Method for measuring temperature response characteristic function of stimulated emission cross section of gain medium

Abstract

The invention discloses a method for measuring a temperature response characteristic function of a stimulated emission cross section of a gain medium, and the method comprises the steps: (1) measuring the intensity noise spectrum of a laser at different temperatures, and obtaining a relaxation oscillation frequency measurement value of the laser; (2) drawing a function curve graph taking the stimulated emission cross section as an independent variable and the relaxation oscillation frequency as a dependent variable according to actual parameters of the laser; (3) calibrating the function curve graph in the step (2) according to the relaxation oscillation frequency measurement value in the step (1) to obtain an actual stimulated emission cross section; and (4) drawing a function curve and fitting an equation according to the temperature of the gain medium in the step (1) and the corresponding actual stimulated emission cross section in the step (3). According to the fitting equation, the actual stimulated emission cross section can be rapidly and accurately calculated in real time through the temperature of the gain medium in the operation process of the laser, the state of the laser is dynamically represented and managed in real time, and guidance is provided for optimization of the laser.

Description

Technical Field

[0001] The invention relates to the field of laser technology, and in particular to a method for measuring a temperature response characteristic function of a gain medium stimulated emission cross section. Background Art

[0002] All-solid-state lasers, due to their ability to combine low noise, narrow linewidth, and excellent beam quality while achieving high-power output, are widely used in fundamental scientific research fields such as quantum information, cold atom physics, precision spectroscopy, and precision measurement; in cutting-edge applications such as low-altitude detection, biomedicine, and industrial manufacturing; and in defense and security fields such as lidar, laser remote sensing, and optoelectronic countermeasures. The gain medium, the carrier for generating oscillating laser light, is one of the three key elements of a laser. The stimulated emission cross section (SES) of the gain medium is a crucial parameter, directly determining the small-signal gain coefficient of the gain medium and the saturation laser intensity of the laser, ultimately affecting the laser's output power and optical-to-optical conversion efficiency. In the development of all-solid-state lasers and all-solid-state laser amplifiers, accurately determining the SES of the gain medium under actual operating conditions is crucial for the design of laser and laser amplifier parameters and structures. Furthermore, accurately measuring the SES of the gain medium under actual operating conditions provides a reference for determining the integrity of the gain medium's performance and the proper functioning of the gain medium's temperature control components during laser maintenance, playing a key role in subsequent laser design optimization. Currently, the measurement of the effective emission cross section of a gain medium is primarily based on fluorescence spectroscopy. This involves using a monochromator or fluorescence spectrometer to measure the fluorescence spectrum of the gain medium, obtaining the effective half-width of the fluorescence emission band. The fluorescence lifetime is then determined based on the decay characteristics of the fluorescence intensity over time. To minimize laser interference, the fluorescence spectrum is typically measured under conditions where low pump laser power is injected into the gain medium and the gain medium is kept in a constant temperature chamber. While numerous reports have been published on the measurement of the stimulated emission cross section of a gain medium, significant variations exist for the same type of gain medium due to varying measurement conditions and growth conditions. Furthermore, fluorescence spectroscopy is incapable of measuring the stimulated emission cross section of a laser that is currently being debugged or packaged. During actual laser operation, the pump laser power is well above the laser threshold. Furthermore, the stimulated emission cross section of the gain medium during actual operation is affected by the gain medium's temperature distribution, specifically, a rapid decrease in the gain medium's temperature as the gain medium temperature increases. This temperature distribution is influenced by factors such as the heat load generated by the laser's nonradiative transitions and the effectiveness of the gain medium's temperature control device. The actual stimulated emission cross section of the gain medium is affected by the doping concentration of the gain medium and the pump laser power injected into the gain medium. Higher doping concentration and pump laser power will increase the thermal load of the gain medium, reduce the actual stimulated emission cross section of the gain medium, and reduce the light-to-light conversion efficiency of the laser.

