Narrow-linewidth cascade Raman fiber laser system based on double-grating cooperative feedback

Abstract

The invention provides a narrow linewidth cascade Raman fiber laser system based on double-grating cooperative feedback, which comprises a pumping module, a Raman gain fiber and an N-order cascade Raman fiber resonant cavity structure, and is characterized in that the N-order cascade Raman fiber resonant cavity structure comprises N-order Raman fiber resonant cavities; wherein the first-order Raman optical fiber resonant cavity to the (N-1) th-order Raman optical fiber resonant cavity are all of asymmetric resonant cavity structures, and each of the first-order Raman optical fiber resonant cavity to the (N-1) th-order Raman optical fiber resonant cavity is composed of two corresponding-order Raman high-reflection gratings and a corresponding-order Raman low-reflection grating; the Nth-order Raman fiber resonant cavity is composed of an N-order Raman high-reflection grating and an N-order Raman low-reflection grating. According to the invention, through the double-Raman high-reflectivity grating in the asymmetric resonant cavity structure, the problem of line width broadening caused by mutual competition of multiple nonlinear effects during high-power operation of a cascaded Raman fiber laser system is solved, and narrow line width output under high power is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber lasers, and in particular to a narrow-linewidth cascade Raman optical fiber laser system based on double-grating cooperative feedback. Background Art

[0002] Cascaded Raman fiber laser systems face significant spectral broadening during the multi-stage Stokes conversion process. As the temporal noise of the pump light propagates through each Raman conversion stage, the linewidth of the higher-order Stokes light often broadens significantly. Furthermore, competition between nonlinear effects such as four-wave mixing, cross-phase modulation, and self-phase modulation between different wavelengths in the fiber further exacerbates spectral degradation, leading to a decrease in the spectral quality of the output light and limiting the realization of narrow-linewidth lasers.

[0003] Although existing technical solutions can improve the output power and bandwidth of the system, they often sacrifice linewidth characteristics and cannot simultaneously achieve high power and narrow linewidth output. For example, the patent application with publication number CN111668685A proposed a high-power narrow linewidth Raman fiber amplifier based on phosphorus-doped fiber, which achieved a linewidth output of 0.2nm by utilizing the 40THz Raman gain peak. However, this solution is only applicable to single-stage amplification systems and does not involve the spectrum control problem of the cascade structure. The patent application with publication number CN106299988A discloses a cascade output fiber Raman random laser, which achieves multi-order Stokes light output through a broadband reflector. However, due to the use of a random feedback mechanism, its output linewidth is relatively wide and does not involve linewidth compression technology. While the multi-wavelength pumping solution (Optical Communications Research, 2017 (5): 46) can improve conversion efficiency, it is difficult to effectively suppress the spectrum broadening effect.

[0004] Furthermore, some existing solutions have explored methods for suppressing noise and spectral broadening, but none have fully addressed the narrow linewidth issue in cascaded Raman systems. For example, patent application CN110165530A proposes using single-frequency pumping to suppress noise transmission and improve spectral purity, but does not provide a specific method for linewidth control.

[0005] In summary, existing technologies have yet to effectively address the narrow-linewidth output problem of cascaded Raman laser systems, particularly in suppressing spectral broadening during multi-stage Stokes conversion. Therefore, a new method is urgently needed to effectively suppress noise transmission, reduce nonlinear interference, and achieve narrow-linewidth output in cascaded Raman fiber laser systems. Summary of the Invention

[0006] To address the problem of linewidth broadening caused by competition between multiple nonlinear effects when the cascade Raman fiber laser system operates at high power, the present invention proposes a narrow-linewidth cascade Raman fiber laser system based on dual-grating collaborative feedback, which successfully achieves narrow linewidth output at high power.

