A method and device for measuring high precision of ultra-stable optical reference cavity

Abstract

The invention discloses a method and device for measuring the high precision of an ultra-stable optical reference cavity. The laser output by a laser is coupled into the ultra-stable optical reference cavity through a mode matching lens group and a reflector group. The laser frequency is slowly scanned near the resonance frequency of the ultra-stable optical reference cavity. By adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity, the laser and the cavity mode are spatially matched, which greatly improves the intensity of the transmitted light signal of the ultra-stable optical reference cavity. An optical switch is placed in the optical path. When the intensity of the transmitted light signal of the ultra-stable optical reference cavity reaches a set threshold, the laser is triggered to be turned off, and an exponential decay signal is obtained at the transmission end of the ultra-stable optical reference cavity. The cavity ring-down time is obtained by fitting, and the high precision and line width of the ultra-stable cavity are obtained by numerical analysis. The high precision of the ultra-stable optical reference cavity can be measured without cavity length modulation and frequency stabilization technology. It is simple, fast, and convenient for rapid measurement.

Description

Technical Field

[0001] The present invention relates to the field of optical measurement technology, and in particular to a high-precision measurement method and device for an ultra-stable optical reference cavity. Background Art

[0002] The invention of the laser is a major achievement of science and technology in the 20th century, and narrow-linewidth lasers have become an indispensable tool in many fields such as precision spectral measurement, optical frequency standards, gravitational wave measurement, and quantum computing. At present, the main method to obtain ultra-narrow linewidth lasers is to use PDH (Pound-Drever-Hall) frequency stabilization technology to lock the free-running laser reference to a specially designed Fabry-Perot cavity. There are many parameters to describe the Fabry-Perot cavity, including free spectral range, finesse, resolution, and character dispersion. Among them, finesse has a very important guiding significance for the selection of ultra-stable optical cavities. The higher the finesse, the narrower the linewidth, and the better the effect of narrowing the linewidth of the laser to be stabilized. In the process of making ultra-stable cavities, it is usually necessary to measure the finesse of the Fabry-Perot cavity mirrors to verify whether the high-precision cavity mirrors are contaminated during the installation process and vacuum preparation process, and whether the finesse of the cavity mirrors meets the requirements.

[0003] Ultra-stable optical reference cavities with high reflectivity and high precision are also called high-fineness cavities. Traditional methods for measuring low-fineness cavities of ultra-stable optical reference cavities usually measure the reflectivity of the cavities first, such as spectrophotometry and reflectance ratio methods, which are based on measuring the ratio of light intensities, have large errors and can only give three significant figures at most. When the reflectivity exceeds 99.9%, such methods usually cannot measure very accurate reflectivity results, and therefore cannot meet people's current requirements for precision measurement, especially in ultra-precision optical measurements.

[0004] At present, the measurement of high-fineness of ultra-stable cavities usually requires cavity length modulation or frequency stabilization technology. The method based on cavity length modulation uses piezoelectric ceramics to modulate the reference cavity length so that the laser frequency and the cavity longitudinal mode are randomly coupled. This method is not easy to make the laser frequency resonate with the cavity, so the output signal amplitude of the ultra-stable optical reference cavity is small, which is not conducive to improving the measurement accuracy. There are two common measurement methods based on frequency stabilization technology: one is directly called "scanning cavity linewidth measurement method", which requires the ultra-narrow linewidth laser source that has been locked on the ultra-high fineness cavity to be injected into the Fabry-Perot cavity whose cavity mirror fineness is to be measured; the other is called "cavity ring-down measurement method", which requires the laser to be locked on the Fabry-Perot cavity through PDH frequency stabilization technology. The frequency locking process is difficult and complicated in experiments.

[0005] Therefore, considering the reality, it is very necessary to have a method to measure the parameters of a high-precision reference cavity. It is necessary to find a method to measure the high-precision of an ultra-stable optical reference cavity. This method is simple, convenient, and can be quickly applied. It is of great significance for any experimental measurement using a Fabry-Perot cavity. Summary of the invention

[0006] In view of this, in order to solve the above-mentioned problems in the prior art, the present invention proposes a method and device for measuring the high finesse of an ultra-stable optical reference cavity, which can measure the high finesse of an ultra-stable optical reference cavity without cavity length modulation and frequency stabilization technology. This method is simple and fast, and is convenient for quickly measuring the high finesse of an ultra-stable optical reference cavity.