[0003] Therefore, the existing measurement method of the effective emission cross section of the gain medium has the problem of inaccurate measurement, and it is not easy to accurately measure the stimulated emission cross section of the gain medium in real time under the actual operation state of the all-solid-state laser. Summary of the Invention

[0004] The present invention aims to solve the above problems of the prior art. It proposes a method for measuring the temperature response characteristic function of the stimulated emission cross section of a gain medium. The technical solution of the present invention is as follows:

[0005] A method for measuring a temperature response characteristic function of a stimulated emission cross section of a gain medium comprises the following steps:

[0006] (1) When the laser is in stable operation and the gain medium temperature is at different temperatures, the intensity noise of the laser is measured, and the relaxation oscillation frequency measurement value of the laser is obtained from the intensity noise spectrum of the laser;

[0007] (2) based on the actual parameters of the laser when measuring the intensity noise of the laser, a function curve is drawn using a theoretical function of the laser relaxation oscillation frequency, wherein the function curve is a function curve with a theoretical value of the stimulated emission cross section of the laser gain medium as an independent variable and a theoretical value of the laser relaxation oscillation frequency as a dependent variable;

[0008] (3) Substituting the relaxation oscillation frequency measured value described in (1) into the function curve diagram described in (2) to calibrate the actual stimulated emission cross section;

[0009] (4) A function curve is drawn based on the actual stimulated emission cross section and the corresponding temperature of the gain medium. The function curve is a function curve with the temperature of the laser gain medium as the independent variable and the actual stimulated emission cross section of the gain medium as the dependent variable, and finally a fitting equation is obtained.

[0010] Furthermore, the actual parameters of the laser include: the laser single round trip cavity length L2, the gain medium doping length L1, the gain medium refractive index n, the inversion population fluorescence lifetime τ f , lower energy level particle lifetime τ, gain medium doping ion concentration c w , pump power P in , pump laser wavelength λ p , pump laser frequency ν p , laser frequency ν l , the waist radius of the pump laser at the center of the gain medium ω p , output coupling mirror transmittance t, laser cavity loss δ, laser wavelength λ l , pump laser transmission efficiency η a , quantum efficiency η q , the absorption coefficient of the gain medium to the pump laser α, the atomic density ρ corresponding to a doping atom concentration of 1.0% c .

[0011] Furthermore, the relaxation oscillation frequency is calculated based on the theoretical value of the actual parameters of the laser resonant cavity as follows:

[0012]

[0013] Where, is the cavity attenuation rate caused by the laser output mirror coupling mirror, t is the output coupling mirror transmittance, is the cavity attenuation rate caused by the laser cavity loss, δ is the cavity loss, is the lifetime of the oscillating laser in the laser resonant cavity, L2 is the cavity length of a single round trip of light in the resonant cavity, σ s is the laser stimulated emission cross section, ρ lm =ρ c *c w is the density of dopant atoms in the gain medium, ρ c is the atomic density corresponding to the doping atomic concentration of 1.0%, c w is the doping concentration of the gain medium, c is the speed of light, L1 is the atomic doping length of the gain medium, L2 is the cavity length of a single round trip of light in the resonant cavity, n is the refractive index of the gain medium, is the spontaneous emission rate of the lower energy level, is the spontaneous emission rate of the upper energy level, τ f is the fluorescence lifetime of the upper energy level inversion particle, is the pumping rate, p in is the laser diode pump power in the laser corresponding to the measurement of laser intensity noise, η t is the pump light transmission efficiency, η a =1-exp(-αL1) is the absorption efficiency of the gain medium, α is the absorption coefficient of the gain medium to the pump laser, is the quantum efficiency, ν l is the output laser frequency, ν p is the pump laser frequency, h is the Planck constant, N lm =ρ l, *V m is the number of doping ions used in the laser medium, is the mode volume of the pump laser in the gain medium, ω p is the waist radius of the pump laser at the center of the gain medium, λ p is the wavelength of the pumping laser, and z represents the axial position coordinate along the length of the laser crystal.

