Long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths and suppression method

Abstract

The invention belongs to the field of devices for high-power fiber lasers, particularly relates to a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths and a suppression method, and aims to solve the problem that the stimulated Raman scattering effect under different wavelengths cannot be dynamically adapted and suppressed. A pumping source is connected with one end of a beam combiner, the other end of the beam combiner is connected with one end of a resonant cavity, the other end of the resonant cavity is connected with one end of a long-period fiber bragg grating, the other end of the long-period fiber bragg grating is connected with one end of a stripper, the other end of the stripper is connected with an output end, and the output end is connected with a spectrograph. Wherein the resonant cavity is used for oscillating and amplifying spontaneous radiation light with a set wavelength; the two ends of the long-period fiber grating are fixed to the stress field applying device, and the long-period fiber grating is arranged in the temperature field applying device. According to the invention, stimulated Raman scattering of different wavelengths is effectively inhibited, the output power, the beam quality and the system stability of the laser are improved, and the service life is prolonged.

Description

Technical Field

[0001] The invention belongs to the field of devices for high-power fiber lasers, and in particular relates to a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths and a suppression method. Background Art

[0002] As high-power fiber lasers move toward higher output powers, nonlinear effects, especially stimulated Raman scattering, have become a major reason limiting the power increase of fiber lasers. Nonlinear effect suppression methods based on fiber Bragg gratings are currently a relatively excellent choice because fiber Bragg gratings have the advantages of relatively low cost, simple preparation process, mass production, and full compatibility with optical fibers, which enables the entire laser system to maintain a high degree of integration.

[0003] However, as my country's manufacturing industry gradually develops towards intelligent manufacturing, the output power and output wavelength of high-power fiber lasers often need to change dynamically with the changes in application scenarios and application objects. At this time, traditional fiber gratings cannot effectively suppress the stimulated Raman scattering effects of different wavelengths in fiber lasers. For example, the common central wavelengths of industrial fiber lasers in the 1-micron band are 1060nm, 1070nm, 1080nm and 1090nm, and the central wavelengths of stimulated Raman scattering effects stimulated by them are 1115nm, 1125nm, 1135nm and 1145nm, respectively. As the industrial application scenarios switch, lasers with different output wavelengths will produce stimulated Raman scattering effects in different bands, thereby hindering the increase in the output power of high-power fiber lasers, deteriorating the quality of the laser output beam, and reducing the stability and life of the laser. Therefore, a new method is needed that can dynamically suppress the stimulated Raman scattering effects stimulated by lasers of different wavelengths as the different wavelengths of the fiber laser are switched.

[0004] Based on this, the present invention proposes a long-period fiber grating and a suppression method that can dynamically suppress stimulated Raman scattering of different wavelengths. Summary of the invention

[0005] In order to solve the above-mentioned problem in the prior art, that is, as the output wavelength changes, the traditional fiber Bragg grating cannot dynamically adapt to and effectively suppress the stimulated Raman scattering effect at different wavelengths, thereby limiting the further increase of power and having a negative impact on the performance, stability and life of the laser. The present invention provides a long-period fiber Bragg grating and a suppression method that can dynamically suppress stimulated Raman scattering at different wavelengths.

[0006] In one aspect of the present invention, there is provided a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths, comprising a pump source, a beam combiner, a resonant cavity, a long-period fiber grating, a stress field applying device, a temperature field applying device, a stripper, an output end and a spectrometer; The pump source is connected to one end of the beam combiner, the other end of the beam combiner is connected to one end of the resonant cavity, the other end of the resonant cavity is connected to one end of the long-period fiber grating, the other end of the long-period fiber grating is connected to one end of the stripper, the other end of the stripper is connected to the output end, and the output end is connected to the spectrometer; wherein the resonant cavity is used to oscillate and amplify spontaneous radiation light of a set wavelength; The two ends of the long-period fiber grating are fixed to the stress field applying device, and the long-period fiber grating is arranged in the temperature field applying device. The stress field applying device and the temperature field applying device are used to adjust the micro-nano structure parameters of the long-period fiber grating in real time, so as to realize dynamic control of the period of the long-period fiber grating, so as to make the central wavelength of the cladding mode resonance peak drift, and strip the stimulated Raman scattering effect from the optical fiber core, thereby dynamically suppressing the stimulated Raman scattering effects of different wavelengths in the optical fiber laser.

