Gain fiber refractive index distribution optimization design method, gain fiber and application

Abstract

According to the gain optical fiber refractive index distribution optimization design method, the gain optical fiber and the application, mode distribution in a fiber core is adjusted through fiber core refractive index distribution with a concave parabola, and the mode field area of a fundamental mode can be remarkably increased on the premise that the number of high-order modes and the mode field area of the high-order modes are almost unchanged. On one hand, the fundamental mode area is increased, and the nonlinear effect in the optical fiber can be effectively relieved. And on the other hand, when the area of the fundamental mode is increased and the mode field area and the mode number of the high-order mode are not changed, the fundamental mode can obtain more gains, and the high-order mode can only obtain less gains, so that the purpose of inhibiting the mode instability effect is achieved, and finally, the output power is improved.

Description

Technical Field

[0001] The present invention mainly relates to the technical field of gain optical fiber optimization design, in particular to a gain optical fiber refractive index distribution optimization design method, gain optical fiber and application. Background Art

[0002] High-power fiber lasers have the advantages of good beam quality, efficient thermal management, flexible operation, and good robustness. They have been widely used in industrial processing, national defense, military, and basic scientific research. At present, the core limiting factors for the performance improvement of high-power fiber laser systems mainly include nonlinear effects and mode instability effects. Among them, nonlinear effects, such as stimulated Brillouin scattering and stimulated Raman scattering, will induce unstable Stokes pulse light, reverse the pumping laser power, and limit the power improvement of the fiber laser system; while the mode instability effect will induce dynamic energy coupling between the fundamental mode and the higher-order mode, limiting the brightness improvement of the fiber laser system.

[0003] In order to weaken the nonlinear effect and mode instability effect in the gain fiber and further improve the output power of the laser, it is necessary to find a method to comprehensively suppress the two effects based on the induction principle of the nonlinear effect and mode instability effect. Traditional gain fiber designs all use a single-step refractive index distribution, such as Figure 1 The red line in the figure shows the conventional refractive index distribution. The design of this scheme uses a uniform high-refractive index core and a low-refractive index cladding to form a refractive index ladder to achieve total reflection of the laser in the core, thereby completing the power boost and transmission of the laser. The conventional refractive index distribution design can directly calculate the numerical aperture (NA) of the optical fiber core through the following formula: (1) Thus, the number of modes and mode field area supported by the optical fiber can be calculated using the core radius and numerical aperture. and Represent the refractive index of the core and cladding respectively. However, the disadvantage of this design is that when the core size and NA value of multiple optical fibers are the same, the number of modes and mode field area supported by the optical fiber are the same, that is, they have the same performance. However, in the actual optical fiber performance test, it is found that the performance of gain fibers with the same core size and NA is also significantly different. This is because the core refractive index distribution of the optical fiber has an important influence on the performance of the optical fiber, and this influence cannot be reflected by the NA value calculated using the average refractive index. Therefore, starting from the refractive index distribution of the optical fiber, exploring the optimal optical fiber design under the same core size and NA is of great significance to comprehensively suppress the nonlinear effects and mode instability effects in the gain fiber and further improve the output power of the laser. Summary of the invention

[0004] In view of the technical problems existing in the prior art, the present invention proposes a gain fiber refractive index distribution optimization design method, a gain fiber and an application thereof.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows: The method for optimizing the refractive index distribution of a gain optical fiber comprises the following steps: Determine the gain optical fiber to be optimized and the average refractive index of its core; Calculate the average NA value of the core of the gain fiber using the average refractive index of the core; According to the average NA value of the core of the gain fiber, the normalized frequency value of the gain fiber is calculated. V , according to the size of the fiber core and the normalized frequency value V Determine all modes supported by the gain fiber; When the core size remains unchanged, the average refractive index of the core remains unchanged, and the average NA Under the premise that the value remains unchanged, the core refractive index distribution is adjusted, and the finite element simulation calculation method is used to obtain the light spot distribution in the core under different core refractive index distributions, and the mode field area of ​​the corresponding mode under different core refractive index distributions is calculated; Under the premise that the core size remains unchanged, the core average refractive index remains unchanged, and the core average NA value remains unchanged, the core refractive index distribution that can increase the fundamental mode mode area in the core while keeping the mode field area of ​​the high-order mode unchanged is determined as the core refractive index distribution of the optimized gain optical fiber.

