CN116735156A - A multi-degree-of-freedom telescope testing system - Google Patents

Abstract

The invention is a multi-degree-of-freedom telescope testing system, including: laser, acousto-optic modulator, beam splitter, transverse beam splitter, plane reflector, half-wave plate, quarter-wave plate, pyramid retroreflector, Half mirror, photodetector, four-quadrant detector and digital phase meter; with the help of heterodyne interferometry, combined with heterodyne interference signal, differential wavefront phase detection signal, differential power detection signal, it provides high-precision measurement with multiple degrees of freedom. The changes in the structure of the sample to be tested in each degree of freedom are obtained. The interferometer used in this method has a compact structure, high precision and high efficiency. In multiple degrees of freedom, it effectively ensures the high-precision requirements for telescope structural stability testing.

Description

A multi-degree-of-freedom telescope testing system

Technical field

The invention relates to a multi-degree-of-freedom telescope testing system, which can measure structural changes of a sample to be tested in multiple degrees of freedom with high efficiency and high precision.

Background technique

In order to successfully complete space measurement tasks, spaceborne optical devices, such as telescopes, need to be highly stable, lightweight, and resistant to deformation to cope with external influences such as temperature changes and vibrations and ensure measurement accuracy. Therefore, it is particularly important to pre-test the structural stability of spaceborne optical devices. Optical interferometers are often used as the first choice for testing the structural stability of telescopes due to their high precision and high signal-to-noise ratio. The test light paths currently used are either limited to single degree of freedom direction measurement, or are limited by measurement accuracy. Therefore, it is necessary to study a testing method that is compact, efficient, high-precision and has multiple degrees of freedom.

Therefore, this high-precision, multi-degree-of-freedom test method is proposed. With the help of a transverse beam splitter, the reference light and the measurement light travel along the same path as much as possible, thereby eliminating common mode noise as much as possible; combined with the differential wavefront phase detection signal, The differential power detection signal and phase information can simultaneously measure changes in the structure of the sample to be tested in various degrees of freedom, which is highly efficient; in addition, the combination of retroreflectors, quarter-wave plates and half mirrors can ensure While the optical path structure is compact, it can effectively measure changes in the rolling freedom of the telescope. Therefore, this testing system can well meet the structural stability testing requirements of spaceborne optical devices and ensure efficient and accurate testing.

Contents of the invention

The technical problem solved by this invention is: in view of the current existing technology, common structural stability testing methods have insufficient accuracy and are limited to a single degree of freedom. The existing solutions are optimized and a high-precision, efficient method is proposed. Multi-degree-of-freedom telescope testing method.

The present invention solves the above technical problems by implementing the following technical solutions:

A multi-degree-of-freedom telescope testing system, including a first laser, a second laser, a first acousto-optic modulator, a second acousto-optic modulator, a first beam splitter, a second beam splitter, a third beam splitter, The first transverse beam splitter, the second transverse beam splitter, the third transverse beam splitter, the first plane reflector, the second plane reflector, the third plane reflector, a half-wave plate, a quarter-wave plate, Corner retroreflector, half mirror, first photodetector, second photodetector, first four-quadrant detector, second four-quadrant detector and digital phase meter;

The first laser and the second laser each output a laser beam with a wavelength of 1064nm. After passing through the first acousto-optic modulator and the second acousto-optic modulator respectively, a reference light with a frequency of f ₁ and a frequency of f ₂ are obtained. The light is measured, after which both beams enter the interferometer;

