A method for suppressing high-power laser shutter srs effect of lpfg-ctfbg cascade

Abstract

The application discloses a kind of LPFG-CTFBG cascade high-power laser optical shutter SRS effect inhibition method, comprising: establishing SRS effect inhibition mathematical model, obtaining the expression of the transmission spectrum and the reflection spectrum of cascade grating;Determine the structure parameter of LPFG and CTFBG;LPFG and CTFBG are inscribed on the core of double-end QBH cable, form cascade grating, and package, complete the preparation of double-end QBH cable with cascade grating;Double-end QBH cable is connected to high-power laser optical shutter output end, and double-end QBH cable with cascade grating inhibits the SRS effect of high-power laser optical shutter.The application can inhibit the SRS effect generated by high-power laser optical shutter and double-end QBH cable, avoid the signal laser power drop caused by the accumulation of SRS effect, reduce system stability.

Description

Technical Field

[0001] This invention belongs to the field of high-power fiber laser equipment, specifically relating to a method for suppressing the SRS effect of a high-power laser shutter in an LPFG-CTFBG cascade. Background Technology

[0002] Since their invention, fiber lasers have been widely used in various fields due to their significant advantages such as small size, high stability, and high conversion efficiency. With the increasing demands for output power, stability, and beam quality, suppressing stimulated Raman scattering (SRS) has become a key issue in achieving high-performance fiber lasers. When the output power exceeds a certain threshold, the SRS effect is excited, causing some signal light power to be transferred to a similar wavelength, generating Raman light components. In high-power fiber lasers, the SRS effect mainly affects the following aspects: 1. The Raman light excited by the SRS effect propagates bidirectionally along the fiber. On the one hand, it causes some of the signal laser power to be transferred to the Raman light, reducing the signal laser power. On the other hand, the backscattered Raman light can damage the fiber optic components in the system. Simultaneously, the generation of Raman light degrades the output beam quality, affecting the processing effect. 2. For near-infrared industrial lasers, the wavelength of the Raman light generated by the SRS effect differs from the signal light wavelength by approximately 50 nm. The optical components in the system are typically not specifically designed for high transmittance at this wavelength; for example, the quartz pillars on the output surface of the power transmission cable, the protective lenses inside the cutting head, and the collimating and focusing lenses. At high power, Raman light is prone to reflection at these locations, causing heat generation and reduced application performance. 3. The threshold of the SRS effect is inversely correlated with fiber length, which limits the length of the power transmission cable for high-power fiber lasers, failing to meet the needs of long-distance laser transmission processing scenarios.

[0003] High-power laser shutters are crucial components in laser manufacturing and processing. They enable fiber lasers to switch from single-channel to multi-channel output through time-division or energy-division methods, thus meeting the energy reuse requirements of manufacturing equipment and achieving multi-purpose functionality. High-power fiber lasers, especially those exceeding 10,000 watts, are often used in conjunction with high-power laser shutters. These shutters can be flexibly configured with output cables of different core diameters and lengths to expand the functionality of the high-power fiber laser and improve its utilization efficiency. However, higher output power leads to an exponential increase in the SRS effect. Suppressing the SRS effect generated by the laser itself is already extremely difficult. After connecting to a high-power laser shutter, residual Raman components in the laser output are accumulated and amplified in the high-power laser shutter output cable, resulting in reduced operating laser power, deteriorated beam quality, and decreased system stability. When the high-power laser shutter output cable is long, the SRS effect is exponentially amplified. However, existing high-power fiber lasers only suppress the SRS effect generated inside the laser, without considering the SRS effect generated in the laser shutter and double-ended QBH optical cable used in conjunction with it. Therefore, there is an urgent need for a method to suppress the SRS effect generated by the high-power laser shutter and double-ended QBH optical cable, so as to avoid the problems caused by the accumulation of SRS effect, such as the decrease in signal laser power and the reduction in system stability. Summary of the Invention

[0004] Purpose of the invention: The purpose of this invention is to provide a method for suppressing the SRS effect of a high-power laser shutter in an LPFG-CTFBG cascade, which can suppress the SRS effect generated by the high-power laser shutter and the double-ended QBH optical cable, and avoid the signal laser power reduction and system stability reduction caused by the accumulation of the SRS effect.