[0007] In order to achieve the above technical objectives, the technical solution adopted by the present invention is: A narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback includes a pump module, a Raman gain fiber, and N The cascaded Raman fiber resonant cavity structure, N Greater than or equal to 2; N The cascaded Raman fiber resonant cavity structure includes N order Raman fiber resonator, where: First-order Raman fiber resonator to N -1 order Raman fiber resonators are all asymmetric resonant cavity structures, and are composed of two corresponding order Raman high reflection gratings and one corresponding order Raman low reflection grating. N The additional random feedback introduced by the double corresponding order Raman high reflectivity grating in the -1 order Raman fiber resonator optimizes the intra-cavity spectral distribution and reduces the accumulation of spontaneous emission noise; No. N The first-order Raman fiber resonator consists of a N It consists of a 1-order Raman high-reflection grating and an N-order Raman low-reflection grating.

[0008] Pump light injection from the pump source in the pump module N After the cascaded Raman fiber resonator structure is built, the first-order Raman laser is selected under the mode selection effect of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating of the first-order Raman fiber resonator. As the injected pump light power increases, the power level of the first-order Raman light increases continuously and serves as the pump source of the second-order Raman laser. Under the mode selection effect of the second-order Raman high-reflection grating and the second-order Raman low-reflection grating of the second-order Raman fiber resonator, the corresponding second-order Raman laser is selected. According to this rule, finally N The Raman laser N Output of cascaded Raman fiber resonant cavity structure; Since the first-order Raman fiber resonator to the N -1 order Raman fiber resonators are all asymmetric resonant cavity structures, including two corresponding order Raman high reflection gratings and one corresponding order Raman low reflection grating. NThe -1 order Raman fiber resonator undergoes the synergistic feedback effect of two asymmetric corresponding order Raman resonators, which can weaken the time domain noise of the pump light, thereby weakening the accumulation of nonlinear effects in the conversion and transmission process from the previous order Raman light to the next order cascade Raman, and suppressing the spectral broadening of the next order Raman fiber, thus achieving a narrower linewidth at high power. N Cascaded Raman laser output.

[0009] Furthermore, the first-order Raman fiber resonator to the N In the -1 order Raman fiber resonator, the reflectivity of the two corresponding order Raman high reflectivity gratings introducing random feedback is required to be greater than 90%, the reflection bandwidth is 3nm-6nm, and the flatness of the random fluctuation of the reflectivity is 0.5dB.

[0010] Compared with the prior art, the beneficial technical effects of the present invention are: The present invention is achieved by N In the cascaded Raman fiber resonator structure, the first-order Raman fiber resonator is connected to the N -1st order Raman fiber resonator is designed as an asymmetric resonator structure, the first order Raman fiber resonator to the N Each -1-order Raman fiber resonator is constructed from two high-reflectivity gratings (HRGs) of the corresponding order and one low-reflectivity grating of the corresponding order. Linewidth compression is achieved through the following mechanisms: First, the wide bandwidth and randomly fluctuating flat reflection properties of the dual HRGs in the asymmetric resonant cavity broaden the phase-matching range of FWM, enhancing the competition between FWM and SRS during the conversion of pump light to first-order Raman. This in turn weakens the temporal noise of the pump light during the first-order Raman conversion process and reduces the conversion to second-order cascaded Raman. Specifically, the two first-order HRGs must have a reflectivity greater than 90%, a reflection bandwidth of 3nm-6nm, and a reflectivity flatness of 0.5dB. The additional random feedback introduced by the dual HRGs in the asymmetric resonant cavity optimizes the intracavity spectral distribution, reduces spontaneous emission noise accumulation, and mitigates spectral distortion caused by self-phase modulation (SPM) through dynamic power redistribution. In addition, the introduction of dual Raman high-reflection gratings in the symmetrical resonant cavity structure can also effectively increase the reflection ability of the backward-propagating first-order Raman and reduce the damage to the previous stage system.

