A heterodyne interferometry system and method

Abstract

The present application relates to the technical field of heterodyne interferometry, and discloses a heterodyne interferometry system and method, wherein a first photodetector and a second photodetector are arranged in the system to synchronously acquire the light intensity noise of the diffraction light of an acousto-optic modulator, the signal measured by the first photodetector and the second photodetector is used to compensate the signal measured by a third photodetector and a fourth photodetector, so that the crosstalk interference signal is suppressed, that is, the influence of the crosstalk of the acousto-optic modulator driving signal on the heterodyne interferometry is suppressed, and the measurement precision is improved. Moreover, the present application adopts a highly similar double-layer optical path structure to reduce the introduction of the crosstalk and other noises of the acousto-optic modulator from the source, and significantly improves the common-mode rejection effect. The present application further arranges a piezoelectric actuator, so that the optical path is adjustable and controllable, and the system can actively and precisely compensate the drift of the optical path difference.

Description

Technical Field

[0001] This invention relates to the field of heterodyne interferometry technology, and in particular to a heterodyne interferometry system and method. Background Technology

[0002] Heterodyne interferometry, with its advantages of high sensitivity, wide dynamic range, and non-contact measurement, has been widely used in many fields such as precision metrology, semiconductor manufacturing, and optical inspection. In recent years, with the advancement of cutting-edge scientific missions such as space gravitational wave detection, extremely stringent requirements have been placed on the measurement accuracy of heterodyne interferometers, especially in the low-frequency range, where displacement measurement accuracy needs to be improved to picometers (10⁻⁶ ppm). -12 On the order of meters. At this level of precision, noise sources that were previously considered minor in the system begin to become key factors limiting the measurement limits.

[0003] The acousto-optic modulator (AOM) is one of the core components of a heterodyne interferometer, used to generate reference and measurement beams with stable frequency differences. In practical systems, the AOM's drive signal inevitably crosstalks into the interference signal link through electromagnetic coupling, power supply ripple, stray capacitance, and other paths. This AOM drive signal crosstalk introduces a typical periodic nonlinear error, manifested as a fluctuation error with the same period as the interference light wavelength superimposed on the displacement measurement. The magnitude and phase of this error change periodically with the measured displacement, severely degrading the measurement accuracy of the heterodyne interferometer at the picometer level.

[0004] To suppress the effects of this periodic nonlinear error, existing technologies typically employ two measures. First, common-mode suppression design is used, which involves constructing an interferometer structure where the reference and measurement optical paths are as symmetrical as possible. This ensures that the two beams experience common-mode interference during propagation, thus canceling out some of the common-mode noise during differential detection of the interference signal. Second, optical path control strategies are implemented. These strategies precisely control the optical path difference between the reference and measurement arms, either actively or passively, to ensure the system operates in a specific state with low AOM crosstalk noise, thereby reducing the equivalent displacement effect of the noise.

[0005] The aforementioned existing technical solutions have the following problems in practical applications: First, in the actual optical path layout of a heterodyne interferometer, the reference optical path and the measurement optical path are always spatially separated to a certain extent, making it impossible to achieve ideal complete overlap. This spatial separation means that the crosstalk paths and intensities of the AOM drive signals sensed by the two optical paths cannot be completely consistent, resulting in a limited common-mode suppression effect and difficulty in completely eliminating crosstalk noise. Second, the method of suppressing AOM drive signal crosstalk noise through optical path control directly depends on the precise setting and maintenance of the optical path difference. However, the effectiveness of this method is limited by factors such as system thermal drift and mechanical instability, and the suppression capability itself also has an upper limit determined by residual common-mode noise. Summary of the Invention

[0006] The purpose of this invention is to provide a heterodyne interferometric measurement system and method that can achieve good common-mode suppression and high measurement accuracy.

[0007] To achieve the above objectives, the present invention provides a heterodyne interferometry measurement system, comprising a laser (LASER), a first acousto-optic modulator (AOM1), a second acousto-optic modulator (AOM2), a first beam splitter (BS1), a second beam splitter (BS2), a first polarizing beam splitter (PBS1), a second polarizing beam splitter (PBS2), a first reflecting mirror (M1), a second reflecting mirror (M2), a third reflecting mirror (M3), a fourth reflecting mirror (M4), a first lateral displacement beam splitter (LDB1), a second lateral displacement beam splitter (LDB2), a first photodetector (PD1), a second photodetector (PD2), a third photodetector (PD3), and a fourth photodetector (PD4). The laser LASER is used to generate laser light; the first beam splitter BS1 is used to split the laser light into two beams, forming a first laser beam and a second laser beam. The first acousto-optic modulator AOM1 is used to frequency shift the first laser beam to emit a first diffracted light, which is reflected by the first reflector M1 to the first polarization beam splitter PBS1. The first polarization beam splitter PBS1 is used to separate the first diffracted light into a first polarization component and a second polarization component, wherein the first polarization component is incident as the background light to the first photodetector PD1 for light intensity reference measurement. The second polarization component is reflected by the third reflector M3 and then transmitted back to the first polarization beam splitter PBS1, and is guided by the first polarization beam splitter PBS1 to the second lateral displacement beam splitter LDB2. The second lateral displacement beam splitter LDB2 splits the incident beam into a second upper beam and a second lower beam that are spatially parallel to each other. The second upper beam is incident to the third photodetector PD3, and the second lower beam is incident to the fourth photodetector PD4. The second laser beam is reflected by the second mirror M2 to the second acousto-optic modulator AOM2. The second acousto-optic modulator AOM2 is used to frequency-shift the second laser beam to emit second diffracted light. The first lateral displacement beam splitter LDB1 splits the second diffracted light into a first upper beam and a first lower beam that are spatially parallel to each other. The second polarization beam splitter PBS2 is used to separate the first upper beam into a third polarization component and a fourth polarization component, and the first lower beam into a fifth polarization component and a sixth polarization component. The third polarization component is used as a reference light and is directly incident on the second photodetector PD2. The fourth polarization component... The first upper beam and the sixth polarization component are reflected by the fourth reflector M4 and the fifth reflector M5, respectively, and then transmitted back to the second polarization beam splitter PBS2. They then merge with the third polarization component and the fifth polarization component to form the first upper beam and the first lower beam, respectively. The first upper beam is split into the first upper component and the second upper component by the second beam splitter BS2. The first upper component is received by the third photodetector PD3 to form the first heterodyne signal. The first lower beam is split into the first lower component and the second lower component by the second beam splitter BS2. The second lower component is received by the fourth photodetector PD4 to form the fourth heterodyne signal.

