Phase-shifting interference measuring device and testing method based on liquid crystal delayer

Abstract

The invention discloses a phase-shifting interference measuring device and a phase-shifting interference testing method based on a liquid crystal retarder. The device comprises a short coherent light source module and a Fizeau interference measurement module. A short coherent light source in the light source module is divided into p light and s light through a polarization splitting prism, and linearly polarized light is subjected to phase modulation through a liquid crystal variable retarder. Modulated polarized light is coupled through the optical fiber coupler and enters the interference module, interference occurs in the interference cavity, and finally an interferogram is obtained by the CCD camera for calculation. According to the invention, the problem of complex installation and adjustment caused by integration of a light source and an interference system of a traditional interferometer device is solved, the situation of unstable environment caused by movement of mechanical parts in traditional phase shift is avoided, and the influence of parasitic fringes is inhibited by using the characteristics of a short coherent light source. The device utilizes the characteristics of the liquid crystal variable delayer to realize phase modulation so as to realize phase shift interference and surface shape detection. According to the phase-shifting interference surface shape measuring device, a high-precision surface shape measuring result can be obtained.

Description

Technical Field

[0001] The present invention belongs to the field of optical interference measurement, and in particular relates to a phase-shifting interference measurement device and a testing method based on a liquid crystal retarder. Background Art

[0002] Optical components are widely used in astronomical observation, military detection, aerospace and other fields. Modern precision optical systems have very high requirements for surface accuracy, and how to efficiently and accurately detect optical components has also become a research hotspot. Interferometer is a measuring instrument with interference detection as its core. It has the advantages of non-contact and high-precision measurement. The use of interferometer to detect optical components can effectively guarantee the actual system accuracy. At present, interferometers mostly use phase-shifting interferometry to measure the phase. By stepping or continuously changing the phase of one of the two interfering light waves, the interference fringe pattern is analyzed and solved to obtain the phase information of the object being measured. Among them, the time phase shifting method continuously collects images in a time series, so that a fixed phase difference is formed between each frame image to solve the phase.

[0003] In the design of interference devices, Fizeau interferometers have been widely used due to their high precision, high stability, common optical path and other characteristics, eliminating the measurement errors caused by mechanically moving optical elements and avoiding the influence of environmental vibrations introduced by the separate propagation of light beams. This application proposes a phase-shifting interferometer measurement device and test method based on a liquid crystal retarder, which uses the Fizeau interferometer measurement method to obtain two lights with zero optical path difference, combines the parasitic fringes in the short coherent light source suppression device, and the liquid crystal variable retarder to modulate the phase delay of orthogonal linear polarized light, and obtains better surface interferometer measurement results without mechanically moving optical elements. Summary of the invention

[0004] The purpose of the present invention is to provide a phase-shifting interferometry measurement device and test method based on a liquid crystal retarder, which realizes phase modulation of a light source module through the driving control of a liquid crystal variable retarder, thereby generating a phase shift, thereby ensuring the accuracy of the results in the actual measurement and phase solution process. The collimated light output by a short coherent laser enters the short coherent light source module, and the output orthogonal linearly polarized light is phase modulated after passing through a liquid crystal variable retarder, and the modulated light enters the Fizeau interferometry measurement module through a fiber coupler to obtain an interference pattern.

[0005] The technical solution to achieve the purpose of the present invention is: a phase-shifting interferometry measurement device based on a liquid crystal retarder, comprising a short coherence light source module and a Fizeau interferometry measurement module, wherein the short coherence light source module comprises a short coherence laser, a first fiber collimator, a polarizer, a half-wave plate, a polarization beam splitter, a first quarter-wave plate, a first corner cube prism, a second quarter-wave plate, a reflector, a second corner cube prism, a precision electric translation stage, a liquid crystal variable retarder, a fiber coupler, and a polarization-maintaining fiber; the Fizeau interferometry measurement module comprises a second fiber collimator, a beam splitter, a reflector, a collimating lens, a standard reference flat crystal, a reflective flat crystal, an aperture stop, an imaging lens, an analyzer, and a CCD camera.

