Z-shaped cascade quasi-parametric amplification method and device

Abstract

According to the Z-shaped cascade quasi-parametric amplification method and device, pump light and signal light are incident from one end of a crystal and are returned and transmitted in the crystal through a Z-shaped light path, and the energy flow collinear state of walk-off compensation is always kept; the front surface and the rear surface of the crystal are subjected to film coating treatment, so that the crystal has high reflectivity on incident pump light and signal light and has high transmissivity on newly generated idler frequency light; during each turn-back, the idler frequency light is effectively guided out, and meanwhile, the pump light and the signal light enter the cascade amplification process of the next stage; and finally, the enhanced signal light and the residual pump light are emitted from the other end of the crystal. The device has the advantages that non-linear backflow is effectively inhibited through transmission and export of idler frequency light, and the signal light efficiency is remarkably improved; when small wedge angles exist on the front and rear surfaces of the crystal, each cascade process corresponds to different phase matching angles, and the gain bandwidth can be expanded; the crystal lath structure is beneficial to heat dissipation and supports high-average-power operation; and the method is suitable for various large-size crystals and optical frequency bands.

Description

Technical Field

[0001] The present invention belongs to the field of laser technology, and particularly relates to a Z-shaped cascaded quasi-parametric amplification (Z-QPA) method and device based on a nonlinear crystal slab. Background Art

[0002] Lasers are increasingly widely used in the fields of scientific research and industrial manufacturing. The continuous growth of actual demand drives the continuous advancement of laser technology towards higher energy, shorter pulse width, more wavelengths, and higher power. This trend poses more stringent requirements for laser amplifiers, specifically including achieving high conversion efficiency, expanding the gain bandwidth, and optimizing the heat dissipation performance. Compared with laser amplifiers using energy-level gain media, optical parametric amplifiers (OPAs) using nonlinear crystals show more significant potential in achieving the above performances: their gain wavelengths are no longer limited by material energy levels and can achieve a wide range of wavelength tuning; at the same time, since the nonlinear crystal only acts as a medium in the energy exchange process and does not directly participate, this provides more possibilities for achieving higher-power amplification.

[0003] However, OPAs also have their own limitations. On the one hand, the OPA process is restricted by the intrinsic backward flow effect (i.e., the phenomenon that the signal light and the idler light combine to generate the pump light), resulting in the energy conversion efficiency from the pump light to the signal light being much lower than the quantum limit efficiency (i.e., the ratio of the signal light frequency to the pump light frequency), and a large amount of pump energy fails to be effectively utilized. On the other hand, in order to achieve broadband amplification, OPAs usually adopt collinear degenerate amplification or non-collinear amplification methods to achieve the group velocity matching of the signal light and the idler light, but this method is only applicable to the case where the group velocity of the signal light is less than or equal to the group velocity of the idler light. In most application scenarios of OPAs, the group velocity matching condition is difficult to meet, thus restricting the realization of broadband amplification. Summary of the Invention

[0004] The present invention aims to solve the above bottleneck problems of OPAs, and proposes a Z-shaped cascaded quasi-parametric amplification method and device based on a nonlinear crystal slab. By performing specific coating on the crystal surface, the intermittent depletion of the idler light triggered blocks the backward flow process, and thus the amplification efficiency can be increased to nearly the quantum conversion limit level. At the same time, by adjusting the wedge angle design of the crystal surface, different phase matching angles can be provided for each internal cascaded unit, so as to achieve the amplification of different parts of the signal spectrum. This serial synthesis method of the spectrum significantly expands the gain bandwidth. In addition, the slab configuration design of the crystal not only facilitates heat dissipation, but also the heat dissipation direction is close to the beam transmission direction, which is beneficial to maintaining high beam quality while achieving high average power.

[0005] The present invention proposes an innovative Z-shaped cascaded quasi-parametric amplification method, which has general applicability and flexibility and is easy to implement.

