Synergistic Suppression Method of Stimulated Raman Scattering and Mode Instability Effect in Fiber Lasers

Abstract

This invention relates to the field of high-power fiber laser technology and discloses a method for synergistically suppressing stimulated Raman scattering and mode instability effects in fiber lasers. The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. By inserting a section of all-solid-state photonic bandgap fiber in the middle of the gain fiber, the synergistic suppression of stimulated Raman scattering and mode instability effects is achieved. The high-loss characteristic of all-solid-state photonic bandgap fiber in the Raman band is used to suppress stimulated Raman scattering. At the same time, the high-loss characteristic of all-solid-state photonic bandgap fiber for higher-order modes is used to filter out higher-order modes, reducing the interference intensity between the fundamental mode and higher-order modes to effectively suppress mode instability effects and achieve high-power single-mode laser output.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, and in particular to a method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers. Background Technology

[0002] High-power single-mode fiber lasers have significant applications in numerous fields. However, their power improvement is constrained by physical phenomena such as stimulated Raman scattering (SRS) and mode instability, making progress quite difficult. Due to the unique waveguide structure of optical fibers, laser energy is primarily confined within the micrometer-scale fiber core. The high power density and long interaction distance between the light and fiber material make stimulated Raman scattering (SRS) highly likely. The effects of SRS are mainly manifested in three aspects: First, SRS causes energy to be transferred from the signal light to the Raman scattered light, leading to a decrease in the output beam power and energy conversion efficiency. Second, since the Stokes light generated by SRS is bidirectional, when the backscattered 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 studies have shown that a high Raman light content in the output laser may trigger static mode degradation or mode instability, resulting in a sharp decline in beam quality. Mode instability refers to the dynamic coupling of energy between the fundamental mode and higher-order modes on a millisecond scale after the laser power exceeds a threshold. Macroscopically, this manifests as beam jitter and beam quality degradation. For all-fiber lasers with cladding optical filters, output power may hysteresis 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 (SRS) include increasing the core diameter, shortening the fiber length, reducing seed power, increasing the core-cladding ratio, and improving pump absorption. Conversely, common strategies for suppressing mode instability include decreasing the core diameter, increasing the fiber length, increasing seed power, decreasing the core-cladding ratio, and reducing pump absorption. Clearly, these strategies conflict in their effectiveness in suppressing both SRS and mode instability. Therefore, it is urgent to develop a method that can effectively suppress both SRS and mode instability simultaneously. Specialized designs for the gain fiber structure, such as using tapered fibers, partially doped fibers, or low numerical aperture fibers, can achieve a compromise between SRS and mode instability. However, the development and fabrication of these special fibers are complex and costly, limiting their large-scale application. Therefore, researching new methods for synergistically suppressing SRS and mode instability is crucial for improving the power and beam quality of fiber lasers. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention proposes a method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers.

[0005] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows:

[0006] On the one hand, a method for synergistic suppression of stimulated Raman scattering and mode instability effects in a fiber laser is provided. The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. By inserting a section of all-solid-state photonic bandgap fiber in the middle of the gain fiber, the synergistic suppression of stimulated Raman scattering and mode instability effects can be achieved.

[0007] Furthermore, the insertion position of the all-solid-state photonic bandgap fiber in the gain fiber is determined through the following steps:

[0008] The first step is to identify fiber lasers that are not yet connected to all-solid-state photonic bandgap fibers and require optimization.

[0009] 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.

[0010] Third, let the total length of the gain fiber be L. Compare the positional relationship between the end boundary of the region where the mode instability occurs in the gain fiber and the midpoint of the gain fiber. If the end boundary of the region where the mode instability occurs in the gain fiber is before the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber should be before the end boundary of the region where the mode instability occurs in the gain fiber. If the end boundary of the region where the mode instability occurs in the gain fiber is at the midpoint of the gain fiber or after the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber should be at the midpoint of the gain fiber or before the midpoint of the gain fiber.

[0011] 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 all-solid-state photonic bandgap fiber is fused between the disconnected gain fibers.

[0012] Compared with the prior art, the beneficial effects of the present invention include at least the following:

[0013] This invention, by inserting an all-solid-state photonic bandgap fiber into the gain fiber, allows for the control of the power of Raman light and higher-order modes during amplification. This simultaneously effectively suppresses two key limiting factors in high-power fiber lasers: stimulated Raman scattering and mode instability, thus optimizing the output beam quality. Specifically, the high-loss characteristic of the all-solid-state photonic bandgap fiber in the Raman band is utilized to suppress stimulated Raman scattering. Simultaneously, the high-loss characteristic of the all-solid-state photonic bandgap fiber for higher-order modes is used to filter out these modes, reducing the interference intensity between the fundamental mode and higher-order modes to effectively suppress mode instability effects, thereby achieving high-power single-mode laser output.

