Method and device for calculating TMI effect in high-power fiber laser

Abstract

The invention relates to the technical field of optical fiber communication, and discloses a TMI effect calculation method and device in a high-power optical fiber laser. The method comprises the following steps: acquiring mode field distribution of a to-be-measured optical fiber in a bending state, further solving mode field distribution under a thermal load by taking the mode field distribution of the to-be-analyzed optical fiber in the bending state as a substrate, and calculating a TMI coupling coefficient and a threshold value by utilizing the mode field distribution. An electromagnetic wave intrinsic problem is solved by using a full-vector finite element method, refractive indexes and mode field distribution of different optical fibers under different bending radiuses and thermal load states are calculated, and the optical fiber laser is simulated to amplify different optical fibers according to different coiling modes in the amplification process of the optical fiber laser. Through the influence of the thermal lens effect caused by amplification on the final bending loss, mode field distribution and TMI coupling coefficient, the comprehensive effect of the influence of the bending and thermal lens effect induced mode change on the TMI is obtained, the theory is closer to the experiment, and the design of the fiber laser and the improvement of the nonlinear threshold value are further guided.

Description

Technical Field

[0001] The present application relates to the field of optical fiber communication technology, and in particular to a method and device for calculating the TMI effect in a high-power optical fiber laser. Background Art

[0002] At present, the output power of fiber lasers has been greatly improved, but the further power scaling of single-mode fiber lasers is still hindered by optical nonlinear effects. To solve this problem, large mode area (LMA) fiber increases the size of the fiber core to reduce the light intensity in the fiber. However, under the same numerical aperture, increasing the core size means that the fiber can guide more modes, so that the large mode area fiber no longer works in a single-mode state. The presence of higher-order modes will reduce the beam quality of the laser output, causing a nonlinear effect called transverse mode instability (TMI). This nonlinear effect will reduce the output performance of high-power fiber lasers, causing the output beam to have spot jitter at a certain threshold power level and inhibiting further increase in output power.

[0003] In view of this, how to provide a method to calculate the impact of the TMI effect and improve the nonlinear threshold of TMI has become a technical problem that needs to be solved urgently. Summary of the invention

[0004] The embodiments of the present application provide a method for calculating the TMI effect in a high-power fiber laser, a device for calculating the TMI effect in a high-power fiber laser, an electronic device, and a computer storage medium, which are used to solve the problem of poor performance in current network traffic anomaly detection.

[0005] In a first aspect of an embodiment of the present application, a method for calculating the TMI effect in a high-power fiber laser is provided, comprising:

[0006] Obtain the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range;

[0007] Taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and the first thermal load state, and calculating the TMI first coupling coefficient and the TMI first threshold based on the first mode field distribution;

[0008] Taking the first mode field distribution as a basis, sequentially calculating the second mode field distribution corresponding to the optical fiber to be tested at different bending radii and second thermal load states, and calculating the TMI second coupling coefficient and the TMI second threshold based on the second mode field distribution;

[0009] Taking the second mode field distribution as the basis, the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and third thermal load states is calculated in sequence, and based on the third mode field distribution, the TMI third coupling coefficient and the TMI third threshold are calculated, and the above steps are repeated until the mode field distribution, TMI coupling coefficient and TMI threshold corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

[0010] In an optional embodiment, obtaining the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range includes:

[0011] Calculate the refractive index distribution of the optical fiber to be tested based on conformal transformation and coordinate system conversion method;

[0012] Based on the refractive index distribution of the optical fiber to be tested, the normalized mode field distribution corresponding to the fundamental mode and the higher-order mode, and the bending loss corresponding to each bending radius are calculated using the full vector finite element method;

[0013] According to the actual amplification requirements, determine the target bending radius range of the optical fiber to be tested, and obtain the target refractive index distribution corresponding to the target bending radius range and the target normalized mode field distribution and target bending loss corresponding to the fundamental mode and the higher-order mode;

[0014] Calculate the TMI target coupling coefficient corresponding to the target normalized mode field distribution state;

[0015] The mode field distribution to be analyzed is determined based on the target refractive index distribution, the target normalized mode field distribution, the target bending loss, and the TMI target coupling coefficient.