[0011] The present invention utilizes the first-order Raman fiber resonant cavity to the N The additional random feedback introduced by the double corresponding order Raman high reflectivity grating in the -1 order Raman fiber resonator optimizes the intracavity spectral distribution and reduces the accumulation of spontaneous emission noise. N The principle analysis of the model in which the -1-order Raman fiber resonator optimizes the intracavity spectral distribution and reduces the accumulation of spontaneous emission noise by introducing additional random feedback through the dual-order Raman high-reflectivity grating is as follows: The signal light spectrum intensity evolution equation in the cascade Raman fiber laser can be expressed as: (1) in: is the spectral intensity distribution of the target Raman laser mode (signal light); is the spectral intensity distribution of spontaneous emission noise (incoherent noise); is the wavelength, corresponding to the central wavelength of the Raman laser and the noise spectrum range. is the Raman gain coefficient; is the cavity loss coefficient; is the random feedback intensity parameter (reflecting the additional random feedback efficiency introduced by the double-corresponding-order Raman high-reflectivity grating, ; is the random feedback perturbation term (statistical characteristics: mean is 0, variance is proportional to); The first term in the above formula (1) represents the natural evolution of the signal light under the balance of gain and loss; the second term The modulation effect of random feedback introduced by the double-order Raman high-reflectivity grating on the signal light is shown as follows: It represents the convolution in the spectral domain, reflecting the "smoothing" or "redistribution" effect of random feedback on the spectral distribution, so that the energy is concentrated in the target mode (such as near the central wavelength). The light field scattering caused by the random fluctuation of reflectivity of the double-corresponding-order Raman high-reflectivity grating is simulated to suppress spectral broadening.

[0012] The principle of spontaneous emission noise accumulation suppression in cascaded Raman fiber lasers is as follows: (2) in is the initial generation rate of spontaneous emission noise (constant term), in the second term, represents the natural amplification / attenuation of noise in the cavity, It indicates that the random feedback introduced by the double-corresponding-order Raman high-reflection grating suppresses noise accumulation through the additional loss mechanism: the random feedback of the double-corresponding-order Raman high-reflection grating introduces additional "virtual loss", which reduces the effective gain coefficient of the noise, thereby reducing its accumulation in the cavity.

[0013] Based on the above model, the regulation mechanism of spectrum and noise by double-corresponding-order Raman high-reflectivity grating through random feedback is qualitatively described. The core lies in the effect of random perturbation on the rearrangement of energy distribution and the suppression of noise gain.

[0014] When there is no random feedback ( ): Both signal light and noise evolve according to the natural gain-loss law, and noise may broaden the spectrum due to mode competition.

[0015] When random feedback is introduced by a double-order Raman high-reflectivity grating, that is, when there is random feedback ( ): Random feedback through convolution ( ) “scatters” the energy of non-target modes to the vicinity of target modes, making More concentrated (narrower spectrum).

[0016] Random feedback items The introduction of is equivalent to adding a factor related to the feedback strength into the noise equation. A negative gain term proportional to , weakening the transmission of time-domain noise. Further analysis of the impact of spontaneous emission noise (ASE) accumulation on three key parameters, reflectivity, reflection bandwidth, and reflectivity random fluctuation flatness, reveals that optimizing these three parameters can maximize suppression of various noise sources within the cavity, effectively reducing the accumulation of spontaneous emission noise (ASE). The requirements for the two corresponding-order Raman high-reflectivity gratings introducing random feedback are that the reflectivity is greater than 90%, the reflection bandwidth is 3nm-6nm, and the reflectivity random fluctuation flatness is 0.5dB.

[0017] The present invention solves the problem of linewidth broadening caused by the competition of multiple nonlinear effects when the cascade Raman fiber laser system operates at high power, and realizes narrow linewidth output at high power. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] To more clearly illustrate the embodiments of the present invention or the technical solutions in 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. Those skilled in the art can also derive other drawings based on the structures shown in these drawings without inventive effort.

