A femtosecond laser preparation method of a pump light recycling grating and a spectrum measuring device

Abstract

The application relates to the field of high-power fiber lasers, and particularly discloses a femtosecond laser preparation method of a pump light recycling grating and a spectrum measuring device. The preparation method takes a double-clad fiber as a substrate, adopts femtosecond laser to be focused in the inner cladding through a coating layer, writes a fiber Bragg grating through a multi-layer line scanning technology, prepares a uniform grating or a chirped grating, and flexibly controls a grating modulation area by adjusting parameters. The spectrum measuring device is composed of an ASE broadband light source, a multimode fiber coupler, a multimode fiber matched with the inner cladding diameter and a double spectrometer, and realizes real-time and accurate measurement of the transmission and reflection spectrum of the inner cladding grating. The application solves the problems of low coupling efficiency, poor reliability and difficulty in reflecting specific wavelengths in the traditional pump light recycling technology, the prepared grating is uniform in modulation, the pump light reflection effect is good, residual pump light can be recycled efficiently, and the system efficiency and integration level of the high-power fiber laser are significantly improved.

Description

Technical Field

[0001] This invention relates to the field of high-power fiber laser technology, specifically to a method for fabricating a femtosecond laser with a pump light recovery grating and a spectral measurement device. Background Technology

[0002] High-power fiber lasers are widely used due to their excellent beam quality, high energy conversion efficiency, good heat dissipation performance, and compact structure. Currently, mainstream high-power fiber lasers generally employ a double-clad fiber structure, where the rare-earth-doped core serves as the laser gain medium, while the inner cladding transmits multimode pump light. During its propagation along the inner cladding, the pump light passes through the core multiple times, exciting the rare-earth ions to a high-energy state, thus achieving population inversion and laser amplification.

[0003] In practical applications, pump light cannot be completely absorbed by the fiber core. Even with long-gain fibers, 10%-30% of residual pump light still leaks from the fiber end. This unused pump light not only significantly reduces the overall system efficiency, increases energy consumption and operating costs, but also damages subsequent optical components and generates excess heat, seriously affecting the laser's operational stability and long-term reliability.

[0004] To improve pump light utilization, various pump light recovery technologies have been proposed in the industry, but all of them have obvious drawbacks: The reflector solution involves coating or attaching a high-reflectivity film to the end of the optical fiber to directly reflect the residual pump light. This method has a simple structure, but it suffers from problems such as limited reflection bandwidth, low fiber coupling efficiency, poor high-power tolerance, and insufficient long-term operational reliability.

[0005] Fiber end face processing solution: A specific angle is formed on the fiber end face through grinding and fusion splicing techniques, and the pump light is recovered by total internal reflection. Although no additional optical components are required, the processing precision requirements are extremely high, and it is impossible to achieve efficient and accurate reflection of pump light of a specific wavelength.

[0006] Volume Bragg grating (VBG) scheme: VBG recovers pump light by utilizing its narrowband high reflectivity. It has high thermal stability and damage threshold, but fiber coupling requires a complex optical system, which greatly increases the system complexity and cost, and is not conducive to the integration and miniaturization of fiber lasers.

[0007] Traditional UV-written fiber Bragg grating schemes rely on the photosensitivity of the fiber core and use UV lasers combined with phase masking technology to write fiber Bragg gratings on the fiber core. This technology is mature but only applicable to the fiber core. The inner cladding of double-clad fibers is mostly pure quartz or low-refractive-index polymers, which have extremely poor photosensitivity. Traditional UV writing technology cannot form an effective grating structure in it, making it difficult to meet the pump light recovery requirements of the inner cladding.

[0008] In recent years, femtosecond laser micromachining technology has developed rapidly. Its extremely short pulse width and ultra-high peak power can induce permanent refractive index changes within transparent materials through nonlinear absorption. Compared with traditional ultraviolet writing technology, femtosecond laser direct writing has advantages such as strong material universality, three-dimensional precision machining, submicron resolution, maskless flexible control, and excellent thermal stability and long-term reliability, providing a new technological possibility for the direct fabrication of gratings in the inner cladding of optical fibers.