The reference light with frequency f ₁ , output from the first laser and frequency-shifted by the first acousto-optic modulator, first enters the first beam splitter. After being split by the beam splitter, it is divided into a reflection component and a transmission component; the transmission component is incident on the first beam splitter. A transverse beam splitter, after being transmitted and reflected by the first transverse beam splitter, the emitted light is two beams of parallel light; after the two beams are transmitted by the second beam splitter, they are reflected by the second plane mirror and will be incident on the second beam splitter again. beam mirror; after that, the two beams are reflected by the second beam splitter to the second transverse beam splitter, and then are received by the first four-quadrant detector and the first photodetector after two more reflections; the reflected component will pass through the half-wave The beam is incident on the first plane mirror, and then reflected to the third beam splitter through the third transverse beam splitter; after the beam passes through the third beam splitter, the transmitted component is received by the second four-quadrant detector; its reflected component is received by the second four-quadrant detector. Photodetector reception;

The measuring light with frequency f ₂ is output by the second laser and then frequency-shifted by the second acousto-optic modulator. It is first transmitted and reflected by the first transverse beam splitter to form two parallel components; both beams are transmitted through the second beam splitter. Then, the component reflected from the first transverse beam splitter to the second beam splitter is reflected by the third plane mirror and then enters the second beam splitter again, and then is reflected to the second transverse beam splitter through the second beam splitter. After being transmitted by the second transverse beam splitter, it is received by the first photodetector; the component transmitted from the first transverse beam splitter to the second beam splitter will first pass through the half mirror, and then a part of the beam will be reflected back to the second beam splitter through the half mirror. The beam splitter is then reflected by the second beam splitter to the second transverse beam splitter, and then transmitted through the second transverse beam splitter and received by the first four-quadrant detector; the other component passes through the half mirror to the retroreflector; The beam component incident on the retroreflector, after being reflected by the retroreflector, will pass through the quarter-wave plate and enter the second beam splitter again, and then be reflected by the beam splitter to the third beam splitter; the beam passes through the third beam splitter. After the beam splitter, the transmitted component will be received by the second photodetector, and the reflected component will be received by the second four-quadrant detector; the light beams incident on the first photodetector and the first four-quadrant detector will interfere respectively to form a light beat. Thereafter, they are respectively received by the first photodetector and the first four-quadrant detector; the light beams incident on the second photodetector and the second four-quadrant detector will respectively interfere to form light beats, and are thereafter respectively received by the second photodetector and the second four-quadrant detector. Four-quadrant detector reception;

The signals output by the first photodetector, the second photodetector, the first four-quadrant detector, and the second four-quadrant detector are input into the digital phase meter after analog-to-digital conversion. After processing, the phase information and differential Wavefront phase detection signal and differential power detection signal;

The obtained differential wavefront phase detection signal is processed to obtain the change θ _y of the sample in the azimuth direction and the change θ _x in the pitch direction; by processing the differential power detection signal, the change Δx of the sample in the x-axis and the change in the elevation direction θ x are obtained The change of the y-axis Δy; by processing the output phase results, the change of the sample in the axial direction Δz and the change of the rolling degree of freedom θ _z are obtained; thus, the sample to be tested has three translational degrees of freedom and three rotational degrees of freedom. The deformation and deflection are detected to achieve the purpose of measuring the stability of multi-degree-of-freedom structures.

The frequency difference Δf between the reference light and the measurement light is =f ₁ -f ₂ =1 MHz.

The beam splitter and transverse beam splitter used are non-polarizing beam splitters, and the transmission-reflection ratio is 50:50.

A half mirror and a quarter-wave plate are respectively adhered to the surface of the retroreflector, each occupying half of the area.

The third plane reflector and the retroreflector are respectively fixed at both ends of the sample to be measured, so that the size changes of the sample to be measured along each degree of freedom can be measured.

