An intracavity loss measurement device and method for a locked cavity optical cavity

Abstract

A device and method for measuring the intracavity loss of a locked cavity optical cavity. By using the steady-state optical field distribution of a four-mirror ring optical cavity to measure the incident optical field and reflected light power on the input mirror, and according to the proportional relationship between the incident optical field and the reflected light power on the input mirror, it is possible to measure the intracavity loss of the optical cavity under the condition of cavity length locking of the optical cavity. Compared with the conventional method, the measurement method proposed by the present invention reduces other sources of additional measurement errors 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 cavity length locking of the four-mirror ring optical cavity.

Description

Technical Field

[0001] The present invention relates to a device and method for measuring the intracavity loss of a locked cavity optical cavity, belonging 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 accuracy 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, attention is paid to 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 during the propagation process, it will interact with the air, mirror surface, and nonlinear crystal in the cavity multiple times, 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 very 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 parametrically describe 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 stabilizes, and a 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 intracavity loss. Therefore, this method can achieve accurate in-situ measurement of 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 fewer.

[0004] For example, in the generation and research of optical squeezed states through optical parametric conversion, the nonlinear effect of the nonlinear crystal in the 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 conventional techniques for measuring intracavity loss in the 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, under the normal operating state of the optical cavity. Therefore, the present intracavity loss measurement technique for the locked cavity optical cavity is of great significance in the fields of optical squeezed state generation and laser technology, etc. 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, which can in-situ measure the intracavity loss of a four-mirror ring optical cavity by measuring the input light intensity and reflected light intensity at the input mirror, improving the accuracy and simplicity of measuring the intracavity loss of the four-mirror ring optical cavity.

[0006] The technical solution of the present invention is as follows:

[0007] A device for measuring intracavity loss of a locked cavity optical cavity, comprising 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.

[0008] 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 shifter and a piezoelectric ceramic controller to lock and control the cavity length of the four-mirror ring optical cavity.

[0009] 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.

[0010] Including the following expressions:

[0011]

[0012] where E r is the reflected light field of the input endoscope, r1 is the reflectivity of the first mirror, r2 is the reflectivity of the second mirror, r3 is the reflectivity of the third mirror, r4 is the reflectivity of the fourth mirror, and ε 2 is the linear loss in the optical cavity, and 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 the natural constant, i is the imaginary unit, Φ is the optical propagation phase in the optical cavity, U1 is the peak value of the voltage shown on the oscilloscope of the reflected light, U2 is the valley value of the voltage shown on the oscilloscope of the reflected light, E0 is the input light field of the input mirror, and P i is the input optical power of the first mirror, and P r is the reflected optical power of the first mirror.

[0013] A method for measuring the intracavity loss of a locked optical cavity, using the above-mentioned device for measuring the intracavity loss of a locked optical cavity, includes the following steps:

[0014] Step 1, install the pump light generation module, the optical cavity module, the cavity length locking module, and the internal loss measurement module;

[0015] Step 2, input the 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 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;

[0016] Step 3, input the reflected light at the incident mirror of the four-mirror ring optical cavity into the reflected light photodetector, record the peak value U1 of the voltage shown on the reflected light oscilloscope and the valley value U2 of the voltage, and finely adjust the four-mirror ring optical cavity to make the ratio of the peak value U1 of the voltage and the valley value U2 of the voltage under the current measurement conditions the largest;

[0017] Step 4, solve the peak value U1 of the voltage and the valley value U2 of the voltage shown on the reflected light oscilloscope, and then obtain the intracavity loss value of the four-mirror ring optical cavity through fitting and solving.

[0018] Step 4 includes: taking the ratio of the peak value U1 of the voltage and the valley value U2 of the voltage shown on the reflected light oscilloscope as the independent variable, and the intracavity loss ε 2 of the four-mirror ring optical cavity as the dependent variable for a square root fitting.

[0019] The technical effects of the present invention are as follows: A device and method for measuring the intracavity loss of a locked-cavity 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.

[0020] The characteristics of the present invention are as follows:

[0021] (1) The present invention uses 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.

[0022] (2) The present invention ensures the high precision of the intracavity loss measurement and the lock-cavity signal measurement by using a high-precision photodetector and oscilloscope.

[0023] (3) The present invention reduces technical errors and improves the precision of the intracavity measurement process by adjusting the intracavity and input finesse of the four-mirror ring optical cavity.

