A method for measuring the equivalent cavity length of a self-injected seed laser

Abstract

The application claims a method for measuring the equivalent cavity length of a self-injection seed laser, comprising a self-injection seed laser (1) and a balanced homodyne noise detection device (2). The method comprises the following steps: first, measuring the intensity noise of the self-injection seed laser under the stable operation state of the self-injection seed laser by using a photoelectric detector, and reading the relaxation oscillation frequency of the laser in the intensity noise spectrum; then, according to the actual parameters of the self-injection seed laser during the laser intensity noise measurement, using a theoretical function of the laser relaxation oscillation frequency as a function curve graph with the laser equivalent cavity length as the independent variable and the laser relaxation oscillation frequency as the dependent variable; finally, equating the laser relaxation oscillation frequency value obtained in the first step with the relaxation oscillation frequency value in the theoretical simulation function curve graph in the second step, and reading the value of the corresponding abscissa in the theoretical function curve graph, which is the equivalent cavity length of the self-injection seed laser. The application is suitable for measuring the equivalent cavity length of the self-injection seed laser under the stable operation state.

Description

Technical Field

[0001] This invention relates to the field of laser technology, specifically a method for measuring the equivalent cavity length of a self-injected seed laser. It is particularly applicable to the measurement of the equivalent cavity length of a self-injected seed laser under actual operating conditions, and will provide theoretical reference and technical guidance for the fabrication of low-intensity noise lasers. Background Technology

[0002] Self-injected seed lasers are a technique that optimizes output characteristics by feeding back a portion of the laser's output light into the resonant cavity as a seed source. By feeding back one arm of the bidirectional output laser path into the laser resonant cavity, a laser intensity difference is created between the two laser beams, enabling unidirectional laser operation. Compared to ring lasers that use optical unidirectional devices for unidirectional operation, the self-injection end of a self-injected seed laser can compensate for noise introduced by the loss of the output coupler, which is beneficial for low-intensity noise laser output. Currently, the engineering fabrication of low-intensity noise self-injected seed lasers still faces technical challenges, primarily because the equivalent cavity length of the laser required for the engineering model fabrication cannot be determined. The equivalent cavity length of a self-injected seed laser is also affected by factors such as the transmittance of the laser output coupler, the optical cavity length of the self-injection feedback end, and the reflectivity of the self-injection feedback end mirror. Currently, due to the influence of the composite cavity structure of self-injected seed lasers, the intensity noise of the output laser can only be characterized through intensity noise measurement methods. This means that the fabrication of low-intensity noise self-injected seed lasers can only be passively achieved by repeatedly changing the resonant cavity parameters and adjusting the resonant cavity manually. The actual intensity noise of a self-injected seed laser is not only related to the cavity structure parameters of the laser itself, but also affected by the coherence characteristics of the self-injected feedback laser (which are simultaneously related to the transmittance of the output coupling mirror, the optical cavity length of the self-injection feedback end, and the reflectivity of the self-injection feedback end mirror). Characterizing the intensity noise of a self-injected seed laser requires measuring the equivalent cavity length of the laser. Furthermore, the engineering fabrication of low-intensity noise self-injected seed lasers also requires measuring the equivalent cavity length. Therefore, this invention develops a measurement method capable of measuring the equivalent cavity length of a self-injected seed laser under actual operating conditions, which can provide a reference for key parameters in the engineering fabrication of low-intensity noise self-injected seed lasers.

[0003] CN201010173127.5 describes a device for measuring the characteristic length of an acoustic resonant cavity. This device calculates the cavity length by utilizing the diameter variation of the equal-inclination interference rings of a He-Ne laser and a semiconductor laser, and then combines this with multi-wavelength synthesis to extend the measurement range. The drawback of this method is the complexity of the device structure. It requires extremely high parallelism between the flat crystals at both ends of the acoustic resonant cavity, which is difficult to achieve with current manufacturing techniques. Insufficient parallelism leads to deformation or blurring of the equal-inclination interference rings, significantly increasing the difficulty of image processing and ultimately affecting the accuracy of the cavity length measurement. Furthermore, this measurement method is only applicable to straight cavities (FP cavity structures). It greatly increases the measurement difficulty for lasers with ring cavity structures, especially for self-injected seed lasers with composite ring cavity structures (consisting of a ring cavity and a seed self-feedback section).

