Micro-displacement measurement system and method based on reflection ball F-P cavity

Abstract

The invention discloses a micro-displacement measurement system and method based on a reflection ball F-P cavity, and the system employs a reflection ball to construct a low-fineness F-P cavity, enables a response curve of the F-P cavity to be close to a sine curve, and eliminates the multi-beam reflection in a distance measurement light path. By adopting the technical scheme of the invention, the problem of measurement error caused by multi-beam interference due to over-high reflectivity of two reflecting surfaces of the F-P cavity in the prior art can be solved; and meanwhile, the measurement precision and sensitivity are improved, and high-precision nanoscale micro-displacement measurement in a large working range can be met.

Description

Technical Field

[0001] The invention belongs to the technical field of sensors, and in particular relates to a micro-displacement measurement system and method based on a reflection ball FP cavity. Background Art

[0002] Length, as one of the seven basic physical units, plays an important role in many frontier fields such as aerospace technology, nanometer (nm) scale measurement, materials science research, ultra-precision manufacturing process, biological exploration and microelectronics industry as human beings' cognition and exploration of the world deepen. In addition, accurate measurement of length also plays a basic supporting role in the quality of high-end equipment manufacturing and plays a decisive role in the entire manufacturing process. Therefore, building nanometer-level measurement standards is a key and fundamental challenge in promoting the development of nanotechnology in the current field of metrology science. In nano-measurement technology, resolutions of hundreds of nanometers to 0.1 nanometers can be achieved, which provides key technical support for many high-precision and high-resolution requirements and constitutes an important cornerstone of nano-technology research.

[0003] At present, the methods of nano-scale measurement can be divided into non-optical and optical types. Among them, non-optical methods mainly include capacitive sensors, inductive sensors, and scanning probe microscopes. These methods have the advantages of high resolution, low power consumption, and good temperature stability. However, the measurement range of these methods is small, only tens of nm to micrometers (μm), and they are mainly used for short-distance micro-displacement measurement. In contrast, optical methods have the advantages of high precision, high resolution, small size, strong anti-electromagnetic interference ability, non-contact measurement, and traceability, showing a wider application potential. Optical methods mainly include Michelson interferometer, grating interferometer and Fabry-Perot (FP) interferometer. Among them, the Michelson and grating interferometer structures have independent reference arms and measurement arms. By measuring the difference between the two optical paths, high-precision micro-displacement measurement (nm or sub-nm level) is achieved, which requires the stability of the reference optical path to reach the nm or sub-nm level. The FP interferometer cancels the independent reference arm and integrates the reference optical path into the measurement system, effectively avoiding the influence of the instability of the reference arm on the measurement results. Nevertheless, in the existing micro-displacement measurement method based on the FP cavity, there are multiple reflections in the FP cavity, forming multi-beam interference, which leads to nonlinear errors in the ranging system. In addition, the FP cavity is usually composed of two parallel reflective film layers. In the actual installation process, the parallelism of the two reflective surfaces is required to be high, which increases the difficulty of application. Summary of the invention

[0004] The technical problem to be solved by the present invention is to provide a micro-displacement measurement method and system based on a reflecting ball FP cavity.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] The present invention also provides a micro-displacement measurement system based on a reflecting ball FP cavity, which utilizes a reflecting ball to construct a low-fineness FP cavity so that the response curve of the FP cavity to the wavelength is close to a sine curve, thereby eliminating multi-beam reflections in the ranging optical path.

[0007] The present invention also provides a micro-displacement measurement system based on a reflecting ball FP cavity, comprising: a laser, a temperature control module, an optical coupler, an acetylene gas absorption cell, a first photodetector, a demodulator, a radio frequency source, a driving current source, a circulator, a laser collimator, a beam expander lens, a reflecting ball, an optical amplifier, an optical filter, a photodetector, a power divider, a first electric amplifier, a second electric amplifier, a first mixer, a second mixer, a first low-pass filter, a second low-pass filter, and a processor.