[0014] Furthermore, the overall implementation device of the method includes: a pump source of an integrated coupling system, an input coupling mirror, a gain medium, an optical isolator, a plano-convex lens, a plano-concave lens, an output coupling mirror and an intensity noise detector, wherein the pump source of the integrated coupling system is an 880nm fiber-coupled laser diode, the core diameter and numerical aperture of the coupled optical fiber are 400μm and 0.22 respectively, the coupling system uses two lenses with focal lengths of 30mm and 80mm respectively to achieve laser shaping, so that the waist spot of the pump laser at the center of the gain medium is 500μm to achieve good mode matching; the input coupling mirror is a plano-convex cavity mirror with a curvature radius of 1500mm, the convex surface is coated with an 880nm high-transmittance film, and the flat surface is coated with an 880nm high-transmittance film and a 1047-1064nm high-reflectivity film; the gain medium is composed of a neodymium-doped yttrium lithium fluoride (Nd:YLF) crystal with a length of 30mm. 3+ The doping concentration is 1.0at.%; the optical isolator consists of an 8mm long terbium gallium garnet (TGG) crystal and a half-wave plate, which can eliminate the spatial hole burning effect and ensure unidirectional propagation of laser light; the plano-convex lens is a plano-convex cavity mirror with a convex surface coated with a 1047-1064nm high-reflectivity film; the plano-concave lens is a plano-concave cavity mirror with a curvature radius of 100mm and a concave surface coated with a 1047-1064nm high-reflectivity film; the output coupling mirror is a plano-concave cavity mirror with a curvature radius of 100mm, a concave surface coated with a 1047-1064nm partially transmitting film, and a flat surface coated with a 1047-1064nm anti-reflection film; the intensity noise detector mainly consists of a photodetector, an oscilloscope, and a spectrum analyzer, and uses a balanced zero-beat detection method for measurement.

[0015] The advantages and beneficial effects of the present invention are as follows:

[0016] The existing methods for measuring the stimulated emission cross section of the gain medium are all direct measurement methods, which have the problem of inaccurate measurement and are not easy to measure the stimulated emission cross section of the gain medium in real time, quickly and accurately under the actual operation state of the all-solid-state laser. The present invention takes into account that in the total quantum noise theoretical function of the all-solid-state laser, the stimulated emission cross section of the laser gain medium directly affects the stimulated emission rate of the coupling of the gain medium atomic transition and the laser cavity mode; the number of oscillation photons in the laser cavity is a function of the stimulated emission rate of the coupling of the atomic transition and the laser cavity mode; the relaxation oscillation frequency of the laser is a function of the stimulated emission rate of the coupling of the laser gain medium atomic transition and the laser cavity mode and the number of oscillation photons in the laser cavity. Therefore, there is a functional correlation characteristic between the laser relaxation oscillation frequency and the stimulated emission cross section of the gain medium. In the present invention, when the laser is in stable operation, the intensity noise of the laser when the gain medium is at different temperatures can be measured using an intensity noise detector, and the laser relaxation oscillation frequency measurement value can be obtained from the laser intensity noise spectrum. According to the actual parameters of the laser when measuring the intensity noise of the laser, the stimulated emission cross section σ of the laser gain medium is theoretically used. s As the independent variable, the laser relaxation oscillation frequency ω f The function curve diagram is a dependent variable. The obtained laser relaxation oscillation frequency measurement value is made equal to the theoretical value of the laser relaxation oscillation frequency in the theoretical function curve diagram, and the value of the corresponding horizontal axis is read, which is the actual stimulated emission cross section of the laser gain medium under the operating state. The actual stimulated emission cross section of the laser gain medium obtained is plotted as a function curve diagram with the corresponding temperature of the gain medium, and a fitting equation is obtained. The function curve diagram is a function curve diagram with the temperature of the laser gain medium as the independent variable and the actual stimulated emission cross section of the gain medium as the dependent variable. The present invention is particularly suitable for real-time, rapid and accurate measurement of the stimulated emission cross section of the gain medium of an all-solid-state laser under actual operating conditions.

