Device and method for measuring loss in optical cavity of lock cavity

Abstract

The invention relates to a device and a method for measuring the loss in a lock cavity optical cavity, which are characterized in that the incident light field and the reflected light power of an input cavity mirror are measured by using the steady-state light field distribution of a four-mirror annular optical cavity, and the loss in the optical cavity can be measured under the condition that the cavity length of the optical cavity is locked according to the proportional relation between the incident light field and the reflected light power of the input cavity mirror. Compared with a conventional method, the measurement method provided by the invention reduces other additional measurement error sources, and can be used for realizing stable and accurate measurement of intra-cavity loss under the condition of cavity length locking of the four-mirror annular optical cavity in a low-frequency high-compression-degree compressed light generation process.

Description

Technical Field

[0001] The present invention relates to a device and method for measuring the intracavity loss of a locked optical cavity, and belongs to the technical field of optical detection. Background Art

[0002] Optical cavity technology is widely used in laser technology, quantum optical communication and processing, cavity optomechanics, and quantum precision measurement. The measurement technology of optical cavity loss helps to evaluate and improve optical cavity technology, improve the photon energy information transmission efficiency and lifetime, improve the performance of lasers, and the precision of quantum precision measurement communication, etc. The intracavity loss of an optical cavity can be roughly divided into geometric loss, diffraction loss, diaphragm loss, and other losses. In a complete optical resonator, there are many factors causing intracavity loss, but when measuring the loss of the entire optical resonator, the focus is on the loss of the entire resonator. The intracavity loss in an optical cavity is one of the important evaluation parameters for evaluating the performance of an optical cavity. In a high-reflectivity optical cavity, Gaussian light circulates and oscillates in the cavity, and will interact with the air, mirror, and nonlinear crystal in the cavity multiple times during the propagation process, and a certain amount of energy will be lost each time, resulting in loss. The four-mirror ring optical cavity is mainly applied in the field of quantum optics, such as the generation of optical squeezed states, the preparation of high-energy long-lifetime single photons, and the mode selection and shaping of beam quality. Usually, it is difficult for the conventional intracavity loss measurement technology of an optical cavity to achieve in-situ measurement of the actual intracavity loss under the condition of cavity length locking of the optical cavity, that is, under the normal working state of the optical cavity. To describe the formation process of the intracavity loss of the four-mirror ring optical cavity, we need to establish an ideal four-mirror ring optical cavity model and parameterize the propagation process therein. The four-mirror ring optical cavity in this study can be regarded as an optical reflector with high reflectivity and low absorption. After the circulating light in the cavity stabilizes, its optical and electric field distribution is stable, and the stable intracavity loss is formed after the intracavity absorption, scattering, and nonlinear effects stabilize.

[0003] Since the four-mirror ring optical cavity is in a cavity length locking state when it is working normally, this method can still measure the incident optical power and reflected optical power of the incident light at the incident mirror of the four-mirror ring optical cavity with high precision under the locked cavity state, and then realize the measurement of the intracavity loss. Therefore, this method can achieve accurate in-situ measurement of the intracavity loss under the condition of cavity length locking of the four-mirror ring optical cavity. And compared with other measurement methods, this measurement method can measure the actual intracavity loss when the optical cavity is actually working, and because the measurement is carried out at the same input mirror, the error sources of this method are less.

[0004] For example, in the generation and research of optical squeezed states through optical parametric conversion, the nonlinear effect of a nonlinear crystal in an optical cavity is utilized to generate squeezed light. The problem of intracavity loss in the optical cavity is very important for the generation of squeezed light. Thus, it can be seen that in-situ measurement of intracavity loss in the optical cavity is of great significance. However, the usual techniques for measuring intracavity loss in an optical cavity are difficult to achieve in-situ measurement of the actual intracavity loss under the condition of locking the cavity length of the optical cavity, that is, in the normal operating state of the optical cavity. Therefore, the present intracavity loss measurement technique for a locked cavity optical cavity is of great significance in the fields such as the generation of optical squeezed states and laser technology. Summary of the Invention

[0005] The technical problem to be solved by the present invention: Provide a device and method for measuring intracavity loss of a locked cavity optical cavity. By measuring the input light intensity and reflected light intensity at the input mirror, in-situ measurement of intracavity loss in a four-mirror ring optical cavity is carried out, improving the accuracy and simplicity of measuring intracavity loss in the four-mirror ring optical cavity.

