System and method for directly pumping a single mode optical fiber

Abstract

The application discloses a system and method for directly pumping a single-mode optical fiber, and belongs to the technical field of single-mode laser fiber coupling. The system comprises a laser emission collimation module, a fast-axis focusing side coupling module and an optical fiber arrangement module arranged along an optical path. The laser emission collimation module comprises a linear array semiconductor laser, a fast-axis collimation mirror and a slow-axis collimation mirror, and is used for outputting collimated laser with controlled divergence angles in the fast-axis direction and the slow-axis direction. The fast-axis focusing side coupling module is used for focusing the collimated laser in the fast-axis direction, so that the laser in the fast-axis direction enters the fiber core along the side of the single-mode optical fiber. The optical fiber arrangement module is used for fixing the single-mode optical fiber, so that the axial direction of the single-mode optical fiber is parallel to the slow-axis direction, and the fiber core is located at the focal point position of the fast-axis focusing side coupling module. The application makes full use of the advantages of good beam quality of the semiconductor laser in the fast-axis direction, solves the problem of low pumping efficiency caused by mode mismatch in the traditional end-pumping mode through direct side coupling, and can be applied to the field of mid-infrared fiber lasers.

Description

Technical Field

[0001] This application relates to the field of single-mode laser fiber coupling technology, and more specifically to a system and method for directly pumping a single-mode fiber. Background Technology

[0002] Semiconductor laser pumping technology is one of the key technologies driving the development of high-power, high-energy, and high-beam-quality fiber lasers. As fiber lasers evolve towards higher performance, higher requirements are placed on the brightness, efficiency, and mode-matching characteristics of the pump source.

[0003] Semiconductor lasers and fiber lasers differ significantly in gain structure, resonant cavity design, and pumping mechanism, making mode matching between them a technical challenge. To improve the pumping efficiency of semiconductor lasers, multimode fiber and double-clad fiber are widely used. Multimode fiber increases the core diameter to accommodate higher-power pump light, while double-clad fiber uses an inner cladding structure to confine the pump light within the cladding, gradually coupling it into the core. These two approaches effectively improve the efficiency of semiconductor laser-pumped multimode fiber lasers.

[0004] However, in applications requiring efficient single-mode pumping, such as semiconductor laser-pumped mid-infrared fiber lasers, the above solutions suffer from a sharp decline in efficiency. Single-mode fibers have extremely small core sizes, typically only a few micrometers to tens of micrometers, making them highly sensitive to the mode characteristics of the incident beam. While multimode and double-clad fibers can improve coupling power, they cannot solve the matching problem between the pump light mode and the fundamental mode of the single-mode fiber, resulting in the pump light being difficult to efficiently enter the core and be absorbed by the gain medium.

[0005] To address this issue, existing technologies attempt to use mode modulation techniques to shape the beam of semiconductor lasers, such as spatial filtering. Spatial filtering physically removes higher-order mode components from the beam, improving beam quality and bringing it closer to the diffraction limit. However, this mode optimization method comes at the cost of power; the filtering process leads to significant power loss, and the overall pump efficiency is not substantially improved.

[0006] Therefore, there is an urgent need for a system and method for directly pumping single-mode optical fibers to solve the technical problem of low coupling efficiency of single-mode optical fibers in the existing technology. Summary of the Invention

[0007] The purpose of this application is to provide a system and method for directly pumping single-mode optical fiber, which can solve at least one of the technical problems mentioned above. The specific solution is as follows: This application provides a system for directly pumping a single-mode optical fiber, comprising: [the following components are arranged along the optical path:] The laser emission collimation module includes a linear array semiconductor laser, a fast-axis collimating lens, and a slow-axis collimating lens; The linear array semiconductor laser is used to generate a linear array semiconductor laser having a fast axis direction and a slow axis direction; the fast axis collimating lens and the slow axis collimating lens are used to collimate the linear array semiconductor laser in the fast axis direction and the slow axis direction respectively, so as to output a collimated laser with controlled divergence angle in the fast axis direction and the slow axis direction. A fast-axis focusing side coupling module is disposed on the output optical path of the laser emission collimation module, and is used to focus the collimated laser in the fast-axis direction so that the laser in the fast-axis direction enters the single-mode fiber core along the side of the single-mode fiber. The fiber optic deployment module is used to fix the single-mode fiber so that the axis of the single-mode fiber is parallel to the slow axis direction of the linear array semiconductor laser, and the core of the single-mode fiber is located at the focal point of the fast axis focusing side coupling module, so as to form a side coupling region for pump light incident on the side of the single-mode fiber.

[0008] Furthermore, the fast-axis focusing-side coupling module includes a cylindrical lens; The cylindrical lens is used to image the effective gain region of the linear array semiconductor laser output structure in the fast axis direction onto the core of the single-mode fiber.