[0017] Compared with the existing direct measurement method, it has the following advantages:

[0018] 1. The present invention is an indirect measurement method with simple device, convenient operation, low cost, and easy to measure the stimulated emission cross section of the gain medium in real time, quickly and accurately under the actual operation state of the all-solid-state laser;

[0019] 2. The present invention has universal applicability and is applicable to the measurement of stimulated emission cross sections of the gain media of visible light and near-infrared lasers pumped by laser diodes, mid-infrared lasers pumped by all-solid-state lasers, and mid-infrared lasers pumped by fiber lasers under actual conditions;

[0020] 3. The present invention is applicable to the measurement of stimulated emission cross sections of gain media in high-power, medium-power, and low-power stable-operating lasers;

[0021] 4. The present invention is applicable to the measurement of stimulated emission cross sections of all-solid-state laser gain media that are being debugged or packaged in actual operation. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a flow chart of a method for measuring a temperature response characteristic function of a gain medium stimulated emission cross section according to a preferred embodiment of the present invention;

[0023] Figure 2 Schematic diagram of the structure of the device for measuring the temperature response characteristic function of the stimulated emission cross section of the gain medium of the all-solid-state laser in the implementation scheme;

[0024] Figure 3 The intensity noise spectra of the laser measured by the intensity noise detector are as follows when the gain medium temperature is 18℃, 28℃, 38℃, 48℃, 58℃, and 68℃ respectively;

[0025] Figure 4 is the theoretical value of the stimulated emission cross section σ obtained from the measured intensity noise spectrum and the gain medium s and the theoretical value of laser relaxation oscillation frequency ω f The function curve diagram of is used to obtain a demonstration diagram of the stimulated emission cross section of the gain medium under actual operating conditions.

[0026] Figure 5 This is a function curve of the actual stimulated emission cross section of the gain medium and the gain medium temperature when the laser is operating normally.

[0027] Reference numerals and corresponding component names:

[0028] 1- Pump source of the integrated coupling system, 2- Input coupling mirror, 3- Gain medium, 4- Optical isolator, 5- Plano-convex lens, 6- Plano-concave lens, 7- Output coupling mirror, 8- Intensity noise detector. DETAILED DESCRIPTION

[0029] The following will describe the technical solutions in the embodiments of the present invention in detail with reference to the accompanying drawings. The described embodiments are only a part of the embodiments of the present invention.

[0030] The technical solution of the present invention to solve the above technical problems is:

[0031] like Figure 1 As shown, the present invention is a method for measuring the temperature response characteristic function of the stimulated emission cross section of a gain medium, the method comprising the following steps:

[0032] (1) When the laser is in stable operation and the gain medium temperature is at different temperatures, the intensity noise of the laser is measured, and the relaxation oscillation frequency measurement value of the laser is obtained from the intensity noise spectrum of the laser;

[0033] (2) According to the actual parameters of the laser when measuring the intensity noise of the laser (including: laser single round-trip cavity length L2, gain medium doping length L1, gain medium refractive index n, inversion particle number fluorescence lifetime τ) f , lower energy level particle lifetime τ, gain medium doping ion concentration c w , pump power P in , pump laser wavelength λ p , pump laser frequency ν p , laser frequency ν l , the waist radius of the pump laser at the center of the gain medium ω p , output coupling mirror transmittance t, laser cavity loss δ, laser wavelength λ l , pump laser transmission efficiency η a , quantum efficiency η q , the absorption coefficient of the gain medium to the pump laser α, the atomic density ρ corresponding to a doping atom concentration of 1.0% c ), using the theoretical function of the laser relaxation oscillation frequency to draw a function graph, wherein the function graph uses the theoretical value of the stimulated emission cross section of the laser gain medium as the independent variable and the theoretical value of the laser relaxation oscillation frequency as the dependent variable;