[0004] The present invention proposes a method for measuring the equivalent cavity length of a self-injected seed laser. This method is applicable to lasers with straight cavity, standing wave cavity, and ring cavity structures, and is particularly suitable for measuring the equivalent cavity length of a self-injected seed laser. Summary of the Invention

[0005] This invention aims to solve the problems of the prior art mentioned above. It proposes a method for measuring the equivalent cavity length of a self-injected seed laser. The technical solution of this invention is as follows:

[0006] A method for measuring the equivalent cavity length of a self-injected seed laser, comprising the following steps:

[0007] (1) Measure the intensity noise of the laser using a photodetector, and read the relaxation oscillation frequency ω of the laser from the laser intensity noise spectrum. m ;

[0008] (2) Based on the actual parameters of the laser when measuring the laser intensity noise, the theoretical function of the laser relaxation oscillation frequency is used, with the laser equivalent cavity length L as the independent variable and the laser relaxation oscillation frequency ω as the independent variable. off The graph of the function of the dependent variable;

[0009] (3) Let the relaxation oscillation frequency ω in the function curve be... off Equal to the relaxation oscillation frequency ω in the measured intensity noise spectrum. c Read ω from the function curve graph m The corresponding horizontal axis value is the actual equivalent cavity length of the laser crystal in this operating state.

[0010] Furthermore, with the equivalent cavity length L of the laser as the independent variable and the laser relaxation oscillation frequency ω as the variable... off In the curve of the dependent variable function, ω off The calculation is performed using formula (1):

[0011]

[0012] in, The cavity attenuation rate caused by the laser output mirror coupling mirror. Let t be the cavity attenuation rate caused by the cavity loss of the laser, t and δ be the transmittance of the output coupling mirror and the linear loss in the cavity, respectively, and L be the equivalent cavity length of the light in the self-injected seed laser resonator.

[0013]

[0014] Where g is the stimulated emission rate of the coupling between atomic transitions in the laser crystal and the laser cavity mode, and σ s ρ is the stimulated emission cross section of the laser. lm =ρ c *c w ρ represents the density of doped atoms in the gain medium. c c represents the atomic density corresponding to a doping concentration of 1.0%. w Where c is the doping concentration of the gain medium, l is the speed of light, and n is the atomic doping length of the laser crystal.

[0015]

[0016] Where α is the number of photons in the cavity, The spontaneous emission rate of the lower energy level. Let τ be the spontaneous emission rate of the upper energy level. f Let j2 be the fluorescence lifetime of the inverted particle at the upper energy level, and j2 be the population probability of the ground state particle. j2 is expressed as: Where Γ is the pump rate, Γ is expressed as: Where, p in η is the pump power of the laser diode in the laser corresponding to the laser intensity noise. t The pump light transmission efficiency, η, is the ratio of the pump light power entering the gain medium to the pump light power output by the laser diode. a =1-exp(-αl) is the absorption efficiency of the gain medium, and α is the absorption coefficient of the gain medium for the pump laser. For quantum efficiency, ν l To output laser frequency, ν p Where is the pump laser frequency, h is Planck's constant, and N is... lm The number of doped ions utilized in the laser medium is denoted as N. lm =ρ lm *V m , where V m The pump laser mode volume at the laser crystal is expressed as: Where, ω p λ is the beam waist radius at the center of the laser crystal, representing the pump laser beam. pThe wavelength of the pump laser.

[0017] Furthermore, the equivalent cavity length measurement of the laser crystal was performed in a self-injected seeded all-solid-state continuous 1064nm continuous laser with a four-mirror ring cavity structure. The laser resonant cavity is a butterfly-shaped ring cavity composed of four mirrors, two of which are two plane mirrors and the other two are two plano-concave lenses with a radius of curvature R = -50mm.

[0018] The advantages and beneficial effects of this invention are as follows:

[0019] 1. When measuring the equivalent cavity length of a self-injected seed laser, this invention only requires using a photodetector to measure the intensity noise spectrum of the laser and read the relaxation oscillation frequency value. The device is simple and easy to operate.

[0020] 2. This invention has universal applicability and is applicable to lasers with straight cavity, standing wave cavity, ring cavity, and composite ring cavity structures.

[0021] 3. This invention can measure the equivalent cavity length of lasers with different power levels and different wavelengths (visible light to mid-infrared), and in particular, it can measure the equivalent cavity length of self-injected seed lasers under actual conditions. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of the structure of the preferred embodiment of the present invention for measuring the equivalent cavity length of a self-injected seed laser crystal;

[0023] Figure 2 This is a cavity structure diagram of a self-injected seed laser;

[0024] Figure 3 The intensity noise spectrum of a laser obtained using a photodetector.

