Laser interferometer nano-displacement measurement device and method with self-compensation for nonlinear error

Abstract

The present invention discloses a laser interference nano-displacement measurement device and method with self-compensation for nonlinear errors. The light beam output by the single-frequency laser in the present invention is converted into 45° linearly polarized light by a polarizer and directed to a laser interference unit composed of a polarization beam splitter, a quarter-wave plate, a fixed mirror, and a movable mirror to form an interference signal. The light beam then enters an electro-optical control unit with self-compensation for nonlinear errors, which is composed of an electro-optical crystal modulator, a polarization beam splitter, a photodetector, and a voltage controller. The difference is taken by taking the light intensity signal of the orthogonal photodetector, and the sum of the difference and the working voltage is used as the driving voltage of the electro-optical modulator. The PID control of the light intensity difference is adopted to realize automatic phase compensation, thereby compensating for the optical nonlinear error mainly caused by polarization aliasing. The present invention combines the polarization modulation detection method of the electro-optical effect with the single-frequency polarization interference length measurement, and realizes high-magnification subdivision of the interference fringes based on the precise modulation and demodulation of polarized light by the electro-optical crystal.

Description

Technical Field

[0001] The present invention belongs to the field of precision measurement technology, and in particular relates to a laser interference nano-displacement measurement device and method with self-compensation for nonlinear errors. Background Art

[0002] Laser interferometry nano-measurement technology, as a core detection method in the field of ultra-precision manufacturing, plays an irreplaceable role in semiconductor lithography equipment, nanoimprinting equipment and precision optical component processing. The displacement resolution of the laser interferometer depends on the interference fringe subdivision method. Existing high-resolution interferometers often use optical range-doubling subdivision combined with electronic subdivision.

[0003] Traditional optical rangefinder methods have certain advantages, but they also suffer from numerous issues, such as system complexity and sensitivity to alignment. Furthermore, imperfect optical components can generate harmonic components during optical rangefinder subdivision, limiting further improvements in interferometer resolution. Furthermore, optical nonlinear errors directly limit the advancement of laser interferometers toward sub-nanometer precision, making the measurement and compensation of nonlinear errors a research hotspot in interferometry.

[0004] Currently, the main methods for compensating and correcting the nonlinear errors of single-frequency laser interferometers include active compensation and passive compensation, which are used to suppress the three errors of unequal amplitude, non-orthogonality of phase, and DC offset in the interference signal. The main problem with active compensation, which adaptively corrects the nonlinear error signal, is its lack of real-time performance, making it difficult to achieve rapid compensation and high-precision processing of orthogonal signals. Passive compensation methods have strong real-time performance, but before measurement, the compensator needs to adjust factors such as the distance to the target and the surface reflectivity. It is also significantly affected by the measurement environment and requires regular calibration. Summary of the Invention

[0005] In view of the deficiencies in the prior art, the present invention provides a laser interferometer nano-displacement measurement device and method with self-compensation for nonlinear errors.

[0006] In a first aspect, the present invention provides a laser interferometer nano-displacement measurement device with self-compensation for nonlinear errors, comprising:

[0007] A light source module, used for providing a single-frequency laser beam;

[0008] a polarized light generating module, configured to convert the single-frequency laser beam into linearly polarized light;

[0009] a beam splitting module, for separating the linearly polarized light into two beams of polarized light and guiding them to a fixed mirror and a movable mirror respectively;

[0010] a polarization state conversion module, configured to convert the two polarized light beams into circularly polarized light, and then convert them back into linearly polarized light after reflection;

[0011] An interference synthesis module, used for synthesizing the two beams of linearly polarized light into one beam of linearly polarized light;

[0012] a polarization state modulation module, configured to perform polarization state modulation on the synthesized linearly polarized light;

[0013] A light intensity detection module, used to detect the light intensity in the X-axis direction and the light intensity in the Y-axis direction of the modulated linearly polarized light;

[0014] a signal processing module for processing the detection signal of the light intensity detection module to calculate the ellipticity angle and orientation angle of the polarized light; the orientation angle is used to determine the displacement of the movable mirror to achieve nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate for optical nonlinear errors caused by polarization aliasing in real time;

[0015] The error compensation control module is used to dynamically adjust the working state of the polarization state modulation module according to the ellipticity angle and orientation angle information to achieve self-compensation of optical nonlinear errors.