[0019] Figure 1 Schematic diagram of the structure of a narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback provided in one embodiment; Figure 2 Schematic diagram of the reflection spectra of the first and second first-order Raman high-reflection gratings used in a narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback in one embodiment, wherein Figure 2 (a) is a schematic diagram of the reflection spectrum of the first-order Raman high-reflectivity grating. Figure 2 (b) is a schematic diagram of the reflection spectrum of the second first-order Raman high-reflectivity grating; Figure 3 Schematic diagram of the traditional second-order cascaded Raman fiber resonator structure based on single grating feedback; Figure 4 Schematic diagram of the structure of a narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback provided in another embodiment; Figure 5 In one embodiment, based on Figure 3 The output characteristic diagram of the traditional second-order cascaded Raman fiber resonant cavity structure based on single grating feedback is shown in FIG. Figure 5 (a) is the evolution trend of the output power of the second-order cascaded Raman fiber resonator structure based on single grating feedback as the injected pump light power is applied. Figure 5 (b) is the evolution trend of the output spectrum of the second-order cascaded Raman fiber resonator structure based on single grating feedback at different signal light power levels; Figure 6 In one embodiment, based on Figure 1 The output characteristic diagram of the narrow linewidth cascade Raman fiber laser system based on dual grating cooperative feedback is shown in FIG. Figure 6 (a) is the evolution trend of the output power of the narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback as the injected pump light power is applied. Figure 6 (b) is the evolution trend of the output spectrum of the narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback at different signal light power levels; Figure 7 An embodiment based on Figure 3 The traditional two-stage cascaded Raman fiber resonator structure based on single grating feedback and the Figure 1 The figure shows a comparison of the output linewidth of the cascade Raman laser output by the narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback. DETAILED DESCRIPTION

[0020] 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.

[0021] Reference Figure 4 , a narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback, including a pump module, a Raman gain fiber and N The cascaded Raman fiber resonant cavity structure, N Greater than or equal to 2; N The cascaded Raman fiber resonant cavity structure includes Norder Raman fiber resonator, where: First-order Raman fiber resonator to N -1 order Raman fiber resonators are all asymmetric resonant cavity structures, and are composed of two corresponding order Raman high reflection gratings and one corresponding order Raman low reflection grating. N The additional random feedback introduced by the double corresponding order Raman high reflectivity grating in the -1 order Raman fiber resonator optimizes the intra-cavity spectral distribution and reduces the accumulation of spontaneous emission noise; No. N The first-order Raman fiber resonator consists of a N It consists of a 1-order Raman high-reflection grating and an N-order Raman low-reflection grating.

[0022] Pump light injection from the pump source in the pump module N After the cascaded Raman fiber resonator structure is built, the first-order Raman laser is selected under the mode selection effect of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating of the first-order Raman fiber resonator. As the injected pump light power increases, the power level of the first-order Raman light increases continuously and serves as the pump source of the second-order Raman laser. Under the mode selection effect of the second-order Raman high-reflection grating and the second-order Raman low-reflection grating of the second-order Raman fiber resonator, the corresponding second-order Raman laser is selected. According to this rule, finally N The Raman laser N Output of cascaded Raman fiber resonant cavity structure; Since the first-order Raman fiber resonator to the N -1 order Raman fiber resonators are all asymmetric resonant cavity structures, including two corresponding order Raman high reflection gratings and one corresponding order Raman low reflection grating. N The -1 order Raman fiber resonator undergoes the synergistic feedback effect of two asymmetric corresponding order Raman resonators, which can weaken the time domain noise of the pump light, thereby weakening the accumulation of nonlinear effects in the conversion and transmission process from the previous order Raman light to the next order cascade Raman, and suppressing the spectral broadening of the next order Raman fiber, thus achieving a narrower linewidth at high power. N Cascaded Raman laser output.

[0023] Furthermore, by optimizing the three key parameters of reflectivity, reflection bandwidth, and reflectivity random fluctuation flatness, various noise sources in the cavity can be suppressed to the greatest extent, thereby effectively reducing the accumulation of spontaneous emission noise (ASE). It is required that the reflectivity of the two corresponding orders of Raman high-reflectivity gratings introducing random feedback is greater than 90%, the reflection bandwidth is 3nm-6nm, and the reflectivity random fluctuation flatness is 0.5dB.