[0009] Furthermore, the generatrix direction of the cylindrical lens is parallel to the slow axis direction of the linear array semiconductor laser. The cylindrical lens is used to apply optical power to the collimated laser in the fast axis direction, but not to apply optical power to the collimated laser in the slow axis direction.

[0010] Furthermore, the imaging magnification of the fast-axis imaging optical path is determined by the ratio of the focal length of the cylindrical lens to the focal length of the fast-axis collimating lens. The imaging magnification is configured so that the image size of the effective gain region of the linear array semiconductor laser in the fast-axis direction matches the core size of the single-mode fiber.

[0011] Furthermore, it also includes at least one reflection re-injection module; The reflection and re-injection module is disposed on the pump light emission path of the single-mode fiber and is used to reflect the pump light that has passed through the core of the single-mode fiber, so that the reflected pump light re-enters the core of the single-mode fiber from the side of the single-mode fiber.

[0012] Furthermore, the reflection re-injection module includes: a beam shaping unit and a beam reflection unit; The beam shaping unit includes at least one set of cylindrical lens groups, and each set of cylindrical lens groups includes two cylindrical lenses, which are used to collimate and focus the pump light that passes through the core of the single-mode fiber. The beam reflection unit includes at least one set of reflectors, each set of which includes two reflectors. The reflectors are arranged along the length of the single-mode fiber on the pump light emission path of the single-mode fiber, and are used to reflect the pump light that has passed through the core of the single-mode fiber back to the side of the single-mode fiber.

[0013] Furthermore, the number of laser emission collimation modules is N, and the N laser emission collimation modules are arranged along the axial direction of the single-mode fiber to increase the pump light power injected into the core of the single-mode fiber; where N is an integer greater than or equal to 2.

[0014] Furthermore, the side coupling region is provided with an antireflective film layer; the side coupling region is ground smooth to form a planar coupling window.

[0015] This application also provides a method for directly pumping a single-mode optical fiber, comprising the following steps: Acquire a linear semiconductor laser array, wherein the linear semiconductor laser array has a fast axis direction and a slow axis direction; The linear array semiconductor laser is collimated in both the fast and slow axis directions to obtain a collimated laser with controlled divergence angles in both directions. The collimated laser is focused in the fast axis direction; The focused laser is coupled from the side of the single-mode fiber into the core of the single-mode fiber; The pump light entering the fiber core is extended and propagated along the axial direction of the single-mode fiber in the slow axis direction.

[0016] Furthermore, it also includes the following steps: The pump light that has passed through the core of the single-mode fiber is reflected, so that the reflected pump light re-enters the core from the side of the single-mode fiber.

[0017] Compared with the prior art, the above-described solutions of this application have at least the following beneficial effects: 1. This application discloses a system and method for directly pumping single-mode fiber. The system focuses a collimated laser beam along the fast axis using a fast-axis focusing side-coupling module, allowing the laser to directly enter the fiber core from the side of the single-mode fiber. This fully utilizes the inherent characteristic of semiconductor lasers where the beam quality along the fast axis is close to the diffraction limit, precisely injecting high-brightness laser light into the single-mode fiber core, which has a diameter of only a few micrometers. A fast-axis collimating lens and a cylindrical lens constitute the fast-axis imaging optical path. By configuring the imaging magnification, the image size of the effective gain region of the linear array semiconductor laser along the fast axis is matched to the size of the single-mode fiber core. This fundamentally solves the problem of low pumping efficiency caused by the mismatch between the pump light mode and the fundamental mode of the single-mode fiber in traditional end-pumping methods.

[0018] 2. This application discloses a system and method for directly pumping single-mode optical fiber. By employing a reflection and re-injection module, the pump light exiting the single-mode fiber core is reflected, allowing the reflected pump light to re-enter the core from the side, forming a multiple-pass path. Two sets of cylindrical lenses in the beam shaping unit collimate the pump light exiting the core and refocus the reflected pump light, ensuring high-quality multiple coupling. The two mirrors are positioned opposite each other to achieve a compact optical path layout, significantly increasing the effective pump light length within a limited fiber length. This significantly improves the core's absorption efficiency of the pump light and avoids energy waste caused by incomplete absorption during a single pass.

[0019] 3. This application discloses a system and method for directly pumping a single-mode fiber. It utilizes a fast-axis collimating lens and a slow-axis collimating lens to independently control two directions: the laser in the fast-axis direction is focused into the fiber core to resolve the mode matching problem, while the laser in the slow-axis direction remains collimated and propagates along the fiber axis to form distributed pumping. Since the spot diameter after collimation in the slow-axis direction is smaller than the actual length of the single-mode fiber, multiple laser emission collimating modules can be arranged along the fiber axis to linearly increase the pump power injected into the fiber core without affecting the coupling efficiency in the fast-axis direction. This application achieves both high-efficiency coupling and power expansion with a simple optical structure, exhibiting good engineering feasibility and cost advantages. Attached Figure Description

[0020] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. It is obvious that the drawings described below are merely some embodiments of this application, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings: Figure 1 This is a schematic diagram of the first structure of a laser emission collimation module for a system that directly pumps a single-mode fiber, as provided in an embodiment of this application.

