Optimization design method of doped optical fiber

Abstract

The present invention proposes a method for optimizing the design of doped optical fibers, including the steps of core size, NA size design, output power threshold estimation, refractive index distribution refinement design, and optical fiber comprehensive performance evaluation. The present invention can simultaneously consider the influence of basic optical fiber parameters and optical fiber refractive index distribution on the performance of doped optical fibers in the optical fiber design link. By introducing the finite element simulation method, the effective mode field area, high-order mode loss and other characteristic indicators of the optical fiber can be accurately analyzed, and finally a good doped optical fiber design is obtained. The present invention has versatility, and the present invention can simultaneously take into account the threshold analysis of other nonlinear effects that limit the increase in optical fiber laser power, such as stimulated Raman scattering effect, four-wave mixing effect, self-phase modulation effect, thermal lens effect, etc. The present invention can provide a direct technical path for the efficient design and performance evaluation of high-performance doped optical fibers, and can serve the performance optimization design and efficient research and development of high-power doped optical fibers.

Description

Technical Field

[0001] The invention mainly relates to the technical field of optical fiber design, in particular to a doped optical fiber optimization design method. Background Art

[0002] High-power fiber lasers have the advantages of good beam quality, efficient thermal management, flexible operation, and good robustness. At present, the performance improvement of high-power fiber laser systems is still mainly restricted by various nonlinear effects, such as stimulated Brillouin scattering and mode instability. Among them, stimulated Brillouin scattering will induce unstable Stokes pulse light, limiting 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. In order to comprehensively evaluate the influence of stimulated Brillouin scattering and mode instability effects on the output laser threshold, and then improve the performance of high-power fiber laser systems, it is necessary to establish a theoretical model to evaluate the threshold characteristics of fiber lasers, so as to efficiently guide the design and optimization of high-power fiber laser systems.

[0003] Traditional technical solutions for evaluating fiber performance are limited to studying the impact of a single parameter on a certain nonlinear effect of a high-power laser system. However, the effects of fiber parameter changes on different nonlinear effects may be completely opposite. For example, increasing the core diameter of the gain fiber will increase the threshold of the stimulated Brillouin scattering effect of the system, while the threshold of the mode instability effect will decrease. It can be seen that the traditional method has obvious limitations and cannot comprehensively evaluate the output power threshold of the high-power fiber laser system. In addition, it is also found in the actual fiber development process that the refractive index distribution of the fiber also has a huge impact on the thresholds of various nonlinear effects, and the traditional fiber design scheme cannot analyze the fiber performance differences caused by such detailed parameter fluctuations. Therefore, the fiber developed based on the traditional fiber design scheme usually requires multiple rounds of fiber iteration and optimization to obtain a fiber with excellent performance. However, this type of verification process usually has the disadvantages of a long development cycle and the parameter optimization direction of the iterative process is heavily dependent on the experience of the staff. In order to further improve the performance of high-power doped fibers and improve the efficiency of doped fiber development, it is necessary to propose a theoretical model that can comprehensively evaluate the output power threshold of the fiber, and optimize and improve the design, development process and performance evaluation method of doped gain fibers. Summary of the invention

[0004] In view of the technical problems existing in the prior art, the present invention proposes a doped optical fiber optimization design method.

[0005] To achieve the above purpose, the technical solution adopted by the present invention is as follows:

[0006] In one aspect, the present invention provides a method for optimizing the design of a doped optical fiber, comprising:

[0007] Rough estimate of different core sizes and NA The fiber characteristic indicators of doped optical fibers of different sizes include the modes supported by the optical fiber, the effective mode field area, and the bending loss coefficient of the high-order mode;

[0008] Based on rough estimates of different core sizes and NA The fiber characteristics of doped fibers under different sizes are predicted to be different for different core sizes and NA The nonlinear effect threshold of doped fibers with different core sizes and sizes is obtained after they are used as gain fibers in the target laser system. NA The system power threshold of the doped fiber with different sizes is applied as the gain fiber to the target laser system. The core size and NA size, as the ideal core size of doped optical fiber and NA size;

[0009] Based on the ideal core size of doped optical fiber and NA Size, using finite element simulation, obtain the mode field distribution of doped optical fibers with different refractive index distributions, and calculate the fiber characteristic indicators of doped optical fibers under different refractive index distributions, including effective mode field area, mode field center energy distribution ratio and mode loss coefficient;

[0010] Based on the fiber characteristic indicators of doped optical fibers under different refractive index distributions, the nonlinear effect thresholds of the doped optical fibers under different refractive index distributions are predicted after being applied as gain fibers to the target laser system. The system power thresholds of the doped optical fibers under different refractive index distributions are obtained after being applied as gain fibers to the target laser system. The refractive index distribution corresponding to the maximum system power threshold is selected from all the system power thresholds as the ideal refractive index distribution of the doped optical fiber.