[0008] In addition, the present invention also provides a heterodyne interferometry method, based on the above-mentioned heterodyne interferometry system, comprising: The first photodetector PD1, the third photodetector PD3, the fourth photodetector PD4, the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4 were calibrated respectively, and the calibration results were obtained. The calibration results are the light intensity ratio relationship and the phase delay relationship between each photodetector. Using the fourth reflecting mirror M4 as the test mirror and the fifth reflecting mirror M5 as the reference mirror, the laser is activated to allow the laser to enter the heterodyne interferometry system and acquire the signals measured by the first photodetector PD1, the second photodetector PD2, the third photodetector PD3 and the fourth photodetector PD4. The signal measured by the first photodetector PD1 is extracted by bandpass filtering to obtain the first crosstalk interference signal. The first compensation signal is obtained by adjusting the first crosstalk interference signal according to the calibration result. The signal measured by the second photodetector PD2 is extracted by bandpass filtering to obtain the second crosstalk interference signal. The second compensation signal is obtained by adjusting the second crosstalk interference signal according to the calibration results. The first heterodyne signal measured by the third photodetector PD3 and the second heterodyne signal measured by the fourth photodetector PD4 are acquired. The first heterodyne signal and the second heterodyne signal are compensated according to the first compensation signal and the second compensation signal to obtain the compensated first heterodyne signal and the compensated second heterodyne signal; The displacement test results are obtained based on the compensated first heterodyne signal and the compensated second heterodyne signal.

[0009] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention suppresses crosstalk interference signals by simultaneously acquiring the intensity noise of the diffracted light from the acousto-optic modulator using a first and second photodetector within the system. The signals measured by the first and second photodetectors are then used to compensate for the signals measured by the third and fourth photodetectors, thereby reducing the impact of acousto-optic modulator drive signal crosstalk on heterodyne interferometry measurements and improving measurement accuracy. Furthermore, this invention employs a highly similar dual-layer optical path structure, maximizing common-mode suppression at the hardware level. This effectively attenuates the common-mode noise level reaching the photodetectors, reducing the introduction of noise such as acousto-optic modulator crosstalk at the source and significantly improving common-mode suppression performance. The invention also incorporates a piezoelectric actuator, making the optical path adjustable and controllable. This allows the system to actively and precisely compensate for optical path difference drift, ensuring the dual-layer optical path always operates at the optimal point for common-mode suppression and providing the ability to flexibly adjust the optical path in different operating modes to adapt to varying measurement requirements. Attached Figure Description

[0010] Figure 1 This is a schematic diagram of the optical path of the heterodyne interferometry measurement system according to an embodiment of the present invention; Figure 2 This is a flowchart of the heterodyne interferometry measurement method according to an embodiment of the present invention.

[0011] In the diagram, LASER - laser, AOM1 - first acousto-optic modulator, AOM2 - second acousto-optic modulator, BS1 - first beam splitter, BS2 - second beam splitter, PBS1 - first polarizing beam splitter prism, PBS2 - second polarizing beam splitter prism, M1 - first reflector, M2 - second reflector, M3 - third reflector, M4 - fourth reflector, LDB1 - first lateral displacement beam splitter prism, LDB2 - second lateral displacement beam splitter prism, PD1 - first photodetector, PD2 - second photodetector, PD3 - third photodetector, PD4 - fourth photodetector, BD - beam terminator. Detailed Implementation

[0012] The specific embodiments of the present invention will be described in further detail below with reference to the accompanying drawings and examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

[0013] In the description of this invention, it should be noted that the terms "center," "longitudinal," "lateral," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention. Furthermore, the terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0014] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; and they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in this invention based on the specific circumstances.