[0008] The temperature control module is connected to the temperature control pin of the laser; the output of the laser is connected to the input of the optical coupler; the first output end of the optical coupler is connected to the input end of the acetylene gas absorption cell; the second output end of the optical coupler is connected to the first port of the circulator; the output end of the acetylene gas absorption cell is connected to the input end of the first photodetector; the output end of the first photodetector is connected to the input end of the demodulator; the output end of the demodulator is connected to the output end of the driving current source; the output end of the driving current source is connected to the current drive of the laser; the first output end of the radio frequency source is connected to the radio frequency input end of the demodulator; the second port of the circulator is connected to the input end of the laser collimator; the output end of the laser collimator is connected to the input end of the beam expander lens, and the output end of the beam expander lens is used as the transmitting end to be incident on the surface of the reflecting ball of the measured target; the third port of the circulator is connected to the input end of the optical amplifier; the output end of the optical amplifier connected to the input end of the optical filter; the output end of the optical filter is connected to the input end of the second photodetector; the output end of the second photodetector is connected to the input end of the power divider; the first output end of the power divider is connected to the input end of the first electrical amplifier; the second output end of the power divider is connected to the input end of the second electrical amplifier; the output end of the first electrical amplifier is connected to the radio frequency input end of the first mixer; the output end of the second electrical amplifier is connected to the radio frequency input end of the second mixer; the second output end of the radio frequency source is connected to the local oscillator input end of the first mixer; the third output end of the radio frequency source is connected to the local oscillator input end of the second mixer; the output end of the first mixer is connected to the input end of the first low-pass filter; the output end of the second mixer is connected to the input end of the second low-pass filter; the output end of the first low-pass filter is connected to the first input end of the processor; the output end of the second low-pass filter is connected to the second input end of the processor.

[0009] Preferably, the input end of the laser collimator is an optical fiber, and the output end is free space light.

[0010] The present invention also provides a micro-displacement measurement method based on a reflecting ball FP cavity, which uses a reflecting ball to construct a low-fineness FP cavity so that the response curve of the FP cavity to the wavelength is close to a sine curve, thereby eliminating multi-beam reflections in the ranging optical path.

[0011] The present invention also provides a micro-displacement measurement method based on a reflecting ball FP cavity, comprising:

[0012] Turn on the RF source, the output frequency is f 0 The modulation signal;

[0013] The laser output wavelength is λ 0 The laser is converted into free-space light through the laser collimator. There is about 4% end-face reflection at the output end face of the laser collimator. This part of the reflected light returns to the optical fiber of the laser collimator and returns to the second port of the circulator. This part of the light field is recorded as the reference light field.

[0014] About 96% of the spatially transmitted light emitted by the laser collimator is incident on the surface of the reflection ball through the beam expansion lens and reflected. The reflection ball is placed on the piezoelectric nanopositioning stage. Then the light field returns to the end face of the laser collimator along the original path. This part of the light field is recorded as the measurement light field. At the end face of the laser collimator, the reference light field and the measurement light field interfere with each other. The light field after interference returns to the second port of the circulator and is output from the third port of the circulator.

[0015] The photodetector converts the light signal into an electrical signal, the frequency component of which is f 0 and 2f 0 ;

[0016] The second frequency of the RF source output is f 0 The signal is used as the local oscillator input signal of the first mixer, and the third frequency output of the RF source is 2f 0 The signal is used as the local oscillator input signal of the second mixer;

[0017] Set the movement parameters of the piezoelectric nanopositioning stage so that the reflective ball vibrates in a square wave with a peak-to-peak value of 50 nm;

[0018] In the first mixer, only the local oscillator signal f 0 Only signals with the same frequency can be mixed into a DC signal. This DC signal is transmitted to the first low-pass filter. After filtering out the sum frequency signal, the signal is recorded as A. 0 ;

[0019] The second mixer only has the local oscillator signal 2f 0Only signals with the same frequency can be mixed into a DC signal. This DC signal is transmitted to the second low-pass filter. After filtering out the sum frequency signal, the signal is recorded as B 0 ;

[0020] Adjust the amplitude of the modulated signal output by the RF source so that A 0 =B 0 ;

[0021] The processor converts two DC signals A 0 and B 0 Substitute into the formula Among them, φ 1 is the initial phase of the light field when the reflecting ball does not move, arccos is the inverse trigonometric function of the cosine function;

[0022] Set the reflection ball to move a fixed distance, and the moving distance is recorded as ΔL;

[0023] After the reflecting ball moves, the output signal of the first low-pass filter is recorded as A 1 The output signal of the second low-pass filter is recorded as B 2 ;

[0024] The processor converts the two DC signals A after the reflection ball moves 1 and B 2 Substitute into the formula Among them, φ 1 is the phase of the light field after the reflection ball moves;

[0025] The processor will 0 and φ 1 Do the difference and get Δφ L =φ 1 -φ 0 , where Δφ L is the change in the phase of the light field before and after the reflection ball moves;

[0026] The processor will Δφ L Substitute into the formula The micro-displacement parameter ΔL to be measured is obtained.