[0011] Further, different fiber core sizes and NA The method comprises: for the first i Fiber core size and NA Doped fiber under the size, get the first i Fiber core size and NA The various nonlinear effect thresholds of the doped fiber with different sizes are used as the gain fiber in the target laser system, and the smallest nonlinear effect threshold is selected as the first i Fiber core size and NA The system power threshold after the doped fiber with a certain size is used as a gain fiber in the target laser system.

[0012] Further, obtaining the system power threshold after the doped optical fiber with different refractive index distribution is applied as the gain optical fiber to the target laser system, the method comprises: for the first j The doped fiber with the refractive index distribution is obtained j The various nonlinear effect thresholds after the doped fiber with a certain refractive index distribution is used as the gain fiber in the target laser system, and the smallest nonlinear effect threshold is selected as the first j The system power threshold after the doped fiber with a certain refractive index distribution is used as a gain fiber in the target laser system.

[0013] The traditional doped fiber design process includes core size, NA design, fiber supported mode number and mode field area evaluation, and finally fiber drawing. In the traditional doped fiber design process, the fiber design process is simple and cannot accurately evaluate the power load capacity of the fiber. It usually requires multiple rounds of fiber design and drawing iterations to develop a high-power doped fiber with stable performance. Compared with the prior art, the present invention has the following beneficial effects:

[0014] The present invention includes the steps of core size, NA size design, output power threshold estimation, refractive index distribution refinement design, optical fiber comprehensive performance evaluation, etc. The present invention can simultaneously consider the influence of optical fiber basic parameters and optical fiber refractive index distribution on doped optical fiber performance in the optical fiber design link. By introducing the finite element simulation method, the effective mode field area, high-order mode loss and other characteristic indicators of the optical fiber can be accurately analyzed, and finally a good doped optical fiber design is obtained. The present invention can theoretically evaluate the output power threshold of the doped optical fiber, and can provide theoretical guidance for the actual optical fiber development. The present invention has versatility, and the present invention can simultaneously take into account the threshold analysis of other nonlinear effects that limit the increase of optical fiber laser power, such as stimulated Raman scattering effect, four-wave mixing effect, self-phase modulation effect, thermal lens effect, etc. The present invention can provide a direct technical path for the efficient design and performance evaluation of high-performance doped optical fibers, and can serve the performance optimization design and efficient research and development of high-power doped optical fibers.

[0015] The present invention explores an efficient optical fiber design and development scheme and an optical fiber threshold evaluation theory from optical fiber parameter design to actual optical fiber development to optical fiber performance evaluation, which is of great significance to promoting the development of high-power optical fiber lasers, and has important practical significance for improving the performance of high-power optical fiber lasers and improving the production capacity of high-performance doped optical fibers. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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.

[0017] Figure 1 is a flow chart of a method for optimizing design of a doped optical fiber in an embodiment;

[0018] Figure 2 is a schematic diagram of a system power threshold obtained in an embodiment;

[0019] Figure 3 This is a comparison chart of the refractive index distribution corresponding to the two doped optical fibers. DETAILED DESCRIPTION

[0020] 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.

[0021] On the one hand, the present invention provides a method for optimizing the design of doped optical fibers, which uses a finite element simulation method to calculate some characteristic indicators that can reflect the performance of optical fibers based on basic optical fiber design parameters and optical fiber refractive index distribution. On the one hand, a power threshold model that simultaneously considers multiple nonlinear effects is constructed, and based on the above characteristic indicators, the multiple nonlinear effect threshold characteristics of the optical fiber are calculated through further theoretical simulation models, and finally a comprehensive evaluation of the output power threshold of the gain optical fiber is achieved.

[0022] In one embodiment, a method for optimizing the design of a doped optical fiber is provided, which includes two parts: the core size and NA Size design and refractive index distribution design.

[0023] S100 roughly estimates the different core sizes and NA Fiber characteristic indicators of doped optical fibers under different sizes.

[0024] (1) Roughly estimate the different fiber core sizes and NA Characteristic indicators of doped optical fibers of different sizes include the modes supported by the optical fiber, the effective mode field area, and the bending loss coefficient of the high-order modes.

