Optimization design method of erbium-ytterbium co-doped fiber and single-frequency erbium-ytterbium co-doped fiber amplifier

Abstract

The application provides an erbium-ytterbium co-doped optical fiber optimization design method and a single-frequency erbium-ytterbium co-doped optical fiber amplifier. From the actual application requirement of the single-frequency erbium-ytterbium co-doped optical fiber amplifier, under the condition of determining the working parameters of the amplifier, three main limiting factors of amplified spontaneous emission, stimulated Brillouin scattering and mode instability are comprehensively considered, the power threshold under the three limiting factors is respectively calculated, and the system threshold power and the system threshold deviation coefficient are further introduced as the optical fiber design evaluation indexes. The application does not pursue the improvement of a single performance index, but through multiple rounds of iterative design, the erbium-ytterbium co-doped optical fiber reaches a relative balance among the amplified spontaneous emission suppression ability, the stimulated Brillouin scattering suppression ability and the mode instability suppression ability, and the optical fiber parameters with higher system threshold power are selected from the candidate schemes meeting the threshold balance requirement.

Description

Technical Field

[0001] This invention belongs to the field of fiber laser technology, and specifically relates to an optimized design method for erbium-ytterbium co-doped optical fiber and a single-frequency erbium-ytterbium co-doped optical fiber amplifier. Background Technology

[0002] Single-frequency lasers refer to lasers that oscillate and output a single longitudinal mode within a laser resonant cavity, exhibiting extremely high coherence. 1.5μm single-frequency fiber lasers, located in the eye-safe and atmospheric transmission bands, have broad application prospects in fields such as space optical communication, lidar, and gravitational wave detection. These practical applications place stringent requirements on the laser's output power, spectral linewidth, beam quality, and optical signal-to-noise ratio. For example, lidar with a detection range exceeding 100 kilometers requires a laser output power greater than 100 W, a spectral linewidth less than 1 kHz, and near-single-mode laser output. Currently, single-frequency fiber lasers typically employ a master oscillator power amplifier (MOPA) scheme to enhance power, with erbium-ytterbium co-doped fiber being a core component. However, the power amplification of current 1.5 μm single-frequency fiber amplifiers is simultaneously limited by multiple factors, including stimulated Brillouin scattering (SBS), amplified spontaneous emission (ASE), and thermally induced mode instability (TMI).

[0003] To overcome the limitations of power enhancement in 1.5 μm single-frequency fiber amplifiers and further improve the performance of the output laser, it is necessary to design an erbium-ytterbium co-doped fiber for 1.5 μm single-frequency fiber amplifiers. To address the SBS effect in fiber lasers, a common approach is to increase the mode area of ​​the fiber, i.e., use a large mode area fiber. However, this increases the number of transverse modes that can be transmitted, leading to the premature onset of the TMI effect. To address the ASE effect in 1.5 μm single-frequency fiber lasers, increasing the rare-earth ion doping concentration can improve the energy transfer efficiency between ytterbium and erbium ions, thereby suppressing spontaneous emission of ytterbium ions. However, increasing the rare-earth ion doping concentration increases the refractive index of the fiber core, thus increasing the numerical aperture of the core. This also leads to an increase in the number of transverse modes that can be transmitted in the fiber, thereby lowering the TMI effect threshold. Therefore, in 1.5 μm band single-frequency fiber amplifiers, there is an internal contradiction between the suppression methods of stimulated Brillouin scattering (SBS), amplified spontaneous emission, and thermally induced mode instability.

[0004] Therefore, there is an urgent need to develop an optimized design method for erbium-ytterbium co-doped fibers for single-frequency fiber amplifiers to achieve comprehensive suppression of stimulated Brillouin scattering (SBS), amplified spontaneous emission (ASE), and thermally induced mode instability (TMI). Summary of the Invention

[0005] To address the technical problems existing in the prior art, this invention provides an optimized design method for erbium-ytterbium co-doped optical fiber and a single-frequency erbium-ytterbium co-doped optical fiber amplifier.

