Method for improving absorption and conversion efficiency of pump light

Abstract

The invention discloses a method for improving pump light absorption and conversion efficiency, a gain optical fiber adopts a three-core optical fiber structure and comprises two main fiber cores and an auxiliary fiber core, and the three fiber cores are separated at the middle section of the gain optical fiber and are gradually close to each other at the position close to the output end to form a conical coupling area; a pump light recoverer and a reverse optical fiber combiner are arranged; the pump light recoverer comprises an input signal fiber, a conical area, an output signal fiber and a pump arm, the fiber core diameter of the input signal fiber is smaller than that of the output signal fiber, the input signal fiber is connected with the output signal fiber through the conical area, the number of the pump fibers of the pump light recoverer is larger than or equal to one, and the pump fibers are combined with the output signal fiber and then connected with the gain fiber; the reverse optical fiber beam combiner is an (N + 1) * 1 beam combiner, the (N + 1) end is connected with the laser output head and N pumping fibers, N is greater than 2, and the beam combining end is connected with the gain optical fiber; and a pumping fiber of the reverse optical fiber combiner is connected with a pumping fiber of the pumping light recoverer and a pumping source.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, and specifically relates to a method for improving the absorption and conversion efficiency of pump light. Background Technology

[0002] In recent years, high-power lasers have developed rapidly. Among the many types of high-power lasers, fiber lasers have been widely used in industrial and military fields due to their advantages such as good beam quality, small size, high conversion efficiency, and good heat dissipation. However, the cross-sectional area of ​​the fiber core is relatively small, and when the transmission power reaches a certain level, it is easy to generate transverse mode instability (TMI) and various nonlinear effects, such as stimulated Raman scattering (SRS) and stimulated Brillouin scattering (SBS). TMI is caused by the thermal effect of the gain fiber, while the SBS and SRS effects are related to the total length of the fiber used in the laser. When the pump absorption coefficient of the gain fiber used in the fiber laser is high, the heat generation per unit length of the gain fiber will increase, exacerbating the TMI effect; while using a gain fiber with a low absorption coefficient will inevitably increase the length of the gain fiber, thus leading to a decrease in the SBS and SRS thresholds.

[0003] On the other hand, the absorption efficiency of the pump light and the extraction efficiency of the signal light directly affect the overall efficiency of the laser. In end-pumped structures, the pump light needs to pass through the end face of the gain fiber multiple times to achieve sufficient absorption, while the signal light also needs to be output with low loss. Traditional solutions use a single end cap or simple coating, which makes it difficult to simultaneously achieve optimized optical characteristics for both the pump light and the signal light, resulting in problems such as high pump light reflection loss or signal light backlight affecting efficiency. Therefore, there is an urgent need for a novel fiber laser design that can simultaneously overcome the limitations of power and efficiency.

[0004] Patents CN202211562612.0 and CN202211566122.8 respectively describe a high-efficiency fiber laser and a fiber laser with high-efficiency pump light absorption. The implementation principle of both requires the use of an inner cladding fiber grating that reflects the pump light wavelength. After adding one grating device each in the forward and reverse directions, the optical structure becomes more complex, and the insertion loss of the inner cladding fiber grating is relatively large, which limits the improvement of the overall efficiency of the fiber laser.

[0005] Patent CN202211704818.2 describes a fiber cladding light processing device and method with recoverable pump light. This device replaces the traditional cladding light stripper. Its principle is to reverse the unidirectional residual pump light of the laser oscillator. However, for the laser amplifier, when there is a lot of residual pump light, the unidirectional pumping structure will be approximately equivalent to bidirectional or forward pumping, which will increase the effective length of the fiber and easily excite the SBS effect, causing damage to the fiber laser.

[0006] Therefore, it is necessary to design a high-efficiency fiber laser structure that can simultaneously improve the efficiency of the fiber oscillator and the fiber amplifier. Summary of the Invention

[0007] This invention addresses the shortcomings of existing technologies by providing a method for improving pump light absorption and conversion efficiency, as well as a fiber laser amplifier and fiber laser using this method. Through a special fiber structure design and pump light recovery technology, a fiber laser structure with higher output power and conversion efficiency is achieved, resulting in a compact laser structure, high conversion efficiency, and high output power.