[0027] Preferably, the temperature control module is connected to the temperature control pin of the laser; the output of the laser is connected to the input of the optical coupler; the first output end of the optical coupler is connected to the input end of the acetylene gas absorption cell; the second output end of the optical coupler is connected to the first port of the circulator; the output end of the acetylene gas absorption cell is connected to the input end of the first photodetector; the output end of the first photodetector is connected to the input end of the demodulator; the output end of the demodulator is connected to the output end of the driving current source; the output end of the driving current source is connected to the current drive of the laser; the first output end of the radio frequency source is connected to the radio frequency input end of the demodulator; the second port of the circulator is connected to the input end of the laser collimator; the output end of the laser collimator is connected to the input end of the beam expander lens, and the output end of the beam expander lens is used as the transmitting end to be incident on the surface of the reflecting ball of the measured target; the third port of the circulator is connected to the input end of the optical amplifier; the output end of the optical amplifier is connected to the input end of the optical amplifier; The output end of the optical filter is connected to the input end of the optical filter; the output end of the optical filter is connected to the input end of the second photodetector; the output end of the second photodetector is connected to the input end of the power divider; the first output end of the power divider is connected to the input end of the first electrical amplifier; the second output end of the power divider is connected to the input end of the second electrical amplifier; the output end of the first electrical amplifier is connected to the radio frequency input end of the first mixer; the output end of the second electrical amplifier is connected to the radio frequency input end of the second mixer; the second output end of the radio frequency source is connected to the local oscillator input end of the first mixer; the third output end of the radio frequency source is connected to the local oscillator input end of the second mixer; the output end of the first mixer is connected to the input end of the first low-pass filter; the output end of the second mixer is connected to the input end of the second low-pass filter; the output end of the first low-pass filter is connected to the first input end of the processor; the output end of the second low-pass filter is connected to the second input end of the processor.

[0028] The present invention uses a reflective ball to construct a low-fineness FP cavity, so that the response curve of the FP cavity to the wavelength should be close to a sine curve, which can effectively eliminate the multi-beam reflection in the ranging optical path and reduce the nonlinear error of the system. In addition, thanks to the symmetrical structure of the reflective ball itself, the construction of the system optical path has the characteristic of being insensitive to installation deviations. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying creative work.

[0030] Figure 1Schematic diagram of the structure of a micro-displacement measurement system based on a reflecting ball FP cavity according to an embodiment of the present invention;

[0031] Figure 2 This is a flow chart of the micro-displacement measurement method according to an embodiment of the present invention.

[0032] Figure 3 is the demodulation signal when the central wavelength of the laser output is at different positions of the acetylene gas absorption peak; Figure 3 (a) is the central wavelength of the laser output aligned with the acetylene gas absorption peak, Figure 3 (b) is the central wavelength of the laser output, which is on the left side of the acetylene gas absorption peak. Figure 3 (c) is the central wavelength of the laser output, which is on the right side of the acetylene gas absorption peak.

[0033] Figure 4 These are the experimental results when the reflecting ball moves in a square wave form with a peak-to-peak value of 50 nm; (a) is the f and 2f terms, (b) is the displacement curve, and (c) is 18 groups of micro-displacement values. DETAILED DESCRIPTION

[0034] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. 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.

[0035] In order to make the above-mentioned objects, features and advantages of the present invention more obvious and easy to understand, the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] like Figure 1 As shown, an embodiment of the present invention provides a micro-displacement measurement system based on a reflecting ball FP cavity, including: a laser 1, a temperature control module 2, an optical coupler 3, an acetylene gas absorption cell 4, a first photodetector 5, a demodulator 6, a radio frequency source 7, a driving current source 8, a circulator 9, a laser collimator 10, a beam expander lens 11, a reflecting ball 12, an optical amplifier 13, an optical filter 14, a photodetector 15, a power divider 16, a first electric amplifier 17, a second electric amplifier 18, a first mixer 19, a second mixer 20, a first low-pass filter 21, a second low-pass filter 22, and a processor 23.