[0007] The present invention solves the above problems by the following technical means:

[0008] In one aspect, the present invention provides a method for measuring high finesse of an ultrastable optical reference cavity, comprising the following steps:

[0009] The laser outputs laser light, which is adjusted to meet the mode matching conditions by the mode matching lens group, and the reflector group is adjusted to couple the laser light into the ultra-stable optical reference cavity. The laser frequency sweep module is turned on, and the laser frequency is slowly scanned near the resonance frequency of the ultra-stable optical reference cavity. By adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity, the laser light and the cavity mode of the ultra-stable optical reference cavity are spatially matched, thereby greatly improving the transmitted light signal intensity of the ultra-stable optical reference cavity.

[0010] An optical switch is placed in the optical path. When the intensity of the transmitted light signal of the ultra-stable optical reference cavity reaches the set threshold, the laser is triggered to turn off. Due to the loss caused by the absorption and scattering of the cavity medium or cavity mirrors, the light will gradually decay during the back-and-forth oscillation in the cavity. An exponential decay signal is obtained at the transmission end of the ultra-stable optical reference cavity. The cavity ring-down time is obtained by fitting, and the average value is taken from multiple measurements. Then, the high finesse and linewidth of the ultra-stable cavity are calculated through the formula.

[0011] Preferably, the laser outputs a single-frequency continuous laser, which enters from the center of the front cavity mirror, reflects back and forth multiple times in the ultra-stable optical reference cavity, and a photodetector is used to receive an exponential decay signal of the light intensity escaping from the transmission end of the ultra-stable optical reference cavity.

[0012] Preferably, the transmission signal of the transmission end of the ultra-stable optical reference cavity is divided into two paths, one path is provided with a camera for monitoring the resonance mode signal, and the other path is provided with a photodetector for monitoring the transmission light power intensity signal.

[0013] Preferably, the response time of the optical switch for cutting off the incident laser and the photodetector for detecting the ring-down signal are both shorter than the cavity ring-down time of the light in the ultra-stable optical reference cavity.

[0014] Preferably, the exponential decay signal obtained is a signal in the form of e exponential decay, using the formula: The fitting will obtain the cavity ring-down time τ of the high-precision ultra-stable cavity, where x is the data acquisition time, y is the acquired attenuation signal, y0 and A are constants related to the photodetector; the photon lifetime in the cavity can be measured by taking the average value of multiple measurements; and then the high-precision and linewidth parameters of the ultra-stable optical reference cavity can be obtained through theoretical calculation; the high-precision measurement is based on the following formula: F = 2πτf FSR , where f FSR is the free spectrum path of the ultra-stable optical reference cavity; then the line width of the ultra-stable optical reference cavity is:

[0015] On the other hand, the present invention provides a high-precision measurement device for an ultra-stable optical reference cavity, comprising a laser, a mode matching lens group, a reflector group, an optical switch, an ultra-stable optical reference cavity, a polarization beam splitter prism, a camera, and a photodetector;

[0016] The laser output by the laser is coupled into the ultra-stable optical reference cavity through the mode matching lens group and the reflector group in turn. The laser at the transmission end of the ultra-stable optical reference cavity is divided into two paths by the polarization beam splitter prism. One optical signal is monitored by a camera to obtain the resonance mode signal and the resonance mode signal of the ultra-stable cavity is displayed on a display screen. The other optical signal is monitored by a photodetector to obtain a transmitted light power intensity signal. The photodetector is connected to an oscilloscope to record the cavity ring-down curve to obtain an exponential decay signal. The laser sweep module is turned on to slowly scan the laser frequency near the resonance frequency of the ultra-stable optical reference cavity. By adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity, the contrast of the transmission peak becomes larger and larger.

[0017] When the frequency is slowly scanned near the resonant frequency of the ultra-stable optical reference cavity, the modes of the laser and the ultra-stable optical reference cavity resonate, and the optical switch in the optical path quickly cuts off the incident laser. The photodetector at the transmission end of the ultra-stable optical reference cavity detects the exponential decay signal of the light intensity escaping from the cavity. The cavity ring-down time is obtained by fitting, and the average value is taken after multiple measurements. The high finesse and linewidth of the ultra-stable cavity are then calculated through the formula.