[0006] A short coherence laser, a first fiber collimator, a polarizer, a half-wave plate, a polarization beam splitter prism, a first quarter-wave plate, and a first corner cube prism are sequentially arranged along the optical axis direction of the short coherence light source module; a second quarter-wave plate, a reflector, and a second corner cube prism are sequentially arranged along the reflection direction of the polarization beam splitter prism, and the second corner cube prism is arranged on a precision electric displacement table; the output light of the short coherence laser in the short coherence light source module is converted into spatial collimated light through the first fiber collimator, passes through the polarizer and the half-wave plate in sequence to form linearly polarized light and enters the polarization beam splitter prism, and after being split by the polarization beam splitter prism, forms a first transmitted light along the optical axis direction and a first reflected light in the direction perpendicular to the optical axis; wherein, the first A transmitted light passes through the first quarter wave plate, enters the first corner cube prism and then turns back, passes through the first quarter wave plate again and then emerges from the polarization beam splitter prism to form a second reflected light; the first reflected light passes through the second quarter wave plate and the reflector, enters the second corner cube prism and then turns back, passes through the second quarter wave plate again and then emerges from the polarization beam splitter prism to form a second transmitted light, the second reflected light and the second transmitted light are both linearly polarized lights, the above two linearly polarized lights are combined and then emerge from the polarization beam splitter prism, enter the liquid crystal variable retarder for modulation, and obtain two modulated orthogonal linear polarized lights, the two orthogonal linear polarized lights enter the fiber coupler, are coupled into the polarization-maintaining fiber and are connected to the Fizeau interferometer measurement module.

[0007] The outgoing light of the polarization-maintaining optical fiber is collimated by the second optical fiber collimator of the Fizeau interferometer measurement module and then split by the beam splitter prism. The decomposed third transmitted light is reflected by the reflector and then expanded to the collimating lens. The collimated light emitted by the collimating lens passes through the standard reference flat crystal and then reaches the reflective flat crystal. The light beam carrying the surface shape information of the front surface of the reflective flat crystal and the surface shape information of the rear surface of the standard reference flat crystal passes through the standard reference flat crystal, collimating lens, reflector, beam splitter prism, aperture stop, imaging lens, and analyzer, and reaches the target surface of the CCD camera and forms an image.

[0008] A testing method for a phase-shifting interferometry device based on a liquid crystal retarder, the steps are as follows:

[0009] Step 1: Turn on the short coherence laser and connect it to the first fiber collimator to form a spatially collimated beam output.

[0010] Step 2: Adjust the transmission axis direction of the polarizer to output linearly polarized light with maximum light intensity, adjust the fast axis direction of the half-wave plate to make the first transmitted light and the first reflected light separated by the polarization splitter prism consistent in intensity; adjust the fast axis direction of the first quarter-wave plate and the second quarter-wave plate to make the polarization directions of the output second reflected light and the second transmitted light orthogonal.

[0011] Step 3: Adjust the position of the fiber coupler so that the orthogonal linear polarized light in step 2 can pass completely, fine-tune the pitch tilt of the fiber coupler to maximize the intensity of the outgoing light passing through it, and couple the outgoing light into the polarization-maintaining fiber.

[0012] Step 4: Connect the other end of the polarization-maintaining fiber to the second fiber collimator of the Fizeau interferometer measurement module so that collimated light can be emitted from the second fiber collimator; set the interference cavity length to L, adjust the position of the CCD camera target surface so that it can form a clear image; set a polarizer in front of the CCD camera, rotate the polarizer transmission axis to 45° to make the two lights interfere, and observe the interference fringe pattern.

[0013] Step 5: According to the interference cavity length L set in step 4, adjust the position of the second corner cube prism through the precision electric translation stage so that the distance difference L2-L1 between the test arm and the reference arm and the interference cavity length L meet the optical path matching condition; adjust the step distance of the precision electric translation stage, step τ each time, observe the change in the contrast of the interference pattern, and adjust it multiple times until the interference pattern with the best contrast is found.

[0014] Step 6: Adjust the driving voltage of the liquid crystal variable retarder to achieve a change in the λ / 4 delay amount each time and synchronously collect multiple interference images.

[0015] Step 7: Use the phase shifting algorithm to solve the interference image and obtain the actual surface measurement result.

[0016] Among them, the peak-to-valley value (PV) of the surface shape measurement result of the transmitted wavefront of the Fizeau interferometry module is better than 1 / 4λ.