[0008] This invention achieves this objective through the following technical solution: A method for improving the absorption conversion efficiency of pump light includes the following: The gain fiber adopts a three-core fiber structure, including two main cores and one auxiliary core. The main cores are rare earth-doped gain cores used to generate and transmit laser light, and the auxiliary core is a passive core used to receive and transmit pump light. The three cores are kept separate in the middle section of the gain fiber and gradually move closer together near the output end to form a tapered coupling region. Configure the pump light recoverer and the reverse fiber combiner: The pump optical recovery unit includes an input signal fiber, a tapered region, an output signal fiber, and pump arms. The core diameter of the input signal fiber is smaller than that of the output signal fiber. The input signal fiber is connected to the output signal fiber through the tapered region. The number of pump arms in the pump optical recovery unit is ≥1 and they are bundled together with the output signal fiber. The bundled end of the pump optical recovery unit is connected to the gain fiber. The reverse fiber combiner is an (N+1)×1 combiner, with the (N+1) end connected to the laser output head and N pump fibers, where N>2, and the combining end of the reverse fiber combiner connected to the gain fiber. The pump fiber of the reverse fiber combiner is connected to the pump arm and pump source of the pump light recoverer.

[0009] Furthermore, the pump light incident end of the three-core optical fiber is provided with an input end film layer, and the laser output end of the three-core optical fiber is provided with an output end film layer; both the input end film layer and the output end film layer are prepared using a multilayer dielectric film coating process, wherein the input end film layer is highly reflective to the pump light band and anti-reflective to the signal light band, and the output end film layer is highly reflective to the pump light band and partially anti-reflective to the signal light band.

[0010] Furthermore, the gain fiber is a double-clad doped fiber, and the two main fiber cores are one of ytterbium-doped fiber, erbium-doped fiber, erbium-ytterbium co-doped fiber, thulium-doped fiber, or thulium-holmium co-doped fiber. The cross-sections of the two main fiber cores, one auxiliary fiber core, and the outer cladding are all circular. The cross-section of the inner cladding of the gain fiber is one of hexagonal, octagonal, D-shaped, or quincunx-shaped.

[0011] Furthermore, the diameter of the main fiber core is 20-100μm, the diameter of the auxiliary fiber core is 60-800μm, the maximum outer diameter of the inner cladding is 100-1000μm, and the diameter of the outer cladding is 200-2000μm.

[0012] Furthermore, the two main fiber cores and one auxiliary fiber core are arranged in parallel on the cross-section of the gain fiber, forming a D-shaped, circular, or eccentric structure.

[0013] Furthermore, the pump source is a semiconductor laser or a fiber laser, and the pump wavelength is 790-1100nm.

[0014] Furthermore, the laser output head is an optical fiber end cap or a QBH.

[0015] The present invention also provides a fiber laser amplifier, including the optical path structure set by the method described above, wherein the input signal fiber is connected to a seed source laser.

[0016] The present invention also provides a fiber laser, including the optical path structure set by the method described above. The resonant cavity adopts an all-fiber structure, which is composed of a pair of fiber gratings, serving as an input high-reflection grating and an output low-reflection grating, respectively. The input high-reflection grating is connected to the input signal fiber, and the output low-reflection grating is connected to the output end of the gain fiber. Alternatively, a spatial resonant cavity can be used, consisting of a dichroic mirror at the input end and an output coupling mirror at the output end.

[0017] Furthermore, the center wavelengths of the high-reflectivity fiber grating and the low-reflectivity fiber grating are any wavelengths within the range of 1050-2090nm.

[0018] Compared with the prior art, the beneficial effects of this invention are as follows: 1. High Power Output: Employing a three-core fiber structure, the functions of pump light transmission (auxiliary core) and laser generation / transmission (main core) are separated. The auxiliary core can effectively carry higher pump power and reduce pump brightness requirements. At the same time, the two main cores can be designed to better suit high-power laser transmission and large mode field output, effectively reducing fiber length while maximizing pump light absorption. This physically breaks through the power limit of single-core fiber and effectively suppresses nonlinear effects such as SRS and TMI.

[0019] 2. High-efficiency conversion: The two main fiber cores effectively increase the mode field area compared to a single fiber core, increasing the absorption of pump light over the same length. Simultaneously, a unidirectional loop formed by connecting the pump light recoverer and the reverse fiber combiner repeatedly couples the residual reverse pump light not absorbed by the gain fiber into the gain fiber until it is completely absorbed, increasing the utilization rate of the pump light, achieving full utilization of the pump light, improving conversion efficiency, and shortening the length of the gain fiber while maintaining gain.

[0020] 3. Simplified pump light processing: Typically, fiber optic oscillators / amplifiers require at least one CPS (Continuous Power Filter) in the optical path to handle residual pump light, increasing system complexity and reducing reliability. By combining a three-core fiber with a pump light recoverer, the utilization rate of pump light is significantly improved, with minimal residual pump light in the optical path, eliminating the need for additional CPS or similar devices.