[0006] The technical solution of the present invention is as follows: A device for measuring intracavity loss of a locked cavity optical cavity includes a pump light generation module, an optical cavity module, and a cavity length locking module connected in sequence. The optical cavity module is connected to an intracavity loss measurement module. The optical cavity module includes a first mirror located in the upper left, a second mirror located in the upper right, a third mirror located in the lower left, and a fourth mirror located in the lower right. The second mirror changes the transmitted light of the input cavity mirror input light field from the first mirror into an output cavity mirror transmitted light field and a second reflected light. The third mirror changes the second reflected light into a third reflected light. After the third reflected light forms squeezed light through a nonlinear crystal, it is reflected by the fourth mirror to form a fourth reflected light and reflected to the first mirror. The first mirror inputs the input cavity mirror reflected light field formed by combining the transmitted light formed by the fourth reflected light and the first reflected light formed by the input cavity mirror input light field into the reflected light photodetector connected to the reflection light oscilloscope in the intracavity loss measurement module.

[0007] The cavity length locking module includes a transmission light oscilloscope connected to the output cavity mirror transmitted light field through a transmission light photodetector. The transmission light oscilloscope is sequentially connected to the second mirror through a phase retarder and a piezoelectric ceramic controller to lock and control the cavity length of the four-mirror ring optical cavity.

[0008] The pump light generation module includes a laser, a laser light intensity control module, an isolator, an electro-optic modulator, and a mode matching lens connected in sequence. The pump light forms an input cavity mirror input light field after passing through the mode matching lens and is connected to the input side of the first mirror.

[0009] Including the following expressions: , where Er is the reflected light field of the input endoscope, r 1 is the reflectivity of the first mirror, r 2 is the reflectivity of the second mirror, r 3 is the reflectivity of the third mirror, r 4 is the reflectivity of the fourth mirror, ε 2 is the linear loss inside the optical cavity, Γ·P 2 is the non - linear loss inside the optical cavity, Γ is the non - linear coefficient, P is the circulating optical power inside the cavity, e is the natural constant, i is the imaginary unit, Φ is the optical propagation phase inside the optical cavity, U 1 is the peak value of the voltage shown on the oscilloscope of the reflected light, U 2 is the valley value of the voltage shown on the oscilloscope of the reflected light, E 0 is the input light field of the input endoscope, P i is the input optical power of the first mirror, P r is the reflected optical power of the first mirror.

[0010] A method for measuring the internal loss of a locked - cavity optical cavity, using the above - mentioned device for measuring the internal loss of a locked - cavity optical cavity, includes the following steps: Step 1, install the pump - light generation module, the optical - cavity module, the cavity - length locking module and the internal - loss measurement module; Step 2, input the standard - parameter pump laser into the four - mirror ring - shaped optical cavity to enhance the resonance of the standard - parameter pump laser in the four - mirror ring - shaped optical cavity, and make it transmit from the output mirror of the four - mirror ring - shaped optical cavity. The transmitted light enters the transmitted - light photodetector, and adjust the phase - delay device and the piezoelectric - ceramic controller by the parameters of the transmitted - light oscilloscope to make the four - mirror ring - shaped optical cavity in the cavity - length locked state; Step 3, input the reflected light at the incident mirror of the four - mirror ring - shaped optical cavity into the reflected - light photodetector, and record the peak value U 1 of the voltage shown on the reflected - light oscilloscope and the valley value U 2 , finely adjust the four - mirror ring - shaped optical cavity to make the ratio of the peak value U 1 and the valley value U 2 of the voltage under the current measurement condition the largest; Step 4, calculate the peak value U 1 and the valley value U 2 of the voltage shown on the reflected - light oscilloscope, and then obtain the internal - loss value of the four - mirror ring - shaped optical cavity through fitting and solving.

[0011] In Step 4, it includes: taking the ratio of the peak value U 1 and the valley value U 2 of the voltage shown on the reflected - light oscilloscope as the independent variable, and the internal loss ε 2 of the four - mirror ring - shaped optical cavity as the dependent variable for square - root fitting.