[0016] In an optional embodiment, the method of taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and the first thermal load state, and calculating the TMI first coupling coefficient and the TMI first threshold based on the first mode field distribution, includes:

[0017] Taking the mode field distribution to be analyzed as a basis, calculating a first thermo-induced refractive index distribution of the mode field distribution to be analyzed under a first heat load state;

[0018] Based on the first thermo-induced refractive index distribution, first mode field distributions corresponding to different bending radii and first thermal load states are calculated in sequence, and a first TMI loss under the first mode field distribution state is calculated;

[0019] Based on the first mode field distribution, a first TMI coupling coefficient and a first TMI threshold are calculated.

[0020] In a second aspect of an embodiment of the present application, a device for calculating TMI effect in a high-power fiber laser is provided, comprising:

[0021] An acquisition module is configured to acquire the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range;

[0022] A first calculation module is configured to calculate the first mode field distribution corresponding to the optical fiber under test at different bending radii and the first thermal load state based on the mode field distribution to be analyzed, and calculate the first TMI coupling coefficient and the first TMI threshold based on the first mode field distribution;

[0023] A second calculation module is configured to calculate the second mode field distribution corresponding to the optical fiber to be tested at different bending radii and a second thermal load state in sequence based on the first mode field distribution, and calculate the TMI second coupling coefficient and the TMI second threshold based on the second mode field distribution;

[0024] The iteration module is configured to use the second mode field distribution as a basis to sequentially calculate the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and a third thermal load state, and based on the third mode field distribution, calculate the TMI third coupling coefficient and the TMI third threshold, and repeat the above steps until the mode field distribution, the TMI coupling coefficient and the TMI threshold corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

[0025] According to a third aspect of an embodiment of the present application, a computer-readable storage medium is provided, which stores computer-executable instructions, and when the instructions are executed by a processor, the steps of the above-mentioned method for calculating the TMI effect in a high-power fiber laser are implemented.

[0026] The present application provides a method for calculating the TMI effect in a high-power fiber laser, comprising: first, obtaining a mode field distribution to be analyzed corresponding to the optical fiber to be tested within a target bending radius range; second, taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and a first thermal load state, and calculating a first TMI coupling coefficient and a first TMI threshold based on the first mode field distribution; third, taking the first mode field distribution as a basis, sequentially calculating the second mode field distribution corresponding to the optical fiber to be tested at different bending radii and a second thermal load state, and calculating a second TMI coupling coefficient and a second TMI threshold based on the second mode field distribution; finally, taking the second mode field distribution as a basis, sequentially calculating the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and a third thermal load state, and calculating a third TMI coupling coefficient and a third TMI threshold based on the third mode field distribution, and repeating the above steps until the mode field distribution, the TMI coupling coefficient and the TMI threshold corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

[0027] A method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application is applied, and the full-vector finite element method is used to solve the eigenvalue problem of electromagnetic waves, and the refractive index and mode field distribution of different optical fibers under different bending radii and thermal load conditions are calculated. The thermal lens effect caused by amplification on the final bending loss, mode field distribution and TMI coupling coefficient is simulated for different optical fibers and different winding methods during the amplification process of the fiber laser, so as to obtain the comprehensive effect of the influence of the modal change induced by the bending and thermal lens effects on the TMI, so as to make the theory closer to the experiment, and further guide the design of the fiber laser and the improvement of the nonlinear threshold.