[0037] The temperature control module 2 is connected to the temperature control pin of the laser 1; the output of the laser 1 is connected to the input of the optical coupler 3; the first output end of the optical coupler 3 is connected to the input end of the acetylene gas absorption cell 4; the second output end of the optical coupler 3 is connected to the first port of the circulator 9; the output end of the acetylene gas absorption cell 4 is connected to the input end of the first photodetector 5; the output end of the first photodetector 5 is connected to the input end of the demodulator 6; the output end of the demodulator 6 is connected to the output end of the driving current source 8; The output end of the driving current source 8 is connected to the current drive of the laser 1; the first output end of the radio frequency source 7 is connected to the radio frequency input end of the demodulator 6; the second port of the circulator 9 is connected to the input end of the laser collimator 10; the output end of the laser collimator 10 is connected to the input end of the beam expander lens 11, and the output end of the beam expander lens 11 is used as the transmitting end of the device and incident on the surface of the reflective ball 12 of the measured target; the third port of the circulator 9 is connected to the input end of the optical amplifier 13; the output end of the optical amplifier 13 is connected to the optical filter 14 is connected; the output end of the optical filter 14 is connected to the input end of the second photodetector 15; the output end of the second photodetector 15 is connected to the input end of the power divider 16; the first output end of the power divider 16 is connected to the input end of the first electric amplifier 17; the second output end of the power divider 16 is connected to the input end of the second electric amplifier 18; the output end of the first electric amplifier 17 is connected to the radio frequency input end of the first mixer 19; the output end of the second electric amplifier 18 is connected to the radio frequency input end of the second mixer 20; the second output end of the radio frequency source 7 is connected to the local oscillator input end of the first mixer 19; the third output end of the radio frequency source 7 is connected to the local oscillator input end of the second mixer 20; the output end of the first mixer 19 is connected to the input end of the first low-pass filter 21; the output end of the second mixer 20 is connected to the input end of the second low-pass filter 22; the output end of the first low-pass filter 21 is connected to the first input end of the processor 23; the output end of the second low-pass filter 22 is connected to the second input end of the processor 23.

[0038] The input end of the laser collimator 10 is an optical fiber, and the output end is free space light. The function of the beam expander lens 11 is to expand the laser beam to make its propagation performance better. The optical amplifier 13 is used to amplify the power of the optical signal. The optical filter 14 is used to filter out noise and improve signal quality.

[0039] like Figure 2 As shown, the embodiment of the present invention further provides a micro-displacement measurement method using a micro-displacement measurement system based on a reflection ball FP cavity, comprising:

[0040] Step 1: Turn on the RF source 7, the output frequency is f 0When the output wavelength of laser 1 is located at different positions of the absorption peak of the acetylene gas absorption cell, the phase difference of the feedback signal is 180°, such as Figure 3 As shown, the demodulator 6 determines the phase of the feedback signal to identify the output wavelength of the laser 1, thereby controlling the output current of the driving current source 8, and then feedback controls the output wavelength of the laser 1, and finally accurately controls the stability of the wavelength to achieve stable output;

[0041] Step 2: Let the central wavelength of the laser output from laser 1 be λ 0 , because the wavelength is affected by the modulation signal, the phase introduced by the wavelength change will change, and its change law is sinusoidal, expressed as: Δφ(t)=φ 0 +ΔφsinΩt, where φ 0 =2nΔL·2π / λ 0 ,φ 0 represents the additional phase introduced by the central wavelength. ΔL is the moving distance of the reflecting ball, i.e. the micro displacement to be measured, Δλ is the maximum change of wavelength, and Δφ is the maximum value of the additional phase change;

[0042] Step 3: The output wavelength of laser 1 is λ 0 The laser light is converted into free space light through the laser collimator 10. There is about 4% end face reflection at the output end face of the laser collimator 10. This part of the reflected light returns to the optical fiber of the laser collimator 10 along the original path and returns to the second port of the circulator 9. This part of the light field is recorded as the reference light field.

