A phase-shifting interferometry apparatus and method based on liquid crystal retarders

Abstract

This invention discloses a phase-shifting interferometry measurement device and testing method based on a liquid crystal retarder. The device includes a short-coherence light source module and a Fizeau interferometry measurement module. In the light source module, the short-coherence light source is split into p-beams and s-beams by a polarizing beam splitter, and the linearly polarized light is phase-modulated by a liquid crystal variable retarder. The modulated polarized light is coupled into the interferometry module via a fiber coupler, where interference occurs in the interferometry cavity. Finally, the interferogram is acquired by a CCD camera for calculation. This invention solves the problem of complex assembly and adjustment in traditional interferometer devices that integrate the light source and the interferometric system, avoids the environmental instability caused by the movement of mechanical parts in traditional phase-shifting methods, and suppresses the influence of parasitic fringes by utilizing the characteristics of the short-coherence light source. This device utilizes the characteristics of the liquid crystal variable retarder to achieve phase modulation to realize phase-shifting interferometry and surface shape detection. The phase-shifting interferometry surface shape measurement device can obtain high-precision surface shape measurement results.

Description

Technical Field

[0001] This invention belongs to the field of optical interferometry, specifically a phase-shifting interferometry device and testing method based on a liquid crystal delay unit. Background Technology

[0002] Optical components have wide applications in various fields such as astronomical observation, military inspection, and aerospace. Modern precision optical systems place high demands on surface accuracy, making efficient and precise inspection of optical components a research hotspot. Interferometers, measuring instruments based on interferometric detection methods, offer advantages such as non-contact and high-precision measurement. Using interferometers to inspect optical components can effectively ensure the accuracy of actual systems. Currently, interferometers mostly employ phase-shifting interferometry to measure phase. By performing step-by-step or continuous phase changes on one of two interferometric light waves, the interference fringe pattern is analyzed, and the phase information of the measured object is calculated. Among these methods, the time-shifting phase method continuously acquires images in a time series, creating a fixed phase difference between each frame to calculate the phase.

[0003] In the design of interferometers, the Fizeau interferometer is widely used due to its high precision, high stability, and common optical path characteristics, which eliminate measurement errors caused by mechanically moving optical elements and avoid the influence of environmental vibrations introduced by beam split propagation. This application proposes a phase-shifting interferometric measurement device and testing method based on a liquid crystal retarder. By applying the Fizeau interferometric measurement method to obtain two beams with zero optical path difference, and combining the parasitic fringes in the short coherence light source suppression device with the phase delay of the orthogonally linearly polarized light modulated by the liquid crystal variable retarder, good surface interferometric measurement results are obtained without the need for mechanically moving optical elements. Summary of the Invention

[0004] The purpose of this invention is to provide a phase-shifting interferometric measurement device and testing method based on a liquid crystal retarder. Phase modulation of the light source module is achieved through the driving control of the liquid crystal variable retarder, thereby generating a phase shift and ensuring the accuracy of the results in the actual measurement and phase calculation process. Collimated light from a short-coherent laser enters the short-coherent light source module. The output orthogonally linearly polarized light is phase-modulated after passing through the liquid crystal variable retarder. The modulated light enters the Fizeau interferometric measurement module through an optical fiber coupler to obtain an interferogram.

[0005] The technical solution to achieve the purpose of this invention is as follows: a phase-shifting interferometric measurement device based on a liquid crystal delay unit, comprising a short coherent light source module and a Fizeau interferometric measurement module, wherein the short coherent light source module includes a short coherent laser, a first fiber collimator, a polarizer, a half-wave plate, a polarizing beam splitter, a first quarter-wave plate, a first pyramidal prism, a second quarter-wave plate, a reflector, a second pyramidal prism, a precision electric displacement stage, a liquid crystal variable delay unit, a fiber coupler, and a polarization-maintaining fiber; the Fizeau interferometric measurement module includes a second fiber collimator, a beam splitter, a reflector, a collimating lens, a standard reference flat, a reflecting flat, an aperture stop, an imaging lens, an analyzer, and a CCD camera.