[0015] Furthermore, in the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0016] Example 1 like Figure 1 As shown, a preferred embodiment of the present invention provides a heterodyne interferometry system, comprising a laser (LASER), a first acousto-optic modulator (AOM1), a second acousto-optic modulator (AOM2), a first beam splitter (BS1), a second beam splitter (BS2), a first polarizing beam splitter (PBS1), a second polarizing beam splitter (PBS2), a first reflecting mirror (M1), a second reflecting mirror (M2), a third reflecting mirror (M3), a fourth reflecting mirror (M4), a first lateral displacement beam splitter (LDB1), a second lateral displacement beam splitter (LDB2), a first photodetector (PD1), a second photodetector (PD2), a third photodetector (PD3), and a fourth photodetector (PD4). The laser LASER is used to generate laser light; the first beam splitter BS1 is used to split the laser light into two beams, forming the first laser beam and the second laser beam. The first acousto-optic modulator AOM1 is used to shift the frequency of the first laser beam to emit the first diffracted light. The first diffracted light is reflected by the first mirror M1 to the first polarization beam splitter PBS1. The first polarization beam splitter PBS1 is used to separate the first diffracted light into a first polarization component and a second polarization component. The first polarization component is used as the background light and is incident on the first photodetector PD1 for light intensity reference measurement. The second polarization component is reflected by the third mirror M3 and then transmitted back to the first polarization beam splitter PBS1, and is guided by the first polarization beam splitter PBS1 to the second lateral displacement beam splitter LDB2. The second lateral displacement beam splitter LDB2 splits the incident beam into a second upper beam and a second lower beam that are spatially parallel to each other. The second upper beam is incident on the third photodetector PD3, and the second lower beam is incident on the fourth photodetector PD4. The second laser beam is reflected by the second mirror M2 to the second acousto-optic modulator AOM2. AOM2 is used to frequency-shift the second laser beam, emitting the second diffracted light. The first lateral displacement beam splitter LDB1 splits the second diffracted light into a spatially parallel upper beam and a lower beam. The second polarization beam splitter PBS2 separates the upper beam into a third and fourth polarization component, and the lower beam into a fifth and sixth polarization component. The third polarization component serves as a reference light directly incident on the second photodetector PD2; the fourth polarization component and... The sixth polarization component is reflected by the fourth reflector M4 and the fifth reflector M5 and then transmitted back to the second polarization beam splitter PBS2, where it merges with the third polarization component and the fifth polarization component to form the first upper beam and the first lower beam, respectively. The first upper beam is split into the first upper component and the second upper component by the second beam splitter BS2. The first upper component is received by the third photodetector PD3 to form the first heterodyne signal. The first lower beam is split into the first lower component and the second lower component by the second beam splitter BS2. The second lower component is received by the fourth photodetector PD4 to form the fourth heterodyne signal.

[0017] This embodiment achieves simultaneous acquisition of the intensity and noise of the diffracted light from the first acousto-optic modulator AOM1 and the second acousto-optic modulator AOM2 by setting up a first photodetector PD1 and a second photodetector PD2 in the system. Based on the signals measured by the first and second photodetectors, the signals measured by the third and fourth photodetectors are compensated, thereby suppressing crosstalk interference signals. This suppresses the influence of crosstalk from the acousto-optic modulator drive signal on heterodyne interferometry measurements, improving measurement accuracy. Furthermore, this embodiment employs a highly similar dual-layer optical path structure, maximizing common-mode suppression at the hardware level. This effectively attenuates the common-mode noise level reaching the photodetectors, reducing the introduction of noise such as crosstalk from the acousto-optic modulators at the source and significantly improving the common-mode suppression effect.

[0018] Furthermore, the heterodyne interferometry system in this embodiment also includes a piezoelectric actuator (PZT), which drives the third reflecting mirror M3 to perform linear motion. The piezoelectric actuator (PZT) makes the optical path adjustable and controllable, enabling the system to actively and precisely compensate for the drift of the optical path difference, ensuring that the dual-layer optical path always maintains the operating point with optimal common-mode suppression, and possessing the ability to flexibly adjust the optical path in different operating modes to adapt to different measurement requirements.

[0019] Furthermore, the heterodyne interferometry system of this embodiment also includes: The first half-wave plate is positioned on the optical path between the first acousto-optic modulator AOM1 and the first beam splitter BS1. The second half-wave plate is positioned in the optical path between the second acousto-optic modulator AOM2 and the second reflector M2. The first quarter-wave plate is positioned in the optical path between the first polarizing beam splitter prism PBS1 and the third reflecting mirror M3; The second quarter-wave plate is positioned in the optical path between the second polarizing beam splitter prism PBS2 and the fourth and fifth reflecting mirrors M4 and M5.

[0020] The first half-wave plate is used to rotate the polarization direction of the beam emitted from the first beam splitter BS1 to align with the working principal axis of the crystal of the first acousto-optic modulator AOM1, ensuring maximum diffraction efficiency. The second half-wave plate is used to adjust the polarization direction of the beam emitted from the second acousto-optic modulator AOM2 to the specific angle required for subsequent optical paths. The first quarter-wave plate is used to change the light reflected back from the third mirror M3 from being transmitted through the first polarizing beam splitter PBS1 to being reflected by the first polarizing beam splitter PBS1. The second quarter-wave plate is used to allow the light reflected back by the fourth mirror M4 and the fifth mirror M5 in sequence to pass through the second polarizing beam splitter PBS2 in an orthogonal polarization state.

[0021] Furthermore, the heterodyne interferometry system in this embodiment also includes a polarizer LP, which is disposed in the optical path between the laser LASER and the first beam splitter BS1. The polarizer LP is used to achieve high-precision polarization state alignment to ensure the purity and signal quality of the subsequent optical path.

[0022] The optical path of the heterodyne interferometry system in this embodiment is as follows: Figure 1 As shown in Table 1, the paths to the first photodetector PD1, the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4 are all as shown in the table.

