Synergistic inhibition method for stimulated Raman scattering and mode instability effect of fiber laser

Abstract

The invention relates to the technical field of high-power fiber laser, and discloses a fiber laser stimulated Raman scattering and mode instability effect cooperative inhibition method, the fiber laser comprises a pumping module and a gain module, the gain module comprises a gain fiber, and the pumping module comprises a pumping module and a pumping module. A section of all-solid-state photonic band gap optical fiber is connected to the middle of a gain optical fiber to achieve collaborative suppression of stimulated Raman scattering and a mode instability effect, and the characteristic of high loss of the all-solid-state photonic band gap optical fiber in a Raman band is utilized to suppress stimulated Raman scattering. And meanwhile, the high-order mode is filtered by utilizing the characteristic that the all-solid-state photonic band gap optical fiber has high loss on the high-order mode, so that the interference intensity of the fundamental mode and the high-order mode is reduced to effectively inhibit the mode instability effect, and high-power single-mode laser output is realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical fiber lasers, and in particular to a method for collaboratively suppressing stimulated Raman scattering and mode instability effects of optical fiber lasers. Background Art

[0002] High-power single-mode fiber lasers have important applications in numerous fields. However, increasing their power is hampered by physical phenomena such as stimulated Raman scattering (SRS) and mode instability, making progress difficult. Due to the unique waveguide structure of optical fibers, the laser energy is primarily confined within the micrometer-scale fiber core. This results in high power density and a long interaction distance between light and the fiber material, making SRS highly susceptible to generation. The impact of SRS is primarily manifested in three ways: First, SRS transfers energy from the signal light to the scattered Raman light, resulting in a decrease in the output beam power and energy conversion efficiency. Second, because the Stokes light generated by SRS is bidirectional, when the backward Stokes light power reaches a certain value, it increases the risk of damage to optical components in the laser system, reducing the stability and reliability of the laser system. Third, recent research has shown that high Raman light content in the output laser can trigger static mode degradation or mode instability, leading to a sharp decline in beam quality. Mode instability refers to the millisecond-scale dynamic coupling of energy between the fundamental and higher-order modes after the laser power exceeds a threshold. This manifests itself macroscopically as spot jitter and beam quality degradation. For all-fiber lasers with cladding light filters, the output power may experience stagnation due to the removal of higher-order modes from the cladding. Therefore, suppressing stimulated Raman scattering and mode instability is crucial for improving output power and beam quality.

[0003] Currently, common methods for suppressing stimulated Raman scattering include increasing the core diameter, shortening the fiber length, reducing the seed power, increasing the core-to-cladding ratio, and improving pump absorption. Common strategies for suppressing mode instability include reducing the core diameter, increasing the fiber length, increasing the seed power, reducing the core-to-cladding ratio, and reducing pump absorption. Clearly, these strategies conflict with each other in suppressing stimulated Raman scattering and mode instability. Therefore, there is an urgent need to develop a method that can effectively suppress both stimulated Raman scattering and mode instability simultaneously. Special design of the gain fiber structure, such as using tapered fibers, partially doped fibers, or low-numerical-aperture fibers, can achieve a compromise between stimulated Raman scattering and mode instability. However, the development and preparation of specialized optical fibers is complex and costly, limiting their large-scale application. Therefore, developing new methods for synergistically suppressing stimulated Raman scattering and mode instability is crucial for improving the power and beam quality of fiber lasers. Summary of the Invention

[0004] In view of the defects of the existing technology, the present invention proposes a method for synergistically suppressing stimulated Raman scattering and mode instability effects of fiber lasers.

[0005] In order to achieve the above technical objectives, the technical solutions adopted by the present invention are as follows: On the one hand, a method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser is provided. The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. A section of all-solid-state photonic bandgap fiber is connected in the middle of the gain fiber to achieve synergistic suppression of stimulated Raman scattering and mode instability effects.

[0006] Furthermore, the access position of the all-solid-state photonic bandgap fiber in the gain fiber is determined by the following steps: The first step is to identify fiber lasers to be optimized that have not yet been connected to all-solid-state photonic bandgap fibers; The second step is to determine the occurrence region of the mode instability effect in the gain fiber of the fiber laser to be optimized through simulation or physical experiments, and determine the starting boundary and the ending boundary of the occurrence region of the mode instability effect in the gain fiber; In the third step, assuming the total length of the gain fiber is L, compare the positional relationship between the end boundary of the region where the mode instability effect occurs in the gain fiber and the midpoint of the gain fiber. If the end boundary of the region where the mode instability effect occurs in the gain fiber is located before the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber is located before the end boundary of the region where the mode instability effect occurs in the gain fiber; if the end boundary of the region where the mode instability effect occurs in the gain fiber is located at the midpoint of the gain fiber or the end boundary of the region where the mode instability effect occurs in the gain fiber is located after the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber should be located at the midpoint of the gain fiber or before the midpoint of the gain fiber.