[0043] Step 4: About 96% of the spatially transmitted light emitted by the laser collimator 10 is incident on the surface of the reflecting ball 12 through the beam expander lens 11 and reflected. The reflecting ball 12 is placed on the piezoelectric nanopositioning stage. Then the light field returns to the end face of the laser collimator 10 along the original path. This part of the light field is recorded as the measurement light field. At the end face of the laser collimator 10, the reference light field and the measurement light field interfere with each other. The light field after interference returns to the second port of the circulator 9 and is output from the third port of the circulator 9.

[0044] Step 5: The photodetector 15 converts the optical signal into an electrical signal, the frequency component of which is f 0 and 2f 0 ;

[0045] Step 6: The second frequency output by the RF source 7 is f 0 The signal is used as the local oscillator input signal of the first mixer 19, and the third frequency output of the RF source 7 is 2f 0 The signal is used as the local oscillator input signal of the second mixer 20;

[0046] Step 7: Setting the movement parameters of the piezoelectric nanopositioning stage to make the reflective ball 12 vibrate with a square wave with a peak-to-peak value of 50 nm;

[0047] Step 8: The first mixer 19 only contains the local oscillator signal f 0 Only signals with the same frequency can be mixed into a DC signal. This DC signal is transmitted to the first low-pass filter 21. After filtering out the sum frequency signal, the signal is recorded as A. 0 ;

[0048] Step 9: The second mixer 20 only contains the local oscillator signal 2f 0 Only signals with the same frequency can be mixed into a DC signal. This DC signal is transmitted to the second low-pass filter 22. After filtering out the sum frequency signal, the signal is recorded as B 0 ;

[0049] Step 10: Adjust the amplitude of the modulated signal output by the RF source 7 so that A 0 =B 0 , thus satisfying the following relationship: 1 (Δφ)=J 2 (Δφ), where J 1 is the value of the first-order Bessel function. This step completes the optical path calibration process of the system and ensures that the amplitudes of the two orthogonal terms measured are completely equal as the distance changes.

[0050] Step 11: The processor 23 converts the two DC signals A 0 and B 0 Substitute the following formula Among them, φ 1 is the initial phase of the light field when the reflecting ball does not move, arccos is the inverse trigonometric function of the cosine function;

[0051] Step 12: Set the reflection ball 12 to move a fixed distance, and the moving distance is recorded as ΔL;

[0052] Step 13: After the reflection ball 12 moves, the output signal of the first low-pass filter 21 is recorded as A 1 The output signal of the second low-pass filter 22 is recorded as B 2 ;

[0053] Step 14: The processor 23 converts the two DC signals A after the reflection ball 12 moves 1 and B 2 Substitute the following formula Among them, φ 1 is the phase of the light field after the reflection ball moves;

[0054] Step 15: Processor 23 converts φ 0 and φ 1 Do the difference and get Δφ L =φ1 -φ 0 , where Δφ L is the change in the phase of the light field before and after the reflection ball 12 moves;

[0055] Step 16: The processor 23 converts Δφ L Substitute the following formula The micro displacement parameter ΔL to be measured can be obtained, and the result is as follows Figure 3 As shown in (a), the high and low steps of each complete square wave are averaged with the same array length, and the result is Figure 3 As shown in the white dots in (b), the adjacent values ​​are subtracted to obtain 18 sets of displacement values, such as Figure 3 As shown in (c), the average of these 18 groups of values ​​gives a micro-displacement value of 53nm measured by the final system.

[0056] The present invention has the following technical effects:

[0057] 1. Only one RF source is needed, which simplifies the structure and reduces the cost of practical application;

[0058] 2. The FP cavity is constructed using the end face of the laser collimator and the surface of the reflecting ball, and its fineness can be flexibly adjusted;

[0059] 3. The FP cavity greatly simplifies the optical alignment process and does not require adjusting the angle of the reflective surface, thus reducing the difficulty of its application;

[0060] 4. Space ranging is achieved through laser collimator and reflective ball. The device is simple and flexible, easy to implement and integrate;

[0061] 5. The low-fineness FP cavity realized by the reflecting ball makes the wavelength response curve close to the sine curve, which can effectively eliminate the multi-beam reflection in the ranging optical path and reduce the measurement error of the system.