[0023] Table 1. Light paths reaching each photodetector

[0024] exist Figure 1 In the optical path, "Laser" refers to the laser, polarizer is represented by "LP", half-wave plate is represented by "λ / 2", and quarter-wave plate is represented by "λ / 4". Polarizers and waveplates in the optical path are used to adjust the polarization direction of the beam. The optical path includes two beam splitters, denoted as "BS1" and "BS2," where "BS1" is a beam-splitting prism and "BS2" is a beam-splitting plate, with a transmittance-reflectance ratio of 1:1. Two acousto-optic modulators, denoted as "AOM1" and "AOM2," are driven by 99.999MHz and 100MHz radio frequency signals, respectively, to frequency-shift the beam. The shift amount is equal to the radio frequency signal frequency. The output light from the acousto-optic modulators in the optical path is the first-order diffracted light. The optical path also includes two polarization beam-splitting prisms, "PBS1" and "PBS2," to separate a portion of the AOM diffracted light for background intensity measurement. Five mirrors, denoted as "M1," "M2," "M3," "M4," and "M5," are used to reflect the beam. Finally, two lateral displacement beam-splitting prisms, "LDB1" and "LDB2," are used to reflect the light beam. The beam is split into two parallel beams, dividing the optical path into two layers of similar optical paths. The upper optical path forms a measurement interferometer, and the lower optical path forms a reference interferometer. Four photodetectors, denoted as "PD1", "PD2", "PD3", and "PD4", are used in the optical path. PD1 measures the intensity of the diffracted light from AOM1, PD2 measures the intensity of the diffracted light from AOM2, PD3 detects the interference signal from the measurement interferometer, and PD4 detects the interference signal from the reference interferometer. All signals detected by the photodetectors are voltage signals, acquired by a computer, and their magnitude is proportional to the intensity of the detected beam. A piezoelectric actuator, denoted as "PZT" in the diagram, is used in the optical path to drive the third reflecting mirror M3, enabling it to move linearly. The three small rectangular blocks not labeled in the optical path are beam terminators, denoted as "BD", used to absorb unwanted light.

[0025] All light rays in the optical path are spatial light, and the beams propagate in straight lines. In this embodiment, after being reflected by the mirrors, all light rays are perpendicular or parallel to the incident light (there are no other angular relationships). Therefore, once the position of the laser (LASER) is determined, the positional relationships of other devices can be basically determined. It is important to note that the fifth mirror (M5) is located directly below the fourth mirror (M4). The directions of the first acousto-optic modulator (AOM1) and the second acousto-optic modulator (AOM2) need to be adjusted so that their outgoing light is first-order diffracted light (or both are -1st-order diffracted light). This ensures that the interference signal frequency is the frequency difference between the RF drive signals of the two acousto-optic modulators. Furthermore, the interferometer is an equal-arm length interferometer (equal-arm length interference reduces the influence of laser frequency noise), meaning that the two beams reaching the third photodetector (PD3) have equal optical path lengths, and the two beams reaching the fourth photodetector (PD4) also have equal optical path lengths. Equal-arm length interference can be achieved simply by adjusting the position of the third mirror (M3).

[0026] It should be pointed out that, Figure 1 The optical path shown does not represent the only positional relationship in the optical path design. This is because in actual applications, the devices also need to be installed on the support structure and adjustment structure, which have certain dimensions. For example, the distance between the first acousto-optic modulator AOM1 and the first beam splitter BS1 is limited to 5cm. The specific operation is determined according to the actual situation. However, the more compact the optical path, the smaller the impact of thermal noise and the higher the measurement accuracy. In actual operation, the optical path should be made as compact as possible.

[0027] Example 2 like Figure 2 As shown, a preferred embodiment of the present invention provides a heterodyne interferometry method, based on the heterodyne interferometry system of Embodiment 1, comprising: S1. Calibrate the first photodetector PD1, the third photodetector PD3, the fourth photodetector PD4, the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4 respectively, and obtain the calibration results. The calibration results are the light intensity ratio relationship and the phase delay relationship between each photodetector. S2. Using the fourth reflecting mirror M4 as the test mirror and the fifth reflecting mirror M5 as the reference mirror, start the laser to allow the laser to enter the heterodyne interferometry system and acquire the signals measured by the first photodetector PD1, the second photodetector PD2, the third photodetector PD3 and the fourth photodetector PD4. S3. The signal measured by the first photodetector PD1 is extracted by bandpass filtering to obtain the first crosstalk interference signal. The first crosstalk interference signal is adjusted according to the calibration result to obtain the first compensation signal. S4. The signal measured by the second photodetector PD2 is extracted by bandpass filtering to obtain the second crosstalk interference signal. The second crosstalk interference signal is adjusted according to the calibration results to obtain the second compensation signal. S5. Acquire the first heterodyne signal measured by the third photodetector PD3 and the second heterodyne signal measured by the fourth photodetector PD4; S6. Compensate the first heterodyne signal and the second heterodyne signal according to the first compensation signal and the second compensation signal to obtain the compensated first heterodyne signal and the compensated second heterodyne signal. S7. Obtain the displacement test results based on the compensated first heterodyne signal and the compensated second heterodyne signal.

[0028] In this embodiment, the heterodyne signals measured by the first photodetector PD1 and the second photodetector PD2 are used to compensate for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4. The crosstalk of the AOM drive signal in the heterodyne signals measured by the first photodetector PD1 and the second photodetector PD2 is removed based on these signals. The signals measured by the first photodetector PD1 and the second photodetector PD2 represent the intensity noise of the AOM diffraction light. This embodiment achieves accurate measurement by measuring the intensity of the AOM diffraction light and using it to suppress crosstalk in the heterodyne signal.

