Reverse Pumped Signal Combiner, Preparation Method and Application

Abstract

The present invention provides a reverse-pumped signal combiner, a preparation method and an application thereof. An air-core fiber is used as the signal fiber of the combiner. The signal fiber and one end of multiple pump fibers form a combined end of a fiber bundle through a fused taper bunching. The signal fiber is located at the center of the fiber bundle, and the remaining multiple pump fibers are uniformly arranged around the signal fiber. The output fiber is fused to the combined end of the fiber bundle, and at the melting point formed by the alignment of the core of the air-core fiber in the fiber bundle and the core of the output fiber and the tight abutment and fusion of their end faces, Fresnel reflection can occur, and the mode field of the output fiber matches that of the air-core fiber. The prepared reverse-pumped signal combiner can be applied to a fiber laser system, undertaking the tasks of both a reverse-pumped signal combiner and a low-reflectivity grating at the same time. The air-core fiber therein can more flexibly achieve the control of the output light mode and beam quality.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of optical fiber devices, and in particular to a reverse pumping signal combiner, a preparation method and an application thereof. Background Art

[0002] The hollow-core fiber can confine light in the hollow core by virtue of its unique structure, providing a near-free-space propagation environment. Compared with traditional solid-core fibers, it has higher damage thresholds, lower optical nonlinear effects and lower waveguide dispersion, and has broad application prospects in the fields of sensing, communication, fiber gas lasers and high-power laser transmission. In order to apply excellent-performance hollow-core fibers in traditional fiber laser systems, two methods of spatial optical path coupling and fiber fusion splicing have been developed to achieve high-efficiency coupling of solid-core fibers and hollow-core fibers. Among them, the fiber fusion splicing method has developed technical means that can achieve mode field matching, withstand high-power lasers, and have low loss, and is promising to achieve miniaturization and all-fiber integration.

[0003] Fiber lasers have the advantages of compact structure, high conversion efficiency, good beam quality and flexible transmission, and have been widely used in many fields such as industrial processing, scientific research, and biomedicine. With the improvement of the pumping scheme and the performance of fiber passive devices, the output power of fiber laser systems has been continuously broken through. However, with the increase of optical power, the influence of nonlinear effects in the system becomes more obvious, which limits the increase of the output power of the fiber laser system and affects the beam quality of the output light. Research has found that adopting a reverse pumping scheme, reducing the fiber length and applying fibers with a large mode field diameter can effectively improve the nonlinear threshold of the system. The reverse pumping signal combiner is the core device for realizing reverse pumping in fiber lasers, and undertakes the tasks of efficiently coupling pump light into the active fiber cladding and high-quality transmission of signal light. Its performance directly determines the pumping ability of the laser system and the output optical power level.

[0004] The input signal fiber of the reverse pumping signal combiner generally serves as the output fiber of the entire fiber laser system. In order to reduce the nonlinear effect, the length of the signal fiber of the reverse pumping signal combiner is short, usually only 2-3 m in length, which limits the application scenarios of high-power fiber lasers. Moreover, the important fiber devices in the all-fiber structure fiber laser system include fiber gratings, pump signal combiners, cladding light filters, end caps, etc. They are connected by melting points. Excessive melting points will affect the stability of the system and the beam quality of the output to a certain extent. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the present invention provides a reverse pumping signal combiner, a preparation method and an application thereof.

[0006] Specifically, the technical solution of the present invention is as follows:

[0007] On the one hand, a preparation method of a reverse pumping signal combiner is proposed, including the following steps:

[0008] According to the requirements of specific application scenarios, a solid-core optical fiber and a hollow-core optical fiber are respectively selected as the output optical fiber and the signal optical fiber of the reverse pumping signal combiner;

[0009] The output optical fiber is obtained by heating and expanding the core of the solid-core optical fiber or performing a fused tapering operation and then performing a flat-angle cutting, so that the output optical fiber can achieve mode field matching with the hollow-core optical fiber;

[0010] Prepare pumping optical fibers and placeholder optical fibers, and use a porous tube optical fiber fixture to assist in bundling the pumping optical fibers and the placeholder optical fibers to obtain a first optical fiber bundle, wherein one end of each pumping optical fiber and the placeholder optical fiber is stripped of a certain length of coating, and multiple pumping optical fibers are evenly arranged around the placeholder optical fiber;

[0011] The first optical fiber bundle is inserted into a pre-tapered glass tube to obtain a second optical fiber bundle in a sleeve;

[0012] The placeholder optical fiber is pulled out from the second optical fiber bundle to obtain a third optical fiber bundle;

[0013] The third optical fiber bundle is subjected to fused tapering to obtain a fourth optical fiber bundle, and a central hole is maintained in the center of the fourth optical fiber bundle;

[0014] The waist region of the fourth optical fiber bundle obtained after fused tapering is cut by an optical fiber cutter, ensuring that the waist region of the optical fiber bundle has a certain length after cutting and the end face of the waist region is flat and of good quality, to obtain a fifth optical fiber bundle, and a central hole is maintained in the center of the fifth optical fiber bundle;

[0015] One end of the hollow-core optical fiber is stripped of a certain length of coating, and at the same time, the cladding of the end of the hollow-core optical fiber where the coating is stripped is etched according to the size of the central hole of the fifth optical fiber bundle and a flat-angle cutting is performed after etching, so that the hollow-core optical fiber can be inserted into the central hole;

[0016] The etched hollow-core optical fiber is inserted into the central hole of the fifth optical fiber bundle to complete bundling, obtaining a sixth optical fiber bundle;

[0017] The sixth optical fiber bundle is fused to the output optical fiber: first, the core alignment of the hollow-core optical fiber and the output optical fiber is realized by using a beam quality feedback alignment system, then the hollow-core optical fiber is moved away from the melting point (i.e., the melting point when the fifth optical fiber bundle is fused to the output optical fiber) by using a precision displacement stage, the fifth optical fiber bundle is fused to the output optical fiber by using an optical fiber fusion splicer, and finally the hollow-core optical fiber in the sixth optical fiber bundle is moved by using a precision displacement stage to make the end faces of the hollow-core optical fiber and the output optical fiber in contact and then fused;

[0018] The production of the reverse pumping signal combiner is completed.