[0005] Technical solution: The present invention provides a method for suppressing the SRS effect of a high-power laser optical shutter in an LPFG-CTFBG cascade, comprising:

[0006] A mathematical model for suppressing the SRS effect of the LPFG-CTFBG cascaded grating was established, and the expressions for the transmission spectrum and reflection spectrum of the cascaded grating were obtained using the mathematical model for suppressing the SRS effect.

[0007] Based on the expressions for the transmission and reflection spectra of cascaded gratings, a simulation program for an LPFG-CTFBG cascaded grating was established. The influence of the structural parameters of the cascaded grating on the spectral parameters was obtained from the simulation program. The structural parameters of LPFG and CTFBG were input into the simulation program, which output the transmission and reflection spectra of the cascaded grating. The center wavelength and bandwidth of the LPFG-CTFBG cascaded grating were obtained from the transmission spectrum. The structural parameters of LPFG and CTFBG were modified according to the influence rules until the center wavelength and bandwidth of the LPFG-CTFBG cascaded grating matched the center wavelength and bandwidth of the SRS effect, thus determining the structural parameters of LPFG and CTFBG.

[0008] According to the structural parameters of LPFG and CTFBG, LPFG and CTFBG are sequentially engraved on the fiber core of the double-ended QBH optical cable along the forward transmission direction of the laser to form an LPFG-CTFBG cascaded grating. After the LPFG-CTFBG cascaded grating is integrated into a desensitization package, the fabrication of the double-ended QBH optical cable with cascaded grating is completed.

[0009] By connecting a double-ended QBH optical cable with a cascaded grating to the output end of a high-power laser shutter, the double-ended QBH optical cable with a cascaded grating can suppress the SRS effect of the high-power laser shutter.

[0010] Furthermore, the LPFG is used to filter out the residual Raman light main peak in the laser output laser and prevent the reverse-propagating Raman light from returning to the laser.

[0011] Furthermore, the CTFBG is used to supplement the filtering of the remaining forward-transmitting Raman light after LPFG filtering, so as to avoid the cumulative amplification of Raman light in long-distance optical fiber transmission, while expanding the suppression bandwidth and suppression rate.

[0012] Furthermore, the structural parameters of the LPFG include period, gate length, and refractive index modulation depth, while the structural parameters of the CTFBG include period, gate length, refractive index modulation depth, chirp rate, and tilt angle.

[0013] Furthermore, the mathematical model for suppressing the SRS effect of the LPFG-CTFBG cascaded grating is established as follows:

[0014] ;

[0015] ;

[0016] The above equation is the mode coupling equation for LPFG, where, for The complex amplitude of the forward transmission of the core mode at the location; for The complex amplitude of the cladding mode propagating forward at the location; The self-coupling coefficient; represents the mutual coupling coefficient of the cladding modes; The location of the fence area; Let be the detuning factor; assuming the amplitude of the core mode at the initial position of the LPFG is 1 and the amplitude of the cladding mode is 0, then calculate the transmission spectrum expression of the LPFG:

[0017] ;

[0018] in, for The complex amplitude of the forward transmission of the core mode at the location; The complex amplitude of the forward-transmitted core mode at position 0; As the first intermediate variable; ; These are the mutual coupling coefficients; for Conjugate; As the second intermediate variable, ;

[0019] The expressions for the transmission and reflection spectra obtained from the coupled-mode equations of CTFBG are as follows:

[0020] ;

[0021] ;

[0022] in, The complex amplitude of the forward-transmitted core mode at the * position; for The complex amplitude of the backward transmission fiber core mode at the location; based on the composition of the cascaded fiber grating, the transmission spectrum of the cascaded grating is composed of the transmission spectra of the two fiber grating segments, which can be expressed as:

[0023] ;

[0024] in, The transmission spectrum of the cascaded grating as a whole; and The transmission spectra of the LPFG and CTFBG structures are shown respectively.

[0025] The reflection spectrum of a cascaded grating mainly consists of the core mode after reflection by the CTFBG structure and subsequent filtering by the LPFG, which can be represented as:

[0026] .

[0027] Further, an LPFG and a CTFBG are sequentially inscribed on the core of the double-ended QBH optical cable along the forward transmission direction of the laser by femtosecond laser.

[0028] Further, when the transmission length of the double-ended QBH optical cable supporting the high-power laser shutter is ≥ 100 meters, a cascaded structure of one LPFG and two CTFBGs is adopted.

