Double-frequency laser heterodyne interference displacement measurement system and method based on electro-optical modulation

Abstract

The invention discloses a double-frequency laser heterodyne interference displacement measurement system and method based on electro-optical modulation, and mainly solves the problems that optical aliasing causes nonlinear errors and speed measurement is limited by frequency difference in the existing method. Comprising the following steps: 1) a laser generates fixed wavelength laser with stable frequency; 2) controlling the radio frequency modulation frequency through a signal generator so as to adjust the frequency difference; 3) MZM carrier suppression double-sideband modulation generates a double-sideband optical signal as double-frequency measurement light; 4) establishing a measurement light path, and converting the displacement of the target to be measured into Doppler frequency shift; according to the method, non-linear errors caused by double-optical-frequency orthogonal polarization aliasing are effectively avoided, the measurement accuracy is remarkably improved, and the requirements of different measurement scenes such as linear motion and vibration can be met.

Description

Technical Field

[0001] This invention belongs to the field of optical precision measurement technology, and further relates to a dual-frequency laser heterodyne interferometric displacement measurement method, specifically a dual-frequency laser heterodyne interferometric displacement measurement system and method based on electro-optic modulation, used for high-precision nanometer displacement measurement and high-speed target displacement measurement. Background Technology

[0002] Laser interferometry (DMLI) is a measurement technique that uses laser wavelength as a scale to recover target displacement information by acquiring the phase change of the interference spot. Due to its advantages such as large range, high resolution, high precision, non-contact operation, and traceability, it is widely used in precision metrology, microelectronic integrated equipment, and large scientific facilities, and is one of the important technologies in the field of ultra-precision displacement measurement. Typical interferometry systems can be divided into two main categories: single-frequency (zero-difference) laser interferometers and dual-frequency (heterodyne) laser interferometers. Heterodyne laser interferometry uses a dual-frequency laser as the light source, and loads the motion information of the measured target onto a stable carrier wave through the Doppler effect, enabling direct determination of the motion direction. High-precision phase detection technology can achieve nanometer-precision displacement measurement, making it even more widely applicable.

[0003] Since the detector cannot respond to high-frequency light waves, the interference beat frequency generated by two laser beams with different frequencies can only be used as the carrier. Dual-frequency light sources are the core of laser heterodyne interferometry technology. Traditional methods for generating dual-frequency lasers for interferometry include the Zeeman effect, acousto-optic modulation frequency shifting, dual longitudinal mode frequency locking, stress birefringence Zeeman effect, and double-reflective film optical frequency splitting. Among these, dual-frequency laser interferometers based on the Zeeman effect have a frequency difference of less than 4MHz, which greatly limits the measurement speed. Acousto-optic modulation frequency shifting technology can overcome the frequency difference limitation, obtaining dual-frequency lasers with controllable frequency difference by changing device parameters, thus improving the maximum measurement speed. However, its frequency stabilization, modulation, and beam combining processes make the optical structure relatively complex and difficult to adjust.

[0004] With the continuous development of ultra-precision engineering and nanotechnology, increasingly higher requirements are being placed on measurement accuracy, measurement speed, and system miniaturization. In addition to being highly susceptible to changes in temperature and refractive index in the measurement environment, laser heterodyne interferometry systems still face many bottlenecks: the frequency difference of the laser source limits the maximum measurement speed; the laser frequency stability introduces an error component proportional to the displacement; and optical aliasing caused by the non-orthogonality of dual-frequency lasers and polarization leakage leads to optical nonlinear errors of several nanometers, which cannot all be properly resolved. Summary of the Invention

[0005] The purpose of this invention is to address the shortcomings of existing technologies by proposing a dual-frequency laser heterodyne interferometric displacement measurement system and method based on electro-optic modulation. This primarily solves the problems of nonlinear errors caused by optical aliasing and speed limitations due to frequency difference in existing dual-frequency laser heterodyne interferometric measurement methods. The proposed solution avoids the nonlinear errors caused by orthogonal polarization aliasing of the two optical frequencies. The frequency difference can be flexibly adjusted from 0 to tens of GHz, and the theoretical maximum measurement speed can reach hundreds of meters per second, meeting the requirements of different measurement applications and possessing enormous application potential.

