A non-mechanical fast phase-shifting interferometry device and method

Abstract

This invention discloses a non-mechanical, rapid phase-shifting interferometric measurement device and method. The device includes a laser, a non-polarizing beam splitter, and a reflector. The laser emits a parallel laser beam, which is expanded by a beam expander and its polarization state is adjusted to the horizontal direction by a polarizer. Then, it passes through a first non-polarizing beam splitter and splits into two horizontally linearly polarized beams of equal intensity. One beam passes through a rapid phase shifter and a second reflector, then is transmitted through a second non-polarizing beam splitter and incident on the sensor surface of an area array detector. The other beam passes through a first reflector and the sample under test, and is reflected by the second non-polarizing beam splitter before incident on the sensor surface of the area array detector. The two beams interfere on the sensor surface of the area array detector, are collected by the detector, and transmitted to a computer for data processing. This invention eliminates mechanical inertia, increases the measurement bandwidth to the 10kHz level, and significantly reduces costs.

Description

Technical Field

[0001] This invention relates to the field of optoelectronic testing technology, and in particular to a non-mechanical device and method for rapidly performing phase-shifting interferometry. Background Technology

[0002] Optical interferometry is widely used in scientific research and production due to its ability to perform non-contact measurements at the wavelength level. However, traditional interferometers rely on interpreting the shape of interference fringes to analyze the phase of the measured object. Since Buring et al. proposed phase-shifting interferometry in 1974, extracting phase information from interferograms with high precision has become a relatively mature technical solution. The core of phase-shifting interferometry is to add a phase-shifting module to a traditional interferometer. By modulating the interference field through "phase shifting," multiple frames of phase-shifted interferograms are acquired, and the compression phase of the measured object is extracted using a phase-shifting algorithm. Subsequently, the complete uncompressed phase is obtained using an uncompressing algorithm.

[0003] Phase-shifting interferometry has many advantages. The most commonly used technical solution is to use piezoelectric ceramics to drive the movement of a reflector as a phase shifter. By applying different voltages to the piezoelectric ceramic PZT, the reflector is driven to achieve different stretching distances, thereby realizing the phase change.

[0004] The shortcomings of piezoelectric ceramic PZT as a phase shifter are as follows: (1) The phase shifting method of this device is mechanical motion. When the diameter of the reflector is large, the load weight increases, the mechanical inertia also increases, and it will cause turbulence interference in the surrounding air, which limits the frequency bandwidth of the phase shifter and is not suitable for rapid phase shifting interferometry of large-diameter devices; (2) This device is a reflective optical path. When used in Mach-Zehnder interference optical paths, more than three-quarters of the light intensity needs to be sacrificed. Moreover, passing through the beam splitter multiple times will introduce unnecessary phase difference, which wastes the power of the laser source and reduces the measurement accuracy; (3) Piezoelectric ceramic PZT is expensive, which increases the manufacturing cost of the phase shifter and is not conducive to the further popularization of the product. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a non-mechanical device and method for rapidly performing phase-shifting interferometry.

[0006] Technical Solution: The non-mechanical fast phase-shifting interferometric measurement device of the present invention includes a laser, a beam expander, a polarizer, a non-polarizing beam splitter, a reflector, a fast phase shifter, a test sample, an area array detector, a computer, and a drive control circuit. The non-polarizing beam splitter includes a first non-polarizing beam splitter and a second non-polarizing beam splitter. The reflector includes a first reflector and a second reflector. The laser emits a parallel laser beam, which is expanded by the beam expander. The polarizer adjusts the polarization state of the laser beam to the horizontal direction. Then, it passes through the first non-polarizing beam splitter and is split into two horizontally linearly polarized beams of equal intensity. One beam passes through the fast phase shifter and the second reflector, is transmitted through the second non-polarizing beam splitter, and then incident on the sensor surface of the area array detector. The other beam passes through the first reflector and the test sample, is reflected by the second non-polarizing beam splitter, and then incident on the sensor surface of the area array detector. The two beams interfere on the sensor surface of the area array detector, and the resulting interference fringe image is acquired by the area array detector and transmitted to the computer for data processing.

[0007] Furthermore, the fast phase shifter includes a first module and a second module, both of which sequentially include a quarter-wave plate, a ferroelectric liquid crystal half-wave plate, and a quarter-wave plate, forming a QHQ module.

[0008] Furthermore, the QHQ module can achieve geometric phase modulation, and the magnitude of the phase change... It depends on the angle β between the crystal axis direction of the half-wave plate and the horizontal direction, and is related to:

[0009] Furthermore, the crystal axis direction of the ferroelectric liquid crystal half-wave plate is its slow axis direction. Under different voltage driving conditions, two slow axis states are generated, with corresponding geometric phases of 0° and -90°.