[0029] The photodetector PD calibration in step S1 specifically includes: The diffraction path of the second acousto-optic modulator AOM2 is blocked, and the signals measured by the first photodetector PD1, the third photodetector PD3, and the fourth photodetector PD4 are obtained. Based on the signals measured by the first photodetector PD1, the third photodetector PD3, and the fourth photodetector PD4, the ratio of light intensity measured by the first photodetector PD1, the third photodetector PD3, and the fourth photodetector PD4, as well as the phase delay of the same signal, are determined. The diffraction path of the first acousto-optic modulator AOM1 is blocked, and the signals measured by the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4 are obtained. Based on the signals measured by the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4, the ratio of light intensity measured by the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4, as well as the phase delay of the same signal, are determined.

[0030] In steps S3 and S4, the crosstalk interference signal is adjusted according to the calibration results, including: Based on the phase delay relationship, the time starting point of the crosstalk interference signal measured by the first photodetector PD1 and the second photodetector PD2 is adjusted to compensate for the phase deviation of the crosstalk interference signal. Based on the light intensity ratio, the overall amplitude of the crosstalk interference signal is multiplied by a preset calibration coefficient to compensate for the amplitude deviation of the crosstalk interference signal.

[0031] Furthermore, the heterodyne interferometry measurement method in this embodiment also includes: During the system power-on initialization phase, the piezoelectric actuator PZT is controlled to drive the third reflector M3 to produce continuous displacement. At the same time, the amplitude of the heterodyne signal measured by the third photodetector PD3 and the fourth photodetector PD4 is monitored in real time. When the amplitude of the heterodyne signal reaches the preset peak threshold, the displacement of the third reflector M3 is stopped and the piezoelectric actuator PZT is locked at the current voltage position to complete the initial optical path matching between the two interference beams.

[0032] During the measurement phase, the phase information of the AC beat frequency component in the signal measured by the third photodetector PD3 and the fourth photodetector PD4 is extracted in real time and compared with the preset target reference phase to generate a phase error signal. Based on the phase error signal, a dynamic compensation driving voltage is generated and applied to the piezoelectric actuator PZT to control the piezoelectric actuator PZT to generate nanoscale micro-displacement, so as to compensate for the optical path difference change caused by external environmental disturbances in real time and maintain the phase stability of the interference signal.

[0033] In this embodiment, a piezoelectric actuator (PZT) is used in the optical path to control the optical path. In addition to suppressing the periodic nonlinear error caused by crosstalk of the acousto-optic modulator (AOM) drive signal, it can also suppress the periodic nonlinear error caused by polarization device defects and stray light.

[0034] Therefore, this embodiment first needs to calibrate the differences in phase delay and gain between different photodetectors (PDs) for the same signal in order to obtain the correct compensation signal. To complete this process, the diffracted light of the second acousto-optic modulator (AOM2) is blocked, but the diffracted light of the first acousto-optic modulator (AOM1) is not blocked. Then, only the first photodetector (PD1), the third photodetector (PD3), and the fourth photodetector (PD4) have signals, and the ratio of their measured light intensity can be determined. The "crosstalk interference signal" is then extracted by digital bandpass filtering, and the phase is compared to obtain the difference in phase delay of the three photodetectors (PDs) for the same signal. Next, the diffracted light of the first acousto-optic modulator (AOM1) is blocked, but the diffracted light of the second acousto-optic modulator (AOM2) is not blocked. Based on the data measured by the second photodetector (PD2), the third photodetector (PD3), and the fourth photodetector (PD4), the ratio of their measured light intensity is determined, and the phase delay relationship is determined in the same way. This completes the PD calibration.

[0035] After PD calibration is completed, heterodyne interferometry experiments can be performed. The heterodyne interferometry measurement system in this embodiment is actually an interferometer. During the heterodyne interferometry experiment, the fourth reflecting mirror M4 and the fifth reflecting mirror M5 are the test mirror and the reference mirror, respectively. In actual use, the test mirror needs to be attached to the test object (it can only measure the displacement of the object along the beam propagation direction). The reference mirror should be placed directly below the fifth reflecting mirror M5 and fixed in place. After adjusting the positions of other components, they should be fixed in place. The interferometer can still work normally when the fourth reflecting mirror M4 is not directly below the fifth reflecting mirror M5, but the interferometer arm lengths will no longer be equal, and the influence of laser frequency noise will be amplified. It can be used normally when the measurement accuracy requirement is not high. After the measurement is completed, the signals measured by the first photodetector PD1 and the second photodetector PD2 are bandpass filtered to obtain the "crosstalk interference signal." The phase and amplitude are then adjusted to obtain the "compensation signal," which is used to compensate for noise in the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4. The compensation process involves subtracting the "compensation signal" from the interference signals measured by the third and fourth photodetectors PD3 and PD4, thus suppressing the "crosstalk interference signal" and inhibiting the influence of crosstalk from the acousto-optic modulator (AOM) drive signal on the heterodyne interference measurement. After noise compensation, the phase of the heterodyne signal is extracted, and the displacement test result is obtained by subtracting the phases of the two interference signals. In this embodiment, the heterodyne signal measured by the third photodetector PD3 and the fourth photodetector PD4 is also called the interference signal.

