A few-mode fiber Bragg grating and fiber oscillator capable of filtering out high-order modes

Abstract

A few-mode fiber Bragg grating and a fiber oscillator capable of filtering out high-order modes, wherein the few-mode fiber Bragg grating comprises a high-reflection grating and a low-reflection grating, both of which are made by tapering a large-core double-clad energy transfer fiber; the high-reflection grating and the low-reflection grating both comprise an input double-clad fiber containing a coating layer, an input fiber after stripping the coating layer, an input taper region, a waist region, an output taper region, an output fiber after stripping the coating layer, and an output double-clad fiber containing a coating layer, wherein the waist region, the output taper region, and the output fiber after stripping the coating layer constitute a grating writing region; and the fiber oscillator comprises a high-reflection grating, a forward pump source, a forward pump / signal combiner, a gain fiber, a backward pump / signal combiner, a backward pump source, a low-reflection grating, a fiber end cap, and a laser output end of a laser.

Description

Technical Field

[0001] The invention relates to the field of laser technology, in particular to a few-mode fiber Bragg grating and a fiber oscillator capable of filtering out high-order modes. Background Art

[0002] Fiber lasers have the advantages of high efficiency, low heat load, high beam quality, and flexible transmission, and have broad application prospects in industrial processing, material processing, national defense research, and other fields. Compared with fiber amplifiers with master oscillator power amplification structures, fiber oscillators have the advantages of compact structure, simple control logic, low cost, strong anti-reflection and return light ability, and good stability. With the development of fiber optic devices and processes, the output power and beam quality of fiber oscillators have been continuously improved, and have received widespread attention and application in scientific research and industrial fields.

[0003] As the power of the fiber oscillator continues to increase, an extremely high energy density will be formed in the fiber core, causing various harmful nonlinear effects. Increasing the fiber core size and reducing the power density in the fiber core is one of the effective ways to avoid nonlinear effects. However, increasing the fiber core size will cause the fiber to support multiple modes. If the gain fiber supports multiple modes, another harmful effect will occur: mode instability effect. Mode instability effect means that when the output signal light power exceeds a certain threshold, the output mode of the fiber laser changes from a stable fundamental mode to a non-steady-state mode in which the relative components of the fundamental mode and the higher-order mode change rapidly and randomly over time. The beam quality of the output light will be seriously deteriorated, which will have a bad impact on the application of the laser.

[0004] Bending the gain fiber to suppress the higher-order mode is the most commonly used method to achieve high beam quality output. By bending the gain fiber, the loss of the higher-order mode in the gain fiber is increased, and the generation of higher-order modes is reduced. However, if the bending diameter of the fiber is too large, it will not play a good role in filtering out the higher-order modes. If the bending diameter is too small, the loss of the fiber fundamental mode will increase, allowing part of the fundamental mode to leak into the fiber cladding, making the laser efficiency lower.

[0005] By designing a variable core diameter gain fiber, the quality of the laser output beam can be maintained while suppressing the nonlinear effect of the fiber, and the mode instability effect can be suppressed. Patent CN209640528U proposes a spindle-shaped gain fiber with a core cladding size that changes continuously and gradually in the longitudinal direction. By limiting the core size at both ends of the gain fiber, the excitation of high-order modes is reduced, and stimulated Raman scattering and mode instability effects are suppressed. Patent CN214069074U proposes a saddle-shaped gain fiber and an all-fiber oscillator based on the saddle-shaped gain fiber. The diameter of the core of the gain fiber is small in the middle and large at both ends in the length direction. The fiber end cap is used for laser feedback, and it has a strong fundamental mode retention ability and nonlinear suppression ability. However, the drawing process of variable core diameter gain fiber is relatively complex and costly, and it is currently difficult to apply on a large scale. Patent CN206096546U proposes a fiber high-order mode stripping device, which strips the high-order mode of the fiber by fiber taper, but does not involve stimulated Raman scattering suppression and mode instability suppression of the fiber oscillator.

[0006] Patent CN212379605U proposes a few-mode fiber Bragg grating and laser system that can suppress high-order mode reflection. A circular refractive index modulation area concentric with the core is set in the core of the few-mode fiber to suppress LP 01 Module and LP 11 Mutual coupling between modes controls LP 11 The reflectivity of the mode is lower than that of LP 01 mode reflectivity, thereby achieving LP 11 However, this method requires precise control of the fiber Bragg grating writing range, has high requirements on the manufacturing process, and can only reduce the LP 11 The reflectivity of LP can be reduced by reducing its power ratio, but it cannot completely suppress LP 11 The generation of mold. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a few-mode fiber Bragg grating capable of filtering out high-order modes.

