A 2.8 mu m single-frequency fiber laser based on pump-enhanced saturable absorber

Abstract

The application provides a 2.8 mu m single-frequency fiber laser based on a pump-enhanced saturable absorber and belongs to the technical field of mid-infrared laser. The linear resonant cavity of the laser is composed of a low-refractivity fiber grating and a high-refractivity fiber grating, and a first pump coupler, an erbium-doped fluoride gain fiber, a pump light stripper and a pump-enhanced saturable absorber filter device are sequentially arranged in the cavity. The filter device contains enhanced pump light injected by a second pump source through a second pump coupler and used for exciting the absorption characteristics of the erbium-doped saturable absorption fiber; the laser of the first pump source is used for pumping the gain fiber to generate laser oscillation, and the enhanced pump light is used for forming a MHz-level ultra-narrow band dynamic grating in the saturable absorption fiber. The dynamic grating and the static filter of the fiber grating work cooperatively to realize stable single-longitudinal-mode selection, and finally the single-frequency laser is output from the first fiber end cap. The application realizes compact structure and high stability of 2.8 mu m single-frequency laser output.

Description

Technical Field

[0001] This invention relates to the field of mid-infrared laser technology, and more specifically to a 2.8μm single-frequency fiber laser based on a pump-enhanced saturable absorber. Background Technology

[0002] Single-frequency mid-infrared fiber lasers in the 2.8μm band have shown great application potential in cutting-edge fields such as laser medicine, gas sensing, spectroscopy, and optoelectronic countermeasures, becoming a research hotspot in the current laser technology field. Traditional techniques for achieving single-frequency laser output in this band typically rely on space optical elements, such as using a composite cavity structure composed of a germanium (Ge) etalon and a fiber grating for spectral filtering and mode selection. Although such schemes can theoretically select a single longitudinal mode, their core filtering mechanism has inherent limitations.

[0003] The main technical bottleneck of existing solutions lies in their filtering mechanisms. Traditional spatial optical filters (such as Ge etalons) typically have bandwidths in the GHz range, while the longitudinal mode spacing of the 2.8μm fiber laser resonator is only in the MHz range. This bandwidth mismatch leads to instability in the mode selection process, making the output laser prone to mode switching and hindering true, stable single-frequency operation. Furthermore, the introduction of spatial optical components such as Ge etalons disrupts the all-fiber structure of the resonator, resulting in a more complex and larger laser system that is extremely sensitive to environmental vibrations and temperature drift, severely limiting the long-term stability, reliability, and practical application in compact integrated systems. Therefore, there is an urgent need to develop a new all-fiber 2.8μm single-frequency laser solution capable of achieving ultra-narrowband filtering in the MHz range, with a compact structure and stable performance. Summary of the Invention

[0004] This invention provides a 2.8μm single-frequency fiber laser based on a pump-enhanced saturable absorber to solve technical problems in the prior art, such as the difficulty in achieving all-fiber operation, the lack of MHz-level ultra-narrowband filtering methods, and poor single-frequency output stability.

[0005] To achieve the above objectives, embodiments of the present invention provide a 2.8 μm single-frequency fiber laser based on a pump-enhanced saturable absorber, comprising a resonant cavity body, a pumping system, and an output component; the resonant cavity body includes: a low-reflectivity fiber grating, a first pump coupler, and an Er³-doped fiber optic cable. +The system comprises a fluoride-gain fiber, a pump light stripper, a pump-enhanced saturable absorber filter, and a high-reflectivity fiber grating, wherein the low-reflectivity fiber grating and the high-reflectivity fiber grating constitute a linear resonant cavity; the pumping system includes a first pump source and a second pump source, wherein the first pump source is a laser diode with an output wavelength of 976 nm or 980 nm, and the pump light output from the first pump coupler is injected into the Er³-doped fiber. + Fluoride-gain fiber; the center wavelength of the second pump source is located in the 1.5-1.6 μm band, used to provide pre-pump to the pump-enhanced saturable absorber filter; the pump-enhanced saturable absorber filter includes a second pump coupler and an Er³-doped section. + The fluoride saturable absorber fiber is used, and the laser output from the second pump source is injected into the saturable absorber fiber via the second pump coupler. The output component includes a first fiber end cap and a second fiber end cap. The first fiber end cap is connected to the low reflectivity fiber grating for outputting 2.8μm single-frequency laser, and the second fiber end cap is connected to the high reflectivity fiber grating.