[0006] Along the optical axis of the short coherent light source module, a short coherent laser, a first fiber collimator, a polarizer, a half-wave plate, a polarizing beam splitter, a first quarter-wave plate, and a first pyramidal prism are arranged sequentially. Along the reflection direction of the polarizing beam splitter, a second quarter-wave plate, a mirror, and a second pyramidal prism are arranged sequentially, with the second pyramidal prism mounted on a precision electric displacement stage. The emitted light from the short coherent laser in the short coherent light source module is converted into spatially collimated light by the first fiber collimator, then sequentially passes through the polarizer and half-wave plate to form linearly polarized light before entering the polarizing beam splitter. After being split by the polarizing beam splitter, it forms a first transmitted light along the optical axis and a first reflected light perpendicular to the optical axis. The first... A transmitted light beam passes through a first quarter-wave plate, enters a first cornerstone prism, is reflected back, passes through the first quarter-wave plate again, and exits from a polarizing beam splitter to form a second reflected light. The first reflected light passes through a second quarter-wave plate and a reflector, enters a second cornerstone prism, is reflected back, passes through the second quarter-wave plate again, and exits from a polarizing beam splitter to form a second transmitted light. Both the second reflected light and the second transmitted light are linearly polarized. The two linearly polarized beams merge and exit from the polarizing beam splitter, enter a liquid crystal variable delay unit for modulation, and obtain two modulated orthogonal linearly polarized beams. The two orthogonal linearly polarized beams enter an optical fiber coupler, are coupled into a polarization-maintaining fiber, and are connected to a Fizeau interferometric measurement module.

[0007] The emitted light from the polarization-maintaining fiber is collimated by the second fiber collimator of the Fizeau interferometry module and then split by the beam splitter prism. The split third transmitted light is reflected by the mirror and then expanded to the collimating lens. The collimated light emitted from the collimating lens passes through the standard reference flat and then to the reflecting flat. The beam carrying the surface shape information of the front surface of the reflecting flat and the surface shape information of the back surface of the standard reference flat passes through the standard reference flat, collimating lens, mirror, beam splitter prism, aperture stop, imaging lens, and analyzer, and finally reaches the target surface of the CCD camera and forms an image.

[0008] A test method for a phase-shifting interferometry measurement device based on a liquid crystal delayer, comprising the following steps:

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

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

[0011] Step 3: Adjust the position of the fiber coupler so that the orthogonally linearly polarized light from step 2 can pass through completely. Fine-tune the pitch 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 interferometry module so that collimated light is emitted from the second fiber collimator; set the length of the interference cavity as L, adjust the position of the CCD camera target surface so that it can form a clear image; set up an analyzer in front of the CCD camera, rotate the transmission axis of the analyzer to 45° so that the two beams interfere, and observe the interference fringe pattern.

[0013] Step 5: Based on the interference cavity length L set in Step 4, adjust the position of the second corner prism using a precision electric displacement stage so that the distance difference L2-L1 between the test arm and the reference arm satisfies the optical path matching condition with the interference cavity length L; adjust the step distance of the precision electric displacement stage, stepping τ each time, and observe the change in the contrast of the interference pattern. Repeat the adjustment until the interference pattern with the best contrast is found.

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

[0015] Step 7: Use a phase-shifting algorithm to solve the interference image to obtain the actual surface shape measurement results.

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

[0017] Compared with the prior art, the significant advantages of this invention are:

[0018] (1) The phase-shifting interference surface shape measuring device of the present invention has a transmitted wavefront PV value of less than 1 / 4λ, and the surface shape measurement results are accurate.

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

[0020] (3) The phase-shifting interference surface shape measuring device of the present invention does not require moving the Fizeau interferometric measurement module. Phase shifting can be achieved simply by adjusting the liquid crystal variable delay device with fast response speed. Moreover, only a common CCD camera is needed to collect the interference pattern, which reduces the overall measurement difficulty and device cost. Attached Figure Description

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

[0022] Figure 2 This is a specific embodiment of the interferogram and phase calculation result diagram acquired by the phase-shifting interferometry device based on liquid crystal delay unit of the present invention. In this embodiment, the length L of the interferometer cavity is set to 500 mm, the single step distance τ of the precision electric displacement stage is 0.02 mm, the length of the reference arm L1 is 150 mm, and the length of the test arm L2 is 650 mm, which satisfies the optical path matching condition.

