Preparation method of a longitudinally diameter-varying gain optical fiber and the gain optical fiber

Abstract

The present invention proposes a method for preparing a longitudinally variable-diameter gain fiber, comprising the following steps: S1, preparing a tapered fiber with a long taper region having a constant cladding outer diameter and a gradually changing core diameter; S2, fusing an auxiliary fiber at each end of the long taper region; S3, heating and softening the fusion regions at both ends of the long taper region, while simultaneously moving the auxiliary fibers at both ends in opposite directions to a preset distance, so that the diameters at both ends of the long taper region gradually decrease longitudinally, forming a short taper region at each end of the long taper region; S4, cutting off the auxiliary fibers at both ends to obtain a longitudinally variable-diameter gain fiber. By preparing a short taper region at each end of the tapered fiber with a long taper region, the present invention, compared to conventional tapered fibers, increases the nonlinear volume-to-length ratio of the fiber with a constant cladding and a gradually changing core, thereby suppressing stimulated Raman scattering. The short tapers at both ends can reduce the number of high-order modes excited during amplification and cause them to leak out of the core, thereby optimizing beam quality.

Description

Technical Field

[0001] The present invention relates to the technical field of optical fiber preparation, and in particular to a preparation method of a longitudinally variable-diameter gain optical fiber and the gain optical fiber. Background Art

[0002] As third-generation lasers, fiber lasers boast numerous advantages, including high efficiency and high integration. Ytterbium-doped pulsed fiber lasers, in particular, are widely used in industry, medicine, military, and defense due to their small size, low cost, and high power. However, stimulated Raman scattering (SRS) caused by high peak power in optical fibers limits the power increase of pulsed fiber lasers. Although large-mode-area optical fibers suppress SRS, the increased mode area increases the number of higher-order modes in the fiber, leading to degradation of beam quality.

[0003] For a double-clad optical fiber, the larger its "nonlinear volume length ratio", the stronger its nonlinear suppression ability. The nonlinear volume length ratio of a conventional double-clad large mode field optical fiber is equal to its effective mode field area. The nonlinear volume length ratio calculation formula of a tapered optical fiber is: R = ∫ L A eff dl / L eff Compared to conventional fibers with uniform core-cladding dimensions, tapered fibers have a larger nonlinear volume-to-length ratio, which increases the stimulated Raman scattering threshold. The smaller core-cladding size at the signal input reduces the content of excited high-order modes, while the smaller core-cladding size at the signal output facilitates higher beam quality. Therefore, they have great application value in high-power laser technology.

[0004] Generally speaking, due to the limitations of the preparation process, the taper ratio of tapered optical fibers is usually controlled below 2. The current common taper method is to heat the optical fiber preform to make the taper. If the taper ratio is too high, it will not only greatly increase the difficulty of drawing, but also there is a risk of preform breakage during the drawing process; resulting in mediocre suppression of stimulated Raman scattering in high-power laser systems, especially the lack of obvious suppression effect on beam quality degradation. Summary of the Invention

[0005] The present invention provides a method for preparing a longitudinally variable-diameter gain fiber and a gain fiber, which solves the technical problem in the prior art that tapered fibers have limited effect on suppressing stimulated Raman scattering in high-power laser systems, especially having an insignificant effect on suppressing beam quality degradation.

[0006] The technical solution of the present invention is achieved as follows:

[0007] According to one aspect of the present invention, a method for preparing a longitudinally variable-diameter gain optical fiber is provided, comprising the following steps:

[0008] S1, preparing a tapered optical fiber with a constant cladding outer diameter and a gradually changing core diameter in a long tapered region;

[0009] S2, fusing a section of auxiliary optical fiber at each end of the long taper region;

[0010] S3, heating and softening the fusion zone at both ends of the long tapered zone, and simultaneously moving the auxiliary optical fibers at both ends in opposite directions to a preset distance, so that the diameters of both ends of the long tapered zone gradually decrease in the longitudinal direction, forming a short tapered zone at both ends of the long tapered zone;

[0011] S4, cutting off the auxiliary optical fibers at both ends to obtain a gain optical fiber with a longitudinally variable diameter.

[0012] The present invention prepares a short tapered region at both ends of a tapered optical fiber in a long tapered region. By combining the long and short tapered regions, the size of the input and output ends of the active optical fiber is further reduced. This can break through the limitations of traditional preform rod preparation and drawing processes on the tapered ratio of the tapered optical fiber. The tapered ratio of the prepared optical fiber can reach 5, which can be applied to the field of optical fiber amplifiers. Compared with traditional tapered optical fibers, the middle cladding-unchanged core gradient long tapered region can increase the nonlinear volume-to-length ratio of the optical fiber, thereby suppressing stimulated Raman scattering. The short tapered regions at both ends can reduce the number of high-order modes excited during the amplification process and leak them out of the fiber core, thereby optimizing the beam quality.

