Simulation analysis method for polarization evolution characteristics in high-power fiber amplifier

Abstract

The application provides a simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier, considers power amplification and nonlinear transmission processes of laser beams with different polarization directions in the high-power fiber amplifier, establishes a polarization evolution model in the high-power fiber amplifier, performs numerical simulation on the high-power fiber amplifier by using the model, and analyzes and obtains influences of mode birefringence and transmission principal axis rotation of a ytterbium-doped fiber on the polarization evolution characteristics in the high-power fiber amplifier. The model established by the application can obtain accurate simulation results, and has important reference value for optimization design of a high-power linearly polarized fiber amplifier system.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, and in particular to a simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier. Background Technology

[0002] Polarization characteristics are a key performance indicator for high-power linearly polarized fiber amplifiers. However, there is currently no research on how the polarization characteristics of the signal laser evolve during power amplification. Summary of the Invention

[0003] To address the problems existing in the prior art, this invention proposes a simulation analysis method for polarization evolution characteristics in high-power fiber amplifiers.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: Simulation and analysis methods for polarization evolution characteristics in high-power fiber amplifiers include: Considering the power amplification and nonlinear transmission processes of lasers with different polarization directions in a high-power fiber amplifier, a polarization evolution model for a high-power fiber amplifier is established as follows: When the main propagation axis along the ytterbium-doped fiber remains constant in a high-power fiber amplifier, the slowly varying amplitudes of the optical field in two mutually perpendicular polarization directions satisfy the following equation: (1) (2) Equations (1) and (2) above describe the slowly varying amplitude of the optical field in two mutually perpendicular polarization directions as a function of the fiber position. z and time t The evolutionary process; A x , A y They represent x , y The amplitude of the light field in two mutually perpendicular polarization directions; and They represent x , y Complex amplitudes in two mutually perpendicular polarization directions; β 1x and β 1y for x , y The first-order dispersion coefficient in the polarization direction, β 2 is the second-order dispersion coefficient. g and α These are the gain coefficient and loss coefficient in an optical fiber, respectively. γ The Kernel coefficient is a nonlinear coefficient. ; ;β 0x and β 0y for x , y The propagation constant in the polarization direction, B m Mode birefringence; The fill factor is the signal optical power. , These represent the emission and absorption cross sections of ytterbium ions in ytterbium-doped optical fibers, respectively. N 1 and N 2 represents the total number of ytterbium ions in the ground state and excited state, respectively. N 0= N 1+ N 2 represents the concentration of doped ions; When the propagation principal axis along the direction of the ytterbium-doped fiber changes in a high-power fiber amplifier, the slowly varying amplitudes of the optical field in two mutually perpendicular polarization directions satisfy the following equation: (3) (4) parameter B , C and D With elliptical angle θ The relationship is: Numerical simulations of a high-power fiber amplifier were performed based on the above model to analyze the influence of mode birefringence of ytterbium-doped fiber and transmission axis rotation on polarization evolution characteristics in the high-power fiber amplifier.

[0005] The model established based on this invention can be used to simulate high-power fiber amplifiers, and accurate simulation results can be obtained, which has important reference value for the optimized design of high-power linearly polarized fiber amplifier systems. Attached Figure Description