[0024] (4) The present invention obtains the accurate value of the in-situ intracavity loss of the four-mirror ring optical cavity from the experimental data by the 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.

[0025] (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

[0026] Figure 1 is a schematic structural diagram of a device for measuring the intracavity loss of a locked-cavity optical cavity according to the present invention.

[0027] Descriptions of the reference numerals in the drawings are 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; r1 - reflectivity of M1; r2 - reflectivity of M2; r3 - reflectivity of M3; r4 - reflectivity of M4; E o - incident optical field of the input cavity mirror; E r - reflected optical field of the input cavity mirror (including the reflected light of M1 of E o and the transmitted light of the reflected light from M4); Et - Output endoscopic transmitted light field; M1 - The first endoscope, also known as the input endoscope or incident endoscope; M2 - The second endoscope, also known as the output endoscope; M3 - The third endoscope; M4 - The fourth endoscope; PPKTP - Nonlinear crystal (using its nonlinear effect to generate squeezed light). Detailed implementation mode

[0028] The following combines the accompanying drawings ( Figure 1 ) and embodiments to illustrate the present invention.

[0029] Figure 1 is a schematic structural diagram of a device for measuring intracavity loss of a locked optical cavity for implementing the present invention. Refer to Figure 1 As shown, a device for measuring intracavity loss of a locked 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 endoscope M1 located in the upper left part, a second endoscope M2 located in the upper right part, a third endoscope M3 located in the lower left part, and a fourth endoscope M4 located in the lower right part. The second endoscope M changes the input light field E o of the input endoscope from the first endoscope M1 into an output endoscopic transmitted light field E t and a second reflected light. The third endoscope M3 changes the second reflected light into a third reflected light. The third reflected light forms squeezed light through the nonlinear crystal PPKTP and is then reflected by the fourth endoscope M4 to form a fourth reflected light, which is reflected to the first endoscope M1. The first endoscope M1 combines the transmitted light formed by the fourth reflected light with the first reflected light formed by the input light field E o formed by the input endoscope to form an input endoscope reflected light field E r and inputs it into the reflected light photodetector 7 connected to the reflected light oscilloscope 6 in the intracavity loss measurement module.

[0030] The cavity length locking module includes a transmitted light oscilloscope 10 connected to the output endoscopic transmitted light field E t through a transmitted light photodetector 9. The transmitted light oscilloscope 10 is sequentially connected to the second endoscope M2 through a phase shifter 11 and a piezoelectric ceramic controller 12 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 intensity control module 2, an isolator 3, an electro-optic modulator 4, and a mode matching lens 5 connected in sequence. The pump light forms an input light field E o after passing through the mode matching lens 5 and is connected to the input side of the first endoscope M1.

[0031] Includes the following expressions:

[0032]

[0033] Where Er is the input endoscopic reflected light field, r1 is the reflectivity of the first mirror, r2 is the reflectivity of the second mirror, r3 is the reflectivity of the third mirror, r4 is the reflectivity of the fourth mirror, ε 2 is the linear loss inside the optical cavity, Γ·P 2 is the nonlinear loss inside the optical cavity, Γ is the nonlinear 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, U1 is the peak value of the voltage shown on the oscilloscope for the reflected light, U2 is the valley value of the voltage shown on the oscilloscope for the reflected light, E0 is the input light field of the input mirror, P i is the input optical power of the first mirror, P r is the reflected optical power of the first mirror.

[0034] A method for measuring the internal loss of a locked optical cavity, using the above-mentioned device for measuring the internal loss of a locked 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, 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 from the output mirror of the four-mirror ring optical cavity, and 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 locked state; Step 3, let the reflected light at the incident mirror of the four-mirror ring optical cavity enter the reflected light photodetector, record the peak value U1 and the valley value U2 of the voltage shown on the reflected light oscilloscope, and finely adjust the four-mirror ring optical cavity to make the ratio of the peak value U1 and the valley value U2 of the voltage under the current measurement conditions the largest; Step 4, solve the peak value U1 and the valley value U2 of the voltage shown on the reflected light oscilloscope, and then obtain the internal loss value of the four-mirror ring optical cavity through fitting and solving.

[0035] In Step 4, it includes: taking the ratio of the peak value U1 and the valley value U2 of the voltage shown on the reflected light oscilloscope as the independent variable, and the internal loss ε of the four-mirror ring optical cavity 2 as the dependent variable for square root fitting.