[0025] Calculate the current core size and NA Cut-off frequency of doped fiber under the size :

[0026] ;

[0027] in is the cut-off frequency, is the numerical aperture, is the core radius, is the wavelength;

[0028] when When doped fiber only supports model;

[0029] when , doped fiber single mode only supports , model;

[0030] when , doped fiber single mode only supports , , and model.

[0031] in The mode is the fundamental mode in the optical fiber, i.e., the lowest-order mode, with a radial quantum number of 0 and an angular quantum number of 1; The mode is the first higher-order mode in the fiber, with a radial quantum number of 1 and an angular quantum number of 1. and mode is the second higher order mode in the optical fiber, The radial quantum number of the mode is 0, the angular quantum number is 2, The radial quantum number of the mode is 2, and the angular quantum number is 1. The radial quantum number and angular quantum number represent the number of oscillations of the mode in the radial and angular directions, respectively.

[0032] (2) Roughly estimate the different core sizes and NA The effective mode field area of ​​the doped fiber under the size ,as follows:

[0033] ;

[0034] in, is the cut-off frequency, is the fiber core radius.

[0035] (3) Roughly estimate the different fiber core sizes and NA The bending loss coefficient of doped optical fiber under the following size is as follows:

[0036] ;

[0037] in is the bending loss coefficient of the higher-order mode, is the core bending radius, is the core radius, β for The propagation constant corresponding to the mode, ,in k is the wave vector, n core is the core refractive index, ,in n clad is the cladding refractive index, K m-1 (W) and K m+1 (W) represents (m-1)-order and (m+1)-order Bessel functions.

[0038] S200 is based on a rough estimate of different core sizes and NA The fiber characteristics of doped fibers under different sizes are predicted to be different for different core sizes and NA The nonlinear effect threshold of doped fibers with different core sizes and sizes is obtained after they are used as gain fibers in the target laser system. NA The system power threshold of the doped fiber with different sizes is applied as the gain fiber to the target laser system. The core size and NA size, as the ideal core size of doped optical fiber and NA size.

[0039] Further, different fiber core sizes and NA The method comprises: for the first i Fiber core size and NA Doped fiber under the size, get the first i Fiber core size and NA The various nonlinear effect thresholds of the doped fiber of different sizes after being applied as the gain fiber to the target laser system. If there are n kinds of nonlinear effects, then n kinds of nonlinear effect thresholds can be obtained. , select the smallest nonlinear effect threshold as the first i Fiber core size and NA The system power threshold after the doped fiber with a certain size is applied as a gain fiber to the target laser system is expressed as:

[0040] ;

[0041] S300 is based on the ideal core size and NASize, using finite element simulation, obtain the mode field distribution of doped optical fibers with different refractive index distributions, and calculate the fiber characteristic indicators of doped optical fibers under different refractive index distributions, including effective mode field area, mode field center energy distribution ratio and mode loss coefficient;

[0042] S400 predicts the nonlinear effect threshold of doped optical fibers with different refractive index distributions after they are applied as gain fibers to the target laser system based on the fiber characteristic indicators of doped optical fibers with different refractive index distributions, obtains the system power threshold of doped optical fibers with different refractive index distributions after they are applied as gain fibers to the target laser system, and selects the refractive index distribution corresponding to the maximum system power threshold among all the system power thresholds as the ideal refractive index distribution of the doped optical fiber.

[0043] In S300, the fiber characteristic indicators of doped fibers under different refractive index distributions are calculated, including the effective mode field area , energy distribution ratio of mode field center and mode loss coefficient α , the calculation formula is as follows:

[0044] ;

[0045] ;

[0046] ;

[0047] In the formula, E ( z , r , φ ) is the coordinate The distribution of the electric field at imag represents the imaginary part, represents the distribution of the effective refractive index, is the wavelength, center_core represents the inner core, wherein the inner core is the central area of ​​the core and the central area of ​​the core occupies 50% of the area of ​​the entire core area.

[0048] Further, obtaining the system power threshold after the doped optical fiber with different refractive index distribution is applied as the gain optical fiber to the target laser system, the method comprises: for the first j The doped fiber with the refractive index distribution is obtained j The various nonlinear effect thresholds after the doped fiber with a certain refractive index distribution is used as the gain fiber in the target laser system, and the smallest nonlinear effect threshold is selected as the first j The system power threshold after the doped fiber with a certain refractive index distribution is used as a gain fiber in the target laser system.