[0006] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0007] Figure 1 This is a schematic diagram of a structure according to an embodiment of the present invention; Figure 2 This refers to the polarization extinction ratio of the output signal light when the modal birefringence of the ytterbium-doped fiber is different in one embodiment of the present invention. Figure 3This is a diagram showing the signal power distribution and output spectral characteristics of an optical fiber amplifier when the ytterbium-doped fiber is a circular birefringent fiber, according to one embodiment of the present invention. Figure 4 These are signal light distribution characteristic diagrams in a high-power fiber optic amplifier when the maximum value of the random tiny deflection of the transmission axis ellipse angle takes different values, according to an embodiment of the present invention. (a) shows the signal light distribution characteristic diagram in the high-power fiber optic amplifier when the maximum value of the random tiny deflection of the transmission axis ellipse angle is π / 200; (b) shows the signal light distribution characteristic diagram in the high-power fiber optic amplifier when the maximum value of the random tiny deflection of the transmission axis ellipse angle is π / 100; (c) shows the signal light distribution characteristic diagram in the high-power fiber optic amplifier when the maximum value of the random tiny deflection of the transmission axis ellipse angle is π / 50; and (d) shows the signal light distribution characteristic diagram in the high-power fiber optic amplifier when the maximum value of the random tiny deflection of the transmission axis ellipse angle is π / 25. Figure 5 This is a distribution diagram of the polarization extinction ratio of the signal light in the fiber amplifier when there is a random slight deflection in the transmission principal axis ellipse angle in one embodiment of the present invention. Specific implementation methods The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0008] Simulation and analysis methods for polarization evolution characteristics in high-power fiber amplifiers include: (1) Considering the power amplification and nonlinear transmission process of lasers with different polarization directions in high-power fiber amplifiers, a polarization evolution model in high-power fiber amplifiers was established.

[0009] To describe the polarization evolution characteristics in high-power fiber amplifiers, it is necessary to describe the power amplification and nonlinear propagation processes of the optical field in two mutually perpendicular polarization directions. During power amplification, the output signal light of the high-power fiber amplifier exhibits significant changes along the fiber direction, which makes the integral contribution of the coupling term non-zero within a beat length. This leads to significant coupling of the laser light in the two polarization directions.

[0010] When the main propagation axis along the ytterbium-doped fiber remains constant in a high-power fiber amplifier, the slowly varying amplitudes of the optical field in two mutually perpendicular polarization directions satisfy the following equation: (1) (2) Equations (1) and (2) above describe the slowly varying amplitude of the optical field in two mutually perpendicular polarization directions as a function of the fiber position. zand time t The evolutionary process; A x , A y They represent x , y The amplitude of the light field in two mutually perpendicular polarization directions; and They represent x , y Complex amplitudes in two mutually perpendicular polarization directions; β 1x and β 1y for x , y The first-order dispersion coefficient in the polarization direction, β 2 is the second-order dispersion coefficient. g and α These are the gain coefficient and loss coefficient in an optical fiber, respectively. γ The Kernel coefficient is a nonlinear coefficient. ; ; β 0x and β 0y for x , y The propagation constant in the polarization direction, B m Mode birefringence; The fill factor is the signal optical power. , These represent the emission and absorption cross sections of ytterbium ions in ytterbium-doped optical fibers, respectively. N 1 and N 2 represents the total number of ytterbium ions in the ground state and excited state, respectively. N 0= N 1+ N 2 represents the concentration of doped ions; When the propagation principal axis along the direction of the ytterbium-doped fiber changes in a high-power fiber amplifier, the slowly varying amplitudes of the optical field in two mutually perpendicular polarization directions satisfy the following equation: (3) (4) parameter B , C and D With elliptical angle θ The relationship is: Numerical simulations of a high-power fiber amplifier were performed based on the above model to analyze the influence of mode birefringence of ytterbium-doped fiber and transmission axis rotation on polarization evolution characteristics in the high-power fiber amplifier.

[0011] The high-power fiber amplifier described in this invention has no structural limitations and can be a forward-pumped, backward-pumped, or bidirectional-pumped high-power fiber amplifier.

[0012] Numerical simulation yields the following results: Whether the main propagation axis along the ytterbium-doped fiber direction remains constant in the high-power fiber amplifier or changes, the polarization extinction ratio of the output signal light of the high-power fiber amplifier gradually increases with the increase of the mode birefringence of the ytterbium-doped fiber, and the degradation of the polarization extinction ratio of the output signal light becomes weaker.

[0013] The ytterbium-doped fiber can be a circular birefringent fiber or a linear birefringent fiber, wherein the circular birefringent fiber has better polarization retention characteristics than the linear birefringent fiber.