[0006] Furthermore, the method of adjusting the core refractive index distribution includes: The core refractive index distribution always maintains central symmetry with the core center as the center. The core center is the core refractive index distribution center. By adjusting the refractive index size of the core refractive index distribution center , controls the shape of the core refractive index distribution, It represents the difference between the refractive index at the center of the core refractive index distribution and the average refractive index of the core. When it is less than 0, the center of the core refractive index distribution is concave, and the core refractive index distribution is low in the middle and high on both sides; When it is greater than 0, the core refractive index distribution center is convex, and the core refractive index distribution is high in the middle and low on both sides; =0, that is, conventional refractive index distribution, the core and cladding form a conventional step-type refractive index distribution.

[0007] Furthermore, in the process of controlling the shape of the core refractive index distribution, the core refractive index distribution always presents a triangular or parabolic refractive index distribution.

[0008] Furthermore, when When the core refractive index distribution center is convex, the fundamental mode field area of ​​the triangular or parabolic refractive index distribution will increase with The increase of is decreasing; when When the core refractive index distribution center is concave, the fundamental mode field area of ​​the triangular or parabolic refractive index distribution will decrease with The mode field area of ​​the high-order mode of the triangular or parabolic refractive index distribution increases with the decrease of The changes in the mode field area of ​​the overall high-order mode are very small, and the change rate of the mode field area of ​​the overall high-order mode is less than 6%; It is thus determined that the optimized core refractive index distribution is a triangular or parabolic refractive index distribution with a concave center and a low center and high sides.

[0009] Furthermore, when Less than 0 and Under the same conditions, the parabolic refractive index distribution can achieve a greater increase in fundamental mode area than the triangular refractive index distribution; Compared with the conventional refractive index distribution, when the core refractive index distribution is triangular, the maximum fundamental mode field area is increased by 44.6%; Compared with the conventional refractive index distribution, when the core refractive index distribution is parabolic, the maximum fundamental mode field area is increased by 62.9%; Therefore, it is determined that the optimized core refractive index distribution is a parabolic refractive index distribution with a concave center and high sides, which is low in the middle.

[0010] Furthermore, the average refractive index of the cladding portion of the gain optical fiber is 1.4500.

[0011] Furthermore, the gain optical fiber is a large mode field doped gain optical fiber, and the fiber core is doped with gain doping ions.

[0012] Furthermore, the gain doping ions are one or more of ytterbium, erbium, neodymium, holmium, thulium, germanium, and phosphorus.

[0013] On the other hand, a gain optical fiber is provided, and the optimization design of the refractive index distribution of the core of the gain optical fiber is realized based on the above-mentioned optimization design method of the refractive index distribution of the gain optical fiber.

[0014] On the other hand, the application of the above-mentioned gain fiber in the laser system. The above-mentioned optimized gain fiber is applied to the laser system, and the gain fiber has an optimized core refractive index distribution, which is a parabolic refractive index distribution with a concave center, low in the middle and high on both sides, which can significantly increase the mode field area of ​​the fundamental mode while keeping the number of high-order modes and the mode field area of ​​the high-order modes almost unchanged.

[0015] The beneficial effects of the present invention are: 1. The present invention can effectively increase the mode field area of ​​the fundamental mode while maintaining the fundamental mode transmission and amplification characteristics, thereby obtaining good nonlinear effect suppression capability.

[0016] 2. The present invention can effectively increase the loss coefficient of the high-order mode while maintaining the fundamental mode transmission and amplification characteristics, thereby obtaining a good ability to suppress mode instability effects.

[0017] 3. The present invention is universal and compatible with technical solutions such as optimizing the distribution of gain dopants and adjusting the transverse and longitudinal structures of the fiber core to suppress nonlinear effects and mode instability effects. It can further improve the output power and beam quality of large mode field doped gain optical fibers based on existing technologies.

[0018] 4. The refractive index distribution of the optical fiber of the present invention is centrally symmetrical, and can be directly mass-produced using a mature chemical vapor deposition optical fiber drawing process (MCVD). BRIEF DESCRIPTION OF THE DRAWINGS

[0019] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. 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 the structures shown in these drawings without paying creative work.