[0029] Further, the LPFG and one of the CTFBGs are integrated at both ends of the double-ended QBH optical cable, and the other CTFBG is integrated in the middle of the double-ended QBH optical cable to achieve segmented filtering of the SRS effect.

[0030] Further, when the length of the double-ended QBH optical cable connected to the high-power laser shutter is ≤ 100 meters, a cascaded structure of one LPFG and one CTFBG is adopted. The LPFG and the CTFBG are integrated at both ends of the double-ended QBH optical cable. The LPFG is integrated first and then the CTFBG along the forward transmission direction of the laser.

[0031] Further, the LPFG is a transmissive fiber grating.

[0032] Beneficial effects: Compared with the prior art, the remarkable effects of the present invention are as follows:

[0033] (1) By designing the LPFG and the CTFBG and sequentially inscribing the designed LPFG and CTFBG on the core of the double-ended QBH optical cable along the forward transmission direction of the laser, an LPFG-CTFBG cascaded grating is formed, which can effectively suppress the SRS effect generated in the laser shutter and the double-ended QBH optical cable, and avoid problems such as the decrease of the signal laser power and the deterioration of the beam quality caused by the accumulation of the SRS effect.

[0034] (2) This method combines the advantages of the LPFG and the CTFBG: The CTFBG has the characteristics of high environmental stability, no need for desensitization packaging, easy preparation and integration. Therefore, the present invention combines an appropriate number of CTFBGs according to the length of the energy transmission optical cable to filter the SRS effect while avoiding its accumulation in the optical fiber; adding an LPFG in the cascaded structure can block the transmission of the Raman light reflected by the CTFBG structure to the laser, fundamentally avoiding the damage to the laser device caused by the cumulative amplification of the backward Raman light, further improving the reliability and service life of the entire high-power laser transmission system, and making up for the technical shortcomings of the simple cascaded CTFBG scheme.

[0035] (3) The SRS effect will increase exponentially with the increase of the length of the double-ended QBH connected to the optical shutter. This invention can control the SRS effect at a low level while increasing the length of the double-ended QBH through targeted and efficient suppression of the SRS effect, thereby eliminating the limitation of the SRS effect on the length of the output optical cable of the high-power fiber laser. Different lengths of optical shutter output optical cables can be flexibly selected as needed. Attached Figure Description

[0036] Figure 1 This is a schematic diagram of the process of the present invention;

[0037] Figure 2 This is a schematic diagram of the system structure for suppressing the SRS effect using a high-power laser shutter, as described in this invention.

[0038] Figure 3 The simulated transmission spectrum of the cascaded grating is obtained based on the determined structural parameters and cascade sequence.

[0039] Figure 4 The transmission spectrum of the cascaded grating is actually written based on the structural parameters;

[0040] Figure 5 The output spectrum of a high-power fiber laser after being connected to a laser shutter and then passing through a 60-meter double-ended QBH.

[0041] Figure 6 The output spectrum of a high-power fiber laser after passing through a laser shutter connected to a 60-meter double-ended QBH with integrated cascaded gratings. Detailed Implementation

[0042] The technical solution of the present invention will now be described in detail with reference to specific embodiments and accompanying drawings.

[0043] like Figure 1 As shown, a method for suppressing the SRS effect of a high-power laser optical shutter in an LPFG-CTFBG cascade according to the present invention includes the following steps:

[0044] S1. Establish a mathematical model for suppressing the SRS (stimulated Raman scattering) effect of the LPFG-CTFBG cascaded grating, and use the SRS effect suppression mathematical model to obtain the expressions for the transmission spectrum and reflection spectrum of the cascaded grating; the specific implementation process is as follows:

[0045] The mathematical model for suppressing the SRS effect of LPFG-CTFBG cascaded gratings is established as follows:

[0046] ;

[0047] ;

[0048] The above equation is the mode coupling equation for LPFG, where, The complex amplitude of the forward-transmitted core mode at the * position; for The complex amplitude of the cladding mode propagating forward at the location; The self-coupling coefficient; represents the mutual coupling coefficient of the cladding modes; The location of the fence area; Let the detuning be denoted by 1; assume the amplitude of the core mode at the initial position of the LPFG is 1 and the amplitude of the cladding mode is 0 (i.e., ... and Then, the transmission spectrum expression of LPFG can be calculated:

[0049] ;

[0050] in, for The complex amplitude of the forward transmission of the core mode at the location; The complex amplitude of the forward-transmitted core mode at position 0; As the first intermediate variable; ; These are the mutual coupling coefficients; for Conjugate; As the second intermediate variable, ;

[0051] The expressions for the transmission and reflection spectra obtained from the coupled-mode equations of CTFBG are as follows:

[0052] ;

[0053] ;

[0054] in, for The complex amplitude of the forward transmission of the core mode at the location; for The complex amplitude of the backward transmission fiber core mode at the location; based on the composition of the cascaded fiber grating, the transmission spectrum of the cascaded grating can be composed of the transmission spectra of two fiber grating segments, which can be expressed as:

[0055] ;

[0056] in, The transmission spectrum of the cascaded grating as a whole; and The transmission spectra are for the LPFG structure and the CTFBG structure, respectively.

[0057] The reflection spectrum of a cascaded grating mainly consists of the core mode after reflection by the CTFBG structure and subsequent filtering by the LPFG, which can be represented as:

[0058] .

[0059] S2. Based on the expressions for the transmission and reflection spectra of the cascaded gratings, a simulation program for the LPFG-CTFBG cascaded grating is established. The influence of the structural parameters of the cascaded grating on the spectral parameters can be obtained from the simulation program. The structural parameters of the long-period fiber grating (LPFG) and the chirped tilted Bragg fiber grating (CTFBG) are input into the simulation program, which outputs the transmission and reflection spectra of the cascaded grating. The center wavelength and bandwidth of the LPFG-CTFBG cascaded grating are obtained from the transmission spectrum. The structural parameters of the LPFG and CTFBG are modified according to the influence law until the center wavelength and bandwidth of the LPFG-CTFBG cascaded grating match the center wavelength and bandwidth of the SRS effect, thus determining the structural parameters of the LPFG and CTFBG.

[0060] LPFG is used to filter out the residual Raman light main peak in the laser output and prevent the back-propagating Raman light from returning to the laser. CTFBG is used to supplement the filtering of the remaining forward-propagating Raman light after LPFG, avoiding the cumulative amplification of Raman light during long-distance fiber transmission, while also expanding the suppression bandwidth and suppression rate.

[0061] To achieve optimal performance, the design of LPFG-CTFBG cascaded gratings should follow these principles: 1. The center wavelength of the LPFG-CTFBG cascaded grating should match the center wavelength of the SRS effect; 2. The bandwidth of the LPFG-CTFBG cascaded grating should cover the range of the Raman spectrum; 3. Minimize signal light band loss.

[0062] Using the expressions for the transmission and reflection spectra of the cascaded gratings from step S1, a simulation program for the LPFG-CTFBG cascaded grating is established. Design requirements are proposed based on the center wavelength and bandwidth needed to suppress Raman light. The structural parameters of the LPFG and CTFBG are determined separately to ensure that the center wavelength, bandwidth, and SRS suppression rate of the cascaded grating structure meet the design requirements. The structural parameters of the LPFG include period, grating length, and refractive index modulation depth, while the structural parameters of the CTFBG include period, grating length, refractive index modulation depth, chirp rate, and tilt angle.

[0063] S3. According to the structural parameters of LPFG and CTFBG, LPFG and CTFBG are sequentially inscribed on the fiber core of the double-ended QBH optical cable along the forward transmission direction of the laser to form an LPFG-CTFBG cascaded grating. After the LPFG-CTFBG cascaded grating is integrated into a desensitization package, the fabrication of the double-ended QBH optical cable with the cascaded grating is completed.

[0064] The specific implementation process of step S3 is as follows:

[0065] According to simulations, when the center wavelength of the working laser output from the fiber laser is 1080nm, the center wavelength of the Raman light excited by the SRS effect is approximately 1135nm, with a 3dB bandwidth of about 15nm. Therefore, the center wavelength of the cascaded fiber grating should be set to 1135nm, and the 3dB bandwidth should cover the bandwidth of the SRS effect. Based on the mathematical model established in step S1, the structural design parameters of the cascaded LPFG and CTFBG are determined as follows:

[0066]

[0067] The transmission spectrum of the LPFG-CTFBG cascaded grating obtained from the simulation based on the above structural parameters is as follows: Figure 3 As shown, the center wavelength is 1135.4nm, the maximum suppression rate is 38.57dB, and the 3dB bandwidth is 20nm.