[0023] Figure 3 This is a flowchart of a test method for a phase-shifting interferometry measurement device based on a liquid crystal delay unit according to the present invention. Detailed Implementation

[0024] The present invention will now be described in further detail with reference to the accompanying drawings.

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

[0026] Along the optical axis of the short coherent light source module, a short coherent laser 1, a first fiber collimator 2, a polarizer 3, a half-wave plate 4, a polarizing beam splitter 5, a first quarter-wave plate 6, and a first pyramidal prism 7 are arranged sequentially. Along the reflection direction of the polarizing beam splitter 5, a second quarter-wave plate 8, a reflector 9, and a second pyramidal prism 10 are arranged sequentially, with the second pyramidal prism 10 mounted on a precision electric displacement stage 11. The emitted light from the short coherent laser 1 in the short coherent light source module is converted into spatially collimated light by the first fiber collimator 2, then sequentially passes through the polarizer 3 and the half-wave plate 4 to form linearly polarized light before entering the polarizing beam splitter 5. After being split by the polarizing beam splitter 5, it forms a first transmitted light along the optical axis and a first reflected light perpendicular to the optical axis. The first transmitted light passes through the first quarter-wave plate 6 and enters the first pyramidal prism 7, then is reflected back. After passing through the first quarter-wave plate 6 again, it exits from the polarizing beam splitter 5 to form the second reflected light. The first reflected light passes through the second quarter-wave plate 8 and the reflector 9, then enters the second pyramidal prism 10, then is reflected back. After passing through the second quarter-wave plate 8 again, it exits from the polarizing beam splitter 5 to form the second transmitted light. Both the second reflected light and the second transmitted light are linearly polarized. The two linearly polarized lights that exit are combined and exit from the polarizing beam splitter 5. They enter the liquid crystal variable delay unit 12 for modulation to obtain two orthogonally linearly polarized lights. The two orthogonally linearly polarized lights enter the fiber coupler 13 and are coupled into the polarization-maintaining fiber 14 and connected to the Fizeau interferometry module.

[0027] The emitted light from the polarization-maintaining fiber 14 is collimated by the second fiber collimator 15 of the Fizeau interferometric measurement module and then split by the beam splitter prism 16. The split third transmitted light is reflected by the mirror 21 and then expanded to the collimating lens 22. The collimated light emitted from the collimating lens 22 passes through the standard reference flat 23 and then to the reflecting flat 24. The beam carrying the surface shape information of the front surface of the reflecting flat 24 and the surface shape information of the rear surface of the standard reference flat 23 passes through the standard reference flat 23, the collimating lens 22, the mirror 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 to form an image and obtain an interferogram.

[0028] The short coherence laser 1 has a coherence length of 1 mm and outputs spatially collimated light through the first 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 liquid crystal molecules through controlling an external driving voltage. The resulting delay can cover 0 to 3λ / 4. The phase delay δ generated by the two orthogonally linearly polarized lights through the liquid crystal variable retarder 12 is... LCVR Satisfy: δ LCVR =2πdΔn / λ.

[0030] Where d is the thickness of the liquid crystal layer, λ is the output wavelength of the short coherent laser 1, and 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 crystals satisfies:

[0032] Where, n o To achieve the ordinary light refractive index of the liquid crystal, n e To utilize the unusual light refractive index of the liquid crystal, n ⊥ n is the refractive index of the liquid crystal in the vertical direction. || This represents the refractive index of the liquid crystal in the parallel direction.

[0033] In the short coherent light source module, along the optical axis, the distance between the first quarter-wave plate 6 and the first corner 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, the reflector 9, and the second corner prism 10, i.e., the test arm, is L2; ​​and the distance between the standard reference flat 23 and the reflective flat 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, thus achieving optical path matching.