[0021] 4. Input end coating: High reflectivity to pump light, which can reflect the pump light that was not absorbed after passing through the gain region for secondary or even multiple absorptions back into the fiber, greatly reducing the waste of pump light.

[0022] 5. Output end coating: High reflectivity to pump light, preventing residual pump light from escaping and reflecting it back into the fiber for further absorption; partial anti-reflection to signal light, serving as a highly efficient output coupling mirror for precise control of laser output.

[0023] 6. Tapered coupling region: It efficiently guides the remaining pump light in the auxiliary fiber core into the main fiber core. Together with the reflection function of the end face coating and the pump light collector, it forms a "pump light trap" to ensure that the pump light is almost completely absorbed.

[0024] 7. Compact structure and reliability: The core function of this invention is achieved through special optical fiber and end face coating without adding unnecessary optical components. The introduction of the pump light recovery unit can eliminate the CPS in the optical path, making it easy to integrate into an all-fiber industrial laser system. The structure is simple and compact, with good stability and high reliability. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of a high-efficiency fiber optic oscillator structure according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the pump-driven light recovery device of the present invention; Figure 3 This is a schematic diagram of the three-core fiber structure for gain fiber of the present invention; Figure 4 This is a schematic diagram of the high-efficiency fiber amplifier structure according to an embodiment of the present invention.

[0026] In the figure, the following labels are used: 1-High reflectivity fiber grating, 2-Pump light recoverer, 3-Three-core fiber, 4-Low reflectivity fiber grating, 5-Reverse fiber combiner, 6-Pump source, 7-Laser output head, 8-Seed source, 21-Input signal fiber, 22-Cone region, 23-Output signal fiber, 24-Pump arm, 25-Pump light recoverer combining end, 31-Main fiber core, 32-Auxiliary fiber core, 33-Inner cladding, 34-Outer cladding. Detailed Implementation

[0027] Exemplary embodiments of the present invention will now be described in more detail with reference to the accompanying drawings. While exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a more thorough understanding of the present invention and to fully convey the scope of the invention to those skilled in the art. It should be noted that, unless otherwise specified, the embodiments and features described herein can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments. Example 1

[0028] This embodiment discloses a high-efficiency fiber laser using the method of improving pump light absorption and conversion efficiency according to the present invention, including a high-reflectivity fiber grating 1, a pump light recoverer 2, a three-core fiber 3, a low-reflectivity fiber grating 4, a reverse fiber combiner 5, a pump source 6, and a laser output head 7; wherein the pump light recoverer 2 is connected to the high-reflectivity fiber grating 1 through an input signal fiber 21, and the output signal light from the small-core input signal fiber 21 is transitioned to the output signal fiber 23 via a tapered region 22, and the pump arm 24 and the output signal fiber 23 are connected in the pump light recovery... The pump light is combined at the combiner end 25 and fused to the three-core fiber 3. A low-reflectivity fiber grating 4 is fused to the other side of the three-core fiber 3. The reverse fiber combiner 5 couples the pump light output from the pump source 6 to the three-core fiber 3. The two main fiber cores 31 of the three-core fiber 3 absorb the pump light and amplify the signal light, which is then output through the laser output head 7. The residual reverse pump light that is not absorbed by the three-core fiber 3 enters the three-core fiber 3 repeatedly until it is completely absorbed, through the loop formed by the pump arm 24 of the pump light recoverer 2 and the pump fiber connection of the reverse fiber combiner 5.

[0029] The gain fiber is a three-core fiber 3, including two main fiber cores 31 and one auxiliary fiber core 32. The main fiber core 31 is a gain fiber core doped with rare earth elements (such as ytterbium Yb and erbium Er), which is used to generate and transmit laser light. The auxiliary fiber core 32 is a passive fiber core, which can achieve efficient reception and transmission of pump light by reasonably designing its size and numerical aperture.

[0030] The input end of the three-core fiber 3 (i.e., the pump light incident end connected to the reverse fiber combiner side) is provided with an input end coating layer; the output end of the three-core fiber 3 (i.e., the laser output end connected to the pump light recoverer side) is provided with an output end coating layer; wherein, the input end coating layer is highly reflective (HR) for the pump light band and partially anti-reflective (AR) for the signal light band, serving as an output coupling mirror, and the transmittance can be designed to be 5%, 10%, etc., according to requirements; the output end coating layer is highly reflective (HR) for both the pump light band and the signal light band, used to control the signal light transmission direction and fully absorb the pump light.

[0031] The input and output end films of the three-core fiber 3 are both prepared using a multilayer dielectric film coating process to achieve high reflectivity and anti-reflection effect at specific wavelengths.