[0068] When the transmission length of the dual-ended QBH optical cable matched with the high-power laser shutter is ≥100 meters, a structure of one LPFG and two CTFBG cascaded is adopted. The LPFG and one of the CTFBG are integrated at both ends of the dual-ended QBH optical cable, and the other CTFBG is integrated in the middle of the dual-ended QBH optical cable to achieve segmented filtering of the SRS effect.

[0069] When the length of the double-ended QBH optical cable connected to the high-power laser optical shutter is ≤100 meters, a structure of one cascaded LPFG and one CTFBG is adopted. The LPFG and CTFBG are integrated at both ends of the double-ended QBH optical cable, and the LPFG is integrated first, followed by the CTFBG, according to the direction of laser forward transmission. In this embodiment, the length of the double-ended QBH optical cable connected to the high-power laser optical shutter is 60 meters, and the LPFG is a transmission-type fiber optic grating.

[0070] LPFG and CTFBG are sequentially inscribed on the fiber core of a double-ended QBH optical cable along the forward propagation direction using a femtosecond laser. In this embodiment, the LPFG structure in the cascaded grating is fabricated using a point-by-point scanning method with a femtosecond laser, and the CTFBG structure is fabricated using a femtosecond laser combined with a phase mask. The femtosecond laser inscription method has advantages such as high power handling capacity, simple inscription operation, good spectral quality, and easy reproducibility. The laser parameters used for inscription are as follows:

[0071]

[0072] Using a spherical mirror to focus the light spot and a precision displacement stage, the LPFG structure was fabricated. Using a cylindrical mirror to focus the light spot and a phase mask, piezoelectric vibration stage, and precision displacement stage, the CTFBG structure was fabricated. The transmission spectrum of the cascaded fiber grating after fabrication is shown below. Figure 4 The center wavelength shown is 1135nm, and the 3dB bandwidth is 15dB. Due to the low intensity of the light source in this band, the suppression rate is greater than 24.4dB.

[0073] After the writing is completed, the LPFG gate area is water-cooled and desensitized by encapsulation. The encapsulation shell can effectively prevent gate deformation, and the water-cooling structure design can maintain the gate temperature. This is to prevent bending, heat generated during operation, and changes in external temperature from affecting the SRS effect suppression rate. The optical fiber is then further encapsulated into a double-ended QBH optical cable.

[0074] S4. Connect the double-ended QBH optical cable with cascaded grating to the output end of the high-power laser shutter. The double-ended QBH optical cable with cascaded grating suppresses the SRS effect of the high-power laser shutter.

[0075] Combination Figure 2 The mechanism of cascaded fiber gratings is as follows: Residual SRS effects in the emitted laser light from the fiber laser are transmitted to the output optical cable of the high-power laser shutter. The LPFG located on one side of the high-power laser shutter output port couples the residual SRS effect into the forward propagation cladding mode, where it is stripped by the cladding stripper (CPS1), preventing amplification during subsequent long-distance optical cable propagation. When the laser propagates to the other side of the optical cable, the accumulated SRS effect is coupled into the backward propagation cladding mode by the CTFBG, and removed by CPS2. Through the multiple filtering effects of the cascaded gratings, the SRS threshold of the system can be effectively improved. Since the LPFG is a transmission-type fiber grating, it does not generate backward Raman reflection light when filtering residual SRS effects from the fiber laser, fundamentally avoiding damage to the laser device caused by the cumulative amplification of backward Raman light.

[0076] To verify the effectiveness of this method in suppressing the SRS effect, the output laser spectrum was tested under different conditions. Figure 5 The output spectrum of a high-power fiber laser connected to a high-power laser shutter, with a 60m double-ended QBH optical cable connected to the output end of the high-power laser shutter, shows that the residual Raman light is accumulated and amplified, forming a Raman peak at 1135nm. The Raman light peak value is 22dB lower than the signal light peak value. Figure 6The output spectrum of a high-power fiber laser connected to a high-power laser shutter, with a 60m double-ended QBH optical cable integrating the aforementioned cascaded grating connected to the shutter's output end, is shown. It can be seen that the Raman component is filtered out; the Raman peak is no longer visible in the spectrum, and the Raman peak intensity is more than 40dB lower than the signal peak intensity. This demonstrates that the proposed method has a good Raman suppression effect.