[0007] After the access position of the all-solid-state photonic bandgap fiber in the gain fiber is determined through the above steps, the gain fiber is disconnected at the access position, and a section of the all-solid-state photonic bandgap fiber is fused between the gain fibers at the disconnected position.

[0008] Compared with the prior art, the beneficial effects of the present invention include at least: By inserting an all-solid-state photonic bandgap fiber into the gain fiber, the present invention regulates the power of Raman light and higher-order modes during the amplification process. This effectively suppresses stimulated Raman scattering and mode instability, two key limiting factors in high-power fiber lasers, thereby optimizing the output beam quality. Specifically, the high loss of the all-solid-state photonic bandgap fiber in the Raman band is utilized to suppress stimulated Raman scattering. Simultaneously, the high loss of the all-solid-state photonic bandgap fiber for higher-order modes is utilized to filter out higher-order modes, reducing the interference intensity between the fundamental mode and higher-order modes to effectively suppress the mode instability effect and achieve high-power single-mode laser output.

[0009] All-solid-state photonic bandgap fibers offer exceptional flexibility and customizability, enabling precise adjustment of parameters to meet specific application requirements. This not only allows for perfect matching with various gain fibers but also allows for specialized designs for transmission bands and mode-differentiated transmission. The all-solid-state structure simplifies cleaving and splicing while ensuring exceptional system stability and safety. Furthermore, its all-fiber construction ensures high compatibility with other system components, simplifying system integration and upgrades.

[0010] In summary, the present invention has important application value in the field of high-power fiber lasers, especially in the field of system design and performance optimization of high-power fiber laser amplifiers. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the structures shown in these drawings without paying any creative work.

[0012] Figure 1 is a schematic diagram of the cross-sectional structure of an all-solid-state photonic bandgap fiber used in one embodiment; Figure 2 2. It is a schematic structural diagram of a system for collaboratively suppressing stimulated Raman scattering and mode instability in a high-power optical fiber amplifier provided in one embodiment; Figure 3 This is a schematic diagram of the spectrum when the output power is 2.9 kW after inserting all-solid-state photonic bandgap fibers at different positions of the gain fiber of the high-power fiber amplifier; Figure 4 Schematic diagram of the mode instability threshold after inserting all-solid-state photonic bandgap fiber at different positions of the gain fiber of a high-power fiber amplifier.

[0013] Numbers in the figure: 1. All-solid-state photonic bandgap fiber; 101. Fiber core; 102. High-refractive-index rod; 103. Solid-state substrate; 104. Outer cladding; 2. Seed laser; 3. Forward-pump combiner; 4. Pump source; 5. Gain fiber; 6. Backward-pump combiner; 7. Cladding light filter; 8. Output end cap.

[0014] The purpose, features and advantages of the present invention will be further described with reference to the accompanying drawings and in conjunction with the embodiments. DETAILED DESCRIPTION

[0015] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making any creative efforts shall fall within the scope of protection of the present invention.

[0016] In one embodiment, a method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser is provided. The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. A section of all-solid-state photonic bandgap fiber is inserted into the middle of the gain fiber to achieve synergistic suppression of stimulated Raman scattering and mode instability effects.

[0017] In order to achieve good suppression of stimulated Raman scattering and mode instability effects, it is crucial to determine the insertion position of the all-solid-state photonic bandgap fiber in the gain fiber, which determines the synergistic suppression effect of stimulated Raman scattering and mode instability effects. Specifically, the steps are as follows: The first step is to identify fiber lasers to be optimized that have not yet been connected to all-solid-state photonic bandgap fibers; The second step is to determine the region where the mode instability effect occurs in the gain fiber of the fiber laser to be optimized through simulation or physical experiments, and to determine the starting and ending boundaries of the region where the mode instability effect occurs in the gain fiber. The starting and ending boundaries of the region where the mode instability effect occurs in the gain fiber are the starting and ending positions of the region where the mode instability effect occurs in the gain fiber. Taking the input end of the gain fiber as the origin, the length distance between the starting boundary of the region where the mode instability effect occurs in the gain fiber and the origin is z1, and the length distance between the ending boundary of the region where the mode instability effect occurs in the gain fiber and the origin is z2.