[0006] The basic idea behind this invention is as follows: the essence of dual-frequency laser heterodyne interferometric displacement measurement is to construct and solve a system of two linear equations involving the frequency difference and Doppler frequency shift. To obtain the heterodyne beat frequency containing displacement information, traditional methods can only rely on orthogonal polarization techniques for spatial beam separation design. This invention uses a Mach-Zehnder intensity modulator (MZM) to perform suppressed carrier double-sideband modulation (DSB) on the frequency-stabilized light, generating a double-sideband light source. Utilizing its unique symmetrical amplitude-frequency characteristics, a common-path design can also construct and solve the displacement equations, fundamentally eliminating the nonlinear errors caused by optical aliasing. First, an experimental system for measuring displacement using electro-optic modulated double-sideband dual-frequency laser heterodyne interferometry was established. A 1550nm frequency-stabilized laser emitted continuous light, which was split into a reference beam and a measurement beam by a beam splitter. The upper arm reference beam was directly connected to a 90° optical mixer. The lower arm measurement beam was first subjected to suppressed carrier double-sideband modulation by a Mach-Zehnder electro-optic modulator, with the radio frequency modulation frequency set by a signal generator. Then, the modulated beam entered the displacement generation section through a polarization-maintaining circulator, and the reflected light carrying displacement information was transmitted to the 90° optical mixer. The four interferometric light signals were transmitted to two balanced detectors (BPDs) for differential detection to obtain the in-phase (I) and quadrature (Q) components of the displacement signal. The signals were then synchronously sampled by a data acquisition card and uploaded to an FPGA for offline processing.

[0007] To achieve the above objectives, the technical solution of the present invention includes the following:

[0008] A dual-frequency laser heterodyne interferometric displacement measurement system based on electro-optic modulation includes an electro-optic modulation module, a measurement optical path module, and a photoelectric detection module; wherein the electro-optic modulation module provides dual-frequency measurement light and reference light to the measurement optical path module and the photoelectric detection module, respectively; the measurement optical path module outputs the dual-frequency measurement light carrying Doppler displacement information to the photoelectric detection module;

[0009] The electro-optic modulation module consists of a signal generator, a frequency-stabilized narrow-linewidth laser, a Mach-Zehnder intensity modulator (MZM), an optical fiber coupler, and an optical fiber circulator. It is used to generate a frequency-stabilized laser and positive and negative double-sideband lasers as dual-frequency measurement light.

[0010] The measurement optical path module consists of a collimator, a corner cube prism, a high-precision displacement guide rail, a signal generator, and a nanometer displacement stage. It is used to simulate different motion displacements of the target under test and generate corresponding Doppler frequency shifts.

[0011] The photoelectric detection module consists of a 90° optical mixer, two balanced detectors (BPDs), a data acquisition card, and an FPGA. It is used to perform optical digital coherent detection of the input dual-frequency measurement light and the reference light, and calculate the nanometer displacement using a high-precision Ap-FFT displacement algorithm.

[0012] Furthermore, in the aforementioned electro-optic modulation module, a frequency-stabilized narrow-linewidth laser generates a frequency-stabilized laser with a fixed wavelength of 1550nm and outputs it to an optical fiber coupler. The optical fiber coupler splits the frequency-stabilized laser into two beams: one beam is used as a reference beam and output to a 90° mixer in the photodetector module, and the other beam is output to an MZM modulator. The signal generator generates a fixed-frequency radio frequency signal as the dual-frequency difference and outputs it to the MZM modulator. The MZM modulator modulates the input fixed-frequency radio frequency signal with the frequency-stabilized laser to generate positive and negative double-sidebands as dual-frequency measurement light, which is output to an optical fiber circulator. The optical fiber circulator includes three ports: port 1 receives the dual-frequency measurement light, which is then sent to a collimator in the measurement optical path module through port 2, and simultaneously receives measurement light carrying Doppler frequency shift information, which is then sent to a 90° mixer in the photodetector module through port 3.