[0006] The technical solution of the present invention is outlined as follows:

[0007] A Z-shaped cascaded quasi-parametric amplification device, characterized by comprising:

[0008] A plate-shaped nonlinear crystal, the front and rear surfaces of which are coated to form a first coating surface and a second coating surface, and the first coating surface and the second coating surface have a high reflectivity for incident pump light and signal light and a high transmittance for the idler light (4) generated during the amplification process;

[0009] The pump light and the signal light are incident from one end of the crystal as an incident wedge surface and are transmitted back and forth multiple times in a Z-shaped optical path between the first coating surface and the second coating surface. Each time it turns back, the idler light is led out through the front and rear surfaces of the crystal, and the pump light and the signal light enter the next stage of cascaded amplification;

[0010] After being amplified through multiple stages of cascading, the enhanced signal light and the remaining pump light are output from the other end of the crystal as an output wedge surface;

[0011] The principle for determining the angle between the incident wedge surface and the first coating surface and the angle between the output wedge surface and the second coating surface is to minimize the angles between the incident light and the normal of the incident wedge surface and between the output light and the normal of the output wedge surface, and it is advisable that the incident light is close to normal incidence and the output light is close to normal emergence;

[0012] The pump light and the signal light enter the crystal from the incident wedge surface at an appropriate angle to ensure that the energy flow directions of the pump light and the signal light inside the crystal are collinear within a preset interaction region.

[0013] Furthermore, the first coating surface and the second coating surface are parallel to each other, and the optical axis of the crystal is perpendicular or parallel to the first coating surface and the second coating surface.

[0014] Furthermore, a wedge angle is formed between the first coating surface and the second coating surface, and the range of this wedge angle is greater than 0° and less than 2°.

[0015] Furthermore, the incident angles of the pump light and the signal light compensate for the walk-off angle through non-collinear phase matching to ensure that the energy flow directions of the two inside the crystal are collinear.

[0016] Furthermore, the polarization states of the pump light and the signal light are a combination of ordinary light or extraordinary light, satisfying one type or two types of phase matching conditions.

[0017] Furthermore, the material selection of the crystal has a wide range, including but not limited to BaGa4Se7 (BGSe), BaB2O4 (BBO), LiNbO3 or KTiOPO4 (KTP), etc.

[0018] Second, the present invention also provides a Z-shaped cascaded quasi-parametric amplification method, which is characterized by including the following steps:

[0019] Step 1: Calculate the phase matching condition, determine the wavelengths of the pump light and the signal light, select a nonlinear crystal with specific birefringence characteristics, and calculate the angles that satisfy the phase matching condition.

[0020] Step 2: Design the nonlinear crystal. Based on the phase matching condition in Step 1, determine the cutting dimensions of the crystal, including the dimensions and angles of the front and rear surfaces, wedge surfaces, and other side surfaces of the crystal, as well as the specific requirements for coating. Coat the front and rear surfaces of the crystal so that it has a high reflectivity for the pump light and signal light of a specific wavelength, and at the same time has a high transmittance for the idler light.

[0021] Step 3: Adjust the incident angles of the pump light and the signal light to satisfy the phase matching condition described in Step 1, and keep the energy flow directions of the two collinear.

[0022] Step 4: Adjust the time delay of the pump light and the signal light by optimizing devices such as translation stages to achieve time synchronization to trigger the Z-QPA process.

[0023] In Step 1, when calculating the angles that satisfy the phase matching condition, the walk-off angle caused by the birefringence effect of the nonlinear crystal and the non-collinear angle introduced to achieve phase matching also need to be considered. Further adjust the calculated angles to ensure the consistency of the energy flow directions of the pump light and the signal light in the interaction region, and at the same time minimize the influence of the spatial walk-off effect on the amplification efficiency.

[0024] In Step 2, when designing the nonlinear crystal, the front and rear surfaces of the crystal are parallel. At this time, it is applicable to both narrow-band amplification and wide-band amplification, depending on the crystal type and the phase matching condition. If it is difficult to support wide-band amplification when the front and rear surfaces of the crystal are parallel, a small wedge angle can be designed on the front and rear surfaces to expand the bandwidth.

[0025] In Step 3, when adjusting the incident angles of the pump light and the signal light, it is also necessary to ensure according to the geometric relationship that the width of the light spot in the plane formed by the front and rear surfaces of the crystal does not exceed the product of the distance between the front and rear surfaces of the crystal and the cosine value of the angle between the light beam and the front and rear surfaces.

[0026] Compared with the prior art, the technical effects of the present invention are as follows:

[0027] 1) By performing specific coating on the crystal surface, intermittent depletion of the idler light is induced, which can block the reverse flow process, enabling the energy conversion efficiency from the pump light to the signal light to be increased to a level close to the quantum conversion limit, thereby greatly improving the energy utilization efficiency.