[0012] The technical effects of the present invention are as follows: A device and method for measuring the intracavity loss of a locked optical cavity according to the present invention utilize the steady-state optical field distribution of a four-mirror ring optical cavity to measure the incident optical field and reflected light power of the input cavity mirror. According to the proportional relationship between the incident optical field and reflected light power of the input cavity mirror, it is possible to measure the intracavity loss of the optical cavity under the condition of locking the cavity length of the optical cavity. Compared with the conventional method, the measurement method proposed by the present invention reduces other additional sources of measurement error and can be used in the process of generating low-frequency and high-compression squeezed light to achieve stable and accurate measurement of the intracavity loss under the condition of locking the cavity length of the four-mirror ring optical cavity.

[0013] The characteristics of the present invention are as follows: (1) The present invention utilizes a phase retarder to perform phase delay on the input signal, improving the stability of the cavity length locking of the four-mirror ring optical cavity and making the change of the intracavity loss in the working state more stable.

[0014] (2) The present invention ensures the high accuracy of intracavity loss measurement and cavity locking signal measurement by using high-precision photodetectors and oscilloscopes.

[0015] (3) The present invention reduces technical errors and improves the accuracy of the intracavity measurement process by adjusting the intracavity and input finesse of the four-mirror ring optical cavity.

[0016] (4) The present invention obtains the accurate value of the in-situ intracavity loss of the four-mirror ring optical cavity from experimental data through a fitting method. The measurement method itself has few error sources and the fitting method eliminates other errors in the experiment, ensuring the accuracy of the measurement results.

[0017] (5) The present invention can perform stable, accurate and rapid in-situ measurement of the intracavity loss under the condition of locking the cavity length of the four-mirror ring optical cavity. Description of the Drawings

[0018] Figure 1 It is a schematic structural diagram of a device for measuring the intracavity loss of a locked optical cavity according to the present invention.

[0019] Description of the reference numerals in the drawings is as follows: 1 - laser; 2 - laser light intensity control module; 3 - isolator; 4 - electro-optic modulator; 5 - mode matching lens; 6 - reflected light oscilloscope; 7 - reflected light photodetector; 8 - four-mirror ring optical cavity; 9 - transmitted light photodetector; 10 - transmitted light oscilloscope; 11 - phase retarder; 12 - piezoelectric ceramic controller; r 1 -M 1 reflectivity; r 2 -M 2 reflectivity; r 3 - M 3 reflectivity; r 4 -M 4Reflectivity; E o - Input cavity mirror input optical field; E r - Input cavity mirror reflected optical field (including the M o reflected light of E and the transmitted light of the reflected light from M 1 ); E 4 - Output cavity mirror transmitted optical field; M t - The first cavity mirror is also called the input cavity mirror or the incident cavity mirror; M 1 - The second cavity mirror is also called the output cavity mirror; M 2 - The third cavity mirror; M 3 - The fourth cavity mirror; PPKTP - nonlinear crystal (using its nonlinear effect to generate squeezed light). 4 - The fourth cavity mirror; PPKTP - nonlinear crystal (using its nonlinear effect to generate squeezed light). Specific implementation mode

[0020] The present invention will be described below with reference to the accompanying drawings ( Figure 1 ) and embodiments.

[0021] Figure 1 is a schematic structural diagram of a device for measuring the intracavity loss of a locked - cavity optical cavity. Referring to Figure 1 as shown, a device for measuring the intracavity loss of a locked - cavity optical cavity includes a pump - light generation module, an optical - cavity module, and a cavity - length locking module connected in sequence. The optical - cavity module is connected to an intracavity - loss measurement module. The optical - cavity module includes a first cavity mirror M 1 located in the upper left, a second cavity mirror M 2 located in the upper right, a third cavity mirror M 3 located in the lower left, and a fourth cavity mirror M 4 located in the lower right. The second cavity mirror M changes the transmitted light of the input cavity - mirror input optical field E 1 from the first cavity mirror M o into the output cavity - mirror transmitted optical field E t and a second reflected light. The third cavity mirror M3 changes the second reflected light into a third reflected light. The third reflected light forms squeezed light through the nonlinear crystal PPKTP and then forms a fourth reflected light reflected by the fourth cavity mirror M4 to the first cavity mirror M1. The first cavity mirror M 1 inputs the transmitted light formed by the fourth reflected light and the input - cavity - mirror reflected optical field E o formed by the combination of the first reflected light formed by the input cavity - mirror input optical field E r into the reflected - light photodetector 7 connected to the reflected - light oscilloscope 6 in the intracavity - loss measurement module.