[0028] The above description is only an overview of the technical solution of the present application. In order to more clearly understand the technical means of the present application, which can be implemented in accordance with the contents of the specification, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the specific implementation methods of the present application are listed below. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Various other advantages and benefits will become apparent to those of ordinary skill in the art by reading the detailed description of the preferred embodiments below. The accompanying drawings are only for the purpose of illustrating the preferred embodiments and are not to be considered as limiting the present application. Also, the same reference symbols are used throughout the accompanying drawings to represent the same components. In the accompanying drawings:

[0030] Figure 1 A schematic diagram of a flow chart of a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application;

[0031] Figure 2 A schematic diagram of a framework of a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application;

[0032] Figure 3 A schematic diagram of the refractive index distribution of an optical fiber under the combined effects of bending and thermal lensing effects in a method for calculating the TMI effect in a high-power optical fiber laser provided in an embodiment of the present application;

[0033] Figure 4 A graph showing the change in bending loss of two sizes of optical fibers with increasing thermal load at specific and different bending radii in a method for calculating the TMI effect in a high-power optical fiber laser provided in an embodiment of the present application;

[0034] Figure 5 A graph showing the variation of TMI coupling coefficients of two sizes of optical fibers with increasing thermal load at specific and different bending radii in a method for calculating TMI effect in a high-power fiber laser provided in an embodiment of the present application;

[0035] Figure 6A schematic diagram showing the relationship between the proportion of high-order mode components caused by quantum noise and intensity noise and the output power of two sizes of optical fibers under a specific radius and thermal load in a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application;

[0036] Figure 7 A schematic diagram of the structure of a TMI effect calculation device in a high-power fiber laser provided in an embodiment of the present application. DETAILED DESCRIPTION

[0037] The exemplary embodiments of the present disclosure will be described in more detail below with reference to the accompanying drawings. Although the exemplary embodiments of the present disclosure are shown in the accompanying drawings, it should be understood that the present disclosure can be implemented in various forms and should not be limited by the embodiments set forth herein. On the contrary, these embodiments are provided to enable a more thorough understanding of the present disclosure and to fully convey the scope of the present disclosure to those skilled in the art.

[0038] It is generally believed that TMI is mainly caused by thermal effects in optical fibers. Through photo-darkening and quantum loss, transverse mode interference in the gain fiber will cause local thermal effects in the fiber, thereby locally modulating the fiber refractive index and generating a refractive index grating. Then, the longitudinal phase shift between the transverse mode interference pattern and the thermal refractive index grating allows energy to be dynamically transferred between the fiber fundamental mode and the higher-order modes, resulting in dynamic fluctuations in the output light spot. In order to gain a deeper understanding of the TMI effect and the influencing factors of the TMI threshold, many semi-analytical models have been proposed. In these models, the TMI coupling coefficient derived from the nonlinear coupling equation is a crucial parameter, which can quantify the intensity of the mode coupling effect. Without considering the gain saturation effect, the TMI coupling coefficient can be calculated from the mode field distribution of the fiber transverse mode and the frequency shift between the modes, thereby guiding the design of the gain fiber.

[0039] Generally speaking, the fewer the number of modes that can be guided in the fiber, the lower the TMI coupling coefficient, and the higher the TMI threshold of the fiber laser. In traditional step-index LMA fibers, a simple and effective way to filter out high-order modes is to bend the fiber. Since the bending loss of high-order modes is higher than that of the fundamental mode, bending the fiber can effectively suppress the power growth of high-order modes in the amplifier. Theoretical and experimental work on the effect of fiber bending on the TMI threshold has confirmed that the threshold of the TMI effect can be increased by properly bending the fiber to a certain bending radius. Most of the previous work focused on obtaining the relationship between the threshold of the TMI effect and the fiber bending radius by introducing the bending loss at each bending radius into the TMI theoretical model. However, in addition to causing mode loss, bending the fiber will also cause mode field distortion of each mode in the fiber, which will change the TMI coupling coefficient and thus affect the TMI threshold of the fiber laser or amplifier. In addition, as the power increases, the thermal lens effect will further change the mode field distribution of the mode guided by the bent fiber, which will also cause the TMI coupling coefficient to change. The combined effect of the mode field change caused by bending and thermal lensing on the TMI effect has not been fully studied.