[0014] All-solid-state photonic bandgap fiber offers exceptional flexibility and customizability, allowing for precise adjustment of parameters to meet specific application requirements. This characteristic not only enables it to perfectly match various gain fibers but also allows for specialized design in terms of transmission band and mode differentiation. The all-solid-state structure simplifies dicing and splicing processes while ensuring superior system stability and security. Furthermore, its all-fiber structure ensures high compatibility with other system components, thereby simplifying system integration and upgrades.

[0015] In summary, this invention has significant application value in the field of high-power fiber lasers, especially in the system design and performance optimization of high-power fiber laser amplifiers. Attached Figure Description

[0016] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0017] Figure 1 This is a schematic diagram of the cross-sectional structure of an all-solid-state photonic bandgap fiber used in one embodiment;

[0018] Figure 2 This is a schematic diagram of the structure of a system for the coordinated suppression of stimulated Raman scattering and mode instability of a high-power fiber amplifier provided in one embodiment;

[0019] Figure 3 This is a schematic diagram of the spectrum when an all-solid-state photonic bandgap fiber is inserted at different positions in the gain fiber of a high-power fiber amplifier, and the output power is 2.9 kW.

[0020] Figure 4 This is a schematic diagram illustrating the mode instability threshold after inserting all-solid-state photonic bandgap fiber at different positions in the gain fiber of a high-power fiber amplifier.

[0021] Numbering on the map:

[0022] 1. All-solid-state photonic bandgap fiber; 101. Core; 102. High-refractive-index rod; 103. Solid-state substrate; 104. Cladding; 2. Seed laser; 3. Forward-pumped combiner; 4. Pump source; 5. Gain fiber; 6. Backward-pumped combiner; 7. Cladding optical filter; 8. Output cap.

[0023] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0025] In one embodiment, a method for synergistically suppressing stimulated Raman scattering and mode instability effects in a fiber laser is provided. The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. By inserting a section of all-solid-state photonic bandgap fiber in the middle of the gain fiber, the synergistic suppression of stimulated Raman scattering and mode instability effects can be achieved.

[0026] To achieve effective suppression of stimulated Raman scattering and mode instability, determining the insertion position of the all-solid-state photonic bandgap fiber within the gain fiber is crucial, as it determines the synergistic suppression effect of stimulated Raman scattering and mode instability. Specifically, the steps are as follows:

[0027] The first step is to identify fiber lasers that are not yet connected to all-solid-state photonic bandgap fibers and require optimization.

[0028] The second step involves determining the region of mode instability in the gain fiber of the fiber laser to be optimized through simulation or physical experiments, and identifying the starting and ending boundaries of this region. These starting and ending boundaries represent the initial and final positions of the mode instability region. With the input end of the gain fiber as the origin, the distance between the starting boundary and the origin is z1, and the distance between the ending boundary and the origin is z2.

[0029] The third step involves assuming the total length of the gain fiber is L. The positional relationship between the end boundary of the region where the mode instability occurs and the midpoint of the gain fiber is compared. If the end boundary of the region is before the midpoint, the all-solid-state photonic bandgap fiber must be positioned before the end boundary of the region. Conversely, if the end boundary is at the midpoint or after the midpoint, the all-solid-state photonic bandgap fiber should be positioned at or before the midpoint. This effectively suppresses Raman scattering, reducing the Raman scattering generated by the preceding gain fiber to noise levels, effectively shortening the laser gain fiber length. Theoretically, the closer the insertion point is to the output end, the better the suppression of stimulated Raman scattering. However, stimulated Raman scattering is 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 it can significantly improve the Raman threshold of the later section, the gain fiber of the front section will reach the Raman threshold first due to its longer length, and the backscattered light generated by it may threaten the system safety.

[0030] Fourth, 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 all-solid-state photonic bandgap fiber is fused between the disconnected gain fibers.