[0013] As a preferred embodiment of the present invention, in step S1, the method for preparing a tapered optical fiber with a long taper region includes the following steps:

[0014] S101, fusing a section of quartz rod to both ends of the preform rod;

[0015] S102, heating and softening the tapered region of the preform, and moving the quartz rods at both ends in opposite directions to a preset distance, so that the diameter of the softened tapered region gradually changes along the longitudinal direction;

[0016] S103, cutting off the quartz rods at both ends and grinding the tapered preform rod so that the outer diameter of the cladding of the preform rod remains consistent in the longitudinal direction;

[0017] S104, drawing and cutting the polished preform rod to obtain a tapered optical fiber with a long tapered region and a constant cladding outer diameter and a gradually changing core diameter.

[0018] The taper ratio of the long-taper region tapered optical fiber prepared by the above method is in the range of 1 to 2.

[0019] As a preferred embodiment of the present invention, in step S102, a blowtorch is used to heat and soften the tapered zone of the preform rod. During heating, the quartz rods at both ends rotate synchronously, driving the preform rod to rotate axially, so that the preform rod is heated more evenly. The flame width of the blowtorch determines the width of the heated and softened zone, thereby determining the length of the final tapered zone.

[0020] As a preferred embodiment of the present invention, in step S2, before fusing the auxiliary optical fiber, the coating layer corresponding to the length of the short taper region at both ends of the long taper region is first stripped off; the material of the coating layer is an ester polymer. The purpose of stripping the coating layer is, on the one hand, to facilitate the subsequent cutting of the optical fiber, and on the other hand, to avoid the softening and burning of the unstripped coating layer when fusing the optical fiber, thereby damaging the optical fiber or even the optical fiber fusion splicer.

[0021] As a preferred solution of the present invention, in step S3, laser heating is used to heat and soften the weld zone; heating and softening the weld zone by laser heating can ensure accuracy.

[0022] As a preferred solution of the present invention, in step S3, the moving speed of the auxiliary optical fibers at both ends is 0.02 mm / s to 0.08 mm / s. The moving speed of the auxiliary optical fibers during the tapering process should not be too fast, as it will cause optical fiber jitter and lead to errors.

[0023] As a preferred solution of the present invention, the short taper regions at both ends of the long taper region serve as the input and output ends of the gain fiber, respectively, and the core diameter of the input end is smaller than the core diameter of the output end. The sizes of the input and output ends of the optical fiber can be flexibly adjusted. The input end can reduce the number of excited modes through the shorter taper region with a smaller size, and the output end can leak out higher-order modes through the shorter taper region, thereby achieving laser output with better beam quality.

[0024] As a preferred solution of the present invention, the core-to-wrapper ratio of the short cone region remains constant along the longitudinal direction, further optimizing the sizes of the input end and the output end.

[0025] According to another aspect of the present invention, a gain optical fiber is provided, which is manufactured using the above-mentioned preparation method and includes a tapered optical fiber with a long tapered region and a constant cladding outer diameter and a gradually changing core diameter, wherein a short tapered optical fiber is provided at each end of the long tapered region.

[0026] Compared with general tapered optical fibers, the gain optical fiber of the present invention has a middle cladding-unchanged core gradually tapered long tapered region that can increase the nonlinear volume-to-length ratio of the optical fiber, thereby suppressing stimulated Raman scattering. The short tapered regions at both ends can reduce the number of high-order modes excited during the amplification process and cause them to leak out of the core, thereby optimizing the beam quality.

[0027] As a preferred embodiment of the present invention, the taper ratio of the gain fiber is in the range of 1 to 5. The taper ratio of the gain fiber prepared by the above method can reach 5, breaking through the limitations of traditional preform rod preparation and drawing process on the taper ratio of tapered optical fibers, and greatly improving the optical fiber's suppression effect on stimulated Raman scattering.