The phase information, differential wavefront phase detection signal and differential power detection signal are obtained after the processing, including:

The light beat signals received by the four detection pixels on the surface of the first four-quadrant detector are processed by the digital phase meter and four phase results can be obtained, which are recorded as Φ _A , Φ _B , Φ _C and Φ _D respectively; the first Total phase results for a four-quadrant detector can be defined as:

Calculate the differential wavefront phase detection signal; the differential wavefront phase detection signal is used to measure the wavefront deflection angle of the two beams, and can be divided into a horizontal signal DWS _h and a vertical signal DWS _v . The calculation method is:

By area integrating the received light intensity, the average optical power received by each quadrant of the second and fourth quadrant detectors is obtained.

where Z is the dielectric impedance; E _R is the reference optical electric field component; E _M is the measured optical electric field component; record the average power of each quadrant as/> DPS signals can be divided into horizontal and vertical signals, denoted as DPS _h and DPS _v respectively, defined as:

The change of the sample in the azimuth direction θ _y and the change in the elevation direction θ _x are obtained by processing the differential wavefront phase detection signal; the change of the sample in the x-axis Δx and the change in the y-axis Δy are obtained by processing the differential power detection signal ; The change in the axial direction of the sample Δz and the change in the rolling degree of freedom θ _z are obtained by processing the output phase results. The specific calculation steps are as follows:

The axial change Δz is the phase signal obtained by the first photodetector and the first four-quadrant detector. and/> Find:

Δx, Δy are obtained from the differential power detection signal; due to the use of a retroreflector to reflect the beam, when the sample structure changes in the horizontal or vertical direction, the position of the retroreflector will change accordingly, causing the measurement beam to The position on the surface of the quadrant detector moves, causing the DPS signal to change; for small-scale changes, there are:

Δx=C ₁ ·DPS _h

Δy＝C ₂ ·DPS _v

Among them, C ₁ and C ₂ are proportional coefficients;

θ _y , θ _x are obtained from the differential wavefront phase detection signal; for a small range of offset, the DWS signal is proportional to the corresponding deflection angle, so it can be obtained:

θ _y ∝DWS _h

θ _x ∝DWS _v

The differential wavefront phase detection signal can be calibrated in advance to find the relationship between the differential wavefront phase detection signal and the variation θ, so that the variation can be calculated in subsequent measurements;

The calculation of θ _z is as follows: when the sample to be tested changes along the rolling direction, the retroreflector will rotate accordingly, which will drive the quarter-wave plate on the surface of the retroreflector to rotate; therefore, the fast axis of the quarter-wave plate The angle with the horizontal direction will change; if the laser incident on the quarter-wave plate is linearly polarized light, when the polarization direction of the incident polarized light forms a certain angle α with the fast axis of the quarter-wave plate, and When , the emitted light is elliptically polarized light; when the quarter-wave plate is rotated, the ellipsometry of the emitted elliptically polarized light will change accordingly; for a small range of angle changes, the declination angle and ellipsometry are approximately linear, so Available:

k is the proportional coefficient, a is the semi-major axis of elliptical polarization, and b is the minor semi-axis of elliptical polarization; the reference light reflected by beam splitter 1 is linearly polarized light, and the half-wave plate is rotated so that the polarization direction of the reference light is consistent with the length of the elliptical polarization light. The axis is at an angle of 45°, and the polarization direction can be obtained by a polarization measuring instrument; at this time, the phase signal change obtained by the second photodetector can be written as:

where a is the semi-major axis of the elliptical polarized light; b is the semi-minor axis of the elliptical polarized light; γ is the angle between the polarization direction of the reference light and the major axis of the elliptically polarized light; is the common mode part; when the initial angle is 45°, the phase change caused by polarization is:

That is to say, the phase change is equal to the change of ellipticity; and because the angle change is small, it can be regarded that the phase change is equal to the change of ellipticity during the measurement process; thus it can be obtained:

in Obtained from the first photodetector, subtract/> The noise introduced by the acousto-optic modulator can be removed, thus obtaining Then we get θ _z .

The advantages of the present invention compared with the prior art are:

(1) A multi-degree-of-freedom telescope structural stability testing method provided by the present invention uses heterodyne interferometry, combined with phase information, differential wavefront phase detection (DWS) signals and differential power detection (DPS) signals, to measure the Measuring the structural changes of the sample in three translational degrees of freedom and three rotational degrees of freedom has the characteristics of high precision and efficiency;

(2) The present invention adopts a method of combining a retroreflector, a half mirror and a quarter-wave plate, so that while meeting the measurement needs, the optical path design is more compact and the occupied volume is reduced, thus obtaining More stable optical path structure. Effectively improve the stability of the measurement system and its ability to respond to environmental changes.