[0034] Combination Figure 3 A test method for a phase-shifting interferometry measurement device based on a liquid crystal delay unit, comprising the following steps:

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

[0036] Step 2: In the short coherence light source module, a polarizing beam splitter 5 is used to split the light to obtain a pair of linearly polarized lights with orthogonal polarization directions. To obtain an interference pattern with relatively consistent contrast, the relative light intensity of the two beams needs to be adjusted. The specific implementation steps are as follows: Adjust the transmission axis direction of the polarizer 3 to output linearly polarized light with maximum intensity; adjust the fast axis direction of the half-wave plate 4 to make the intensity of the first transmitted light and the first reflected light generated from the polarizing beam splitter 5 consistent; adjust 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 orthogonally linearly polarized light in step 2 can pass through completely. Fine-tune the pitch 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 short-coherence light source conditions, the optical path difference between the two beams must be matched with the interference cavity length for interference to occur and form interference fringes. The second reflected light and the second transmitted light emitted from the short-coherence light source module will form six different interference combinations on the rear surface of the standard reference flat 23 and the front surface of the reflective flat 24. Therefore, the interference cavity length and the distance between the two arms in the light source module need to be strictly set to meet the optical path matching condition. To obtain clearer interference fringes, a linearly polarized light combination with a zero optical path difference is selected to achieve the best interference fringe contrast. In addition, since the polarization directions of the two beams interfering are orthogonal, the transmission axis angle of the analyzer 19 needs to be adjusted 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 fiber 14 to the second fiber collimator 15 of the Fizeau interferometric measurement module so that collimated light is emitted from the second fiber collimator 15; set the interference cavity length to L = 500 mm, adjust the target surface position of the CCD camera 20 so that it can form a clear image; set the analyzer 19 in front of the CCD camera 20, rotate the transmission axis of the analyzer 19 to the 45° direction to make the two beams interfere, and observe the interference fringe pattern.

[0039] Step 5: Based on the interference cavity length L set in Step 4, adjust the position of the second corner bevel prism 10 using the precision electric displacement stage 11 so that the distance difference L2-L1 between the test arm and the reference arm satisfies the optical path matching condition with the interference cavity length L. The specific implementation steps are as follows: adjust the step distance of the precision electric displacement stage 11, with each step τ = 0.02 mm, 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: This invention is the first to apply a liquid crystal variable delay unit to an interferometer system, avoiding the need to adjust the interferometric measurement module and reducing actual measurement errors. There is a corresponding relationship between the external driving voltage of the liquid crystal delay unit and the delay amount. This relationship curve can be used to determine and set the magnitude of the delay generated by the liquid crystal delay unit to achieve equal-step phase shifting. The specific implementation steps are as follows: Adjust the driving voltage of the liquid crystal variable delay unit 12 to achieve a change in the delay amount of λ / 4 each time, simultaneously acquiring multiple interferometric images.

[0041] Step 7: Use a phase-shifting algorithm to solve the interferometric image to obtain the actual surface shape measurement results, as shown below. Figure 2 As shown.

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

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

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

[0045]

[0046] Where a represents the amplitude of one branch of linearly polarized light returning from the reflective flat crystal 24, b represents the amplitude of the other branch of linearly polarized light returning from the reflective flat crystal 24, i represents the imaginary part, exp represents the exponential function, and λ is the output wavelength of the short coherent laser 1.

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

[0048]

[0049] Where θ represents the angle between the transmission axis of the analyzer 19 and the x-axis, and θ is set to 45°;

[0050] Therefore, the expression for the interference light intensity I is obtained as follows:

[0051]

[0052] The driving voltage of the liquid crystal variable delay unit 12 is adjusted by combining the voltage-delay test curve to change the introduced delay δ. LCVR To achieve phase modulation with equal step size, multiple sets of different light intensity distributions I1 to I4 were obtained:

[0053]

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

[0055]

[0056] In summary, this invention proposes an efficient and convenient phase-shifting interferometric measurement device and testing method based on a liquid crystal retarder. It is the first to apply a liquid crystal variable retarder to a Fizeau interferometric measurement device, employing a short-coherence light source to effectively suppress the influence of parasitic fringes in the device, thus significantly improving the device's stability. Furthermore, phase shifting can be achieved through modulation of the light source module without moving other components within the device. Applying this phase-shifting interferometric surface shape measurement device yielded good surface shape measurement and phase determination results.