[0019] Fuse the hollow-core fiber in the sixth fiber bundle with the output fiber: First, use the beam quality feedback alignment system to align the cores of the hollow-core fiber and the output fiber. Then, use a precision displacement stage to move the hollow-core fiber away from the melting point to avoid deformation of the microstructure of the hollow-core fiber caused by the fusion of the fifth fiber bundle and the output fiber. Use a fiber fusion splicer to fuse the fifth fiber bundle with the output fiber. Finally, use the precision displacement stage to tightly press the end faces of the hollow-core fiber and the output fiber in the sixth fiber bundle together and then fuse them. During the fusion process, control the discharge amount so that the microstructure of the hollow-core fiber does not deform and collapse;

[0020] Complete the fabrication of the backward-pumped signal combiner.

[0021] Furthermore, the present invention further includes: A hollow-core fiber roughened section with a certain length is provided on the signal fiber at a certain distance from the combining end of the backward-pumped signal combiner as a cladding light filter. The hollow-core fiber roughened section is formed by stripping the coating layer of a certain length of the hollow-core fiber and then corroding and roughening the cladding of the hollow-core fiber with a corrosive agent.

[0022] Furthermore, the hollow-core fiber is one of an anti-resonant hollow-core fiber, a kagome fiber, a nested anti-resonant hollow-core fiber, a hollow-core photonic bandgap fiber, a capillary fiber, and a multi-hole fiber. And according to specific application requirements, the length of the hollow-core fiber can be flexibly selected, and its length can be 3 - 100 m, or even longer. According to the requirements of the output beam quality of the fiber laser, select the appropriate number of modes supported by the hollow-core fiber, such as a single-mode hollow-core fiber or a few-mode hollow-core fiber, and use the number of modes supported by the hollow-core fiber at the backward-pumped signal combiner to perform mode filtering and selection functions to achieve precise control of the number of modes and beam quality of the laser output light.

[0023] On the other hand, the present invention provides a backward-pumped signal combiner prepared by the preparation method of a backward-pumped signal combiner, including a signal fiber, an output fiber, and multiple pump fibers. The signal fiber is a hollow-core fiber, the output fiber is a solid-core fiber. The same ends of the signal fiber and the multiple pump fibers are formed into a combining end of a fiber bundle through a fused taper bunching. The combining end of the fiber bundle has a taper region and a waist region. The signal fiber is located at the central position of the fiber bundle, and the remaining multiple pump fibers are uniformly arranged around the signal fiber. The output fiber is fused with the combining end of the fiber bundle, and at the melting point formed by aligning the cores of the hollow-core fiber and the output fiber in the fiber bundle and tightly pressing their end faces together for fusion, Fresnel reflection can occur, and the mode field of the output fiber matches that of the hollow-core fiber.

[0024] Further, a hollow fiber texturing section with a certain length is provided on the signal optical fiber at a certain distance from the beam combining end as a cladding light filter. The hollow fiber texturing section is formed by stripping the coating layer of a certain length of hollow fiber and then corroding and texturing the cladding of the hollow fiber with an etchant.

[0025] The backward pumping signal combiner prepared by the preparation method of the above backward pumping signal combiner can be applied to a fiber laser system, and simultaneously undertake the tasks of the backward pumping signal combiner and the low reflectivity grating, which can simplify the structure of the fiber laser, realize high-quality long-distance transmission of laser in the output optical fiber of the laser, meet the requirements of some remote operations of high-power fiber lasers, and expand its application scenarios.

[0026] Compared with the traditional backward pumping signal combiner with a solid core optical fiber as the signal optical fiber, the beneficial effects of the present invention are as follows:

[0027] (1) A hollow fiber with a lower nonlinear effect is adopted. In a high-power fiber laser system, the length of the signal optical fiber of the backward pumping signal combiner can be taken longer, and the fiber laser system and the output port can be distributed at a long distance, which can ensure the remote operation of the laser system in a special environment and expand the application scenarios of high-power fiber lasers;

[0028] (2) According to specific application requirements, a hollow fiber supporting different mode numbers can be flexibly selected, including: single-mode hollow fiber, few-mode hollow fiber, multi-mode hollow fiber, etc., which can realize the control of the mode number and beam quality of the output light of the fiber laser system and meet the application requirements of different scenarios;

[0029] (3) The output optical fiber and the hollow fiber are directly fusion spliced at a flat angle, and the Fresnel reflection at the melting point can play the role of a low reflectivity grating and participate in forming the resonant cavity of the fiber laser;

[0030] (4) Applying the backward pumping signal combiner prepared by the method of the present invention to a fiber laser, compared with the traditional fiber laser system, the present invention integrates the functions of the traditional backward pumping signal combiner, the low reflectivity grating and the cladding light filter into one backward pumping signal combiner based on a hollow fiber, reduces the melting points in the fiber laser system, and is beneficial to improving the stability of the system and reducing the deterioration of the beam quality of the output light. Description of the Drawings

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or in the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on the structures shown in these drawings.