[0024] Reference Figure 1In one embodiment, a narrow-linewidth cascaded Raman fiber laser system based on dual-grating cooperative feedback is provided. It is a second-order cascaded Raman fiber resonator structure, comprising a pump source array 1-1, a return light detection port 1-2, a backward return light protector 2, a multimode pump combiner 3, a first-order Raman high-reflection grating 4-1, a second-order Raman high-reflection grating 4-3, a first-order Raman low-reflection grating 4-2, a second-order Raman high-reflection grating 5-1, a second-order Raman low-reflection grating 5-2, a Raman gain fiber 6, and an output end cap 7. The two first-order Raman high-reflection gratings each have a reflectivity greater than 90%, a reflection bandwidth of 3-6 nm, and a reflectivity random fluctuation flatness of 0.5 dB.

[0025] The pump source array 1-1 is the output end of the pump source. The output pigtail of the pump source is connected to the input end of the backward return light protector 2. The output pigtail of the backward return light protector 2 is connected to the input arm of the multimode pump combiner 3. The remaining pump arms of the multimode pump combiner 3 are output through an oblique angle and are used together with the backward return light monitoring device to monitor the backward return light. The output arm of the multimode pump combiner 3 is connected to the input end of the first first-order Raman high-reflection grating 4-1, the output end of the first first-order Raman high-reflection grating 4-1 is connected to the input end of the second first-order Raman high-reflection grating 4-3, the output end of the second first-order Raman high-reflection grating 4-3 is connected to the input end of the second-order Raman high-reflection grating 5-1, the output end of the second-order Raman high-reflection grating 5-1 is connected to the Raman gain fiber 6, the output end of the Raman gain fiber 6 is connected to the input end of the second-order Raman low-reflection grating 5-2, the output end of the second-order Raman low-reflection grating 5-2 is connected to the input end of the first-order Raman low-reflection grating 4-2, and the output end of the first-order Raman low-reflection grating 4-2 is connected to the output end cap 7.

[0026] After the pump light output by the pump source is injected into the second-order cascaded Raman fiber resonator structure through the pump arm of the multimode pump combiner 3, the corresponding first-order Raman laser is first selected under the mode selection effect of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating 4-2 of the first-order Raman fiber resonator. As the power of the injected pump light increases, the power level of the first-order Raman light continues to increase, and serves as the pump source for the second-order Raman laser. Under the mode selection effect of the second-order Raman high-reflection grating 5-1 and the second-order Raman low-reflection grating 5-2 of the second-order Raman fiber resonator, the corresponding second-order Raman laser is selected. Finally, the second-order Raman laser is output through the output end cap 7.

[0027] Since the first-order Raman fiber resonator has an asymmetric resonant cavity structure, including two first-order Raman high-reflection gratings and a corresponding order Raman low-reflection grating 4-2, the synergistic feedback effect of the two asymmetric first-order Raman resonators in the first-order Raman fiber resonator can weaken the time domain noise of the pump light, thereby weakening the accumulation of nonlinear effects in the conversion and transmission process of the first-order Raman light to the second-order cascade Raman, suppressing the spectral broadening of the second-order Raman fiber, and thus achieving a second-order cascade Raman laser output with a narrower linewidth at high power.

[0028] Figure 2 Schematic diagram of the reflection spectra of the first and second first-order Raman high-reflection gratings used in a narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback in one embodiment, wherein Figure 2 (a) is a schematic diagram of the reflection spectrum of the first-order Raman high-reflectivity grating. Figure 2 (b) is a schematic diagram of the reflection spectrum of the second first-order Raman high-reflectivity grating. In the above embodiment, by introducing a second first-order Raman high-reflectivity grating with a reflectivity greater than 90%, a reflection bandwidth of 4nm, and a reflectivity random fluctuation flatness of 0.5dB into the first-order Raman fiber resonant cavity, an asymmetric resonant cavity structure is constructed, and linewidth compression is achieved through the following mechanism: First, the wide bandwidth and randomly fluctuating flat reflection characteristics of the first first-order Raman high-reflection grating 4-1 and the second first-order Raman high-reflection grating 4-3 broaden the phase matching range of FWM, enhance the competition between the FWM process and SRS during the conversion of pump light to first-order Raman, and then weaken the time domain noise of the pump light in the first-order Raman conversion process, weakening the conversion and transmission process to second-order cascade Raman.