[0009] However, a complete technical solution is still lacking for directly fabricating pump light recovery gratings with large modulation areas and adjustable parameters within the inner cladding of double-clad optical fibers, and for accurately characterizing the grating's spectral properties in real time. Based on this, this invention proposes a femtosecond laser fabrication method and spectral measurement device for pump light recovery gratings to address many shortcomings of existing technologies. Summary of the Invention

[0010] To address the aforementioned problems in the prior art, this invention provides a method for fabricating femtosecond lasers with pump light recovery gratings and a spectral measurement device. This effectively solves the problems of low coupling efficiency, poor reliability, and difficulty in reflecting specific wavelengths in traditional pump light recovery technology, thereby improving the system efficiency and integration level of high-power fiber lasers.

[0011] To achieve the above objectives, this invention proposes a femtosecond laser fabrication method for a pump light recovery grating, comprising: A double-clad optical fiber is provided, wherein the double-clad optical fiber has an inner cladding; a femtosecond laser is directly focused through the coating layer of the double-clad optical fiber onto the interior of the inner cladding using a focusing objective lens, and a multi-layer line-by-line scanning technique is used to fabricate a fiber Bragg grating in the inner cladding; the multi-layer line-by-line scanning technique includes: scanning layer by layer along the y-axis to form multiple modulation planes, each modulation plane being superimposed in the depth direction, and adjacent modulation planes having an overlapping region; repeating the above layer-by-layer scanning along the x-axis with a preset grating period step to form multiple grating planes; the grating period between each grating plane is kept consistent to form a uniform grating, or the grating period between each grating plane is linearly increased to form a chirped grating; the modulation area of ​​the inner cladding fiber grating is controlled by adjusting the length of the modulation plane, the interlayer spacing, and the number of layers, thereby obtaining the inner cladding fiber grating.

[0012] Preferably, the multi-layer line-by-line scanning technology specifically involves: during each layer scan, the femtosecond laser scans a straight line along the positive y-axis to form a rectangular modulation plane; after completing one layer, the femtosecond laser stops emitting light, moves back to the origin, and moves a preset distance d in the z-axis direction before scanning the next layer; the preset distance d is less than the depth formed by a single modulation plane in the z-axis direction.

[0013] This invention also proposes a spectral measurement device for a pump light recovery fiber Bragg grating, comprising: The system comprises an ASE broadband light source, a first multimode fiber, a third fiber coupling assembly, a fourth fiber coupling assembly, a first spectrometer, a second spectrometer, and a multimode fiber coupler. The input end of the multimode fiber coupler is coupled to the ASE broadband light source via the first fiber coupling assembly; the input end of the first multimode fiber is coupled to the output end of the multimode fiber coupler via the second fiber coupling assembly; the third fiber coupling assembly couples the output end of the first multimode fiber to one end of the inner cladding of a double-clad fiber for which an inner cladding grating is to be written; the fourth fiber coupling assembly couples the other end of the inner cladding of the double-clad fiber to the input end of the second multimode fiber; the first spectrometer is connected to the output end of the second multimode fiber; and the second spectrometer is connected to the other input end of the multimode fiber coupler.

[0014] Preferably, the core diameter of the multimode fiber coupler, the core diameter of the first multimode fiber, and the core diameter of the second multimode fiber are all consistent with the inner cladding diameter of the double-clad fiber.

[0015] Preferably, the broadband light emitted from the ASE broadband light source is coupled to one input end of the multimode fiber coupler via the first fiber coupling component, and then enters the inner cladding of the double-clad fiber from the output end of the multimode fiber coupler via the second fiber coupling component, the first multimode fiber, and the third fiber coupling component.

[0016] Preferably, the light emitted from the inner cladding passes through the fourth fiber coupling component and the second multimode fiber into the first spectrometer; the reflected light from the inner cladding grating returns along the original path to the multimode fiber coupler, and is output from the other input end of the multimode fiber coupler to the second spectrometer.

[0017] Preferably, the first multimode fiber and the second multimode fiber are used to stabilize the mode excitation state and extend the optical path length.

[0018] Therefore, this invention proposes a femtosecond laser fabrication method and spectral measurement device for a pump light recovery grating, the advantages of which are as follows: (1) Using femtosecond laser multilayer line-by-line scanning direct writing technology, uniform or chirped gratings can be directly prepared in the inner cladding through the fiber coating layer without fiber pretreatment. The grating modulation area and period can be flexibly adjusted, the grating modulation is uniform and the scattering loss is low. It breaks through the limitation of traditional ultraviolet writing that depends on the photosensitivity of the material, further efficiently reflects residual pump light, and significantly improves the pump utilization and operation stability of fiber lasers.