[0036] The working principle of the optical path and the principle of the measurement method of the heterodyne interferometry system in this embodiment are as follows: The angular frequency of the driving signal for the first acousto-optic modulator AOM1 is: The angular frequency of the driving signal for the second acousto-optic modulator AOM2 is... The complex amplitude of the diffraction photoelectric field of the first acousto-optic modulator AOM1 The expression is: ; Among them, A and These are the amplitude and phase of the AOM1 diffracted light, respectively; the second term Noise introduced for signal crosstalk, a and It is its amplitude and phase; similarly, the complex amplitude of the photoelectric field in AOM2 diffraction. The expression is: ; Among them, B and These are the amplitude and phase of the AOM2 diffracted light, respectively; the second term Noise introduced for signal crosstalk, b and It is its amplitude and phase, taking the angular frequency (beat frequency) of the interference signal. ; The intensity signal of the diffracted light from the first acousto-optic modulator AOM1 measured by the first photodetector PD1 in the heterodyne interferometry system is as follows: ; The intensity signal of the diffracted light from the second acousto-optic modulator AOM2, measured by the second photodetector PD2 in the heterodyne interferometry system, is as follows: ; The expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ; Ignoring low-frequency components and higher-order small quantities, the expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ; In the interference signal and The noise signal caused by crosstalk of the AOM drive signal is called the "crosstalk interference signal". It can be obtained by measuring the signals of PD1 and PD2 and then compensating for it, so as to obtain the ideal interference signal. Therefore, by compensating the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 based on the signals measured by the first photodetector PD1 and the second photodetector PD2, the ideal heterodyne signal is obtained as follows: .

[0037] It is important to note that different photodetectors may exhibit slight differences in phase delay and gain for the same weak signal. In other words, the phase and relative amplitude of the "crosstalk interference signal" measured by the first photodetector PD1 and the second photodetector PD2 may differ slightly from the signal. The phase and relative amplitude of the "crosstalk interference signal" may differ to some extent. In this embodiment, this difference is calibrated before testing. Based on the calibration results, adjusting the starting point of the time of the "crosstalk interference signal" measured by the first photodetector PD1 and the second photodetector PD2 can adjust its phase, and multiplying the whole by the calibrated coefficient can adjust its relative amplitude. The above is the principle of the present invention for suppressing crosstalk noise in the AOM drive signal.

[0038] Example 3 The difference between this embodiment and embodiment two is that, based on embodiment two, this embodiment introduces a spatial term in the expression for the complex amplitude of the electric field of the beam.

[0039] In the derivation of the formula in Example 2, to simplify the process and ignore the phase difference of the beam at different spatial positions, the third photodetector PD3 and the fourth photodetector PD4 measure the same signal. If a correct phase relationship is desired, it is only necessary to introduce a spatial term into the expression for the complex amplitude of the beam's electric field.

[0040] This embodiment introduces a spatial term into the expression for the complex amplitude of the electric field of the beam, as follows: Since the beam has a phase difference at different spatial locations, a spatial term is introduced into the expression for the complex amplitude of the beam's electric field. ,but: The angular frequency of the driving signal for the first acousto-optic modulator AOM1 is: The angular frequency of the driving signal for the second acousto-optic modulator AOM2 is The complex amplitude of the diffraction photoelectric field of the first acousto-optic modulator AOM1 The expression is: ; After the optical path Upon reaching the first photodetector PD1, the complex amplitude of the electric field at the first photodetector PD1 becomes: ; in The wavelength of the light beam is given by the signal measured by the first photodetector PD1: ; Similarly, the complex amplitude of the AOM2 diffraction photoelectric field of the second acousto-optic modulator The expression is: ; After the optical path Upon reaching the second photodetector PD2, the complex amplitude of the electric field at the second photodetector PD2 becomes: ; The signal measured by the second photodetector PD2 is: ; The expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ; Ignoring low-frequency components and higher-order minor quantities, the spatial phase terms will not cancel each other out. Therefore, the expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ; Based on the signals measured by the first photodetector PD1 and the second photodetector PD2, the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are compensated to obtain the ideal heterodyne signal: .

[0041] The symbol in the formula The symbol "proportional" indicates that there is a direct proportional relationship between two quantities. This is because the photodetector outputs a voltage signal whose magnitude is proportional to the intensity of the light beam.

[0042] Furthermore, step S7 obtains the displacement test results based on the compensated first heterodyne signal and the compensated second heterodyne signal, including: Phase extraction is performed on the compensated first heterodyne signal to obtain the first phase information. Phase extraction is performed on the compensated second heterodyne signal to obtain the second phase information; The displacement test result of the fourth reflector M4 is obtained by subtracting the first phase information and the second phase information.

[0043] The other steps in this embodiment are the same as in Embodiment 2, and will not be repeated here.