[0020] Figure 1 is a schematic diagram of a parabolic refractive index distribution in an embodiment; Figure 2 is a schematic diagram of a triangular refractive index distribution in an embodiment; Figure 3 This is a comparison chart of the fundamental mode field area changes obtained by finite element simulation; Figure 4 Calculated by finite element simulation LP 11 Comparison of mode field area changes of the modes. DETAILED DESCRIPTION

[0021] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0022] In one embodiment, a method for optimizing the refractive index profile of a gain fiber is provided, comprising the following steps: Determine the gain optical fiber to be optimized and the average refractive index of its core; Calculate the average NA value of the core of the gain fiber using the average refractive index of the core; According to the average NA value of the core of the gain fiber, the normalized frequency value of the gain fiber is calculated. ; According to the size of the core and the normalized frequency value V Determine all modes supported by the gain fiber; Under the premise that the core size remains unchanged, the core average refractive index remains unchanged, and the core average NA value remains unchanged, the core refractive index distribution is adjusted, and the finite element simulation calculation method is used to obtain the light spot distribution in the core under different core refractive index distributions, and the mode field area of ​​the corresponding mode under different core refractive index distributions is calculated; Under the premise that the core size remains unchanged, the core average refractive index remains unchanged, and the core average NA value remains unchanged, the core refractive index distribution that can increase the fundamental mode mode area in the core while keeping the mode field area of ​​the high-order mode unchanged is determined as the core refractive index distribution of the optimized gain optical fiber.

[0023] The method of adjusting the core refractive index distribution in the present invention includes: the core refractive index distribution always maintains the central symmetry with the core center as the center, the core center is the core refractive index distribution center, and the refractive index size of the core refractive index distribution center is adjusted. , controls the shape of the core refractive index distribution, It represents the difference between the refractive index at the center of the core refractive index distribution and the average refractive index of the core. When it is less than 0, the center of the core refractive index distribution is concave, and the core refractive index distribution is low in the middle and high on both sides; When it is greater than 0, the core refractive index distribution center is convex, and the core refractive index distribution is high in the middle and low on both sides; =0, that is, conventional refractive index distribution, the core and the cladding form a conventional step-type refractive index distribution. Further, in the process of controlling the morphology of the core refractive index distribution, the core refractive index distribution always presents a triangular or parabolic refractive index distribution.

[0024] Furthermore, when When the core refractive index distribution center is convex, the fundamental mode field area of ​​the triangular or parabolic refractive index distribution will increase with The increase of is decreasing; when When the core refractive index distribution center is concave, the fundamental mode field area of ​​the triangular or parabolic refractive index distribution will decrease with The mode field area of ​​the high-order mode of the triangular or parabolic refractive index distribution increases with the decrease of The changes in the mode field area of ​​the overall high-order mode are very small, and the change rate of the mode field area of ​​the overall high-order mode is less than 6%; It is thus determined that the optimized core refractive index distribution is a triangular or parabolic refractive index distribution with a concave center and a low center and high sides.

[0025] Furthermore, when Less than 0 and Under the same conditions, the parabolic refractive index distribution can achieve a greater increase in fundamental mode area than the triangular refractive index distribution; Therefore, it is determined that the optimized core refractive index distribution is a parabolic refractive index distribution with a concave center and high sides, which is low in the middle.

[0026] Furthermore, the average refractive index of the cladding portion of the gain optical fiber is 1.4500.

[0027] Furthermore, the gain optical fiber is a large mode field doped gain optical fiber, and the fiber core is doped with gain doping ions.

[0028] Furthermore, the gain doping ions are one or more of ytterbium, erbium, neodymium, holmium, thulium, germanium, and phosphorus.

[0029] The present invention proposes a fiber refractive index distribution design that improves the ability of gain fiber to suppress nonlinear effects and mode instability effects. The fiber refractive index distribution that can improve the ability of gain fiber to suppress nonlinear effects and mode instability effects is a parabolic refractive index distribution with a concave center of the core refractive index distribution and a low middle and high sides. Figure 1 As shown in FIG. 1 , a schematic diagram of a parabolic refractive index distribution in an embodiment is shown. The advantage is that the mode field area of ​​the fundamental mode can be significantly increased while the number of high-order modes and the mode field area of ​​the high-order modes remain almost unchanged. On the one hand, the increase in the fundamental mode area can effectively alleviate the nonlinear effect in the optical fiber. On the other hand, when the fundamental mode area increases while the mode field area and the number of modes of the high-order modes remain unchanged, the fundamental mode can obtain more gain, while the high-order modes can only obtain less gain, thereby achieving the purpose of suppressing the mode instability effect and ultimately achieving an increase in output power.