[0019] (2) The matching spectral measurement device adopts a multimode fiber and a dual spectrometer that matches the diameter of the inner cladding. It can accurately characterize the transmission and reflection spectra of the grating in real time, simplify the process of optimizing processing parameters, and the system has stable coupling and high integration. It solves the problems of low coupling efficiency, poor reliability and difficult spectral characterization of traditional pump recovery schemes, and is suitable for the miniaturization and industrial application of high-power fiber laser systems.

[0020] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the structure of the inner cladding fiber grating based on femtosecond laser multilayer line-by-line scanning technology of the present invention; Figure 2 This is a schematic diagram of the real-time spectral measurement device with inner cladding fiber grating of the present invention; Figure 3 This is a schematic diagram of the fabrication and spectral testing process of the pump light recovery fiber grating of the present invention; Figure 4 These are top and side microscopic images of the inner cladding fiber grating of the spectral measurement device of the pump light recovery fiber grating of the present invention. Figure 5 This is a cross-sectional refractive index distribution diagram of the inner cladding fiber grating of the present invention; Figure 6 This is the transmission and reflection spectrum of the inner cladding fiber grating of this invention.

[0022] Figure Labels 1. ASE broadband light source; 2. First fiber coupling component; 3. Multimode fiber coupler; 4. Second fiber coupling component; 5. First multimode fiber; 6. Third fiber coupling component; 7. Double-clad fiber; 8. Fourth fiber coupling component; 9. Second multimode fiber; 10. First spectrometer; 11. Second spectrometer. Detailed Implementation

[0023] To make the technical solutions, advantages, and objectives of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below. The described embodiments are only some, not all, of the embodiments of the present invention. All other embodiments obtained by those skilled in the art based on the described embodiments of the present invention without creative effort are within the protection scope of this application.

[0024] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0025] like Figures 1-6As shown, the present invention provides a femtosecond laser fabrication method and spectral measurement device for a pump light recovery grating.

[0026] A method for fabricating a femtosecond laser pump light recovery grating includes: Provides double-clad optical fiber, the processing principle is as follows Figure 1 As shown, the double-clad optical fiber has an inner cladding.

[0027] The specific processing principle is as follows: First, the femtosecond laser is focused on one side above the inner cladding of the double-clad fiber and scans a straight line along the positive y-axis. During the scanning process, the Rayleigh length of the femtosecond laser will produce a certain depth of focus in the z-axis direction. Therefore, a rectangular modulation plane with a certain length and depth can be obtained in a single scan. Then, the femtosecond laser stops emitting light, moves back to the origin, and moves downward a distance d, which is usually slightly smaller than the depth formed by a single modulation plane; Next, the femtosecond laser scans out a second modulation plane along the positive y-axis. This plane overlaps with the first modulation plane to a certain extent, so the two superimposed can form a larger modulation region. Subsequently, the above process is repeated to obtain a grating plane formed by the superposition of several modulation planes; Then, the femtosecond laser moves along the x-axis for one grating period Ʌ and begins to write the second grating plane at the same position above the inner cladding; By repeating the above process, an inner cladding fiber grating composed of several grating planes can be obtained.

[0028] The grating periods of each grating plane are kept consistent, forming a uniform grating; the grating periods of each grating plane increase linearly, forming a chirped grating. The modulation area of ​​the inner cladding fiber grating is controlled by adjusting the length of the modulation plane, the interlayer spacing, and the number of layers.

[0029] The present invention also discloses a real-time spectral measurement device for inner cladding fiber gratings. This device is used to accurately characterize the spectral properties of the prepared inner cladding gratings in real time, which can significantly improve the optimization efficiency of grating processing parameters during the preparation process.

[0030] like Figure 2 As shown, the core components of the device include: ASE broadband light source 1, first fiber coupling component 2, multimode fiber coupler 3, second fiber coupling component 4, first multimode fiber 5, third fiber coupling component 6, double-clad fiber 7 for writing inner cladding gratings, fourth fiber coupling component 8, second multimode fiber 9, first spectrometer 10, and second spectrometer 11.

[0031] After the broadband light emitted by the ASE light source is output through a single-mode fiber, it is first coupled to an input end of a multimode fiber coupler 3 through a first fiber coupling component 2. The fiber core diameter of the multimode fiber coupler is the same as or equivalent to the inner cladding diameter of the double-clad fiber.