[0022] The cavity - length locking module includes a transmitted - light photodetector 9 connected to the output cavity - mirror transmitted optical field E tThe transmission light oscilloscope 10, and the transmission light oscilloscope 10 is connected to the second mirror M through a phase retarder 11 and a piezoelectric ceramic controller 12 in sequence 2 , so as to lock and control the cavity length of the four-mirror ring optical cavity. The pump light generation module includes a laser 1, a laser light intensity control module 2, an isolator 3, an electro-optic modulator 4, and a mode matching lens 5 that are connected in sequence. After the pump light passes through the mode matching lens 5, an input cavity mirror input optical field E is formed o is connected to the input side of the first mirror M 1 .

[0023] including the following expressions: , where E r is the input cavity mirror reflected optical field, r 1 is the reflectivity of the first mirror, r 2 is the reflectivity of the second mirror, r 3 is the reflectivity of the third mirror, r 4 is the reflectivity of the fourth mirror, ε 2 is the linear loss in the optical cavity, Γ·P 2 is the nonlinear loss in the optical cavity, Γ is the nonlinear coefficient, P is the intracavity circulating optical power, e is the natural constant, i is the imaginary unit, Φ is the optical propagation phase in the optical cavity, U 1 is the peak value of the voltage value displayed by the reflection light oscilloscope, U 2 is the valley value of the voltage value displayed by the reflection light oscilloscope, E 0 is the input cavity mirror input optical field, P i is the input optical power of the first mirror, P r is the reflected optical power of the first mirror.

[0024] A method for measuring the intracavity loss of a cavity-locked optical cavity, using the above-mentioned device for measuring the intracavity loss of a cavity-locked optical cavity, includes the following steps: Step 1, install a pump light generation module, an optical cavity module, a cavity length locking module, and an internal loss measurement module; Step 2, input a standard parameter pump laser into the four-mirror ring optical cavity to enhance the resonance of the standard parameter pump laser in the four-mirror ring optical cavity, and transmit it out from the output mirror of the four-mirror ring optical cavity, so that the transmitted light enters a transmission light photodetector, and adjust the phase retarder and the piezoelectric ceramic controller by the parameters of the transmission light oscilloscope, so that the four-mirror ring optical cavity is in a cavity length locked state; Step 3, input the reflected light at the incident mirror of the four-mirror ring optical cavity into a reflection light photodetector, and record the peak value U of the voltage value displayed by the reflection light oscilloscope 1 and the valley value U of the voltage value 2 , finely adjust the four-mirror ring optical cavity so that the peak value U of the voltage value under the current measurement conditions 1 and the valley value U of the voltage value 2has the largest ratio; Step 4, calculate the peak value U of the voltage value displayed by the reflected light oscilloscope 1 and the valley value U of the voltage value 2 , and then obtain the intracavity loss value of the four-mirror ring optical cavity through fitting and solving.

[0025] Step 4 includes: taking the ratio of the peak value U of the voltage value displayed by the reflected light oscilloscope 1 and the valley value U of the voltage value 2 as the independent variable, and the intracavity loss ε of the four-mirror ring optical cavity 2 as the dependent variable for square root fitting.

[0026] In Step 2, finely adjust the phase delay amount of the phase retarder to achieve precise control of the locked cavity length of the four-mirror ring optical cavity.

[0027] In Step 3, it is necessary to repeatedly and coordinately adjust the mode matching lens and the four cavity mirrors of the four-mirror ring optical cavity to improve its fineness, so that the ratio of the peak value U of the voltage value displayed by the reflected light oscilloscope measured in the experiment 1 and the valley value U of the voltage value 2 is the largest.