[0006] To achieve the above-mentioned technical objectives, the technical solution adopted by the present invention is as follows: This invention provides an optimized design method for erbium-ytterbium co-doped optical fibers, comprising the following steps: S1. The application of the erbium-ytterbium co-doped fiber to be optimized is determined to be a single-frequency erbium-ytterbium co-doped fiber amplifier. The erbium-ytterbium co-doped fiber is used as the gain fiber in the single-frequency erbium-ytterbium co-doped fiber amplifier. The operating parameters of the single-frequency erbium-ytterbium co-doped fiber amplifier are determined. The operating parameters include signal light wavelength, signal light width, seed light power, target output power, pump light wavelength, and pumping mode. S2, Input the initial erbium-ytterbium co-doped fiber parameters for the erbium-ytterbium co-doped fiber to be optimized; S3, Calculate the absorption coefficient of erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier based on the parameters of the erbium-ytterbium co-doped fiber. S4. Based on the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in the single-frequency erbium-ytterbium co-doped fiber amplifier and the preset target pump absorption rate, calculate the fiber length required to reach the target pump absorption. S5, based on the steady-state rate equation and power transmission equation of the erbium-ytterbium co-doped fiber amplifier, numerical calculations are performed on the optical field distribution inside the erbium-ytterbium co-doped fiber to obtain the power distribution of pump light, signal light, and amplified spontaneous emission light along the fiber length. S6, calculate the threshold power corresponding to the amplified spontaneous emission effect, the threshold power of stimulated Brillouin scattering and the threshold power of mode instability, and then calculate the system threshold power and the system threshold deviation coefficient. S7, compare the system threshold power and the system threshold deviation coefficient with the preset system output power value. P set and system threshold deviation coefficient D set Compare and determine whether the convergence condition is met. P sys ≥ P set and D sys ≤D set If the convergence condition is met, the current erbium-ytterbium co-doped fiber parameters are output; otherwise, the erbium-ytterbium co-doped fiber parameters are automatically adjusted and returned to S3 to continue the fiber parameter optimization process.

[0007] Compared with the prior art, the present invention has the following beneficial effects: Traditional erbium-ytterbium co-doped fiber design methods typically optimize for a single limiting factor. For example, patent application CN120010052A discloses an erbium-ytterbium co-doped single-mode fiber. By specially designing the refractive index distribution of the fiber, it helps ensure good beam quality of the laser, and this method has a certain suppression effect on mode instability effects. For example, patent application CN121857131A discloses an erbium-ytterbium co-doped fiber with a highly phosphorus-doped core, its preparation method, and applications. By precisely controlling the concentration and distribution of doping elements in the core, the gain performance and gain consistency of the erbium-ytterbium co-doped fiber are improved, and this method has a certain suppression effect on amplified spontaneous emission. However, for practical single-frequency high-power fiber amplifiers, amplified spontaneous emission, stimulated Brillouin scattering, and mode instability effects often coexist, and the intensity of these three effects does not correspond consistently with the fiber parameters, sometimes even showing completely opposite trends. For example, increasing the area of ​​the fiber core can suppress stimulated Brillouin scattering, but it leads to beam quality degradation and premature mode instability. Therefore, traditional design methods often only provide relatively simple structural designs or empirical evaluation criteria, making it difficult to directly guide the optimization of actual erbium-ytterbium co-doped fibers. In the actual development of optical fibers, multiple rounds of iterative corrections are typically required based on test results such as conversion efficiency, output power, laser spectrum, and output beam quality.

[0008] This invention addresses the practical application requirements of single-frequency erbium-ytterbium co-doped fiber amplifiers. Under the condition of fixed amplifier operating parameters, it comprehensively considers three main limiting factors: amplified spontaneous emission, stimulated Brillouin scattering, and mode instability. Power thresholds are calculated for each of these factors, and system threshold power and system threshold deviation coefficient are further introduced as evaluation indicators for fiber design. Specifically, the system threshold power characterizes the actual power upper limit of the current fiber design scheme, and the threshold deviation coefficient characterizes the degree of matching between the three limiting mechanisms.