[0016] In a second aspect, the present invention provides a laser interferometer nano-displacement measurement method with self-compensation for nonlinear errors, using the above-mentioned measurement device, comprising the following steps:

[0017] Converting a single-frequency laser beam into linearly polarized light;

[0018] Splitting the linearly polarized light into two beams of polarized light, and guiding them to a fixed mirror and a movable mirror respectively;

[0019] Converting the two polarized lights into circularly polarized lights respectively, and converting them into linearly polarized lights again after reflection;

[0020] Combining the two linearly polarized light beams into one linearly polarized light beam;

[0021] performing polarization state modulation on the synthesized linearly polarized light;

[0022] Detecting the light intensity in the X-axis direction and the light intensity in the Y-axis direction of the modulated linearly polarized light;

[0023] Calculating the ellipticity angle and orientation angle of polarized light; the orientation angle is used to determine the displacement of the movable mirror to achieve nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate for optical nonlinear errors caused by polarization aliasing in real time;

[0024] According to the ellipticity angle and orientation angle information, the parameters of polarization state modulation are dynamically adjusted to achieve self-compensation of optical nonlinear errors.

[0025] Compared with the prior art, the present invention has the following beneficial effects:

[0026] 1. This invention studies the conversion of laser polarization states and how this is converted using optical components such as electro-optical crystal modulators (EOCMs). This method implements a homodyne polarization interferometer on a simple optical path. This method uses an EOCM to measure the polarization state of the linearly polarized light, thereby achieving interference fringe subdivision. For example, with a resolution of 0.001° for measuring the orientation angle of linearly polarized light, the interference fringe subdivision factor can reach 360,000.

[0027] 2. The present invention realizes automatic phase compensation through dual closed-loop control of the electro-optical crystal driving voltage, achieves precise modulation of the polarization state of the synthetic interference light, effectively controls the ellipticity angle to keep it at 0°, and compensates for the optical nonlinear error mainly caused by polarization aliasing.

[0028] 3. The present invention measures the ellipticity angle and orientation angle of linearly polarized light in real time through precise modulation and demodulation of polarized light, and obtains a high-speed, high-magnification interference fringe subdivision method with the help of existing electro-optic crystal phase modulation technology. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 This is a schematic diagram of the device according to an embodiment of the present application.

[0030] Figure 2 The ellipticity angle in the embodiment of this application within the measurement range of 1 μm is The amount of change.

[0031] Figure 3 Different ellipticity angles of the embodiments of this application Displacement curve under numerical value. DETAILED DESCRIPTION

[0032] The embodiments of the present application are further described below with reference to the accompanying drawings.

[0033] This application utilizes a single-frequency polarization phase-shifting optical path and electro-optical crystal laser polarization parameter phase modulation and demodulation technology to achieve high-resolution, low-noise measurement of the orientation angle of the interferometer's outgoing circularly polarized light, thereby achieving high-magnification segmentation of the interference fringes. By controlling the intensity difference between the circularly polarized light in the measurement and reference arms, the interferometer's polarization state is controlled, compensating for optical nonlinear errors primarily caused by polarization aliasing. This achieves self-compensation for optical nonlinear errors while simultaneously achieving high-magnification segmentation of the interference fringes, achieving nanometer measurement precision.

[0034] like Figure 1 As shown, the present application consists of a single-frequency laser 1, a polarizer 2, a first polarization beam splitter 3, a first quarter-wave plate 4, a fixed mirror 5, a second quarter-wave plate 6, a movable mirror 7, a third quarter-wave plate 8, a first electro-optical crystal modulator 9, a second electro-optical crystal modulator 10, a second polarization beam splitter 11, a voltage controller 12, a first photodetector 13 and a second photodetector 14.