[0032] Figure 1 It is a schematic structural diagram of a hollow fiber after cladding corrosion, texturing, and cutting in an embodiment;

[0033] Figure 2 It is a schematic structural diagram of a reverse pump signal combiner after fusion splicing in an embodiment;

[0034] Figure 3 It is a schematic diagram of an optical fiber laser system built in an embodiment;

[0035] Figure 4 It is a mode field distribution diagram of the optical wave modes that can be stably transmitted in a triple-nested anti-resonant hollow fiber used in an embodiment, where Figure 4 (a) is the mode field distribution diagram of the fundamental mode of the hollow fiber, Figure 4 (b) is the mode field distribution diagram of the LP 11 mode of the hollow fiber.

[0036] Reference numerals in the drawings:

[0037] 1. Hollow fiber; 2. Reverse pump signal combiner; 11. Corroded section of the hollow fiber; 12. Textured section of the hollow fiber; 21. Output fiber; 22. Expanded core area of the output fiber; 23. Waist area; 24. Taper area; 25. Pump fiber; 31. Forward pump combiner; 32. High reflectivity fiber grating; 33. Gain fiber; 34. End cap. Detailed implementation manners

[0038] The following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the drawings in the embodiments of the present invention. Obviously, the described embodiments are only some, rather than all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention.

[0039] Referring to Figure 1 , an embodiment of the present invention provides a method for preparing a reverse pump signal combiner, including the following steps:

[0040] (1) Prepare materials:

[0041] According to the requirements of specific application scenarios, a solid-core optical fiber and a hollow-core optical fiber are respectively selected as the output optical fiber and the signal optical fiber of the reverse pumping signal combiner;

[0042] The output optical fiber is obtained by heating and expanding the core of the solid-core optical fiber or performing a fused biconical taper operation and then performing a flat-angle cutting, so that the output optical fiber and the hollow-core optical fiber can achieve mode field matching;

[0043] Prepare pump optical fibers and placeholder optical fibers, and use a porous tube optical fiber fixture to assist in bundling the pump optical fibers and the placeholder optical fibers to obtain a first optical fiber bundle, wherein one end of each pump optical fiber and the placeholder optical fiber is stripped of a certain length of coating, and multiple pump optical fibers are evenly arranged around the placeholder optical fiber.

[0044] (2) Bundling:

[0045] Insert the first optical fiber bundle into a pre-fused biconical tapered glass tube to obtain a second optical fiber bundle in the sleeve;

[0046] Withdraw the placeholder optical fiber from the second optical fiber bundle to obtain a third optical fiber bundle;

[0047] Perform a fused biconical taper on the third optical fiber bundle to obtain a fourth optical fiber bundle, and a central hole is maintained in the center of the fourth optical fiber bundle;

[0048] Use an optical fiber cutter to cut the waist area of the fourth optical fiber bundle obtained after the fused biconical taper, ensure that the waist area of the optical fiber bundle has a certain length after cutting and the end face of the waist area is flat and of good quality to obtain a fifth optical fiber bundle, and a central hole is maintained in the center of the fifth optical fiber bundle;

[0049] Strip a certain length of coating from one end of the hollow-core optical fiber, and at the same time, corrode the cladding of the end of the hollow-core optical fiber where the coating is stripped according to the size of the central hole of the fifth optical fiber bundle and perform a flat-angle cutting after corrosion, so that the hollow-core optical fiber can be inserted into the central hole;

[0050] Insert the hollow-core optical fiber after the corrosion treatment into the central hole of the fifth optical fiber bundle to complete the bundling and obtain a sixth optical fiber bundle;

[0051] Fuse the hollow-core optical fiber in the sixth optical fiber bundle with the output optical fiber: First, use a beam quality feedback alignment system to align the cores of the hollow-core optical fiber and the output optical fiber, then use a precision displacement stage to move the hollow-core optical fiber away from the melting point to avoid deformation of the microstructure of the hollow-core optical fiber when the fifth optical fiber bundle is fused with the output optical fiber, use an optical fiber fusion splicer to fuse the fifth optical fiber bundle with the output optical fiber, and finally use a precision displacement stage to press the end faces of the hollow-core optical fiber and the output optical fiber in the sixth optical fiber bundle tightly and then fuse them. During the fusion process, control the discharge amount so that the microstructure of the hollow-core optical fiber will not deform and collapse;

[0052] Complete the production of the reverse pumping signal combiner.

[0053] In the present invention, an air-core fiber is used as the signal fiber of the reverse pumping signal combiner, which has the following advantages: The air-core fiber has a lower non-linear effect. When the reverse pumping signal combiner with an air-core fiber as the signal fiber is applied to a high-power fiber laser system, the length of the signal fiber of the reverse pumping signal combiner can be longer, and the fiber laser system and the output port can be distributed at a long distance, which can ensure the remote operation of the laser system in a special environment and expand the application scenarios of high-power fiber lasers. Flexibly selecting an air-core fiber that supports different mode numbers can realize the control of the mode number and beam quality of the output light of the fiber laser system, meeting the requirements of various application scenarios. Both the output fiber and the air-core fiber are cut at a flat angle, that is, the cutting end faces of the output fiber and the air-core fiber are flat angles, so as to ensure that Fresnel reflection occurs at the melting point formed by the fusion of the output fiber and the air-core fiber. During the fusion of the air-core fiber in the sixth fiber bundle and the output fiber, the end faces (both are planes) of the air-core fiber and the output fiber are closely attached for fusion, and the Fresnel reflection at the melting point can play the role of a low-reflectivity grating and participate in the formation of the resonant cavity of the fiber laser. In this way, the reverse pumping signal combiner and the low-reflectivity grating can be integrated onto one reverse pumping signal combiner, reducing the melting points in the fiber laser system, which is beneficial to improving the stability of the system and reducing the deterioration of the beam quality of the output light.