[0025] Figure 4 The intensity noise spectrum obtained from the measurement is based on the equivalent cavity length L of the laser and the laser relaxation oscillation frequency ω. off The function curve with the dependent variable is used to obtain a demonstration diagram of the equivalent cavity length of the self-injected seed laser under actual operating conditions. Detailed Implementation

[0026] The technical solutions of the embodiments of the present invention will be clearly and thoroughly described below with reference to the accompanying drawings. The described embodiments are merely some embodiments of the present invention.

[0027] The technical solution of the present invention to solve the above-mentioned technical problems is:

[0028] The principle of this invention is as follows: In the full quantum noise theory of self-injected seed lasers, the equivalent cavity length of the laser affects the relaxation oscillation frequency of the laser by influencing the lifetime of external feedback photons, the number of oscillating photons within the composite laser resonator, and the coupling strength between the number of inverted particles at the upper energy level and the cavity membrane. Therefore, there is a functional correlation between the relaxation oscillation frequency and the equivalent cavity length of the self-injected seed laser. Under stable laser operation, the intensity noise spectrum of the laser can be measured using a photodetector, and the laser relaxation oscillation frequency value can be read from the laser intensity noise spectrum. Based on the measured actual parameters of the self-injected seed laser, a function curve is theoretically plotted with the equivalent cavity length as the independent variable and the laser relaxation oscillation frequency as the dependent variable. Setting the measured laser relaxation oscillation frequency value to be the same as the theoretical function curve's laser relaxation oscillation frequency value, the corresponding horizontal axis value is read, which is the actual equivalent cavity length of the laser crystal under this operating condition.

[0029] According to the full quantum noise theory function of the laser, under stable operating conditions, the relaxation oscillation frequency ω of the laser is... off Represented as:

[0030]

[0031] in, The cavity attenuation rate caused by the laser output mirror coupling mirror. Let t be the cavity attenuation rate caused by the cavity loss of the laser, t and δ be the transmittance of the output coupling mirror and the linear loss in the cavity, respectively, and L be the equivalent cavity length of the light in the self-injected seed laser resonator.

[0032]

[0033] Where g is the stimulated emission rate of the coupling between atomic transitions in the laser crystal and the laser cavity mode, and σ s ρ is the stimulated emission cross section of the laser. lm =ρ c *c w ρ represents the density of doped atoms in the gain medium. c c represents the atomic density corresponding to a doping concentration of 1.0%. w Where c is the doping concentration of the gain medium, l is the speed of light, and n is the atomic doping length of the laser crystal.

[0034]

[0035] Where α is the number of photons in the cavity, The spontaneous emission rate of the lower energy level. Let τ be the spontaneous emission rate of the upper energy level. fLet j2 be the fluorescence lifetime of the inverted particle at the upper energy level, and j2 be the population probability of the ground state particle. j2 is expressed as: Where Γ is the pump rate, Γ is expressed as: Where, p in η is the pump power of the laser diode in the laser corresponding to the laser intensity noise. t η is the pump light transmission efficiency (the ratio of the pump light power entering the gain medium to the pump light power output by the laser diode). a =1-exp(-αl) is the absorption efficiency of the gain medium, and α is the absorption coefficient of the gain medium for the pump laser. For quantum efficiency, ν l To output laser frequency, ν p Where is the pump laser frequency, h is Planck's constant, and N is... lm The number of doped ions utilized in the laser medium is denoted as N. lm =ρ lm *V m , where V m The pump laser mode volume at the laser crystal is expressed as: Where, ω p λ is the beam waist radius at the center of the laser crystal, representing the pump laser beam. p The wavelength of the pump laser.

[0036] From equations (1), (2), and (3), it can be seen that, under the condition that the pump power and cavity structure parameters of the self-injected seed laser are determined, the relaxation oscillation frequency of the laser is a function of the equivalent cavity length of the laser. Therefore, when the laser is operating stably (p in To determine the value, based on the actual parameters of the laser, we can obtain a value with the equivalent cavity length L of the laser as the independent variable and the laser relaxation oscillation frequency ω as the variable. off The graph shows the function curve of the dependent variable.

[0037] When the laser is operating stably (p in To determine the value, the intensity noise spectrum of the laser is measured using a photodetector. The actual relaxation oscillation frequency ω of the laser can then be read from the laser intensity noise spectrum. m .

[0038] When the ω in the actual measured intensity noise spectrum m The oscillation frequency ω in the function curve graph off At the same time, ω in the function curve graph m The corresponding value on the horizontal axis represents the laser crystal's pump power p at the injection point. in The equivalent laser cavity length under this operating condition.