[0013] Furthermore, in the aforementioned measurement optical path module, the collimator is connected to port 2 of the fiber optic circulator of the electro-optic modulation module, realizing the coupling of the measurement light in the optical fiber and free space; the dual-frequency measurement light is incident on the corner prism, and the Doppler measurement light reflected back from the corner prism is received and output back to the fiber optic circulator; the corner prism is installed on the target to be measured, and the Doppler measurement light is reflected back to the collimator parallel to the original optical path; the high-precision displacement guide is used to generate uniform displacement; the nano-displacement stage is used to generate nano-vibration displacement; the signal generator generates a sinusoidal voltage to drive the nano-displacement stage to generate displacement.

[0014] Furthermore, in the aforementioned photoelectric detection module, the 90° optical mixer performs interference mixing on the received reference light and Doppler measurement light to generate four optical signals with phase differences of 0°, 90°, 180°, and 270°; the balanced detector is used for photoelectric conversion and DC elimination to generate two orthogonal electrical signals with phase differences of 0° and 90°; the acquisition card performs AD sampling to obtain the interference electrical signal; the FPGA integrates a high-precision Ap-FFT displacement algorithm to process the interference signal and obtain the displacement of the target under test with nanometer precision.

[0015] Furthermore, the present invention also proposes a ranging method based on the system described in claim 1, characterized by comprising the following steps:

[0016] (1) The laser generates a frequency-stable 1550nm fixed-wavelength laser. The power of the beam is split into two beams by an optical fiber coupler. One beam is used as a reference beam and output to the 90° mixer in the photodetector module. The other beam is used as a measurement beam and output to the MZM modulator.

[0017] (2) The signal generator produces a fixed frequency of Based on the actual measurement scenario's requirements for measurement speed, the minimum required RF modulation frequency and sampling frequency are calculated for the RF signal; and the dual-frequency difference is directly adjusted by adjusting the RF modulation frequency to achieve high-speed dynamic displacement measurement.

[0018] (3) The radio frequency signal generated by the signal generator and the frequency-stabilized laser generated by the laser are subjected to carrier-suppressed double-sideband modulation at the Mach-Zehnder intensity modulator (MZM). The bias voltage is set at the minimum bias point to generate the dual-frequency measurement light. The positive and negative double-sided bands are transmitted to the measurement optical path module via an optical fiber circulator;

[0019] (4) The measurement optical path module uses an integrated transceiver optical path. The corner cube prism is installed on the target to be measured. The motion of the target to be measured is simulated by a high-precision displacement guide rail and a nano-displacement stage. The corner cube prism transmits the incident dual-frequency measurement light. Parallel reflection back to the collimator yields Doppler measurement light superimposed with the Doppler frequency shift. This enables the coupling of measurement light between optical fiber and free space;

[0020] (5) The Doppler measurement light and the reference light interfere and beat at the 90° mixer. The interference light is detected by two balanced detectors (BPDs) to obtain orthogonal interference signals containing displacement measurement information. And collect data using a data acquisition card;

[0021] (6) The two orthogonal interference signals are sent into the FPGA, and the interference signals are processed by the high-precision Ap-FFT displacement algorithm to identify and obtain the phase difference corresponding to the target. and The implementation steps are as follows:

[0022] (6.1) Construct the complex signal form of the two interference signals, perform windowing and superposition preprocessing on the original data, and obtain the full phase spectrum and phase spectrum after FFT;

[0023] (6.2) Based on the spectral distribution, the center spectral lines of the upper and lower sidebands are identified. The frequency interval between the upper and lower sidebands is always equal to the frequency difference. The initial phase corresponding to the center spectral line of the sideband is approximately the phase at the data point. The phase invariance of the algorithm ensures the phase estimation accuracy of the center spectral line.

[0024] (6.3) The phase of the upper and lower sidebands is obtained by solving point by point. The phase ambiguity problem causes the calculated phase to have periodic jumps.