[0014] The random, minute deflection of the elliptic angle of the transmission principal axis of the ytterbium-doped fiber causes a degradation in the polarization extinction ratio of the output signal light. Specifically, the larger the random, minute deflection of the elliptic angle of the transmission principal axis of the ytterbium-doped fiber, the more significant the degradation in the polarization extinction ratio of the output signal light of the high-power fiber amplifier.

[0015] The following numerical simulation of a high-power fiber optic amplifier is performed based on the model constructed according to the present invention. In this embodiment, as shown... Figure 1 As shown, the high-power fiber amplifier employs a forward-pumped structure, including a seed source 1, a preamplifier 2, a pump source 3, a combiner 4, a gain fiber 5, and an output cap 6. The pump power output from the pump source of the fiber amplifier is set to 2kW. The gain fiber 5 is ytterbium-doped fiber, specifically a double-clad ytterbium-doped fiber with a core / inner cladding diameter of 20 / 400 μm.

[0016] Furthermore, to avoid the temporal stability of the seed laser output from the seed source affecting the simulation results, the injected seed laser is a phase-modulated single-frequency seed laser, i.e., a phase-modulated single-frequency seed source is used. The main simulation parameters of the high-power fiber amplifier in this embodiment are shown in Table 1: Table 1. Main simulation parameters and their values Based on the above-mentioned model (Equations (1) to (4)) constructed in this invention, the high-power amplifier in this embodiment is simulated to obtain the influence of the mode birefringence of ytterbium-doped fiber on polarization evolution characteristics, including: Based on the model constructed in this invention (Equations (1) to (4)), the polarization extinction ratio changes of the high-power fiber amplifier output signal light under different modes of birefringence of ytterbium-doped fiber, when the polarization extinction ratio of the seed laser is 25 dB and 15 dB respectively, were calculated. The numerical simulation results are as follows: Figure 2 As shown. From Figure 2 It can be seen that, in both cases, as the modal birefringence of the ytterbium-doped fiber increases, the polarization extinction ratio of the output signal light also gradually increases, and the degradation of the polarization extinction ratio of the output signal light becomes weaker. In particular, when the modal birefringence of the ytterbium-doped fiber is greater than 7 × 10⁻⁶, the polarization extinction ratio of the output signal light gradually increases. -8 At this time, the polarization extinction ratio degradation of the output signal light is less than 1 dB. This result indicates that, ideally, in fiber amplifiers based on polarization-maintaining gain fibers, the polarization extinction ratio degradation of the output signal light is negligible.

[0017] Based on the above-mentioned model (Equations (1) to (4)) constructed in this invention, the high-power amplifier in this embodiment is simulated to obtain the influence of the transmission principal axis rotation of the ytterbium-doped fiber on the polarization evolution characteristics, including: In practical fiber laser systems, another factor that can cause degradation of the polarization extinction ratio of the output signal is the rotation of the transmission spindle of the ytterbium-doped fiber.

[0018] First, consider the case where the ytterbium-doped fiber is a circularly birefringent fiber, and the corresponding fiber elliptic angle. θ At a value of π / 2, the polarization extinction ratio of the seed laser is 25 dB, and the mode birefringence of the ytterbium-doped fiber is 0. Numerical calculations were performed to obtain the signal light distribution and output spectral characteristics in the fiber amplifier at this point. The calculation results are as follows: Figure 3 As shown. From Figure 3 (a) It can be seen that the output signal light power in the two polarization directions is approximately 1.78 kW and 0.01 kW, respectively, and the polarization extinction ratio of the output signal light is approximately 25 dB, showing almost no degradation. Figure 3 As shown in (b), the spectrum of the output signal light is almost identical to that of the seed laser. This result demonstrates that the circularly birefringent fiber has good polarization preservation characteristics.