[0029] Secondly, the additional random feedback introduced by the first first-order Raman high-reflection grating 4-1 and the second first-order Raman high-reflection grating 4-3 optimizes the intra-cavity spectral distribution, reduces the accumulation of spontaneous emission noise, and alleviates the spectral line distortion caused by self-phase modulation (SPM) through dynamic power redistribution.

[0030] In addition, the introduction of the first first-order Raman high-reflection grating 4-1 and the second first-order Raman high-reflection grating 4-3 can also effectively increase the reflection capability of the backward-propagating first-order Raman and reduce the damage to the front-stage system.

[0031] Furthermore, the pump source may be a semiconductor laser, an ASE source, or a fiber laser.

[0032] Furthermore, the above-mentioned return light protector can be a tilted grating, or an optical fiber device composed of filtering optical fibers, etc.

[0033] Furthermore, the multi-mode pump combiner is an N×1 pump signal combiner, where N can be 3, 5, or other commercial models.

[0034] Furthermore, the Raman gain fiber can be either a graded-index fiber or a step-index fiber, either a single-clad fiber or a multi-clad fiber, or a silicate fiber or a fiber made of other matrices such as tellurite.

[0035] Furthermore, the central wavelengths of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating are located near the Raman gain peak of the pump light in the Raman gain fiber. Specifically, the Raman gain peak of the silicate fiber is located at 440 cm -1 The Raman gain peak of tellurite fiber is located at 750cm -1 The Raman gain peak of fluoride fiber is located at 580 cm -1 wait.

[0036] Furthermore, the center wavelengths of the second-order Raman high-reflection grating and the second-order Raman low-reflection grating are located near the Raman gain peak of the first-order Raman laser output wavelength in the Raman gain fiber. Specifically, the Raman gain peak of the silicate fiber is located at 440 cm -1 The Raman gain peak of tellurite fiber is located at 750cm -1 The Raman gain peak of fluoride fiber is located at 580 cm -1 wait.

[0037] Furthermore, the above is not limited to second-order Raman fiber Bragg gratings, and third-order Raman fiber Bragg gratings can be cascaded to form a third-order cascaded Raman fiber resonator. However, the output wavelength of the third-order Raman fiber laser is located before the zero-dispersion wavelength of the Raman fiber. The zero-dispersion wavelength of commercial optical fiber is located near the 1310nm or 1550nm band. The zero-dispersion wavelength of commercial Raman fiber used for high-order cascades can reach near the 2000nm band.

[0038] Furthermore, the Raman grating pair can be either a grating written by ultraviolet or a grating written by femtosecond.

[0039] Furthermore, the Raman grating can be a fiber grating based on a graded-index fiber or a fiber grating based on a step-index fiber.

[0040] Furthermore, the output end cap is plated with an anti-reflection film covering the wavelength range of the signal light and the residual pump light, which can reduce the end face feedback of the signal light and the residual pump light and avoid end face damage of the optical fiber.