[0006] Furthermore, the output power of the first pump source is 0-100W; the output power of the second pump source is 0-10W.

[0007] Furthermore, the Er³ doping + Fluoride gain fiber and the Er³-doped fiber + Fluoride-saturable absorber optical fiber is made of Er³. + AlF3, Er³ + InF3 or Er³ + The fiber is selected from any one or a combination of two of ZrF4-BaF2-LaF3-AlF3-NaF optical fibers; the length of the gain fiber is 10-500 cm, and the Er³⁺ content is... + The doping concentration is 0.5-10 mol%; the length of the saturable absorbable optical fiber is 50-1000 cm, and Er³... + The doping concentration is 0.5-10 mol.

[0008] Furthermore, the low-reflectivity fiber grating has a reflectivity of 10%-90% in the 2.8μm band and a reflection bandwidth of no more than 0.5nm; the high-reflectivity fiber grating has a reflectivity of no less than 90% in the 2.8μm band and a reflection bandwidth of 0.1-3.0nm.

[0009] Furthermore, the low-reflectivity fiber grating and the high-reflectivity fiber grating are polarization-maintaining fiber gratings or non-polarization-maintaining fiber gratings; when a polarization-maintaining fiber grating is used, the slow-axis reflection peak of the low-reflectivity fiber grating is located within the reflection bandwidth of the high-reflectivity fiber grating.

[0010] Furthermore, the first pump coupler is a pump coupler with a wavelength adapted to 976nm / 980nm pump light and 2.8μm signal light; the second pump coupler is a pump coupler with a wavelength adapted to 1.5-1.6μm pump light and 2.8μm signal light.

[0011] Furthermore, the pumped optical stripper is used to strip and filter out the Er³-doped material. + Pump light from the residual first pump source after transmission through fluoride-gain fiber.

[0012] Furthermore, the end face bevel angle of the first fiber end cap and the second fiber end cap is 4-10°, and their materials are selected from quartz, fluoride glass, chalcogenide glass or tellurate glass.

[0013] Furthermore, the center wavelength of the single-frequency laser output by the laser is 2.7-2.9μm, the linewidth is less than 100kHz, and the root mean square value of long-term power fluctuation is less than 1%.

[0014] Furthermore, when the low-reflectivity fiber grating and the high-reflectivity fiber grating are both polarization-maintaining fiber gratings, the laser outputs a linearly polarized single-frequency laser with a polarization extinction ratio of not less than 20dB and a linewidth of less than 50kHz.

[0015] The 2.8μm single-frequency fiber laser based on a pump-enhanced saturable absorber provided by this invention has the following beneficial effects:

[0016] (1) This invention abandons the traditional space optical filtering elements and realizes the all-fiber design of a 2.8μm single-frequency laser for the first time. All core functions are realized by fiber devices through fusion splicing. The structure is extremely compact and highly integrated, which fundamentally overcomes the drawbacks of the stringent requirements of space optical paths on mechanical stability and environmental cleanliness, and lays the foundation for the engineering and practical application of lasers.

[0017] (2) This invention proposes a pump enhancement mechanism, which activates an erbium-doped fluoride fiber into a high-performance saturable absorber by pumping a section of erbium-doped fluoride fiber with a 1.5μm laser. This fiber forms an ultra-narrowband dynamic grating with a bandwidth on the order of MHz in the cavity, which perfectly fills the gap of lacking an effective narrowband filter in the mid-infrared band.