[0007] Preferably, the resonant cavity comprises a high-reflection fiber Bragg grating, a gain fiber and a low-reflection fiber Bragg grating; One end of the high-reflection fiber grating is connected to the other end of the beam combiner, the other end of the high-reflection fiber grating is connected to one end of the gain fiber, the other end of the gain fiber is connected to one end of the low-reflection fiber grating, and the other end of the low-reflection fiber grating is connected to one end of the long-period fiber grating.

[0008] Preferably, the stress field applying device comprises an adjustment frame and a stepping motor; One end of the long-period fiber grating is fixed to the adjustment end of the adjustment frame, and the other end of the long-period fiber grating is fixed to the moving end of the stepping motor, and the stress direction applied by the stepping motor is the axial direction of the long-period fiber grating; The adjustment frame is used to ensure that one end of the long-period fiber grating is fixed and parallel to the temperature field applying device, thereby ensuring that the temperature field applied by the temperature field applying device acts uniformly on the long-period fiber grating.

[0009] Preferably, the temperature field applying device includes a semiconductor refrigerator.

[0010] Preferably, the stepper motor and the semiconductor refrigerator in the stress field applying device are both connected to an industrial computer, and the industrial computer is used to control the displacement of the moving end of the stepper motor and the temperature applied by the semiconductor refrigerator.

[0011] Preferably, the gain optical fiber is an ytterbium-doped optical fiber.

[0012] Another aspect of the present invention provides a method for suppressing long-period fiber gratings (LPBs) that can dynamically suppress stimulated Raman scattering of different wavelengths. The method comprises: The strain sensitivity factor of the resonant wavelength is calculated based on the elastic-optic coefficient and effective refractive index of the core and cladding of the long-period fiber grating; Calculate the temperature sensitivity factor of the optical fiber based on the thermo-optic coefficient and effective refractive index of the core and cladding of the long-period fiber grating; The stress change and temperature change are calculated by combining the strain sensitivity factor, the temperature sensitivity factor, the original wavelength, the pre-acquired wavelength offset, the thermal expansion coefficient and the Poisson's ratio; According to the temperature and stress detected in real time, and based on the stress change amount and the temperature change amount, the stress field and the temperature field to be applied are obtained; According to the stress field and temperature field to be applied, the parameters of the stress field applying device and the temperature field applying device are adjusted to apply the corresponding stress field and temperature field to achieve dynamic suppression of stimulated Raman scattering effects of different wavelengths in the fiber laser.

[0013] Preferably, the strain sensitivity factor of the resonant wavelength is calculated as follows: ; in, represents the elastic-optic coefficient of the fiber core, represents the elastic-optical coefficient of the cladding, is the effective refractive index of the fiber core, represents the effective refractive index of the cladding.

[0014] Preferably, the temperature sensitivity factor is calculated as follows: ; in, is the thermo-optic coefficient of the fiber core, is the thermo-optical coefficient of the cladding, is the effective refractive index of the fiber core, represents the effective refractive index of the cladding.

[0015] Preferably, the stress variation and temperature variation are calculated as follows: ; in, is the wavelength shift of the long-period fiber grating, is the original wavelength, is the coefficient of thermal expansion, is the temperature change, is the dependent variable, is the stress change, is the Poisson's ratio, is the strain sensitivity factor, is the temperature sensitivity factor.

[0016] Beneficial effects of the present invention: Dynamic adaptability: The micro-nanostructure parameters of the long-period fiber grating are adjusted in real time by using the stress field application device and the temperature field application device, realizing the dynamic suppression of stimulated Raman scattering effect at different wavelengths. This dynamic adjustment capability enables the laser to flexibly adjust the output wavelength according to the changes in the application scenario without affecting the performance.

[0017] Improve output power: It effectively solves the nonlinear effect problem encountered when high-power fiber lasers move towards higher output power, especially the problem of stimulated Raman scattering effect, thereby further improving the output power of the laser.

[0018] Improved beam quality: By removing the stimulated Raman scattering effect from the fiber core, unnecessary energy loss and beam distortion are reduced, and the quality of the laser output beam is improved.

[0019] Enhance system stability and life: The successful suppression of the impact of stimulated Raman scattering on the laser not only improves the stability and reliability of the system, but also extends the service life of the laser.

