Method for measuring equivalent cavity length of self-injection seed laser

Abstract

The invention relates to a method for measuring the equivalent cavity length of a self-injection seed laser. The method comprises a self-injection seed laser (1) and a balanced zero beat noise detection device (2). The method is characterized by comprising the following steps: firstly, measuring the intensity noise of the self-injection seed laser by using a photoelectric detector in a stable operation state of the self-injection seed laser, and reading the relaxation oscillation frequency of the laser in an intensity noise spectral line; then, according to actual parameters of a self-injection seed laser during laser intensity noise measurement, a theoretical function of the relaxation oscillation frequency of the laser is used for drawing a function curve graph with the equivalent cavity length of the laser as an independent variable and the relaxation oscillation frequency of the laser as a dependent variable; and finally, enabling the relaxation oscillation frequency value of the laser measured in the first step to be equal to the relaxation oscillation frequency value in the theoretical simulation function curve graph in the second step, and reading the value of a corresponding abscissa in the theoretical simulation function curve graph, namely the equivalent cavity length of the self-injection seed laser. The method is suitable for measuring the equivalent cavity length of the self-injection seed laser under stable operation.

Description

Technical Field

[0001] The invention relates to the field of laser technology, and specifically to a method for measuring the equivalent cavity length of a self-injection seed laser. The method is particularly suitable for measuring the equivalent cavity length of a self-injection seed laser under actual operation, and will provide theoretical reference and technical guidance for the preparation of a low-intensity noise laser. Background Art

[0002] Self-injection seed laser is a technology that optimizes the output characteristics of the laser by feeding back a part of the light output by the laser itself into the resonant cavity as a seed light source. Self-injection seed laser feeds back one arm of the bidirectional output laser path into the laser resonant cavity to form a laser intensity difference between the bidirectional lasers and realize unidirectional operation of the laser. Compared with the ring laser that uses an optical isolator to realize unidirectional operation, the self-injection end of the self-injection seed laser can compensate for the noise introduced by the loss of the output coupling mirror, which is conducive to low-intensity noise laser output. At present, the engineering preparation of low-intensity noise self-injection seed laser still faces technical difficulties. The main reason is that it is impossible to determine the equivalent cavity length of the laser required for the preparation of the engineering model of the self-injection seed laser. The equivalent cavity length of the self-injection seed laser is affected by factors such as the transmittance of the laser output coupling mirror, the optical cavity length of the self-injection feedback end, and the reflectivity of the reflector at the self-injection feedback end. At present, due to the influence of the composite cavity structure of the self-injection seed laser, the intensity noise of the output laser can only be characterized by intensity noise measurement means, resulting in the preparation of low-intensity noise self-injection seed laser can only be passively realized by manually changing the resonant cavity parameters and adjusting the resonant cavity multiple times. The actual intensity noise of the self-injection 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-injection feedback laser (also related to the transmittance of the output coupling mirror of the laser, the optical cavity length of the self-injection feedback end, and the reflectivity of the reflector at the self-injection feedback end). The characterization of the intensity noise of the self-injection seed laser requires the measurement of the equivalent cavity length of the laser. At the same time, the engineering preparation of low-intensity noise self-injection seed lasers also requires the measurement of the equivalent cavity length of the laser. To this end, the present invention develops a measurement method that can measure the equivalent cavity length of a self-injection seed laser under actual operating conditions, which can provide a reference for key parameters in the engineering preparation of low-intensity noise self-injection seed lasers.

[0003] CN201010173127.5, an acoustic resonance cavity characteristic length measurement device, uses the change in diameter of the equal-inclination interference ring of He-Ne and semiconductor laser to calculate the cavity length, and combines multi-wavelength synthesis to expand the measurement range. The defect of this method is that the device structure is complex, and the device requires extremely high parallelism of the flat crystals at both ends of the acoustic resonance cavity, which is difficult to meet with existing processing technology. Insufficient parallelism will cause the equal-inclination interference ring to deform or blur, significantly increase the difficulty of image processing, and ultimately affect the accuracy of cavity length measurement. In addition, this measurement method is only applicable to straight cavities (FP cavity structure), which greatly increases the measurement difficulty for lasers with annular cavity structures, especially for self-injected seed lasers with composite annular cavity structures (consisting of annular cavities and seed self-feedback segments).