[0017] Compared with the prior art, the present invention has the following significant advantages:

[0018] (1) The PV value of the transmitted wavefront of the phase-shifting interferometry surface shape measurement device of the present invention is less than 1 / 4λ, and the surface shape measurement result is accurate.

[0019] (2) The phase-shifting interferometry surface shape measuring device of the present invention uses a short coherent light source, which can effectively suppress the parasitic fringes generated in the device.

[0020] (3) The phase-shifting interferometric surface shape measurement device of the present invention does not need to move the Fizeau interferometric measurement module. Phase shifting can be achieved by adjusting the liquid crystal variable delay device with a fast response speed, and only an ordinary CCD camera is needed to collect the interference pattern, thereby reducing the overall measurement difficulty and device cost. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 This is an optical path diagram of a phase-shifting interferometry measurement device based on a liquid crystal retarder according to the present invention.

[0022] Figure 2 This is a specific embodiment of the interference pattern and phase solution result diagram collected by a phase-shifting interferometry measurement device based on a liquid crystal delay device of the present invention, wherein the interference cavity length L is set to 500mm, the single step distance τ of the precision electric translation stage is 0.02mm, the reference arm L1 length is 150mm, and the test arm L2 length is 650mm, satisfying the optical path matching condition.

[0023] Figure 3 The present invention is a flow chart of a testing method of a phase-shifting interferometry measuring device based on a liquid crystal retarder. DETAILED DESCRIPTION

[0024] The present invention is further described in detail below in conjunction with the accompanying drawings.

[0025] Combination Figure 1 A phase-shifting interferometry device based on a liquid crystal retarder includes a short coherence light source module and a Fizeau interferometry module. The short coherence light source module includes a short coherence laser 1, a fiber collimator 2, a polarizer 3, a half-wave plate 4, a polarization beam splitter 5, a first quarter-wave plate 6, a first corner cube 7, a second quarter-wave plate 8, a reflector 9, a second corner cube 10, a precision electric translation stage 11, a liquid crystal variable retarder 12, a fiber coupler 13, and a polarization-maintaining fiber 14; the Fizeau interferometry module includes a fiber collimator 15, a beam splitter 16, a reflector 21, a collimating lens 22, a standard reference flat crystal 23, a reflective flat crystal 24, an aperture stop 17, an imaging lens 18, an analyzer 19, and a CCD camera 20.

[0026] A short coherence laser 1, a first fiber collimator 2, a polarizer 3, a half-wave plate 4, a polarization beam splitter 5, a first quarter-wave plate 6, and a first corner cube 7 are sequentially arranged along the optical axis direction of the short coherence light source module; a second quarter-wave plate 8, a reflector 9, and a second corner cube 10 are sequentially arranged along the reflection direction of the polarization beam splitter 5, and the second corner cube 10 is arranged on a precision electric translation stage 11; the output light of the short coherence laser 1 in the short coherence light source module is converted into spatial collimated light through the first fiber collimator 2, passes through the polarizer 3 and the half-wave plate 4 in sequence to form linearly polarized light and enters the polarization beam splitter prism 5, and after being split by the polarization beam splitter prism 5, a first transmitted light along the optical axis direction and a first reflected light in a direction perpendicular to the optical axis are formed; wherein The first transmitted light passes through the first quarter wave plate 6, enters the first corner cube prism 7, and then turns back. After passing through the first quarter wave plate 6 again, it is emitted from the polarization beam splitter prism 5 to form the second reflected light; the first reflected light passes through the second quarter wave plate 8 and the reflector 9, enters the second corner cube prism 10, and then turns back. After passing through the second quarter wave plate 8 again, it is emitted from the polarization beam splitter prism 5 to form the second transmitted light. Both the second reflected light and the second transmitted light are linearly polarized lights. The above-mentioned two linearly polarized lights are combined and then emitted from the polarization beam splitter prism 5, enter the liquid crystal variable retarder 12 for modulation, and obtain two modulated orthogonal linear polarized lights. The two orthogonal linear polarized lights enter the fiber coupler 13, are coupled into the polarization-maintaining fiber 14, and are connected to the Fizeau interferometer measurement module.