[0018] In the third step, assuming the total length of the gain fiber is L, the positional relationship between the end boundary of the region where the mode instability effect occurs and the midpoint of the gain fiber is compared. If the end boundary of the region where the mode instability effect occurs is before the midpoint of the gain fiber, the insertion position of the all-solid-state photonic bandgap fiber must be before the end boundary of the region where the mode instability effect occurs. If the end boundary of the region where the mode instability effect occurs is at the midpoint of the gain fiber or after the midpoint of the gain fiber, the insertion position of the all-solid-state photonic bandgap fiber should be at or before the midpoint of the gain fiber. In this way, the all-solid-state photonic bandgap fiber can effectively suppress Raman light, suppressing the Raman light generated by the preceding gain fiber to the noise level, which is equivalent to shortening the length of the laser gain fiber. In theory, the closer the insertion position is to the output end, the better the suppression effect on stimulated Raman scattering. However, stimulated Raman scattering is a bidirectional scattering (including forward and backward scattered light). If the access position of the all-solid-state photonic bandgap fiber is too close to the output end, although the Raman threshold of the rear section can be significantly improved, the front-end gain fiber will reach the Raman threshold first due to its longer length, and the backscattered light generated by it may threaten the safety of the system.

[0019] In the fourth step, after determining the access position of the all-solid-state photonic bandgap fiber in the gain fiber through the above steps, the gain fiber is disconnected at the access position, and a section of the all-solid-state photonic bandgap fiber is fused between the gain fibers at the disconnected position.

[0020] The fiber laser in the above embodiment is of any type and may be a fiber amplifier, a fiber oscillator, or a cascade structure consisting of a fiber oscillator and a fiber amplifier. The fiber amplifier includes a seed laser, a pump module, a gain module, and a laser output module. The pumping mode of the fiber amplifier may be forward pumping, backward pumping, or bidirectional pumping. The type of laser used in the seed laser of the present invention is not limited and may include different types such as a fiber laser oscillator, a superfluorescent fiber laser, a random fiber laser, a single-frequency fiber laser, or a phase-modulated single-frequency fiber laser. The fiber oscillator includes a pump module, a high-reflectivity fiber Bragg grating, a gain fiber, a low-reflectivity fiber Bragg grating, and a laser output module. The pumping mode of the fiber oscillator may be forward pumping, backward pumping, or bidirectional pumping.

[0021] The all-solid-state photonic bandgap fiber is a passive optical fiber comprising a core and a microstructured cladding. The core is a solid-state core, and the microstructured cladding surrounds the core. The microstructured cladding comprises an array of high-refractive-index rods arranged in a regular hexagonal lattice around the solid-state core, and a solid substrate filling the spaces between the high-refractive-index rods. The all-solid-state photonic bandgap fiber produces a photonic bandgap effect through antiresonant coupling of the high-refractive-index rods in the microstructured cladding: wavelengths outside the photonic bandgap cannot be transmitted due to high loss, while wavelengths within the bandgap are confined to the core and stably transmitted. By varying the diameter and spacing of the high-refractive-index rods, the transmission spectrum of the all-solid-state photonic bandgap fiber can be customized to achieve high transmittance for signal laser transmission and high loss for Raman laser transmission, thereby suppressing stimulated Raman scattering. The greater the transmission loss in the Raman band, the better the stimulated Raman scattering suppression effect. At the same time, by improving the cladding structure and selectively removing some high-refractive-index rods in the array, the coupling of high-order modes to the cladding region can be increased, and the photonic band gap can be used to achieve efficient fundamental mode transmission of the signal wavelength and high-loss transmission of high-order modes, thereby suppressing the mode instability effect in the laser. The higher the loss of the high-order mode, the better the suppression effect of the mode instability effect, the lower the loss of the fundamental mode, and the higher the efficiency of the laser.

[0022] Preferably, the all-solid-state photonic bandgap fiber has a loss of less than 0.1 dB / m for the fundamental mode laser in the signal band, a loss of more than 10 dB / m for the high-order mode in the signal band, and a loss of more than 20 dB / m for the Raman laser.

[0023] The length of the all-solid-state photonic bandgap fiber is 1m to 2m, which can ensure high loss of high-order modes and Raman lasers in the signal band to achieve good suppression of mode instability and stimulated Raman scattering effects, while also ensuring low loss of fundamental mode lasers in the signal band to achieve high-efficiency operation of the laser.