[0034] (3) Substituting the relaxation oscillation frequency measured value described in (1) into the function curve diagram described in (2) to calibrate the actual stimulated emission cross section;

[0035] (4) A function curve is drawn based on the actual stimulated emission cross section and the corresponding temperature of the gain medium. The function curve is a function curve with the temperature of the laser gain medium as the independent variable and the actual stimulated emission cross section of the gain medium as the dependent variable, and finally a fitting equation is obtained.

[0036] The working principle is: the present invention takes into account that in the total quantum noise theoretical function of the all-solid-state laser, the stimulated emission cross section of the laser gain medium directly affects the stimulated emission rate of the coupling of the gain medium atomic transition and the laser cavity mode; the number of oscillation photons in the laser cavity is a function of the stimulated emission rate of the coupling of the atomic transition and the laser cavity mode; the relaxation oscillation frequency of the laser is a function of the stimulated emission rate of the coupling of the laser gain medium atomic transition and the laser cavity mode and the number of oscillation photons in the laser cavity. Therefore, there is a functional correlation characteristic between the laser relaxation oscillation frequency and the stimulated emission cross section of the gain medium. In the present invention, when the laser is in a stable operating state, the intensity noise of the laser can be measured using an intensity noise detector, and the laser relaxation oscillation frequency measurement value can be obtained from the laser intensity noise spectrum. According to the actual parameters of the laser when measuring the intensity noise of the laser, the stimulated emission cross section σ of the laser gain medium is theoretically used. s As the independent variable, the laser relaxation oscillation frequency ωoff The function curve diagram is a dependent variable. Let the measured value of the laser relaxation oscillation frequency be the same as the theoretical value of the laser relaxation oscillation frequency in the theoretical function curve diagram, and read the value of the corresponding horizontal axis, which is the actual stimulated emission cross section of the laser gain medium under the operating state. The actual stimulated emission cross section of the measured laser gain medium is plotted as a function curve diagram with the corresponding temperature of the gain medium, and a fitting equation is obtained. The function curve diagram is a function curve diagram with the temperature of the laser gain medium as the independent variable and the actual stimulated emission cross section of the gain medium as the dependent variable. The present invention is particularly suitable for real-time, rapid and accurate measurement of the stimulated emission cross section of a stably operating all-solid-state laser gain medium under the actual operating state.

[0037] Figure 2 The schematic diagram of the structure of the device for measuring the temperature response characteristic function of the stimulated emission cross section of the gain medium of the all-solid-state laser in the implementation scheme. The overall implementation device includes: a pump source 1 of the integrated coupling system, an input coupling mirror 2, a gain medium 3, an optical isolator 4, a plano-convex lens 5, a plano-concave lens 6, an output coupling mirror 7 and an intensity noise detector 8. The following is a detailed introduction to each optical component and device: the pump source 1 of the integrated coupling system is an 880nm fiber-coupled laser diode, the core diameter and numerical aperture of the coupled optical fiber are 400μm and 0.22 respectively, and the coupling system uses two lenses with focal lengths of 30mm and 80mm respectively to achieve laser shaping, so that the waist spot of the pump laser at the center of the gain medium 3 is 500μm to achieve good mode matching; the input coupling mirror 2 is a plano-convex cavity mirror with a curvature radius of 1500mm, and the convex surface is coated with an 880nm high transmittance film (T 880nm >99.5%), the plane is coated with a high transmittance film at 880nm and a high reflectivity film at 1047-1064nm (T 880nm >99.5%&R 1047-1064nm >99.7%); the gain medium 3 is composed of a neodymium-doped yttrium lithium fluoride (Nd:YLF) crystal with a length of 30 mm. 3+ The doping concentration is 1.0at.%; the optical isolator 4 is composed of an 8mm long terbium gallium garnet (TGG) crystal and a half-wave plate, which can eliminate the spatial hole burning effect and the unidirectional propagation of the laser; the plano-convex lens 5 is a plano-convex cavity mirror (R 1047-1064nm >99.7%); Plano-concave lens 6 is a plano-concave cavity mirror with a curvature radius of 100 mm, and the concave surface is coated with a high-reflection film (R 1047-1064nm >99.7%); the output coupling mirror 7 is a plano-concave cavity mirror with a curvature radius of 100 mm, and the concave surface is coated with a 1047-1064 nm partially transparent film (T 1047-1064nm = 20%), the plane is coated with an anti-reflection film (AR1047-1064nm >99.7%); the intensity noise detector 8 is mainly composed of a photodetector, an oscilloscope and a spectrum analyzer, and is measured using a balanced zero-beat detection method.