[0040] In view of this, the embodiment of the present application provides a method for calculating the TMI effect in a high-power fiber laser, that is, by calculating the influence of fiber bending and thermal lens effect on the TMI coupling coefficient, the possible TMI effect is avoided and suppressed, and the optimization design of gain fibers and fiber amplifiers in each band is guided. Specifically, it includes: obtaining the mode field distribution of the fiber to be tested in a bent state, and using the mode field distribution of the fiber to be analyzed in a bent state as a basis, further obtaining the mode field distribution under thermal load, and using the mode field distribution to calculate the TMI coupling coefficient and threshold.

[0041] See also Figure 1 , Figure 1 The following is a flow chart of a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application. Figure 1 As shown, the specific steps include the following steps.

[0042] Step S102: obtaining the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range.

[0043] Step S104: taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and first thermal load states, and calculating the TMI first coupling coefficient and the TMI first threshold based on the first mode field distribution.

[0044] In an embodiment of the present application, the method of obtaining the mode field distribution to be analyzed corresponding to the target bending radius range of the optical fiber to be tested includes: calculating the refractive index distribution of the optical fiber to be tested based on the conformal transformation and the coordinate system conversion method; calculating the normalized mode field distribution corresponding to the fundamental mode and the higher-order mode, and the bending loss corresponding to each bending radius using the full vector finite element method based on the refractive index distribution of the optical fiber to be tested, that is, solving the eigenvalue problem of the electromagnetic wave; determining the target bending radius range of the optical fiber to be tested according to the actual amplification requirements, and obtaining the target refractive index distribution corresponding to the target bending radius range and the target normalized mode field distribution and target bending loss corresponding to the fundamental mode and the higher-order mode; calculating the corresponding TMI target coupling coefficient under the target normalized mode field distribution state; and determining the mode field distribution to be analyzed based on the target refractive index distribution, the target normalized mode field distribution, the target bending loss, and the TMI target coupling coefficient.

[0045] Step S106: taking the first mode field distribution as a basis, sequentially calculating the second mode field distributions of the optical fiber to be tested corresponding to different bending radii and second thermal load states, and calculating the TMI second coupling coefficient and the TMI second threshold based on the second mode field distribution;

[0046] Step S108: Taking the second mode field distribution as a basis, the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and third thermal load states is calculated in sequence, and based on the third mode field distribution, the TMI third coupling coefficient and the TMI third threshold are calculated, and the above steps are repeated until the mode field distribution, TMI coupling coefficient and TMI threshold corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

[0047] In an embodiment of the present application, the method of taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and a first thermal load state, and calculating the first TMI coupling coefficient and the first TMI threshold value based on the first mode field distribution, includes: taking the mode field distribution to be analyzed as a basis, calculating the first thermo-induced refractive index distribution of the mode field distribution to be analyzed under the first thermal load state; based on the first thermo-induced refractive index distribution, sequentially calculating the first mode field distribution corresponding to different bending radii and the first thermal load state, and calculating the first TMI loss under the first mode field distribution state; and calculating the first TMI coupling coefficient and the first TMI threshold value based on the first mode field distribution.

[0048] See also Figure 2 , Figure 2 A schematic diagram of the framework of a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application.

[0049] Taking LMA20 / 400μm fiber and LMA25 / 400μm fiber as examples, the embodiment of the present application provides a simulation method for numerically analyzing the influence of mode distortion caused by bending and thermal lens effect on TMI effect in LMA step-index fiber. The refractive index distribution of LMA20 / 400μm fiber and LMA25 / 400μm fiber at different bending radii is calculated by full vector finite element method (i.e. FEM) and conformal transformation, and the fundamental mode LP is calculated. 01 and higher order modes LP 11 The corresponding bending loss is calculated, and the bending radius range is selected according to the single-mode operation conditions required for actual amplification; the mode field distribution and TMI coupling coefficient of each mode are calculated according to the refractive index distribution within the bending radius range, and the mode field distribution and refractive index distribution under bending are used as the basis to further calculate the thermally induced refractive index change and thermally induced mode field distortion caused by the increase of thermal load under bending state, and calculate the TMI coupling coefficient and TMI threshold of two sizes of optical fiber under different thermal loads. Specifically, the following steps are included:

[0050] Step 1: First calculate LP at different bending radii 01 and LP 11 The refractive index distribution of the bent fiber is transformed into an equivalent straight fiber in the (x, y, z) coordinate system through conformal transformation. The bending radius of the fiber in the x direction is R. The refractive index distribution of the bent fiber after conformal transformation is:

[0051]

[0052] R eff =(1+δ)R,δ:{0.28,0.31};

[0053] Among them, n eq (x, y) is the effective refractive index distribution of the equivalent straight fiber; n(x, y) is the effective refractive index distribution of the bent fiber; R is the bending radius; R eff For conformal transformation, the bending radius R is replaced by the effective bending radius with an elasto-optic correction factor, where for silica fiber, R eff / R=1.28.

[0054] Step 2: Based on the equivalent refractive index distribution obtained in step 1, the normalized LP under a given bending radius can be obtained by solving the eigenvalue problem of electromagnetic waves using the full vector finite element method. 01 and LP 11 Mode field distribution. A perfect matching layer is introduced outside the fiber cladding, which can absorb the leakage mode and introduce an imaginary part in the solved effective refractive index, representing the bending loss α of the corresponding eigenmode in the fiber, which can be expressed as:

[0055]

[0056] Among them, R eff For conformal transformation, the bending radius R is replaced by the effective bending radius with an elasto-optic correction factor; α is the bending loss of the corresponding eigenmode in the optical fiber.

[0057] Step 3: Further, based on the calculated mode field distribution, the TM I coupling coefficient under each bending state can be calculated:

[0058]

[0059] Among them, B 21 The Green's function G(r ⊥ ,r ⊥ ′,ω) and LP 01 and LP 11 The normalized mode field distribution ψ1(r ⊥ ) and ψ2(r ⊥ ) decision, which can be expressed as:

[0060]

[0061] G(r ⊥ ,r ⊥ ′,ω)=G(r ⊥ -r ⊥ ′,ω);

[0062]

[0063] Among them, q=iρCΔω / κ, Δω=ω1-ω2 is LP 01 and LP 11 The frequency shift between ρC and S is the product of density and specific heat capacity. d and subscript ⊥ represent the doping region and the abscissas x and y respectively. The external integral in the above formula acts on the entire fiber cross section; K0 is the second-kind zero-order modified Bessel function.

[0064] From the above equation, it can be seen that the Green's function is determined by the optical fiber itself. Bending the optical fiber will change the mode field distribution ψ1(r ⊥ ) and ψ2(r ⊥ ), which will affect B in the equation 21 , thereby changing the TMI coupling coefficient and affecting the gain of TMI.

[0065] Step 4: Based on the mode field distribution of the bent optical fiber obtained in step 2, use it as the basis to calculate the thermally induced refractive index change and mode field distribution of the bent optical fiber. The heat load generated by the amplification process will cause thermal distribution in the optical fiber. Due to the thermo-optical effect, the thermal distribution will determine the refractive index change in the optical fiber, that is, β = dn / dT, β is the thermo-optical coefficient. For simplicity, it is assumed that the heat deposition in the optical fiber is uniform, and the heat is only deposited in the doped core area. The cladding region has the same thermal properties as the core, and the coating is ignored. By solving the heat diffusion equation in the steady state, the expression of the thermal distribution T(r) of the core and cladding regions can be obtained, and then combined with the proportional relationship between the refractive index change and the temperature change Δn β (r) = β(T(r)-T c ), T c is the coolant temperature, and the fiber refractive index distribution under heat load can be expressed as:

[0066]

[0067] Among them, q h =Q / πa 2 is the thermal power density; Q is the heat load of the optical fiber; a and b are the core radius and cladding radius; n co and n cl is the refractive index of the core and cladding; k si is the thermal conductivity of silicon dioxide; h q is the heat transfer coefficient.