[0035] The device of the present application can be divided into two parts: a laser interference unit and an electro-optical control unit for self-compensation of nonlinear errors. The laser interference unit comprises a single-frequency laser 1, a polarizer 2, a first polarization beam splitter 3, a first quarter-wave plate 4, a fixed mirror 5, a second quarter-wave plate 6, a movable mirror 7, and a third quarter-wave plate 8. The light beam output by the single-frequency laser 1 is converted into 45° linearly polarized light after passing through the polarizer 2 and is emitted to the first polarization beam splitter 3, where it is reflected and transmitted, forming S-polarized light and P-polarized light. The S-polarized light is reflected by the first quarter-wave plate 4 and converted into circularly polarized light, which is then sent to the fixed mirror 5. The P-polarized light is transmitted through the second quarter-wave plate 6 and converted into circularly polarized light, which is then sent to the movable mirror 7. The two beams are reflected by the fixed mirror 5 and the movable mirror 7, respectively, and then pass through the first quarter-wave plate 4 and the second quarter-wave plate 6 again before returning to the first polarization beam splitter 3 and merging into two linearly polarized beams. The two beams then pass through the third quarter-wave plate 8 to convert them into left-handed circularly polarized light and right-handed circularly polarized light. The two circularly polarized beams are then combined into linearly polarized light, which is the output light of the laser interference component.

[0036] Furthermore, the orientation angle of the linear polarized light emitted by the laser interference unit is The phase difference will change by ±Φ / 2 with the displacement direction of the movable mirror 7 (Φ is the phase difference caused by the movement of the movable mirror). The positive and negative signs are determined by the movement direction of the movable mirror. The phase change can be detected by detecting the orientation angle of the polarized light vibration plane. Measure the displacement of the moving mirror and orientation angle The relational expression is:

[0037] (1)

[0038] In the formula is the vacuum laser wavelength, is the refractive index of air.

[0039] The electro-optical control unit for self-compensation of nonlinear errors of the present application comprises a first electro-optical crystal modulator 9, a second electro-optical crystal modulator 10, a second polarization beam splitter 11, a voltage controller 12, a first photodetector 13, and a second photodetector 14. The output light of the laser interferometer unit enters the electro-optical control unit for self-compensation of nonlinear errors. The linearly polarized light passes through the first electro-optical crystal modulator 9, the second electro-optical crystal modulator 10, and the second polarization beam splitter 11. The second polarization beam splitter 11 splits the light beam into two orthogonal beams of P-polarized light and S-polarized light. The first photodetector 13 and the second photodetector 14 detect the light intensity in the X-axis and Y-axis directions. By taking the difference between the orthogonal photodetector light intensity signals, if the difference is not zero, it indicates that the amplitude of the output light of the laser interferometer unit is unequal or the polarization state is non-circularly polarized light. The light intensity difference signal detected by the two photodetectors is fed back to the voltage controller 12. The voltage controller dynamically adjusts the output voltage based on the feedback signal and acts on the two electro-optical crystal modulators, forming a dual closed-loop control system.

[0040] Furthermore, the nonlinear error self-compensation electro-optical control unit realizes dynamic control of the polarization state through electro-optical crystal phase modulation. Based on the voltage-sensitive characteristics of the electro-optical crystal, the birefringence coefficient of the crystal is changed by adjusting the applied voltage, thereby accurately controlling the linear polarization state output by the laser interferometer system. Among them, the phase delay of the first electro-optical crystal modulator 9 and the second electro-optical crystal modulator 10 is controlled by the amplitude output by the EOM driver control circuit. The voltage controller 12 dynamically adjusts the output voltage applied to the two electro-optical crystal modulators according to the feedback signal. This changes the driving voltage of the two electro-optical crystal modulators, and then changes the phase delay in the fast axis direction. By changing the phase delay of the EOM driver of the two electro-optical crystal modulators, the two electro-optical crystal modulators produce different phase delays.