[0020] High integration: The use of long-period fiber gratings and other fiber components maintains a high degree of integration of the entire laser system, which is conducive to compact design and easy compatibility with other fiber optical components.

[0021] Cost-effectiveness: Long-period fiber gratings have the advantages of relatively low cost and simple preparation process, which helps to reduce the cost of the overall solution while ensuring the feasibility of mass production.

[0022] Intelligent control: Both the stepper motor and the semiconductor cooler are connected to the industrial computer, allowing precise control and adjustment, providing the possibility of intelligent management for applications in smart manufacturing environments.

[0023] Wide applicability: This solution is applicable to multiple industrial fiber laser center wavelengths (such as 1060nm, 1070nm, 1080nm and 1090nm), and can be customized according to specific application requirements, with wide applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Other features, objects and advantages of the present application will become more apparent by reading the detailed description of non-limiting embodiments made with reference to the following drawings: Figure 1 It is a schematic structural diagram of a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to the present invention; Figure 2 It is a spectrum diagram of a long-period fiber grating with a central wavelength of 1115 nm after being regulated by a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to the present invention; Figure 3 It is a spectrum diagram of a long-period fiber grating with a central wavelength of 1125 nm after being regulated by a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to the present invention; Figure 4 It is a spectrum diagram of a long-period fiber grating with a central wavelength of 1135 nm after being regulated by a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to the present invention; Figure 5 The spectrum diagram is a long-period fiber grating with a central wavelength of 1145 nm after being regulated by the long-period fiber grating of the present invention which can dynamically suppress stimulated Raman scattering of different wavelengths.

[0025] In the figure: 1. Pump source; 2. Beam combiner; 3. High-reflection fiber Bragg grating; 4. Gain fiber; 5. Low-reflection fiber Bragg grating; 6. Adjustment stand; 7. Long-period fiber Bragg grating; 8. Semiconductor refrigerator; 9. Stepper motor; 10. Stripper; 11. Output end; 12. Spectrometer; 13. Industrial computer. DETAILED DESCRIPTION

[0026] The present application will be further described in detail below in conjunction with the accompanying drawings and embodiments. It is to be understood that the specific embodiments described herein are only used to explain the relevant invention, rather than to limit the invention. It should also be noted that, for ease of description, only the parts related to the relevant invention are shown in the accompanying drawings.

[0027] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present application will be described in detail below with reference to the accompanying drawings and in combination with the embodiments.

[0028] like Figure 1-Figure 5 For details, see Figure 1 The first embodiment of the present invention provides a long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths, comprising a pump source 1, a beam combiner 2, a resonant cavity, a long-period fiber grating 7, a stress field applying device, a temperature field applying device, a stripper 10, an output end 11 and a spectrometer 12; The pump source 1 is connected to one end of the beam combiner 2, the other end of the beam combiner 2 is connected to one end of the resonant cavity, the other end of the resonant cavity is connected to one end of the long-period fiber grating 7, the other end of the long-period fiber grating 7 is connected to one end of the stripper 10, the other end of the stripper 10 is connected to the output end 11, and the output end 11 is connected to the spectrometer 12; wherein the resonant cavity is used to oscillate and amplify spontaneous radiation light of a set wavelength; The two ends of the long-period fiber grating 7 are fixed to the stress field applying device, and the long-period fiber grating 7 is arranged in the temperature field applying device. The stress field applying device and the temperature field applying device are used to adjust the micro-nano structure parameters of the long-period fiber grating 7 in real time, so as to realize dynamic control of the period of the long-period fiber grating 7, so as to make the central wavelength of the cladding mode resonance peak drift, and strip the stimulated Raman scattering effect from the optical fiber core, thereby dynamically suppressing the stimulated Raman scattering effects of different wavelengths in the optical fiber laser.