[0027] The outgoing light of the polarization-maintaining optical fiber 14 is collimated by the second optical fiber collimator 15 of the Fizeau interferometer measurement module and then split by the beam splitter prism 16. The decomposed third transmitted light is reflected by the reflector 21 and then expanded to the collimating lens 22. The collimated light emitted by the collimating lens 22 passes through the standard reference flat crystal 23 and then reaches the reflecting flat crystal 24. The light beam carrying the surface shape information of the front surface of the reflecting flat crystal 24 and the surface shape information of the rear surface of the standard reference flat crystal 23 passes through the standard reference flat crystal 23, the collimating lens 22, the reflector 21, the beam splitter prism 16, the aperture stop 17, the imaging lens 18, and the analyzer 19, and then reaches the target surface of the CCD camera 20, forms an image, and obtains an interference pattern.

[0028] The coherence length of the short coherent laser 1 is 1 mm, and it outputs spatially collimated light through the first optical fiber collimator 2, wherein the diameter of the output spatially collimated light spot is 2 mm.

[0029] The liquid crystal variable retarder 12 modulates the phase delay of linearly polarized light by changing the birefringence of the liquid crystal molecules by controlling the external driving voltage. The delay generated can cover 0 to 3λ / 4. The phase delay δ generated by the two orthogonal linearly polarized lights of the liquid crystal variable retarder 12 LCVR Satisfaction: δ LCVR =2πdΔn / λ.

[0030] Wherein, d is the thickness of the liquid crystal layer, λ is the output wavelength of the short coherent laser 1, Δn is the birefringence difference of the liquid crystal molecules, Δn=n e -n o =n ⊥ -n || .

[0031] The principal refractive index relationship of liquid crystal satisfies:

[0032] Among them, n o is the ordinary refractive index of light passing through the liquid crystal, n e is the extraordinary refractive index of light passing through the liquid crystal, n ⊥ is the refractive index of the liquid crystal in the vertical direction, n || is the refractive index in the direction parallel to the liquid crystal.

[0033] Assume that in the short coherent light source module, along the optical axis direction, the distance between the first quarter wave plate 6 and the first corner cube prism 7, that is, the reference arm, is L1; along the direction perpendicular to the optical axis, the distance between the second quarter wave plate 8 via the reflector 9 and the second corner cube prism 10, that is, the test arm, is L2; ​​suppose that the distance between the standard reference flat crystal 23 and the reflective flat crystal 24, that is, the interference cavity length, is L, then the relationship between the interference cavity length and the distance between the two arms satisfies: L=L2-L1, thereby achieving optical path matching.

[0034] Combination Figure 3 , a testing method for a phase-shifting interferometry device based on a liquid crystal retarder, the steps are as follows:

[0035] Step 1: Turn on the short coherent laser 1 and connect it to the first fiber collimator 2 to form a spatially collimated light output;

[0036] Step 2: In the short coherent light source module, a polarization beam splitter prism 5 is required to split the light to obtain a pair of linear polarized lights with orthogonal polarization directions. In order to obtain an interference pattern with relatively consistent contrast, the relative light intensity of the two lights needs to be adjusted. The specific implementation steps are: adjusting the transmission axis direction of the polarizer 3 to output linear polarized light with maximum light intensity, adjusting the fast axis direction of the half-wave plate 4 to make the first transmitted light and the first reflected light separated by the polarization beam splitter prism 5 consistent; adjusting the fast axis direction of the first quarter-wave plate 6 and the second quarter-wave plate 8 to make the polarization directions of the output second reflected light and the second transmitted light orthogonal.

[0037] Step 3: Adjust the position of the fiber coupler 13 so that the orthogonal linear polarized light in step 2 can pass completely, fine-tune the pitch tilt of the fiber coupler 13 to maximize the intensity of the outgoing light passing through it, and couple the outgoing light into the polarization-maintaining fiber 14.

[0038] Step 4: Under the condition of short coherence light source, the optical path difference of the two lights must match the interference cavity length in order to interfere and form interference fringes. The second reflected light and the second transmitted light emitted from the short coherence light source module will form 6 different interference combinations on the rear surface of the standard reference flat crystal 23 and the front surface of the reflection flat crystal 24. Therefore, it is necessary to strictly set the interference cavity length and the distance between the two arms in the light source module to meet the optical path matching conditions; in order to obtain clearer interference fringes, a linear polarized light combination with a zero optical path difference is selected here to meet the best interference fringe contrast. In addition, since the polarization directions of the two interfering lights are orthogonal, it is necessary to adjust the transmission axis angle of the analyzer 19 to obtain interference of light components in the same direction. The specific implementation steps are as follows: connect the other end of the polarization-maintaining optical fiber 14 to the second optical fiber collimator 15 of the Fizeau interferometer measurement module so that collimated light is emitted from the second optical fiber collimator 15; set the interference cavity length to L=500mm, adjust the target surface position of the CCD camera 20 so that it can form a clear image; set a polarizer 19 in front of the CCD camera 20, rotate the light transmission axis of the polarizer 19 to a 45° direction to allow the two lights to interfere, and observe the interference fringe pattern.