[0028] Beneficial effects

[0029] Compared with the prior art, the beneficial effects of the present invention are as follows: by preparing a short tapered region at both ends of a tapered optical fiber in a long tapered region, the present invention further reduces the size of the input and output ends of the active optical fiber by combining the long and short tapers, and can break through the limitations of the traditional preform rod preparation and drawing process on the taper ratio of the tapered optical fiber. The taper ratio of the prepared optical fiber can reach 5, which greatly improves the optical fiber's suppression effect on stimulated Raman scattering; compared with traditional tapered optical fibers, the input end can reduce the number of excited modes through the smaller short tapered region, and the output end can leak out high-order modes through the short tapered region, thereby achieving laser output with better beam quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0031] Figure 1 This is a flow chart of a method for preparing a longitudinally variable-diameter gain optical fiber according to the present invention;

[0032] Figure 2 Schematic diagram of the process of fusing quartz rods at both ends of a preform rod in an embodiment of the present invention;

[0033] Figure 3 Schematic diagram of the process of heating and taper-drawing a preform in an embodiment of the present invention;

[0034] Figure 4 Schematic diagram of the polished preform structure in an embodiment of the present invention;

[0035] Figure 5 Schematic diagram of the structure of the tapered optical fiber with a long taper region obtained after cutting in an embodiment of the present invention;

[0036] Figure 6 Schematic diagram of the process of fusing auxiliary optical fibers at both ends of the long tapered region in an embodiment of the present invention;

[0037] Figure 7 Schematic diagram of the process of drawing short tapered regions at both ends of a long tapered region in an embodiment of the present invention;

[0038] Figure 8 Schematic diagram of the structure of the gain optical fiber obtained after cutting in an embodiment of the present invention;

[0039] In the figure: 1. Quartz rod; 2. Preform rod; 3. Blowtorch; 4. Taper region; 5. Long taper region; 6. Cladding; 7. Fiber core; 8. Auxiliary optical fiber; 9. Laser; 10. Input end; 11. Output end. DETAILED DESCRIPTION

[0040] The following will clearly and completely describe the technical solutions of the present invention in conjunction with the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts shall fall within the scope of protection of the present invention.

[0041] Reference Figure 1 As shown, this embodiment provides a method for preparing a longitudinally variable-diameter gain optical fiber, comprising the following steps:

[0042] S1, preparing a tapered optical fiber with a long tapered region 5 and a constant outer diameter of the cladding 6 and a gradually changing diameter of the core 7, including the following steps:

[0043] S101, a section of quartz rod 1 is welded to both ends of preform rod 2, such as Figure 2 As shown;

[0044] During the specific implementation process, a preform rod 2 with an outer diameter of 20 mm and a quartz rod 1 of the same size are fixed to the chucks on both sides of the welding lathe. A blowtorch 3 is lit, and the chuck is set to rotate the preform rod 2 and the quartz rod 1 at a constant speed of 30 r / min. The blowtorch 3 is moved to the joint of the preform rod 2 and the quartz rod 1 to heat them. After the two are softened, welding is carried out. After welding is completed, the blowtorch 3 is turned off to allow them to cool.

[0045] Fix the other end of the preform rod 2 and the quartz rod 1 of the same size on the chucks on both sides of the welding lathe, and repeat the above steps to weld both ends of the preform rod 2 to the quartz rod 1;

[0046] S102, the tapered area 4 of the preform rod 2 is heated and softened, and the quartz rods 1 at both ends are moved in opposite directions at a constant speed to a preset distance, so that the diameter of the softened tapered area 4 gradually changes along the longitudinal direction (the diameter of the softened area is parabolic distribution), as shown in FIG. Figure 3 As shown;

[0047] During the specific implementation process, after lighting the blowtorch 3, the flame width is adjusted to 10 mm, and the blowtorch 3 is moved to the tapered area 4 of the preform rod 2. The blowtorch 3 is kept in place. After the tapered area 4 is softened, the lathe motor is set to make the quartz rods 1 at both ends move in opposite directions at a uniform speed, and stop moving after a certain distance; the movement speed of the quartz rods 1 cannot be too slow or too fast to prevent the preform rod 2 from volatilizing or insufficient heating and softening; during heating, the quartz rods 1 at both ends keep rotating synchronously, driving the preform rod 2 to rotate axially, so that the preform rod 2 is heated more evenly. The flame width of the blowtorch 3 determines the width of the heated and softened zone, thereby determining the length of the final long tapered area 5.