[0009] This invention does not solely pursue improvements in a single performance indicator. Instead, through multiple rounds of iterative design, it achieves a relative balance between amplified spontaneous emission suppression, stimulated Brillouin scattering suppression, and mode instability suppression in erbium-ytterbium co-doped optical fibers. Furthermore, it selects fiber parameters with higher system threshold power from candidate schemes that meet threshold balance requirements. This provides clearer evaluation criteria and iterative directions for the design of doping ratios, absorption coefficients, core sizes, core-to-cladding ratios, numerical apertures, and fiber lengths for gain fibers used in single-frequency erbium-ytterbium co-doped fiber amplifiers, improving the matching between fiber design and actual fabrication. Attached Figure Description

[0010] 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 these drawings without creative effort. Figure 1 This is a flowchart of an erbium-ytterbium co-doped optical fiber optimization design method in one embodiment; Figure 2 This is a schematic diagram of the structure of a single-frequency erbium-ytterbium co-doped fiber amplifier in one embodiment; Figure 3 This is a diagram showing the ion doping concentration of erbium-ytterbium co-doped optical fiber before and after optimization in one embodiment. Figure 3 (a) is a diagram showing the ion doping concentration of the erbium-ytterbium co-doped fiber before optimization. Figure 3 (b) is a diagram of the optimized ion doping concentration of the erbium-ytterbium co-doped fiber. Detailed Implementation

[0011] The technical solution of the present invention will now be clearly and completely described through specific embodiments. Obviously, the described embodiments are merely some embodiments of the present invention, and not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0012] In one embodiment, the erbium-ytterbium co-doped optical fiber optimization design method includes the following steps: S1. The application of the erbium-ytterbium co-doped fiber to be optimized is determined to be a single-frequency erbium-ytterbium co-doped fiber amplifier. The erbium-ytterbium co-doped fiber is used as the gain fiber in the single-frequency erbium-ytterbium co-doped fiber amplifier. The operating parameters of the single-frequency erbium-ytterbium co-doped fiber amplifier are determined. The operating parameters include signal light wavelength, signal light width, seed light power, target output power, pump light wavelength, and pumping mode. S2, Input the initial erbium-ytterbium co-doped fiber parameters for the erbium-ytterbium co-doped fiber to be optimized; S3, Calculate the absorption coefficient of erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier based on the parameters of the erbium-ytterbium co-doped fiber. S4. Based on the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in the single-frequency erbium-ytterbium co-doped fiber amplifier and the preset target pump absorption rate, calculate the fiber length required to reach the target pump absorption. S5, based on the steady-state rate equation and power transmission equation of the erbium-ytterbium co-doped fiber amplifier, numerical calculations are performed on the optical field distribution inside the erbium-ytterbium co-doped fiber to obtain the power distribution of pump light, signal light, and amplified spontaneous emission light along the fiber length. S6. Calculate the threshold power corresponding to the amplified spontaneous emission effect, the threshold power of stimulated Brillouin scattering, and the threshold power of mode instability, and then calculate the system threshold power. P sys And system threshold deviation coefficient D sys ; S7, system threshold power P sys And system threshold deviation coefficient D sys With the preset system output power value P set and system threshold deviation coefficient D set Compare and determine whether the convergence condition is met. P sys ≥ P set and D sys ≤D set If the convergence condition is met, the current erbium-ytterbium co-doped fiber parameters are output; otherwise, the erbium-ytterbium co-doped fiber parameters are automatically adjusted and returned to S3 to continue the fiber parameter optimization process.

[0013] The erbium-ytterbium co-doped fiber parameters described in the above embodiments include one or more of the following: Er / Yb ion ratio, rare earth ion doping concentration, P / Al co-doping ratio, core diameter, cladding diameter, and numerical aperture.

[0014] In step S3 of the above embodiment, the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in the single-frequency erbium-ytterbium co-doped fiber amplifier is calculated based on the parameters of the erbium-ytterbium co-doped fiber. Specifically, the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in the single-frequency erbium-ytterbium co-doped fiber amplifier is determined based on the pump wavelength, Er / Yb ion ratio, rare earth ion doping concentration, absorption cross-section, and overlap factor between the pump light and the doped region.

[0015] Specifically, the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier is estimated based on the parameters of the erbium-ytterbium co-doped fiber, as follows:

[0016] in, Pump wavelength of erbium-ytterbium co-doped fiber in single-frequency erbium-ytterbium co-doped fiber amplifier The absorption coefficient at that location, expressed in dB / m; The overlap factor between the pump light and the rare-earth ion-doped region; For ytterbium ions at the pump wavelength Absorption cross section at the location; The ytterbium ion doping concentration; For erbium ions at the pump wavelength Absorption cross section at the location; This represents the erbium ion doping concentration.