[0039] Step 5: According to the interference cavity length L set in step 4, the position of the second corner cube prism 10 is adjusted by the precision electric translation stage 11 so that the distance difference L2-L1 between the test arm and the reference arm and the interference cavity length L meet the optical path matching condition; the specific implementation steps are: adjust the step distance of the precision electric translation stage 11, each step τ=0.02mm, observe the change in the contrast of the interference pattern, and adjust it multiple times until the interference pattern with the best contrast is found.

[0040] Step 6: The present invention applies the liquid crystal variable retarder to the interferometer system for the first time, avoiding the adjustment of the interference measurement module and reducing the actual measurement error. There is a corresponding relationship between the external driving voltage of the liquid crystal retarder and the delay amount. The delay amount generated by the liquid crystal retarder can be determined and set by the relationship curve therebetween to achieve equal step phase shifting. The specific implementation steps are: adjusting the driving voltage of the liquid crystal variable retarder 12 to achieve a change in the λ / 4 delay amount each time, and synchronously collecting multiple interference images.

[0041] Step 7: Use the phase shift algorithm to solve the interference image and obtain the actual surface measurement result. Figure 2 shown.

[0042] When the second reflected light and the second transmitted light emitted from the short coherent light source module reach the CCD camera 20 through the Fizeau interferometry module, they carry the surface information of the standard reference flat crystal 23. and the surface information of the reflective flat crystal 24 After passing through the polarizer 19, the light field vector E out satisfy:

[0043] Eout =E P (E TF +E RF )

[0044] Among them, E P is the Jones vector of the polarizer 19, E TF is the Jones vector of the standard reference flat crystal 23, E RF is the Jones vector of the reflective flat crystal 24:

[0045]

[0046] Wherein, a represents the amplitude of a branch of linear polarized light returned from the reflection flat crystal 24 , b represents the amplitude of another branch of linear polarized light returned from the reflection flat crystal 24 , i represents the imaginary part, exp represents the exponential function, and λ is the output wavelength of the short coherence laser 1 .

[0047] Substituting the above expression into the light field vector E out :

[0048]

[0049] Wherein, θ represents the angle between the transmission axis direction of the polarizer 19 and the x-axis, and θ is assumed to be 45°;

[0050] The expression of interference light intensity I is obtained as follows:

[0051]

[0052] The driving voltage of the liquid crystal variable retarder 12 is adjusted in combination with the voltage-delay test curve to change the introduced delay δ LCVR , realize phase modulation with equal step length, and obtain multiple groups of different light intensity distributions I1~I4:

[0053]

[0054] The phase solution result δ obtained by using the four-step phase shift algorithm is:

[0055]

[0056] In summary, the present invention proposes an efficient and convenient phase-shifting interferometry measurement device and test method based on liquid crystal retarders. For the first time, liquid crystal variable retarders are applied to Fizeau interferometry measurement devices. Short coherent light sources are used to effectively suppress the influence of parasitic fringes in the device, effectively improve the stability of the device, and can achieve phase shifting through light source module modulation without moving other components in the device. The phase-shifting interferometry surface measurement device was used to obtain better surface measurement and phase solution results.