[0030] The present invention is described by taking an optical fiber with a core diameter of 20 microns and an average NA value of 0.066 as an example. Figure 1 As shown in FIG. 1 , a schematic diagram of a parabolic refractive index distribution in an embodiment is shown, wherein the black dashed line is a cross-sectional schematic diagram of the parabolic refractive index distribution, and its average core NA value is 0.066, as shown by the red line. In order to facilitate comparison of the advantages of the present invention, a triangular refractive index distribution is used as an example for comparison and explanation, as shown in FIG. Figure 2, which is a schematic diagram of a triangular refractive index distribution in an embodiment, wherein the black dashed line is a cross-sectional schematic diagram of the triangular refractive index distribution, and its average core NA value is 0.066, as shown by the red line. Figure 2 The triangular refractive index distribution shown is Figure 1 The parabolic refractive index profile shown is provided for comparison.

[0031] By adjusting the central refractive index size of the triangular and parabolic refractive index distribution , to control the shape of the refractive index distribution (here, It represents the difference between the central refractive index of triangular and parabolic refractive index profiles and the average refractive index of the core. When it is negative, the refractive index at the center of the core is concave; When it is positive, the refractive index at the center of the core is convex; =0, it is a conventional step-type refractive index distribution). It is necessary to control the average NA value of the fiber core to be constant at 0.066 to ensure that the calculation results of the optical fiber performance parameters are comparable.

[0032] Here, the average refractive index of the core is calculated as shown in equation (2): (2) Based on the calculated average NA value of the fiber core, the normalized frequency value of the optical fiber can be further calculated using formula (3): . The size of the fiber core and the normalized frequency value The size of the fiber core determines the number of modes supported by the fiber. For example, the core diameter of the example is 20 microns, and the average core NA is 0.066, and its normalized frequency value is 3.8975, the fiber will be able to support LP 01 , LP 11 , LP 21 Three modes, (3) according to Figure 1 and Figure 2 Refractive index distribution design, using finite element simulation calculation method, to obtain the spot distribution in the core of the refractive index distribution, and calculate the mode field area of ​​the corresponding mode according to equation (4), (4) Where A is the mode field area of ​​the optical fiber, E mn ( z , r , φ )for LP mn The electric field distribution of the mode. LP 01 Mode and LP 11 The mode field area of ​​the higher-order mode is explained.

[0033] when When the refractive index changes from -0.0015 to 0.0015, the fundamental mode field area change of the triangular and parabolic refractive index distributions is calculated using formula (4). The results are as follows: Figure 3 shown. Figure 3 This is a comparison of the fundamental mode area changes calculated by finite element simulation; the red dotted line is the parabolic refractive index distribution under different The fundamental mode field area changes under different conditions; the black dotted lines represent the distribution of the triangle refractive index under different conditions. The fundamental mode field area changes under When the core refractive index distribution center is convex, the fundamental mode field area of ​​both the triangular and parabolic refractive index distributions will decrease, and the smaller trends of the two are the same. When the core refractive index distribution center is concave, the fundamental mode area of ​​both the triangular and parabolic refractive index distributions will increase, and the parabolic refractive index distribution proposed in the present invention can achieve a greater increase in the fundamental mode area. =0), the maximum fundamental mode area of ​​the core triangular refractive index distribution is increased by 44.6%, while the maximum fundamental mode area of ​​the core parabolic refractive index distribution is increased by 62.9%. It can be seen that the center of the core refractive index distribution is concave ( Less than 0) is more conducive to increasing the fundamental mode mode area, especially the parabolic refractive index distribution design. The increase in the fundamental mode mode area obtained by the parabolic refractive index distribution is more obvious.

[0034] Further calculations of triangular and parabolic refractive index profiles LP 11 The mode field area of ​​the mode changes, and the results are as follows Figure 4 shown. Figure 4 Calculated by finite element simulation LP 11 Comparison of the mode field area changes of the modes; the red dotted line is the parabolic refractive index distribution under different Next LP 11 The mode field area of ​​the mode changes; the black dotted line is the triangle refractive index distribution in different Next LP 11 The mode field area of ​​the mode changes; when When it is less than 0, LP11 The mode field area of ​​the mode has no significant change compared to the normal mode field area. When the core refractive index distribution center is concave, both the triangular and parabolic refractive index distribution LP 11 The changes in the modes are very small, and the overall mode field change rate is less than 6%. It can be seen that the center of the core refractive index distribution is concave ( The refractive index design of less than 0 will not LP 11 The mode area of ​​the mode has a significant impact.

[0035] The parabola-shaped concave core refractive index distribution of the present invention can adjust the mode distribution in the core to increase the fundamental mode field area in the core under the premise of keeping the equivalent NA and core size unchanged. LP 11 The mode field area of ​​the mode hardly changes, so that the fundamental mode can extract more gain during the power amplification process, achieving the purpose of suppressing both nonlinear effects and mode instability effects, and ultimately achieving an increase in output power.