[0018] (3) This invention can accurately and stably screen and lock a single longitudinal mode from multiple longitudinal modes, solving the mode switching problem caused by the mismatch between the filter bandwidth and the longitudinal mode interval in traditional solutions. Ultimately, the laser outputs a pure single-frequency laser with narrow linewidth and high power stability, fundamentally achieving the goal of stable output. Attached Figure Description

[0019] 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 some embodiments of this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort. In the drawings: Figure 1 This is a schematic diagram of the structure of the 2.8μm single-frequency fiber laser of this invention. Figure 2 For Er³ + Ion energy level structure and Er³ for generating 2.8 μm laser light + Schematic diagram of energy level transition; Figure 3 This is a schematic diagram of the principle of forming a pump-enhanced saturable absorber dynamic grating in a bulk, wherein (a) is a schematic diagram of the principle that a dynamic grating cannot be formed without active pumping, and (b) is a schematic diagram of the principle of forming a dynamic grating after active pumping is applied. Figure 4 A schematic diagram illustrating the principle of achieving single longitudinal mode selection by combining dynamic and static gratings; In the figure, 1. Low reflectivity fiber grating; 2. First pump source; 3. First pump coupler; 4. Er³ doped fiber optic cable. + 5. Fluoride-gain fiber; 6. Pump light stripper; 7. Pump-enhanced saturable absorber filter; 8. Second pump source; 9. Second pump coupler; 10. Er³-doped fiber + 10. Fluoride-saturable absorber fiber; 11. High-reflectivity fiber grating; 12. First fiber end cap; 13. Second fiber end cap. Detailed Implementation

[0020] The specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings. It should be understood that the specific embodiments described herein are for illustration and explanation only and are not intended to limit the scope of the present invention.

[0021] It should be noted that the acquisition, transmission, storage, use, and processing of data in the technical solution of this application all comply with the relevant provisions of national laws and regulations. In the embodiments of this application, certain existing industry solutions such as software, components, and models may be mentioned. These should be considered exemplary, intended only to illustrate the feasibility of implementing the technical solution of this application, and do not imply that the applicant has already used or necessarily used such solutions.

[0022] The following is combined Figures 1-4 This invention is described in detail.

[0023] Example 1: like Figure 1As shown, this embodiment of the invention provides a 2.8μm single-frequency fiber laser based on a pump-enhanced saturable absorber, including a resonant cavity body, a pumping system, and an output component; the resonant cavity body includes: a low-reflectivity fiber grating 1, a first pump coupler 3, and an Er³-doped fiber optic coupler 4. + The system comprises a fluoride-gain fiber 4, a pump light stripper 5, a pump-enhanced saturable absorber filter 6, and a high-reflectivity fiber grating 10, wherein the low-reflectivity fiber grating 1 and the high-reflectivity fiber grating 10 constitute a linear resonant cavity; the pumping system includes a first pump source 2 and a second pump source 7, wherein the first pump source 2 is a laser diode with an output wavelength of 976nm or 980nm, and its output pump light is injected into the Er³-doped fiber optic cable via the first pump coupler 3. + Fluoride-gain fiber 4; the center wavelength of the second pump source 7 is located in the 1.5-1.6 μm band, used to provide pre-pump to the pump-enhanced saturable absorber filter device 6; the pump-enhanced saturable absorber filter device 6 includes a second pump coupler 8 and an Er³-doped section. + The fluoride saturable absorber fiber 9 is used, and the laser output from the second pump source 7 is injected into the saturable absorber fiber 9 via the second pump coupler 8. The output component includes a first fiber end cap 11 and a second fiber end cap 12. The first fiber end cap 11 is connected to the low reflectivity fiber grating 1 for outputting 2.8μm single-frequency laser, and the second fiber end cap 12 is connected to the high reflectivity fiber grating 10.

[0024] Specifically, the main function of the first pump source 2 is to provide energy. Its output 976nm / 980nm laser is injected into the gain fiber 4 after cladding pumping by the first pump coupler 3, thus transferring the Er³ laser in the gain fiber 4. + Ion pumping to the upper energy level to produce population inversion is the basis for achieving 2.8 μm laser oscillation. The main function of the second pump source 7 is to perform active modulation, and its output laser in the 1.5-1.6 μm band (corresponding to Er³) + Ionic 4 I 15 / 2 → 4 I 13 / 2 (Transition) used to excite Er³ in saturable absorber fiber 9 + Ions, thereby significantly enhancing the fiber's response to the subsequently generated 2.8μm laser (corresponding to...). 4 I 13 / 2 → 4 I 11 / 2 The absorption capacity of the transition (the process called pre-pumping or pump enhancement) is a key step in transforming ordinary doped optical fibers into highly efficient saturable absorbers.