[0028] 2) By ingeniously designing the wedge angle between the front and back surfaces of the crystal plate, the present invention can adjust the phase-matching angles of the cascaded processes, achieve the amplification of different parts of the signal spectrum, expand the gain bandwidth through the serial synthesis method of the spectrum, and realize broadband amplification without satisfying the group velocity matching, greatly enhancing the universality of the present invention.

[0029] 3) Adopting the plate configuration design of the crystal is not only convenient for heat dissipation, but also the heat dissipation direction is close to the beam transmission direction. This is beneficial to maintaining high beam quality while achieving high average power.

[0030] 4) The present invention can achieve extremely high energy conversion efficiency, much higher than that of traditional OPAs, and even reach a conversion efficiency close to the quantum limit in the case of narrowband amplification. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The above and other objects, features, and advantages of the exemplary embodiments of the present invention will become more clearly understood by reading the following detailed description and referring to the drawings. In the drawings, several embodiments of the present invention are shown in an exemplary rather than restrictive manner, specifically including:

[0032] Figure 1 : Schematic diagram of the crystal configuration and beam transmission of the embodiment of the Z-shaped cascaded quasi-parametric amplification method based on a nonlinear crystal plate of the present invention;

[0033] Figure 2 : Schematic diagram of the cutting of the crystal plate in Embodiment 1 of the present invention;

[0034] Figure 3 : (a) Curve of the conversion efficiency of broadband amplification in Application Example 1 varying with the equivalent crystal length; (b) Comparison diagram of the input signal spectrum and the amplified signal spectrum in Example 1;

[0035] Figure 4 : Schematic diagram of the cutting of the crystal plate in Embodiment 2 of the present invention;

[0036] Figure 5 : Curve of the conversion efficiency of narrowband high-efficiency amplification in Example 2 varying with the equivalent crystal length, showing the situation under different pump average powers.

[0037] In the figures, 1 - crystal, 2 - pump light, 3 - signal light, 4 - idler light, 5 - first coated surface, 6 - second coated surface, 7 - incident wedge surface, 8 - exit wedge surface, 9 - remaining pump light, 10 - enhanced signal light. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0038] The technical solutions of the present invention will be further elaborated below in conjunction with the drawings and embodiments, but please note that this does not mean a limitation to the protection scope of the present invention.

[0039] A specially coated strip-shaped nonlinear crystal is used. The coating design on the front and back surfaces of the crystal makes it highly reflective to the incident pump light and signal light, and highly transmissive to the newly generated idler light. The pump light and signal light are incident from one end of the crystal, and then return along the Z-shaped optical path in the crystal. At each return, the idler light is guided out from the front and back surfaces of the coating and no longer participates in subsequent interactions (such as Figure 1 As shown). This design cleverly avoids the nonlinear backflow effect, thereby achieving high-efficiency amplification. In addition, the two coated surfaces of the crystal slab can remain parallel to each other, in which case the optical axis of the crystal is perpendicular (or parallel) to the two surfaces; or, a small wedge angle is formed between one coated surface and the other coated surface, thereby increasing the gain bandwidth of the entire cascade amplification process. The crystal size must ensure that the pump light and signal light can be reflected multiple times inside the crystal without being blocked, and the width of the light spot must also meet a specific geometric relationship, that is, the light spot is Figure 1 The width in the plane shown shall not exceed the product of the distance between the front and back surfaces of the crystal and the cosine of the angle between the beam and the front and back surfaces.

[0040] In actual application, the following steps should be followed:

[0041] 1) According to the wavelengths of the pump light and the signal light and the crystal properties, calculate the angle that makes the energy flow directions of the pump light and the signal light in the crystal collinear and satisfies the phase matching condition.

[0042] 2) Based on the calculation results of step 1, determine the cutting size of the crystal strip, including the size and angle of the front and back surfaces, wedge surfaces and other side surfaces, as well as the specific requirements for coating.

[0043] 3) Allow the pump light and signal light to be incident on the crystal, and adjust their incident angles to meet the phase matching condition described in step 1, while ensuring that the energy flow directions of the two remain collinear

[0044] 4) Adjusting the time delay of the pump light and the signal light to synchronize their time so as to cause the Z-QPA process.