[0019] Based on the above model (Equations (1) to (4)) constructed in this invention, the high-power amplifier in this embodiment is simulated. Considering the random slight deflection of the transmission principal axis ellipse angle of the ytterbium-doped fiber, the signal light distribution characteristics in the fiber amplifier include: Assuming the elliptic angle of the transmission principal axis of the ytterbium-doped fiber has a uniform distribution between 0 and 1, and the maximum values ​​of the elliptic angle are π / 200, π / 100, π / 50, and π / 25, the numerical calculation results are as follows: Figure 4 As shown. Figure 4As shown in (a), when the maximum value of the random small deflection of the ellipse angle is π / 200, the output signal optical power in the two polarization directions is approximately 1.60 kW and 0.18 kW, respectively, corresponding to a total power conversion efficiency of approximately 86.6%; Figure 4 As shown in (b), when the maximum value of the random small deflection of the ellipse angle is π / 100, the output signal optical power in the two polarization directions is approximately 1.56 kW and 0.22 kW, respectively, corresponding to a total power conversion efficiency of approximately 86.6%; Figure 4 As shown in (c), when the maximum value of the random small deflection of the ellipse angle is π / 50, the output signal optical power in the two polarization directions is approximately 1.40 kW and 0.38 kW, respectively, corresponding to a total power conversion efficiency of approximately 86.6%; Figure 4 As shown in (d), when the maximum value of the random tiny deflection of the ellipse angle is π / 25, the output signal optical power in the two polarization directions is approximately 0.93 kW and 0.86 kW, respectively, and the corresponding total power conversion efficiency is approximately 86.6%. The total power conversion efficiency remains almost unchanged in all four cases.

[0020] The corresponding calculations yielded the polarization extinction ratio distribution of the signal light in the fiber amplifier under four different conditions. The calculation results are as follows: Figure 5 As shown. From Figure 5 It can be seen that when the maximum random micro-deflection of the elliptic angle is π / 200, the polarization extinction ratio of the signal light in the fiber amplifier gradually decreases from 25 dB to about 9.5 dB, a reduction of about 15.5 dB, similar to the result when there is no random micro-deflection. When the maximum random micro-deflection of the elliptic angle is π / 100, the polarization extinction ratio of the signal light in the fiber amplifier gradually decreases from 25 dB to about 8.4 dB, a reduction of about 16.6 dB. When the maximum random micro-deflection of the elliptic angle is π / 50, the polarization extinction ratio of the signal light in the fiber amplifier gradually decreases from 25 dB to about 5.7 dB, a reduction of about 19.3 dB. When the maximum random micro-deflection of the elliptic angle is π / 50, the polarization extinction ratio of the signal light in the fiber amplifier gradually decreases from 25 dB to about 0.3 dB, a reduction of about 24.7 dB. This result indicates that as the random micro-deflection of the elliptic angle of the transmission axis of the ytterbium-doped fiber increases, the polarization extinction ratio of the output signal light degrades more significantly.

[0021] This invention comprehensively considers the power amplification and nonlinear transmission processes of lasers with different polarization directions in high-power fiber amplifiers, establishes a polarization evolution model for high-power fiber amplifiers, and studies the influence of the mode birefringence of ytterbium-doped fiber and the rotation of the transmission principal axis on the polarization evolution characteristics of high-power fiber amplifiers. The results show that: firstly, as the mode birefringence of ytterbium-doped fiber increases, the polarization extinction ratio of the output signal light gradually increases, and the degradation of the polarization extinction ratio of the output signal light becomes weaker; secondly, random, minute deflections of the elliptic angle of the transmission principal axis of ytterbium-doped fiber cause degradation of the polarization extinction ratio of the output signal light, and the larger the random, minute deflection, the more significant the degradation of the polarization extinction ratio of the output signal light. The research results obtained based on this technology have important reference value for the optimized design of high-power linearly polarized fiber amplifiers.

[0022] Matters not covered in this invention are common knowledge.

[0023] The technical features of the above embodiments can be combined in any way. For the sake of brevity, 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.