[0025] Specifically, the second pump coupler 8 is a pump coupler whose function is to combine the 1.5-1.6μm modulated pump light from the second pump source 7 with the 2.8μm signal light oscillating within the resonant cavity, and inject them together into the saturable absorbable fiber 9. (Er³ doped) + Fluoride saturable absorbable fiber 9 is the core filter medium after being excited by the second pump source 7. Its absorption coefficient for 2.8μm laser is greatly improved due to the pump enhancement effect, so that it can form periodic loss / refractive index modulation in the standing wave field formed by the laser in the cavity through the saturable absorption effect and Kerr nonlinear effect, that is, an ultra-narrowband dynamic grating with a bandwidth on the order of MHz.

[0026] Specifically, both the first fiber end cap 11 and the second fiber end cap 12 are optical elements with beveled ends. The bevel is typically 4-10°, designed to effectively suppress Fresnel reflections at the fiber end face and prevent reflected light from re-entering the resonant cavity and interfering with the stable oscillation of the laser. The first fiber end cap 11 serves as the laser output port, extracting the stable 2.8μm single-frequency laser obtained after the two-stage filtering. The second fiber end cap 12, in addition to protecting the end face, is primarily used to safely guide the residual 1.5-1.6μm modulated pump light after transmission through the saturable absorber fiber 9, preventing it from accumulating at the end face of the high-reflectivity fiber grating 10 and causing damage.

[0027] The core technical effect of the technical solution provided by this invention lies in achieving high-performance, high-stability single-frequency laser output in the 2.8μm band through an innovative, actively controllable all-fiber structure combined with a two-stage composite mode selection mechanism of static grating + dynamic grating. This solution introduces a pump-enhanced saturable absorber filter 6, utilizing the 1.5-1.6μm pre-pump light from the second pump source 7 to actively transform a section of erbium-doped fluoride fiber 9 from a conventional fiber with weak intrinsic absorption into a saturable absorber with strong absorption capability for 2.8μm laser light. Under the action of the standing wave field formed by the laser within the cavity, the enhanced fiber combines the saturable absorption effect with the Kerr nonlinear effect, forming an ultra-narrowband dynamic grating with a bandwidth on the order of MHz. This dynamic grating works in synergy with a static grating composed of a low-reflectivity fiber grating 1 and a high-reflectivity fiber grating 10 for mode selection. Its bandwidth perfectly matches the longitudinal mode spacing of the laser, enabling precise and stable selection and locking of a single longitudinal mode from multiple modes. This fundamentally solves the core problem of mode switching and unstable single-frequency output caused by the severe mismatch between GHz-level filtering bandwidth and MHz-level longitudinal mode spacing in traditional space optical filter schemes. Furthermore, the entire scheme is constructed entirely from fused fiber optic components, achieving a fully fiber-based, compact design that avoids the sensitivity of the space optical path to environmental disturbances, significantly improving the practicality and reliability of the laser.

[0028] Preferably, the output power of the first pump source 2 is 0-100W; the output power of the second pump source 7 is 0-10W.

[0029] The preferred embodiment of the present invention defines the available output power range of the first pump source 2 and the second pump source 7. The power of the first pump source 2, 0-100W, primarily determines the power that can be supplied to the Er³ doped pump. + The upper limit of the pump energy of the fluoride-gain fiber 4 directly affects the output power level of the final 2.8μm single-frequency laser; the higher the power, the higher the potential output power. The power of the second pump source 7 (0-10W) is a key parameter for controlling the performance of the pump-enhanced saturable absorber filter 6. By adjusting this power, the Er³ doping can be dynamically controlled. + Fluoride-saturable absorber fiber 9 enhances the absorption of 2.8μm laser light, thereby finely adjusting the modulation depth and bandwidth of the resulting dynamic grating to optimize the threshold and stability of single-frequency selection. In practice, the specific power value within this range is selected and set according to the desired laser output characteristics and system optimization objectives.

[0030] Preferably, the Er³ doping + Fluoride-gain fiber 4 and the Er³-doped fiber + The material of fluoride saturable absorber fiber 9 is Er³. + AlF3, Er³ + InF3 or Er³ + : Any one or a combination of two of the following: ZrF4-BaF2-LaF3-AlF3-NaF optical fibers; the length of the gain fiber 4 is 10-500cm, Er³ + The doping concentration is 0.5-10 mol%; the length of the saturable absorbable optical fiber 9 is 50-1000 cm, and Er³... + The doping concentration is 0.5-10 mol.