[0021] Figure 2 This is a schematic diagram of the second structure of a laser emission collimation module for a system that directly pumps a single-mode fiber, as provided in an embodiment of this application.

[0022] Figure 3 This is a schematic diagram of the structure of multiple laser emission collimation modules in a system for directly pumping a single-mode fiber, as provided in an embodiment of this application.

[0023] Figure 4 This is a schematic diagram of the reflection and re-injection module structure of a system for directly pumping single-mode optical fiber, provided in an embodiment of this application.

[0024] Figure 5This application provides a schematic diagram of the structure of a system for directly pumping a single-mode fiber in an infrared fiber laser.

[0025] Explanation of reference numerals in the attached figures: Laser emission collimation module 100, first laser emission collimation module 101, second laser emission collimation module 102, third laser emission collimation module 103, fourth laser emission collimation module 104, fifth laser emission collimation module 105, linear array semiconductor laser LD, fast-axis collimating lens FC, slow-axis collimating lens SC, fiber optic FB, first cylindrical lens FL1, first reflection and re-injection module 201, second reflection and re-injection module 202, second cylindrical lens FL2, third cylindrical lens FL3, fourth cylindrical lens FL 4. Fifth cylindrical lens FL5, first reflecting mirror M1, second reflecting mirror M2, third reflecting mirror M3, fourth reflecting mirror M4, cavity reflecting mirror M5, sixth reflecting mirror M6, seventh reflecting mirror M7, eighth reflecting mirror M8, ninth reflecting mirror M9, output coupling mirror M10, first linear array semiconductor laser LD1, second linear array semiconductor laser LD2, third linear array semiconductor laser LD3, 976nm semiconductor laser LD4, first aspherical coupling lens L1, second aspherical coupling lens L2, dichroic mirror DM. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this application clearer, the application will be further described in detail below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0027] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a product or device that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a product or device. Without further limitation, an element defined by the phrase "comprising one" does not exclude the presence of other identical elements in the product or device that includes that element.

[0028] The embodiments of this application are described in detail below with reference to the accompanying drawings.

[0029] Example 1: like Figure 1 As shown, this application provides a system for directly pumping a single-mode fiber FB, such as... Figure 1 and Figure 2As shown, this is a single-side coupling structure, which includes: a laser emission collimation module 100, a fast-axis focusing side coupling module, and an optical fiber FB deployment module arranged along the optical path.

[0030] The laser emission collimation module 100 includes a linear array semiconductor laser LD, a fast-axis collimating lens FC, and a slow-axis collimating lens SC. The linear array semiconductor laser LD is used to generate a linear array semiconductor laser with fast-axis and slow-axis directions. The fast-axis collimating lens FC and the slow-axis collimating lens SC are used to collimate the linear array semiconductor laser in the fast-axis and slow-axis directions, respectively, so as to output a collimated laser with controlled divergence angles in the fast-axis and slow-axis directions.

[0031] The fast-axis focusing side coupling module is set on the output optical path of the laser emission collimation module 100. It is used to focus the collimated laser in the fast-axis direction so that the laser in the fast-axis direction enters the single-mode fiber FB core along the side of the single-mode fiber FB.

[0032] The fiber optic FB (fiber optic fiber) deployment module is used to fix the single-mode fiber FB, so that the axis of the single-mode fiber FB is parallel to the slow axis direction of the linear array semiconductor laser LD, and the fiber core of the single-mode fiber FB is located at the focal point of the fast axis focusing side coupling module, so as to form a side coupling region for pump light incident on the side of the single-mode fiber FB.

[0033] In the technical solution of this application embodiment, the linear array semiconductor laser LD is composed of 19 individual laser tubes. The effective gain region size of each individual laser tube is 2μm (fast axis direction) × 100μm (slow axis direction), the divergence angle in the fast axis direction is approximately 30°, the divergence angle in the slow axis direction is 10°, and the spacing between adjacent individual tubes is 500μm. The effective gain region, divergence angle, and spacing between adjacent individual tubes of the linear array semiconductor laser LD all depend on the manufacturing process of the linear array semiconductor laser LD and have no impact on the technical solution in this application embodiment. This is only used as an example for illustration and does not constitute a limitation on the technical solution of this application.