[0029] The resonant cavity in the present invention is composed of a pair of fiber Bragg gratings and a gain medium, including a high-reflection fiber Bragg grating 3 with a reflectivity greater than 99% and a low-reflection fiber Bragg grating 5 with a reflectivity of about 10%. The gain medium gain fiber 4 usually adopts ytterbium-doped fiber. After absorbing the energy of the pump source 1, the Yb3+ ions in the fiber core will transition from the ground state to the excited state, thereby achieving a population inversion. For the spontaneous radiation light generated by the energy level transition, the frequency selection effect of the resonant cavity composed of the high-reflection fiber Bragg grating 3, the gain fiber 4, and the low-reflection fiber Bragg grating 5 will cause the spontaneous radiation light of a specific wavelength to be oscillated and amplified, thereby achieving high-power laser output. Specifically: The resonant cavity comprises a high-reflection fiber Bragg grating 3, a gain fiber 4 and a low-reflection fiber Bragg grating 5; One end of the high-reflection fiber grating 3 is connected to the other end of the combiner 2, the other end of the high-reflection fiber grating 3 is connected to one end of the gain fiber 4, the other end of the gain fiber 4 is connected to one end of the low-reflection fiber grating 5, and the other end of the low-reflection fiber grating 5 is connected to one end of the long-period fiber grating 7.

[0030] The stress field applying device in the present invention includes an adjustment frame 6 and a stepping motor 9, and the temperature field applying device includes a semiconductor refrigerator 8; One end of the long-period fiber grating 7 is fixed to the adjustment end of the adjustment frame 6, and the other end of the long-period fiber grating 7 is fixed to the moving end of the stepping motor 9, and the stress direction applied by the stepping motor 9 is the axial direction of the long-period fiber grating 7; The adjustment frame 6 is used to ensure that one end of the long-period fiber grating 7 is fixed and positioned parallel to the temperature field applying device, thereby ensuring that the temperature field applied by the temperature field applying device acts uniformly on the long-period fiber grating 7 .

[0031] The adjustment frame 6 can adjust the position of the long-period fiber grating 7 in the X-axis and Y-axis directions. Figure 1 The axial directions of the two adjustment handles installed on the middle adjustment frame 6, the specific structure of the adjustment frame 6 is prior art and will not be described in detail here.

[0032] The stepper motor 9 and the semiconductor refrigerator 8 are both connected to an industrial computer 13 , and the industrial computer 13 is used to control the displacement of the moving end of the stepper motor 9 and the temperature applied by the semiconductor refrigerator 8 .

[0033] The present invention aims at stimulated Raman scattering of different wavelengths in a fiber laser, and uses an industrial computer 13 to control the movement of a stepper motor 9 and a semiconductor refrigerator 8 to apply a stress field and a temperature field to a long-period fiber grating 7, thereby accurately and dynamically controlling the period of the long-period fiber grating 7 so that the central wavelength of the cladding mode resonance peak drifts, so that it matches the central wavelength of Stokes light of different wavelengths excited by the stimulated Raman scattering effect, so that the stimulated Raman scattering effect can be stripped from the fiber core, thereby achieving the purpose of dynamically suppressing the stimulated Raman scattering effects of different wavelengths in the fiber laser.

[0034] The micro-nano structure of the long-period fiber grating 7 is fused before the stripper 10, and the grating structure is placed in the semiconductor refrigerator 8, and the two ends are fixed on the adjustment frame 6 and the stepper motor 9 respectively. The industrial computer 13 is used to control the movement of the stepper motor 9 and the semiconductor refrigerator 8 to apply stress field and temperature field to the long-period fiber grating 7. Through the coarse and fine two-level long-period fiber grating micro-nano structure closed-loop real-time precision control technology, the dynamic stress field is combined with the temperature field to perform real-time dynamic adjustment of the micro-nano structure parameters of the long-period fiber grating 7, so as to accurately and dynamically control the period of the long-period fiber grating 7 so that the central wavelength of the cladding mode resonance peak drifts. Figure 2-Figure 5 As shown in FIG. 1 , the result after regulation is as follows: the spectrum of the long period fiber grating with the first central wavelength to the fourth central wavelength being 1115nm, 1125nm, 1135nm and 1145nm respectively. As long as they are matched with the central wavelengths of Stokes light of different wavelengths excited by stimulated Raman scattering effect, namely 1115nm, 1125nm, 1135nm and 1145nm, respectively, the stimulated Raman scattering effect can be stripped from the fiber core, so as to achieve the purpose of dynamically suppressing the stimulated Raman scattering effect of different wavelengths in the fiber laser. In this process, as Figure 2 As shown, a stress of 5N and a temperature field of 20°C are applied to a long period fiber grating with a first central wavelength of 1115nm; like Figure 3 As shown in FIG. 1 , a stress of 10 N and a temperature field of 22.5° C. are applied to the long-period fiber grating with a second central wavelength of 1125 nm; Figure 4As shown in FIG. 1 , a stress of 15 N and a temperature field of 25.12° C. are applied to the third long-period fiber grating with a central wavelength of 1135 nm; Figure 5 As shown, a stress of 20N and a temperature field of 27.43°C were applied to the long-period fiber grating with a fourth central wavelength of 1145nm.