[0010] Furthermore, the first module and the second module are connected in series, and the total geometric phase generated is the sum of the geometric phases generated by the first module and the second module individually, and the set of their value states is as follows: It includes three possible values ​​for the geometric phase.

[0011] Furthermore, the ferroelectric liquid crystal half-wave is connected to a computer via a drive control circuit.

[0012] Furthermore, the transmission to reflection ratio of the second non-polarizing beam splitter is 50:50.

[0013] Furthermore, the parallel laser beam emitted by the laser is either unpolarized or polarized light.

[0014] The non-mechanical rapid phase-shifting interferometry measurement method of the present invention includes the following steps:

[0015] (1) Open the image acquisition program on the computer and send the command to the area array detector to acquire the interference fringe image;

[0016] (2) The computer issues instructions to drive the control circuit to generate two positive and negative voltages, which respectively control the liquid crystal orientation direction of the ferroelectric liquid crystal half-wave plate, thereby realizing the adjustment of geometric phase;

[0017] (3) The computer issues a command, the array detector acquires the interference fringe image, and saves the acquired interference image into the computer's memory. The light intensity distribution of the interference image is I = I i , i = 1 to 3;

[0018] (4) Repeat steps (2) and (3) three times to obtain three interference images I1, I2, and I3;

[0019] (5) The encapsulation phase distribution of the test sample is calculated based on the three-step phase shifting algorithm;

[0020] (6) The unwrapping phase distribution of the sample under test is obtained by using the unwrapping algorithm, and the measurement is completed.

[0021] Furthermore, the calculation formula for the three-step phase-shifting algorithm in step (5) is as follows:

[0022]

[0023] If the three geometric phases are not exactly {-90°, 0°, 90°}, but rather {-α, 0°, α}, then a more general calculation formula is used:

[0024]

[0025] Where α is the phase step size of the three-step phase shift.

[0026] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: The phase switching time in the three-step phase shifting of this application depends on the response time of the ferroelectric liquid crystal half-wave plate, with a typical time of less than 100us and a corresponding bandwidth on the order of ~10kHz, which can realize non-mechanical fast phase shifting interferometry measurement; while eliminating mechanical inertia, the measurement bandwidth can be increased to the order of 10kHz, and the cost is significantly reduced. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the structure of the present invention;

[0028] Figure 2 This is a diagram showing the cascade relationship between two QHQ modules. Detailed Implementation

[0029] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0030] like Figure 1 As shown, the structure of the non-mechanical fast phase-shifting interferometric measurement device of the present invention includes a laser 1, a beam expander 2, a polarizer 3, an unpolarized beam splitter NPBS 4, including a first unpolarized beam splitter NPBS 401 and a second unpolarized beam splitter NPBS 402, a reflector 5, including a first reflector 501 and a second reflector 502, a fast phase shifter 6, including a first module 61 and a second module 62, a test sample 7, an area array detector 8, a computer 9, and a drive control circuit 10, etc.

[0031] The laser 1 emits a parallel laser beam with a wavelength of λ, which can be selected from 405nm, 450nm, 532nm, 635nm, 1064nm, 1550nm, etc. This laser beam has a certain beam diameter, which is increased after being expanded by a beam expander 2, becoming comparable to the aperture of the area array detector 8, for example, 10mm. This laser beam can be unpolarized or polarized. If it is linearly polarized, its polarization direction cannot be perpendicular or nearly perpendicular to the transmission axis of the subsequent polarizer 3; otherwise, extinction or insufficient light intensity will occur, hindering the acquisition of the light intensity signal. If the laser is linearly polarized, the angle between its polarization direction and the transmission axis of the subsequent polarizer 3 can be selected as 45°, 60°, 75°, or other angles, ensuring that the received light intensity of the area array detector 8 does not saturate.

[0032] Polarizer 3 adjusts the polarization state of the laser beam to a horizontal direction, but it can also be vertical, facilitating unified polarization control and management of the subsequent interference optical path. Here, it is uniformly set to a horizontal direction.

[0033] Horizontally linearly polarized light is incident perpendicularly on the non-polarizing beam splitter NPBS401 and splits into two beams of equal intensity. One beam passes sequentially through the fast phase shifter 6 and the reflector 502, then is transmitted through the non-polarizing beam splitter NPBS402 and incident on the sensor surface of the area array detector 8. The other beam passes sequentially through the reflector 501 and the sample under test 7, and is reflected by the non-polarizing beam splitter NPBS402 before incident on the sensor surface of the area array detector 8. The transmission to reflection ratio of the non-polarizing beam splitter NPBS402 is 50:50, meaning the transmitted and reflected light intensities are equal and the polarization state of the incident light is not changed. The two beams interfere on the sensor surface of the area array detector 8, and the resulting interference fringe image is acquired by the area array detector 8 and transmitted to the computer 9 for data processing.