[0031] In a preferred embodiment of the present invention, Er³ + AlF3, Er³ + InF3, Er³ + ZrF4-BaF2-LaF3-AlF3-NaF refers to three typical erbium-doped fluoride glass fiber matrix materials. +ZrF4-BaF2-LaF3-AlF3-NaF is a commonly used chemical composition of ZBLAN optical fibers. Due to its low phonon energy, wide transmission range, high rare-earth ion solubility, and low intrinsic loss, it is a high-performance medium for realizing 2.8μm band lasers and is a preferred option. Lengths of 10-500cm and 50-1000cm define the physical length ranges of gain fiber 4 and saturable absorber fiber 9, respectively. These specific values ​​determine the effective length of the laser gain, the pump absorption efficiency, and the fineness and bandwidth of the dynamic grating. A doping concentration of 0.5-10 mol% defines the Er³⁺ content in the fiber core. + The relative content of ions, this concentration range is designed to ensure that the optical fiber has a sufficiently high gain or absorption coefficient, while avoiding negative effects such as concentration quenching caused by excessive concentration.

[0032] Preferably, the low reflectivity fiber grating 1 has a reflectivity of 10%-90% in the 2.8μm band and a reflection bandwidth of no more than 0.5nm; the high reflectivity fiber grating HR-FBG10 has a reflectivity of no less than 90% in the 2.8μm band and a reflection bandwidth of 0.1-3.0nm.

[0033] More preferably, the low-reflectivity fiber grating 1 and the high-reflectivity fiber grating 10 are non-polarization-maintaining fiber gratings.

[0034] Among them, reflectivity refers to the percentage of reflection intensity of a specific center wavelength laser by a fiber grating, and is a key parameter determining the feedback efficiency and output coupling ratio of the resonant cavity. Reflection bandwidth refers to the wavelength range in the grating's reflection spectrum where the reflectivity is not less than half of its peak reflectivity; its size determines the width of the wavelength range for the initial screening of longitudinal modes by the grating. The low-reflectivity fiber grating 1, with a reflectivity of 10%-90% and a reflection bandwidth ≤0.5nm, is designed to act as an output coupling mirror while simultaneously performing a preliminary coarse selection of the wavelength of the intracavity oscillating laser, initially screening out a relatively narrow longitudinal mode group. The high-reflectivity fiber grating 10, with a high reflectivity ≥90% and a relatively wide reflection bandwidth of 0.1-3.0nm, primarily functions as an end mirror of the resonant cavity, providing high feedback and ensuring complete coverage of the reflection peak of the low-reflectivity fiber grating for effective resonance. In this embodiment, both the high-reflectivity fiber grating 10 and the low-reflectivity fiber grating 1 are non-polarization-maintaining fiber gratings, which is a basic and feasible implementation scheme for achieving single-frequency output.

[0035] Preferably, the first pump coupler 3 is a pump coupler with a wavelength adapted to 976nm / 980nm pump light and 2.8μm signal light; the second pump coupler 8 is a pump coupler with a wavelength adapted to 1.5-1.6μm pump light and 2.8μm signal light.

[0036] More preferably, the pump-driven photostripper 5 is used to strip and filter out the Er³-doped material. + The residual pump light after transmission through the fluoride gain fiber 4. The end face bevel angles of the first fiber end cap 11 and the second fiber end cap 12 are 4-10°, and their materials are selected from quartz, fluoride glass, chalcogenide glass, or tellurate glass. The center wavelength of the single-frequency laser output by the laser is 2.7-2.9 μm, the linewidth is less than 100 kHz, and the root mean square value of long-term power fluctuation is less than 1%.