[0035] The long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths of the present invention can couple the parasitic laser from the forward-transmitting core mode to the forward-transmitting cladding mode.

[0036] The industrial computer 13 is used to control the movement of the stepping motor 9 and the semiconductor refrigerator 8 to perform real-time dynamic coarse and fine dual adjustment on the micro-nano structure inside the long-period fiber grating 7.

[0037] The coarse-fine two-stage fiber grating micro-nano structure closed-loop real-time precision control technology applies a dynamic stress field or temperature field to the fiber grating, and a technical means combining the two to adjust the structural parameters of the long-period fiber grating 7 in real time. The main parameter is the period, so that the central wavelength of the cladding mode resonance peak of the long-period fiber grating 7 dynamically drifts, so as to match the central wavelength of Stokes light of different wavelengths stimulated by the stimulated Raman scattering effect according to actual application requirements, and remove the stimulated Raman scattering effect from the fiber core, thereby achieving the purpose of dynamically suppressing the stimulated Raman scattering effect of different wavelengths in the fiber laser. Since the micro-nano structure inside the long-period fiber grating 7 is affected by the stress field, but is affected by the temperature field. The change is small, so the industrial computer 13 is used to control the movement of the stepping motor 9 and the semiconductor refrigerator 8 to apply the stress field and temperature field to the long-period fiber grating 7 to perform coarse-fine dual adjustment on the micro-nano structure of the fiber grating, thereby ensuring the control accuracy.

[0038] Applying a dynamic stress field or temperature field to the long-period fiber grating 7 is to calculate the change function of the internal micro-nano structure parameter pitch and tilt angle of the long-period fiber grating 7 when it is subjected to the stress field and temperature field according to the elastic coefficient and thermal expansion coefficient of the optical fiber, and calculate the influence of the stress field and temperature field on the fiber refractive index core and cladding refractive index according to the elastic-optic effect and the thermal-optic effect. By introducing the change amount of the micro-nano structure parameters of the long-period fiber grating 7 and the change of the fiber refractive index into the established mathematical analysis model of the long-period fiber grating 7 precisely regulating the central wavelength of the cladding mode resonance peak, a theoretical mapping model of the relationship between the micro-nano structure of the long-period fiber grating 7 precisely regulating the central wavelength of the cladding mode resonance peak and the external dynamic temperature field and stress field parameters is established. Then, according to the demand of the fiber laser to suppress the stimulated Raman scattering effect of different wavelengths, the parameters of the applied stress field and temperature field can be quantitatively calculated by using this model, so that the industrial computer 13 is used to control the movement of the stepping motor 9 and the semiconductor refrigerator 8 to perform coarse and fine dual adjustment on the internal micro-nano structure of the long-period fiber grating 7.

[0039] The second embodiment of the present invention provides a method for suppressing long-period fiber gratings (LPBFs) that can dynamically suppress stimulated Raman scattering of different wavelengths. The method is based on the long-period fiber grating (LPBFs) that can dynamically suppress stimulated Raman scattering of different wavelengths of the first embodiment. The method includes: Calculate the strain sensitivity factor of the resonance wavelength based on the elastic-optic coefficient and effective refractive index of the core and cladding of the long-period fiber grating 7; Calculate the temperature sensitivity factor of the optical fiber according to the thermo-optic coefficient and effective refractive index of the core and cladding of the long-period fiber grating 7; The stress change and temperature change are calculated by combining the strain sensitivity factor, the temperature sensitivity factor, the original wavelength, the pre-acquired wavelength offset, the thermal expansion coefficient and the Poisson's ratio; According to the temperature and stress detected in real time, and based on the stress change amount and the temperature change amount, the stress field and the temperature field to be applied are obtained; According to the stress field and temperature field to be applied, the parameters of the stress field applying device and the temperature field applying device are adjusted, and the corresponding stress field and temperature field are applied to achieve dynamic suppression of stimulated Raman scattering effects of different wavelengths in the fiber laser. When the long-period fiber grating 7 is subjected to the stress field and temperature field, its structural parameters will change accordingly. The change function is described by the elastic coefficient and thermal expansion coefficient of the optical fiber, and is used to quantitatively calculate the changes in the grating pitch and tilt angle of the optical fiber grating under the stress field and temperature field. Based on the changes in these structural parameters and the changes in the refractive index of the optical fiber, a mathematical model for regulating the central wavelength of the cladding mode resonance peak of the long-period fiber grating 7 is established, thereby describing the relationship between the drift characteristics of the central wavelength of the cladding mode resonance peak of the long-period fiber grating 7 and the external dynamic temperature and stress field.