[0036] Matters not covered by the present invention are known technologies.

[0037] The technical features of the above embodiments may be arbitrarily combined. To make the description concise, 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, they should be considered to be within the scope of this specification.

[0038] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be understood as limiting the scope of the invention. It should be pointed out that, for a person of ordinary skill in the art, several modifications and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the attached claims.

[0039] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. 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 optimizing the refractive index distribution of a gain optical fiber, characterized in that: The following steps are involved: Determine the gain optical fiber to be optimized and the average refractive index of its core; Calculate the average NA value of the core of the gain fiber using the average refractive index of the core; According to the average NA value of the core of the gain fiber, the normalized frequency value of the gain fiber is calculated , according to the size of the fiber core and the normalized frequency value Determine all modes supported by the gain fiber; When the core size remains unchanged, the average refractive index of the core remains unchanged, and the average NA Under the premise that the value remains unchanged, the core refractive index distribution is adjusted, and the finite element simulation calculation method is used to obtain the light spot distribution in the core under different core refractive index distributions, and the mode field area of ​​the corresponding mode under different core refractive index distributions is calculated; Under the premise that the core size remains unchanged, the core average refractive index remains unchanged, and the core average NA value remains unchanged, the core refractive index distribution that can increase the fundamental mode mode area in the core while keeping the mode field area of ​​the high-order mode unchanged is determined as the core refractive index distribution of the optimized gain optical fiber.

2. The method for optimizing the refractive index distribution of a gain optical fiber according to claim 1, characterized in that: Ways to adjust the core refractive index profile include: The core refractive index distribution always maintains central symmetry with the core center as the center. The core center is the core refractive index distribution center. By adjusting the refractive index size of the core refractive index distribution center , controls the shape of the core refractive index distribution, It represents the difference between the refractive index at the center of the core refractive index distribution and the average refractive index of the core. When it is less than 0, the center of the core refractive index distribution is concave, and the core refractive index distribution is low in the middle and high on both sides; When it is greater than 0, the core refractive index distribution center is convex, and the core refractive index distribution is high in the middle and low on both sides; =0, that is, conventional refractive index distribution, the core and cladding form a conventional step-type refractive index distribution.

3. The method for optimizing the refractive index distribution of a gain optical fiber according to claim 2, characterized in that: In the process of controlling the morphology of the core refractive index distribution, the core refractive index distribution always presents a triangular or parabolic refractive index distribution.

4. The method for optimizing the refractive index distribution of a gain optical fiber according to claim 3, characterized in that: when When the core refractive index distribution center is convex, the fundamental mode field area of ​​the triangular or parabolic refractive index distribution will increase with decreases with the increase of when When the core refractive index distribution center is concave, the fundamental mode field area of ​​the triangular or parabolic refractive index distribution will decrease with The mode field area of ​​the high-order mode of the triangular or parabolic refractive index distribution increases with the decrease of The changes in the mode field area of ​​the overall high-order mode are very small, and the change rate of the mode field area of ​​the overall high-order mode is less than 6%; It is thus determined that the optimized core refractive index distribution is a triangular or parabolic refractive index distribution with a concave center and a low center and high sides.

5. The method for optimizing the refractive index distribution of a gain optical fiber according to claim 4, characterized in that: when Less than 0 and Under the same conditions, the parabolic refractive index distribution can achieve a greater increase in fundamental mode area than the triangular refractive index distribution; Therefore, it is determined that the optimized core refractive index distribution is a parabolic refractive index distribution with a concave center and high sides, which is low in the middle.

6. The method for optimizing the refractive index distribution of a gain optical fiber according to any one of claims 1 to 5, characterized in that: The average refractive index of the cladding portion of the gain fiber is 1.4500.

7. The method for optimizing the refractive index distribution of a gain optical fiber according to claim 6, characterized in that: The gain optical fiber is a large mode field doped gain optical fiber, and the fiber core is doped with gain doping ions.

8. The method for optimizing the refractive index distribution of a gain optical fiber according to claim 7, characterized in that: The gain doping ions are one or more of ytterbium, erbium, neodymium, holmium, thulium, germanium, and phosphorus.

9. A gain optical fiber, characterized in that: The optimization design of the refractive index distribution of the gain optical fiber is achieved based on the optimization design method of the refractive index distribution of the gain optical fiber as described in any one of claims 1 to 5.

10. Use of the gain optical fiber according to claim 9 in a laser system.

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