[0036] 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.

[0037] In Step 3, it is necessary to repeatedly and coordinately adjust the mode matching lens and the four mirrors of the four-mirror ring optical cavity to improve its fineness, so that the ratio of the peak value U1 and the valley value U2 of the voltage shown on the reflected light oscilloscope measured in the experiment is the largest.

[0038] A device for measuring the intracavity loss of 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.

[0039] The measurement of the intracavity loss of the four-mirror ring optical cavity is in a locked cavity state.

[0040] By solving the optical electric field distribution at each part of the four-mirror ring optical cavity, the reflected optical electricity is obtained as:

[0041] ,

[0042] where E0 is the optical electric field input to the optical cavity, E r is the superposition of the reflected optical electric field at the input cavity mirror M1 and the transmitted optical electric field of the circulating light in the optical cavity at the cavity mirror M1, r1~r4 are the respective reflectivities of the cavity mirrors M1~M4, ε 2 is the linear loss in the optical cavity, Γ·P 2 is the non-linear loss in the optical cavity, Γ is the non-linear 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 =1, that is, there is no phase difference in the cavity.

[0043] It can be seen from equation (1) that the input light power P i of the M1 plane cavity mirror measured experimentally and the reflected light power P r have the following relationship:

[0044] ,

[0045] By taking the modulus of the right side of the equation and solving, the equations for the linear loss and nonlinear loss in the cavity can be obtained. and the reflected light power P r and the ratio P i / P r of the reflected light power to the input light power P i . Furthermore, by measuring P r / P i , the linear loss ε 2 in the cavity and the nonlinear loss Γ·P 2 can be obtained. Moreover, before installing the nonlinear crystal in the cavity, the linear loss ε 2 in the cavity can be obtained by this method, and after installing the nonlinear crystal, the nonlinear loss Γ·P 2 can be obtained by this method again. This optical cavity loss measurement method does not restrict the working state of the optical cavity, and the intracavity loss measurement can be carried out under both the cavity length scanning and cavity length locking conditions. In particular, this method can measure the intracavity loss without destroying the working conditions when the optical cavity is working normally under the cavity length locking condition, which 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.

[0046] During the experiment, the peak voltage value U1 and the valley voltage value U2 shown on the oscilloscope of the reflected light we measured are respectively proportional to the reflected light power P r at the incident mirror M1 and the input light power P i , that is, U1∝P r and U2∝P i . There is U1 / U2 =P r / P i . Therefore, the linear loss ε 2 and the nonlinear loss Γ·P 2 in the four-mirror ring optical cavity can be measured in-situ by the peak voltage value U1 and the valley voltage value U2 shown on the oscilloscope of the reflected light.

[0047] Such as Figure 1As shown in the figure, the device for measuring the intracavity loss of the optical cavity of the present invention includes four parts: a pump light generation module, an intracavity loss measurement module, an optical cavity module, and a cavity length locking module. The pump light generation module consists 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 the generated pump light with standard parameters serves as the pump light source for the four-mirror ring optical cavity. The intracavity loss measurement module consists 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 pump light with quasi-parameters 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.

[0048] 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 pump laser with standard parameters. After the pump laser with standard parameters enters the four-mirror ring optical cavity 8, it is enhanced in resonance in the four-mirror ring optical cavity 8. A part of the light is transmitted out of the four-mirror ring optical cavity 8 and enters the transmitted light photodetector 9. After 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 for the cavity length locking 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 intracavity loss of the four-mirror ring optical cavity is realized by using the reading information of the reflected light oscilloscope 6.

[0049] The specific implementation method of the present invention is as follows:

[0050] a. Install the laser, the laser light intensity control module, the isolator, the electro-optic modulator, the mode matching lens, the four-mirror ring optical cavity, the reflected light photodetector, the reflected light oscilloscope, the transmitted light photodetector, the transmitted light oscilloscope, the phase retarder, and the piezoelectric ceramic controller; turn on the laser, the laser light intensity control module, the isolator, and the electro-optic modulator, and calibrate the power of the pump laser with standard parameters;

[0051] b. Let the pump laser with standard parameters enter the four-mirror ring optical cavity to enhance the resonance of the pump laser with standard parameters in the four-mirror ring optical cavity, and transmit it out from the output cavity mirror of the four-mirror ring optical cavity, and let the transmitted light enter the transmitted light photodetector. Adjust the phase retarder and the piezoelectric ceramic controller according to the parameters of the transmitted light oscilloscope to make the four-mirror ring optical cavity in the cavity length locking state;