[0031] The fiber laser type described in the above embodiments is not limited and can be a fiber amplifier, a fiber oscillator, or a cascaded 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 method of the fiber amplifier can be forward pumping, backward pumping, or bidirectional pumping. The type of laser used in the seed laser of this invention is not limited and includes different types such as fiber laser oscillators, superfluorescent fiber lasers, random fiber lasers, single-frequency fiber lasers, or phase-modulated single-frequency fiber lasers. The fiber oscillator includes a pump module, a high-reflectivity fiber grating, a gain fiber, a low-reflectivity fiber grating, and a laser output module. The pumping method of the fiber oscillator can be forward pumping, backward pumping, or bidirectional pumping.

[0032] The all-solid-state photonic bandgap fiber is a passive fiber comprising a core and a microstructure cladding. The core is solid, and the microstructure cladding surrounds the core. The microstructure cladding includes an array of high-refractive-index rods arranged in a hexagonal lattice around the solid core and a solid substrate filling the spaces between the high-refractive-index rods. The all-solid-state photonic bandgap fiber generates a photonic bandgap effect through anti-resonant coupling of the high-refractive-index rods in the microstructure cladding: wavelengths outside the photonic bandgap cannot propagate due to high loss, while wavelengths within the bandgap are confined to the core for stable propagation. By changing 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 lasers and high-loss transmission for Raman lasers, thus suppressing stimulated Raman scattering. The greater the transmission loss in the Raman band, the better the suppression effect of stimulated Raman scattering. Meanwhile, by improving the cladding structure, selectively removing some high-refractive-index rods in the array can increase the coupling of higher-order modes to the cladding region. This allows for efficient transmission of the fundamental mode and high-loss transmission of higher-order modes using the photonic bandgap, thereby suppressing mode instability in the laser. The higher the loss of higher-order modes, the better the suppression of mode instability; the lower the loss of the fundamental mode, the higher the efficiency of the laser.

[0033] Preferably, the loss of the all-solid-state photonic bandgap fiber for the fundamental mode laser in the signal band is preferably below 0.1 dB / m, the loss for the higher-order modes in the signal band is preferably above 10 dB / m, and the loss for the Raman laser is preferably above 20 dB / m.

[0034] The length of the all-solid-state photonic bandgap fiber is 1m to 2m, which ensures both high loss for higher-order modes and Raman lasers in the signal band to achieve good suppression of mode instability and stimulated Raman scattering, and low loss for the fundamental mode laser in the signal band to achieve high-efficiency operation of the laser.

[0035] The core NA of the all-solid-state photonic bandgap fiber is the same as that of the gain fiber, ensuring that the fundamental mode field of the all-solid-state photonic bandgap fiber matches that of the gain fiber. The core / cladding diameter of the all-solid-state photonic bandgap fiber is the same as that of the gain fiber.

[0036] 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.

[0037] like Figure 1 As shown, Figure 1This 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 diameter of the all-solid-state photonic bandgap fiber is 30 / 250 μm. In the 1060 nm-1080 nm range, 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 LP (LP bandgap) at the 1080 nm band... 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.

[0038] To demonstrate the effectiveness of the synergistic suppression method for stimulated Raman scattering and mode instability effects in fiber lasers provided by this invention, refer to... Figure 2 , Figure 2 This is a schematic diagram of a system for the coordinated suppression of stimulated Raman scattering and mode instability in a high-power fiber amplifier. Specifically, a section of all-solid-state photonic bandgap fiber is inserted into the gain fiber of the high-power fiber amplifier to achieve coordinated suppression of stimulated Raman scattering and mode instability. Figure 2 As shown, the fiber laser includes: an all-solid-state photonic bandgap fiber 1, a seed laser 2, a forward-pumped combiner 3, a pump source 4, a gain fiber 5, a backward-pumped combiner 6, a cladding optical filter 7, and an output cap 8. The gain fiber 5 is a large-mode-field double-clad ytterbium-doped fiber.

[0039] The parameters, type, operating wavelength, and output power of the seed laser 2 vary depending on the specific application and have no special requirements. Following the output pigtail of the seed laser 2, a forward pump combiner 3, a gain fiber 5, an all-solid-state photonic bandgap fiber 1, another gain fiber 5, a backward pump combiner 6, a cladding optical filter 7, and an output cap 8 are sequentially fused together. Multiple pump sources 4 are fused to the pump fibers of the forward pump combiner 3 and the backward pump combiner 6, respectively.