[0054] In the present invention, the output fiber is obtained by heating and expanding the core or performing a fused biconical taper operation on the solid-core fiber and then cutting it at a flat angle, so that the output fiber and the air-core fiber can achieve mode field matching and reduce the loss caused by mode field mismatch. Specifically, it includes the following steps:

[0055] Calculate the mode field diameters of the solid-core fiber and the air-core fiber;

[0056] Based on the calculated mode field diameters of the solid-core fiber and the air-core fiber, determine whether to perform a fused biconical taper operation or a heating and expanding operation on the solid-core fiber. If the mode field diameter of the solid-core fiber is larger than that of the air-core fiber, perform a fused biconical taper on the solid-core fiber. If the mode field diameter of the solid-core fiber is smaller than that of the air-core fiber, perform a heating and expanding operation on the solid-core fiber;

[0057] Calculate the mode field diameter of the output fiber that can achieve mode field matching between the output fiber and the air-core fiber, determine the core diameter of the output fiber that achieves mode field matching, and further determine the heating and expanding or fused biconical taper parameters of the solid-core fiber;

[0058] According to the determined fused biconical taper or heating and expanding parameters, perform a heating and expanding or fused biconical taper operation on one end of the solid-core fiber to obtain the output fiber, and the output fiber and the air-core fiber can achieve mode field matching.

[0059] Among them, the fusion tapering method uses a tapering machine to taper the output optical fiber, heats the optical fiber with a burner head, and the clamps on both sides of the burner head move at different speeds to achieve precise tapering of the optical fiber. By setting parameters such as the burner head fire volume, the movement speed of the optical fiber clamp, and the dimensions and lengths of the taper region and the waist region, the shape and size of the tapered optical fiber can be precisely controlled, the sizes of the cladding and the core can be reduced, and the mode field diameter can be reduced. The tapering parameters mainly include: the lengths and dimensions of the tapering waist region and the taper region, and the length of the taper region needs to meet the adiabatic tapering condition.

[0060] The heating and core expanding method heats the output optical fiber through a hydrogen-oxygen flame, enables the doping in the core of the optical fiber to diffuse into the cladding, expands the core diameter, and thus realizes the expansion of the mode field diameter.

[0061] As Figure 1 Shown is a schematic structural diagram of a hollow-core optical fiber after cladding corrosion, texturing, and cutting in an embodiment. A certain length of the coating layer is stripped from one end of the hollow-core optical fiber 1, and the section of the hollow-core optical fiber with the coating layer stripped is the section of the hollow-core optical fiber to be corroded. Before corroding the cladding of the section of the hollow-core optical fiber with the coating layer stripped according to the size of the central hole of the fifth optical fiber bundle, a solid-core optical fiber is first fused to one end of the hollow-core optical fiber 1 to be corroded to prevent the corrosive liquid from entering the hollow-core optical fiber and damaging the internal structure. The cladding size of the fused solid-core optical fiber must be larger than the size of the microstructural region of the hollow-core optical fiber. Then, the cladding of the hollow-core optical fiber is corroded using a hydrofluoric acid solution, and by controlling the corrosion time, a corroded section 11 of the hollow-core optical fiber with a fiber cladding size matching the size of the central hole of the fifth optical fiber bundle is obtained. Further, in the preparation method of the backward pumping signal combiner provided in an embodiment, in addition to the steps provided in the above embodiment, it further includes: a hollow-core optical fiber texturing section 12 with a certain length is provided on the signal optical fiber at a certain distance from the combining end of the backward pumping signal combiner as a cladding light filter. The hollow-core optical fiber texturing section 12 is formed by stripping a certain length of the coating layer of the hollow-core optical fiber and then corroding and texturing the cladding of the hollow-core optical fiber using a corrosive agent. In this way, the functions of the backward pumping signal combiner, the low-reflectivity grating, and the cladding light filter can be integrated into one backward pumping signal combiner. When the backward pumping signal combiner is applied to a fiber laser, the melting points in the fiber laser system will be reduced, which is beneficial to improving the stability of the system and reducing the degradation of the output light beam quality.

[0062] Preferably, when corroding the cladding of the hollow-core optical fiber using a hydrofluoric acid solution, multiple hollow-core optical fibers can be set as controls at the same time. In the early stage, one control optical fiber is taken out every same time to measure the diameter, a more accurate corrosion speed is obtained, and the accurate corrosion time is calculated. In the later stage, when approaching the calculated corrosion time, one optical fiber is taken out every shorter time to measure its diameter until a suitable fiber cladding size is obtained.

[0063] In the present invention, the type of the hollow-core optical fiber is not limited and can be any one of an anti-resonant hollow-core optical fiber, a kagome optical fiber, a nested anti-resonant hollow-core optical fiber, a hollow-core photonic bandgap optical fiber, a capillary optical fiber, and a multi-hole optical fiber. And according to specific application requirements, the length of the hollow-core optical fiber can be flexibly selected, and its length can be 3 - 100 m, or even longer. According to the requirements for the output beam quality of the fiber laser, the number of modes supported by a suitable hollow-core optical fiber is selected, such as a single-mode hollow-core optical fiber or a few-mode hollow-core optical fiber. At the reverse-pumping signal combiner, the mode filtering and selection functions are performed using the number of modes supported by the hollow-core optical fiber to achieve precise control over the number of modes and the beam quality of the output light of the laser.