[0037] A pump coupler is an optical device that allows two or more optical signals of different wavelengths to be combined or separated and transmitted without interference in the same optical fiber. The first pump coupler 3 functions to combine 976 / 980nm pump light with a counter-propagating 2.8μm signal light to achieve pumping. The second pump coupler 8 functions to inject 1.5-1.6μm modulated pump light into a saturable absorber fiber 9, and transmit it in the same or opposite direction as the 2.8μm signal light. A pump light stripper 5 is a device used to remove residual pump light from the fiber cladding. In this embodiment, it specifically refers to filtering out the unabsorbed 976 / 980nm pump light in the cladding after transmission through the gain fiber 4, preventing it from returning to the pump source or interfering with subsequent devices. End face beveling refers to grinding the fiber end face to a specific angle of 4-10°. Its core purpose is to greatly reduce the vertical reflectivity of the end face, thereby suppressing Fresnel reflection light from re-entering the resonant cavity, ensuring the stability of laser oscillation, and preventing damage to the pump source. All materials used are optical glass suitable for mid-infrared transmission. The center wavelength of 2.7-2.9 μm is determined by the energy level transitions of gain fiber 4 and the center reflection wavelength of the fiber grating. A linewidth of less than 100 kHz is a direct measure of the spectral purity of single-frequency lasers; the narrower the linewidth, the longer the coherence. A root mean square value of long-term power fluctuations of less than 1% is a key indicator of the stability of output laser power; the smaller the value, the stronger the laser's anti-interference capability and the more stable its single-frequency operation.

[0038] In a preferred embodiment of this invention, the specific wavelength adaptation design of the pump coupler ensures precise injection of the pump light and control light, which is fundamental to the realization of laser generation and dynamic filtering. The introduction of the pump light stripper effectively purifies the optical environment within the resonant cavity, removing interference from residual pump light, thereby improving the signal-to-noise ratio and long-term operational stability of the output laser. The oblique cut design and specific material selection of the fiber end cap significantly suppress harmful reflections at the end face, which is a key structural measure to maintain stable laser oscillation and prevent mode jumps. This achieves single-frequency laser output with extremely narrow linewidth and ultra-high power stability within a specific wavelength range.

[0039] In this embodiment, both the gain fiber 4 and the saturable absorber fiber 9 are Er³+:ZBLAN fibers. The gain fiber 4 has a length of 250 cm and an Er³+ doping concentration of 7.0 mol%. The saturable absorber fiber 9 has a length of 200 cm and an Er³+ doping concentration of 7.0 mol%. The center wavelength of the low-reflectivity fiber grating 1 is matched with that of the high-reflectivity fiber grating 10, with a reflectivity of 50% and a reflection bandwidth of 0.25 nm. The center wavelength of the high-reflectivity fiber grating 10 is 2795 nm, with a reflectivity >99% and a reflection bandwidth of 0.35 nm. The first pump source 2 is a 976 nm multimode semiconductor laser with a maximum output power of 10 W. The second pump source 7 is a 1530 nm multimode semiconductor laser with a maximum output power of 1000 mW. The first and second pump couplers are dedicated pump couplers for the corresponding wavelengths. The fiber end caps 11 and 12 are fluoride glass end caps made of AlF3 material with an 8° bevel.

[0040] In this embodiment, the selection of single-frequency laser is achieved through a two-stage filtering mechanism: First-stage static filtering process: After starting the first pump source 2, the 976nm pump light is absorbed by the gain fiber 4, causing Er3+ to... 4 I 11 / 2 and 4 I 13 / 2 Population inversion is established between energy levels, resulting in spontaneous emission in the 2.8 μm band. The energy level transition relationship is as follows: Figure 2 As shown. Under the feedback effect of the low-reflectivity fiber grating 1 and the high-reflectivity fiber grating 10, only the longitudinal modes whose wavelengths fall within the reflection bandwidth range of the low-reflectivity fiber grating 1 can obtain sufficient feedback to form laser oscillation, thereby achieving the initial screening of the longitudinal modes.