[0028] An intracavity loss measurement device for a locked cavity optical cavity, the device includes a laser, a laser intensity control module, an isolator, an electro-optic modulator, a mode matching lens, a four-mirror ring optical cavity, a reflected light photodetector, a reflected light oscilloscope, a transmitted light photodetector, a transmitted light oscilloscope, a phase retarder, and a piezoelectric ceramic controller; the laser, the laser intensity control module, the isolator, the electro-optic modulator, and the mode matching lens form a pump light generation module; the standard parameter pump laser enters the four-mirror ring optical cavity and resonates and enhances in the four-mirror ring optical cavity; the transmitted light photodetector, the transmitted light oscilloscope, the phase retarder, and the piezoelectric ceramic controller form a cavity length locking module; the resonantly enhanced laser in the four-mirror ring optical cavity is transmitted from the incident cavity mirror and converges with the reflected light of the incident laser into the reflected light photodetector, and the in-situ measurement of the intracavity loss of the four-mirror ring optical cavity is realized by using the reading information of the reflected light oscilloscope; the standard parameter pump laser enters the transmitted light photodetector after passing through the four-mirror ring optical cavity, the transmitted light photodetector is connected to the transmitted light oscilloscope, the phase retarder performs phase delay through the indication of the transmitted light oscilloscope, and then inputs to the piezoelectric ceramic controller to realize the cavity length locking of the four-mirror ring optical cavity; the control signal of the piezoelectric ceramic controller is provided by the phase retarder, and the precise value of the intracavity loss of the four-mirror ring optical cavity can be quickly calculated through the ratio of the reflected light power to the incident light power displayed by the oscilloscope.

[0029] The intracavity loss measurement of the four-mirror ring optical cavity is in a locked cavity state.

[0030] The reflected photoelectric field is obtained by solving the distribution of the photoelectric field at various places in the four-mirror ring optical cavity: , where E 0 is the photoelectric field input to the optical cavity, and E r is the superposition of the reflected photoelectric field at the input mirror M 1 and the transmitted photoelectric field of the circulating light in the optical cavity at the mirror M 1 . r 1 ~r 4 are the respective reflectivities of the mirrors M 1 ~M 4 , ε 2 is the linear loss in the optical cavity, Γ·P 2 is the nonlinear loss in the optical cavity, Γ is the nonlinear coefficient, P is the circulating light power in the cavity, Φ is the phase of the light propagation in the optical cavity. For a closed cavity, the phase factor e iΦ of the light propagation in the optical cavity is 1, that is, there is no phase difference in the cavity.

[0031] It can be seen from Equation (1) that the input light power P 1 of the plane mirror input in the experimental measurement and the reflected light power P i have the following relationship: r , By taking the modulus of the right side of the equation and solving, the linear loss and nonlinear loss equations in the cavity and the ratio P r of the reflected light power P i to the input light power P r / P i can be obtained. Furthermore, the linear loss ε r / P i in the cavity can be obtained by measuring P 2 and the nonlinear loss Γ·P 2 . Moreover, before installing the nonlinear crystal in the cavity, the linear loss ε 2 in the cavity can be obtained by this method. After installing the nonlinear crystal, the nonlinear loss Γ·P 2 can be obtained by this method again. This method for measuring the loss of the optical cavity does not restrict the working state of the optical cavity, and the loss in the cavity can be measured under both the cavity length scanning and cavity length locking conditions. In particular, this method can measure the loss in the cavity without destroying the working conditions when the cavity length is locked, that is, when the optical cavity is working normally. It belongs to an in-situ measurement method in the normal working state of the optical cavity. The application of this method has many application scenarios, which can improve the efficiency of optical cavity adjustment and testing and is of great significance for the research on improving the performance of squeezed light.

[0032] ​During the experiment, the peak voltage value U 1 and the valley voltage value U 2 measured by us are respectively proportional to the reflected light power P r at the incident cavity mirror M1 and the input light power P i , that is, U 1 ∝P r and U 2 ∝P i , so there is U 1 / U 2 =P r / P i . Therefore, the in-situ measurement of the linear loss ε 1 and the nonlinear loss Γ·P 2 inside the four-mirror ring optical cavity can be carried out through the peak voltage value U 2 and the valley voltage value U 2 displayed by the reflected light oscilloscope.

[0033] As Figure 1 shown, the in-cavity loss measurement device of the locked cavity optical cavity of the present invention includes four parts: a pump light generation module, an in-cavity loss measurement module, an optical cavity module, and a cavity length locking module. The pump light generation module is composed of a laser 1, a laser light intensity control module 2, an isolator 3, an electro-optic modulator 4, and a mode matching lens 5, and generates a standard parameter pump light as the pump light source of the four-mirror ring optical cavity. The in-cavity loss measurement module is composed of a reflected light oscilloscope 6 and a reflected light photodetector 7 to collect the optical cavity reflected light signal. The optical cavity module includes a four-mirror ring optical cavity 8 to enhance the resonance of the generated quasi-parameter pump light in the cavity. Its main structure includes four cavity mirrors and a nonlinear crystal PPKTP. The cavity length locking module includes a transmitted light photodetector 9, a transmitted light oscilloscope 10, a phase retarder 11, and a piezoelectric ceramic controller 12 to lock and control the cavity length of the four-mirror ring optical cavity.