[0064] In one embodiment, taking a double-clad output optical fiber with a core / cladding size of 20 / 400 μm (NA = 0.06 / 0.46), a hollow-core optical fiber with a size of 30 / 250 μm, and six double-clad pumping optical fibers with structural parameters of 220 / 242 μm (NA = 0.22) as examples, the preparation method of the reverse-pumping signal combiner provided by the present invention is used for production description. The specific steps are as follows:

[0065] (1) Prepare materials:

[0066] Prepare an output optical fiber, a signal optical fiber, a pumping optical fiber, and a glass tube. Among them, the output optical fiber is selected as a double-clad solid-core optical fiber with a size of 20 / 400 μm (NA = 0.06 / 0.46), the signal optical fiber is selected as a hollow-core optical fiber 1 with a core of 30 μm, an inner cladding of 87 μm, an outer cladding of 240 μm, and a capillary thickness of 1.15 - 1.2 μm. The pumping optical fiber 25 is six double-clad solid-core optical fibers with a size of 220 / 242 μm (NA = 0.22), and the glass tube is selected as a thin-walled semi-fluorinated glass tube with an inner / outer diameter of 850 / 1000 μm.

[0067] Calculate the mode field diameters of the output fiber and the hollow core fiber according to the wavelength to be applied. Taking the wavelength to be applied as 1080 nm as an example, it can be obtained that the mode field diameter of the selected double-clad fiber of 20 / 400 μm (NA = 0.06 / 0.46) is smaller than that of the hollow core fiber 1. Therefore, it is necessary to heat-expand the core of the selected double-clad fiber of 20 / 400 μm (NA = 0.06 / 0.46) to match the mode fields of the output fiber and the hollow core fiber. It can be calculated that the core diameter of the 20 / 400 μm (NA = 0.06 / 0.46) double-clad fiber used as the output fiber needs to be heat-expanded to about 26 μm. The heat-expansion method is used to obtain the core expansion region 22 of the output fiber, realizing the mode field matching between the solid-core output fiber 21 and the hollow core fiber 1. Heat-expansion is achieved by discharging at the end face of the fiber through the electrodes of a fusion splicer or heating with a hydrogen-oxygen flame, so that the doping substances in the fiber core diffuse into the cladding, and thus the fiber mode field becomes larger as the core size increases. During the heating process, the size of the cladding remains almost unchanged. The core diameter of the core expansion region 22 of the output fiber changes gradually, and decreases to the original core diameter size as the distance from the heat source increases. In this embodiment, the mode field diameter of the output fiber is smaller than that of the hollow core fiber. By precisely controlling the heating time, the core diameter of the output fiber is expanded to 26 μm, and then the output fiber is cut in the middle region of the heat-expansion, and the core diameter of the cut fiber is observed with a microscope to ensure that the core of the output fiber meets the mode field matching conditions.

[0068] Furthermore, it can be determined that the hollow core fiber needs to be etched to 133 μm, and the glass tube needs to be pre-tapered to 726 / 854 μm.

[0069] Strip the coating layer with a length of about 8 cm at one end of the hollow core fiber 1, and fuse a solid-core fiber with a cladding diameter of 250 μm at the end where the coating layer is stripped, and then insert it into a hydrofluoric acid solution for cladding etching. At the same time, set multiple control hollow core fibers to detect the etching process. After etching is completed, perform a flat-angle cut to obtain a hollow core fiber etched section 11 with a diameter of 133 μm. Strip the coating layer with a length of about 5 cm at about 1 m before the other end of the hollow core fiber 1, and then use an etchant prepared by mixing 55% concentrated hydrofluoric acid and glass frosting paste to roughen the cladding in the coating layer stripping section to obtain a hollow core fiber roughened section 12 with a length of 3 cm, as Figure 1 shown is a schematic structural diagram of the hollow core fiber after cladding etching, roughening, and cutting in an embodiment.

[0070] Prepare 7 220 / 242 μm (NA=0.22) double-clad solid-core optical fibers, and insert the 7 optical fibers into a 7-hole optical fiber fixture to obtain the first optical fiber bundle, wherein the optical fiber inserted from the central hole of the 7-hole optical fiber fixture is the placeholder optical fiber, and the other optical fibers inserted from the 6 holes evenly distributed around are pump optical fibers 25, wherein one end of each pump optical fiber and the placeholder optical fiber is stripped of a coating layer of about 8 cm in length, and the placeholder optical fiber inserted into the central hole can also be stripped of a slightly longer length of the coating layer to facilitate the distinction between the pump optical fiber and the placeholder optical fiber, and then wipe it clean with anhydrous ethanol to ensure that the surface of the optical fiber cladding is clean.

[0071] Use anhydrous ethanol to clean the wall of the glass tube, and then pre-tap it to an inner / outer diameter of 726 / 854 μm, a waist length of about 3 cm, and a taper length of about 8 mm at both ends for use.