Description of drawings

Figure 1 is a flow chart of the present invention;

Figure 2 is a schematic diagram of the optical path of the present invention;

Figure 3 is a top view of a schematic diagram of the optical path of the present invention;

Specific implementation methods

A multi-degree-of-freedom telescope testing system of the present invention includes a first laser, a second laser, a first acousto-optic modulator, a second acousto-optic modulator, a first beam splitter, a second beam splitter, and a third beam splitter. Mirror, first transverse beam splitter, second transverse beam splitter, third transverse beam splitter, first plane reflector, second plane reflector, third plane reflector, half-wave plate, quarter-wave plate plate, pyramid retroreflector, half mirror, first photodetector, second photodetector, first four-quadrant detector, second four-quadrant detector and digital phase meter;

First, the retroreflector RR and plane mirror M3 are fixed at both ends of the sample to be measured, and then the reference light and measurement light are introduced by lasers 1 and 2 respectively. The reference light and the measurement light pass through the acousto-optic modulator respectively, thereby obtaining a frequency difference of 1MHz.

The reference light first enters the beam splitter BS1 and is divided into a reflection component and a transmission component after being split by the beam splitter. The transmitted component is incident on the transverse beam splitter LBS1, and after being transmitted and reflected by the transverse beam splitter LBS1, the emergent light is two beams of parallel light. After the two beams are transmitted through the beam splitter BS2, they are reflected by the plane mirror M2 and will enter the beam splitter BS2 again. After that, the two beams are reflected by the beam splitter BS2 to the transverse beam splitter LBS2, and are received by the four-quadrant detector QPD1 and the photodetector PD1 respectively after two reflections; the reflection component formed by the reference light after passing through the beam splitter BS1 will It is incident on the plane mirror M1 through the half-wave plate HWP, and then reflected to the beam splitter BS3 through the transverse beam splitter LBS3. After the beam passes through the beam splitter BS3, the transmitted component is received by the four-quadrant detector QPD2; its reflected component is received by the photodetector PD2;

The measurement light is first transmitted and reflected by the transverse beam splitter LBS1 to form two parallel components. After the two light beams pass through the beam splitter BS2, the component reflected from the transverse beam splitter LBS1 to the beam splitter BS2 is reflected by the plane mirror M3 and then enters the beam splitter BS2 again, and then is reflected by the beam splitter BS2 to the transverse beam splitter LBS2. After being transmitted through the transverse beam splitter BS2, it is received by the photodetector PD1; the component transmitted from the transverse beam splitter LBS1 to the beam splitter BS2 will first pass through the half mirror SRM, and then a part of the beam will be reflected back to the BS beam splitter through the half mirror 2. Then it is reflected by the beam splitter BS2 to the transverse beam splitter LBS2, and then transmitted through the transverse beam splitter LBS2 and received by the four-quadrant detector QPD1; the other component passes through the half mirror to the retroreflector RR; the incident retroreflector The beam component, after being reflected by the retroreflector, will pass through the quarter wave plate QWP and enter the beam splitter BS2 again, and will then be reflected by the beam splitter to the beam splitter BS3. After the beam passes through the beam splitter BS3, the transmitted component will be received by the photodetector PD2; the reflected component will be received by the four-quadrant detector QPD2;

The light beams incident on the photodetector PD1 and the four-quadrant detector QPD1 will interfere respectively to form a light beat, which will then be received by the photodetector PD1 and the four-quadrant detector QPD1 respectively; the light beam incident on the photodetector PD2 and the four-quadrant detector QPD2 The light beats will be formed by interference respectively, and will then be received by the photodetector PD2 and the four-quadrant detector QPD2 respectively;