[0044] In summary, this invention provides a heterodyne interferometry system and method that suppresses noise caused by crosstalk in the AOM drive signal by measuring the intensity of the diffracted light from the acousto-optic modulator (AOM). Specifically, this is achieved by simultaneously acquiring the intensity noise of the diffracted light from the AOM using a first photodetector PD1 and a second photodetector PD2. The signals measured by the first and second photodetectors PD1 and PD2 are then used to compensate for the signals measured by the third and fourth photodetectors PD3 and PD4, thereby suppressing the crosstalk interference signal. This effectively suppresses the influence of AOM drive signal crosstalk on heterodyne interferometry, improving measurement accuracy. Furthermore, this invention employs a highly similar dual-layer optical path structure, maximizing common-mode suppression at the hardware level. This effectively attenuates the common-mode noise level reaching the photodetectors, reducing the introduction of noise such as AOM crosstalk at the source and significantly improving the common-mode suppression effect. This invention also incorporates a piezoelectric actuator (PZT), enabling adjustable and controllable optical path length. This allows the system to actively and precisely compensate for optical path difference drift, ensuring the dual-layer optical path always operates at the point of optimal common-mode suppression. Furthermore, it provides the ability to flexibly adjust the optical path length in different operating modes to adapt to varying measurement requirements. The measurement system and method of this invention utilize fewer components to meet the design requirements of a dual-optical-path equal-arm-length interferometer, while simultaneously providing optical path length control and synchronous measurement of interference light background noise. It also effectively suppresses crosstalk noise from the AOM drive signal, resulting in high measurement accuracy.

[0045] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and substitutions can be made without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present invention.

Claims

1. A heterodyne interferometric measurement system, characterized in that, It includes a laser (LASER), a first acousto-optic modulator (AOM1), a second acousto-optic modulator (AOM2), a first beam splitter (BS1), a second beam splitter (BS2), a first polarizing beam splitter (PBS1), a second polarizing beam splitter (PBS2), a first reflecting mirror (M1), a second reflecting mirror (M2), a third reflecting mirror (M3), a fourth reflecting mirror (M4), a first lateral displacement beam splitter (LDB1), a second lateral displacement beam splitter (LDB2), a first photodetector (PD1), a second photodetector (PD2), a third photodetector (PD3), and a fourth photodetector (PD4). The laser LASER is used to generate laser light; the first beam splitter BS1 is used to split the laser light into two beams, forming the first laser beam and the second laser beam. The first acousto-optic modulator AOM1 is used to shift the frequency of the first laser beam to emit the first diffracted light. The first diffracted light is reflected by the first mirror M1 to the first polarization beam splitter PBS1. The first polarization beam splitter PBS1 is used to separate the first diffracted light into a first polarization component and a second polarization component. The first polarization component is used as the background light and is incident on the first photodetector PD1 for light intensity reference measurement. The second polarization component is reflected by the third mirror M3 and then transmitted back to the first polarization beam splitter PBS1, and is guided by the first polarization beam splitter PBS1 to the second lateral displacement beam splitter LDB2. The second lateral displacement beam splitter LDB2 splits the incident beam into a second upper beam and a second lower beam that are spatially parallel to each other. The second upper beam is incident on the third photodetector PD3, and the second lower beam is incident on the fourth photodetector PD4. The second laser beam is reflected by the second mirror M2 to the second acousto-optic modulator AOM2. AOM2 is used to frequency-shift the second laser beam, emitting the second diffracted light. The first lateral displacement beam splitter LDB1 splits the second diffracted light into a spatially parallel upper beam and a lower beam. The second polarization beam splitter PBS2 separates the upper beam into a third and fourth polarization component, and the lower beam into a fifth and sixth polarization component. The third polarization component serves as a reference light directly incident on the second photodetector PD2; the fourth polarization component and... The sixth polarization component is reflected by the fourth reflector M4 and the fifth reflector M5 and then transmitted back to the second polarization beam splitter PBS2, where it merges with the third polarization component and the fifth polarization component to form the first upper beam and the first lower beam, respectively. The first upper beam is split into the first upper component and the second upper component by the second beam splitter BS2. The first upper component is received by the third photodetector PD3 to form the first heterodyne signal. The first lower beam is split into the first lower component and the second lower component by the second beam splitter BS2. The second lower component is received by the fourth photodetector PD4 to form the fourth heterodyne signal.

2. The heterodyne interferometry measurement system according to claim 1, characterized in that, It also includes a piezoelectric actuator PZT, which is used to drive the third reflector M3 to perform linear motion.

3. The heterodyne interferometry measurement system according to claim 1, characterized in that, Also includes: The first half-wave plate is disposed on the optical path between the first acousto-optic modulator AOM1 and the first beam splitter BS1; The second half-wave plate is disposed in the optical path between the second acousto-optic modulator AOM2 and the second reflector M2; The first quarter-wave plate is disposed in the optical path between the first polarizing beam splitter prism PBS1 and the third reflecting mirror M3; The second quarter-wave plate is disposed in the optical path between the second polarizing beam splitter prism PBS2 and the fourth reflector M4 and the fifth reflector M5.

4. A heterodyne interferometry method, based on the heterodyne interferometry system according to any one of claims 1-3, characterized in that, include: The first photodetector PD1, the third photodetector PD3, the fourth photodetector PD4, the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4 were calibrated respectively, and the calibration results were obtained. The calibration results are the light intensity ratio relationship and the phase delay relationship between each photodetector. Using the fourth reflecting mirror M4 as the test mirror and the fifth reflecting mirror M5 as the reference mirror, the laser is activated to allow the laser to enter the heterodyne interferometry system and acquire the signals measured by the first photodetector PD1, the second photodetector PD2, the third photodetector PD3 and the fourth photodetector PD4. The signal measured by the first photodetector PD1 is extracted by bandpass filtering to obtain the first crosstalk interference signal. The first compensation signal is obtained by adjusting the first crosstalk interference signal according to the calibration result. The signal measured by the second photodetector PD2 is extracted by bandpass filtering to obtain the second crosstalk interference signal. The second compensation signal is obtained by adjusting the second crosstalk interference signal according to the calibration results. The first heterodyne signal measured by the third photodetector PD3 and the second heterodyne signal measured by the fourth photodetector PD4 are acquired. The first heterodyne signal and the second heterodyne signal are compensated according to the first compensation signal and the second compensation signal to obtain the compensated first heterodyne signal and the compensated second heterodyne signal; The displacement test results are obtained based on the compensated first heterodyne signal and the compensated second heterodyne signal.