[0018] Preferably, the response time of the optical switch for cutting off the incident laser and the photodetector for detecting the ring-down signal are both shorter than the cavity ring-down time of the light in the ultra-stable optical reference cavity.

[0019] Preferably, the exponential decay signal obtained is a signal in the form of e exponential decay, using the formula: The fitting will obtain the cavity ring-down time τ of the high-precision ultra-stable cavity, where x is the data acquisition time, y is the acquired attenuation signal, y0 and A are constants related to the photodetector; the photon lifetime in the cavity can be measured by taking the average value of multiple measurements; and then the high-precision and linewidth parameters of the ultra-stable optical reference cavity can be obtained through theoretical calculation; the high-precision measurement is based on the following formula: F = 2πτf FSR , where f FSR is the free spectrum path of the ultra-stable optical reference cavity; then the line width of the ultra-stable optical reference cavity is:

[0020] Preferably, the mode matching lens group includes a first convex lens and a second convex lens arranged along the optical path, and the laser output by the laser passes through the first convex lens and the second convex lens in sequence and enters the reflector group; the reflector group includes a first reflector and a second reflector arranged along the optical path, and the laser passes through the first reflector and the second reflector in sequence and enters the ultra-stable optical reference cavity through optical switch coupling.

[0021] Preferably, coarse adjustment is performed through camera observation, and by adjusting the reflector group, changing bright light is observed on the camera display; the angle of the reflector group is optimized so that the laser light passing through the ultra-stable optical reference cavity is distributed as evenly as possible, and high-order modes of the laser light can be observed.

[0022] Compared with the prior art, the beneficial effects of the present invention include at least:

[0023] 1. Simple device. The experimental optical path consists of a light source, a mode matching optical group and a pair of reflectors, avoiding a complex laser system.

[0024] 2. The experimental process is convenient and the measurement accuracy is high. The present invention does not need to modulate the cavity length and laser frequency stabilization. It slowly scans near the resonance frequency of the ultra-stable optical reference cavity, and the mode matching lens group makes the light beam and the cavity mode achieve spatial mode matching, thereby greatly improving the output signal amplitude of the ultra-stable optical reference cavity. Multiple measurements are averaged, which is conducive to improving the measurement accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following briefly introduces the drawings required for use in the description of the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0026] Figure 1 It is a flow chart of the high-fineness measurement method of the ultra-stable optical reference cavity of the present invention;

[0027] Figure 2 It is a schematic diagram of the structure of the high-precision measurement device of the ultra-stable optical reference cavity of the present invention;

[0028] Description of reference numerals:

[0029] 1. Laser; 2. First convex lens; 3. Second convex lens; 4. First reflector; 5. Second reflector; 6. Optical switch; 7. Ultra-stable optical reference cavity; 8. Polarization beam splitter; 9. Camera; 10. Photodetector. DETAILED DESCRIPTION

[0030] In order to make the above-mentioned purposes, features and advantages of the present invention more obvious and easy to understand, the technical solution of the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be pointed out that the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0031] Example 1

[0032] like Figure 1 and Figure 2 As shown, the present invention provides a method for measuring high finesse of an ultra-stable optical reference cavity, comprising the following steps:

[0033] S1, laser 1 outputs laser, and the mode matching lens group adjusts the laser to meet the mode matching conditions, adjusts the reflector group to couple the laser into the ultra-stable optical reference cavity 7, turns on the frequency scanning module of laser 1, and slowly scans the laser frequency near the resonance frequency of the ultra-stable optical reference cavity 7. By adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity 7, the laser and the cavity mode of the ultra-stable optical reference cavity 7 are spatially matched, thereby greatly improving the transmitted light signal intensity of the ultra-stable optical reference cavity 7;

[0034] S2. Place an optical switch 6 in the optical path. When the intensity of the transmitted light signal of the ultra-stable optical reference cavity reaches a set threshold, the laser is triggered to be turned off. Since the absorption and scattering of the cavity medium or cavity mirror will cause losses, the light will gradually decay during the back-and-forth oscillation in the cavity. An exponential decay signal is obtained at the transmission end of the ultra-stable optical reference cavity 7. The cavity ring-down time is obtained by fitting, and the average value is taken for multiple measurements. Then, the high finesse and line width and other parameters of the ultra-stable cavity are calculated by formula.