[0040] According to the central wavelength control requirements of the long-period fiber grating 7, the required stress and temperature control parameters can be quantitatively calculated through the mathematical model and the variation function. For the long-period fiber grating 7, the relationship between the central wavelength and the period can be expressed by the following formula: ; in is the effective refractive index of the core mode, is the effective refractive index of the i-th cladding mode, is the period of the long-period fiber grating 7. By controlling the period, length, refractive index modulation depth and other structural parameters of the grating, the cladding mode resonance peak of the long-period fiber grating 7 is aligned with the stimulated Raman scattering effect spectrum of different wavelengths excited by the fiber laser, so that the stimulated Raman scattering effect can be coupled from the core mode to the forward-transmitted cladding mode. Then, the stimulated Raman scattering effect is stripped from the optical fiber using a cladding light stripper, thereby achieving the purpose of suppressing the stimulated Raman scattering effect of the fiber laser.

[0041] The axial strain characteristics of the long-period fiber grating 7 can be analyzed based on the elasto-optic effect and elastic mechanics. The effective refractive index of the core fundamental mode, the effective refractive index of the cladding mode and the grating period can all affect the axial strain, so its phase matching condition can be expressed as: ; In the formula, is the dependent variable, is the resonance peak wavelength, Λ is the grating period, represents the effective refractive index of the fiber Bragg grating core, represents the effective refractive index of the fiber Bragg grating cladding.

[0042] When the long-period fiber grating 7 is axially strained, the effective refractive index of the core and the effective refractive index of the cladding will change due to the elastic-optic effect, and the degree of change is related to the elastic-optic coefficient. , Λ is the grating period, and the change of effective refractive index caused by the photoelastic effect is: ; ; In the formula, represents the FBG core elastic-optic coefficient, represents the elastic-optical coefficient of the fiber Bragg grating cladding, represents the effective refractive index of the fiber Bragg grating core, represents the effective refractive index of the fiber Bragg grating cladding, is the dependent variable.

[0043] Considering the reasons of mode dispersion and waveguide dispersion, the above formula can also be expressed as: ; In the formula, is the waveguide dispersion factor of the optical fiber, is the dependent variable, is the resonance peak wavelength, The strain sensitivity factor representing the resonant wavelength is defined as: ; in, represents the elastic-optic coefficient of the fiber core, represents the elastic-optical coefficient of the cladding, is the effective refractive index of the fiber core, represents the effective refractive index of the cladding.

[0044] Long-period fiber grating realizes the coupling between the core guided mode and the first-order cladding modes propagating in the same direction. Its phase matching condition is: ; In the formula, , The core guided mode and the first-order m The propagation constant of the sub-cladding mode, Λ is the grating period. , the above formula becomes: ; In the formula, is the resonance peak wavelength of the grating, represents the effective refractive index of the fiber Bragg grating core, represents the effective refractive index of the fiber Bragg grating cladding, and Λ is the grating period.

[0045] When the long-period fiber grating is only affected by the environmental parameter of temperature, the temperature coefficient of the resonance wavelength of the long-period fiber grating can be obtained by taking the differential of the temperature T in the above formula: for: ; In the formula, is the difference in effective refractive index between the core and cladding, is the grating length, is the resonance peak wavelength of the grating, represents the effective refractive index of the fiber Bragg grating core, represents the effective refractive index of the fiber Bragg grating cladding, Λ is the grating period, and T is the temperature.

[0046] The change in grating length caused by thermal expansion effect is: ; In the formula, is the thermal expansion coefficient of the optical fiber, is the grating length and T is the temperature.

[0047] The change in effective refractive index caused by the thermo-optic effect is: ; ; In the formula, , are the thermo-optic coefficients of the core and cladding, respectively. represents the effective refractive index of the fiber Bragg grating core, represents the effective refractive index of the fiber Bragg grating cladding, and T is the temperature.