[0034] In the technical solution of this application embodiment, a fast-axis collimating lens FC and a slow-axis collimating lens SC are sequentially arranged in the output optical path of the linear array semiconductor laser LD. The fast-axis collimating lens FC is used to collimate the laser with a divergence angle of 30° in the fast-axis direction, and the slow-axis collimating lens SC is used to collimate the laser with a divergence angle of 10° in the slow-axis direction. After processing by the fast-axis collimating lens FC and the slow-axis collimating lens SC, a collimated laser with controlled divergence angles in both the fast and slow axes is output. In this application embodiment, the divergence angle in the fast-axis direction is relatively large at approximately 30°. If the laser were first collimated by the slow-axis collimating lens SC and then by the fast-axis collimating lens FC, a larger fast-axis collimating lens FC would be required for collimation. Therefore, this application embodiment adopts a technical solution that sequentially arranges the fast-axis collimating lens FC and the slow-axis collimating lens SC.

[0035] The numerical aperture of the fast-axis collimating lens FC needs to cover a 30° divergence angle along the fast axis of the linear array semiconductor laser LD to ensure that the entire beam is effectively collimated. The focal length and aperture of the slow-axis collimating lens SC are configured to receive beams with a 10° divergence angle along the slow axis and to collimate them in that direction. The specific parameters of the slow-axis collimating lens SC can be conventionally selected based on the beam size and divergence angle along the slow axis of the linear array semiconductor laser LD, with the basic requirement being the ability to receive the entire beam spot.

[0036] In the technical solution of this application embodiment, a fast-axis focusing side coupling module is provided in the output optical path of the laser emission collimation module 100. In this embodiment, the fast-axis focusing side coupling module includes a first cylindrical lens FL1. The first cylindrical lens FL1 is used to focus the collimated laser in the fast-axis direction, so that the laser in the fast-axis direction enters the fiber core of the single-mode fiber FB along the side of the single-mode fiber FB. Specifically, the generatrix direction of the first cylindrical lens FL1 is parallel to the slow-axis direction of the linear array semiconductor laser LD, so that the first cylindrical lens FL1 only applies optical power to the collimated laser in the fast-axis direction to achieve focusing, so that the laser in the fast-axis direction is accurately focused into the fiber core; no optical power is applied to the collimated laser in the slow-axis direction, thereby maintaining the collimation characteristics of the beam in the slow-axis direction to propagate along the fiber FB axis.

[0037] In the technical solution of this application embodiment, the first cylindrical lens FL1 and the fast-axis collimating lens FC in the laser emission collimation module 100 are combined to form a fast-axis imaging optical path. The imaging magnification of the imaging optical path is determined by the ratio of the focal length of the cylindrical lens to the focal length of the fast-axis collimating lens FC. The imaging magnification is configured so that the effective gain region of the linear array semiconductor laser LD in the fast-axis direction is imaged onto the core position of the single-mode fiber FB, and the image size after imaging matches the core size of the single-mode fiber FB, ensuring that the pump light enters the core efficiently.

[0038] In a linear array semiconductor laser (LD), the effective gain region of each single-tube laser has a size of 2 μm along the fast axis. The core diameter of the single-mode fiber (FB) is 8 μm. To match the image size of the effective gain region along the fast axis with the core size, a magnification scheme with an imaging ratio of 1:4 is adopted. That is, the 2 μm effective gain region along the fast axis is magnified and imaged into an 8 μm spot, making it consistent with the core diameter.

[0039] According to the Gaussian imaging formula, the imaging magnification is determined by the ratio of the focal length of the first cylindrical lens FL1 to the focal length of the fast-axis collimating lens FC. To achieve an imaging magnification of 1:4, the ratio of the focal length of the first cylindrical lens FL1 to the focal length of the fast-axis collimating lens FC needs to be configured as 4:1. In this embodiment, the focal length of the fast-axis collimating lens FC is selected as 0.375mm, and the focal length of the cylindrical lens is selected as 1.5mm, with a ratio of exactly 1:4, satisfying the imaging magnification requirement.

[0040] In the technical solution of this application embodiment, the fiber optic FB (FB) deployment module is used to fix the single-mode fiber FB, making the axis of the single-mode fiber FB parallel to the slow axis direction of the linear array semiconductor laser LD, and positioning the core of the single-mode fiber FB at the focal point of the fast axis focusing side coupling module, thereby forming a side coupling region for pump light incident on the side of the single-mode fiber FB. The laser light, focused in the fast axis direction, enters the core of the single-mode fiber FB from this side coupling region. The pump light entering the core extends and propagates along the axis of the single-mode fiber FB in the slow axis direction, forming a pumping region distributed along the length of the single-mode fiber FB.