[0039] This invention provides a method for measuring the equivalent cavity length of a self-injected seed laser under actual operating conditions, comprising the following steps:

[0040] 1. Measure the intensity noise of the laser using a photodetector, and read the relaxation oscillation frequency ω of the laser from the laser intensity noise spectrum. m .

[0041] 2. Based on the actual operating parameters of the laser when measuring laser intensity noise, plot the laser's equivalent cavity length L as the independent variable and the laser's relaxation oscillation frequency ω as the variable. off The graph shows the function curve of the dependent variable.

[0042] 3. Let the oscillation frequency ω in the function curve of the second step be... off Equal to ω in the intensity noise spectrum measured in the first step m Read ω from the function curve graph m The corresponding horizontal axis value is the equivalent cavity length of the laser in actual operation.

[0043] Figure 1 In the overall implementation scheme of the present invention, under the stable operation state of the self-injected seed laser, a photodetector is used to measure the intensity noise of the laser. Figure 2 This is a schematic diagram of the structure of the equivalent cavity length measurement device for the self-injected seed laser in the specific implementation scheme. The specific implementation scheme for measuring the equivalent cavity length of the laser crystal is carried out in a self-injected seed all-solid-state continuous 1064nm continuous laser with a four-mirror annular cavity structure. The laser resonant cavity is a butterfly-shaped annular cavity composed of four mirrors (4, 5, 6, 7), where (4) and (5) are two plane mirrors, and (6) and (7) are two plano-concave lenses with a radius of curvature of R = -50mm. The input coupling mirror (4) is coated with an 808nm high-transmittance film, (T 808nm >99.5%), and a 1064nm high reflectance film (R 1064nm >99.7%). A 1064nm high-reflectivity film (R) was deposited on (6) and (7). 1064nm >99.7%). The output coupling mirror (5) is coated with a 1064nm transmittance T 1064nm =4% film. The pump source (1) is an 808nm fiber-coupled laser diode, with a core diameter and numerical aperture of 200μm and 0.22, respectively. The pump laser (1) is focused by the coupling system (3) onto the center of the laser crystal (8) with a beam waist diameter of 0.510mm. The laser crystal (8) is a 5mm Nd:YVO4 (S1,S2:AR) doped with 0.5 at.% Nd. 808nm；1064nm The back end of the laser crystal is cut at a small angle of 1.2° to ensure stable polarization of the laser. The self-injected seed feedback cavity mirror is a plane mirror (9) coated with a 1064nm high-reflectivity film (R).1064nm >99.7%). The geometric length between the reflective cavity mirror (9) at the injection end and the output mirror (5) is 55 mm.

[0044] The geometric cavity length of the butterfly-shaped annular cavity is 315 mm. Under stable operation, the injected 808 nm pump laser power is 2 W, and the output power of the self-injected seed laser is 0.68 W. The intensity noise spectrum of the laser obtained using a photodetector is as follows... Figure 3 As shown, the relaxation oscillation frequency ω of the laser m =200kHz. With an 808nm pump laser power of 2W, based on the actual laser parameters: laser crystal length L1 = 5 × 10⁻⁶ -3 m, laser crystal refractive index n = 1.976, fluorescence lifetime τ f =1×10 -4 s, the lifetime of the lower energy level particle τ = 3 × 10 -8 s, laser crystal doped with Nd +3 Ion concentration c w =0.5at.%, Avogadro's constant n a =6.02×10 23 The speed of light, c = 2.997 × 10⁻⁶ m / s, and Planck's constant, h = 6.63 × 10⁻⁶ m / s. -34 The radius of the waist spot of the pump laser at the center of the laser crystal is 240 × 10⁻⁶. -6 m, laser cavity loss δ=0.02, η t =0.96 (Pump laser transmission efficiency η) a =0.98, quantum efficiency η q =0.76, the pump power P of the laser diode in =2W, pump laser wavelength λ p =808×10 -9 m, pump laser frequency laser frequency λ l =1064×10 -9 m, the density of doped atoms in the gain medium ρ lm =ρ c *c w =1.26*10 26 *c w Based on the actual parameters of the laser above, a plot is made with the equivalent cavity length L of the self-injected seed laser as the independent variable and the laser relaxation oscillation frequency ω as the variable. off The graph of the function of the dependent variable, such as Figure 4 As shown. Figure 4 In the middle, let ω m =ω offAt that time, the actual measured laser relaxation oscillation frequency straight line intersects with the function curve, and the horizontal axis corresponding to this intersection point is the equivalent cavity length of the self-injected seed laser: 522mm.