[0048] Taking into account the modal dispersion and waveguide dispersion of the optical fiber, the formula becomes: ; In the formula, and are the waveguide dispersion factor and temperature sensitivity factor of the optical fiber, is the thermal expansion coefficient of the optical fiber, T is the temperature, is the resonance peak wavelength of the grating, where and The two can be expressed as: ; ; In the formula, , are the thermo-optic coefficients of the core and cladding, respectively. represents the effective refractive index of the fiber Bragg grating core, represents the effective refractive index of the fiber Bragg grating cladding, is the resonance peak wavelength of the grating, and Λ is the grating period.

[0049] The optical properties of long period gratings are affected by temperature and stress and are described by the following formula: ; In the formula, is the wavelength shift of the long-period fiber grating 7, is the original wavelength, is the coefficient of thermal expansion, is the temperature change, is the dependent variable, is the stress change, is the Poisson's ratio, is the strain sensitivity factor, is the temperature sensitivity factor.

[0050] Among them, the theoretical mapping model of the relationship between the central wavelength of the cladding mode resonance peak precisely regulated by the micro-nano structure of the long-period fiber grating 7 and the parameters of the dynamic temperature field and stress field applied by the outside world is used. According to the requirement of the fiber laser to suppress the stimulated Raman scattering effect of different wavelengths, the model is used to quantitatively calculate the parameters of the applied stress field and temperature field, and the industrial computer 13 is used to control the movement of the stepper motor 9 and the semiconductor refrigerator 8 to apply the stress field and temperature field to the long-period fiber grating 7, and the micro-nano structure of the grating is coarsely and finely adjusted to ensure the control accuracy. Finally, the pure laser stripped of the stimulated Raman scattering effect by the stripper 10 is output from the output end 11, and the spectrum information is transmitted to the spectrometer 12 for display.

[0051] In the description of the present invention, the terms "center", "upper", "lower", "left", "right", "vertical", "horizontal", "inner", "outer" and the like indicating directions or positional relationships are based on the directions or positional relationships shown in the drawings, which are only for the convenience of description, and do not indicate or imply that the device or element must have a specific orientation, be constructed and operated in a specific orientation, and therefore cannot be understood as limiting the present invention. In addition, the terms "first", "second", and "third" are used for descriptive purposes only, and cannot be understood as indicating or implying relative importance.

[0052] In addition, it should be noted that in the description of the present invention, unless otherwise clearly specified and limited, the terms "installation", "connection" and "connection" should be understood in a broad sense, for example, it can be a fixed connection, a fusion connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium, or it can be the internal communication of two components. For those skilled in the art, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0053] The term "comprise" or any other similar term is intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus / device that includes a list of elements includes not only those elements but also other elements not expressly listed, or also includes elements inherent to such process, method, article, or apparatus / device.

[0054] So far, the technical solutions of the present invention have been described in conjunction with the preferred embodiments shown in the accompanying drawings. However, it is easy for those skilled in the art to understand that the protection scope of the present invention is obviously not limited to these specific embodiments. Without departing from the principle of the present invention, those skilled in the art can make equivalent changes or substitutions to the relevant technical features, and the technical solutions after these changes or substitutions will fall within the protection scope of the present invention.

Claims

1. A long-period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths, characterized in that: It comprises a pump source (1), a beam combiner (2), a resonant cavity, a long-period fiber grating (7), a stress field applying device, a temperature field applying device, a stripper (10), an output end (11) and a spectrometer (12); The pump source (1) is connected to one end of a beam combiner (2), the other end of the beam combiner (2) is connected to one end of a resonant cavity, the other end of the resonant cavity is connected to one end of a long-period fiber grating (7), the other end of the long-period fiber grating (7) is connected to one end of a stripper (10), the other end of the stripper (10) is connected to an output end (11), and the output end (11) is connected to a spectrometer (12); wherein the resonant cavity is used to oscillate and amplify spontaneous radiation light of a set wavelength; The two ends of the long-period fiber grating (7) are fixed to the stress field applying device, and the long-period fiber grating (7) is arranged in the temperature field applying device. The stress field applying device and the temperature field applying device are used to adjust the micro-nano structure parameters of the long-period fiber grating (7) in real time, so as to dynamically control the period of the long-period fiber grating (7), so as to cause the central wavelength of the cladding mode resonance peak to drift, and to remove the stimulated Raman scattering effect from the optical fiber core, thereby dynamically suppressing the stimulated Raman scattering effects of different wavelengths in the optical fiber laser.