[0041] Secondary dynamic filtering process: The second pump source 7 is activated, and the 1530nm pump light pumps Er3+ ions in the saturable absorbable fiber 9 from the ground state to the 4I13 / 2 energy level, increasing the particle population of this level and thus enhancing the fiber's absorption capacity for 2.8μm laser light (corresponding to…). 4 I 13 / 2 → 4 I 11 / 2 The absorption capacity of (leap) such as Figure 3As shown, when the laser light after primary filtering forms a standing wave field in the saturable absorber fiber 9, the loss is low at the intensity antinodes due to absorption saturation, while the loss is high at the intensity nodes. This creates periodic loss modulation in the fiber, equivalent to generating an equivalent refractive index / loss grating, i.e., a dynamic grating. The bandwidth of this dynamic grating is on the order of several MHz, comparable to the longitudinal mode spacing (tens of MHz) of a fiber laser several meters long. The bandwidth of the dynamic grating can be controlled by adjusting the power of the second pump light and the length of the saturable absorber fiber 9. When the bandwidth of the dynamic grating is less than half of the longitudinal mode spacing, a single longitudinal mode can be precisely selected from the longitudinal mode group filtered by the static grating composed of the low reflectivity fiber grating 1 and the high reflectivity fiber grating 10, while strongly suppressing adjacent longitudinal modes, thereby achieving stable single-mode operation.

[0042] Finally, a stable 2.8μm single-frequency laser is output from the first fiber end cap 11 at one end of the low-reflectivity fiber grating 1, while the residual 1530nm modulated pump light is output from the second fiber end cap 12 at one end of the high-reflectivity fiber grating 10.

[0043] The laser constructed in this embodiment achieves all-fiber 2.8μm single-frequency laser output, with a maximum output power of 800mW, a laser linewidth of less than 80kHz, a long-term power fluctuation root mean square (RMS) of less than 0.8%, and an optical signal-to-noise ratio of greater than 50dB. It can achieve continuous and stable operation without active temperature control and without mode switching phenomenon.

[0044] Example 2: Based on the technical solution provided in Embodiment 1, this embodiment further introduces a polarization-maintaining fiber grating to specifically realize a preferred embodiment capable of outputting highly stable linearly polarized single-frequency laser. This embodiment corresponds to the relevant descriptions in the technical solution regarding the use of a polarization-maintaining fiber grating and the realization of linearly polarized, narrow-linewidth output.

[0045] To achieve linearly polarized single-frequency output, both the low-reflectivity fiber grating 1 and the high-reflectivity fiber grating 10 in this embodiment are polarization-maintaining fiber gratings. During fabrication and selection, it is ensured that the center wavelength of the slow-axis reflection peak of the low-reflectivity fiber grating 1 falls precisely within the reflection bandwidth of the high-reflectivity fiber grating 10 and that the two are matched. The remaining fibers in the cavity (including the gain fiber 4, the saturable absorber fiber 9, and the connecting pigtail) are all polarization-maintaining fibers. During the splicing process, a precise polarization axis alignment process is used to ensure that the slow axis of all polarization-maintaining fibers is strictly aligned with the slow axis of the grating, thus constructing a complete polarization-maintaining linear resonant cavity. This ensures that laser oscillation is confined to the slow axis of the polarization-maintaining fibers, laying the foundation for obtaining pure linearly polarized output.

[0046] like Figure 4As shown, during operation, the first pump source 2 is first turned on, and its output 976nm pump light excites the gain fiber 4 to generate a broadband 2.8μm fluorescence. With feedback from the polarization-maintaining fiber gratings at both ends, only wavelength-matched light with a slow-axis polarization state can form effective oscillations, completing the initial screening of oscillation wavelengths and polarization states. Next, the second pump source 7 is turned on, and its output 1530nm modulated pump light is injected into the saturable absorber fiber 9 via the second pump coupler 8, significantly enhancing the fiber's absorption of the 2.8μm laser through the pump enhancement effect. Under the influence of the laser standing wave field formed within the cavity, this enhanced saturable absorber fiber 9 forms an ultra-narrowband dynamic grating in the slow-axis polarized light. This dynamic grating, in conjunction with the static polarization-maintaining fiber grating, ultimately precisely selects and locks a single longitudinal mode from the initially screened slow-axis polarized longitudinal mode group.

[0047] The laser constructed using the above embodiments not only fully possesses all the advantages of the all-fiber, compact structure, and stable single-frequency output achieved using a MHz-level dynamic grating described in Example 1, but also further achieves higher-order output characteristics. This laser achieves continuously tunable linearly polarized single-frequency laser output within the 0-500mW range, with a polarization extinction ratio better than 20dB. Thanks to the effective suppression of polarization mode competition and noise by the polarization-maintaining structure, the frequency stability of the output laser is further improved, with a measured linewidth narrower than 45kHz and a long-term power fluctuation root mean square value of less than 0.5%, exhibiting extremely high spectral purity and power stability. Finally, pure linearly polarized single-frequency laser light is output from the first fiber end cap 11.