[0077] Both LPFG and CTFBG can filter out the SRS effect. LPFG couples the forward-propagating core mode to the forward-propagating cladding mode, while CTFBG couples the forward-propagating core mode to the backward-propagating cladding mode. This separates the SRS-excited Raman light component from the core to the cladding, which is then stripped by the CPS stripper, thus suppressing the SRS effect. CTFBG is easy to integrate and has high environmental stability, but it reflects some Raman light. At extremely high laser power, this reflected light can accumulate and amplify, causing a decrease in laser stability or even damage to the laser. LPFG, on the other hand, has the advantage of no backscattering, avoiding the generation of backscattered reflected Raman light. However, changes in the external environment (such as stress, bending, and temperature variations) can cause the center wavelength of the LPFG to drift, reducing the SRS suppression rate. This invention designs the structural parameters of LPFG and CTFBG, cascades them in a specific order, and integrates them into a double-ended QBH optical cable. Utilizing the efficient filtering of forward-transmitting Raman light and the absence of backward backlighting by LPFG, combined with the strong environmental stability and easily tunable wavelength bandwidth of CTFBG, it achieves multiple and efficient suppressions of the SRS effect. This solves the problem of SRS effect suppression in high-power laser optical shutters.

[0078] This invention integrates LPFG and CTFBG cascaded gratings into a double-ended QBH optical cable at the output end of a high-power laser shutter. By matching the SRS effect to excite the Raman light and the center wavelength and bandwidth of the cascaded grating, the Raman light in the laser output is stripped, achieving highly stable and reliable laser output.

[0079] The innovation of this invention lies in the first application of the cascaded structure of LPFG and CTFBG to the optical output cable of the optical shutter. The key point is to establish a mathematical model of the cascaded structure of LPFG and CTFBG, and on this basis, to build a simulation program. Through reasonable parameter design, the design requirements such as high SRS suppression rate and center wavelength and bandwidth matching Raman spectrum are achieved. Furthermore, the stability of the system is improved through desensitization packaging.

[0080] The core of the present invention lies in cascading a long-period fiber grating (LPFG) and a chirped tilted fiber Bragg grating (CTFBG) in a specific order and integrating them into the double-ended QBH optical cable supporting a high-power laser shutter. By leveraging the characteristics of the LPFG, which can efficiently filter forward-propagating Raman light and has no backward reflected light, and combining with the characteristics of the CTFBG, which has high environmental stability and easily tunable wavelength bandwidth, the synergistic effect of the LPFG and the CTFBG is utilized to filter the Raman light accumulated and amplified during the transmission of the laser shutter and the double-ended QBH, achieving the suppression of the SRS effect and realizing the double-high-efficiency suppression of Raman light during the transmission of the laser shutter and the double-ended QBH.

[0081] The present invention can solve the technical problems of the cumulative amplification of the SRS effect during the transmission of existing high-power lasers in the shutter and the double-ended QBH optical cable, resulting in poor output stability of the laser, reduced power, and beam quality.

[0082] The present invention significantly increases the SRS excitation threshold of the high-power laser shutter system, effectively blocks the cumulative amplification and backward propagation amplification of Raman light, and can still ensure high stability of laser output, high power retention rate, and high beam quality in long-distance transmission scenarios. It is suitable for the supporting use of high-power fiber lasers and laser shutters above ten thousand watts, with strong adaptability to transmission distances and high environmental tolerance. The present invention also has the characteristics of high SRS effect suppression rate and no Raman backward light.