[0041] In another embodiment, based on Figure 1 The narrow linewidth cascade Raman fiber laser system based on dual grating cooperative feedback is shown in the figure. Figure 2The reflection spectrum diagram of the first first-order Raman high-reflection grating 4-1 and the second first-order Raman high-reflection grating 4-3 is shown, that is, random feedback is introduced through two first-order Raman high-reflection gratings. The pump source is a 1080nm ytterbium-doped fiber laser, the center wavelength of the first-order cascaded Raman fiber grating is 1130nm, and the center wavelength of the second-order cascaded Raman fiber grating is 1185nm. In this embodiment, the bandwidth of the two first-order cascaded Raman fiber gratings is 4nm, and the flatness of the random fluctuation of the reflectivity is 0.5dB. Figure 3 The conventional second-order cascaded Raman fiber resonator structure based on single grating feedback is shown as a comparative example. The only difference in the comparative example is that it only contains one first-order Raman high-reflection grating. The pump source in the comparative example is a 1080nm ytterbium-doped fiber laser. The central wavelength of the first-order cascaded Raman fiber grating is 1130nm and it adopts Figure 2 Any of the gratings in the cascaded Raman fiber Bragg grating is used as the first-order cascaded Raman fiber Bragg grating in the comparative example. The central wavelength of the second-order cascaded Raman fiber Bragg grating is also 1185. In this comparative example, there is no random feedback. The output characteristics of the second-order cascaded Raman laser at the same signal light power level are analyzed by comparison. Specifically, Figure 5 For comparative example based on Figure 3 The output characteristic diagram of the traditional second-order cascaded Raman fiber resonant cavity structure based on single grating feedback is shown in FIG. Figure 5 (a) is the evolution trend of the output power of the second-order cascaded Raman fiber resonator structure based on single grating feedback as the injected pump light power is applied. Figure 5 (b) is the evolution trend of the output spectrum of the second-order cascaded Raman fiber resonant cavity structure based on single grating feedback at different signal light power levels. Figure 6 In the embodiment based on Figure 1 The output characteristic diagram of the narrow linewidth cascade Raman fiber laser system based on dual grating cooperative feedback is shown in FIG. Figure 6 (a) is the evolution trend of the output power of the narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback as the injected pump light power is applied. Figure 6 (b) shows the evolution trend of the output spectrum of a narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback at different signal light power levels. By comparison, it can be found that at the same signal light output power level, the backward return light power of the dual-grating cooperative feedback is lower than that of the single-grating feedback, and has a better return light suppression effect. In addition, the high-order Raman suppression ratio of 1248nm in the second-order 1185nm cascade Raman fiber resonator with dual-grating cooperative feedback is 46dB, while the high-order Raman suppression ratio of the second-order 1185nm cascade Raman fiber resonator with single grating feedback is 43dB. Figure 7The output linewidth of the cascade Raman laser based on single grating feedback and dual grating cooperative feedback is compared. At the same signal light output power level, the output linewidth of the cascade Raman laser based on dual grating cooperative feedback is narrower than that of the single grating feedback.

[0042] Matters not covered by the present invention are known technologies.

[0043] The technical features of the above embodiments can be combined arbitrarily. To make the description concise, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0044] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art may make various modifications and improvements without departing from the spirit of the present application, and such modifications and improvements are all within the scope of protection of the present application.

[0045] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that the present invention is susceptible to various modifications and variations. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

Claims

1. Narrow linewidth cascade Raman fiber laser system based on dual grating cooperative feedback, characterized by: Including pump module, Raman gain fiber and N The cascaded Raman fiber resonant cavity structure, N Greater than or equal to 2; N The cascaded Raman fiber resonant cavity structure includes N order Raman fiber resonator, where: First-order Raman fiber resonator to N -1 order Raman fiber resonators are all asymmetric resonant cavity structures, and are composed of two corresponding order Raman high reflection gratings and one corresponding order Raman low reflection grating. N The additional random feedback introduced by the double corresponding order Raman high reflectivity grating in the -1 order Raman fiber resonator optimizes the intra-cavity spectral distribution and reduces the accumulation of spontaneous emission noise; No. N The first-order Raman fiber resonator consists of a N It consists of a 1-order Raman high-reflection grating and an N-order Raman low-reflection grating.

2. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 1, characterized in that: Pump light injection from the pump source in the pump module N After the cascaded Raman fiber resonator structure is built, the first-order Raman laser is selected under the mode selection effect of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating of the first-order Raman fiber resonator. As the injected pump light power increases, the power level of the first-order Raman light increases continuously and serves as the pump source of the second-order Raman laser. Under the mode selection effect of the second-order Raman high-reflection grating and the second-order Raman low-reflection grating of the second-order Raman fiber resonator, the corresponding second-order Raman laser is selected. According to this rule, finally N The Raman laser N Output of cascaded Raman fiber resonant cavity structure; Since the first-order Raman fiber resonator to the N -1 order Raman fiber resonators are all asymmetric resonant cavity structures, including two corresponding order Raman high reflection gratings and one corresponding order Raman low reflection grating. N The -1 order Raman fiber resonator undergoes the synergistic feedback effect of two asymmetric corresponding order Raman resonators, which can weaken the time domain noise of the pump light, thereby weakening the accumulation of nonlinear effects in the conversion and transmission process from the previous order Raman light to the next order cascade Raman, and suppressing the spectral broadening of the next order Raman fiber, thus achieving a narrower linewidth at high power. N Cascaded Raman laser output.