After processing by the phase meter, the phase information, differential wavefront phase detection DWS signal and differential power detection DPS signal can be obtained. The differential wavefront phase detection signal is used to measure the wavefront deflection angle of the two beams, and can be divided into horizontal signal DWS _h and vertical signal The direct signal DWS _v is calculated as:

The differential power detection DPS signal is divided into horizontal and vertical signals, which are recorded as DPS _h and DPS _v respectively, and are defined as:

Δz can be obtained from the phase signal of photodetector PD1 and four-quadrant detector QPD1 and/> Find:

θ _y , θ _x can be obtained by differential wavefront phase detection of DWS signals. For a small range of offset, the DWS signal is proportional to the corresponding deflection angle, so we can get:

θ _y ∝DWS _h

θ _x ∝DWS _v

The differential wavefront phase detection signal can be calibrated in advance to find the relationship between the differential wavefront phase detection signal and the change amount θ, so that the change amount can be calculated in subsequent measurements; Δx, Δy can be obtained from the differential power detection DPS signal, For small-scale changes, there are:

Δx=C ₁ ·DPS _h

Δy＝C ₂ ·DPS _v

C ₁ and C ₂ are proportional coefficients; when the sample to be tested changes along the rolling direction, for a small range of angular changes, the declination angle and ellipsoidality have an approximately linear relationship, so it can be obtained:

k is the proportional coefficient, a is the major semi-axis of the elliptical polarization, and b is the minor semi-axis of the elliptical polarization. The reference light reflected by the beam splitter BS1 is linearly polarized light. The half-wave plate is rotated so that the polarization direction of the reference light is at an angle of 45° with the long axis of the elliptically polarized light. The polarization direction can be obtained by a polarization measuring instrument. At this time, the phase signal change obtained by detector PD2 can be written as:

where a is the semi-major axis of the elliptical polarized light; b is the semi-minor axis of the elliptical polarized light; γ is the angle between the polarization direction of the reference light and the major axis of the elliptically polarized light; For the common mode part. When the initial angle is 45°, the phase changes caused by polarization are:

That is, the phase change is equal to the ellipsoidality change; and because the angle change is small, it can be regarded that the phase change is equal to the ellipsoidality change during the measurement process. Therefore:

Can be obtained from detector PD1, minus/> The noise introduced by the acousto-optic modulator can be removed, so we can get/> Then we get θ _z .

From this, the changes Δx, Δy, Δz of the sample to be measured in the x, y, and z directions can be obtained; as well as the changes θ _y , θ _x , θ _z in the azimuth, pitch, and rolling directions.

Claims (7)