[0041] To improve pump power, this application provides a preferred technical solution, which can employ multiple laser emission collimation modules 100. For example... Figure 3 As shown, N laser emission collimation modules 100 are arranged along the axial direction of the single-mode fiber FB, where N is an integer greater than or equal to 2. In this embodiment, N is 5, representing the first laser emission collimation module 101, the second laser emission collimation module 102, the third laser emission collimation module 103, the fourth laser emission collimation module 104, and the fifth laser emission collimation module 105. Since the diameter of the light spot after collimation in the slow axis direction is smaller than the actual usable length of the single-mode fiber FB, arranging the five laser emission collimation modules 100 along the axial direction can increase the pump power injected into the core of the single-mode fiber FB, achieving distributed side pumping.

[0042] This application provides a system for directly pumping a single-mode fiber FB (fiber optic cable). It employs a fast-axis focusing side-coupling structure to focus the fast-axis laser beam from a linear semiconductor laser LD (laser array) with near-diffraction-limited beam quality, allowing the fast-axis laser to directly enter the fiber core from the side of the single-mode fiber FB. This structure fully utilizes the inherent characteristics of semiconductor lasers—large divergence angle and high beam quality along the fast axis—effectively matching the high-brightness laser beam along the fast axis to the tiny core of the single-mode fiber FB, fundamentally solving the problem of low coupling efficiency caused by mode mismatch in end-face pumping. Compared to complex spatial beam combining and beam shaping schemes, this application only requires a fast-axis collimating lens FC, a slow-axis collimating lens SC, and a first cylindrical lens FL1 to achieve efficient side-coupling. It has fewer optical components, a simpler optical path, and lower assembly and adjustment difficulty, offering excellent engineering feasibility and cost advantages. This simple optical structure solves the technical problem of low pumping efficiency caused by mode mismatch in traditional end-face pumping methods, achieving efficient direct pumping of a single-mode fiber FB by a semiconductor laser.

[0043] Example 2: Example 2 has the same laser emission collimation module 100, fast-axis focusing side coupling module, and fiber FB deployment module as Example 1. The difference is that, based on Example 1, it further includes a reflection and re-injection module to improve pump light absorption efficiency. The specific structure is as follows: Figure 4 As shown. The following only describes in detail the unique structure and optical path of the reflection and re-injection module; the similarities with Embodiment 1 will not be repeated.

[0044] When a semiconductor linear array directly pumps a single-mode fiber (FB), although the power injected into the fiber core can be increased, the absorption efficiency of the pump light by the fiber core is limited during a single pass due to the shortened operating distance. To solve this problem, the technical solution of this application embodiment sets a reflection and re-injection module on the pump light emission path of the single-mode fiber FB.

[0045] In the technical solution of this application embodiment, there are two sets of reflection re-injection modules, each set of reflection re-injection modules is the same, namely the first reflection re-injection module 201 and the second reflection re-injection module 202. Each set of reflection re-injection modules includes a beam shaping unit and a beam reflection unit.

[0046] The beam shaping unit includes at least one set of cylindrical lens groups, each set comprising two cylindrical lenses. The first cylindrical lens is positioned outside the pump light emitting end of the single-mode fiber (FB) and is used to collimate the pump light exiting the FB core in the fast axis direction. The second cylindrical lens is positioned in the reflected light path and is used to refocus the reflected pump light in the fast axis direction, enabling it to re-enter the FB core from the side. In the technical solution of this application embodiment, the second cylindrical lens FL2 and the fourth cylindrical lens FL4 are used to collimate the pump light exiting the FB core in the fast axis direction; the third cylindrical lens FL3 and the fifth cylindrical lens FL5 are used to refocus the reflected pump light in the fast axis direction.

[0047] The beam shaping unit in the reflection and re-injection module includes two sets of cylindrical lenses: the first cylindrical lens collimates the pump light exiting the fiber core along the fast axis, restoring the diverging beam to a collimated state for precise guidance by the reflector; the second cylindrical lens refocuses the reflected pump light along the fast axis, ensuring it re-enters the fiber core from the side with good focus. This complete optical path design of collimation, reflection, and focusing ensures that the coupling quality is not degraded during multiple reflections.

[0048] The beam reflecting unit includes at least one set of mirror groups, each set comprising two mirrors. The mirror groups are positioned along the length of the single-mode fiber (FB) along the pump light emission path of the FB, reflecting the pump light exiting the FB core back to the side of the FB. In the technical solution of this application embodiment, both mirrors are 45° mirrors, positioned opposite each other outside the pump light emission end of the FB, to create a pumping path that passes through the FB core twice. That is, the first mirror M1, the second mirror M2, the third mirror M3, and the fourth mirror M4 are all 45° mirrors.

[0049] The technical solution of this application uses two 45° reflectors positioned opposite each other on the outside of the fiber optic FB output end. This allows the pump light to return to the fiber FB after two reflections in a direction parallel to the original optical path. The optical path layout is compact, occupies little space, and is easy to integrate into the system. The reflectors are arranged along the length of the fiber FB, and the number of reflections can be increased as needed to form a multi-pass pumping path.