[0034] The fast phase shifter consists of two modules, 61 and 62, whose structure is as follows: Figure 2As shown. Module 61, arranged in the direction of light propagation, comprises a quarter-wave plate 611 (45° slow axis), a ferroelectric liquid crystal half-wave plate 612, and a quarter-wave plate 613 (45° slow axis), constituting QHQ module 1; Module 62, arranged in the direction of light propagation, comprises a quarter-wave plate 621 (45° slow axis), a ferroelectric liquid crystal half-wave plate 622, and a quarter-wave plate 623 (45° slow axis), constituting QHQ module 2; the QHQ modules can achieve geometric phase modulation, and the magnitude of the phase change... It depends on the angle β between the crystal axis direction of the half-wave plate and the horizontal direction, and is related to:

[0035] The crystal axis of the ferroelectric liquid crystal half-wave plates 612 and 622 is its slow axis direction. Under different voltage driving, two slow axis states can be generated, which correspond to two geometric phase magnitudes.

[0036] One example of a ferroelectric liquid crystal material is FD4004N (DIC, Japan), which is uniformly aligned with parallel light using Azo-dye SD1 (DIC, Japan).

[0037] The initial slow axis direction, i.e., the crystal axis direction, of the ferroelectric liquid crystal half-wave plate 612 is the orientation direction of the liquid crystal, set to β1 = 22.05°. When a positive voltage is applied (e.g., +5V), the orientation direction of the liquid crystal rotates to β1 = 44.1°, corresponding to a geometric phase of... When a negative voltage (e.g., -5V) is applied, the orientation of the liquid crystal rotates to the β1 = 0° direction, and the corresponding geometric phase is...

[0038] The initial slow axis direction, i.e., the crystal axis direction, of the ferroelectric liquid crystal half-wave plate 622 is the orientation direction of the liquid crystal, set to β2 = -22.05°. When a positive voltage is applied (e.g., +5V), the orientation direction of the liquid crystal rotates to the β2 = 0° direction, corresponding to a geometric phase of... When a negative voltage (e.g., -5V) is applied, the orientation of the liquid crystal rotates to the β2 = -44.1° direction, corresponding to a geometric phase of...

[0039] Modules 61 and 62 are connected in series, generating a total geometric phase. The sum of the geometric phases generated individually for modules 61 and 62, i.e. Under the action of positive and negative voltages respectively, the geometric phase generated by module 61 is: The geometric phase generated by module 62 is

[0040] Ferroelectric liquid crystal half-wave plates 612 and 622 are connected to computer 9 via drive control circuit 10. Computer 9 can issue commands to drive control circuit 10 to generate two positive and two negative voltages respectively, which control the liquid crystal orientation direction of ferroelectric liquid crystal half-wave plates 612 and 622, thereby achieving geometric phase modulation.

[0041] Since modules 61 and 62 can be independently applied with positive and negative voltages, the total geometric phase generated by the series connection of modules 61 and 62 is the sum of the geometric phases generated by modules 61 and 62 individually, and its set of value states is as follows: There are three possible geometric phase values. Therefore, the device of this invention can use a three-step phase-shifting algorithm to calculate the phase distribution.

[0042] The phase switching time in the three-step phase shift depends on the response times of the ferroelectric liquid crystal half-wave plates 612 and 622. For the FD4004N ferroelectric liquid crystal material, the typical time is 82 μs, corresponding to a bandwidth on the order of ~10 kHz. This enables non-mechanical, rapid phase-shifting interferometry measurements.

[0043] The non-mechanical rapid phase-shifting interferometry measurement method of the present invention includes the following steps:

[0044] (1) Open the image acquisition program in computer 9 and be ready to send instructions to the array detector 8 to acquire interference fringe images.

[0045] (2) The computer 9 issues a command to the drive control circuit 10 to generate two positive and two negative voltages respectively, which control the liquid crystal orientation of the ferroelectric liquid crystal half-wave plates 612 and 622, thereby realizing the adjustment of geometric phase. i = 1 to 3. The geometric phases corresponding to the positive and negative voltages are shown in Table 1.

[0046] Table 1. Geometric phases corresponding to positive and negative voltages.

[0047]

[0048] (3) Computer 9 issues a command to the area array detector 8 to acquire the interference fringe image and save the acquired interference image to the computer's memory. The light intensity distribution of the interference image is I = I i , i = 1 to 3.

[0049] (4) Repeat steps 2 and 3 three times to obtain three interference images I1, I2, and I3.

[0050] (5) The phase distribution of the sample under test is calculated based on the three-step phase shifting algorithm.