[0017] In step S4 of the above embodiment, the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in the single-frequency erbium-ytterbium co-doped fiber amplifier and the preset target pump absorption rate are used as the basis for the calculation. Calculate the fiber length required to reach the target pump absorption. L ;

[0018] Among them, erbium-ytterbium co-doped fibers with higher absorption coefficients correspond to shorter fiber lengths, while erbium-ytterbium co-doped fibers with lower absorption coefficients correspond to longer fiber lengths.

[0019] In one embodiment, the target pump absorption rate The value is 80%-98%, preferably 90%-95%.

[0020] In step S6 of the above embodiment, the threshold power corresponding to the amplified spontaneous emission effect, the stimulated Brillouin scattering threshold power, and the mode instability threshold power are calculated. Among these, the threshold power corresponding to the amplified spontaneous emission effect... Defined as follows: When the amplified spontaneous emission optical signal power output by a single-frequency erbium-ytterbium co-doped fiber amplifier reaches 1% of the output signal power, the system is considered to have reached the ASE threshold. The output signal power at this point is the threshold power corresponding to the amplified spontaneous emission effect. .

[0021] Based on the power distribution of the signal light along the length of the erbium-ytterbium co-doped fiber, and combined with the effective mode area, Brillouin gain coefficient, and effective interaction length of the erbium-ytterbium co-doped fiber, the stimulated Brillouin scattering threshold power is calculated. .

[0022] The thermal load distribution is obtained based on quantum loss and background loss, and the mode instability threshold power is calculated by combining fiber mode parameters, heat conduction model, and thermo-optical coupling model. .

[0023] This invention introduces a system threshold power and a system threshold deviation coefficient, wherein the system threshold power To amplify the threshold power corresponding to spontaneous emission, the minimum of the stimulated Brillouin scattering threshold power and the mode instability threshold power is as follows:

[0024] System threshold deviation coefficient Defined as the difference between the maximum and minimum values ​​of the three threshold powers: the threshold power corresponding to amplified spontaneous emission, the threshold power of stimulated Brillouin scattering, and the threshold power of mode instability, divided by the average:

[0025] in The smaller the value, the closer the threshold power corresponding to the three limiting factors ASE, SBS, and TMI is.

[0026] In one specific embodiment, a method for optimizing the design of erbium-ytterbium co-doped fiber for a single-frequency erbium-ytterbium co-doped fiber amplifier is provided. In this embodiment, the injection signal wavelength of the single-frequency erbium-ytterbium co-doped fiber amplifier is 1550 nm, the injection signal beamwidth is 1 kHz, the injection signal power is 10 W, the target signal power is 200 W, the pump wavelength is 940 nm, and the pumping method is back-pumping. The erbium-ytterbium co-doped fiber to be optimized is the gain fiber of the single-frequency erbium-ytterbium co-doped fiber amplifier. (Refer to...) Figure 2 This is a schematic diagram of the single-frequency erbium-ytterbium co-doped fiber amplifier in this embodiment, including a single-frequency seed source 1, a preamplifier 2, a wavelength division multiplexer 3, a mode matcher 4, an erbium-ytterbium co-doped fiber 5, a signal pump combiner 6, a semiconductor pump laser 7, and a fiber end cap 8. The output laser of the single-frequency erbium-ytterbium co-doped fiber amplifier is output through the fiber end cap. The laser passes through a high-reflection mirror, and a power meter is used to record the power of the high-power transmitted light, while a spectrometer is used to measure the spectrum of the low-power transmitted light. The returned light spectrum of the laser is recorded at the pump end of the wavelength division multiplexer. It is understood that the above structure is only an example. The erbium-ytterbium co-doped fiber optimization design method provided by this invention does not limit the specific structure of the single-frequency erbium-ytterbium co-doped fiber amplifier. Its pump structure type can be forward pumping, backward pumping, or bidirectional pumping. The optical path is not limited to the above devices, and isolators, cladding filters, and other functional devices can also be set as needed.

