-focusing length Z of the laser in the current stage single crystal fiber f , expressed as:

[0062]

[0063] Wherein, P is the laser power, which is the data obtained by measurement; ω0 is the laser focusing radius, which is the data obtained by measurement; n0 and n2 are the linear refractive index and nonlinear refractive index of the single crystal fiber, respectively, which are the data known from the single crystal fiber material;

[0064] The length of the current-stage single crystal fiber is set to be less than the self-focusing length and greater than the preset minimum length; the minimum length is set to 30 mm;

[0065] S4: Measure the frequency domain, time domain characteristics, and power level of the laser after the nonlinear cascade of a single-stage single-crystal fiber. The frequency domain characteristics include the spectral width and central wavelength, and the time domain characteristics include the pulse width. The effective mode volume corresponding to different signal light transmission parameters is calculated using a volume integral model. Based on the effective mode volume, the axial laser intensity at any position during the laser transmission in the current-stage single-crystal fiber is calculated.

[0066] S5: Calculate the actual B integral value B′ at different amplification powers based on the B integral formula and the nonlinear refractive index and axial laser intensity of the current-stage single-crystal fiber, which is expressed as:

[0067]

[0068] Where n2 represents the nonlinear refractive index of the single-crystal fiber, which is a known value based on the single-crystal fiber material; I(z) represents the axial laser intensity, which is a measured value; λ represents the wavelength of the seed source emission light, which is a known value based on the selected seed source; B integral is used to calculate the nonlinear phase shift after each level, and the nonlinear accumulation is controlled by optimizing the axial light intensity distribution, thereby controlling the nonlinear broadening of the pulse spectrum during the amplification process.

[0069] S6: If the actual B-integral value is less than the target B-integral value, add a level of single crystal fiber and return to S3 to measure the laser frequency domain and time domain characteristics after the two-level, three-level, and four-level nonlinear cascades in sequence, and calculate the B-integral values ​​at different amplification powers until the actual B-integral value is greater than or equal to the target B-product.

[0070] Furthermore, by controlling the axial laser intensity and SCF length during the nonlinear amplification process, it can be operated in a state between spectrum broadening and pulse splitting, that is, to control the SCF length (gain medium length) L gain Smaller than the self-focusing length Z within the SCF f :

[0071]

[0072] I0 represents the axial laser intensity at the laser focusing radius ω0; the overall system needs to optimize the cascade number, SCF size parameters of each level, and focusing parameters of the pump light and laser based on the amplification power level, spectral nonlinear broadening and beam quality. Among them, the power amplification level is related to the axial laser intensity I(z) of the formula and is proportional. The higher the amplification power level, the larger the B integral; the spectral nonlinear broadening is the change in the spectral half-width before and after amplification, which corresponds to the frequency broadening F; the self-focusing effect during the amplification process may deteriorate the beam quality, and the self-focusing length Z f Affects the beam quality.

[0073] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. Reference can be made to the common and similar parts between the various embodiments. For the devices disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the description is relatively simple, and the relevant parts can be referred to the method description.

[0074] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A single crystal fiber nonlinear cascade laser amplifier, characterized in that: It includes a seed source, a pre-chirp management module, a single crystal fiber cascade module and a pulse compression module arranged along the optical path; A pump source is also provided between the pre-chirp management module and the single-crystal fiber cascade module, and the pump source emits pump light to the optical path between the pre-chirp management module and the single-crystal fiber cascade module; the single-crystal fiber cascade module includes a plurality of cylindrical single-crystal fibers connected in series; By regulating the self-focusing length during nonlinear accumulation and amplification by the size of the single-crystal fiber and the number of cascade stages in the single-crystal fiber cascade module, a pulsed laser with a broadband spectrum and high peak power is generated. The arrangement method of the single crystal optical fiber cascade module is as follows: Step 1: Collect the seed light spectrum width of the seed source, set the target spectrum width of the amplified laser, and calculate the spectrum broadening amount based on the seed light spectrum width and the target spectrum width; Step 2: Calculate the target B integral value based on the spectrum broadening amount, and set a single crystal fiber in the single crystal fiber cascade module; Step 3: Collect the laser power and laser focusing radius of the current single crystal fiber output laser, and calculate the self-focusing length Z of the laser in the current single crystal fiber f , expressed as: Where P is the laser power, ω0 is the laser focusing radius, n0 and n2 are the linear refractive index and nonlinear refractive index of the single crystal fiber, respectively; The length of the current-stage single crystal optical fiber is set to be less than the self-focusing length and greater than the preset minimum length; Step 4: Collect the frequency domain characteristics, time domain characteristics, and power level of the laser output from the current single crystal fiber, and calculate the effective mode volume corresponding to different signal light transmission parameters using the volume integral model. Calculate the axial laser intensity at any position when the laser is transmitting in the current single crystal fiber based on the effective mode volume. Step 5: Calculate the actual B integral value B′ at different amplification powers based on the B integral formula and the nonlinear refractive index and axial laser intensity of the current-stage single crystal fiber, expressed as: Where n2 represents the nonlinear refractive index of the single crystal fiber, I(z) represents the axial laser intensity, and λ represents the wavelength of the seed source emission light; Step 6: If the actual B integral value is less than the target B integral value, add a level of single crystal fiber and return to step 3 until the actual B integral value is greater than or equal to the target B integral value.

2. The single crystal fiber nonlinear cascade laser amplifier according to claim 1, characterized in that: The seed source is a femtosecond laser with adjustable wavelength and repetition rate, including a solid-state laser, a fiber laser or a laser based on frequency conversion technology.

3. The single crystal fiber nonlinear cascade laser amplifier according to claim 1, characterized in that: The pre-chirp management module uses a chirped mirror pair or ZnSe material with positive or negative group delay dispersion.

4. The single crystal fiber nonlinear cascade laser amplifier according to claim 1, characterized in that: The pump source adopts a continuous laser, including a semiconductor laser or a fiber laser.

5. The single crystal fiber nonlinear cascade laser amplifier according to claim 1, characterized in that: The doping ions of single crystal optical fibers include Nd 3+ 、Yb 3+ 、Er 3+ 、Tm 3+ 、Ho 3+ One of the; matrix materials of single crystal optical fiber include garnet and aluminate.

6. The single crystal fiber nonlinear cascade laser amplifier according to claim 1, characterized in that: The pulse compression module adopts a chirped mirror combination, including several groups of chirped mirror pairs, and the multiple groups of chirped mirror pairs are arranged along the optical path.

7. The single crystal fiber nonlinear cascade laser amplifier according to claim 3 or 6, characterized in that: The chirped mirror pair includes two offset and parallel chirped mirrors. The input light is irradiated by the second chirped mirror and then reflected to the first chirped mirror. After multiple reflections between the two chirped mirrors, it is emitted by the first chirped mirror along the transmission path of the input light.

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