[0035] In step S1, laser 1 outputs single-frequency continuous laser, which enters from the center of the front cavity mirror and reflects back and forth multiple times in the ultra-stable optical reference cavity 7. The photodetector 10 receives the exponential decay signal of the light intensity escaping from the transmission end of the ultra-stable optical reference cavity 7.

[0036] In step S1, the laser output by the laser 1 needs to pass through a mode matching lens group to achieve spatial mode matching between the laser and the cavity mode, and the laser adjusted by the mode matching lens group is coupled into the ultra-stable optical reference cavity 7, and the transmission signal at the transmission end of the ultra-stable optical reference cavity 7 is divided into two paths, a camera 9 is placed on one path to monitor the resonance mode signal, and a photodetector 10 is placed on the other path to monitor the transmitted light power intensity signal.

[0037] In step S1, the laser output by the laser 1 does not need to use frequency stabilization technology to lock the free-running laser reference to the ultra-stable optical reference cavity 7, and the laser frequency is slowly scanned near the resonance frequency of the cavity. The laser is coupled into the ultra-stable optical reference cavity 7 through a short optical resonance and enhanced. The resonance mode of the laser and the ultra-stable optical reference cavity 7 and the transmission signal light power intensity signal can be observed by the observation camera 9 and the photodetector 10.

[0038] In step S2, an optical switch 6 is placed in the optical path. When the intensity of the transmitted light signal of the ultra-stable optical reference cavity 7 reaches a set threshold, the incident laser is quickly cut off to achieve attenuation of the optical signal in the cavity and obtain an exponential decay signal.

[0039] In step S2 , the response time of the optical switch 6 for cutting off the incident laser and the photodetector 10 for detecting the ring-down signal are both shorter than the cavity ring-down time of the light in the ultra-stable optical reference cavity 7 .

[0040] In step S2, the exponential decay signal obtained is a signal in the form of e exponential decay, using the formula: The fitting will obtain the cavity ring-down time τ of the high-precision ultra-stable cavity, where x is the data acquisition time, y is the acquired attenuation signal, y0 and A are constants related to the photodetector 10; the photon lifetime in the cavity can be measured by taking the average value of multiple measurements; and then the high finesse and line width parameters of the ultra-stable optical reference cavity 7 are obtained through theoretical calculation; the high finesse measurement is based on the following formula: F = 2πτf FSR , where f FSR is the free spectrum range of the ultra-stable optical reference cavity 7; then the line width of the ultra-stable optical reference cavity 7 is:

[0041] The present invention measures the finesse of the Fabry-Perot cavity by measuring the ring-down time of the resonant laser in the Fabry-Perot cavity. This method does not require cavity length modulation or a complicated frequency locking process, and the measuring device is simple and the measuring process is quick and convenient.

[0042] Example 2

[0043] like Figure 2 As shown, the present invention provides a high-precision measurement device for an ultra-stable optical reference cavity, comprising a laser 1, a mode matching lens group, a reflector group, an optical switch 6, an ultra-stable optical reference cavity 7, a polarization beam splitter prism 8, a camera 9 and a photodetector 10.

[0044] The laser output by the laser 1 is coupled into the ultra-stable optical reference cavity 7 through the mode matching lens group and the reflector group in turn. The laser at the transmission end of the ultra-stable optical reference cavity 7 is divided into two paths by the polarization beam splitter prism 8. One optical signal is monitored by the camera 9 for the resonance mode signal and the resonance mode signal of the ultra-stable cavity is displayed on the display screen (such as TEM 00 TEM mn ); One optical signal is monitored by the photodetector 10 to obtain a transmitted light power intensity signal. The photodetector 10 is connected to an oscilloscope to record the cavity ring-down curve to obtain an exponential decay signal. Rough adjustment is performed by observing with the camera 9. Generally, by adjusting the reflector group, the changing bright light can be observed on the display of the camera 9; the angle of the reflector group is optimized so that the laser passing through the ultra-stable optical reference cavity 7 is distributed as evenly as possible, and the high-order mode of the laser can be observed. Turn on the frequency scanning module of the laser 1, and slowly scan the laser frequency near the resonance frequency of the ultra-stable optical reference cavity 7 to observe the TEM of the laser-coupled supercavity. 00 The transmission signal of the ultra-stable optical reference cavity 7 can be monitored by an oscilloscope, and the contrast of the transmission peak can be increased by adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity 7.