[0027] Reference Figure 1 The erbium-ytterbium co-doped optical fiber optimization design method includes the following steps: S100, Input a set of initial fiber design parameters. The initial fiber parameters include one or more of the following: Er / Yb ion ratio, rare earth ion doping concentration, P / Al co-doping ratio, core diameter, cladding diameter, numerical aperture, and absorption cross section.

[0028] The absorption coefficient of erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier was calculated based on the parameters of the erbium-ytterbium co-doped fiber.

[0029] in, Pump wavelength of erbium-ytterbium co-doped fiber in single-frequency erbium-ytterbium co-doped fiber amplifier The absorption coefficient at that location, expressed in dB / m; The overlap factor between the pump light and the rare-earth ion-doped region; This represents the absorption cross section of ytterbium ions at the pump wavelength; The ytterbium ion doping concentration; For erbium ions at the pump wavelength Absorption cross section at the location; This represents the erbium ion doping concentration.

[0030] S200 calculates the required fiber length based on the calculated absorption coefficient of the pump wavelength and the target pump absorptivity. A smaller absorption coefficient requires a longer fiber length, while a larger absorption coefficient requires a shorter fiber length.

[0031] Specifically, in one embodiment, the target pump absorption rate is set at 95%. The required erbium-ytterbium co-doped gain fiber length... L for:

[0032] S300, based on the steady-state rate equation and power transmission equation of a single-frequency erbium-ytterbium co-doped fiber amplifier, calculates the power distribution along the length of the gain fiber for pump light, signal light, and amplified spontaneous emission light in the 1 μm band.

[0033] The steady-state rate equation for the single-frequency erbium-ytterbium co-doped fiber amplifier is as follows:

[0034]

[0035]

[0036]

[0037] in n 1. n 2. n 3 represents the normalized particle number of erbium ions in the ground state, metastable state, and excited state, respectively. n 5. n 6 represents the normalized number of ytterbium ions in the ground state and excited state, respectively. τ 21 , τ 32 These represent the particle lifetimes of erbium ions transitioning from metastable to ground state and from excited to metastable state, respectively. τ 65 The lifetime of a ytterbium ion as it transitions from an excited state to a ground state. W 12The stimulated absorption transition rate of erbium ions from the ground state to the metastable state is given by . W 21 The stimulated emission transition rate of erbium ions from the metastable energy level to the ground energy level; W 13 The stimulated absorption transition rate of erbium ions from the ground state to the excited state is given by η. W 56 The stimulated absorption transition rate of ytterbium ions from the ground state to the excited state is given by γ. W 65 The stimulated emission rate of the ytterbium ion from the excited state level to the ground state level; C up The erbium ion upconversion coefficient; R 61 The energy transfer coefficient from ytterbium ions to erbium ions; R 35 The reverse energy transfer coefficient from erbium ions to ytterbium ions; N Er This represents the total erbium ion doping concentration. W Yb This represents the total doping concentration of ytterbium ions.

[0038] The power transfer equation for a single-frequency erbium-ytterbium co-doped fiber amplifier is as follows:

[0039]

[0040]

[0041] The "±" sign represents the direction of laser transmission, the "+" sign represents forward transmission, and the "-" sign represents reverse transmission. , , The coordinates along the length of the erbium-ytterbium co-doped fiber represent the power of the pump light, the spontaneous emission light in the ytterbium ion band, and the signal light in the erbium ion band, respectively. z Changes; , and These are the overlap factors between the spontaneous emission light (Yb-ASE light) in the pump light, signal light, and ytterbium ion emission bands and the rare earth ion doped regions, respectively. , These are the absorption and emission cross sections of ytterbium ions at the corresponding wavelengths, respectively. , These represent the absorption and emission cross sections of erbium ions at the corresponding wavelengths; , and These are the background loss coefficients for pump light, signal light, and Yb-ASE light, respectively. This refers to the center wavelength of the k-th discrete channel of the Yb-ASE spectrum used in the calculation; The wavelength spacing between adjacent discrete channels of the Yb-ASE spectrum; h It is Planck's constant; c It is the speed of light in a vacuum.

[0042] Specifically, by solving the steady-state rate equation and power transmission equation of the single-frequency erbium-ytterbium co-doped fiber amplifier, the power distribution of the pump light, Yb-ASE light, and Er-ASE light along the length of the erbium-ytterbium co-doped fiber can be obtained.