[0024] The core NA of the all-solid-state photonic bandgap fiber is the same as the core NA of the gain fiber, ensuring that the fundamental mode field of the all-solid-state photonic bandgap fiber matches the fundamental mode field of the gain fiber. The core / cladding diameter of the all-solid-state photonic bandgap fiber is the same as the core / cladding diameter of the gain fiber.

[0025] The doping distribution of the doped core of the gain fiber is not limited and can be uniformly doped, partially doped, or non-uniformly doped. The gain fiber is a double-clad fiber. The doping elements in the gain fiber are not limited and include ytterbium, erbium, neodymium, holmium, thulium, etc., and can be one or more of ytterbium, erbium, neodymium, holmium, and thulium.

[0026] like Figure 1 As shown, Figure 1The figure is a schematic diagram of the cross-sectional structure of an all-solid-state photonic bandgap fiber used in one embodiment. The all-solid-state photonic bandgap fiber includes: a core 101, a high refractive index rod 102, a solid substrate 103, and an outer cladding 104. The core / cladding diameters of the all-solid-state photonic bandgap fiber are 30 / 250 μm. In the range of 1060 nm to 1080 nm, the transmission loss is <0.01 dB / m. When the wavelength exceeds 1100 nm, the transmission loss is >20 dB / m. When the bending diameter is ≥15 cm, the 1080 nm band LP 01 Mode loss <0.02 dB / m, LP 11o Mode loss>10 dB / m, LP 11e Mode loss>7 dB / m. The core NA of the all-solid-state photonic bandgap fiber is 0.05.

[0027] In order to prove the effectiveness of the method for synergistically suppressing stimulated Raman scattering and mode instability effect of fiber lasers provided by the present invention, Figure 2 , Figure 2 The schematic diagram of the structure of the system for cooperative suppression of stimulated Raman scattering and mode instability in high-power fiber amplifiers is shown in FIG. 1 , which is a schematic diagram of the structure of the system for cooperative suppression of stimulated Raman scattering and mode instability in high-power fiber amplifiers. That is, a section of all-solid-state photonic bandgap fiber is connected to the gain fiber of the high-power fiber amplifier to achieve cooperative suppression of stimulated Raman scattering and mode instability in high-power fiber amplifiers. Figure 2 As shown, the fiber laser comprises: an all-solid-state photonic bandgap fiber 1, a seed laser 2, a forward pump combiner 3, a pump source 4, a gain fiber 5, a backward pump combiner 6, a cladding light filter 7, and an output end cap 8. The gain fiber 5 is a large-mode-area double-clad ytterbium-doped fiber.

[0028] The parameters, type, operating wavelength, and output power of seed laser 2 vary depending on actual needs and do not require specific specifications. The output fiber pigtail of seed laser 2 is then sequentially fused to a forward pump combiner 3, a gain fiber 5, an all-solid-state photonic bandgap fiber 1, a gain fiber 5, a backward pump combiner 6, a cladding light filter 7, and an output end cap 8. Multiple pump sources 4 are fused to the pump fibers of forward pump combiner 3 and backward pump combiner 6, respectively.

[0029] Seed laser 2 is a fiber oscillator with an output wavelength of 1080 nm. The core / cladding diameters of all optical fibers in the signal optical path are 30 / 250 μm. Both forward pump combiner 3 and backward pump combiner 6 can be of the (6+1)×1 type, with pump fibers having core / cladding diameters of 135 / 155 μm, fused to pump source 4. Furthermore, forward pump combiner 3 and backward pump combiner 6 do not necessarily need to be connected simultaneously in the optical path. Different pump combiners can be used depending on the pumping scheme. For example, for forward pumping, only the forward pump combiner can be used, without the backward pump combiner. The same applies to backward pumping. For bidirectional pumping, both forward and backward pump combiners are used. For ease of operation, the forward pumping scheme is used here. Currently, the most commonly used pump source wavelengths are 915 nm, 976 nm, and 1018 nm. A 1018 nm fiber laser is used as pump source 4. The gain fiber 5 has an absorption coefficient of 0.36 dB / m at 1018 nm, a total length of 53 m, and a core numerical aperture (NA) of 0.05.

[0030] A section of all-solid-state photonic bandgap fiber 5 is inserted into different positions of the gain fiber 5 , with a length of 2 m and a bending diameter of >50 cm. Figure 3 This is a schematic diagram of the spectrum when the output power is 2.9 kW after inserting all-solid-state photonic bandgap fiber at different positions of the gain fiber of the high-power fiber amplifier. Figure 3 As can be seen, the Raman suppression ratio is 34 dB without insertion. After insertion, the Raman suppression ratio increases compared to the original state, and the improvement in Raman suppression ratio increases with the distance from the seed input. When inserted at a distance of 15 m, the characteristic Raman peak in the spectrum disappears, and the Raman suppression ratio exceeds 46 dB, effectively suppressing stimulated Raman scattering.