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

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

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

[0007] (1) Use a photodetector to measure the intensity noise of the laser and read the relaxation oscillation frequency value ω of the laser from the laser intensity noise spectrum. m ;

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

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

[0010] Furthermore, taking the laser equivalent cavity length L as the independent variable and the laser relaxation oscillation frequency ω off In the dependent variable function curve, ω off It is calculated using formula (1):

[0011]

[0012] in, is the cavity decay rate caused by the laser output mirror and coupling mirror, is the cavity decay rate caused by the intracavity loss of the laser, t and δ are the output coupling mirror transmittance and the intracavity linear loss respectively, and L is the equivalent cavity length of the light in the resonant cavity of the self-injected seed laser;

[0013]

[0014] Where g is the rate of stimulated emission of light coupled between the atomic transition of the laser crystal and the laser cavity mode, σ s is the laser stimulated emission cross section, ρ lm =ρ c *c w is the density of doped 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, l is the atomic doping length of the laser crystal, and n is the refractive index of the laser crystal;

[0015]

[0016] Where α is the number of photons in the cavity, 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, j2 is the probability of the ground state particle number distribution, and j2 is expressed as: Where Γ is the pump rate and Γ is expressed as: Among them, 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, that is, the ratio of the pump light power entering the gain medium to the pump light power output by the laser diode, η a =1-exp(-αl) 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 Planck's constant, N lm is the number of doping ions used in the laser medium, expressed as: N lm =ρ lm *V m , where V m is the mode volume of the pump laser at the laser crystal, expressed as: Among them, ω p is the waist radius of the pump laser at the center of the laser crystal, λ pis the wavelength of the pumping laser.

[0017] Furthermore, the equivalent cavity length measurement of the laser crystal is carried out 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 curvature radius of R = -50mm.

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

[0019] 1. When measuring the equivalent cavity length of a self-injection seed laser, the present invention only needs to use 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. The present invention has universal applicability and is applicable to lasers with straight cavity, standing wave cavity, ring cavity structure, and composite ring cavity structure.

[0021] 3. The present invention can measure the equivalent cavity length of lasers of different power levels and different wavelength bands (visible light-mid-infrared), and can especially measure the equivalent cavity length of self-injection seed lasers in actual conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 It is a schematic diagram of the structure of a device for measuring the equivalent cavity length of a self-injection seed laser crystal according to a preferred embodiment of the present invention;

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

[0024] Figure 3 This is the intensity noise spectrum of the laser measured using a photodetector;

[0025] Figure 4 is the intensity noise spectrum obtained by measurement and the laser equivalent cavity length L as the independent variable and the laser relaxation oscillation frequency ω off The function curve diagram of the dependent variable is used to obtain a demonstration diagram of the equivalent cavity length of the self-injection seed laser under actual operating conditions. DETAILED DESCRIPTION

[0026] The following will describe the technical solutions in the embodiments of the present invention in detail in conjunction with the accompanying drawings in the embodiments of the present invention. The described embodiments are only part of the embodiments of the present invention.

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

[0028] The principle of the present invention is: in the total quantum noise theory of the self-injection seed laser, the equivalent cavity length of the laser affects the relaxation oscillation frequency of the laser by affecting the lifetime of the external feedback photons, the number of oscillation photons in the cavity of the composite laser resonant cavity, and the mutual coupling strength between the number of upper energy level inversion particles and the cavity membrane. Therefore, there is a functional correlation characteristic between the relaxation oscillation frequency of the self-injection seed laser and the equivalent cavity length of the self-injection seed laser. Under the stable operation state of the laser, the intensity noise spectrum of the laser can be measured by using a photodetector, and the laser relaxation oscillation frequency value can be read from the laser intensity noise spectrum line. According to the actual parameters of the measured self-injection seed laser, a function curve graph with the equivalent cavity length of the laser as the independent variable and the laser relaxation oscillation frequency as the dependent variable is theoretically made. Let the measured laser relaxation oscillation frequency value be the same as the laser relaxation oscillation frequency value of the theoretical function curve graph, and read the corresponding horizontal coordinate value, which is the actual equivalent cavity length of the laser crystal under this operation state.