[0050] The proposed multi-reflection optimized structure in this application introduces a reflection-reinjection module. This module reflects the pump light exiting the single-mode fiber (FB) core, allowing the reflected pump light to re-enter the core from the FB side. Multiple reflections create multiple paths for the pump light within the core, effectively increasing the interaction distance between the pump light and the core gain medium. Taking two reflections as an example, the effective interaction length of the pump light within the core increases, thus improving pump absorption efficiency. The pump light exiting the FB, which would otherwise be lost through dissipation, is recovered and re-injected into the core via a mirror array, achieving energy recycling. This energy recovery mechanism fully utilizes limited pump power, significantly improving overall optical-to-optical conversion efficiency and reducing the system's power requirements for the pump source.

[0051] The reflection and re-injection module adopts a modular design, with each set of cylindrical lens groups and mirror groups forming an independent reflection unit. Multiple reflection and re-injection modules can be arranged along the fiber FB length to achieve multiple recovery and re-injection of pump light, with absorption efficiency increasing linearly with the number of reflections, offering high design flexibility.

[0052] Example 3: Example 3 has the same laser emission collimation module 100, fast-axis focusing side coupling module, and fiber FB deployment module as Examples 1 and 2. Example 3 applies the system of directly pumping a single-mode fiber FB to a 3.5μm mid-infrared fiber FB laser, and the specific structure is as follows. Figure 5 As shown. The similarities with Examples 1 and 2 will not be repeated.

[0053] In the technical solution of this application embodiment, the single-mode fiber FB is an erbium-doped fluoride fiber FB with a core diameter of 16.5μm, a numerical aperture of 0.12, a cladding size of 240μm×260μm, a numerical aperture of 0.4, and a fiber FB length of 18cm.

[0054] The three direct pumping systems and the three reflection re-injection modules from Embodiment 1 are used, such as... Figure 5 As shown in the figure, not all systems and modules are fully represented. Each system uses a 1973nm linear array semiconductor laser (LD) as the pump source, namely the first linear array semiconductor laser LD1, the second linear array semiconductor laser LD2, and the third linear array semiconductor laser LD3, arranged along the axis of the erbium-doped fluoride fiber (FB) and coupled into the fiber core from the side of the FB. Each system employs a reflection-reinjection module to improve pump absorption efficiency. The cylindrical lens in the reflection-reinjection module has been omitted. The sixth mirror M6, the seventh mirror M7, the eighth mirror M8, and the ninth mirror M9 are all 45° mirrors.

[0055] The technical solution of this application directly replaces the complex fiber FB laser pump source in traditional schemes by using multiple direct pumping systems as the main pump source. The 1973nm wavelength precisely matches the absorption peak of erbium ions in erbium-doped fluoride fiber (FB), effectively exciting the upconversion process. Semiconductor lasers are small in size, highly efficient, and low in cost, greatly simplifying the entire laser system. The pump light is efficiently coupled from the side of the erbium-doped fluoride fiber (FB) into the fiber core. Side coupling avoids the mode mismatch problem of end-face pumping, enabling multimode semiconductor lasers to be effectively injected into the single-mode fiber core, significantly improving pump efficiency. Each 1973nm pumping system uses a reflection and re-injection module, allowing the pump light to form multiple paths within the fiber core. In erbium-doped fluoride fiber (FB), multiple reflections effectively increase the interaction distance between the pump light and the fiber core, ensuring sufficient absorption of the pump light and avoiding incomplete absorption due to the limited length of the fiber FB.

[0056] This embodiment also includes an end-face pump optical path as an auxiliary pump source. The laser output from the 976nm semiconductor laser LD4 is coupled into the end face of the erbium-doped fluoride fiber FB through two aspherical coupling lenses: a first aspherical coupling lens L1 and a second aspherical coupling lens L2. The first aspherical coupling lens L1 has a focal length of 10mm, and the second aspherical coupling lens L2 has a focal length of 20mm.

[0057] The erbium-doped fluoride fiber FB has cavity mirrors M5 and output couplers M10 directly mounted at both ends, forming a laser resonant cavity. Cavity mirror M5 has high reflectivity for 3.5μm laser wavelengths (HR@3.5μm) and high transmittance for 976nm laser wavelengths (HT@976nm); output coupler M10 has a reflectivity of 85% for 3.5μm laser wavelengths (R=85%@3.5μm) and high transmittance for 976nm laser wavelengths (HT@976nm).

[0058] Under the combined action of two pump lights—a first linear array semiconductor laser LD1 with a wavelength of 1973 nm, a second linear array semiconductor laser LD2, a third linear array semiconductor laser LD3, and a semiconductor laser LD4 with a wavelength of 976 nm—an erbium-doped fluoride fiber FB generates 3.5 μm mid-infrared laser light. The output laser light is split by a 45° dichroic mirror DM. The dichroic mirror has high reflectivity for the 3.5 μm laser (HR@3.5 μm) and high transmittance for the 976 nm laser (HT@976 nm), thus achieving separation of the 3.5 μm signal light from the residual 976 nm pump light. In this embodiment, the dichroic mirror is a 45° dichroic mirror.