[0049] After the doped fiber is used as a gain fiber in the target laser system, the target laser system is a high-power fiber laser system. The performance improvement of the high-power fiber laser system is mainly restricted by various nonlinear effects, such as stimulated Brillouin scattering effect and mode instability effect. Among them, the stimulated Brillouin scattering effect will induce unstable Stokes pulse light, limiting the power improvement of the fiber laser system; and the mode instability effect will induce dynamic energy coupling between the fundamental mode and the high-order mode, limiting the brightness improvement of the fiber laser system.

[0050] There are some theoretical analysis models about nonlinear effects in the prior art. By selecting a suitable theoretical analysis model of nonlinear effects from the existing theoretical analysis models of nonlinear effects, the theoretical analysis models of different fiber core sizes and NA The threshold values ​​of various nonlinear effects of the doped fiber under the size as the gain fiber applied to the target laser system are obtained, and the threshold values ​​of various nonlinear effects of the doped fiber under the refractive index distribution as the gain fiber applied to the target laser system are obtained. Without loss of generality, the present invention takes the theoretical analysis model that simultaneously considers stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) effects as an example to briefly describe the above steps.

[0051] In step S200, the roughly estimated fiber core sizes and NA Substituting the fiber characteristic indicators of doped optical fibers with different core sizes into the theoretical analysis model of SBS effect and TMI effect, we can obtain the results of NA The SBS effect threshold and TMI effect threshold of the doped fiber with the same size as the gain fiber are applied to the target laser system. i Fiber core size and NA Doped fiber under the size, get the first i Fiber core size and NA The SBS effect threshold and TMI effect threshold of the doped fiber with different sizes are used as the gain fiber in the target laser system, and the smaller one of the SBS effect threshold and the TMI effect threshold is selected as the first i Fiber core size and NA The system power threshold after the doped fiber with a certain size is used as a gain fiber in the target laser system.

[0052] Without loss of generality, a theoretical analysis model of the SBS effect and a theoretical analysis model of the TMI effect used in one embodiment of the present invention are provided below.

[0053] Theoretical analysis model of SBS effect:

[0054] In order to describe the evolution of the SBS effect in the optical fiber, it is necessary to establish a time-dependent three-wave coupling equation involving the signal light field, the Stokes light field, and the acoustic wave field. Under the conditions of line bias approximation and slowly varying amplitude approximation, the time-dependent three-wave coupling equation can be expressed as:

[0055] ;

[0056] ;

[0057] ;

[0058] In the formula, E s , E B and Q represent the complex amplitudes of signal light, Stokes light and acoustic wave field respectively, v s and v B are the group velocities of the signal light and the Stokes light, v A is the speed of sound, γ e is the electrostriction constant of quartz, ρ 0 is the density of quartz. A ao is the overlapping area of ​​the acoustic wave field and the light field. Since the fundamental mode area of ​​the acoustic wave field is much larger than that of the light wave field, A ao The effective mode field area of ​​the signal light can be approximated by A eff )replace 。 and is the active gain of the signal light and the Brillouin-Stokes light, which satisfy the following expressions:

[0059] ;

[0060] ;

[0061] ;

[0062] In the formula, ± represents the positive and negative transmission directions of the pump light, respectively. σ ai 、σ ei ( i = p, s, B ) represent the absorption cross section and emission cross section of pump light, signal light and Stokes light respectively, Г i (i = p, s, B ) is the effective overlap factor of the pump light, signal light, and Stokes light with the core doping area, P i ( i = p, s, B ) are the signal light and Stokes light power, h is Planck's constant, τ is the lifetime of the upper energy level of the ytterbium ion, A co is the core area of ​​the optical fiber. s and α B is the loss coefficient of the signal light and the Stokes light in the optical fiber. Since the frequencies of the signal light and the Stokes light are very close, α s and α B Approximately equal. f is the noise term that causes the SBS effect, κ1 and κ2 are the SBS coupling coefficients, and their specific forms are given by the following formula:

[0063] ;

[0064] Define that when the Stokes optical power in a direction reaches 1‰ of the output power, the output power at this time is the SBS threshold of the system P th(SBS) :

[0065] ;

[0066] Theoretical analysis model of SBS effect:

[0067] The evolution of mode instability in a tapered fiber amplifier is derived using the semi-analytical heat conduction equation. LP 01 and LP 11 During the two modes, in order to simplify the simulation process, the simulation model only considers the LP 01 and LP 11 Two modes. LP 01 and LP 11 Under the premise of two eigenmodes, the light field in the fiber core can be expressed as:

[0068] ;

[0069] In the formula, E represents the total light field in the fiber core, a 0 and a 1 respectively LP01 and LP 11 The mode amplitudes of the two eigenmodes, β 0. β 1 is the transmission constant of the mode, ω 0. ω 1 is the angular frequency of the two modes; ψ 0 and ψ 1 represents the normalized light field distribution of the two intrinsic modes, and then the light intensity distribution in the fiber core can be obtained as:

[0070] ;

[0071] in

[0072] ;

[0073] ;

[0074] ;

[0075] In the above formula, Z 0 ≈ 377 Ω is the vacuum wave impedance; I 0 is the slow variation of light intensity with respect to time, It is the interference term of light intensity, that is, the rapid change with respect to time. β and Ω Respectively represent LP 01 and LP 11 The transmission constant difference and angular frequency difference of the mode can be expressed as:

[0076] ;

[0077] ;

[0078] Ignoring photon darkening and signal light background loss, the heat source in the gain fiber comes entirely from the quantum defect term of the ytterbium ion. Assuming the gain distribution in the doped fiber is g(r, φ, z, t) , then the heat source term in the gain fiber core can be expressed as follows:

[0079] ;

[0080] According to the heat source term, the heat source distribution in the optical fiber can be obtained ΔT Taking into account the fiber gain and thermo-optic effect, the refractive index distribution change caused by it ( ) is expressed as:

[0081] ;

[0082] ;

[0083] in h 00 , h 11 , , is the contribution of the symmetrical component of the temperature distribution in the optical fiber to the refractive index of the optical fiber, that is, the contribution of the average temperature of the optical fiber to the refractive index distribution; h 01 , h 10 , , is the asymmetric component of the temperature distribution in the optical fiber, that is LP 01 and LP 11 The contribution of the temperature oscillation distribution caused by the optical field interference term of the mode to the refractive index distribution of the optical fiber.

[0084] because Much smaller than the refractive index of the optical fiber itself n 0, so the total refractive index of the optical fiber can be expressed as:

[0085] ;

[0086] Set the SBS threshold P th(SBS) The formula and the total refractive index formula of the optical fiber are substituted into the wave equation:

[0087] ;

[0088] By using the slowly varying amplitude approximation, the steady-state mode coupling equation of thermally induced mode instability is solved as follows:

[0089] ;

[0090] ;

[0091] Where χ(Ω,z) is the nonlinear coupling coefficient between the fundamental mode and the higher-order mode. Studies have shown that the refractive index distribution caused by the asymmetric temperature distribution in the optical fiber can cause energy coupling between the fundamental mode and the higher-order mode, so χ(Ω,z) can be expressed as:

[0092] ;

[0093] Definition LP 11 When the power ratio of the mode reaches 5% of the total output power, the output power at this time is the system TMI threshold ( P th(TMI) ):

[0094] ;

[0095] The design of a doped optical fiber suitable for high-power single-frequency laser amplification is used as an example for explanation. The high-power single-frequency optical fiber system is mainly limited by the SBS and TMI effects. According to the theoretical analysis model of the SBS effect, the theoretical analysis model of the TMI effect and the steps S100 and S200 of the present invention, a schematic diagram of the system power threshold can be obtained, as shown in FIG. Figure 2 As shown, it can be found that when designing a doped optical fiber suitable for high-power single-frequency laser amplification, when the core NA is designed to be 0.04, the optimal core diameter is 32.5μm.

[0096] In practical applications, multiple nonlinear effects such as stimulated Brillouin scattering (SBS) and transverse mode instability (TMI) can be considered simultaneously. By comparing the nonlinear power thresholds, the smaller nonlinear threshold power is taken as the output power threshold of the system under this parameter design.

[0097] Figure 3 This is a comparison chart of the refractive index distributions corresponding to two doped optical fibers. The two doped optical fibers have the same core diameter and NA, but are designed with different refractive index distributions, one is a conventional refractive index distribution, and the other is a special refractive index distribution. In the conventional refractive index distribution, the refractive index of the cladding is 1.45, the refractive index of the core is 1.4504, and the average core NA is 0.038; in the special refractive index distribution, the refractive index of the cladding is 1.45, the refractive index of the outer 50% area of ​​the core is 1.4504, the refractive index of the inner 50% area of ​​the core is 1.4506, and the average core NA is 0.038.