[0043] S400 calculates the threshold power corresponding to the amplified spontaneous emission effect, the threshold power of stimulated Brillouin scattering, and the threshold power of mode instability.

[0044] S401, based on the characteristics of the power growth of amplified spontaneous emission, calculate the threshold power corresponding to the amplified spontaneous emission effect. P ASE In this embodiment, the threshold power corresponding to the amplified spontaneous emission effect is... Defined as follows: When the amplified spontaneous emission optical signal power output by a single-frequency erbium-ytterbium co-doped fiber amplifier reaches 1% of the output signal power, the system is considered to have reached the ASE threshold. The output signal power at this point is defined as the threshold power corresponding to the amplified spontaneous emission effect. .

[0045] S402, based on the power distribution of the signal light along the fiber length, the effective mode area, the Brillouin gain coefficient, and the effective interaction length, calculate the stimulated Brillouin scattering threshold power. P SBS for:

[0046] in is the threshold gain factor for stimulated Brillouin scattering, which is usually taken as 21; The effective mode area of ​​the signal light in the gain fiber; The Brillouin gain coefficient of the optical fiber material; For signal beam width; For Brillouin gain bandwidth; The effective interaction length of stimulated Brillouin scattering.

[0047] S403, based on quantum defect and background loss, yields the thermal load distribution, and combines fiber mode parameters, a thermal conduction model, and a thermo-optical coupling model to calculate the mode instability threshold power P. TMI ; The heat load distribution is as follows:

[0048] in, The thermal load per unit length at position z along the length direction of the erbium-ytterbium co-doped optical fiber; Forward pump power; This is the power for the backward pump; Signal optical power; Let Yb-ASE power be the value. This formula means that after deducting the increase in optical power from the signal light and Yb-ASE power from the power reduction of the pump light, the remaining portion is converted into thermal load.

[0049] Mode instability threshold power That is, the output power corresponding to the ratio of the higher-order mode power at the output terminal to the total output power reaches a preset proportion (usually 5%), which can be expressed as:

[0050] in, Input seed light power; The thermal conductivity of the optical fiber material; The radius of the pump cladding; The core radius; k 0 is the vacuum wavenumber of the signal light, satisfying ; k T The thermo-optic coefficient of the optical fiber material; The equivalent TMI correction factor related to erbium-ytterbium energy transfer, used to characterize the combined effects of energy transfer efficiency, equivalent heat load, gain saturation, higher-order mode loss, and mode distribution on the TMI threshold, can be expressed as:

[0051] in, The equivalent absorption-emission cross-section factor of the erbium-ytterbium co-doped fiber is used to characterize the coupling relationship between Yb ion pump absorption, Yb-Er energy transfer, and Er ion signal gain. Factors related to higher-order mode loss and the initial proportion of higher-order modes; The correlation factor is the signal light amplification factor; This is a mode distribution correlation factor used to characterize the distribution of fundamental and higher-order modes in the fiber core and their thermally induced coupling strength; The equivalent thermal loading factor of the erbium-ytterbium co-doped fiber amplifier can be expressed as:

[0052] in, Q(z) Let be the thermal load per unit length of the erbium-ytterbium co-doped fiber at position z, calculated previously; L is the length of the erbium-ytterbium co-doped fiber. To inject the input seed light power into the erbium-ytterbium co-doped fiber amplifier, The output signal optical power; P out -P seed Let be the power increment of the signal light in the amplifier. This formula represents the amount of heat power generated per unit increase in signal light power in an erbium-ytterbium co-doped fiber.

[0053] S500 calculates the system threshold power and the system threshold deviation coefficient.

[0054] System threshold power P sys To amplify the spontaneous emission threshold P ASE Stimulated Brillouin scattering threshold P SBS and mode instability threshold P TMI The minimum value in, i.e.

[0055] Further calculate the system threshold deviation coefficient D sys :

[0056] D sys Used to characterize the degree of dispersion among the threshold powers corresponding to the three confinement mechanisms: ASE, SBS, and TMI. D sys The larger the value, the greater the threshold deviation among the three effects in the single-frequency erbium-ytterbium co-doped fiber amplifier system. D sys The smaller the value, the closer the three threshold powers are, and the better the balance between the three limiting mechanisms in the current fiber optic design.