[0068] The refractive index distribution of the optical fiber after the heat load is introduced can be calculated by further conformal transformation of the thermally induced refractive index distribution at a certain bending radius, such as Figure 3 As shown, the corresponding mode field distribution of LP01 and LP11 under the thermal lens effect is obtained. Figure 3 A schematic diagram of the refractive index distribution of an optical fiber under the combined effects of bending and thermal lensing effects in a method for calculating the TMI effect in a high-power optical fiber laser provided in an embodiment of the present application.

[0069] Step 5, combining the thermally induced refractive index change and mode field distribution under the bending state obtained in step 2, the bending loss and TMI coupling coefficient after the introduction of thermal load can be calculated according to the formulas shown in steps 2 and 3. Further, the TMI threshold can also be determined, considering quantum noise and intensity noise as the noise sources of the TMI effect respectively. For quantum noise, the high-order mode content can be defined as:

[0070]

[0071] Among them, P 0,1 and P1(L) are the input power and the output power at the fiber length L, respectively; is the equivalent quantum noise input optical power density; Γ1, Γ2 are the overlap factors of the fundamental mode and the higher-order mode; Ω p is the frequency at the maximum χ value; χ″ is the second-order derivative of χ with respect to Ω, at a given threshold ξ th =0.05, the TMI threshold power caused by quantum noise can be obtained from the above formula: P1(L)=P th .

[0072] In the case of TMI effect caused by intensity noise, a small amount of signal light exists in the high-order mode LP due to the imperfect matching of the signal light mode in the fiber. 11 The high-order mode content can be given by:

[0073]

[0074] Where ξ(0) is the initial high-order mode content, R N (Ω p ) is the relative intensity noise of the input signal light.

[0075] Step 6, further use the mode field distribution in step 4 as a basis to calculate the optical fiber refractive index distribution, loss, mode field distribution and TMI coupling coefficient under the next thermal load state, and continue to iterate to obtain the bending loss and TMI coupling coefficient under different thermal load states at a specific bending radius, and further calculate the TMI threshold under different thermal load states at a specific bending radius.

[0076] Step 6: Repeat steps 1 to 5 to obtain the TMI coupling coefficient under different thermal load conditions at different bending radii. The calculation results of bending loss and thermal bending loss under different bending radii are as follows: Figure 4 ,in, Figure 4 A graph showing the change in bending loss of two sizes of optical fibers with increasing thermal load at specific and different bending radii in a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application.

[0077] The calculation results of the TM I coupling coefficient under different bending radii and the TM I coupling coefficient and TM I threshold after introducing heat load under a specific bending radius are shown in Figure 5 and Figure 6 .in, Figure 5 A graph showing the variation of TMI coupling coefficients of two sizes of optical fibers with increasing thermal load at specific and different bending radii in a method for calculating TMI effect in a high-power fiber laser provided in an embodiment of the present application; Figure 6 A schematic diagram of the relationship between the proportion of high-order mode components caused by quantum noise and intensity noise and the output power of two sizes of optical fibers under specific radius and thermal load in a method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application.

[0078] A method for calculating the TMI effect in a high-power fiber laser provided in an embodiment of the present application is applied, and the full-vector finite element method is used to solve the eigenvalue problem of electromagnetic waves, and the refractive index and mode field distribution of different optical fibers under different bending radii and thermal load conditions are calculated. The thermal lens effect caused by amplification on the final bending loss, mode field distribution and TMI coupling coefficient is simulated for different optical fibers and different winding methods during the amplification process of the fiber laser, so as to obtain the comprehensive effect of the influence of the modal change induced by the bending and thermal lens effects on the TMI, so as to make the theory closer to the experiment, and further guide the design of the fiber laser and the improvement of the nonlinear threshold.