[0032] Subsequently, the broadband light is output from the output end of the multimode fiber coupler 3, and is transmitted sequentially through the second fiber coupling component 4, the first multimode fiber 5, and the third fiber coupling component 6 to the inner cladding of the double-clad fiber to be inscribed. Then it is transmitted through the fourth fiber coupling component to the second multimode fiber, and finally enters the first spectrometer 10.

[0033] Among them, the first and second multimode fibers are used to stabilize the mode excitation state and extend the optical path length. Their core diameter is also the same as or equivalent to the inner cladding diameter of the double-clad fiber, thereby achieving the matching of the two fiber mode fields and efficiently coupling broadband light into the inner cladding and out from the inner cladding.

[0034] Before writing the inner cladding fiber Bragg grating, the broadband signal transmitted to the first spectrometer 10 is fixed as the original reference spectrum. During the subsequent writing process, the transmission spectrum of the inner cladding grating is obtained by real-time scanning using the first spectrometer 10; simultaneously, the reflection signal of the inner cladding grating is transmitted to the second spectrometer 11 via the aforementioned device, and its reflection spectrum is obtained by real-time spectral scanning.

[0035] Before fabrication, the processing parameters of the grating, as well as the dimensions and operating wavelengths of each transmission fiber in the spectral testing apparatus, are determined based on the inner cladding dimensions of the optical fiber and the designed pump light recovery wavelength, bandwidth, and other spectral parameters. Specifically, the length of the modulation plane, the interlayer spacing, and the number of plane layers are determined based on the inner cladding dimensions, thereby designing the modulation area of ​​the grating to cover the inner cladding region as much as possible; the initial period, chirp rate, and total length of the grating are determined based on the designed spectral parameters.

[0036] like Figure 3 As shown, the fabrication and spectral testing process of the inner cladding fiber grating is as follows: First, fix all the components in the fiber optic loop of the spectral measurement device to ensure stable optical path coupling. Turn on the ASE light source and use the first spectrometer 10 to detect the overall stability of the broadband transmission signal of the fiber optic loop. When the power fluctuation is within an acceptable range, fix the spectral signal as the original reference spectral signal. Then, in the processing software of the host computer, the preset grating processing parameters are input, and the software will automatically generate the preset processing path and import the path into the controller of the displacement stage; Subsequently, the program is executed in the host computer. The controller moves the displacement stage and switches the femtosecond laser according to the preset path. Through the linkage of these two mechanisms, the inner cladding grating is fabricated. During the fabrication process, the inner cladding grating can be fabricated through... Figure 2 The spectral testing device shown can be used to observe the transmission and reflection spectra of the inner cladding grating in real time. Finally, the preparation is completed, and the final transmission and reflection spectral data of the inner cladding grating are calibrated and saved in the first spectrometer 10 and the second spectrometer 11 to obtain the inner cladding grating sample.

[0037] This invention takes a double-clad optical fiber of model LMA-20 / 400-GDF-HP-M as an example, wherein the inner cladding diameter of the optical fiber is 400. .

[0038] First, based on the inner cladding size of the fiber and the pump light recovery spectral parameters, the processing parameters such as the grating modulation plane length, interlayer spacing, number of layers, and grating period are determined. Then, all components of the fiber loop of the spectral measurement device are fixed, the ASE broadband light source is turned on, and the original transmission reference spectrum of the fiber loop is detected and calibrated. The preset processing parameters are input into the host computer processing software, and the software automatically generates a three-dimensional processing path and imports it into the displacement stage controller. The controller controls the movement of the three-axis displacement stage and the switching of the femtosecond laser. Using multi-layer line-by-line scanning technology, the inner cladding fiber grating is directly etched in the inner cladding of the double-clad fiber through the fiber coating. The transmission and reflection spectra of the grating are monitored in real time by a dual spectrometer throughout the fabrication process.

[0039] The final modulation area was approximately 180. ×180 The inner cladding fiber grating has a uniform refractive index distribution across its cross-section. According to the spectral measurement device of this invention, the center wavelength is 976.5nm, the transmission depth is -1.3dB, and the 3dB bandwidth is about 6.6nm. It can effectively reflect residual pump light in the 976nm band.

[0040] The above experimental results verify the flexibility and effectiveness of the femtosecond laser direct writing preparation method of the present invention, as well as the reliability and accuracy of the spectral measurement device.