[0024] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention patent. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

[0025] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A simulation analysis method for polarization evolution characteristics in high-power fiber amplifiers, characterized in that, include: Considering the power amplification and nonlinear transmission processes of lasers with different polarization directions in a high-power fiber amplifier, a polarization evolution model for a high-power fiber amplifier is established as follows: When the propagation axis along the ytterbium-doped fiber remains constant in a high-power fiber amplifier, where the ytterbium-doped fiber serves as the gain fiber, the slowly varying amplitudes of the optical field in two mutually perpendicular polarization directions satisfy the following equation: (1) (2) Equations (1) and (2) above describe the slowly varying amplitude of the optical field in two mutually perpendicular polarization directions as a function of the fiber position. z and time t The evolutionary process; A x , A y They represent x , y The amplitude of the light field in two mutually perpendicular polarization directions; and They represent x , y Complex amplitudes in two mutually perpendicular polarization directions; β 1x and β 1y for x , y The first-order dispersion coefficient in the polarization direction, β 2 is the second-order dispersion coefficient. g and α These are the gain coefficient and loss coefficient in an optical fiber, respectively. γ The Kernel coefficient is a nonlinear coefficient. ; ; β 0x and β 0y for x , y The propagation constant in the polarization direction, B m Mode birefringence; The fill factor is the signal optical power. , These represent the emission and absorption cross sections of ytterbium ions in ytterbium-doped optical fibers, respectively. N 1 and N 2 represents the total number of ytterbium ions in the ground state and excited state, respectively. N 0= N 1+ N 2 represents the concentration of doped ions; When the propagation principal axis along the direction of the ytterbium-doped fiber changes in a high-power fiber amplifier, the slowly varying amplitudes of the optical field in two mutually perpendicular polarization directions satisfy the following equation: (3) (4) parameter B , C and D With elliptical angle θ The relationship is: Numerical simulations of a high-power fiber amplifier were performed based on the above model to analyze the influence of mode birefringence of ytterbium-doped fiber and transmission axis rotation on polarization evolution characteristics in the high-power fiber amplifier.

2. The simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier according to claim 1, characterized in that, The high-power fiber amplifier is a high-power fiber amplifier with a forward-pumped structure, a backward-pumped structure, or a bidirectional-pumped structure.

3. The simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier according to claim 1, characterized in that, The high-power fiber amplifier uses double-clad ytterbium-doped fiber with a core / inner cladding diameter of 20 / 400 μm.

4. The simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier according to claim 1, characterized in that, The seed laser of the high-power fiber amplifier is a phase-modulated single-frequency seed laser.

5. The simulation analysis method for polarization evolution characteristics in a high-power fiber optic amplifier according to claim 1, 2, 3, or 4, characterized in that, Whether the main propagation axis along the ytterbium-doped fiber direction remains constant in the high-power fiber amplifier or changes, the polarization extinction ratio of the output signal light of the high-power fiber amplifier gradually increases with the increase of the mode birefringence of the ytterbium-doped fiber, and the degradation of the polarization extinction ratio of the output signal light becomes weaker.

6. The simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier according to claim 5, characterized in that, When the modal birefringence of ytterbium-doped fiber is greater than 7 × 10 -8 At that time, the polarization extinction ratio degradation of the output signal light is less than 1 dB.

7. The simulation analysis method for polarization evolution characteristics in a high-power fiber optic amplifier according to claim 1, 2, 3, or 4, characterized in that, The ytterbium-doped fiber is either a circular birefringent fiber or a linear birefringent fiber.

8. The simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier according to claim 7, characterized in that, The ytterbium-doped fiber is a circularly birefringent fiber with good polarization retention characteristics.

9. The simulation analysis method for polarization evolution characteristics in a high-power fiber optic amplifier according to claim 1, 2, 3, or 4, characterized in that, Random, minute deflections of the elliptic angle of the transmission principal axis of ytterbium-doped fiber can cause a degradation in the polarization extinction ratio of the output signal light.

10. The simulation analysis method for polarization evolution characteristics in a high-power fiber amplifier according to claim 9, characterized in that, As the random, minute deflection of the transmission axis elliptic angle of ytterbium-doped fiber increases, the polarization extinction ratio of the output signal light of the high-power fiber amplifier degrades more significantly.

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