1. A multi-degree-of-freedom telescope testing system, characterized in that it includes a first laser, a second laser, a first acousto-optic modulator, a second acousto-optic modulator, a first beam splitter, a second beam splitter, The third beam splitter, the first transverse beam splitter, the second transverse beam splitter, the third transverse beam splitter, the first plane reflector, the second plane reflector, the third plane reflector, the half-wave plate, the fourth Half-wave plate, corner pyramid retroreflector, half mirror, first photodetector, second photodetector, first four-quadrant detector, second four-quadrant detector and digital phase meter; The first laser and the second laser each output a laser beam. After passing through the first acousto-optic modulator and the second acousto-optic modulator respectively, a reference light with a frequency of f ₁ and a measurement light with a frequency of f ₂ are obtained. After that, Both beams enter the interferometer; The reference light with frequency f ₁ , output from the first laser and frequency-shifted by the first acousto-optic modulator, first enters the first beam splitter. After being split by the beam splitter, it is divided into a reflection component and a transmission component; the transmission component is incident on the first beam splitter. A transverse beam splitter, after being transmitted and reflected by the first transverse beam splitter, the emitted light is two beams of parallel light; after the two beams are transmitted by the second beam splitter, they are reflected by the second plane mirror and will be incident on the second beam splitter again. beam mirror; after that, the two beams are reflected by the second beam splitter to the second transverse beam splitter, and then are received by the first four-quadrant detector and the first photodetector after two more reflections; the reflected component will pass through the half-wave The beam is incident on the first plane mirror, and then reflected to the third beam splitter through the third transverse beam splitter; after the beam passes through the third beam splitter, the transmitted component is received by the second four-quadrant detector; its reflected component is received by the second four-quadrant detector. Photodetector reception; The measuring light with frequency f ₂ is output by the second laser and then frequency-shifted by the second acousto-optic modulator. It is first transmitted and reflected by the first transverse beam splitter to form two parallel components; both beams are transmitted through the second beam splitter. Then, the component reflected from the first transverse beam splitter to the second beam splitter is reflected by the third plane mirror and then enters the second beam splitter again, and then is reflected to the second transverse beam splitter through the second beam splitter. After being transmitted by the second transverse beam splitter, it is received by the first photodetector; the component transmitted from the first transverse beam splitter to the second beam splitter will first pass through the half mirror, and then a part of the beam will be reflected back to the second beam splitter through the half mirror. The beam splitter is then reflected by the second beam splitter to the second transverse beam splitter, and then transmitted through the second transverse beam splitter and received by the first four-quadrant detector; the other component passes through the half mirror to the retroreflector; The beam component incident on the retroreflector, after being reflected by the retroreflector, will pass through the quarter-wave plate and enter the second beam splitter again, and then be reflected by the beam splitter to the third beam splitter; the beam passes through the third beam splitter. After the beam splitter, the transmitted component will be received by the second photodetector, and the reflected component will be received by the second four-quadrant detector; the light beams incident on the first photodetector and the first four-quadrant detector will interfere respectively to form a light beat. Thereafter, they are respectively received by the first photodetector and the first four-quadrant detector; the light beams incident on the second photodetector and the second four-quadrant detector will respectively interfere to form light beats, and are thereafter respectively received by the second photodetector and the second four-quadrant detector. Four-quadrant detector reception; The signals output by the first photodetector, the second photodetector, the first four-quadrant detector, and the second four-quadrant detector are input into the digital phase meter after analog-to-digital conversion. After processing, the phase information and differential Wavefront phase detection signal and differential power detection signal; The obtained differential wavefront phase detection signal is processed to obtain the change θ _y of the sample in the azimuth direction and the change θ _x in the pitch direction; by processing the differential power detection signal, the change Δx of the sample in the x-axis and the change in the elevation direction θ x are obtained The change of the y-axis Δy; by processing the output phase results, the change of the sample in the axial direction Δz and the change of the rolling degree of freedom θ _z are obtained; thus, the sample to be tested has three translational degrees of freedom and three rotational degrees of freedom. The deformation and deflection are detected to achieve the purpose of measuring the stability of multi-degree-of-freedom structures. 2. A multi-degree-of-freedom telescope testing system according to claim 1, characterized in that: the frequency difference between the reference light and the measurement light is Δf=f ₁ -f ₂ =1 MHz. 3. A multi-degree-of-freedom telescope testing system according to claim 1, characterized in that: the beam splitter and the transverse beam splitter used are non-polarizing beam splitters, and the transmission-reflection ratio is 50:50. 