[0040] Seed laser 2 is an optical fiber oscillator with an output wavelength of 1080 nm. All optical fibers in the signal path have a core / cladding diameter of 30 / 250 μm. Both forward pump combiner 3 and backward pump combiner 6 can be of the (6+1)×1 type, with pump fiber 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 selected depending on the pumping scheme. For example, in a forward pumping scheme, only the forward pump combiner is needed, without the need for a backward pump combiner. The same applies to backward pumping. For bidirectional pumping, both forward and backward pump combiners are used simultaneously. Here, for ease of operation, a forward pumping scheme is adopted. Currently, the most commonly used pump source wavelengths are 915 nm, 976 nm, and 1018 nm. Pump source 4 uses a 1018 nm fiber laser as the pump source. The gain fiber 5 has an absorption coefficient of 0.36 dB / m@1018 nm, a total length of 53 m, and a core numerical aperture (NA) of 0.05.

[0041] A section of all-solid-state photonic bandgap fiber 5 with a length of 2 m and a bending diameter >50 cm is inserted at different positions of the gain fiber 5. Figure 3 This is a schematic diagram of the spectrum when an all-solid-state photonic bandgap fiber is inserted at different positions in the gain fiber of a high-power fiber amplifier, resulting in an output power of 2.9 kW. Figure 3 As can be seen, the Raman suppression ratio (RSR) without insertion is 34 dB. After insertion, the RRS is improved compared to the uninserted RRS, and the improvement is greater the further the insertion position is from the seed input. When inserted at 15 m, the Raman characteristic peaks in the spectrum have disappeared, and the RRS is >46 dB, effectively suppressing stimulated Raman scattering.

[0042] Figure 4 This diagram illustrates the mode instability threshold after inserting all-solid-state photonic bandgap fiber at different locations in the gain fiber of a high-power fiber amplifier. Figure 4 As can be seen, the mode instability threshold is 2565 W when no all-solid-state photonic bandgap fiber is inserted. When inserted at 40 m and 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 position is already outside the trailing boundary of the mode instability region, so it does not help to improve the threshold. On the contrary, the high loss characteristics of higher-order modes lead to a decrease in efficiency, resulting in a decrease in the threshold.

[0043] 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 falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principles of the present invention should be considered within the scope of protection of the present invention.

Claims

1. A method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers, characterized in that: The fiber laser includes a pump module and a gain module. The gain module includes a gain fiber. Stimulated Raman scattering and mode instability effects are synergistically suppressed by inserting a section of all-solid-state photonic bandgap fiber into the middle of the gain fiber. The insertion position of the all-solid-state photonic bandgap fiber within the gain fiber is determined through the following steps: The first step is to identify fiber lasers that are not yet connected to all-solid-state photonic bandgap fibers and require optimization. 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. Third, let the total length of the gain fiber be L. Compare the positional relationship between the end boundary of the region where the mode instability occurs in the gain fiber and the midpoint of the gain fiber. If the end boundary of the region where the mode instability occurs in the gain fiber is before the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber should be before the end boundary of the region where the mode instability occurs in the gain fiber. If the end boundary of the region where the mode instability occurs in the gain fiber is at the midpoint of the gain fiber or after the midpoint of the gain fiber, then the access position of the all-solid-state photonic bandgap fiber should be at the midpoint of the gain fiber or before the midpoint of the gain fiber.

2. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 1, characterized in that: The fiber laser is a cascaded structure consisting of a fiber amplifier, a fiber oscillator, or a fiber oscillator and a fiber amplifier.

3. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 2, characterized in that: The fiber amplifier includes a seed laser, a pump module, a gain module, and a laser output module. The fiber optic oscillator includes a pump module, a high-reflectivity fiber grating, a gain fiber, a low-reflectivity fiber grating, and a laser output module.

4. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 1, 2, or 3, characterized in that: The all-solid-state photonic bandgap fiber includes a core and a microstructure cladding. The core is a solid core, and the microstructure cladding covers the periphery of the core. The microstructure cladding includes an array of high-refractive-index rods arranged in a regular hexagonal lattice around the solid core and a solid substrate filling the spaces between the high-refractive-index rods.

5. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 4, characterized in that: The core NA of the all-solid-state photonic bandgap fiber is the same as that 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.

6. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 4, characterized in that, The core / cladding diameter of the all-solid-state photonic bandgap fiber is the same as that of the gain fiber.

7. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 1, 2, 3, 5, or 6, characterized in that: 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 higher-order modes in the signal band, and a loss of more than 20 dB / m for the Raman laser.

8. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 7, characterized in that: The length of the all-solid-state photonic bandgap fiber is 1m-2m.

9. The method for synergistic suppression of stimulated Raman scattering and mode instability effects in fiber lasers according to claim 8, characterized in that: The doping element in the core of the gain fiber is one or more of ytterbium, erbium, neodymium, holmium, and thulium.

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