[0098] The technical features of the above embodiments may be combined arbitrarily. 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.

[0099] 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.

[0100] 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 design of doped optical fibers, characterized in that: include: Rough estimate of different core sizes and NA The fiber characteristic indicators of doped fibers of different sizes, including the modes supported by the fiber and the effective mode field area and the bending loss coefficients of the higher-order modes ; Based on rough estimates of different core sizes and NA The fiber characteristics of doped fibers under different sizes are predicted to be different for different core sizes and NA The nonlinear effect threshold of doped fibers with different core sizes and sizes is obtained after they are used as gain fibers in the target laser system. NA The system power threshold of the doped fiber with different sizes is applied as the gain fiber to the target laser system. The core size and NA size, as the ideal core size of doped optical fiber and NA size; Based on the ideal core size of doped optical fiber and NA Size, using finite element simulation, obtain the mode field distribution of doped optical fibers with different refractive index distributions, and calculate the fiber characteristic indicators of doped optical fibers under different refractive index distributions, including the effective mode field area , energy distribution ratio of mode field center and mode loss coefficient α ; Based on the fiber characteristic indicators of doped optical fibers under different refractive index distributions, the nonlinear effect thresholds of the doped optical fibers under different refractive index distributions are predicted after being applied as gain fibers to the target laser system. The system power thresholds of the doped optical fibers under different refractive index distributions are obtained after being applied as gain fibers to the target laser system. The refractive index distribution corresponding to the maximum system power threshold is selected from all the system power thresholds as the ideal refractive index distribution of the doped optical fiber.

2. The method for optimizing the design of doped optical fibers according to claim 1, characterized in that: Get different fiber core sizes and NA The method comprises: for the first i Fiber core size and NA Doped fiber under the size, get the first i Fiber core size and NA The various nonlinear effect thresholds of the doped fiber with different sizes are used as the gain fiber in the target laser system, and the smallest nonlinear effect threshold is selected as the first i Fiber core size and NA The system power threshold after the doped fiber with a certain size is used as a gain fiber in the target laser system.

3. The method for optimizing the design of doped optical fibers according to claim 1 or 2, characterized in that: Obtaining a system power threshold after doped optical fibers with different refractive index distributions are applied as gain optical fibers to a target laser system, the method comprising: for the first j The doped fiber with the refractive index distribution is obtained j The various nonlinear effect thresholds after the doped fiber with a certain refractive index distribution is used as the gain fiber in the target laser system, and the smallest nonlinear effect threshold is selected as the first j The system power threshold after the doped fiber with a certain refractive index distribution is used as a gain fiber in the target laser system.

4. The method for optimizing the design of doped optical fibers according to claim 3, characterized in that: Rough estimate of different core sizes and NA The modes supported by the fiber under the size of the doped fiber are as follows: Calculate the current core size and NA Cut-off frequency of doped fiber under the size : in is the cut-off frequency, is the numerical aperture, is the core radius, is the wavelength; when When doped fiber only supports model; when , doped fiber single mode only supports , model; when , doped fiber single mode only supports , , and model.

5. The method for optimizing the design of doped optical fibers according to claim 4, characterized in that: Rough estimate of different core sizes and NA The effective mode field area of ​​the doped fiber under the size ,as follows: in is the cut-off frequency, is the fiber core radius.

6. The method for optimizing the design of doped optical fibers according to claim 4, characterized in that: Rough estimate of different core sizes and NA The bending loss coefficient of the high-order mode of the doped optical fiber with a size of 2 is as follows: in is the bending loss coefficient of the higher-order mode, is the core bending radius, is the core radius, β for The propagation constant corresponding to the mode, ,in k is the wave vector, n core is the core refractive index, ,in n clad is the cladding refractive index, K m-1 (W) and K m+1 (W) represents (m-1)-order and (m+1)-order Bessel functions.

7. The method for optimizing the design of doped optical fibers according to claim 1, 2, 4, 5 or 6, characterized in that: Calculate the fiber characteristics of doped fibers with different refractive index profiles, including effective mode field area , energy distribution ratio of mode field center and mode loss coefficient α , the calculation formula is as follows: In the formula, E ( z , r , φ ) is the coordinate The distribution of the electric field at imag represents the imaginary part, represents the distribution of the effective refractive index, is the wavelength, center_core represents the inner core, wherein the inner core is the central area of ​​the core and the central area of ​​the core occupies 50% of the area of ​​the entire core area.

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