[0062] 6. Since the low-fineness FP cavity avoids the influence of nonlinear errors, the device has the characteristics of high measurement accuracy and high sensitivity;

[0063] 7. The orthogonal demodulation algorithm ensures that the amplitudes of the two orthogonal term signals measured are completely equal as the distance changes, which can meet the measurement accuracy within a large working range.

[0064] 8. By monitoring the system output orthogonal signals, real-time display and real-time measurement can be achieved.

[0065] The present invention can solve the problem in the prior art that the reflectivity of the two reflection surfaces of the FP cavity is too high, resulting in multi-beam interference and causing measurement errors; at the same time, the technical solution provided by the present invention improves the measurement accuracy and sensitivity by utilizing the design of the reflection ball, and can meet the requirements of high-precision nanometer-level micro-displacement measurement within a large working range.

[0066] The embodiments described above are only descriptions of the preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A micro-displacement measurement system based on a reflecting ball FP cavity, characterized in that: A low-fineness FP cavity is constructed using a reflecting sphere, so that the response curve of the FP cavity to the wavelength is close to a sine curve, eliminating multi-beam reflections in the ranging optical path.

2. A micro-displacement measurement system based on a reflecting ball FP cavity, characterized in that: include: Laser, temperature control module, optical coupler, acetylene gas absorption cell, first photodetector, demodulator, radio frequency source, driving current source, circulator, laser collimator, beam expander lens, reflecting ball, optical amplifier, optical filter, photodetector, power divider, first electric amplifier, second electric amplifier, first mixer, second mixer, first low-pass filter, second low-pass filter, processor; The temperature control module is connected to the temperature control pin of the laser; the output of the laser is connected to the input of the optical coupler; the first output end of the optical coupler is connected to the input end of the acetylene gas absorption cell; the second output end of the optical coupler is connected to the first port of the circulator; the output end of the acetylene gas absorption cell is connected to the input end of the first photodetector; the output end of the first photodetector is connected to the input end of the demodulator; the output end of the demodulator is connected to the output end of the driving current source; the output end of the driving current source is connected to the current drive of the laser; the first output end of the radio frequency source is connected to the radio frequency input end of the demodulator; the second port of the circulator is connected to the input end of the laser collimator; the output end of the laser collimator is connected to the input end of the beam expander lens, and the output end of the beam expander lens is used as the transmitting end to be incident on the surface of the reflecting ball of the measured target; the third port of the circulator is connected to the input end of the optical amplifier; the output end of the optical amplifier connected to the input end of the optical filter; the output end of the optical filter is connected to the input end of the second photodetector; the output end of the second photodetector is connected to the input end of the power divider; the first output end of the power divider is connected to the input end of the first electrical amplifier; the second output end of the power divider is connected to the input end of the second electrical amplifier; the output end of the first electrical amplifier is connected to the radio frequency input end of the first mixer; the output end of the second electrical amplifier is connected to the radio frequency input end of the second mixer; the second output end of the radio frequency source is connected to the local oscillator input end of the first mixer; the third output end of the radio frequency source is connected to the local oscillator input end of the second mixer; the output end of the first mixer is connected to the input end of the first low-pass filter; the output end of the second mixer is connected to the input end of the second low-pass filter; the output end of the first low-pass filter is connected to the first input end of the processor; the output end of the second low-pass filter is connected to the second input end of the processor.

3. The micro-displacement measurement system based on the reflecting ball FP cavity as claimed in claim 2, characterized in that: The input end of the laser collimator is optical fiber, and the output end is free space light.

4. A micro-displacement measurement method based on a reflecting ball FP cavity, characterized in that: A low-fineness FP cavity is constructed using a reflecting sphere, so that the response curve of the FP cavity to the wavelength is close to a sine curve, eliminating multi-beam reflections in the ranging optical path.