[0072] (2) Bundle:

[0073] Align the stripped ends of the fiber coating of the first fiber bundle. At this time, the end face of the pump fiber 25 is flush, while the placeholder fiber is slightly longer. Insert the first fiber bundle into anhydrous ethanol, and continuously shake and twist the 7-hole fiber clamp to make the placeholder fiber in the center. The pump fibers are neatly and straightly arranged around the placeholder fiber to avoid staggered twisting of the pump fibers. Then, slowly pull it out of the anhydrous ethanol and insert it into the pre-tapered glass tube to obtain the second fiber bundle of the sleeve. There is a slight resistance during the process of inserting the fiber bundle, indicating that the inner diameter of the glass tube is close to the diameter of the fiber bundle.

[0074] The placeholder optical fiber is extracted from the second optical fiber bundle, and the optical fiber bundle is placed on a heating table and an exhaust fan is used to extract the residual anhydrous ethanol in the glass tube to prevent the glass tube and the optical fiber bundle from being burned in the subsequent taper drawing process, thereby obtaining a third optical fiber bundle.

[0075] Adjust the taper parameters to taper the fiber bundle, including the amount of fire head, taper speed, taper area and waist area size, etc. The parameters of the taper area 24 and waist area 23 of the fiber bundle need to meet the adiabatic taper conditions. The length of the taper area 24 can be set to 1.5 cm, the length of the waist area 23 can be set to 3 cm, and the waist area diameter can be the same as the output fiber cladding diameter of 400 μm. The third fiber bundle is melt tapered to obtain a fourth fiber bundle, and the center of the fourth fiber bundle maintains a center hole.

[0076] The waist region of the fourth optical fiber bundle obtained after the fusion taper is cut with an optical fiber cutter to ensure that the waist region of the optical fiber bundle has a certain length (about 2 cm) after cutting and the end face of the waist region is flat and of good quality, thereby obtaining a fifth optical fiber bundle, wherein the center of the fifth optical fiber bundle maintains a center hole.

[0077] The hollow-core optical fiber after cladding corrosion, roughening and cutting is inserted into the central hole of the fifth optical fiber bundle to complete the bundle assembly and obtain the sixth optical fiber bundle.

[0078] Fuse the sixth optical fiber bundle with the output optical fiber: First, use the beam quality feedback alignment system to align the cores of the hollow-core optical fiber and the output optical fiber. Then, use the precision displacement stage to move the hollow-core optical fiber in the sixth optical fiber bundle, moving the hollow-core optical fiber away from the melting point (i.e., the melting point when the fifth optical fiber bundle is fused with the output optical fiber, where the fifth optical fiber bundle is the optical fiber bundle surrounding the hollow-core optical fiber of the sixth optical fiber bundle), to avoid deformation of the microstructure of the hollow-core optical fiber caused by the fusion of the fifth optical fiber bundle and the output optical fiber. Use an optical fiber fusion splicer to fuse the fifth optical fiber bundle with the output optical fiber. Finally, use the precision displacement stage to move the hollow-core optical fiber in the sixth optical fiber bundle, and press the end faces of the hollow-core optical fiber and the output optical fiber in the sixth optical fiber bundle tightly together. During the fusion process, control the discharge amount to ensure that the microstructure of the hollow-core optical fiber does not deform and collapse. Use beam quality feedback alignment during the fusion of the hollow-core optical fiber and the output optical fiber to ensure the core alignment of the hollow-core optical fiber and the output optical fiber in the sixth optical fiber bundle, so as to reduce the insertion loss and reduce the influence on the spot characteristics of the device.

[0079] Finally, complete the fabrication of the backward pumping signal combiner.

[0080] The present invention adopts a two-step fusion method. After achieving alignment, first fuse the fifth optical fiber bundle with the output optical fiber, then use the precision displacement stage to press the end faces of the hollow-core optical fiber and the output optical fiber tightly together, and perform fusion with a small discharge amount to ensure that the microstructure of the hollow-core optical fiber does not deform and collapse during the fusion process.

[0081] As Figure 2As shown, a backward pumping signal combiner prepared by using the preparation method of the backward pumping signal combiner provided in the above embodiment is provided, which includes a signal optical fiber, an output optical fiber 21, and multiple pump optical fibers 25. The signal optical fiber is a hollow optical fiber 1, and the output optical fiber 21 is a solid optical fiber. The same ends of the signal optical fiber and the multiple pump optical fibers 25 are combined into a fiber bundle through a fused biconical taper to form a combining end of the fiber bundle. The combining end of the fiber bundle has a tapered region 24 and a waist region 23. The signal optical fiber is located at the center of the fiber bundle, and the remaining multiple pump optical fibers 25 are uniformly arranged around the signal optical fiber. The output optical fiber 21 is fused to the combining end of the fiber bundle, and the cores of the hollow optical fiber 1 and the output optical fiber 21 in the fiber bundle are aligned. And Fresnel reflection can occur at the melting point formed by the alignment and tight butt welding of the end faces of the hollow optical fiber 1 and the output optical fiber 21 in the fiber bundle, and the mode field of the output optical fiber 21 matches that of the hollow optical fiber 1. In this embodiment, the output optical fiber 21 has an output optical fiber core expansion region 22 to ensure the mode field matching between the output optical fiber 21 and the hollow optical fiber 1. The cutting end faces of the output optical fiber 21 and the hollow optical fiber 1 are flat angles to ensure that Fresnel reflection will occur at the melting point formed by the fusion of the output optical fiber 21 and the hollow optical fiber 1. Further, a hollow optical fiber roughened section 12 with a certain length is provided on the signal optical fiber at a certain distance from the combining end as a cladding light filter. The hollow optical fiber roughened section 12 is formed by stripping the coating layer of a certain length of the hollow optical fiber and then corroding and roughening the cladding of the hollow optical fiber with a corrosive agent.