[0034] The laser 1, the laser light intensity control module 2, the isolator 3, the electro-optic modulator 4, and the mode matching lens 5 generate a standard parameter pump laser; the standard parameter pump laser enters the four-mirror ring optical cavity 8 and is enhanced in resonance in the four-mirror ring optical cavity 8; a part of the light is transmitted from the four-mirror ring optical cavity 8 and enters the transmitted light photodetector 9, and the lock cavity signal is input through the transmitted light oscilloscope 10, and the signal is input to the piezoelectric ceramic controller 12 after the phase retarder 11 performs phase delay to lock the cavity length of the four-mirror ring optical cavity 8; the laser enhanced in resonance in the four-mirror ring optical cavity is transmitted from the incident cavity mirror and converges with the reflected light of the incident laser to enter the reflected light photodetector 7, and the rapid and accurate calculation of the in-cavity loss of the four-mirror ring optical cavity is realized by using the reading information of the reflected light oscilloscope 6.

[0035] The specific implementation process of the present invention is as follows: a. Install a laser, a laser intensity control module, an isolator, an electro-optic modulator, a mode matching lens, a four-mirror ring optical cavity, a reflected light photodetector, a reflected light oscilloscope, a transmitted light photodetector, a transmitted light oscilloscope, a phase retarder, and a piezoelectric ceramic controller; turn on the laser, the laser intensity control module, the isolator, and the electro-optic modulator, and calibrate the standard parameter pump laser power; b. Let the standard parameter pump laser enter the four-mirror ring optical cavity to enhance the resonance of the standard parameter pump laser in the four-mirror ring optical cavity, and transmit it out from the output mirror of the four-mirror ring optical cavity. Let the transmitted light enter the transmitted light photodetector, and adjust the phase retarder and the piezoelectric ceramic controller by the parameters of the transmitted light oscilloscope to make the four-mirror ring optical cavity in the cavity length locking state; c. Let the reflected light at the incident mirror of the four-mirror ring optical cavity enter the reflected light photodetector, and adjust to improve the cavity finesse, so that the peak value U 1 of the voltage value displayed by the reflected light oscilloscope and the valley value U 2 of the voltage value have the largest ratio, and record the peak value U 1 of the voltage value and the valley value U 2 of the voltage value displayed by the reflected light oscilloscope; d. Calculate the peak value U 1 of the voltage value and the valley value U 2 displayed by the reflected light oscilloscope, and then obtain the intracavity loss value of the four-mirror ring optical cavity through fitting and solving.

[0036] Thus, the measurement of the intracavity loss of the locked optical cavity is realized.

[0037] Based on the functional relationship between the incident and reflected optical fields at the input mirror of the optical cavity and the linear and nonlinear losses in the cavity, the present invention realizes the measurement of the intracavity loss of the locked optical cavity under the condition of cavity length locking. A four-mirror ring optical cavity cavity length locking scheme is proposed to reduce the instability of cavity length locking. An intracavity loss calculation method is proposed to reduce the error of intracavity loss measurement and improve the measurement accuracy. The applied fitting method eliminates other error factors in the experimental process. The rapid and accurate in-situ measurement of the intracavity loss of the four-mirror ring optical cavity under the condition of cavity length locking is realized.

[0038] The content not described in detail in the specification of the present invention belongs to the prior art well-known to those skilled in the art. It is hereby pointed out that the above description helps those skilled in the art to understand the present invention, but does not limit the protection scope of the present invention. Any implementation that is an equivalent replacement, modification, improvement, and / or simplification of the above description without departing from the essence of the present invention falls within the protection scope of the present invention.