4. A multi-degree-of-freedom telescope testing system according to claim 1, characterized in that: a half mirror and a quarter-wave plate are respectively adhered to the surface of the retroreflector, each occupying half of the area. 5. A multi-degree-of-freedom telescope testing system according to claim 1, characterized in that: the third plane reflector and the retroreflector are respectively fixed at both ends of the sample to be tested, thereby measuring the parameters of the sample to be tested. Variation of dimensions along each degree of freedom. 6. A multi-degree-of-freedom telescope testing system according to claim 1, characterized in that: the processed phase information, differential wavefront phase detection signal and differential power detection signal are obtained, including: The light beat signals received by the four detection pixels on the surface of the first four-quadrant detector are processed by the digital phase meter and four phase results can be obtained, which are recorded as Φ _A , Φ _B , Φ _C and Φ _D respectively; the first Total phase results for a four-quadrant detector can be defined as: Calculate the differential wavefront phase detection signal; the differential wavefront phase detection signal is used to measure the wavefront deflection angle of the two beams, and can be divided into a horizontal signal DWS _h and a vertical signal DWS _v . The calculation method is: By area integrating the received light intensity, the average optical power received by each quadrant of the second and fourth quadrant detectors is obtained. where Z is the dielectric impedance; E _R is the reference optical electric field component; E _M is the measured optical electric field component; record the average power of each quadrant as/> DPS signals can be divided into horizontal and vertical signals, denoted as DPS _h and DPS _v respectively, defined as: 7. A multi-degree-of-freedom telescope testing system according to claim 6, characterized in that: the change θ _y of the sample in the azimuth direction and the change θ _x in the pitch direction are obtained by processing the differential wavefront phase detection signal. ; The change of the sample in the x-axis Δx and the change in the y-axis Δy are obtained through the differential power detection signal; the change in the axial direction of the sample Δz and the change in the rolling degree of freedom θ _z are obtained by processing the output phase results, specifically The calculation steps are as follows: The axial change Δz is the phase signal obtained by the first photodetector and the first four-quadrant detector. and/> Find: Δx, Δy are obtained from the differential power detection signal; due to the use of a retroreflector to reflect the beam, when the sample structure changes in the horizontal or vertical direction, the position of the retroreflector will change accordingly, causing the measurement beam to The position on the surface of the quadrant detector moves, causing the DPS signal to change; for small-scale changes, there are: Δx=C ₁ ·DPS _h Δy＝C ₂ ·DPS _v Among them, C ₁ and C ₂ are proportional coefficients; θ _y , θ _x are obtained from the differential wavefront phase detection signal; for a small range of offset, the DWS signal is proportional to the corresponding deflection angle, so it can be obtained: θ _y ∝DWS _h θ _x ∝DWS _v The differential wavefront phase detection signal can be calibrated in advance to find the relationship between the differential wavefront phase detection signal and the variation θ, so that the variation can be calculated in subsequent measurements; The calculation of θ _z is as follows: when the sample to be tested changes along the rolling direction, the retroreflector will rotate accordingly, which will drive the quarter-wave plate on the surface of the retroreflector to rotate; therefore, the fast axis of the quarter-wave plate The angle with the horizontal direction will change; if the laser incident on the quarter-wave plate is linearly polarized light, when the polarization direction of the incident polarized light forms a certain angle α with the fast axis of the quarter-wave plate, and When , the emitted light is elliptically polarized light; when the quarter-wave plate is rotated, the ellipsometry of the emitted elliptically polarized light will change accordingly; for a small range of angle changes, the declination angle and ellipsometry are approximately linear, so Available: k is the proportional coefficient, a is the semi-major axis of elliptical polarization, and b is the minor semi-axis of elliptical polarization; the reference light reflected by beam splitter 1 is linearly polarized light, and the half-wave plate is rotated so that the polarization direction of the reference light is consistent with the length of the elliptical polarization light. The axis is at an angle of 45°, and the polarization direction can be obtained by a polarization measuring instrument; at this time, the phase signal change obtained by the second photodetector can be written as: where a is the semi-major axis of the elliptical polarized light; b is the semi-minor axis of the elliptical polarized light; γ is the angle between the polarization direction of the reference light and the major axis of the elliptically polarized light; is the common mode part; when the initial angle is 45°, the phase change caused by polarization is: That is to say, the phase change is equal to the change of ellipticity; and because the angle change is small, it can be regarded that during the measurement process, the phase change is equal to the change of ellipticity; thus it can be obtained: in Obtained from the first photodetector, subtract/> The noise introduced by the acousto-optic modulator can be removed, thus obtaining/> Then we get θ _z .