[0082] The backward pumping signal combiner prepared in the above embodiment is applied to a high beam quality fiber laser system to simultaneously realize the functions of the backward pumping signal combiner, the low reflectivity grating, and the cladding light filter. Figure 3Schematic diagram of an optical fiber laser system built in an embodiment, including a forward pumping combiner 31, a high reflectivity fiber grating 32, a gain fiber 33, a reverse pumping signal combiner 2 based on a hollow core fiber, and an end cap 34. The forward pumping combiner 31, the high reflectivity fiber grating 32, the gain fiber 33, the reverse pumping signal combiner 2, and the end cap 34 are connected in sequence. The signal fiber of the reverse pumping signal combiner serves as the output fiber of the signal light. The reverse pumping signal combiner plays the functions of pumping light injection, signal light transmission, forming a resonant cavity, cladding light filtering, and long-distance transmission of the output light in the optical fiber laser system. Benefiting from the advantage of the low nonlinear effect of the hollow core fiber 1, a relatively long length of the hollow core fiber 1 can be used as the output fiber of the optical fiber laser system, expanding the application of high-power optical fiber lasers. The reverse pumping signal combiner 2 based on the hollow core fiber 1 realizes multi-functional integration, reduces the melting points in the system, and can effectively improve the system stability and reduce the risk of beam quality degradation. The output fiber core expansion region 22 in the reverse pumping signal combiner 2 may introduce high-order modes, affecting the beam quality of the output light of the optical fiber laser system. To solve this problem, in one embodiment, a triple nested anti-resonant hollow core fiber that only transmits the fundamental mode and LP 11 mode at a wavelength of 1080 nm is selected. It has the same mode as a 20 / 400 μm solid fiber, can effectively filter out the high-order modes introduced by the output fiber core expansion region 22, and ensure low loss and good beam quality, as Figure 4 shown Figure 4 is the mode field distribution diagram of the optical wave modes that can be stably transmitted in the triple nested anti-resonant hollow core fiber used in this embodiment, where Figure 4 (a) is the mode field distribution diagram of the fundamental mode of the hollow core fiber, Figure 4 (b) is the mode field distribution diagram of the LP 11 mode of the hollow core fiber.

[0083] Furthermore, according to specific application requirements, single-mode hollow core fibers, few-mode hollow core fibers, or multi-mode hollow core fibers with different microstructures and sizes can be selected. By selecting the corresponding hollow core fiber, the control of the output light mode and beam quality of the optical fiber laser system can be realized more flexibly, and the laser output with different beam qualities can be achieved to meet diverse application requirements. The present invention can simplify the structure of the optical fiber laser, realize high-power laser high-quality long-distance transmission in the laser output fiber, and effectively control the beam quality of the output light.

[0084] Matters not described in the present invention are well-known technologies.

[0085] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity of description, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, it should be considered as the scope described in this specification.

[0086] The above-described embodiments merely represent several implementation manners of the present application. The description thereof is relatively specific and detailed, but it should not be construed as a limitation to the scope of the invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present application, several modifications and improvements can still be made, and these all fall within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the appended claims.

[0087] The above is only the preferred embodiment of the present invention and is not used to limit the present invention. For those skilled in the art, the present invention can have various changes and modifications. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a backward-pumped signal beam combiner, characterized in that It includes the following steps: According to the requirements of specific application scenarios, select a solid-core optical fiber and a hollow-core optical fiber as the output optical fiber and the signal optical fiber of the backward pumping signal combiner respectively; After heating and expanding the core of the solid-core optical fiber or performing a fused biconical taper operation, perform a flat-angle cutting to obtain the output optical fiber, so that the output optical fiber and the hollow-core optical fiber can achieve mode field matching; Prepare the pump optical fiber and the placeholder optical fiber, and use a porous tube optical fiber fixture to assist in bundling the pump optical fiber and the placeholder optical fiber to obtain a first optical fiber bundle, where one end of each pump optical fiber and the placeholder optical fiber is stripped of a certain length of coating, and multiple pump optical fibers are evenly arranged around the placeholder optical fiber; Insert the first optical fiber bundle into a pre-fused biconical glass tube to obtain a second optical fiber bundle in the sleeve; Withdraw the placeholder optical fiber from the second optical fiber bundle to obtain a third optical fiber bundle; Perform a fused biconical taper on the third optical fiber bundle to obtain a fourth optical fiber bundle, and a central hole is maintained in the center of the fourth optical fiber bundle; Use an optical fiber cutter to cut the waist area of the fourth optical fiber bundle obtained after the fused biconical taper, ensure that the waist area of the optical fiber bundle has a certain length after cutting and the end face of the waist area is flat and of good quality, to obtain a fifth optical fiber bundle, and a central hole is maintained in the center of the fifth optical fiber bundle; Strip a certain length of coating from one end of the hollow-core optical fiber, and at the same time, corrode the cladding of the end of the hollow-core optical fiber where the coating is stripped according to the size of the central hole of the fifth optical fiber bundle and perform a flat-angle cutting after corrosion, so that the hollow-core optical fiber can be inserted into the central hole; Insert the corroded hollow-core optical fiber into the central hole of the fifth optical fiber bundle to complete the bundling and obtain a sixth optical fiber bundle; Fuse the sixth optical fiber bundle and the output optical fiber: first use a beam quality feedback alignment system to align the cores of the hollow-core optical fiber and the output optical fiber, then use a precision displacement stage to move the hollow-core optical fiber away from the melting point, use an optical fiber fusion splicer to fuse the fifth optical fiber bundle and the output optical fiber, and finally use a precision displacement stage to move the hollow-core optical fiber to make the end faces of the hollow-core optical fiber and the output optical fiber in the sixth optical fiber bundle close to each other and then fuse them; Complete the production of the backward pumping signal combiner.