5. A micro-displacement measurement method based on a reflecting ball FP cavity, characterized in that: include: Turn on the RF source to output a modulated signal with a frequency of f0; The laser output wavelength λ0 is converted into free space light through the laser collimator. There is about 4% end face reflection at the output end face of the laser collimator. This part of the reflected light returns to the optical fiber of the laser collimator and returns to the second port of the circulator. This part of the light field is recorded as the reference light field. About 96% of the spatially transmitted light emitted by the laser collimator is incident on the surface of the reflection ball through the beam expansion lens and reflected. The reflection ball is placed on the piezoelectric nanopositioning stage. Then the light field returns to the end face of the laser collimator along the original path. This part of the light field is recorded as the measurement light field. At the end face of the laser collimator, the reference light field and the measurement light field interfere with each other. The light field after interference returns to the second port of the circulator and is output from the third port of the circulator. The photodetector converts the optical signal into an electrical signal, the frequency components of which are f0 and 2f0; The second signal with a frequency of f0 output by the RF source is used as the local oscillator input signal of the first mixer, and the third signal with a frequency of 2f0 output by the RF source is used as the local oscillator input signal of the second mixer; Set the movement parameters of the piezoelectric nanopositioning stage so that the reflective ball vibrates in a square wave with a peak-to-peak value of 50 nm; In the first mixer, only the signal with the same frequency as the local oscillator signal f0 can be mixed into a DC signal, and this DC signal is transmitted to the first low-pass filter. After filtering out the sum frequency signal, the signal is recorded as A0; In the second mixer, only the signal with the same frequency as the local oscillator signal 2f0 can be mixed into a DC signal, which is transmitted to the second low-pass filter. After filtering out the sum frequency signal, the signal is recorded as B0. Adjust the amplitude of the modulation signal output by the RF source so that A0 = B0; The processor puts the two DC signals A0 and B0 into the formula Among them, φ1 is the initial phase of the light field when the reflecting ball does not move, arccos is the inverse trigonometric function of the cosine function; Set the reflection ball to move a fixed distance, and the moving distance is recorded as ΔL; After the reflecting ball moves, the output signal of the first low-pass filter is recorded as A1, and the output signal of the second low-pass filter is recorded as B2; The processor puts the two DC signals A1 and B2 after the reflection ball moves into the formula Among them, φ1 is the phase of the light field after the reflection ball moves; The processor takes the difference between φ0 and φ1 and obtains Δφ L =φ1-φ0, where Δφ L is the change in the phase of the light field before and after the reflection ball moves; The processor will Δφ L Substitute into the formula The micro-displacement parameter ΔL to be measured is obtained.

6. The micro-displacement measurement method based on the reflecting ball FP cavity as claimed in claim 5, characterized in that: The temperature control module is connected to the temperature control pin of the laser; the output of the laser is connected to the input of the optical coupler; the first output end of the optical coupler is connected to the input end of the acetylene gas absorption cell; the second output end of the optical coupler is connected to the first port of the circulator; the output end of the acetylene gas absorption cell is connected to the input end of the first photodetector; the output end of the first photodetector is connected to the input end of the demodulator; the output end of the demodulator is connected to the output end of the driving current source; the output end of the driving current source is connected to the current drive of the laser; the first output end of the radio frequency source is connected to the radio frequency input end of the demodulator; the second port of the circulator is connected to the input end of the laser collimator; the output end of the laser collimator is connected to the input end of the beam expander lens, and the output end of the beam expander lens is used as the transmitting end to be incident on the surface of the reflecting ball of the measured target; the third port of the circulator is connected to the input end of the optical amplifier; the output end of the optical amplifier connected to the input end of the optical filter; the output end of the optical filter is connected to the input end of the second photodetector; the output end of the second photodetector is connected to the input end of the power divider; the first output end of the power divider is connected to the input end of the first electrical amplifier; the second output end of the power divider is connected to the input end of the second electrical amplifier; the output end of the first electrical amplifier is connected to the radio frequency input end of the first mixer; the output end of the second electrical amplifier is connected to the radio frequency input end of the second mixer; the second output end of the radio frequency source is connected to the local oscillator input end of the first mixer; the third output end of the radio frequency source is connected to the local oscillator input end of the second mixer; the output end of the first mixer is connected to the input end of the first low-pass filter; the output end of the second mixer is connected to the input end of the second low-pass filter; the output end of the first low-pass filter is connected to the first input end of the processor; the output end of the second low-pass filter is connected to the second input end of the processor.

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