[0025] (6.4) By performing a phase unwrapping operation, the phase is restored to continuous phase. The phase of all data points is accumulated point by point to obtain the corresponding upper and lower sideband phase difference. and ;

[0026] (7) Construct a double-sideband laser interferometric displacement measurement formula, substitute the phase difference between the upper and lower sidebands into the displacement measurement formula, and calculate the displacement change of the target. .

[0027] Compared with the prior art, the present invention has the following advantages:

[0028] First, addressing the speed measurement limitations of traditional methods, where the Doppler frequency shift must be lower than the inherent frequency difference of the light source, this invention uses an electro-optic modulator to generate a double-sideband, with a dual-frequency difference in the radio frequency. Its adjustment method is simple and flexible, ranging from 0 to tens of GHz, and the theoretical maximum measurement speed can reach hundreds of meters per second. This can meet the displacement measurement requirements of different measurement occasions and solve the problem that the maximum measurement speed of traditional dual-optical-frequency interferometers is limited by the frequency difference of the laser light source.

[0029] Secondly, this invention generates dual-frequency lasers through external modulation, which facilitates the cascading of ultra-stable frequency lasers. The external frequency modulations applied to the upper and lower sidebands are always in opposite directions and equal, which is independent of frequency difference stability, thus locking the accuracy to the external ultra-stable frequency laser. Compared with the traditional internal modulation method, the frequency stability of this invention is significantly improved, effectively reducing Doppler frequency shift error.

[0030] Third, traditional dual-frequency optical paths often suffer from optical interference aliasing due to imperfect orthogonal polarization, which causes optical nonlinear errors, typically ranging from a few nanometers to tens of nanometers. This invention uses a dual-frequency interferometer measurement optical path based on electro-optic modulation double-sideband, which only requires ensuring linear polarization, thereby effectively simplifying the system and avoiding nonlinear errors caused by orthogonal polarization optical aliasing.

[0031] Fourth, since this invention uses an all-fiber optical path, the system structure is simplified compared to traditional rangefinders built with many polarizers; its light combining is simple, the spatial light structure does not require extremely high alignment accuracy, which significantly improves system stability and facilitates miniaturization applications. Attached Figure Description

[0032] Figure 1 This is a flowchart illustrating the implementation of the present invention;

[0033] Figure 2This is a schematic diagram of linear motion measurement provided in the experimental section of this invention;

[0034] Figure 3 This is a schematic diagram of vibration measurement provided in the experimental section of this invention; Detailed Implementation

[0035] The embodiments and effects of the present invention will be described in further detail below with reference to the accompanying drawings:

[0036] Example 1: Refer to Appendix Figure 1 The present invention provides a dual-frequency laser heterodyne interferometric displacement measurement system based on electro-optic modulation, comprising an electro-optic modulation module, a measurement optical path module, and a photoelectric detection module; wherein the electro-optic modulation module provides dual-frequency measurement light and reference light to the measurement optical path module and the photoelectric detection module respectively; the measurement optical path module outputs the dual-frequency measurement light carrying Doppler displacement information to the photoelectric detection module;

[0037] The electro-optic modulation module consists of a signal generator, a frequency-stabilized narrow-linewidth laser, a Mach-Zehnder intensity modulator (MZM), an optical fiber coupler, and an optical fiber circulator. It is used to generate a frequency-stabilized laser and positive and negative double-sideband lasers as dual-frequency measurement light.

[0038] The measurement optical path module consists of a collimator, a corner cube prism, a high-precision displacement guide rail, a signal generator, and a nanometer displacement stage. It is used to simulate different motion displacements of the target under test and generate corresponding Doppler frequency shifts.

[0039] The photoelectric detection module consists of a 90° optical mixer, two balanced detectors (BPDs), a data acquisition card, and an FPGA. It is used to perform optical digital coherent detection of the input dual-frequency measurement light and the reference light, and calculate the nanometer displacement using a high-precision Ap-FFT displacement algorithm.