2. The long period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 1, characterized in that: The resonant cavity comprises a high-reflection fiber Bragg grating (3), a gain fiber (4) and a low-reflection fiber Bragg grating (5); One end of the high-reflection fiber Bragg grating (3) is connected to the other end of the beam combiner (2), the other end of the high-reflection fiber Bragg grating (3) is connected to one end of a gain fiber (4), the other end of the gain fiber (4) is connected to one end of a low-reflection fiber Bragg grating (5), and the other end of the low-reflection fiber Bragg grating (5) is connected to one end of a long-period fiber Bragg grating (7).

3. The long period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 1, characterized in that: The stress field applying device comprises an adjustment frame (6) and a stepping motor (9); One end of the long-period fiber grating (7) is fixed to the adjustment end of the adjustment frame (6), and the other end of the long-period fiber grating (7) is fixed to the moving end of the stepping motor (9), and the stress direction applied by the stepping motor (9) is the axial direction of the long-period fiber grating (7); The adjustment frame (6) is used to ensure that one end of the long-period fiber grating (7) is fixed and positioned parallel to the temperature field applying device, thereby ensuring that the temperature field applied by the temperature field applying device acts uniformly on the long-period fiber grating (7).

4. The long period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 1, characterized in that: The temperature field applying device comprises a semiconductor refrigerator (8).

5. A long period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 3 or 4, characterized in that: The stepper motor (9) and the semiconductor refrigerator (8) in the stress field applying device are both connected to an industrial control computer (13), and the industrial control computer (13) is used to control the displacement of the moving end of the stepper motor (9) and the temperature applied by the semiconductor refrigerator (8).

6. The long period fiber grating capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 2, characterized in that: The gain optical fiber (4) is an ytterbium-doped optical fiber.

7. A method for suppressing long-period fiber gratings (LPBFs) capable of dynamically suppressing stimulated Raman scattering of different wavelengths, based on the LPBFs capable of dynamically suppressing stimulated Raman scattering of different wavelengths as claimed in any one of claims 1 to 6, characterized in that: The method includes: Calculating the strain sensitivity factor of the resonance wavelength based on the elastic-optic coefficient and effective refractive index of the core and cladding of the long-period fiber grating (7); Calculating the temperature sensitivity factor of the optical fiber based on the thermo-optic coefficient and effective refractive index of the core and cladding of the long-period fiber grating (7); The stress change and temperature change are calculated by combining the strain sensitivity factor, the temperature sensitivity factor, the original wavelength, the pre-acquired wavelength offset, the thermal expansion coefficient and the Poisson's ratio; According to the temperature and stress detected in real time, and based on the stress change amount and the temperature change amount, the stress field and the temperature field to be applied are obtained; According to the stress field and temperature field to be applied, the parameters of the stress field applying device and the temperature field applying device are adjusted to apply the corresponding stress field and temperature field to achieve dynamic suppression of stimulated Raman scattering effects of different wavelengths in the fiber laser.

8. The method for suppressing long-period fiber gratings capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 7, characterized in that: The strain sensitivity factor of the resonant wavelength is calculated as: ； in, represents the elastic-optic coefficient of the fiber core, represents the elastic-optical coefficient of the cladding, is the effective refractive index of the fiber core, represents the effective refractive index of the cladding.

9. The method for suppressing long-period fiber gratings capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 7, characterized in that: The temperature sensitivity factor is calculated as follows: ； in, is the thermo-optic coefficient of the fiber core, is the thermo-optical coefficient of the cladding, is the effective refractive index of the fiber core, represents the effective refractive index of the cladding.

10. The method for suppressing long-period fiber gratings capable of dynamically suppressing stimulated Raman scattering of different wavelengths according to claim 7, characterized in that: The calculation method of stress change and temperature change is: ； in, is the wavelength shift of the long-period fiber grating (7), is the original wavelength, is the coefficient of thermal expansion, is the temperature change, is the dependent variable, is the stress change, is the Poisson's ratio, is the strain sensitivity factor, is the temperature sensitivity factor.

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