[0045] The specific steps are as follows:

[0046] Step 1. Calculate the phase matching condition:

[0047] Determine the wavelengths of pump light and signal light, and select nonlinear crystals with specific birefringence characteristics;

[0048] Calculating an angle satisfying a phase matching condition according to the wavelengths of the pump light and the signal light and the birefringence characteristics of the selected nonlinear crystal;

[0049] Considering the walk-off angle caused by the birefringence effect of the nonlinear crystal and the non-collinear angle introduced to achieve phase matching, further adjust the calculated angle to ensure the consistency of the energy flow directions of the pump light and the signal light within the interaction region, while minimizing the impact of spatial walk-off on the amplification efficiency;

[0050] According to the verification results, determine the phase matching conditions, including parameters such as the incident angles and polarization states of the pump light and the signal light.

[0051] Step 2. Design of the nonlinear crystal: Based on the calculation results of the phase matching conditions in Step 1, determine the cutting dimensions of the crystal, including the dimensions and angles of the front and rear surfaces, the wedge surfaces, and other side surfaces of the crystal, as well as the specific requirements for coating. Specifically include:

[0052] The front and rear surfaces of Crystal 1 are coated to have a high reflectivity for the pump light 2 and the signal light 3 of a specific wavelength, and at the same time have a high transmittance for the idler light 4 generated by the interaction of the pump light 2 and the signal light 3.

[0053] The first surface 5 and the second surface 6 can be coated to be parallel to the principal plane of Crystal 1, suitable for narrowband amplification (also suitable for broadband amplification in some special cases), or designed to have a small angle with the principal plane of Crystal 1 to achieve an increase or decrease of the phase matching wavelength within a predetermined range with each reflection, suitable for broadband amplification.

[0054] According to the phase matching angles calculated in Step 1, cut out the first wedge surface 7 and the second wedge surface 8 of Crystal 1 for the incidence of the pump light and the signal light and the emergence of the amplified signal light, ensuring that the pump light 2 and the signal light 3 can completely enter the crystal and maintain phase matching within the crystal, while the amplified signal light 10 can smoothly emerge.

[0055] Step 3. Adjust the incident angles of the pump light and the signal light to meet the phase matching conditions described in Step 1 and keep the energy flow directions of the two collinear. First, adjust the incident angle of the pump light 2 to ensure that it can completely enter Crystal 1 from the first wedge surface 7 and the light spot can completely emerge from the second wedge surface 8 without being blocked by the edges or opaque surfaces of Crystal 1. In addition to adjusting the incident angle, it is also necessary to ensure according to the geometric relationship that the width of the light spot in the Figure 1 plane shown does not exceed the product of the distance between the first surface 5 and the second surface 6 and the cosine value of the angle between the light beam and the front and rear surfaces. After adjusting the pump light 2, then adjust the incident angle of the signal light 3 so that its light spot completely coincides with the light spot of the pump light 2 on the incident surface 7 and the exit surface 8.

[0056] Step 4: Adjust the time delay between the pump light and the signal light by optimizing devices such as translation stages to achieve time synchronization and trigger the Z-QPA process. When the output signal energy reaches the maximum value, it indicates that the pump light and the signal light are time-synchronized.

[0057] Example 1:

[0058] Select the biaxial crystal BaGa4Se7 (BGSe) with broad application prospects in the mid-infrared band as the nonlinear crystal. The BGSe crystal has excellent performance in terms of transparency range, effective nonlinear coefficient, etc. However, when using a mature 1μm pump source, it is difficult to achieve the group velocity matching condition through collinear or non-collinear configurations in most of the transparent range of BGSe, so it is limited in terms of amplification bandwidth.

[0059] The present invention is not limited by the group velocity matching condition and adopts a cascaded amplification method to achieve broadband amplification, which can make full use of the excellent performance of the BGSe crystal. Even when the group velocity matching is not satisfied, efficient parametric amplification can be achieved.

[0060] In this embodiment, a pump light of 1.03μm is selected to amplify a signal light of 10μm. The corresponding idler light wavelength is 1.15μm, and the phase matching is type-I phase matching in the XZ plane. The pump light is an ordinary light (o light), and the signal light and the idler light are extraordinary lights (e lights). The cutting method of the BGSe crystal plate is as Figure 2 shown. The paper surface is perpendicular to the crystal Y axis. The front surface 5 and the rear surface 6 are approximately perpendicular to the crystal Z axis, and form angles of +0.1° and -0.1° with the XY main plane respectively, so there is a wedge angle of 0.2° between the front and rear surfaces to change the phase matching wavelength after each beam folding. The plate thickness is 4mm, the length is 41mm, and the width (perpendicular to the paper surface) is 10mm. A wedge surface is cut at each end for the pump light and the signal light to enter and the signal light to exit after amplification. The inclination angle of the wedge surface is designed based on the criterion that the pump light and the signal light are normally incident or normally exit the crystal. After the inclination angle of the wedge surface is determined, the angle between the two beams in the crystal and the optical axis can be adjusted by finely tuning the incident beam direction during actual use.