[0040] In this embodiment, the electro-optic modulation module described above includes a frequency-stabilized narrow-linewidth laser that generates a 1550nm fixed-wavelength frequency-stabilized laser and outputs it to an optical fiber coupler. The optical fiber coupler splits the frequency-stabilized laser into two beams: one beam is used as a reference beam and output to a 90° mixer in the photodetector module, and the other beam is output to an MZM modulator. The signal generator generates a fixed-frequency radio frequency signal as the dual-frequency difference and outputs it to the MZM modulator. The MZM modulator modulates the input fixed-frequency radio frequency signal with the frequency-stabilized laser to generate positive and negative double-sidebands as dual-frequency measurement light, which is output to an optical fiber circulator. The optical fiber circulator includes three ports: port 1 receives the dual-frequency measurement light, and port 2 sends it to the collimator in the measurement optical path module. It also simultaneously receives measurement light carrying Doppler frequency shift information and port 3 sends the Doppler measurement light to the 90° mixer in the photodetector module.

[0041] The aforementioned measurement optical path module includes a collimator connected to port 2 of the fiber optic circulator of the electro-optic modulation module, enabling coupling of the measurement light between the optical fiber and free space. Dual-frequency measurement light is incident on a corner prism, and the Doppler measurement light reflected back from the corner prism is received and output back to the fiber optic circulator. The corner prism is mounted on the target and reflects the Doppler measurement light parallel to the original optical path back to the collimator. The high-precision displacement guide rail is used to generate uniform displacement. The nanoscale displacement stage is used to generate nanoscale vibration displacement. The signal generator generates a sinusoidal voltage to drive the nanoscale displacement stage to produce displacement.

[0042] In the aforementioned photoelectric detection module, the 90° optical mixer performs interference mixing on the received reference light and Doppler measurement light to generate four optical signals with phase differences of 0°, 90°, 180°, and 270°. The balanced detector is used for photoelectric conversion and DC elimination to generate two orthogonal electrical signals with phase differences of 0° and 90°. The acquisition card performs AD sampling to obtain the interference electrical signal. The FPGA integrates a high-precision Ap-FFT displacement algorithm to process the interference signal and obtain the displacement of the target under test with nanometer precision.

[0043] Example 2: Refer to Appendix Figure 1 The present invention provides a ranging method based on the system described in Embodiment 1, characterized by comprising the following steps:

[0044] Step 1) The laser generates a frequency-stabilized 1550nm fixed-wavelength laser. The power of the beam is split into two beams by an optical fiber coupler. One beam is used as a reference beam and output to the 90° mixer in the photodetector module. The other beam is used as a measurement beam and output to the MZM modulator.

[0045] In this embodiment, the reference signal output by the frequency-stabilized laser is represented as follows:

[0046] ,

[0047] in, It represents the frequency of a narrow-linewidth laser, and exp represents an exponential function with the natural constant e as its base. To represent a complex number, Indicates time.

[0048] Step 2) The signal generator produces a signal at a fixed frequency. Based on the actual measurement scenario's requirements for measurement speed, the minimum required RF modulation frequency and sampling frequency are calculated for the RF signal; and the dual-frequency difference is directly adjusted by adjusting the RF modulation frequency to achieve high-speed dynamic displacement measurement.

[0049] To achieve high-speed displacement measurement and ensure the quality of heterodyne interferometry signals, the radio frequency modulation frequency is required. Greater than the Doppler frequency shift caused by the motion of the target being measured:

[0050] ,

[0051] in, The instantaneous velocity of the target being measured. Let be the refractive index of the medium. The wavelength of the frequency-stabilized laser; in this embodiment, the frequency difference between the two frequencies is equal to the bandwidth of the double-sideband. It can flexibly adjust the modulation frequency, with the frequency difference continuously varying from 0 to tens of GHz, thus removing the limitation of the light source frequency difference on the measurement speed. Although the maximum measurement speed is further limited by the sampling frequency, when the sampling rate is 1 Gsps, the theoretical maximum measurement speed reaches hundreds of meters per second, which can meet the measurement speed requirements of different measurement scenarios.