[0038] Figure 3 The laser intensity noise spectrum, measured using an intensity noise detector, shows the gain medium relaxation oscillation frequencies of 162kHz, 158kHz, 152kHz, 146kHz, 142kHz, and 136kHz, respectively, when the gain medium temperature is 18°C, 28°C, 38°C, 48°C, 58°C, and 68°C, respectively. Analysis shows that the relaxation oscillation frequency decreases with increasing gain medium temperature, and for every 10°C increase in gain medium temperature, the relaxation oscillation frequency decreases by approximately 4-6kHz.

[0039] Figure 4 is the theoretical value of the stimulated emission cross section σ obtained from the measured intensity noise spectrum and the gain medium s and the theoretical value of laser relaxation oscillation frequency ω f The function curve of the gain medium is used to obtain a demonstration diagram of the stimulated emission cross section under actual operation. When the pump laser wavelength is 880nm and the pump power is 44.84W, according to the actual parameters of the laser during stable operation: gain medium length L1 = 30mm, single round-trip optical path length in the laser cavity L2 = 483mm, gain medium refractive index n = 1.47, fluorescence lifetime τ f =4.85×10 -4 s, the lifetime of the lower energy level particle τ=3×10 -8 s, gain medium doped with Nd +3 Ion concentration c w =1.0at.%, Avogadro constant n a =6.02×10 23 , the speed of light c = 2.997 × 10 8 m / s, Planck constant h=6.63×10 -34 , the waist radius of the pump laser at the center of the gain medium ω p =500μm, the absorption coefficient of the gain medium to the pump laser is α=359 / m, the output coupling mirror transmittance T=0.2, the laser cavity loss δ=0.035, and the pump laser transmission efficiency η a =0.98, quantum efficiency η q =0.76, the pump power of the laser diode P in =44.84W, pump laser wavelength λ p =880×10 -9 m, pump laser frequency Laser frequency λ l=1053×10 -9 m, the density of dopant atoms in the gain medium ρ lm =ρ c *c w =1.26*10 26 *c w According to the actual parameters of the laser above, the laser crystal stimulated emission cross section σ s As the independent variable, the laser relaxation oscillation frequency ω f is the function curve of the dependent variable, let ω m =ω f When the gain medium temperature is 18℃, 28℃, 38℃, 48℃, 58℃, and 68℃, the corresponding actual stimulated emission cross sections are 1.97×10 -19 cm 2 , 1.90×10 -19 cm 2 , 1.80×10 -19 cm 2 , 1.70×10 -19 cm 2 , 1.64×10 -19 cm 2 , 1.55×10 -19 cm 2 .