[0008] The technical solution adopted by the present invention to solve this technical problem is:

[0009] A few-mode fiber Bragg grating capable of filtering out high-order modes comprises a high-reflection grating and a low-reflection grating, wherein the high-reflection grating and the low-reflection grating are both made by tapering a large-core double-clad energy-transmitting optical fiber; wherein the high-reflection grating and the low-reflection grating both comprise an input double-clad optical fiber containing a coating layer, an input optical fiber after stripping the coating layer, an input tapered region, a waist region, an output tapered region, an output optical fiber after stripping the coating layer, and an output double-clad optical fiber containing a coating layer, wherein the waist region, the output tapered region, and the output optical fiber after stripping the coating layer constitute a grating writing region; the input optical fiber after stripping the coating layer, the input tapered region, the waist region, the output tapered region, and the output optical fiber after stripping the coating layer constitute a cladding light stripping region; and the lengths of the input tapered region, the waist region, and the output tapered region are all 20 mm.

[0010] The technical solution adopted by the present invention to solve this technical problem is:

[0011] A fiber oscillator comprises a high-reflection grating, a forward pump source, a forward pump / signal combiner, a gain fiber, a backward pump / signal combiner, a backward pump source, a low-reflection grating, a fiber end cap, and a laser output end of a laser, wherein the high-reflection grating, the low-reflection grating, the forward pump / signal combiner, and the backward pump / signal combiner constitute a fiber resonant cavity.

[0012] The input end of the high-reflection grating is fused to the input end of the forward pump / signal combiner, the output end of the forward pump source is fused to the pump arm of the forward pump / signal combiner, and the output end of the forward pump / signal combiner is fused to the gain fiber;

[0013] The output end of the gain fiber is fused to the input end of the backward pump / signal combiner, the output end of the backward pump source is fused to the pump arm of the backward pump / signal combiner, the output end of the backward pump / signal combiner is fused to the input end of the low-reflection grating, the output end of the low-reflection grating is fused to the input end of the optical fiber end cap, and the output end of the optical fiber end cap is fused to the laser output end of the laser.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] 1. The present invention provides a few-mode fiber Bragg grating capable of filtering out high-order modes, which can simply and effectively realize the preparation of few-mode fiber with only LP 01 The high / low reflection grating of the mode output can realize the pure fundamental mode output of the few-mode fiber oscillator with a relatively thick core diameter, thereby improving the laser mode instability threshold and beam quality.

[0016] 2. The present invention integrates multiple functions such as mode filtering, fiber Bragg grating, cladding light stripping, etc. in one device, and there is no fusion point inside. Compared with the fusion of separate devices to achieve the same function, it reduces the number of fiber fusions and the fiber length, reduces the nonlinear effect caused by the fiber length, and avoids fusion loss and beam quality degradation caused by fusion. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the few-mode fiber Bragg grating structure of Example 1 of the present invention;

[0018] Figure 2 For the present invention Figure 1 LP 01 and LP 11 The relationship between mode passing rate and waist core diameter;

[0019] Figure 3 It is a schematic diagram of the structure of the optical fiber oscillator of the present invention.

[0020] In the figure: 11, input end double-clad optical fiber with coating, 12, input end optical fiber after stripping the coating, 13, input end taper, 14, waist, 15, output end taper, 16, output end optical fiber after stripping the coating, 17, output end double-clad optical fiber with coating, 101, high reflection grating, 102, forward pump / signal combiner, 103, gain fiber, 104, backward pump / signal combiner, 105, low reflection grating, 106, optical fiber end cap, 107, laser output end, 108, forward pump source, 109, backward pump source. DETAILED DESCRIPTION

[0021] The present invention is further described below in conjunction with the accompanying drawings and embodiments.

[0022] Example 1

[0023] like Figure 1-2As shown, this embodiment includes a high-reflection grating 101 and a low-reflection grating 105, and both the high-reflection grating 101 and the low-reflection grating 105 are made by tapering a large-core double-clad energy transfer optical fiber; the high-reflection grating 101 and the low-reflection grating 105 both include an input double-clad optical fiber 11 containing a coating, an input optical fiber 12 after stripping the coating, an input taper region 13, a waist region 14, an output taper region 15, an output optical fiber 16 after stripping the coating, and an output double-clad optical fiber 17 containing a coating, wherein the waist region 14, the output taper region 15, and the output optical fiber 16 after stripping the coating constitute a grating writing region 18; the input optical fiber 12 after stripping the coating, the input taper region 13, the waist region 14, the output taper region 15, and the output optical fiber 16 after stripping the coating constitute a cladding light stripping region 19; and the lengths of the input taper region 13, the waist region 14, and the output taper region 15 are all 20 mm.

[0024] Existing fiber Bragg gratings usually include high-reflection gratings and low-reflection gratings. The reflectivity of the high-reflection grating is generally greater than 99%, and it serves as the high-reflection end of the fiber oscillator. The reflectivity of the low-reflection grating is generally between 5-15%, and it serves as the output end of the fiber oscillator. The high-reflection grating 101 and the low-reflection grating 105 of the present application are both made by tapering a large-core double-clad energy-transfer fiber, that is, the high-order mode filtering function is first achieved by tapering the fiber, and then the fiber Bragg grating is made on the tapered fiber.