Claims

1. A phase-shifting interferometry measurement device based on a liquid crystal retarder, characterized in that: The system includes a short coherent light source module and a Fizeau interferometry module. The short coherent light source module includes a short coherent laser (1), a first fiber collimator (2), a polarizer (3), a half-wave plate (4), a polarizing beam splitter (5), a first quarter-wave plate (6), a first pyramidal prism (7), a second quarter-wave plate (8), a mirror (9), a second pyramidal prism (10), a precision electric displacement stage (11), a liquid crystal variable delay unit (12), a fiber coupler (13), and a polarization-maintaining fiber (14). The Fizeau interferometry module includes a second fiber collimator (15), a beam splitter (16), a mirror (21), a collimating lens (22), a standard reference flat (23), a reflecting flat (24), an aperture stop (17), an imaging lens (18), an analyzer (19), and a CCD camera (20). Along the optical axis of the short coherent light source module, a short coherent laser (1), a first fiber collimator (2), a polarizer (3), a half-wave plate (4), a polarizing beam splitter (5), a first quarter-wave plate (6), and a first pyramidal prism (7) are arranged sequentially. Along the reflection direction of the polarizing beam splitter (5), a second quarter-wave plate (8), a reflector (9), and a second pyramidal prism (10) are arranged sequentially. The second pyramidal prism (10) is mounted on a precision electric displacement stage (11). The emitted light from the short coherent laser (1) in the short coherent light source module is converted into spatially collimated light by the first fiber collimator (2), and then passes through the polarizer (3) and the half-wave plate (4) to form linearly polarized light before entering the polarizing beam splitter (5). After being split by the polarizing beam splitter (5), the light forms a first transmitted light along the optical axis and a first transmitted light perpendicular to the optical axis. The first transmitted light passes through the first quarter-wave plate (6) and enters the first pyramidal prism (7) and is reflected back. After passing through the first quarter-wave plate (6) again, it exits from the polarization beam splitter (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 pyramidal prism (10) and is reflected back. After passing through the second quarter-wave plate (8) again, it exits from the polarization beam splitter (5) to form the second transmitted light. Both the second reflected light and the second transmitted light are linearly polarized. The two linearly polarized lights that are emitted above merge and exit from the polarization beam splitter (5), enter the liquid crystal variable delay unit (12) for modulation, and obtain two orthogonally linearly polarized lights after modulation. The two orthogonally linearly polarized lights enter the fiber coupler (13) and are coupled into the polarization-maintaining fiber (14) and connected to the Fizeau interferometric measurement module. The emitted light from the polarization-maintaining fiber (14) is collimated by the second fiber collimator (15) of the Fizeau interferometric measurement module and then split by the beam splitter (16). The split third transmitted light is reflected by the mirror (21) and then expanded to the collimating lens (22). The collimated light emitted from the collimating lens (22) passes through the standard reference flat (23) and then to the reflecting flat (24). The beam carrying the surface shape information of the front surface of the reflecting flat (24) and the surface shape information of the back surface of the standard reference flat (23) passes through the standard reference flat (23), the collimating lens (22), the mirror (21), the beam splitter (16), the aperture stop (17), the imaging lens (18), and the analyzer (19) before reaching the target surface of the CCD camera (20) and forming an image.

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

3. The phase-shifting interferometry measuring device based on a liquid crystal delay unit 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 through controlling the external driving voltage. The resulting retardation can cover 0 to 3λ / 4. The phase delay δ generated by the two orthogonally linearly polarized lights through the liquid crystal variable retarder (12) LCVR Satisfy: δ LCVR =2πdΔn / λ; Where d is the thickness of the liquid crystal layer, λ is the output wavelength of the short coherent laser (1), and Δn is the birefringence difference of the liquid crystal molecules, Δn = n e -n o =n ⊥ -n || ; The principal refractive index relationship of liquid crystals satisfies: Where, n o To achieve the ordinary light refractive index of the liquid crystal, n e To utilize the unusual light refractive index of the liquid crystal, n ⊥ n is the refractive index of the liquid crystal in the vertical direction. || This represents the refractive index of the liquid crystal in the parallel direction.