2. The preparation method of the reverse-pumped signal combiner according to claim 1, characterized in that After heating and expanding the core of the solid-core optical fiber or performing a fused biconical taper operation and then performing a flat-angle cutting to obtain the output optical fiber, so that the output optical fiber and the hollow-core optical fiber can achieve mode field matching, it includes the following steps: Calculate the mode field diameters of the solid-core optical fiber and the hollow-core optical fiber; Based on the calculated mode field diameters of the solid-core optical fiber and the hollow-core optical fiber, determine to perform a heating and expanding core or a fused biconical taper operation on the solid-core optical fiber. If the mode field diameter of the solid-core optical fiber is greater than that of the hollow-core optical fiber, perform a fused biconical taper on the solid-core optical fiber. If the mode field diameter of the solid-core optical fiber is less than that of the hollow-core optical fiber, perform a heating and expanding core on the solid-core optical fiber; Calculate the mode field diameter of the output optical fiber that can make the output optical fiber and the hollow-core optical fiber achieve mode field matching, determine the core diameter of the output optical fiber for achieving mode field matching, and further determine the heating and expanding core or fused biconical taper parameters of the solid-core optical fiber; According to the determined fused biconical taper or heating and expanding core parameters, perform a heating and expanding core or a fused biconical taper operation on one end of the solid-core optical fiber to obtain the output optical fiber, and the output optical fiber and the hollow-core optical fiber can achieve mode field matching.

3. The preparation method of the backward pumping signal combiner according to claim 1, wherein Before etching the cladding at one end of the hollow-core fiber by removing the coating according to the size of the central hole of the fifth fiber bundle, a solid-core fiber is first fused to one end of the hollow-core fiber to be etched to prevent the etching solution from entering the hollow-core fiber and damaging its internal structure. The cladding size of the fused solid-core fiber must be larger than the size of the microstructural region of the hollow-core fiber. Then, the cladding of the hollow-core fiber is etched using a hydrofluoric acid solution, and by controlling the etching time, a hollow-core fiber with a fiber cladding size matching the size of the central hole of the fifth fiber bundle is obtained.

4. The preparation method of the reverse-pumped signal combiner according to claim 1 or 2 or 3, characterized in that, It further includes: A hollow-core fiber texturing section with a certain length is provided on the signal fiber at a certain distance from the beam combining end of the backward pump signal combiner as a cladding light filter. The hollow-core fiber texturing section is formed by removing the coating of a certain length of the hollow-core fiber and then etching and texturing the cladding of the hollow-core fiber using an etching agent.

5. The preparation method of the reverse-pumped signal combiner according to claim 1, characterized in that, The hollow-core fiber is one of an anti-resonant hollow-core fiber, a kagome fiber, and a hollow-core photonic bandgap fiber. According to the requirements of the output beam quality of the fiber laser, the corresponding hollow-core fiber is selected. The hollow-core fiber is used for mode selection and filtering functions in the backward pump signal combiner to control the number of modes and the beam quality of the laser output light.

6. The backward pumping signal combiner prepared by the method for preparing a backward pumping signal combiner according to claim 1, characterized in that, It includes a signal fiber, an output fiber, and multiple pump fibers. The signal fiber is a hollow-core fiber, the output fiber is a solid-core fiber. The same ends of the signal fiber and the multiple pump fibers are formed into a beam combining end of a fiber bundle through a fused biconical taper. The beam combining end of the fiber bundle has a taper region and a waist region. The signal fiber is located at the center of the fiber bundle, and the remaining multiple pump fibers are evenly arranged around the signal fiber. The output fiber is fused to the beam combining end of the fiber bundle, and at the melting point formed by aligning the cores of the hollow-core fiber and the output fiber in the fiber bundle and tightly attaching their end faces for fusion, Fresnel reflection can occur, and the mode field of the output fiber matches that of the hollow-core fiber.

7. The backward pumping signal combiner according to claim 6, wherein The hollow-core fiber is one of an anti-resonant hollow-core fiber, a kagome fiber, and a hollow-core photonic bandgap fiber. According to the requirements of the output beam quality of the fiber laser, the corresponding hollow-core fiber is selected. The hollow-core fiber is used for mode selection and filtering functions in the backward pump signal combiner to control the number of modes and the beam quality of the laser output light.

8. The backward pumping signal combiner according to claim 6, characterized in that A hollow-core fiber texturing section with a certain length is provided on the signal fiber at a certain distance from the beam combining end as a cladding light filter. The hollow-core fiber texturing section is formed by removing the coating of a certain length of the hollow-core fiber and then etching and texturing the cladding of the hollow-core fiber using an etching agent.

9. Application of the backward pump signal combiner prepared by the preparation method of the backward pump signal combiner according to claim 1 in a fiber laser system.

10. The application according to claim 9, wherein The fiber laser system includes a forward pump combiner, a high-reflectivity fiber grating, a gain fiber, a backward pump signal combiner, and an end cap. The forward pump combiner, the high-reflectivity fiber grating, the gain fiber, the backward pump signal combiner, and the end cap are connected in sequence. The signal fiber of the backward pump signal combiner serves as the output fiber of the signal light.

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