[0079] Corresponding to the above method embodiment, this specification also provides an embodiment of a TMI effect calculation device in a high-power fiber laser, Figure 7 This is a schematic diagram of the structure of a TMI effect calculation device in a high-power fiber laser provided in an embodiment of the present application. Figure 7 As shown, the device comprises:

[0080] An acquisition module 702 is configured to acquire a mode field distribution to be analyzed corresponding to the optical fiber to be tested within a target bending radius range;

[0081] The first calculation module 704 is configured to calculate the first mode field distribution corresponding to the optical fiber under test at different bending radii and the first thermal load state based on the mode field distribution to be analyzed, and calculate the first TMI coupling coefficient and the first TMI threshold based on the first mode field distribution;

[0082] The second calculation module 706 is configured to calculate the second mode field distribution corresponding to the optical fiber under test at different bending radii and second thermal load states based on the first mode field distribution, and calculate the TMI second coupling coefficient and the TMI second threshold based on the second mode field distribution;

[0083] The iteration module 708 is configured to calculate the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and a third thermal load state in sequence based on the second mode field distribution, and calculate the TMI third coupling coefficient and the TMI third threshold value based on the third mode field distribution, and repeat the above steps until the mode field distribution, the TMI coupling coefficient and the TMI threshold value corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

[0084] In an optional embodiment, the acquisition module 702 is further configured to:

[0085] Calculate the refractive index distribution of the optical fiber to be tested based on conformal transformation and coordinate system conversion method;

[0086] Based on the refractive index distribution of the optical fiber to be tested, the normalized mode field distribution corresponding to the fundamental mode and the higher-order mode, and the bending loss corresponding to each bending radius are calculated using the full vector finite element method;

[0087] According to the actual amplification requirements, determine the target bending radius range of the optical fiber to be tested, and obtain the target refractive index distribution corresponding to the target bending radius range and the target normalized mode field distribution and target bending loss corresponding to the fundamental mode and the higher-order mode;

[0088] Calculate the TMI target coupling coefficient corresponding to the target normalized mode field distribution state;

[0089] The mode field distribution to be analyzed is determined based on the target refractive index distribution, the target normalized mode field distribution, the target bending loss, and the TMI target coupling coefficient.

[0090] In an optional embodiment, the first calculation module 704 is further configured to:

[0091] Taking the mode field distribution to be analyzed as a basis, calculating a first thermo-induced refractive index distribution of the mode field distribution to be analyzed under a first heat load state;

[0092] Based on the first thermo-induced refractive index distribution, first mode field distributions corresponding to different bending radii and first thermal load states are calculated in sequence, and a first TMI loss under the first mode field distribution state is calculated;

[0093] Based on the first mode field distribution, a first TMI coupling coefficient and a first TMI threshold are calculated.

[0094] A TMI effect calculation device in a high-power fiber laser provided in an embodiment of the present application is applied, and the full-vector finite element method is used to solve the eigenvalue problem of electromagnetic waves, and the refractive index and mode field distribution of different optical fibers under different bending radii and thermal load conditions are calculated. The influence of the thermal lens effect caused by amplification on the final bending loss, mode field distribution and TMI coupling coefficient for different optical fibers and different winding methods during the amplification process of the fiber laser is simulated, so as to obtain the comprehensive effect of the influence of the modal change induced by the bending and thermal lens effects on the TMI, so as to make the theory closer to the experiment, and further guide the design of the fiber laser and the improvement of the nonlinear threshold.

[0095] Each embodiment in this specification is described in a progressive manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the differences from other embodiments. In particular, for the TMI effect calculation device in a high-power fiber laser, since it is basically similar to the embodiment of the TMI effect calculation method in a high-power fiber laser, the description is relatively simple, and the relevant parts can be referred to the partial description of the embodiment of the TMI effect calculation method in a high-power fiber laser.

[0096] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0097] The preferred embodiments of this specification disclosed above are only used to help explain this specification. The optional embodiments do not describe all the details in detail, nor do they limit the invention to only the specific implementation methods described. Obviously, many modifications and changes can be made according to the content of the embodiments of this specification. This specification selects and specifically describes these embodiments in order to better explain the principles and practical applications of the embodiments of this specification, so that technicians in the relevant technical field can well understand and use this specification. This specification is only limited by the claims and their full scope and equivalents.