[0029] According to the total quantum noise theoretical function of the laser, under stable operation, the relaxation oscillation frequency ω of the laser off It is expressed as:

[0030]

[0031] in, is the cavity decay rate caused by the laser output mirror and coupling mirror, is the cavity decay rate caused by the intracavity loss of the laser, t and δ are the output coupling mirror transmittance and the intracavity linear loss respectively, and L is the equivalent cavity length of the light in the resonant cavity of the self-injected seed laser;

[0032]

[0033] Where g is the rate of stimulated emission of light coupled between the atomic transition of the laser crystal and the laser cavity mode, σ s is the laser stimulated emission cross section, ρ lm =ρ c *c w is the density of doped 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, l is the atomic doping length of the laser crystal, and n is the refractive index of the laser crystal;

[0034]

[0035] Where α is the number of photons in the cavity, is the spontaneous emission rate of the lower energy level, is the spontaneous emission rate of the upper energy level, τ fis the fluorescence lifetime of the upper energy level inversion particle, j2 is the probability of the ground state particle number distribution, and j2 is expressed as: Where Γ is the pump rate and Γ is expressed as: Among them, 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 (the ratio of the pump light power entering the gain medium to the pump light power output by the laser diode, η a =1-exp(-αl) 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 Planck's constant, N lm is the number of doping ions used in the laser medium, expressed as: N lm =ρ lm *V m , where V m is the mode volume of the pump laser at the laser crystal, expressed as: Among them, ω p is the waist radius of the pump laser at the center of the laser crystal, λ p is the wavelength of the pumping laser.

[0036] It can be seen from formulas (1), (2), and (3) that, under the conditions of the self-injected seed laser pump power and cavity structure parameters being determined, the laser relaxation oscillation frequency is a function of the laser equivalent cavity length. Therefore, when the laser is operating stably (p in is a determined value), according to the actual parameters of the laser, we can get the laser equivalent cavity length L as the independent variable and the laser relaxation oscillation frequency ω off is a function graph of the dependent variable.

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

[0038] When the ω in the actual measured intensity noise spectrum m The oscillation frequency ω in the function graph off When the function curve is the same, ω m The corresponding value of the horizontal axis is the value of the laser crystal when the pump power p is injected. in The equivalent laser cavity length under this operating state.

[0039] The present invention provides a method for measuring the equivalent cavity length of a self-injection seed laser in an actual operating state, comprising the following steps:

[0040] 1. Use a photodetector to measure the intensity noise of the laser and read the relaxation oscillation frequency value ω of the laser from the laser intensity noise spectrum. m .

[0041] 2. According to the actual working parameters of the laser when measuring the laser intensity noise, the equivalent cavity length L of the laser is used as the independent variable and the laser relaxation oscillation frequency ω is used as the independent variable. off is a function graph of the dependent variable.

[0042] 3. Let the oscillation frequency ω in the second step function curve graph off is equal to ω in the intensity noise spectrum measured in the first step m , read ω in the function curve m The corresponding value of the horizontal axis is the equivalent cavity length of the laser under actual operating conditions.

[0043] Figure 1 According to the general embodiment of the present invention, a photodetector is used to measure the intensity noise of a self-injection seed laser when the laser is in stable operation. Figure 2 The figure is a schematic diagram of the structure of the equivalent cavity length measurement device of the self-injected seed laser to be measured in the implementation scheme. The specific implementation scheme of the laser crystal equivalent cavity length measurement is carried out in a self-injected seed 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 (4, 5, 6, 7), where (4) and (5) are two plane mirrors, (6) and (7) are two plano-concave lenses with a curvature radius of R = -50mm. The input coupling mirror (4) is coated with a 808nm high-transmittance film, (T 808nm >99.5%), and 1064nm high reflection film (R 1064nm >99.7%). (6) and (7) are coated with a 1064nm high reflective film (R 1064nm >99.7%). The output coupling mirror (5) is plated with a 1064nm transmittance of T 1064nm =4% film. The pump source (1) is an 808nm fiber-coupled laser diode, and the core diameter and numerical aperture of the coupling fiber are 200μm and 0.22 respectively. The pump laser (1) is focused at the center of the laser crystal (8) through the coupling system (3) with a beam waist diameter of 0.510mm. The laser crystal (8) is a 5mm piece of Nd:YVO4 (S1, S2:AR 808nm；1064nm ). The rear end of the laser crystal is cut at a small angle of 1.2° to ensure stable polarization of the laser. The self-injection seed feedback cavity mirror is a plane reflector (9) coated with a 1064nm high reflection film (R1064nm >99.7%). The geometric length between the reflection cavity mirror (9) at the injection end and the output mirror (5) is 55 mm.