[0048] S103, cut off the quartz rod 1 at both ends, and grind the preform rod 2 after taper, and grind the outer diameter of the preform rod 2 to the same as the outer diameter of the thinnest part of the long tapered area 5, so that the outer diameter of the cladding 6 of the preform rod 2 is consistent in the longitudinal direction, such as Figure 4 As shown; if the core-to-package ratio does not meet the preset conditions, the preform 2 may be further polished or sleeved;

[0049] S104, drawing and cutting the polished preform rod 2 to obtain a tapered optical fiber with a long tapered region 5 and a constant outer diameter of the cladding 6 and a gradually changing diameter of the core 7, such as Figure 5 As shown;

[0050] In the specific implementation process, the polished preform rod 2 is drawn at a uniform speed through a drawing tower, and the tapered region is cut symmetrically to obtain an optical fiber with a cladding 6, a constant core 7, and a gradually tapered region 5;

[0051] The taper ratio of the tapered optical fiber in the long taper region 5 prepared by the above method is in the range of 1 to 2.

[0052] S2, a section of auxiliary optical fiber 8 is fused at both ends of the long tapered area 5, and the diameter of the cladding 6 of the two sections of auxiliary optical fiber 8 is the same as the diameter of the cladding 6 at both ends of the long tapered area 5, such as Figure 6 As shown;

[0053] In a specific implementation process, before fusing the auxiliary optical fiber 8, the coating layer corresponding to the length of the short taper region (5-8 cm) at both ends of the long taper region 5 is first stripped; the coating layer is made of an ester polymer. The purpose of stripping the coating layer is to facilitate subsequent cutting of the optical fiber and to prevent the unstripped coating layer from softening and burning during the fusing of the optical fiber, thereby damaging the optical fiber or even the optical fiber fusion splicer.

[0054] S3, fix the area where the long taper area 5 and the auxiliary optical fiber 8 are fused at the position of the CO2 laser 9, then use a clamp to fix the optical fibers on both sides, heat the melting point by the CO2 laser 9 to soften them, and adjust the clamp to move in opposite directions at a constant speed for a preset distance on both sides. Draw a 5cm short taper area on the side of the long taper area 5, and reserve a 5cm core diameter constant area at the melting point for cutting and welding, such as Figure 7 As shown; after the taper is completed, turn off the laser 9 and wait for the short taper area to cool;

[0055] During the specific implementation, the moving speed of the auxiliary optical fibers 8 at both ends is 0.02 mm / s to 0.08 mm / s. During the tapering process, the moving speed of the auxiliary optical fibers 8 should not be too fast, as it will cause optical fiber jitter and lead to errors.

[0056] S4, cutting off the auxiliary optical fibers 8 at both ends to obtain a gain optical fiber with a longitudinally variable diameter, such as Figure 8 shown.

[0057] In this embodiment, a short tapered region is prepared at each end of the tapered optical fiber in the long tapered region 5. By combining the long and short tapered regions, the sizes of the input end 10 and the output end 11 of the active optical fiber are further reduced. This can break through the limitations of the traditional preform rod 2 preparation and drawing process on the tapered ratio of the tapered optical fiber. The tapered ratio of the produced optical fiber can reach 5, which can be applied to the field of optical fiber amplifiers. Compared with traditional tapered optical fibers, the middle cladding 6 and the core 7 with a gradual long tapered region 5 can increase the nonlinear volume-to-length ratio of the optical fiber, thereby suppressing stimulated Raman scattering. The short tapered regions at both ends can reduce the number of high-order modes excited during the amplification process and cause them to leak out of the core 7, thereby optimizing the beam quality.

[0058] As a preferred solution of this embodiment, the short taper regions at both ends of the long taper region 5 serve as the input end 10 and the output end 11 of the gain optical fiber, respectively, and the diameter of the fiber core 7 of the input end 10 is smaller than the diameter of the fiber core 7 of the output end 11; the sizes of the input end 10 and the output end 11 of the optical fiber can be flexibly adjusted, and the input end 10 can reduce the number of excited modes through the smaller short taper region, and the output end 11 can leak out high-order modes through the short taper region, thereby achieving laser output with better beam quality.

[0059] As a preferred solution of this embodiment, the core-to-wrapper ratio of the short cone region remains constant along the longitudinal direction, further optimizing the sizes of the input end 10 and the output end 11 .

[0060] This embodiment also provides a gain optical fiber, which is manufactured using the above-mentioned preparation method and includes a tapered optical fiber with a long tapered region 5 having a constant outer diameter of the cladding 6 and a gradually changing diameter of the core 7. A short tapered region of tapered optical fiber is provided at each end of the long tapered region 5, and the length of the two short tapered regions is 5 cm.