Claims

1. A device for measuring intracavity loss in a cavity-locked optical cavity, characterized in that: The invention comprises a pump light generating module, an optical cavity module and a cavity length locking module which are connected in sequence. The optical cavity module is connected to an intracavity loss measuring module. The optical cavity module comprises a first cavity mirror located at the upper left, a second cavity mirror located at the upper right, a third cavity mirror located at the lower left, and a fourth cavity mirror located at the lower right. The second cavity mirror changes the input cavity mirror input light field transmitted light from the first cavity mirror into an output cavity mirror transmitted light field and a second reflected light. The third cavity mirror changes the second reflected light into a third reflected light. The third reflected light is formed into compressed light by a nonlinear crystal and then formed into a fourth reflected light by a fourth cavity mirror and reflected to the first cavity mirror. The first cavity mirror combines the transmitted light formed by the fourth reflected light with the first reflected light formed by the input cavity mirror input light field to form an input cavity mirror reflected light field, which is input into a reflected light photodetector connected to a reflected light oscilloscope in the intracavity loss measuring module.

2. The device for measuring intracavity loss of a cavity-locked optical cavity according to claim 1, characterized in that: The cavity length locking module includes a transmitted light oscilloscope connected to the transmitted light field of the output cavity mirror through a transmitted light photodetector, and the transmitted light oscilloscope is connected to the second cavity mirror through a phase delay device and a piezoelectric ceramic controller in turn to lock and control the cavity length of the four-mirror annular optical cavity.

3. The device for measuring intracavity loss of a cavity-locked optical cavity according to claim 1, characterized in that: The pump light generating module comprises a laser, a laser light intensity control module, an isolator, an electro-optic modulator and a mode matching lens which are connected in sequence. After the pump light passes through the mode matching lens, it forms an input cavity mirror input light field which is connected to the input side of the first cavity mirror.

4. The device for measuring intracavity loss of a cavity-locked optical cavity according to claim 1, characterized in that: Include the following expressions: ， Where E r is the reflected light field of the input cavity mirror, r1 is the reflectivity of the first cavity mirror, r2 is the reflectivity of the second cavity mirror, r3 is the reflectivity of the third cavity mirror, r4 is the reflectivity of the fourth cavity mirror, ε 2 is the linear loss in the optical cavity, Γ·P 2 is the nonlinear loss in the optical cavity, Γ is the nonlinear coefficient, P is the circulating optical power in the cavity, e is a natural constant, i is an imaginary unit, Φ is the phase of light propagation in the optical cavity, U1 is the peak voltage value displayed by the reflected light oscilloscope, U2 is the valley voltage value displayed by the reflected light oscilloscope, E0 is the input optical field of the input cavity mirror, and P i is the optical power input to the first cavity mirror, P r is the light power reflected by the first cavity mirror.

5. A method for measuring intracavity loss in a cavity-locked optical cavity, characterized in that: The device for measuring intracavity loss of a cavity-locked optical cavity according to any one of claims 1 to 4 comprises the following steps: Step 1, installing a pump light generation module, an optical cavity module, a cavity length locking module and an internal loss measurement module; Step 2, introducing a standard parameter pump laser into a four-mirror annular optical cavity so that the standard parameter pump laser is resonated and enhanced in the four-mirror annular optical cavity, and is transmitted from the output cavity mirror of the four-mirror annular optical cavity, so that the transmitted light enters a transmitted light photodetector, and the phase delay device and the piezoelectric ceramic controller are adjusted by the parameters of the transmitted light oscilloscope, so that the four-mirror annular optical cavity is in a cavity length locked state; Step 3, the reflected light at the incident cavity mirror of the four-mirror annular optical cavity enters the reflected light photodetector, the voltage peak value U1 and the voltage valley value U2 displayed by the reflected light oscilloscope are recorded, and the four-mirror annular optical cavity is fine-tuned to maximize the ratio of the voltage peak value U1 to the voltage valley value U2 under the current measurement conditions; Step 4, calculating the voltage peak value U1 and the voltage valley value U2 displayed by the reflected light oscilloscope, and then obtaining the intra-cavity loss value of the four-mirror annular optical cavity by fitting.

6. The method for measuring intracavity loss of a cavity-locked optical cavity according to claim 5, characterized in that: Step 4 includes: taking the ratio of the voltage peak value U1 and the voltage valley value U2 displayed by the reflected light oscilloscope as the independent variable, and the intracavity loss ε of the four-mirror annular optical cavity 2 A squared square fit was performed as the dependent variable.

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