[0032] The three-core fiber 3 is a double-clad doped fiber, wherein the two main fiber cores 31 are one of ytterbium-doped fiber, erbium-doped fiber, erbium-ytterbium co-doped fiber, thulium-doped fiber, or thulium-holmium co-doped fiber, and the cross-sections of the two main fiber cores 31 and one auxiliary fiber core 32 are all circular; the cross-section of the inner cladding 33 of the fiber is one of circular, hexagonal, octagonal, D-shaped, or quincunx, and the cross-section of the outer cladding 34 is circular; the diameter of the main fiber core 31 of the three-core fiber 3 is 20-100μm, the diameter of the auxiliary fiber core 32 is 60-800μm, the maximum outer diameter of the inner cladding 33 is 100-1000μm, and the diameter of the outer cladding 34 is 200-2000μm.

[0033] Two main fiber cores 31 and one auxiliary fiber core 32 are arranged in parallel on the cross-section of the three-core fiber 3, in a D-shape, circular or eccentric structure, and the size is designed such that the sum of the maximum outer diameters of the three fiber cores does not exceed the maximum outer diameter of the inner cladding 33.

[0034] The two main cores 31 and the auxiliary core 32 of the three-core optical fiber 3 remain separated in the middle section of the fiber. They gradually approach each other along a certain length near the output end connected to the reverse fiber combiner, forming a tapered coupling region. The end of the tapered coupling region is fused to the reverse fiber combiner side. In this coupling region, any remaining pump light that is not completely absorbed by the main core 31 is coupled from the auxiliary core 32 into the main core 31 and further absorbed, thus significantly improving the absorption efficiency of the pump light.

[0035] The resonant cavity can be an all-fiber structure or a spatial structure. When an all-fiber structure is used, it consists of a pair of fiber optic gratings (FBGs), which serve as the high-reflection grating at the input end and the low-reflection grating at the output end, respectively.

[0036] When the resonant cavity adopts a spatial structure, it consists of a dichroic mirror at the input end and an output coupling mirror at the output end.

[0037] The center wavelengths of the high-reflectivity fiber grating 1 and the low-reflectivity fiber grating 4 can be any wavelength within the range of 1050-2090nm; the output fiber of the high-reflectivity fiber grating 1 is size-matched with the input fiber of the pump optical receiver 2.

[0038] The pump optical reclaimer 2 includes an input signal fiber 21, a tapered region 22, an output signal fiber 23, a pump arm 24, and a pump optical reclaimer combiner end 25. The core diameter of the input signal fiber 21 is smaller than that of the output signal fiber 23. The fiber size of the input signal fiber 21 matches the size of the output fiber of the high reflectivity fiber grating 1. The output signal fiber 23 and the pump arm 24 are combined at the pump optical reclaimer combiner end 25. The fiber size of the pump optical reclaimer combiner end 25 matches the size of the three-core fiber 3. The fiber size of the pump arm 24 matches the size of the pump fiber of the reverse fiber combiner 5. The number of pump fibers in the pump optical reclaimer 2 is ≥1.

[0039] The reverse fiber combiner 5 is an (N+1)×1 combiner, where N is the number of pump fibers and N>2; the pump fibers of the reverse fiber combiner 5 are connected to the pump fibers 24 of the pump light recoverer 2 and the pump source 6 by fiber optic fusion splicing.

[0040] The pump laser output from pump source 6 is coupled into three-core fiber 3 through the pump fiber of reverse fiber combiner 5. The pump light and the signal light propagate in opposite directions in the optical path. Pump source 6 can be a semiconductor laser or a fiber laser, with a pump wavelength of 790-1100nm. The output fiber of pump source 6 is matched with the pump fiber of reverse fiber combiner 5 in size.

[0041] The laser output head 7 can be either a fiber end cap or a QBH (Quick Beam Head); the laser output head 7 is connected to the reverse fiber combiner 5 via fiber fusion splicing and is located at the very end of the fiber laser for the final laser emission output. Example 2

[0042] This embodiment provides a fiber laser amplifier, including a seed source 8, a pump source 6, a reverse fiber combiner 5, a three-core fiber 3, a pump light recoverer 2, and a laser output head 7.

[0043] The seed source 8 is connected to the input signal fiber 21 of the pump light recoverer 2. The recoverer combiner end 25 is connected to the output end of the three-core fiber 3. The combiner end of the reverse fiber combiner 5 is connected to the input end of the three-core fiber 3. The pump fiber of the reverse fiber combiner 5 is connected to the pump arm 24 and the pump source 6 of the pump light recoverer 2. The reverse fiber combiner 5 is connected to the laser output head 7.