[0031] Figure 4 This is a schematic diagram of the mode instability threshold after inserting all-solid-state photonic bandgap fibers at different positions of the gain fiber of a high-power fiber amplifier. Figure 4 As can be seen, the mode instability threshold is 2565 W without the all-solid-state photonic bandgap fiber. When inserted at 40 m or earlier, the mode instability threshold increases by 5% to 20%. Furthermore, the mode instability threshold decreases after insertion at 42 m. This is because this location is outside the rear boundary of the mode instability region and therefore does not contribute to the threshold increase. On the contrary, the high loss of higher-order modes leads to a decrease in efficiency, resulting in a lower threshold.

[0032] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions based on the principles of the present invention are within the scope of protection of the present invention. It should be noted that for those skilled in the art, various improvements and modifications that do not depart from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for synergistically suppressing stimulated Raman scattering and mode instability effects in fiber lasers, characterized by: The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. A section of all-solid-state photonic bandgap fiber is inserted into the middle of the gain fiber to achieve synergistic suppression of stimulated Raman scattering and mode instability effects.

2. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of fiber lasers according to claim 1, characterized in that: The access position of the all-solid-state photonic bandgap fiber in the gain fiber is determined by the following steps: The first step is to identify fiber lasers to be optimized that have not yet been connected to all-solid-state photonic bandgap fibers; The second step is to determine the occurrence region of the mode instability effect in the gain fiber of the fiber laser to be optimized through simulation or physical experiments, and determine the starting boundary and the ending boundary of the occurrence region of the mode instability effect in the gain fiber; In the third step, assuming the total length of the gain fiber is L, compare the positional relationship between the end boundary of the region where the mode instability effect occurs in the gain fiber and the midpoint of the gain fiber. If the end boundary of the region where the mode instability effect occurs in the gain fiber is located before the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber is located before the end boundary of the region where the mode instability effect occurs in the gain fiber; if the end boundary of the region where the mode instability effect occurs in the gain fiber is located at the midpoint of the gain fiber or the end boundary of the region where the mode instability effect occurs in the gain fiber is located after the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber should be located at the midpoint of the gain fiber or before the midpoint of the gain fiber.

3. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser according to claim 1 or 2, characterized in that: The fiber laser is a fiber amplifier, a fiber oscillator, or a cascade structure consisting of a fiber oscillator and a fiber amplifier.

4. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser according to claim 3, characterized in that: The fiber amplifier includes a seed laser, a pump module, a gain module and a laser output module, and the fiber oscillator includes a pump module, a high-reflectivity fiber Bragg grating, a gain fiber, a low-reflectivity fiber Bragg grating and a laser output module.

5. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser according to claim 1, 2 or 4, characterized in that: The all-solid-state photonic bandgap fiber includes a core and a microstructured cladding, wherein the core is a solid core and the microstructured cladding is coated on the periphery of the core. The microstructured cladding includes an array of high-refractive-index rods arranged in a regular hexagonal lattice around the solid core and a solid substrate filled between the high-refractive-index rods.

6. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser according to claim 5, characterized in that: The core NA of the all-solid-state photonic bandgap fiber is the same as the core NA of the gain fiber, ensuring that the fundamental mode field of the all-solid-state photonic bandgap fiber matches the fundamental mode field of the gain fiber.

7. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of fiber lasers according to claim 5, characterized in that: The core / cladding diameter of the all-solid-state photonic bandgap fiber is the same as the core / cladding diameter of the gain fiber.

8. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of a fiber laser according to claim 1, 2, 4, 6, or 7, characterized in that: The all-solid-state photonic bandgap optical fiber has a loss of less than 0.1 dB / m for fundamental mode laser in the signal band, a loss of more than 10 dB / m for high-order modes in the signal band, and a loss of more than 20 dB / m for Raman laser.

9. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of fiber lasers according to claim 8, characterized in that: The length of the all-solid-state photonic bandgap optical fiber is 1m-2m.

10. The method for synergistically suppressing stimulated Raman scattering and mode instability effects of fiber lasers according to claim 9, characterized in that: The doping element in the core of the gain optical fiber is one or more of ytterbium, erbium, neodymium, holmium and thulium.

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