[0059] The technical solution of this application embodiment achieves high-performance 3.5μm mid-infrared fiber FB laser output with a compact system structure, and can also realize laser output in other bands according to actual needs. Direct pumping with a semiconductor laser replaces pumping with a fiber FB laser, significantly reducing system size and cost; side coupling and multiple reflection structures ensure high pumping efficiency, laying a technical foundation for improving mid-infrared laser output power and providing a new technical path for the miniaturization and practical application of lasers.

[0060] In the above embodiments 1-3, to reduce the interface reflection loss generated when the pump light is incident from the side of the single-mode fiber FB, an antireflection coating is provided in the side coupling region of the single-mode fiber FB. The antireflection coating has a reflectivity of less than 0.5% at the output wavelength of the linear array semiconductor laser LD, such as a wavelength of 1973nm.

[0061] Meanwhile, depending on actual needs, the side coupling area of ​​the single-mode fiber FB can be ground flat to form a planar coupling window, so as to improve the optical contact conditions of the coupling interface and further improve the coupling efficiency.

[0062] Example 4: The technical solution of this application embodiment also provides a method for directly pumping a single-mode fiber FB, including the following steps: S1. Obtain a linear array semiconductor laser, which has a fast axis direction and a slow axis direction.

[0063] A linear array semiconductor laser (LD) is provided, which outputs linear semiconductor laser light with fast and slow axis directions. The fast axis direction corresponds to the thickness direction of the emitting region of the semiconductor laser, and the beam quality is close to the diffraction limit; the slow axis direction corresponds to the width direction of the emitting region, and the beam quality is relatively poor but the divergence angle is small.

[0064] S2. Collimate the linear array semiconductor laser in the fast axis and slow axis directions respectively to obtain a collimated laser with controlled divergence angles in the fast axis and slow axis directions.

[0065] Collimation processing was performed on the linear array semiconductor laser in both the fast and slow axis directions. A fast-axis collimating lens FC was used to collimate the laser with a large divergence angle in the fast axis direction, while a slow-axis collimating lens SC was used to collimate the laser in the slow axis direction, resulting in collimated lasers with controlled divergence angles in both directions.

[0066] S3. Focus the straight laser along the fast axis.

[0067] The collimated laser is focused along the fast axis. A cylindrical lens is used to apply optical power to the collimated laser along the fast axis. The generatrix of the cylindrical lens is parallel to the slow axis of the linear semiconductor laser LD, so that the cylindrical lens only focuses the beam along the fast axis and does not apply optical power to the beam along the slow axis.

[0068] The cylindrical lens and the fast-axis collimating lens FC are combined to form the imaging optical path in the fast-axis direction. The imaging magnification is determined by the ratio of the focal length of the cylindrical lens to the focal length of the fast-axis collimating lens FC, so that the effective gain region of the linear array semiconductor laser LD in the fast-axis direction is imaged to the core position of the single-mode fiber FB, and the image size after imaging matches the core size of the single-mode fiber FB.

[0069] S4. The focused laser is coupled from the side of the single-mode fiber FB into the core of the single-mode fiber FB.

[0070] The focused laser beam is coupled from the side of the single-mode fiber (FB) into the core of the FB. The FB is fixed by a fiber optic FB deployment module, ensuring that its axis is parallel to the slow axis of the linear semiconductor laser (LD), and that the FB core is located at the focal point of the cylindrical lens, thus forming a side coupling region for pump light incidence on the side of the FB.

[0071] S5 causes the pump light entering the fiber core to propagate along the axial direction of the single-mode fiber FB in the slow axis direction.

[0072] This causes the pump light entering the fiber core to propagate along the slow axis of the single-mode fiber (FB) in the axial direction, forming a pumping region distributed along the length of the single-mode fiber (FB).

[0073] This application embodiment also provides a preferred technical solution. After step S5 above, in order to further improve the pump light absorption efficiency, the method for directly pumping a single-mode fiber FB further includes the following steps: S6. The pump light that has been transmitted through the core of the single-mode fiber FB is reflected, so that the reflected pump light enters the core of the single-mode fiber FB again from the side.

[0074] A reflection and re-injection module is installed along the pump light emission path of a single-mode fiber (FB). The reflection and re-injection module includes a beam shaping unit and a beam reflection unit. The first cylindrical lens in the beam shaping unit collimates the pump light exiting the fiber core along the fast axis. A mirror in the beam reflection unit reflects the collimated pump light. The second cylindrical lens in the beam shaping unit refocuses the reflected pump light along the fast axis, allowing it to re-enter the fiber core from the side of the single-mode fiber (FB).