[0041] After the outgoing light of the laser interference unit is modulated by the first electro-optic crystal modulator 9 and the second electro-optic crystal modulator 10, it is split into two orthogonal beams of P-polarized light and S-polarized light by the polarization beam splitter 11. The first photodetector 13 and the second photodetector 14 detect the light intensity in the X-axis direction and the light intensity in the Y-axis direction, respectively. By changing the driving voltage of the first electro-optic crystal modulator 9 and the second electro-optic crystal modulator 10, the phase delay in the fast axis direction can be changed. The first photodetector 13 and the second photodetector 14 then detect the light intensity in the X-axis direction and the light intensity in the Y-axis direction. The Stokes vector is calculated based on the difference in the two detected light intensities. Similarly, by changing the phase delay of the other two groups, the Stokes vector can be calculated. and . Ellipsity angle of polarized light and the orientation angle of polarized light can be 、 and Measured ellipticity angle and orientation angle The relationship expression with the Stokes vector is:

[0042] (2)

[0043] (3)

[0044] In the formula , the ellipticity angle of polarized light and the orientation angle of polarized light .

[0045] Furthermore, the relationship between the driving voltage of the electro-optical crystal and the phase delay is calibrated. The voltage value is changed multiple times, and the light intensity signal is received by the photodetector. The Stokes vector is obtained through the relationship between the light intensity signal and the Stokes parameter. Then, the ellipticity angle can be obtained by formulas (2) and (3). and orientation angle , thereby realizing the measurement of polarization parameters.

[0046] Furthermore, according to the actual driving characteristics of the electro-optic modulator, a high-order nonlinear log function high-frequency signal is used to drive the electro-optic crystal modulator to make the phase response characteristics close to linear. The interference signal is received by the photodetector and processed by the signal conditioning circuit hardware. At the same time, the interference signal data is collected and sent to the computer. The important factor affecting the polarization parameter detection speed is the intelligent algorithm of the polarization parameter. According to equations (2) and (3), the ellipticity angle and orientation angle The polarization state parameters are obtained by calculating the inverse tangent of the Stokes parameters. Considering computational speed, a CORDIC algorithm based on an FPGA and a pipelined architecture is used to implement polarization state parameter detection. Each iteration of the CORDIC algorithm involves only addition and shift operations, so a hardware circuit based on an iteration counter and shift register is used for implementation.

[0047] In the electro-optical control unit with self-compensation for nonlinear errors, the difference between the light intensity in the X-axis direction and the light intensity in the Y-axis direction detected by the orthogonal first photodetector 13 and the second photodetector 14 is taken. If the difference is not zero, the main reason is the optical nonlinear error caused by polarization aliasing, which causes the synthetic light to appear as elliptically polarized light rather than linearly polarized light. When the difference is not zero, the light intensity difference signal detected by the two photodetectors is fed back to the voltage controller 12, and the voltage controller dynamically adjusts the output voltage to act on the two electro-optical crystal modulators according to the feedback signal. The difference signal and the working voltage are summed as the driving voltage of the electro-optical crystal modulator, and the PID control of the light intensity difference is adopted. The phase is automatically compensated by double closed-loop control of the electro-optical crystal driving voltage, and precise modulation of the polarization state of the synthetic interference light is achieved. The ellipticity angle of the linearly polarized light can be effectively controlled by precisely modulating the polarization state of the synthetic interference light. By keeping it at 0°, the displacement value can be kept unaffected by the elliptical polarization of the polarized synthetic light, thereby compensating for the optical nonlinear error caused by polarization aliasing in the system.