[0045] The systems, devices, modules, or units described in the above embodiments can be implemented by computer chips or entities, or by products with certain functions. A typical implementation device is a computer. Specifically, a computer can be, for example, a personal computer, laptop computer, cellular phone, camera phone, smartphone, personal digital assistant, media player, navigation device, email device, game console, tablet computer, wearable device, or any combination of these devices.

[0046] Computer-readable media includes both permanent and non-permanent, removable and non-removable media that can store information using any method or technology. Information can be computer-readable instructions, data structures, modules of programs, or other data. Examples of computer storage media include, but are not limited to, phase-change memory (PRAM), static random access memory (SRAM), dynamic random access memory (DRAM), other types of random access memory (RAM), read-only memory (ROM), electrically erasable programmable read-only memory (EEPROM), flash memory or other memory technologies, CD-ROM, digital versatile optical disc (DVD) or other optical storage, magnetic tape, magnetic magnetic disk storage or other magnetic storage devices, or any other non-transferable medium that can be used to store information accessible by a computing device. As defined herein, computer-readable media does not include transient computer-readable media, such as modulated data signals and carrier waves.

[0047] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0048] The above embodiments should be understood as illustrative only and not as limiting the scope of protection of the present invention. After reading the description of the present invention, those skilled in the art can make various alterations 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 equivalent cavity length of a self-injected seed laser, characterized in that, Includes the following steps: (1) Measure the intensity noise of the laser using a photodetector and read the relaxation oscillation frequency value of the laser from the laser intensity noise spectrum. ; (2) Based on the actual parameters of the laser when measuring the laser intensity noise, the theoretical function of the laser relaxation oscillation frequency is used as the independent variable with the laser equivalent cavity length L and the laser relaxation oscillation frequency. The graph of the function of the dependent variable; (3) Let the relaxation oscillation frequency in the function curve be... Equal to the relaxation oscillation frequency in the measured intensity noise spectrum. Read the function curve graph The corresponding horizontal axis value is the actual equivalent cavity length of the laser crystal in operation. The equation is plotted with the equivalent cavity length L of the laser as the independent variable and the laser relaxation oscillation frequency as the inverse variable. In the dependent variable function curve graph, The calculation is performed using formula (1): （1）； Among them, 2* The cavity attenuation rate caused by the laser output mirror coupling mirror, 2* Let t be the cavity attenuation rate caused by intracavity loss in the laser, and t be the cavity attenuation rate. These represent the output coupling mirror transmittance and the intracavity linear loss, respectively. Let be the equivalent cavity length of light in the resonant cavity of the self-injected seed laser; （2）； in, σ represents the stimulated emission rate of the coupling between atomic transitions in the laser crystal and the cavity mode of the laser. s The stimulated emission cross section of the laser. The density of doped atoms in the gain medium. This represents the atomic density corresponding to a doping concentration of 1.0%. The doping concentration of the gain medium. At the speed of light, The atomic doping length of the laser crystal. The refractive index of the laser crystal; （3）； in, The number of photons in the cavity. The spontaneous emission rate of the lower energy level. The spontaneous emission rate of the upper energy level. The fluorescence lifetime of the particle with the inverted upper energy level. This represents the probability of the ground-state particle number distribution. Represented as: ,in, For pump rate, Represented as: ,in, This is the pump power of the laser diode in the laser that corresponds to the laser intensity noise. Pump light transmission efficiency is the ratio of the pump light power entering the gain medium to the pump light power output by the laser diode. , It is the absorption coefficient of the gain medium for the pump laser. For quantum efficiency, To output laser frequency, The pump laser frequency, Let be Planck's constant. The number of doped ions utilized in the laser medium is expressed as: ,in, The pump laser mode volume at the laser crystal is expressed as: ,in, The beam waist radius at the center of the laser crystal is the radius of the pump laser beam. The wavelength of the pump laser.

2. The method for measuring the equivalent cavity length of a self-injected seed laser according to claim 1, characterized in that, The equivalent cavity length measurement of the laser crystal was performed in a self-injected seeded all-solid-state continuous-wave 1064nm continuous-wave laser with a four-mirror ring cavity structure. The laser resonant cavity is a butterfly-shaped ring cavity composed of four mirrors, two of which are plane mirrors and the other two are mirrors with a radius of curvature of 1064nm. Plano-concave lenses.

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