[0040] Figure 5 This is a function curve of the actual stimulated emission cross section of the gain medium and the gain medium temperature when the laser is operating normally. The function curve is established with the gain medium temperature as the independent variable and the actual stimulated emission cross section of the gain medium as the dependent variable. After linear fitting, the fitting equation σ=2.1×10 -19 (cm 2 )-8.5×10 -22 (cm 2 / ℃)×T(℃). In the temperature range of the gain medium studied, the actual stimulated emission cross section decreases at an average rate of about 4.3% per 10℃. This temperature-dependent variation of the stimulated emission cross section is very close to the 4.0% in neodymium-doped garnet (Nd:YAG) and neodymium-doped yttrium vanadate (Nd:YVO4) measured by fluorescence spectroscopy [J.Opt.Soc.Am.B, 26(11), 2084-2088, 2009; Phys.Status SolidiA202, 2565–2573(2005)], indicating the feasibility of the measurement method and the reliability of the measurement results.

[0041] As can be seen, the present invention provides a simple, easy-to-use, and low-cost method for measuring the temperature response characteristics of the stimulated emission cross section of a gain medium. This method facilitates real-time, rapid, and accurate measurement of the stimulated emission cross section of a gain medium during actual operation of an all-solid-state laser. The present invention has universal applicability and is applicable to the measurement of the stimulated emission cross section of the gain medium of visible and near-infrared lasers pumped by laser diodes, mid-infrared lasers pumped by all-solid-state lasers, and mid-infrared lasers pumped by fiber lasers under actual conditions. The present invention is also applicable to the measurement of the stimulated emission cross section of both packaged and debugged all-solid-state laser gain media under actual operation.

[0042] The systems, devices, modules or units described in the above embodiments may be implemented by computer chips or entities, or by products with certain functions.

[0043] It should also be noted that the terms "comprises," "includes," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, commodity, or apparatus that includes a series of elements includes not only those elements but also other elements not explicitly listed, or includes elements inherent to such process, method, commodity, or apparatus. In the absence of further limitations, an element defined by the phrase "comprises a ..." does not exclude the presence of other identical elements in the process, method, commodity, or apparatus that includes the element.

[0044] The above embodiments should be understood as merely illustrating the present invention and not as limiting the scope of protection of the present invention. After reading the contents of the present invention, technicians may make various changes or modifications to the present invention, and these equivalent changes and modifications also fall within the scope defined by the claims of the present invention.

Claims

1. A method for measuring the temperature response characteristic function of the stimulated emission cross section of a gain medium, characterized in that: The following steps are involved: (1) When the laser is in stable operation and the gain medium temperature is at different temperatures, the intensity noise of the laser is measured, and the relaxation oscillation frequency measurement value of the laser is obtained from the intensity noise spectrum of the laser; (2) based on the actual parameters of the laser when measuring the intensity noise of the laser, a function curve is drawn using a theoretical function of the laser relaxation oscillation frequency, wherein the function curve is a function curve with a theoretical value of the stimulated emission cross section of the laser gain medium as an independent variable and a theoretical value of the laser relaxation oscillation frequency as a dependent variable; (3) Substituting the relaxation oscillation frequency measured value described in (1) into the function curve diagram described in (2) to calibrate the actual stimulated emission cross section; (4) A function curve is drawn based on the actual stimulated emission cross section and the corresponding temperature of the gain medium. The function curve is a function curve with the temperature of the laser gain medium as the independent variable and the actual stimulated emission cross section of the gain medium as the dependent variable, and finally a fitting equation is obtained.

2. The method for measuring the temperature response characteristic function of the stimulated emission cross section of a gain medium according to claim 1, characterized in that: The actual parameters of the laser include: the laser single round trip cavity length L2, the gain medium doping length L1, the gain medium refractive index n, the inversion population fluorescence lifetime τ f , lower energy level particle lifetime τ, gain medium doping ion concentration c w , pump power P in , pump laser wavelength λ p , pump laser frequency ν p , laser frequency ν l , the waist radius of the pump laser at the center of the gain medium ω p , output coupling mirror transmittance t, laser cavity loss δ, laser wavelength λ l , pump laser transmission efficiency η a , quantum efficiency η q , the absorption coefficient of the gain medium to the pump laser α, the atomic density ρ corresponding to a doping atom concentration of 1.0% c .