5. The heterodyne interferometry measurement method according to claim 4, characterized in that, The diffraction path of the second acousto-optic modulator AOM2 is blocked, and the signals measured by the first photodetector PD1, the third photodetector PD3, and the fourth photodetector PD4 are obtained. Based on the signals measured by the first photodetector PD1, the third photodetector PD3, and the fourth photodetector PD4, the ratio of light intensity measured by the first photodetector PD1, the third photodetector PD3, and the fourth photodetector PD4, as well as the phase delay of the same signal, are determined. The diffraction path of the first acousto-optic modulator AOM1 is blocked, and the signals measured by the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4 are obtained. Based on the signals measured by the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4, the ratio of light intensity measured by the second photodetector PD2, the third photodetector PD3, and the fourth photodetector PD4, as well as the phase delay of the same signal, are determined.

6. The heterodyne interferometry measurement method according to claim 4, characterized in that, Also includes: During the system power-on initialization phase, the piezoelectric actuator PZT is controlled to drive the third reflector M3 to produce continuous displacement. At the same time, the amplitude of the heterodyne signal measured by the third photodetector PD3 and the fourth photodetector PD4 is monitored in real time. When the amplitude of the heterodyne signal reaches the preset peak threshold, the displacement of the third reflector M3 is stopped and the piezoelectric actuator PZT is locked at the current voltage position to complete the initial optical path matching between the two interference beams.

7. The heterodyne interferometry method according to claim 4, characterized in that, Also includes: During the measurement phase, the phase information of the AC beat frequency component in the signal measured by the third photodetector PD3 and the fourth photodetector PD4 is extracted in real time and compared with the preset target reference phase to generate a phase error signal. A dynamic compensation driving voltage is generated based on the phase error signal and applied to the piezoelectric actuator PZT. This controls the piezoelectric actuator PZT to produce nanometer-level micro-displacement, thereby compensating for changes in optical path difference caused by external environmental disturbances in real time and maintaining the phase stability of the interference signal.

8. The heterodyne interferometry method according to claim 4, characterized in that, The angular frequency of the driving signal for the first acousto-optic modulator AOM1 is: The angular frequency of the driving signal for the second acousto-optic modulator AOM2 is... The complex amplitude of the diffraction photoelectric field of the first acousto-optic modulator AOM1 The expression is: ； Among them, A and These are the amplitude and phase of the AOM1 diffracted light, respectively; the second term Noise introduced for signal crosstalk, a and It is its amplitude and phase; similarly, the complex amplitude of the photoelectric field in AOM2 diffraction. The expression is: ； Among them, B and These are the amplitude and phase of the AOM2 diffracted light, respectively; the second term Noise introduced for signal crosstalk, b and It is its amplitude and phase, taking the angular frequency (beat frequency) of the interference signal. ; The intensity signal of the diffracted light from the first acousto-optic modulator AOM1 measured by the first photodetector PD1 in the heterodyne interferometry system is as follows: ； The intensity signal of the diffracted light from the second acousto-optic modulator AOM2, measured by the second photodetector PD2 in the heterodyne interferometry system, is as follows: ； The expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ； Ignoring low-frequency components and higher-order small quantities, the expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ； Based on the signals measured by the first photodetector PD1 and the second photodetector PD2, the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are compensated to obtain the ideal heterodyne signal: 。 9. The heterodyne interferometry method according to claim 4, characterized in that, Since the beam has a phase difference at different spatial locations, a spatial term is introduced into the expression for the complex amplitude of the beam's electric field. ,but: The angular frequency of the driving signal for the first acousto-optic modulator AOM1 is: The angular frequency of the driving signal for the second acousto-optic modulator AOM2 is The complex amplitude of the diffraction photoelectric field of the first acousto-optic modulator AOM1 The expression is: ； After the optical path Upon reaching the first photodetector PD1, the complex amplitude of the electric field at the first photodetector PD1 becomes: ； in The wavelength of the light beam is given by the signal measured by the first photodetector PD1: ； Similarly, the complex amplitude of the AOM2 diffraction photoelectric field of the second acousto-optic modulator The expression is: ； After the optical path Upon reaching the second photodetector PD2, the complex amplitude of the electric field at the second photodetector PD2 becomes: ； The signal measured by the second photodetector PD2 is: ； The expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ； Ignoring low-frequency components and higher-order minor quantities, the spatial phase terms will not cancel each other out. Therefore, the expressions for the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are as follows: ； Based on the signals measured by the first photodetector PD1 and the second photodetector PD2, the heterodyne signals measured by the third photodetector PD3 and the fourth photodetector PD4 are compensated to obtain the ideal heterodyne signal: 。 10. The heterodyne interferometry method according to claim 4, characterized in that, The process of obtaining displacement test results based on the compensated first heterodyne signal and the compensated second heterodyne signal includes: Phase extraction is performed on the compensated first heterodyne signal to obtain the first phase information. Phase extraction is performed on the compensated second heterodyne signal to obtain the second phase information; The displacement test result of the fourth reflector M4 is obtained by subtracting the first phase information and the second phase information.

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