[0061] Use a one-dimensional time-domain program to simulate the Z-QPA process designed in Example 1, and focus on demonstrating the performance in terms of bandwidth. The input pump pulse and signal pulse both adopt Gaussian pulse shapes, and the pulse width is 100ps. Among them, the signal pulse is a chirped pulse, and the spectral bottom width (peak 1 / e 2 intensity) is about 3.9μm, and the Fourier transform limited pulse width is equivalent to 2 cycles at 10μm. The peak optical intensity of the input pump light is 3GW / cm 2 , and the peak optical intensity of the signal light is 3MW / cm 2 . As Figure 3(a), since the idler light is filtered out every time the light beam turns back, there is almost no backflow effect in the whole process. Therefore, the conversion efficiency increases continuously as the pulse propagates in the crystal, and finally reaches about 5.4% at the output end. The amplified spectrum is as Figure 3 (b). The bottom width of the spectrum is about 2μm, covering the range of 9 - 11μm, far exceeding the spectral width of ordinary narrowband OPA. The broadband amplification of this embodiment does not depend on the group velocity matching condition required for traditional broadband amplification, greatly expanding the spectral range that can achieve broadband amplification.

[0062] Example 2:

[0063] In this embodiment, a 515nm pump light is selected to amplify an 800nm signal light, and the corresponding idler light wavelength is 1445nm. The nonlinear crystal selects the commonly used uniaxial crystal BaB2O4 (BBO), and type-I phase matching is adopted. The pump light is an extraordinary light (e light), and the signal light and the idler light are ordinary lights (o light). The cutting method of the BBO crystal plate is as Figure 4 shown. The front and back surfaces are parallel and perpendicular to the optical axis. The length of the plate is 34mm, the thickness is 5mm, and the width (perpendicular to the paper surface) is 10mm. A wedge surface is cut at each end for the pump light and the signal light to enter and the signal light to exit after amplification. The included angle between the wedge surface and the front and back surfaces is 29.2°. To compensate for the relative walk-off angle between the pump light and the signal light, the included angle between the pump light wave vector and the optical axis is set to 26.1°, and the included angle between the signal light wave vector and the optical axis is 29.6°.

[0064] A three-dimensional program is used to simulate the Z-QPA process of Example 2, and a heat conduction model is established to calculate the temperature distribution to reflect the high conversion efficiency and the ability to maintain the conversion efficiency under high average power conditions. The input pump pulse and signal pulse are both Gaussian in the time domain, with a pulse width of 100ps and no chirp. In the spatial domain, they are both square super-Gaussian light spots, with a width of 4mm (full width at half maximum, which is also approximately equal to the bottom width for super-Gaussian beams) and a length of 8mm. The peak light intensity of the input pump light and the peak light intensity of the signal light are the same as those in Example 1, which are 3GW / cm 2 and 3MW / cm 2 . As Figure 5 shown, when the average pump power is 1kW, the conversion efficiency at the output end reaches 59.5%, which is very close to the quantum limit efficiency of 64.3%. When the average pump power is increased to 30kW, although the phase mismatch caused by strong temperature rise has significantly reduced the conversion efficiency of ordinary OPA (such as the part where the crystal length is less than 10mm under the condition of 30kW), but due to the characteristic of filtering out the idler light to effectively suppress the backflow effect, the conversion efficiency of the present invention shows an upward trend throughout the process, and finally achieves an ultra-high efficiency of 55% at the output end.

[0065] The above two embodiments and application embodiments fully demonstrate the remarkable advantages of the present invention in improving the efficiency of non-linear parametric amplification and expanding the gain bandwidth.