[0048] Example:

[0049] The laser light source selected for the single-frequency laser 1 in the embodiment of the present application is a single-frequency He-Ne stabilized laser, and its wavelength calibration value is 632.991nm. The interference signal is sampled with high precision by using a 16-bit ADC high-resolution analog-to-digital converter, the analog signal is digitized, and then the accuracy is gradually improved based on the CORDIC algorithm. Each iteration optimizes the subdivision effect. After 12 iterations of the CORDIC algorithm and 16-bit A / D subdivision, the orientation angle is gradually refined. The angle is measured to 0.001°, so that each interference fringe period is subdivided into 360,000 parts. According to formula (1) the displacement and the orientation angle of the outgoing polarized light The theoretical relationship of , by substituting the numerical value, the corresponding displacement measurement resolution is 17pm. Due to the influence of optical nonlinear error on the polarization state of synthetic interference light, it is reflected in the ellipticity angle It is not always zero when moving the mirror. Figure 2 As shown in Figure 2, multiple experiments have shown that the range of the ellipticity angle change over a 1µm travel is [-2.5°, 2.5°]. Figure 3 As shown, different ellipticity angles Therefore, the phase automatic compensation is achieved by controlling the driving voltage of the electro-optical crystal in a double closed loop, and the polarization state of the synthetic interference light is precisely modulated, effectively controlling the ellipticity angle. Keeping it at 0° compensates for optical nonlinear errors caused primarily by polarization aliasing.

[0050] As can be seen from the examples so far, the present application organically integrates laser polarization interference and electro-optical crystal phase modulation methods, establishes a functional relationship between the displacement value of the movable mirror and the polarization parameters of the interference light, and obtains the polarization light ellipticity angle and orientation angle values ​​in real time through precise modulation and demodulation of the polarized light. Combined with the electro-optical crystal phase modulation technology, a high-speed, high-magnification interference fringe subdivision method is obtained. At the same time, an electro-optical control system is constructed based on electro-optical crystal phase modulation, and the polarization state of the interferometer is controlled by controlling the intensity difference of the circularly polarized light in the measuring arm and the reference arm to compensate for the optical nonlinear error mainly caused by optical polarization aliasing. Under strictly controlled experimental environment conditions, nanometer-level displacement measurement accuracy is achieved through self-compensation of optical nonlinear errors.

[0051] The above specific embodiments are used to illustrate the present invention rather than to limit the present invention. Any modifications and changes made to the present invention within the spirit of the present invention and the protection scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A laser interferometer nano-displacement measurement device with self-compensation for nonlinear errors, characterized in that: include: A light source module, used for providing a single-frequency laser beam; a polarized light generating module, configured to convert the single-frequency laser beam into linearly polarized light; a beam splitting module, for separating the linearly polarized light into two beams of polarized light and guiding them to a fixed mirror and a movable mirror respectively; a polarization state conversion module, configured to convert the two polarized light beams into circularly polarized light, and then convert them back into linearly polarized light after reflection; An interference synthesis module, used for synthesizing the two beams of linearly polarized light into one beam of linearly polarized light; a polarization state modulation module, configured to perform polarization state modulation on the synthesized linearly polarized light; A light intensity detection module, used to detect the light intensity in the X-axis direction and the light intensity in the Y-axis direction of the modulated linearly polarized light; a signal processing module for processing the detection signal of the light intensity detection module to calculate the ellipticity angle and orientation angle of the polarized light; the orientation angle is used to determine the displacement of the movable mirror to achieve nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate for optical nonlinear errors caused by polarization aliasing in real time; an error compensation control module, configured to dynamically adjust the working state of the polarization state modulation module according to the ellipticity angle and orientation angle information, thereby achieving self-compensation of optical nonlinear errors; The electro-optical control unit for self-compensation of nonlinear errors is composed of a first electro-optical crystal modulator, a second electro-optical crystal modulator, a second polarization beam splitter, a voltage controller, a first photodetector and a second photodetector; The linearly polarized light passes through the first electro-optic crystal modulator, the second electro-optic crystal modulator, and the second polarization beam splitter. The second polarization beam splitter splits the light beam into two orthogonal beams of P-polarized light and S-polarized light. The first photodetector and the second photodetector detect the light intensity in the X-axis direction and the light intensity in the Y-axis direction. The light intensity difference signal detected by the two photodetectors is fed back to the voltage controller, which dynamically adjusts the output voltage based on the feedback signal to act on the two electro-optic crystal modulators. By controlling the driving voltage of the electro-optical crystal, automatic phase compensation is achieved, precise modulation of the polarization state of the synthetic interference light is achieved, and the ellipticity angle is controlled. Keeping it at 0° compensates for optical nonlinear errors caused primarily by polarization aliasing.