[0044] The geometric cavity length of the butterfly ring 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 measured by the photodetector is as follows: Figure 3 As shown, the relaxation oscillation frequency ω of the laser m =200kHz. When the 808nm pump laser power is 2W, according to the actual laser parameters: laser crystal length L1 = 5×10 -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 constant n a =6.02×10 23 , speed of light c = 2.997 × 10 m / s, Planck constant h = 6.63 × 10 -34 The waist radius of the pump laser at the center of the laser crystal is 240×10 -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 of the laser diode P 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 doping atoms in the gain medium ρ lm =ρ c *c w =1.26*10 26 *c w According to the actual parameters of the above lasers, the equivalent cavity length L of the self-injected seed laser is taken as the independent variable, and the laser relaxation oscillation frequency ω is taken as the independent variable. off is a function graph of the dependent variable, such as Figure 4 shown. Figure 4 In the above example, let ω m =ω offWhen , the actual measured laser relaxation oscillation frequency straight line and the function curve graph produce an intersection, and the horizontal coordinate corresponding to the intersection is the equivalent cavity length of the self-injection seed laser: 522mm.

[0045] 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. A typical implementation device is a computer. Specifically, the computer may be, for example, a personal computer, a laptop computer, a cellular phone, a camera phone, a smart phone, a personal digital assistant, a media player, a navigation device, an email device, a game console, a tablet computer, a wearable device, or a combination of any of these devices.

[0046] Computer readable media include permanent and non-permanent, removable and non-removable media that can be implemented by any method or technology to store information. Information can be computer readable instructions, data structures, program modules 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 technology, compact disk read-only memory (CD-ROM), digital versatile disk (DVD) or other optical storage, magnetic cassettes, magnetic tape magnetic disk storage or other magnetic storage devices or any other non-transmission media that can be used to store information that can be accessed by a computing device. As defined herein, computer readable media does not include temporary computer readable media (transitory media), such as modulated data signals and carrier waves.

[0047] It should also be noted that the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, commodity or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, commodity or device. In the absence of more restrictions, the elements defined by the sentence "comprises a ..." do not exclude the existence of other identical elements in the process, method, commodity or device including the elements.

[0048] The above embodiments should be understood to be only used to illustrate the present invention and not to limit the protection scope of the present invention. After reading the contents of the present invention, technicians can 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 equivalent cavity length of a self-injection seed laser, characterized in that: The following steps are involved: (1) Use a photodetector to measure the intensity noise of the laser and read the relaxation oscillation frequency value ω of the laser from the laser intensity noise spectrum. m ; (2) According to the actual parameters of the laser when measuring the laser intensity noise, the theoretical function of the laser relaxation oscillation frequency is used to calculate the laser equivalent cavity length L as the independent variable and the laser relaxation oscillation frequency ω as the independent variable. off is the function graph of the dependent variable; (3) Let the relaxation oscillation frequency ω in the function curve off is equal to the relaxation oscillation frequency ω in the measured intensity noise spectrum c , read ω in the function curve m The corresponding value of the horizontal axis is the actual equivalent cavity length of the laser crystal in this operating state.

2. A method for measuring the equivalent cavity length of a self-injection seed laser according to claim 1, characterized in that: Taking the laser equivalent cavity length L as the independent variable and the laser relaxation oscillation frequency ω as the independent variable off In the dependent variable function curve, ω off It is calculated using formula (1): in, is the cavity decay rate caused by the laser output mirror and coupling mirror, is the cavity decay rate caused by the intracavity loss of the laser, t and δ are the output coupling mirror transmittance and the intracavity linear loss respectively, and L is the equivalent cavity length of the light in the resonant cavity of the self-injected seed laser; Where g is the rate of stimulated emission of light coupled between the atomic transition of the laser crystal and the laser cavity mode, σ s is the laser stimulated emission cross section, ρ lm =ρ c *c w is the density of doped 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, l is the atomic doping length of the laser crystal, and n is the refractive index of the laser crystal; Where α is the number of photons in the cavity, 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, j2 is the probability of the ground state particle number distribution, and j2 is expressed as: Where Γ is the pump rate and Γ is expressed as: Among them, 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, that is, the ratio of the pump light power entering the gain medium to the pump light power output by the laser diode, η a =1-exp(-αl) 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, v l is the output laser frequency, v p is the pump laser frequency, h is Planck's constant, N lm is the number of doping ions used in the laser medium, expressed as: N lm =ρ lm *V m , where V m is the mode volume of the pump laser at the laser crystal, expressed as: Among them, ω p is the waist radius of the pump laser at the center of the laser crystal, λ p is the wavelength of the pumping laser.

3. The method for measuring the equivalent cavity length of a self-injection seed laser according to claim 1, characterized in that: The equivalent cavity length measurement of the laser crystal is carried out 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 curvature radius of R = -50mm.

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