[0041] Therefore, this invention provides a femtosecond laser fabrication method and spectral measurement device for pump light recovery gratings, effectively solving the problems of low coupling efficiency, poor reliability, difficulty in reflecting at specific wavelengths, and limitations in the fabrication of inner cladding gratings and the inability to accurately characterize the spectrum in real time using traditional pump light recovery schemes. By employing femtosecond laser multilayer line-by-line scanning direct writing technology, a grating with a large modulation area and adjustable parameters is directly fabricated in the inner cladding through the coating layer, overcoming the limitations of material photosensitivity in traditional ultraviolet writing. Combined with a mode-field matched dual-spectrum measurement device, the transmission and reflection spectra of the grating are acquired in real time. This achieves efficient and stable recovery of residual pump light, reduces scattering loss, significantly improves the pump utilization rate and system stability of fiber lasers, simplifies the process of optimizing processing parameters, and adapts to the miniaturization, integration, and industrial application requirements of high-power fiber laser systems.

[0042] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for fabricating a femtosecond laser pump light recovery grating, characterized in that, Includes the following steps: A double-clad optical fiber is provided, wherein the double-clad optical fiber has an inner cladding; a femtosecond laser is directly focused through the coating layer of the double-clad optical fiber onto the interior of the inner cladding using a focusing objective lens, and a fiber Bragg grating is fabricated in the inner cladding using a multilayer line-by-line scanning technique. The multi-layer line-by-line scanning technology includes: scanning layer by layer along the y-axis to form multiple modulation planes, with each modulation plane superimposed in the depth direction and adjacent modulation planes having overlapping regions; The above layer-by-layer scanning is repeated along the x-axis with a preset grating period step to form multiple grating planes; the grating periods of each grating plane are kept consistent to form a uniform grating, or the grating periods of each grating plane are linearly increased to form a chirped grating; the modulation area of ​​the inner cladding fiber grating is controlled by adjusting the length of the modulation plane, the interlayer spacing and the number of layers, thereby obtaining the inner cladding fiber grating. The multi-layer line-by-line scanning technology is specifically as follows: during each layer scan, the femtosecond laser scans a straight line along the positive y-axis to form a rectangular modulation plane; after completing one layer, the femtosecond laser stops emitting light, moves back to the origin, and moves a preset distance d in the z-axis direction before scanning the next layer; the preset distance d is less than the depth formed by a single modulation plane in the z-axis direction.

2. A spectral measurement device for a pump-light-recovery fiber grating, employing the femtosecond laser fabrication method for a pump-light-recovery grating as described in claim 1, characterized in that, include: The system comprises an ASE broadband light source, a first multimode fiber, a third fiber coupling assembly, a fourth fiber coupling assembly, a first spectrometer, a second spectrometer, and a multimode fiber coupler. The input end of the multimode fiber coupler is coupled to the ASE broadband light source via the first fiber coupling assembly; the input end of the first multimode fiber is coupled to the output end of the multimode fiber coupler via the second fiber coupling assembly; the third fiber coupling assembly couples the output end of the first multimode fiber to one end of the inner cladding of a double-clad fiber for which an inner cladding grating is to be written; the fourth fiber coupling assembly couples the other end of the inner cladding of the double-clad fiber to the input end of the second multimode fiber; the first spectrometer is connected to the output end of the second multimode fiber; and the second spectrometer is connected to the other input end of the multimode fiber coupler.

3. The spectral measurement device for a pump light recovery fiber Bragg grating according to claim 2, characterized in that, The core diameter of the multimode fiber coupler, the core diameter of the first multimode fiber, and the core diameter of the second multimode fiber are all consistent with the inner cladding diameter of the double-clad fiber.

4. The spectral measurement device for a pump light recovery fiber Bragg grating according to claim 2, characterized in that, The broadband light emitted from the ASE broadband light source is coupled to one input end of the multimode fiber coupler via the first fiber coupling component, and then enters the inner cladding of the double-clad fiber from the output end of the multimode fiber coupler via the second fiber coupling component, the first multimode fiber, and the third fiber coupling component.

5. The spectral measurement device for a pump light recovery fiber Bragg grating according to claim 4, characterized in that, The light emitted from the inner cladding passes through the fourth fiber coupling component and the second multimode fiber into the first spectrometer; the reflected light from the inner cladding grating returns along the original path to the multimode fiber coupler, and is output from the other input end of the multimode fiber coupler to the second spectrometer.

6. The spectral measurement device for a pump light recovery fiber Bragg grating according to claim 5, characterized in that, The first multimode fiber and the second multimode fiber are used to stabilize the mode excitation state and extend the optical path length.

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