[0075] Step S6 can be repeated multiple times to create multiple pumping paths in the fiber core, thereby increasing the effective interaction length between the pump light and the fiber core and significantly improving the absorption efficiency of the pump light by the fiber core.

[0076] Finally, it should be noted that the various embodiments in this specification are described in a progressive manner, with each embodiment focusing on its differences from other embodiments. Similar or identical parts between embodiments can be referred to interchangeably. For the systems or apparatus disclosed in the embodiments, since they correspond to the methods disclosed in the embodiments, the descriptions are relatively simple, and relevant parts can be referred to the method section.

[0077] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of this application.

Claims

1. A system for directly pumping single-mode optical fiber, characterized in that, Including those set along the optical path: The laser emission collimation module includes a linear array semiconductor laser, a fast-axis collimating lens, and a slow-axis collimating lens; The linear array semiconductor laser is used to generate a linear array semiconductor laser having a fast axis direction and a slow axis direction; the fast axis collimating lens and the slow axis collimating lens are used to collimate the linear array semiconductor laser in the fast axis direction and the slow axis direction respectively, so as to output a collimated laser with controlled divergence angle in the fast axis direction and the slow axis direction. A fast-axis focusing side coupling module is disposed on the output optical path of the laser emission collimation module, and is used to focus the collimated laser in the fast-axis direction so that the laser in the fast-axis direction enters the single-mode fiber core along the side of the single-mode fiber. The fiber optic deployment module is used to fix the single-mode fiber so that the axis of the single-mode fiber is parallel to the slow axis direction of the linear array semiconductor laser, and the core of the single-mode fiber is located at the focal point of the fast axis focusing side coupling module, so as to form a side coupling region for pump light incident on the side of the single-mode fiber.

2. The system according to claim 1, characterized in that, The fast-axis focusing-side coupling module includes a cylindrical lens; The cylindrical lens is used to image the effective gain region of the linear array semiconductor laser output structure in the fast axis direction onto the core of the single-mode fiber.

3. The system according to claim 2, characterized in that, The generatrix direction of the cylindrical lens is parallel to the slow axis direction of the linear array semiconductor laser. The cylindrical lens is used to apply optical power to the collimated laser in the fast axis direction, but not to apply optical power to the collimated laser in the slow axis direction.

4. The system according to claim 2, characterized in that, The imaging magnification of the fast-axis imaging optical path is determined by the ratio of the focal length of the cylindrical lens to the focal length of the fast-axis collimating lens. The imaging magnification is configured so that the image size of the effective gain region of the linear array semiconductor laser in the fast-axis direction matches the core size of the single-mode fiber.

5. The system according to claim 1, characterized in that, It also includes at least one reflection re-injection module; The reflection and re-injection module is disposed on the pump light emission path of the single-mode fiber and is used to reflect the pump light that has passed through the core of the single-mode fiber, so that the reflected pump light re-enters the core of the single-mode fiber from the side of the single-mode fiber.

6. The system according to claim 5, characterized in that, The reflection and re-injection module includes: a beam shaping unit and a beam reflection unit; The beam shaping unit includes at least one set of cylindrical lens groups, and each set of cylindrical lens groups includes two cylindrical lenses, which are used to collimate and focus the pump light that passes through the core of the single-mode fiber. The beam reflection unit includes at least one set of reflectors, each set of which includes two reflectors. The reflectors are arranged along the length of the single-mode fiber on the pump light emission path of the single-mode fiber, and are used to reflect the pump light that has passed through the core of the single-mode fiber back to the side of the single-mode fiber.

7. The system according to claim 1, characterized in that, The number of laser emission collimation modules is N, and the N laser emission collimation modules are arranged along the axial direction of the single-mode fiber to increase the pump light power injected into the core of the single-mode fiber; where N is an integer greater than or equal to 2.

8. The system according to claim 1, characterized in that, The side coupling region is provided with an anti-reflective coating layer; the side coupling region is ground smooth to form a planar coupling window.

9. A method for directly pumping a single-mode optical fiber based on the system according to any one of claims 1-8, characterized in that, Includes the following steps: Acquire a linear array semiconductor laser, wherein the linear array semiconductor laser has a fast axis direction and a slow axis direction; The linear array semiconductor laser is collimated in both the fast and slow axis directions to obtain a collimated laser with controlled divergence angles in both directions. The collimated laser is focused in the fast axis direction; The focused laser is coupled from the side of the single-mode fiber into the core of the single-mode fiber; The pump light entering the fiber core is extended and propagated along the axial direction of the single-mode fiber in the slow axis direction.

10. The method according to claim 9, characterized in that, It also includes the following steps: The pump light that has passed through the core of the single-mode fiber is reflected, so that the reflected pump light re-enters the core from the side of the single-mode fiber.

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