[0052] c. The reflected light at the incident mirror of the four-mirror ring optical cavity enters the reflected-light photodetector, and the cavity finesse is adjusted and increased so that the ratio of the peak voltage value U1 to the valley voltage value U2 shown on the reflected-light oscilloscope is maximized, and the peak voltage value U1 and the valley voltage value U2 shown on the reflected-light oscilloscope are recorded;

[0053] d. Calculate the peak voltage value U1 and the valley voltage value U2 shown on the reflected-light oscilloscope, and then obtain the intracavity loss value of the four-mirror ring optical cavity through fitting and solving.

[0054] Thus, the measurement of the intracavity loss of the locked optical cavity is achieved.

[0055] Based on the functional relationship between the incident and reflected optical electric 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 cavity length locking scheme for the four-mirror ring optical cavity is proposed to reduce the instability of cavity length locking, and a method for solving intracavity losses 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.

[0056] 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 specified 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 essential content of the present invention falls within the protection scope of the present invention.

Claims

1. A measuring device for the intracavity loss of a locked cavity optical cavity, characterized in that It includes a pump light generation module, an optical cavity module, and a cavity length locking module that are connected in sequence. The optical cavity module is connected to an in-cavity loss measurement module. The optical cavity module includes a first cavity mirror located in the upper left, a second cavity mirror located in the upper right, a third cavity mirror located in the lower left, and a fourth cavity mirror located in the lower right. The second cavity mirror changes the transmitted light of the input cavity mirror input light field 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 forms squeezed light through a nonlinear crystal and then is reflected by the fourth cavity mirror to form a fourth reflected light and reflected to the first cavity mirror. The first cavity mirror inputs the transmitted light formed by the fourth reflected light and the input cavity mirror reflected light field formed by combining 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 in-cavity loss measurement module. It includes the following expressions: Among them, E r is the reflected light field of the input endoscope, r1 is the reflectivity of the first endoscope, r2 is the reflectivity of the second endoscope, r3 is the reflectivity of the third endoscope, r4 is the reflectivity of the fourth endoscope, 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 the natural constant, i is the imaginary unit, Φ is the optical propagation phase in the optical cavity, U1 is the peak value of the voltage value displayed by the oscilloscope for the reflected light, U2 is the valley value of the voltage value displayed by the oscilloscope for the reflected light, E0 is the input light field of the input endoscope, P i is the input optical power of the first endoscope, P r is the reflected optical power of the first endoscope.

2. The intracavity loss measurement device for a locked cavity optical cavity according to claim 1, wherein The cavity length locking module includes a transmitted light oscilloscope connected to the output cavity mirror transmitted light field through a transmitted light photodetector. The transmitted light oscilloscope is sequentially connected to the second cavity mirror through a phase retarder and a piezoelectric ceramic controller to lock and control the cavity length of the four-mirror ring optical cavity.

3. The intracavity loss measurement device for a locked cavity optical cavity according to claim 1, characterized in that 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 that are 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 cavity mirror.

4. A method for measuring the intracavity loss of a locked cavity optical cavity, characterized in that, Using the in-cavity loss measurement device for a locked cavity optical cavity according to one of the above claims 1-3, it includes the following steps: Step 1, install the pump light generation module, the optical cavity module, the cavity length locking module, and the in-loss measurement module; Step 2, input the 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 from the output cavity mirror of the four-mirror ring optical cavity. The transmitted light enters the transmitted light photodetector, and the phase retarder and the piezoelectric ceramic controller are adjusted by the parameters of the transmitted light oscilloscope to make the four-mirror ring optical cavity in a cavity length locked state; Step 3, input the reflected light at the incident cavity mirror of the four-mirror ring optical cavity into the reflected light photodetector, record the peak voltage value U1 and the valley voltage value U2 displayed by the reflection light oscilloscope, and finely adjust the four-mirror ring optical cavity to make the ratio of the peak voltage value U1 and the valley voltage value U2 under the current measurement conditions the largest; Step 4, solve the peak voltage value U1 and the valley voltage value U2 displayed by the reflection light oscilloscope, and then obtain the in-cavity loss value of the four-mirror ring optical cavity through fitting and solving.

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

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