[0044] The center wavelength of the laser output from seed source 8 is 1000-1100nm, the core diameter of the output fiber of seed source 8 is 10-20μm, and the size of the output fiber of seed source 8 is matched with that of the input signal fiber of pump light receiver 2.

[0045] The present invention has been described in detail above through embodiments, but the content described is only an exemplary embodiment of the present invention and should not be considered as limiting the scope of the present invention. The scope of protection of the present invention is defined by the claims. Any technical solutions designed by those skilled in the art using the technical solutions described in the present invention, or similar technical solutions designed by those skilled in the art under the inspiration of the technical solutions of the present invention, within the substance and scope of protection of the present invention, to achieve the above-mentioned technical effects, or equivalent changes and improvements made to the scope of the application, should still fall within the patent protection scope of the present invention. It should be noted that, for clarity, descriptions of some components and processes that are not directly and obviously related to the scope of protection of the present invention but are known to those skilled in the art have been omitted in the description of the present invention.

Claims

1. A method for improving the absorption and conversion efficiency of pump light, characterized in that, Includes the following: The gain fiber adopts a three-core fiber structure, including two main cores and one auxiliary core. The main cores are rare earth-doped gain cores used to generate and transmit laser light, and the auxiliary core is a passive core used to receive and transmit pump light. The three cores are kept separate in the middle section of the gain fiber and gradually move closer together near the output end to form a tapered coupling region. Configure the pump light recoverer and the reverse fiber combiner: The pump optical recovery unit includes an input signal fiber, a tapered region, an output signal fiber, and pump arms. The core diameter of the input signal fiber is smaller than that of the output signal fiber. The input signal fiber is connected to the output signal fiber through the tapered region. The number of pump arms in the pump optical recovery unit is ≥1 and they are bundled together with the output signal fiber. The bundled end of the pump optical recovery unit is connected to the gain fiber. The reverse fiber combiner is an (N+1)×1 combiner, with the (N+1) end connected to the laser output head and N pump fibers, where N>2, and the combining end of the reverse fiber combiner connected to the gain fiber. The pump fiber of the reverse fiber combiner is connected to the pump arm and pump source of the pump light recoverer.

2. The method according to claim 1, characterized in that, The pump light incident end of the three-core optical fiber is provided with an input end film layer, and the laser output end of the three-core optical fiber is provided with an output end film layer. Both the input end film layer and the output end film layer are prepared using a multilayer dielectric film coating process. The input end film layer is highly reflective in the pump light band and anti-reflective in the signal light band, while the output end film layer is highly reflective in the pump light band and partially anti-reflective in the signal light band.

3. The method according to claim 1, characterized in that, The gain fiber is a double-clad doped fiber, and the two main fiber cores are one of ytterbium-doped fiber, erbium-doped fiber, erbium-ytterbium co-doped fiber, thulium-doped fiber or thulium-holmium co-doped fiber. The cross-sections of the two main fiber cores, one auxiliary fiber core and the outer cladding are all circular. The inner cladding cross-section of the gain fiber is one of the following: circular, hexagonal, octagonal, D-shaped, or quincunx-shaped.

4. The method according to claim 3, characterized in that, The main fiber core has a diameter of 20-100μm, the auxiliary fiber core has a diameter of 60-800μm, the inner cladding has a maximum outer diameter of 100-1000μm, and the outer cladding has a diameter of 200-2000μm.

5. The method according to claim 3, characterized in that, The two main fiber cores and one auxiliary fiber core are arranged in parallel on the cross-section of the gain fiber, in a D-shaped, circular or eccentric structure.

6. The method according to claim 1, characterized in that, The pump source is a semiconductor laser or a fiber laser, with a pump wavelength of 790-1100nm.

7. The method according to claim 1, characterized in that, The laser output head is either an optical fiber end cap or a QBH.

8. A fiber laser amplifier, characterized in that, The optical path structure includes the method described in any one of claims 1-7, wherein the input signal fiber is connected to the seed source laser.

9. A fiber laser, characterized in that, The optical path structure provided by the method described in any one of claims 1-7 includes a resonant cavity with an all-fiber structure, consisting of a pair of fiber optic gratings, which serve as an input high-reflection grating and an output low-reflection grating, respectively, wherein the input high-reflection grating is connected to the input signal fiber, and the output low-reflection grating is connected to the output end of the gain fiber. Alternatively, a spatial resonant cavity can be used, consisting of a dichroic mirror at the input end and an output coupling mirror at the output end.

10. The fiber laser according to claim 9, characterized in that, The center wavelengths of the high-reflectivity fiber grating and the low-reflectivity fiber grating are any wavelengths within the range of 1050-2090nm.

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