[0057] S600, based on the current erbium-ytterbium co-doped fiber parameters, obtains the system power threshold and system threshold deviation coefficient, and compares them with the preset system output power value. P set and system threshold deviation coefficient D set Compare the results to determine if the convergence condition is met. P sys ≥ P set and D sys ≤D set If the convergence condition is met, proceed to S700; otherwise, the convergence condition is not met, proceed to S800. The S700 outputs optimal fiber optic design parameters.

[0058] In step S800, the minimum power threshold limitation mechanism of the single-frequency erbium-ytterbium co-doped fiber amplifier system is identified, and the parameters of the erbium-ytterbium co-doped fiber are automatically adjusted, including the dopant ion ratio, rare earth concentration, core cladding size, numerical aperture, and other parameters. Then, the process returns to step S200 to continue the fiber parameter optimization process.

[0059] In the above embodiments, under the condition that the operating parameters of the single-frequency erbium-ytterbium co-doped fiber amplifier are determined, characteristic indicators reflecting the absorption, gain, thermal load, and mode characteristics of the erbium-ytterbium co-doped fiber are calculated based on the basic design parameters of the erbium-ytterbium co-doped fiber, the rare earth ion doping concentration, and the ion ratio. Furthermore, a power threshold model that simultaneously considers amplified spontaneous emission, stimulated Brillouin scattering, and mode instability effects is constructed. Based on the above characteristic indicators, the amplified spontaneous emission (ASE) threshold power, stimulated Brillouin scattering (SBS) threshold power, and mode instability (TMI) threshold power of the erbium-ytterbium co-doped fiber after its application in the single-frequency erbium-ytterbium co-doped fiber amplifier are calculated using the steady-state rate equation, power transfer equation, Brillouin scattering threshold model, and thermally induced mode coupling model of the single-frequency erbium-ytterbium co-doped fiber amplifier. This ultimately achieves a comprehensive evaluation of the power threshold of the single-frequency erbium-ytterbium co-doped fiber amplifier system and an optimized design of the erbium-ytterbium co-doped fiber parameters.

[0060] Figure 3 The diagram shows the ion doping concentration before and after optimization of the erbium-ytterbium co-doped fiber based on the above embodiments. Figure 3 (a) is a diagram showing the ion doping concentration of the erbium-ytterbium co-doped fiber before optimization. Figure 3 (b) shows the optimized ion doping concentration of the erbium-ytterbium co-doped fiber. Before optimization, the erbium-ytterbium co-doped fiber had a mass percentage erbium doping concentration of 0.51% and a ytterbium doping concentration of 2.34%. The system power threshold of the single-frequency erbium-ytterbium co-doped fiber amplifier using the unoptimized fiber was limited by the amplified spontaneous emission effect, resulting in an output power of 100 W. Based on the erbium-ytterbium co-doped fiber optimization design method for a single-frequency erbium-ytterbium co-doped fiber amplifier according to this invention, the ion doping concentration of the erbium-ytterbium co-doped fiber is increased. The optimized erbium-ytterbium co-doped fiber has a mass percentage erbium doping concentration of 0.59% and a ytterbium doping concentration of 3.05%. The threshold power of the single-frequency erbium-ytterbium co-doped fiber amplifier using the optimized fiber is increased to 200 W, and the threshold deviation coefficient is reduced to 5%. This meets the system design requirements.

[0061] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of the invention. 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 application should be determined by the appended claims.