Claims

1. A method for suppressing the SRS effect of a high-power laser optobar cascaded with LPFG-CTFBG, characterized in that, Including: Establish a mathematical model for suppressing the SRS effect of the LPFG-CTFBG cascaded grating, and obtain the expressions of the transmission spectrum and reflection spectrum of the cascaded grating by using the mathematical model for suppressing the SRS effect; According to the expressions of the transmission spectrum and reflection spectrum of the cascaded grating, establish a simulation program for the LPFG-CTFBG cascaded grating, and obtain the influence law of the structural parameters of the cascaded grating on the spectral parameters according to the simulation program; input the structural parameters of the LPFG and CTFBG into the simulation program, and the simulation program outputs the transmission spectrum and reflection spectrum of the cascaded grating; obtain the central wavelength and bandwidth of the LPFG-CTFBG cascaded grating according to the transmission spectrum, and modify the structural parameters of the LPFG and CTFBG according to the influence law until the central wavelength and bandwidth of the LPFG-CTFBG cascaded grating match the central wavelength and bandwidth of the SRS effect, so as to determine the structural parameters of the LPFG and CTFBG; According to the structural parameters of the LPFG and CTFBG, write the LPFG and CTFBG in sequence along the forward transmission direction of the laser on the core of the double-ended QBH optical cable to form an LPFG-CTFBG cascaded grating. After integrally desensitizing and packaging the LPFG-CTFBG cascaded grating, complete the preparation of the double-ended QBH optical cable with a cascaded grating; Connect the double-ended QBH optical cable with a cascaded grating to the output end of the high-power laser shutter, and the double-ended QBH optical cable with a cascaded grating suppresses the SRS effect of the high-power laser shutter.

2. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: The LPFG is used to filter the main peak of the residual Raman light in the laser output by the laser and prevent the backward transmitted Raman light from returning to the inside of the laser.

3. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: The CTFBG is used to further filter the forward transmitted Raman light remaining after the LPFG filtering, avoid the cumulative amplification of Raman light in long-distance optical fiber transmission, and at the same time expand the suppression bandwidth and suppression rate.

4. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: The structural parameters of the LPFG include the period, grating region length, and refractive index modulation depth, and the structural parameters of the CTFBG include the period, grating region length, refractive index modulation depth, chirp rate, and tilt angle.

5. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: The establishment process of the mathematical model for suppressing the SRS effect of the LPFG-CTFBG cascaded grating is as follows: ； ； The above equation is the mode coupling equation for LPFG, where, The complex amplitude of the forward-transmitted core mode at the * position; for The complex amplitude of the cladding mode propagating forward at the location; The self-coupling coefficient; represents the mutual coupling coefficient of the cladding modes; The location of the fence area; Let be the detuning factor; assuming the amplitude of the core mode at the initial position of the LPFG is 1 and the amplitude of the cladding mode is 0, then calculate the transmission spectrum expression of the LPFG: ； in, for The complex amplitude of the forward transmission of the core mode at the location; The complex amplitude of the forward-transmitted core mode at position 0; As the first intermediate variable; ; These are the mutual coupling coefficients; for Conjugate; As the second intermediate variable, ; According to the coupled mode equations of the CTFBG, the expressions of its transmission spectrum and reflection spectrum are calculated as: ； ； in, for The complex amplitude of the forward transmission of the core mode at the location; for The complex amplitude of the backward transmission fiber core mode at the location; based on the composition of the cascaded fiber grating, the transmission spectrum of the cascaded grating is composed of the transmission spectra of the two fiber grating segments, which can be expressed as: ； in, The transmission spectrum of the cascaded grating as a whole; and The transmission spectra of the LPFG and CTFBG structures are shown respectively. The reflection spectrum of the cascaded grating is mainly composed of the core modes that pass through the CTFBG structure and are then filtered by the LPFG again, and can be expressed as: 。 6. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: Write the LPFG and CTFBG in sequence along the forward transmission direction of the laser on the core of the double-ended QBH optical cable by using femtosecond laser.

7. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: When the transmission length of the double-ended QBH optical cable supporting the high-power laser shutter is ≥ 100 meters, a structure of one LPFG cascaded with two CTFBGs is adopted.

8. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 7, characterized in that: The LPFG and one of the CTFBGs are integrated at both ends of the double-ended QBH optical cable, and the other CTFBG is integrated in the middle of the double-ended QBH optical cable to achieve segmented filtering of the SRS effect.

9. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: When the length of the dual-ended QBH optical cable connected by the high-power laser shutter is ≤100 meters, a structure of one LPFG and one CTFBG cascaded is adopted. The LPFG and CTFBG are integrated at both ends of the dual-ended QBH optical cable. The LPFG is integrated first and then the CTFBG is integrated according to the direction of laser forward transmission.

10. The method for suppressing the SRS effect of a high-power laser shutter in a cascaded LPFG-CTFBG system according to claim 1, characterized in that: The LPFG is a transmission fiber grating.

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