[0025] In this embodiment, the original double-clad optical fiber supports multiple modes. After the taper, the fiber core of the waist region 14 becomes thinner and only supports fundamental mode transmission. The fundamental mode in the original optical fiber can pass through the input end taper region 13, the waist region 14 and the output end taper region 15 with very low loss. 11 The higher-order modes are completely cut off, converted into cladding modes and stripped off;

[0026] The double-clad energy transfer optical fiber of the present application has a core diameter of 20 μm, a cladding diameter of 400 μm, a core NA of 0.065, a laser wavelength of 1080 nm, and only supports fundamental mode LP 01 and higher-order modes LP 11 Two modes; and the lengths of the input end cone region 14, the waist region 15, and the output end cone region 16 are all 20 mm. When the core diameter of the waist region 5 is reduced, such as Figure 2 As shown, LP 01 The LP mode can pass through the device with very low loss; when the core size is less than 12μm, 11 The model is completely cut off.

[0027] Example 2

[0028] like Figure 3As shown, this embodiment includes a high-reflection grating 101, a forward pump source 108, a forward pump / signal combiner 102, a gain fiber 103, a backward pump / signal combiner 104, a backward pump source 109, a low-reflection grating 105, an optical fiber end cap 106, and a laser output end 107. The high-reflection grating 101, the low-reflection grating 105, the forward pump / signal combiner 102, and the backward pump / signal combiner 104 constitute a fiber resonant cavity. The input end of the high-reflection grating 101 is fused with the input end of the forward pump / signal combiner 102, and the output end of the forward pump source 108 is fused with the input end of the forward pump / signal combiner 102. The end is fused to the pump arm of the forward pump / signal combiner 102, the output end of the forward pump / signal combiner 102 is fused to the gain fiber 103, the output end of the gain fiber 103 is fused to the input end of the backward pump / signal combiner 104, the output end of the backward pump source 109 is fused to the pump arm of the backward pump / signal combiner 104, the output end of the backward pump / signal combiner 104 is fused to the input end of the low-reflection grating 105, the output end of the low-reflection grating 105 is fused to the input end of the fiber end cap 106, and the output end of the fiber end cap 106 is fused to the laser output end 107 of the laser. Thus, the pump lasers output by the forward pump source 108 and the backward pump source 109 are injected into the fiber laser resonant cavity through the pump arms of the forward pump / signal combiner 102 and the backward pump / signal combiner 104, and the cladding light in the resonant cavity output laser is stripped by the cladding light stripping area in the low-reflection grating 105, and then output by the fiber end cap 106.

[0029] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the technical solutions of the present invention have been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A few-mode fiber Bragg grating capable of filtering out high-order modes, characterized in that: The invention comprises a high-reflection grating (101) and a low-reflection grating (105), wherein the high-reflection grating (101) and the low-reflection grating (105) are both made by taper-drawing a large-core double-clad energy-transmitting optical fiber; wherein the high-reflection grating (101) and the low-reflection grating (105) each comprise an input double-clad optical fiber (11) containing a coating, an input optical fiber (12) after the coating is stripped, an input taper region (13), a waist region (14), an output taper region (15), an output optical fiber (16) after the coating is stripped, an input double-clad optical fiber (11) containing a coating, an input optical fiber (12) after the coating is stripped, an input taper region (13), a waist region (14), an output taper region (15), an output optical fiber (16) after the coating is stripped, an input double-clad optical fiber (11) containing a coating, and an output taper region (15). An output double-clad optical fiber (17), wherein the waist region (14), the output end cone region (15), and the output end optical fiber (16) after stripping the coating layer constitute a grating writing region (18); the input end optical fiber (12) after stripping the coating layer, the input end cone region (13), the waist region (14), the output end cone region (15), and the output end optical fiber (16) after stripping the coating layer constitute a cladding light stripping region (19); the lengths of the input end cone region (13), the waist region (14), and the output end cone region (15) are all 20 mm.

2. A fiber oscillator, characterized in that: It comprises a high-reflection grating (101), a forward pump source (108), a forward pump / signal combiner (102), a gain fiber (103), a backward pump / signal combiner (104), a backward pump source (109), a low-reflection grating (105), an optical fiber end cap (106), and a laser output end (107), wherein the high-reflection grating (101), the low-reflection grating (105), the forward pump / signal combiner (102), and the backward pump / signal combiner (104) constitute a fiber resonant cavity; The input end of the high-reflection grating (101) is fused to the input end of the forward pump / signal combiner (102), the output end of the forward pump source (108) is fused to the pump arm of the forward pump / signal combiner (102), and the output end of the forward pump / signal combiner (102) is fused to the gain optical fiber (103); The output end of the gain optical fiber (103) is fused to the pump arm of the backward pump / signal combiner (104), the output end of the backward pump source (109) is fused to the output end of the backward pump / signal combiner (104), the output end of the backward pump / signal combiner (104) is fused to the input end of the low-reflection grating (105), the output end of the low-reflection grating (105) is fused to the input end of the optical fiber end cap (106), and the output end of the optical fiber end cap (106) is fused to the laser output end (107) of the laser.

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