Claims

1. A phase-shifting interferometry device based on a liquid crystal retarder, characterized in that: The invention comprises a short coherent light source module and a Fizeau interferometry module, wherein the short coherent light source module comprises a short coherent laser (1), a first optical fiber collimator (2), a polarizer (3), a half-wave plate (4), a polarization beam splitter (5), a first quarter-wave plate (6), a first corner cube prism (7), a second quarter-wave plate (8), a reflector (9), a second corner cube prism (10), a precision electric translation stage (11), a liquid crystal variable retarder (12), an optical fiber coupler (13), and a polarization-maintaining optical fiber (14); and the Fizeau interferometry module comprises a second optical fiber collimator (15), a beam splitter prism (16), a reflector (21), a collimating lens (22), a standard reference flat crystal (23), a reflective flat crystal (24), an aperture stop (17), an imaging lens (18), an analyzer (19), and a CCD camera (20). A short coherence laser (1), a first optical fiber collimator (2), a polarizer (3), a half-wave plate (4), a polarization beam splitter (5), a first quarter-wave plate (6), and a first corner cube prism (7) are sequentially arranged along the optical axis direction of the short coherence light source module; a second quarter-wave plate (8), a reflector (9), and a second corner cube prism (10) are sequentially arranged along the reflection direction of the polarization beam splitter prism (5); the second corner cube prism (10) is arranged on a precision electric displacement stage (11); the output light of the short coherence laser (1) in the short coherence light source module is converted into spatial collimated light through the first optical fiber collimator (2), passes through the polarizer (3) and the half-wave plate (4) in sequence to form linearly polarized light and enters the polarization beam splitter prism (5), and after being split by the polarization beam splitter prism (5), a first transmitted light along the optical axis direction and a first linearly polarized light in the direction perpendicular to the optical axis are formed. Reflected light; wherein the first transmitted light passes through the first quarter wave plate (6) and enters the first corner cube prism (7) and then turns back, passes through the first quarter wave plate (6) again and then exits from the polarization beam splitter prism (5) to form the second reflected light; the first reflected light passes through the second quarter wave plate (8) and the reflector (9) and enters the second corner cube prism (10) and then turns back, passes through the second quarter wave plate (8) again and then exits from the polarization beam splitter prism (5) to form the second transmitted light, the second reflected light and the second transmitted light are both linearly polarized light, the two emitted linearly polarized lights are combined and then exit from the polarization beam splitter prism (5), enter the liquid crystal variable retarder (12) for modulation, and obtain two modulated orthogonal linear polarized lights, the two orthogonal linear polarized lights enter the optical fiber coupler (13), are coupled into the polarization-maintaining optical fiber (14) and are connected to the Fizeau interferometer measurement module; The output light of the polarization-maintaining optical fiber (14) is collimated by the second optical fiber collimator (15) of the Fizeau interferometer measurement module and then split by the beam splitter (16). The decomposed third transmitted light is reflected by the reflector (21) and then expanded to the collimating lens (22). The collimated light output from the collimating lens (22) passes through the standard reference flat crystal (23) and then reaches the reflecting flat crystal (24). The light beam carrying the surface shape information of the front surface of the reflecting flat crystal (24) and the surface shape information of the rear surface of the standard reference flat crystal (23) passes through the standard reference flat crystal (23), the collimating lens (22), the reflector (21), the beam splitter (16), the aperture stop (17), the imaging lens (18), and the analyzer (19), and then reaches the target surface of the CCD camera (20) and forms an image.

2. The phase-shifting interferometry device based on a liquid crystal retarder according to claim 1, characterized in that: The coherence length of the short coherent laser (1) is 1 mm, and it outputs spatially collimated light through a first optical fiber collimator (2), wherein the spot diameter of the emitted spatially collimated light is 2 mm.

3. The phase-shifting interferometry device based on a liquid crystal retarder according to claim 2, characterized in that: The liquid crystal variable retarder (12) modulates the phase delay of linearly polarized light by changing the birefringence of liquid crystal molecules by controlling an external driving voltage, and the generated delay amount can cover 0 to 3λ / 4; the phase delay amount δ generated by two orthogonal linearly polarized lights of the liquid crystal variable retarder (12) LCVR Satisfaction: δ LCVR =2πdΔn / λ; Wherein, d is the thickness of the liquid crystal layer, λ is the output wavelength of the short coherent laser (1), Δn is the birefringence difference of the liquid crystal molecules, Δn=n e -n o =n ⊥ -n || ; The principal refractive index relationship of liquid crystal satisfies: Among them, n o is the ordinary refractive index of light passing through the liquid crystal, n e is the extraordinary refractive index of light passing through the liquid crystal, n ⊥ is the refractive index of the liquid crystal in the vertical direction, n || is the refractive index in the direction parallel to the liquid crystal.