4. The phase-shifting interferometry measuring device based on a liquid crystal delay unit according to claim 3, characterized in that: In the short coherent light source module, along the optical axis, the distance between the first quarter-wave plate (6) and the first corner 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) and the mirror (9) and the second corner prism (10), i.e., the test arm, is L2; ​​and the distance between the standard reference flat (23) and the reflective flat (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, thus achieving optical path matching.

5. A test method for a phase-shifting interferometry measurement device based on a liquid crystal delay unit, characterized in that, The phase-shifting interferometry measuring device based on a liquid crystal delayer according to any one of claims 1 to 4 comprises the following steps: Step 1: Turn on the short coherent laser (1) and connect it to the first fiber collimator (2) to form a spatially collimated beam output; Step 2: Adjust the transmission axis direction of the polarizer (3) to output linearly polarized light with maximum intensity; adjust the fast axis direction of the half-wave plate (4) to make the intensity of the first transmitted light and the first reflected light generated from the polarizing beam splitter (5) consistent; adjust 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 second reflected light and the second transmitted light orthogonal. Step 3: Adjust the position of the fiber coupler (13) so that the orthogonally linearly polarized light in step 2 can pass through completely. Fine-tune the pitch 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). Step 4: Connect the other end of the polarization-maintaining fiber (14) to the second fiber collimator (15) of the Fizeau interferometry module so that collimated light is emitted from the second fiber collimator (15); set the interference cavity length as L, adjust the target position of the CCD camera (20) so that it can form a clear image; set the analyzer (19) in front of the CCD camera (20), rotate the transmission axis of the analyzer (19) to the 45° direction so that the two beams interfere, and observe the interference fringe pattern; Step 5: Based on the interference cavity length L set in Step 4, adjust the position of the second corner bevel prism (10) using the precision electric displacement stage (11) so that the distance difference L2-L1 between the test arm and the reference arm satisfies the optical path matching condition with the interference cavity length L; adjust the step distance of the precision electric displacement stage (11), step τ each time, observe the change in the contrast of the interference pattern, and adjust multiple times until the interference pattern with the best contrast is found; Step 6: Adjust the driving voltage of the liquid crystal variable delay unit (12) to realize the change of the delay amount of λ / 4 each time, and simultaneously acquire multiple interference images; Step 7: Use a phase-shifting algorithm to solve the interference image to obtain the actual surface shape measurement results.

6. The test method of the phase-shifting interferometry measurement device based on a liquid crystal delay unit according to claim 5, characterized in that: When the second reflected light and the second transmitted light emitted from the short coherence light source module reach the CCD camera (20) via the Fizeau interferometer module, they carry the surface shape information of the standard reference flat crystal (23). and surface shape information of the reflective flat (24) After passing through the analyzer (19), the light field vector E out satisfy: AND out =And P (AND TF +E RF ) Among them, E P E is the Jones vector of the analyzer (19). TF For the Jones vector of the standard reference flat (23), E RF Jones vector for the reflective flat crystal (24): Where a represents the amplitude of one branch of linearly polarized light returning from the reflective flat crystal (24), b represents the amplitude of another branch of linearly polarized light returning from the reflective flat crystal (24), i represents the imaginary part, exp represents the exponential function, and λ is the output wavelength of the short coherent laser (1). Substituting the above expression into the equation yields the light field vector E. out : Where θ represents the angle between the transmission axis of the analyzer (19) and the x-axis, and θ is set to 45°; Therefore, the expression for the interference light intensity I is obtained as follows: The driving voltage of the liquid crystal variable delay device (12) is adjusted by combining the voltage-delay test curve to change the introduced delay δ. LCVR To achieve phase modulation with equal step size, multiple sets of different light intensity distributions I1 to I4 were obtained: The phase solution δ obtained using the four-step phase shifting algorithm is:

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