2. The laser interferometer nano-displacement measuring device with self-compensation for nonlinear error according to claim 1, characterized in that: The beam separation module includes a polarization beam splitter for separating the linearly polarized light into S-polarized light and P-polarized light.

3. The laser interferometer nano-displacement measuring device with self-compensation for nonlinear error according to claim 1 or 2, characterized in that: The polarization state conversion module includes a quarter wave plate, which is used to convert the polarized light into circularly polarized light and then convert it back into linearly polarized light after reflection.

4. The laser interferometer nano-displacement measuring device with self-compensation for nonlinear error according to claim 3, characterized in that: The polarization state modulation module includes an electro-optical crystal modulator, and the phase delay of the electro-optical crystal modulator is dynamically adjusted by a voltage controller.

5. The laser interferometer nano-displacement measuring device with self-compensation for nonlinear error according to claim 4, characterized in that: The signal processing module uses the FPGA-based CORDIC algorithm to calculate the ellipticity angle and orientation angle to improve the calculation speed and accuracy.

6. A laser interferometer nano-displacement measurement method with self-compensation for nonlinear errors, using the device according to any one of claims 1 to 5, characterized in that: The following steps are involved: Converting a single-frequency laser beam into linearly polarized light; Splitting the linearly polarized light into two beams of polarized light, and guiding them to a fixed mirror and a movable mirror respectively; Converting the two polarized lights into circularly polarized lights respectively, and converting them into linearly polarized lights again after reflection; Combining the two linearly polarized light beams into one linearly polarized light beam; performing polarization state modulation on the synthesized linearly polarized light; Detecting the light intensity in the X-axis direction and the light intensity in the Y-axis direction of the modulated linearly polarized light; Calculating the ellipticity angle and orientation angle of polarized light; the orientation angle is used to determine the displacement of the movable mirror to achieve nanometer-level displacement measurement accuracy, and the ellipticity angle is used to monitor and compensate for optical nonlinear errors caused by polarization aliasing in real time; According to the ellipticity angle and orientation angle information, the parameters of polarization state modulation are dynamically adjusted to achieve self-compensation of optical nonlinear errors.

7. The laser interferometer nano-displacement measurement method with self-compensation for nonlinear errors according to claim 6, characterized in that: The step of modulating the polarization state of the synthesized linearly polarized light includes using an electro-optical crystal modulator, wherein the phase delay of the electro-optical crystal modulator is dynamically adjusted by a voltage controller.

8. The laser interferometer nano-displacement measurement method with self-compensation for nonlinear errors according to claim 7, characterized in that: The step of calculating the ellipticity angle and orientation angle of the polarized light includes using an FPGA-based CORDIC algorithm to perform calculations to improve calculation speed and accuracy.

9. The laser interferometer nano-displacement measurement method with self-compensation for nonlinear errors according to claim 7, characterized in that: The voltage controller includes a proportional-integral-differential controller, which is used to dynamically adjust the applied voltage according to the light intensity signal to achieve precise adjustment of the phase delay amount.

10. The laser interferometer nano-displacement measurement method with self-compensation for nonlinear errors according to claim 8, characterized in that: The method further includes calibrating the relationship between the driving voltage and the phase delay of the electro-optical crystal modulator, and establishing a mapping relationship between the voltage and the phase delay by changing the voltage value multiple times and measuring the corresponding light intensity signal.

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