Claims

1. A Z-shaped cascaded quasi-parametric amplifier (Z-QPA) device, characterized in that include: A slab-shaped nonlinear crystal, the front and rear surfaces of which are subjected to coating treatment to form a first coating surface and a second coating surface, wherein the first coating surface and the second coating surface have high reflectivity to incident pump light and signal light, and have high transmittance to idler light (4) generated during the amplification process; The pump light and signal light are incident from one end of the crystal, which serves as an incident wedge surface, and are transmitted multiple times in a zigzag optical path between the first coating surface and the second coating surface. Each time the idler light is turned back, it is led out through the front and back surfaces of the crystal, and the pump light and signal light enter the next stage of cascade amplification. After multi-stage cascade amplification, the enhanced signal light and the remaining pump light are output from the other end of the crystal as an output wedge surface; The angle between the incident wedge surface and the first coating surface, and the angle between the exit wedge surface and the second coating surface are determined in accordance with the principle of minimizing the angle between the incident light and the normal of the incident wedge surface, and the angle between the exit light and the normal of the exit wedge surface, so that the incident light is close to normal incidence, and the exit light is close to normal exit; The pump light and the signal light enter the crystal from the incident wedge surface at a suitable angle, ensuring that the energy flow directions of the pump light and the signal light inside the crystal remain collinear in a preset interaction region.

2. The Z-shaped cascaded quasi-parametric amplification device according to claim 1, characterized in that, The first coating surface and the second coating surface are parallel to each other, and the crystal optical axis is perpendicular or parallel to the first coating surface and the second coating surface.

3. The Z-shaped cascaded quasi-parametric amplification device according to claim 1, characterized in that A wedge angle is formed between the first coating surface and the second coating surface, and the wedge angle is in a range of greater than 0° and less than 2°.

4. The Z-shaped cascaded quasi-parametric amplification device according to claim 1, characterized in that, The incident angles of the pump light (2) and the signal light (3) are compensated for the walk-off angle by non-collinear phase matching, thereby ensuring that the energy flow directions of the two in the crystal are collinear.

5. The Z-shaped cascaded quasi-parametric amplification device according to claim 1, characterized in that The polarization states of the pump light (2) and the signal light (3) are a combination of ordinary light or extraordinary light, satisfying a first-class or a second-class phase matching condition.

6. The Z-shaped cascaded quasi-parametric amplification device according to any one of claims 1-5, characterized in that, The material selection of the crystal (1) is extensive, including but not limited to BaGa4Se7 (BGSe), BaB2O4 (BBO), LiNbO3 or KTiOPO4 (KTP) and the like.

7. A Z-shaped cascaded quasi-parametric amplification method, characterized in that, The following steps are involved: Step 1: Calculate the phase matching condition, determine the wavelengths of the pump light and the signal light, select a nonlinear crystal with specific birefringence characteristics, and calculate the angle that satisfies the phase matching condition; Step 2: Design a nonlinear crystal. Based on the phase matching conditions in step 1, determine the cutting size of the crystal, including the size and angle of the front and back surfaces, wedge surface and other side surfaces of the crystal, as well as the specific requirements for coating. Coating the front and back surfaces of the crystal to make it have high reflectivity for pump light and signal light of a specific wavelength, and high transmittance for idler light. Step 3: Adjust the incident angles of the pump light and the signal light to meet the phase matching condition described in step 1 and keep the energy flow directions of the two collinear; Step 4: Adjust the time delay of pump light and signal light by optimizing the translation stage and other devices to achieve time synchronization to trigger the Z-QPA process.

8. The Z-shaped cascade quasi-parametric amplification method according to claim 7, wherein In step 1, when calculating the angle that satisfies the phase matching condition, the walk-off angle caused by the birefringence effect of the nonlinear crystal and the non-collinear angle introduced to achieve phase matching also need to be considered, and the calculated angle is further adjusted to ensure the consistency of the energy flow directions of the pump light and the signal light in the interaction region, while minimizing the influence of the spatial walk-off effect on the amplification efficiency.

9. The Z-shaped cascade quasi-parametric amplification method according to claim 7, wherein In step 2, when designing the nonlinear crystal, the front and rear surfaces of the crystal are parallel. In this case, it is applicable to both narrowband amplification and broadband amplification, depending on the crystal type and the phase matching conditions; if it is difficult to support broadband amplification when the front and rear surfaces of the crystal are parallel, a small wedge angle can be designed for the front and rear surfaces to expand the bandwidth.

10. The Z-shaped cascaded quasi-parametric amplification method according to claim 7, wherein In step 3, when adjusting the incident angles of the pump light and the signal light, it is also necessary to ensure according to the geometric relationship that the width of the light spot in the plane formed by the front and rear surfaces of the crystal does not exceed the product of the distance between the front and rear surfaces of the crystal and the cosine value of the angle between the light beam and the front and rear surfaces.

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