Claims

1. A method for calculating the TMI effect in a high-power fiber laser, characterized in that: include: Obtain the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range; Taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and the first thermal load state, and calculating the TMI first coupling coefficient and the TMI first threshold based on the first mode field distribution; Taking the first mode field distribution as a basis, sequentially calculating the second mode field distribution corresponding to the optical fiber to be tested at different bending radii and second thermal load states, and calculating the TMI second coupling coefficient and the TMI second threshold based on the second mode field distribution; Taking the second mode field distribution as the basis, the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and third thermal load states is calculated in sequence, and based on the third mode field distribution, the TMI third coupling coefficient and the TMI third threshold are calculated, and the above steps are repeated until the mode field distribution, TMI coupling coefficient and TMI threshold corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

2. The method according to claim 1, characterized in that The step of obtaining the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range includes: Calculate the refractive index distribution of the optical fiber to be tested based on conformal transformation and coordinate system conversion method; Based on the refractive index distribution of the optical fiber to be tested, the normalized mode field distribution corresponding to the fundamental mode and the higher-order mode, and the bending loss corresponding to each bending radius are calculated by using the full vector finite element method; According to the actual amplification requirements, determine the target bending radius range of the optical fiber to be tested, and obtain the target refractive index distribution corresponding to the target bending radius range and the target normalized mode field distribution and target bending loss corresponding to the fundamental mode and the higher-order mode; Calculate the TMI target coupling coefficient corresponding to the target normalized mode field distribution state; The mode field distribution to be analyzed is determined based on the target refractive index distribution, the target normalized mode field distribution, the target bending loss, and the TMI target coupling coefficient.

3. The method according to claim 1, characterized in that The method of taking the mode field distribution to be analyzed as a basis, sequentially calculating the first mode field distribution corresponding to the optical fiber to be tested at different bending radii and the first thermal load state, and calculating the first TMI coupling coefficient and the first TMI threshold based on the first mode field distribution, comprises: Taking the mode field distribution to be analyzed as a basis, calculating a first thermo-induced refractive index distribution of the mode field distribution to be analyzed under a first heat load state; Based on the first thermo-induced refractive index distribution, first mode field distributions corresponding to different bending radii and first thermal load states are calculated in sequence, and a first TMI loss under the first mode field distribution state is calculated; Based on the first mode field distribution, a first TMI coupling coefficient and a first TMI threshold are calculated.

4. A device for calculating TMI effect in a high-power fiber laser, characterized in that: include: An acquisition module is configured to acquire the mode field distribution to be analyzed corresponding to the optical fiber to be tested within the target bending radius range; A first calculation module is configured to calculate the first mode field distribution corresponding to the optical fiber under test at different bending radii and the first thermal load state based on the mode field distribution to be analyzed, and calculate the first TMI coupling coefficient and the first TMI threshold based on the first mode field distribution; A second calculation module is configured to calculate the second mode field distribution corresponding to the optical fiber to be tested at different bending radii and a second thermal load state in sequence based on the first mode field distribution, and calculate the TMI second coupling coefficient and the TMI second threshold based on the second mode field distribution; The iteration module is configured to use the second mode field distribution as a basis to sequentially calculate the third mode field distribution corresponding to the optical fiber to be tested at different bending radii and a third thermal load state, and based on the third mode field distribution, calculate the TMI third coupling coefficient and the TMI third threshold, and repeat the above steps until the mode field distribution, the TMI coupling coefficient and the TMI threshold corresponding to the optical fiber to be tested at different bending radii and different thermal load states are obtained.

5. A computer-readable storage medium, characterized in that: The computer-readable storage medium stores an implementation program for information transmission, and when the program is executed by a processor, the steps of the method according to any one of claims 1 to 3 are implemented.

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