[0048] The above description is merely a preferred embodiment of the technical solution of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A 2.8μm single-frequency fiber laser based on a pump-enhanced saturable absorber, characterized in that, Includes the resonant cavity body, pumping system, and output components; The resonant cavity body includes: a low-reflectivity fiber grating (1), a first pump coupler (3), and an Er³-doped fiber optic cable. + Fluoride gain fiber (4), pump light stripper (5), pump-enhanced saturable absorber filter (6) and high reflectivity fiber grating (10), wherein the low reflectivity fiber grating (1) and the high reflectivity fiber grating (10) constitute a linear resonant cavity. The pumping system includes a first pump source (2) and a second pump source (7). The first pump source (2) is a laser diode with an output wavelength of 976nm or 980nm, and its output pump light is injected into the Er³-doped laser through the first pump coupler (3). + Fluoride-gain fiber (4); the center wavelength of the second pump source (7) is located in the 1.5-1.6μm band, and is used to provide pre-pump to the pump-enhanced saturable absorber filter device (6); The pump-enhanced saturable absorber filter device (6) includes a second pump coupler (8) and an Er³-doped section. + The laser output from the second pump source (7) is injected into the saturable absorber fiber (9) via the second pump coupler (8). The output component includes a first fiber end cap (11) and a second fiber end cap (12). The first fiber end cap (11) is connected to the low reflectivity fiber grating (1) for outputting 2.8μm single-frequency laser, and the second fiber end cap (12) is connected to the high reflectivity fiber grating (10).

2. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The output power of the first pump source (2) is 0-100W; the output power of the second pump source (7) is 0-10W.

3. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The Er³ doping + Fluoride gain fiber (4) and the Er³-doped fiber + The fluoride saturable absorber optical fiber (9) is made of Er³ material. + AlF3, Er³ + InF3 or Er³ + : Any one or a combination of two of the following: ZrF4-BaF2-LaF3-AlF3-NaF optical fibers; the length of the gain fiber (4) is 10-500cm, Er³ + The doping concentration is 0.5-10 mol%; the length of the saturable absorbable optical fiber (9) is 50-1000 cm, Er³ + The doping concentration is 0.5-10 mol.

4. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The low reflectivity fiber grating (1) has a reflectivity of 10%-90% in the 2.8μm band and a reflection bandwidth of no more than 0.5nm; the high reflectivity fiber grating (10) has a reflectivity of no less than 90% in the 2.8μm band and a reflection bandwidth of 0.1-3.0nm.

5. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The low reflectivity fiber grating (1) and the high reflectivity fiber grating (10) are polarization-maintaining fiber gratings or non-polarization-maintaining fiber gratings; when a polarization-maintaining fiber grating is used, the slow-axis reflection peak of the low reflectivity fiber grating (1) is located within the reflection bandwidth of the high reflectivity fiber grating (10).

6. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The first pump coupler (3) is a pump coupler with a wavelength adapted to 976nm / 980nm pump light and 2.8μm signal light; the second pump coupler (8) is a pump coupler with a wavelength adapted to 1.5-1.6μm pump light and 2.8μm signal light.

7. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The pump-driven optical stripper (5) is used to strip and filter out the Er³-doped material. + Pump light from the first pump source (2) remaining after transmission of the fluoride gain fiber (4).

8. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The end face bevel angle of the first fiber end cap (11) and the second fiber end cap (12) is 4-10°, and their materials are selected from quartz, fluoride glass, chalcogenide glass or tellurite glass.

9. The 2.8μm single-frequency fiber laser according to claim 1, characterized in that, The laser outputs a single-frequency laser with a center wavelength of 2.7-2.9 μm, a linewidth of less than 100 kHz, and a root mean square value of less than 1% for long-term power fluctuation.

10. The 2.8μm single-frequency fiber laser according to claim 5, characterized in that, When the low reflectivity fiber grating (1) and the high reflectivity fiber grating (10) are polarization-maintaining fiber gratings, the laser outputs a linearly polarized single-frequency laser with a polarization extinction ratio of not less than 20dB and a linewidth of less than 50kHz.

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