3. The narrow linewidth cascade Raman fiber laser system based on dual grating cooperative feedback according to claim 1, characterized in that: It is a second-order cascaded Raman fiber resonator structure, including a pump source array, a return light detection port, a backward return light protector, a multimode pump combiner, a first-order Raman high-reflection grating, a second-order Raman high-reflection grating, a first-order Raman low-reflection grating, a second-order Raman high-reflection grating, a second-order Raman low-reflection grating, a Raman gain fiber, and an output end cap. The pump source array is the output end of the pump source. The output pigtail of the pump source is connected to the input end of the backward return light protector. The output pigtail of the backward return light protector is connected to the input arm of the multimode pump combiner. The remaining pump arms of the multimode pump combiner are output through an oblique angle and are used together with the backward return light monitoring device to monitor the backward return light. The output arm of the multimode pump combiner is connected to the input end of the first first-order Raman high-reflection grating, the output end of the first first-order Raman high-reflection grating is connected to the input end of the second first-order Raman high-reflection grating, the output end of the second first-order Raman high-reflection grating is connected to the input end of the second-order Raman high-reflection grating, the output end of the second-order Raman high-reflection grating is connected to the Raman gain fiber, the output end of the Raman gain fiber is connected to the input end of the second-order Raman low-reflection grating, the output end of the second-order Raman low-reflection grating is connected to the input end of the first-order Raman low-reflection grating, and the output end of the first-order Raman low-reflection grating is connected to the output end cap.

4. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 2, characterized in that: When the pump light output by the pump source is injected into the second-order cascaded Raman fiber resonator structure through the pump arm of the multimode pump combiner, the corresponding first-order Raman laser is first selected under the mode selection effect of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating of the first-order Raman fiber resonator. As the injected pump light power increases, the power level of the first-order Raman light continues to increase and serves as the pump source for the second-order Raman laser. Under the mode selection effect of the second-order Raman high-reflection grating and the second-order Raman low-reflection grating of the second-order Raman fiber resonator, the corresponding second-order Raman laser is selected. Finally, the second-order Raman laser is output through the output end cap; Since the first-order Raman fiber resonator has an asymmetric resonant cavity structure, including two first-order Raman high-reflection gratings and a corresponding-order Raman low-reflection grating, the synergistic feedback of the two asymmetric first-order Raman resonators in the first-order Raman fiber resonator can weaken the time domain noise of the pump light, thereby reducing the accumulation of nonlinear effects in the conversion and transmission process of the first-order Raman light to the second-order cascade Raman, suppressing the spectral broadening of the second-order Raman fiber, and thus achieving a second-order cascade Raman laser output with a narrower linewidth at high power.

5. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 3 or 4, characterized in that: The center wavelengths of the first-order Raman high-reflection grating and the first-order Raman low-reflection grating are located within a set range near the Raman gain peak of the pump light in the Raman gain fiber, and the center wavelengths of the second-order Raman high-reflection grating and the second-order Raman low-reflection grating are located within a set range near the Raman gain peak of the first-order Raman laser output wavelength in the Raman gain fiber.

6. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 5, characterized in that: The reflectivity of the two first-order Raman high-reflectivity gratings is greater than 90%, the reflection bandwidth is 3nm-6nm, and the flatness of the reflectivity random fluctuation is 0.5dB.

7. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 1, 2, 3, 4, or 6, characterized in that: The Raman gain fiber is a graded-refractive-index fiber or a step-refractive-index fiber.

8. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 7, characterized in that: The Raman gain fiber is a silicate fiber, a tellurite fiber or a fluorite fiber.

9. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 7, characterized in that: The pump source in the pump module adopts a semiconductor laser, an ASE source or a fiber laser.

10. The narrow-linewidth cascade Raman fiber laser system based on dual-grating cooperative feedback according to claim 7, characterized in that: The return light protector adopts an inclined grating.

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