[0051] The three-step phase-shifting algorithm can be calculated using a more general formula as follows:

[0052]

[0053] If the three geometric phases are not exactly {-90°, 0°, 90°}, but rather {-α, 0°, α}, then a more general calculation formula is used:

[0054]

[0055] Where α is the phase step size of the three-step phase shift.

[0056] (6) The unwrapping phase distribution of the sample under test is obtained by using the unwrapping algorithm, and the measurement is completed.

Claims

1. A non-mechanical, rapid phase-shifting interferometric measurement device, characterized in that: The system includes a laser (1), a beam expander (2), a polarizer (3), a non-polarizing beam splitter (4), a reflector (5), a fast phase shifter (6), a test sample (7), an area array detector (8), a computer (9), and a drive control circuit (10). The non-polarizing beam splitter (4) includes a first non-polarizing beam splitter (401) and a second non-polarizing beam splitter (402). The reflector (5) includes a first reflector (501) and a second reflector (502). The laser (1) emits a parallel laser beam, which is expanded by the beam expander (2), and the polarization state of the laser beam is adjusted to the horizontal direction by the polarizer (3). Then, the beam is transmitted through the first non-polarizing beam splitter (401) and the reflector (502). The polarizing beam splitter (401) splits the light into two horizontally polarized beams of equal intensity. One beam passes through the fast phase shifter (6) and the second mirror (502), and is transmitted through the second unpolarized beam splitter (402) before being incident on the sensor surface of the array detector (8). The other beam passes through the first mirror (501) and the sample under test (7), and is reflected by the second unpolarized beam splitter (402) before being incident on the sensor surface of the array detector (8). The two beams interfere on the sensor surface of the array detector (8), and the resulting interference fringe image is acquired by the array detector (8) and transmitted to the computer (9) for data processing. The fast phase shifter (6) includes a first module (61) and a second module (62), both of which include a quarter-wave plate, a ferroelectric liquid crystal half-wave plate and a quarter-wave plate in sequence, forming a QHQ module; The QHQ module can achieve geometric phase modulation. The magnitude of the phase change φ depends on the angle β between the crystal axis direction of the half-wave plate and the horizontal direction, and there is a relationship: φ=2β.

2. The non-mechanical rapid phase-shifting interferometric measurement device according to claim 1, characterized in that: The crystal axis of the ferroelectric liquid crystal half-wave plate is its slow axis direction. Under different voltage driving conditions, two slow axis states are generated, with corresponding geometric phases of 0° and -90°.

3. The non-mechanical rapid phase-shifting interferometric measurement device according to claim 1, characterized in that: The first module (61) and the second module (62) are connected in series. The total geometric phase generated is the sum of the geometric phases generated by the first module (61) and the second module (62) individually. The set of its value states is φ={90°, -90°, 0°}, which includes three geometric phase values.

4. The non-mechanical rapid phase-shifting interferometric measurement device according to claim 1, characterized in that: The ferroelectric liquid crystal half-wave is connected to the computer (9) through the drive control circuit (10).

5. The non-mechanical rapid phase-shifting interferometric measurement device according to claim 1, characterized in that: The transmission to reflection ratio of the second non-polarizing beam splitter (402) is 50:

50.

6. The non-mechanical rapid phase-shifting interferometric measurement device according to claim 1, characterized in that: The parallel laser beam emitted by the laser (1) is either unpolarized or polarized light.

7. A non-mechanical, rapid phase-shifting interferometry method, applied to the apparatus described in any one of claims 1-6, characterized in that: Includes the following steps: (1) Open the image acquisition program in the computer and send the command to the area array detector to acquire the interference fringe image; (2) The computer issues instructions to drive the control circuit to generate two positive and negative voltages, which respectively control the liquid crystal orientation direction of the ferroelectric liquid crystal half-wave plate, thereby realizing the adjustment of geometric phase; (3) The computer issues a command, the array detector acquires the interference fringe image, and saves the acquired interference image into the computer's memory. The light intensity distribution of the interference image is I=I i , i = 1~3; (4) Repeat steps (2) and (3) three times to obtain three interference images I1, I2, and I3; (5) The encapsulation phase distribution of the test sample is calculated according to the three-step phase shifting algorithm; (6) The unwrapping phase distribution of the sample under test is obtained by using the unwrapping algorithm, and the measurement is completed.

8. The non-mechanical rapid phase-shifting interferometry method according to claim 7, characterized in that: The calculation formula for the three-step phase-shifting algorithm in step (5) is as follows: If the three geometric phases are not exactly {-90°, 0°, 90°}, but rather {-α, 0°, α}, then a more general calculation formula is used: Where α is the phase step size of the three-step phase shift.

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