4. The phase-shifting interferometry device based on a liquid crystal retarder according to claim 3, characterized in that: Assume that in a short coherent light source module, along the optical axis direction, the distance between the first quarter wave plate (6) and the first corner cube prism (7), i.e., the reference arm, is L1; along the direction perpendicular to the optical axis, the distance between the second quarter wave plate (8) via the reflector (9) and the second corner cube prism (10), i.e., the test arm, is L2; ​​and assume that the distance between the standard reference flat crystal (23) and the reflective flat crystal (24), i.e., the interference cavity length, is L, then the relationship between the interference cavity length and the distance between the two arms satisfies: L=L2-L1, thereby achieving optical path matching.

5. A testing method for a phase-shifting interferometry device based on a liquid crystal retarder, characterized in that: The phase-shifting interferometry device based on a liquid crystal retarder according to any one of claims 1 to 4, wherein the steps are as follows: Step 1: Turn on the short coherence laser (1) and connect it to the first fiber collimator (2) to form a spatially collimated beam output; Step 2: adjusting the transmission axis direction of the polarizer (3) so as to output linearly polarized light of maximum light intensity, adjusting the fast axis direction of the half-wave plate (4) so ​​as to make the first transmitted light and the first reflected light separated by the polarization beam splitter (5) have the same light intensity; adjusting the fast axis directions of the first quarter-wave plate (6) and the second quarter-wave plate (8) so as to make the polarization directions of the output second reflected light and the second transmitted light orthogonal; Step 3: Adjust the position of the fiber coupler (13) so that the orthogonal linear polarized light in step 2 can pass completely, fine-tune the pitch tilt of the fiber coupler (13) so that the intensity of the outgoing light passing through it is maximized, and couple the outgoing light into the polarization-maintaining fiber (14); Step 4: Connect the other end of the polarization-maintaining optical fiber (14) to the second optical fiber collimator (15) of the Fizeau interferometer measurement module, so that collimated light is emitted from the second optical fiber collimator (15); assume that the interference cavity length is L, adjust the target surface position of the CCD camera (20) so that a clear image can be formed; set a polarizer (19) in front of the CCD camera (20), rotate the light transmission axis of the polarizer (19) to a 45° direction to cause interference between the two lights, and observe the interference fringe pattern; Step 5: According to the interference cavity length L set in step 4, the position of the second corner cube prism (10) is adjusted by the precision electric displacement stage (11) so that the distance difference L2-L1 between the test arm and the reference arm and the interference cavity length L meet the optical path matching condition; the stepping distance of the precision electric displacement stage (11) is adjusted, and each step is τ, and the change in the contrast of the interference pattern is observed, and the adjustment is repeated multiple times until the interference pattern with the best contrast is found; Step 6: adjusting the driving voltage of the liquid crystal variable retarder (12) to achieve a change in the λ / 4 delay amount each time and synchronously collecting multiple interference images; Step 7: Use the phase shifting algorithm to solve the interference image and obtain the actual surface measurement result.

6. The testing method of the phase-shifting interferometry device based on the liquid crystal retarder according to claim 5, characterized in that: When the second reflected light and the second transmitted light emitted from the short coherent light source module reach the CCD camera (20) via the Fizeau interferometer measurement module, they carry the surface shape information of the standard reference flat crystal (23). and the surface information of the reflection flat crystal (24) After passing through the polarizer (19), the light field vector E out satisfy: AND out =And P (AND TF +E RF ) Among them, E P is the Jones vector of the polarizer (19), E TF is the Jones vector of the standard reference flat crystal (23), E RF is the Jones vector of the reflective flat crystal (24): Wherein, a represents the amplitude of a branch of linear polarized light returned from the reflection flat crystal (24), b represents the amplitude of another branch of linear polarized light returned from the reflection flat crystal (24), i represents the imaginary part, exp represents the exponential function, and λ is the output wavelength of the short coherence laser (1); Substituting the above expression into the light field vector E out : Wherein, θ represents the angle between the transmission axis direction of the polarizer (19) and the x-axis, and θ is assumed to be 45°; The expression of interference light intensity I is obtained as follows: The driving voltage of the liquid crystal variable retarder (12) is adjusted in combination with the voltage-delay test curve to change the introduced delay δ LCVR , realize phase modulation with equal step length, and obtain multiple groups of different light intensity distributions I1~I4: The phase solution result δ obtained by using the four-step phase shift algorithm is:

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