3. The method for measuring the temperature response characteristic function of the stimulated emission cross section of a gain medium according to claim 1, characterized in that: The relaxation oscillation frequency is calculated based on the theoretical value of the actual parameters of the laser resonant cavity as follows: Where, is the cavity attenuation rate caused by the laser output mirror coupling mirror, t is the output coupling mirror transmittance, is the cavity attenuation rate caused by the laser cavity loss, δ is the cavity loss, is the lifetime of the oscillating laser in the laser resonator, L2 is the cavity length of the light in the resonator for a single round trip, σ s is the laser stimulated emission cross section, ρ lm =ρ c *c w is the density of dopant atoms in the gain medium, ρ c is the atomic density corresponding to the doping atomic concentration of 1.0%, c w is the doping concentration of the gain medium, c is the speed of light, L1 is the atomic doping length of the gain medium, L2 is the cavity length of a single round trip of light in the resonant cavity, n is the refractive index of the gain medium, is the spontaneous emission rate of the lower energy level, is the spontaneous emission rate of the upper energy level, τ f is the fluorescence lifetime of the upper energy level inversion particle, is the pumping rate, p in is the laser diode pump power in the laser corresponding to the measurement of laser intensity noise, η t is the pump light transmission efficiency, η a =1-exp(-αL1) is the absorption efficiency of the gain medium, α is the absorption coefficient of the gain medium to the pump laser, is the quantum efficiency, ν l is the output laser frequency, ν p is the pump laser frequency, h is the Planck constant, N lm =ρ lm *V m is the number of doping ions used in the laser medium, is the mode volume of the pump laser in the gain medium, ω p is the waist radius of the pump laser at the center of the gain medium, λ p is the wavelength of the pumping laser, and z represents the axial position coordinate along the length of the laser crystal.

4. The method for measuring the temperature response characteristic function of the stimulated emission cross section of a gain medium according to claim 1, characterized in that: The overall implementation device of the method includes: a pump source of an integrated coupling system, an input coupling mirror, a gain medium, an optical isolator, a plano-convex lens, a plano-concave lens, an output coupling mirror and an intensity noise detector. The pump source of the integrated coupling system is an 880nm fiber-coupled laser diode, the core diameter and numerical aperture of the coupling fiber are 400μm and 0.22 respectively, the coupling system uses two lenses with focal lengths of 30mm and 80mm respectively to achieve laser shaping, so that the waist spot of the pump laser at the center of the gain medium is 500μm to achieve good mode matching; the input coupling mirror is a plano-convex cavity mirror with a curvature radius of 1500mm, the convex surface is coated with an 880nm high-transmittance film, and the flat surface is coated with an 880nm high-transmittance film and a 1047-1064nm high-reflectivity film; the gain medium is composed of a neodymium-doped yttrium lithium fluoride (Nd:YLF) crystal with a length of 30mm. 3+ The doping concentration is 1.0at.%; the optical isolator consists of an 8mm long terbium gallium garnet (TGG) crystal and a half-wave plate, which can eliminate the spatial hole burning effect and ensure unidirectional propagation of laser light; the plano-convex lens is a plano-convex cavity mirror with a convex surface coated with a 1047-1064nm high-reflectivity film; the plano-concave lens is a plano-concave cavity mirror with a curvature radius of 100mm and a concave surface coated with a 1047-1064nm high-reflectivity film; the output coupling mirror is a plano-concave cavity mirror with a curvature radius of 100mm, a concave surface coated with a 1047-1064nm partially transmitting film, and a flat surface coated with a 1047-1064nm anti-reflection film; the intensity noise detector mainly consists of a photodetector, an oscilloscope, and a spectrum analyzer, and uses a balanced zero-beat detection method for measurement.

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