[0062] 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. An optimized design method for erbium-ytterbium co-doped optical fibers, characterized in that, Includes the following steps: S1. The application of the erbium-ytterbium co-doped fiber to be optimized is determined to be a single-frequency erbium-ytterbium co-doped fiber amplifier. The erbium-ytterbium co-doped fiber is used as the gain fiber in the single-frequency erbium-ytterbium co-doped fiber amplifier. The operating parameters of the single-frequency erbium-ytterbium co-doped fiber amplifier are determined. The operating parameters include signal light wavelength, signal light width, seed light power, target output power, pump light wavelength, and pumping mode. S2, Input the initial erbium-ytterbium co-doped fiber parameters for the erbium-ytterbium co-doped fiber to be optimized; S3, Calculate the absorption coefficient of erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier based on the parameters of the erbium-ytterbium co-doped fiber. S4. Based on the absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in the single-frequency erbium-ytterbium co-doped fiber amplifier and the preset target pump absorption rate, calculate the fiber length required to reach the target pump absorption. S5, based on the steady-state rate equation and power transmission equation of the erbium-ytterbium co-doped fiber amplifier, numerical calculations are performed on the optical field distribution inside the erbium-ytterbium co-doped fiber to obtain the power distribution of pump light, signal light, and amplified spontaneous emission light along the fiber length. S6. Calculate the threshold power corresponding to the amplified spontaneous emission effect, the threshold power of stimulated Brillouin scattering, and the threshold power of mode instability, and then calculate the system threshold power. P sys And system threshold deviation coefficient D sys ; S7, system threshold power P sys And system threshold deviation coefficient D sys With the preset system output power value P set and system threshold deviation coefficient D set Compare and determine whether the convergence condition is met. P sys ≥ P set and D sys ≤D set If the convergence condition is met, the current erbium-ytterbium co-doped fiber parameters are output; otherwise, the erbium-ytterbium co-doped fiber parameters are automatically adjusted and returned to S3 to continue the fiber parameter optimization process.

2. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 1, characterized in that, The parameters of erbium-ytterbium co-doped optical fibers include one or more of the following: Er / Yb ion ratio, rare earth ion doping concentration, P / Al co-doping ratio, core diameter, cladding diameter, core-cladding ratio, and numerical aperture.

3. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 1 or 2, characterized in that, The absorption coefficient of the erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier is estimated based on the parameters of the erbium-ytterbium co-doped fiber, as follows: in, Pump wavelength of erbium-ytterbium co-doped fiber in single-frequency erbium-ytterbium co-doped fiber amplifier Absorption coefficient at the location; The overlap factor between the pump light and the rare-earth ion-doped region; For ytterbium ions at the pump wavelength Absorption cross section at the location; The ytterbium ion doping concentration; For erbium ions at the pump wavelength Absorption cross section at the location; This represents the erbium ion doping concentration.

4. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 3, characterized in that, Based on the absorption coefficient of erbium-ytterbium co-doped fiber at the pump wavelength in a single-frequency erbium-ytterbium co-doped fiber amplifier and the preset target pump absorption rate, the fiber length required to reach the target pump absorption is calculated. L ; in The preset target pump absorption rate.

5. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 1, 2, or 4, characterized in that, The system threshold power is the minimum of the threshold power corresponding to the amplified spontaneous emission effect, the threshold power of stimulated Brillouin scattering, and the threshold power of mode instability.

6. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 5, characterized in that, The system threshold deviation coefficient is calculated using the following formula: Where the system threshold deviation coefficient D sys Threshold power used to characterize the amplified spontaneous emission effect Stimulated Brillouin scattering threshold power and mode instability threshold power The degree of dispersion between them.

7. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 6, characterized in that, Threshold power corresponding to amplified spontaneous emission effect Defined as: the signal output power when the amplified spontaneous emission optical signal power output by a single-frequency erbium-ytterbium co-doped fiber amplifier reaches 1% of the output signal optical power.

8. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 6, characterized in that, Based on the power distribution of the signal light along the length of the erbium-ytterbium co-doped fiber, and combined with the effective mode area, Brillouin gain coefficient, and effective interaction length of the erbium-ytterbium co-doped fiber, the stimulated Brillouin scattering threshold power is calculated. .

9. The erbium-ytterbium co-doped optical fiber optimization design method according to claim 6, characterized in that, The thermal load distribution is obtained based on quantum loss and background loss, and the mode instability threshold power is calculated by combining fiber mode parameters, heat conduction model, and thermo-optical coupling model. .

10. A single-frequency erbium-ytterbium co-doped fiber amplifier, comprising a gain fiber, wherein the gain fiber is an erbium-ytterbium co-doped fiber, characterized in that, The erbium-ytterbium co-doped optical fiber is designed based on the erbium-ytterbium co-doped optical fiber optimization design method as described in claim 1, 2, 4, 6, 7, 8, or 9.

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