[0061] In this embodiment, the outer diameter of the cladding 6 of the long tapered region 5 is 400 to 600 μm;

[0062] When the outer diameter of the cladding 6 of the long tapered region 5 is 400 μm, the length of the long tapered region 5 is 4.5 to 8.6 m, the diameter of the small end core 7 is 50 to 75 μm, and the diameter of the large end core 7 is 75 to 100 μm; the outer diameter of the cladding 6 of the short tapered regions at both ends is 250 μm;

[0063] When the outer diameter of the cladding 6 of the long tapered region 5 is 600 μm, the length of the long tapered region 5 is 1.8 to 5 m, the diameter of the small end core 7 is 50 to 100 μm, and the diameter of the large end core 7 is 75 to 150 μm; the outer diameter of the cladding 6 of the short tapered regions at both ends is 400 μm;

[0064] The diameter of the cladding 6 in the short tapered regions at both ends should be the same as the diameter of the cladding 6 of commercial optical fibers.

[0065] As a preferred embodiment of the present invention, the taper ratio of the gain fiber is in the range of 1 to 5. The taper ratio of the gain fiber produced by the above method can reach 5, breaking through the limitations of the traditional preform rod 2 preparation and drawing process on the taper ratio of the tapered fiber, and greatly improving the optical fiber's suppression effect on stimulated Raman scattering.

[0066] Compared with general tapered optical fibers, the gain optical fiber of this embodiment has a middle cladding 6 with an unchanged core 7 and a gradually tapered long tapered region 5, which can increase the nonlinear volume-to-length ratio of the optical fiber and thus suppress stimulated Raman scattering. The short tapered regions at both ends can reduce the number of high-order modes excited during the amplification process and leak them out of the core 7, thereby optimizing the beam quality.

[0067] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a longitudinally variable-diameter gain optical fiber, characterized in that: The following steps are involved: S1, preparing a tapered optical fiber with a long tapered region (5) having a constant outer diameter of the cladding (6) and a gradually changing diameter of the core (7); S2, fusing a section of auxiliary optical fiber (8) at both ends of the long tapered region (5); S3, heating and softening the fusion zone at both ends of the long tapered zone (5), and simultaneously moving the auxiliary optical fibers (8) at both ends in opposite directions to a preset distance, so that the diameters of both ends of the long tapered zone (5) gradually shrink in the longitudinal direction, forming a short tapered zone at both ends of the long tapered zone (5); S4, cutting off the auxiliary optical fibers (8) at both ends to obtain a gain optical fiber with a longitudinally variable diameter.

2. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 1, wherein: In step S1, the method for preparing a tapered optical fiber having a long tapered region (5) comprises the following steps: S101, welding a section of quartz rod (1) to both ends of the preform rod (2); S102, heating and softening the tapered region (4) of the preform rod (2), and moving the quartz rods (1) at both ends in opposite directions to a preset distance, so that the diameter of the softened tapered region (4) gradually changes along the longitudinal direction; S103, cutting off the quartz rod (1) at both ends, and grinding the tapered preform (2) so that the outer diameter of the cladding (6) of the preform (2) remains consistent in the longitudinal direction; S104, drawing and cutting the polished preform rod (2) to obtain a tapered optical fiber with a long tapered region (5) having a constant outer diameter of the cladding (6) and a gradually changing diameter of the fiber core (7).

3. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 2, wherein: In step S102, a blowtorch (3) is used to heat and soften the tapered region (4) of the preform rod (2). During heating, the quartz rods (1) at both ends rotate synchronously, driving the preform rod (2) to rotate axially.

4. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 1, wherein: In step S2, before fusing the auxiliary optical fiber (8), the coating layers at both ends of the long taper region (5) corresponding to the length of the short taper region are first stripped off.

5. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 1, wherein: In step S3, laser heating is used to heat and soften the weld zone.

6. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 1, wherein: In step S3, the moving speed of the auxiliary optical fibers (8) at both ends is 0.02 mm / s to 0.08 mm / s.

7. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 1, wherein: The short taper regions at both ends of the long taper region (5) serve as the input end (10) and the output end (11) of the gain optical fiber, respectively. The diameter of the fiber core (7) at the input end (10) is smaller than the diameter of the fiber core (7) at the output end (11).

8. The method for preparing a longitudinally variable-diameter gain optical fiber according to claim 1, wherein: The core-to-shell ratio of the short tapered region remains constant along the longitudinal direction.

9. A gain optical fiber, prepared by the preparation method according to any one of claims 1 to 8, characterized in that: The invention comprises a tapered optical fiber having a long tapered region (5) with a constant outer diameter of a cladding (6) and a gradually changing diameter of a fiber core (7), wherein a short tapered region is provided at each end of the long tapered region (5).

10. The gain optical fiber according to claim 9, characterized in that: The taper ratio of the gain optical fiber is in the range of 1 to 5.

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