[0045] When the frequency is slowly scanned near the resonance frequency of the ultra-stable optical reference cavity 7, the mode of the laser and the ultra-stable optical reference cavity 7 resonates, and the optical switch 6 in the optical path quickly cuts off the incident laser. The photodetector 10 is at the transmission end of the ultra-stable optical reference cavity 7 to detect the exponential decay signal of the light intensity escaping from the cavity. The cavity ring-down time is obtained by fitting, and the average value is taken after multiple measurements, and then the high finesse and line width of the ultra-stable cavity are calculated by formula.

[0046] The response time of the optical switch 6 for cutting off the incident laser and the photodetector 10 for detecting the ring-down signal of the present invention are both shorter than the cavity ring-down time of the light in the ultra-stable optical reference cavity 7 .

[0047] The exponential decay signal obtained is a signal in the form of e exponential decay, using the formula: The fitting will obtain the cavity ring-down time τ of the high-precision ultra-stable cavity, where x is the data acquisition time, y is the acquired attenuation signal, y0 and A are constants related to the photodetector 10; the photon lifetime in the cavity can be measured by taking the average value of multiple measurements; and then the high finesse and line width parameters of the ultra-stable optical reference cavity 7 are obtained through theoretical calculation; the high finesse measurement is based on the following formula: F = 2πτf FSR , where f FSR is the free spectrum range of the ultra-stable optical reference cavity 7; then the line width of the ultra-stable optical reference cavity 7 is:

[0048] In this embodiment, the mode matching lens group includes a first convex lens 2 and a second convex lens 3 arranged along the optical path, and the laser output by the laser 1 passes through the first convex lens 2 and the second convex lens 3 in sequence and enters the reflector group; the reflector group includes a first reflector 4 and a second reflector 5 arranged along the optical path, and the laser passes through the first reflector 4 and the second reflector 5 in sequence and enters the ultra-stable optical reference cavity 7 through coupling through the optical switch 6.

[0049] The device of the present invention is simple. The experimental optical path consists of a light source, a mode matching optical group and a pair of reflectors, thus avoiding a complicated laser system.

[0050] The experimental process of the present invention is convenient and has high measurement accuracy. It does not need to modulate the cavity length and laser frequency stabilization, and slowly scans near the resonance frequency of the ultra-stable optical reference cavity. The mode matching lens group makes the light beam and the cavity mode achieve spatial mode matching, thereby greatly improving the output signal amplitude of the ultra-stable optical reference cavity. The average value is taken after multiple measurements, which is conducive to improving the measurement accuracy.

[0051] The above-mentioned embodiments only express several implementation methods of the present invention, and the description thereof is relatively specific and detailed, but it cannot be understood as limiting the scope of the patent of the present invention. It should be pointed out that, for ordinary technicians in this field, several variations and improvements can be made without departing from the concept of the present invention, which all belong to the protection scope of the present invention. Therefore, the protection scope of the patent of the present invention shall be subject to the attached claims.

Claims

1. A method for measuring high finesse of an ultrastable optical reference cavity, characterized in that: The steps include: The laser outputs laser light, which is adjusted to meet the mode matching conditions by the mode matching lens group, and the reflector group is adjusted to couple the laser light into the ultra-stable optical reference cavity. The laser frequency sweep module is turned on, and the laser frequency is slowly scanned near the resonance frequency of the ultra-stable optical reference cavity. By adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity, the laser light and the cavity mode of the ultra-stable optical reference cavity are spatially matched, thereby greatly improving the transmitted light signal intensity of the ultra-stable optical reference cavity. An optical switch is placed in the optical path. When the intensity of the transmitted light signal of the ultra-stable optical reference cavity reaches the set threshold, the laser is triggered to turn off. The absorption and scattering of the cavity medium or cavity mirrors cause loss. The light will gradually decay during the back-and-forth oscillation in the cavity. An exponential decay signal is obtained at the transmission end of the ultra-stable optical reference cavity. The cavity ring-down time is obtained by fitting. The average value is taken after multiple measurements, and the high finesse and line width of the ultra-stable cavity are calculated by formula. The laser outputs a single-frequency continuous laser, which enters from the center of the front cavity mirror and reflects back and forth multiple times in the ultra-stable optical reference cavity. A photoelectric detector is used to receive the exponential decay signal of the light intensity escaping from the transmission end of the ultra-stable optical reference cavity. The transmission signal of the transmission end of the ultra-stable optical reference cavity is divided into two paths, one path is provided with a camera for monitoring the resonance mode signal, and the other path is provided with a photodetector for monitoring the transmission light power intensity signal; The exponential decay signal obtained is a signal in the form of e exponential decay, using the formula: The fitting will obtain the cavity ring-down time τ of the high-precision ultra-stable cavity, where x is the data acquisition time, y is the acquired attenuation signal, y0 and A are constants related to the photodetector; the photon lifetime in the cavity can be measured by taking the average value of multiple measurements; and then the high-precision and linewidth parameters of the ultra-stable optical reference cavity can be obtained through theoretical calculation; the high-precision measurement is based on the following formula: F = 2πτf FSR , where f FSR For ultrastable optical reference cavity The free spectrum path; then the line width of the ultra-stable optical reference cavity is:

2. The method for measuring high finesse of an ultrastable optical reference cavity according to claim 1, characterized in that: The response time of the optical switch for cutting off the incident laser and the photodetector for detecting the ring-down signal are both shorter than the cavity ring-down time of the light in the ultra-stable optical reference cavity.

3. A high-precision measurement device for an ultra-stable optical reference cavity, characterized in that: It includes laser, mode matching lens group, reflector group, optical switch, ultra-stable optical reference cavity, polarization beam splitter, camera and photodetector; The laser output by the laser is coupled into the ultra-stable optical reference cavity through the mode matching lens group and the reflector group in turn. The laser at the transmission end of the ultra-stable optical reference cavity is divided into two paths by the polarization beam splitter prism. One optical signal is monitored by a camera to obtain the resonance mode signal and the resonance mode signal of the ultra-stable cavity is displayed on a display screen. The other optical signal is monitored by a photodetector to obtain the transmitted light power intensity signal. The photodetector is connected to an oscilloscope to record the cavity ring-down curve to obtain an exponential decay signal. Turn on the laser's frequency sweep module and slowly sweep the laser frequency near the resonance frequency of the ultra-stable optical reference cavity. By adjusting the distance between the mode matching lens groups and optimizing the incident angle of the reflector group coupled into the ultra-stable optical reference cavity, the contrast of the transmission peak becomes larger and larger. When the frequency is slowly scanned near the resonance frequency of the ultra-stable optical reference cavity, the mode of the laser and the ultra-stable optical reference cavity resonates, and the optical switch in the optical path quickly cuts off the incident laser. The photodetector at the transmission end of the ultra-stable optical reference cavity detects the exponential decay signal of the light intensity escaping from the cavity. The cavity ring-down time is obtained by fitting, and the average value is taken after multiple measurements, and then the high finesse and line width of the ultra-stable cavity are calculated by formula. The exponential decay signal obtained is a signal in the form of e exponential decay, using the formula: Fitting will obtain the cavity ring-down time τ of the high-precision ultra-stable cavity, where x is the data acquisition time, y is the acquired attenuation signal, y0 and A are constants related to the photodetector; taking the average value of multiple measurements, the photon lifetime in the cavity can be measured; and then the high finesse and line width of the ultra-stable optical reference cavity can be obtained through theoretical calculations. Parameters: High-precision measurement is based on the following formula: F = 2πτf FSR , where f FSR is the free spectrum path of the ultra-stable optical reference cavity; then the line width of the ultra-stable optical reference cavity is: Rough adjustments are made through camera observation, and by adjusting the reflector group, the changing bright light can be observed on the camera display; the angle of the reflector group is optimized so that the laser light passing through the ultra-stable optical reference cavity is distributed as evenly as possible, and the high-order modes of the laser can be observed.

4. The ultra-stable optical reference cavity high-fineness measurement device according to claim 3, characterized in that: The response time of the optical switch for cutting off the incident laser and the photodetector for detecting the ring-down signal are both shorter than the cavity ring-down time of the light in the ultra-stable optical reference cavity.

5. The ultra-stable optical reference cavity high-fineness measurement device according to claim 3, characterized in that: The mode matching lens group includes a first convex lens and a second convex lens arranged along the optical path, and the laser output by the laser passes through the first convex lens and the second convex lens in sequence to enter the reflector group; the reflector group includes a first reflector and a second reflector arranged along the optical path, and the laser passes through the first reflector and the second reflector in sequence to enter the ultra-stable optical reference cavity through optical switch coupling.

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