[0052] Step 3) The radio frequency signal generated by the signal generator and the frequency-stabilized laser generated by the laser are subjected to carrier-suppressed double-sideband modulation at the Mach-Zehnder intensity modulator (MZM). The bias voltage is set at the minimum bias point to generate the dual-frequency measurement light. The positive and negative double-sided bands are transmitted to the measurement optical path module via an optical fiber circulator;

[0053] Step 4) The measurement optical path module uses a transceiver integrated optical path. A corner cube prism is mounted on the target under test. A high-precision displacement guide rail and a nanometer displacement stage simulate the motion of the target. The corner cube prism directs the incident dual-frequency measurement light... Parallel reflection back to the collimator yields Doppler measurement light superimposed with the Doppler frequency shift. This achieves coupling of the measurement light between the optical fiber and free space; the phase change during its motion is proportional to the displacement; in this embodiment, the meaningless amplitude and initial phase are ignored, and the incident dual-frequency measurement light is obtained according to the following formula. :

[0054] ,

[0055] in, This represents the dual-frequency measurement light at time t. It represents the frequency of a narrow-linewidth laser, and exp represents an exponential function with the natural constant e as its base. To represent a complex number, Indicates time;

[0056] When the target being measured moves, the Doppler effect manifests as a delay in the optical signal; the reflected Doppler measurement light is the Doppler measurement light superimposed with the Doppler frequency shift. , means as follows:

[0057] ,

[0058] in, The Doppler measurement light at time t represents the light emitted during the measurement. At the speed of light, The initial absolute distance difference between the propagation paths of the reference light and the measurement light before they meet; For the target being measured over time The amount of displacement change.

[0059] Step 5) The Doppler measurement light and the reference light interfere and beat at the 90° mixer. The interference light is detected by two balanced detectors (BPDs) to obtain orthogonal interference signals containing displacement measurement information. The data is collected using a data acquisition card; in this embodiment, the orthogonal interference signal is... , means as follows:

[0060] ,

[0061] The upper and lower sideband signals consist of a Doppler frequency shift term containing displacement information, a fixed carrier frequency offset term, and a fixed phase based on the initial absolute distance difference.

[0062] Step 6) Input the two orthogonal interference signals into the FPGA, process the interference signals using a high-precision Ap-FFT displacement algorithm, and identify and obtain the phase difference corresponding to the target. and The implementation steps are as follows:

[0063] (6.1) Construct the complex signal form of the two interference signals, perform windowing and superposition preprocessing on the original data, and obtain the full phase spectrum and phase spectrum after FFT;

[0064] (6.2) Based on the spectral distribution, the center spectral lines of the upper and lower sidebands are identified. The frequency interval between the upper and lower sidebands is always equal to the frequency difference. The initial phase corresponding to the center spectral line of the sideband is approximately the phase at the data point. The phase invariance of the algorithm ensures the phase estimation accuracy of the center spectral line.

[0065] (6.3) The phase of the upper and lower sidebands is obtained by solving point by point. The phase ambiguity problem causes the calculated phase to have periodic jumps.

[0066] (6.4) By performing a phase unwrapping operation, the phase is restored to continuous phase. The phase of all data points is accumulated point by point to obtain the corresponding upper and lower sideband phase difference. and :

[0067] ,

[0068] Among them, in the equations for the phase difference between the upper and lower sidebands, the radio frequency modulation frequency Errors can significantly affect the accuracy of displacement measurements, and this is related to the change in displacement. Treat them as unknown parameters for calculation and solution.

[0069] Step 7) Construct a double-sideband laser interferometric displacement measurement formula, substitute the phase difference between the upper and lower sidebands into the displacement measurement formula, and calculate the displacement change of the target being measured. :

[0070] ,

[0071] in, The wavelength of the frequency-stabilized laser is the benchmark for displacement measurement and directly affects measurement accuracy. The system supports external light source systems with higher power and frequency stability, improving ranging accuracy and facilitating applications in multi-axis measurement scenarios; due to the radio frequency modulation frequency... The difference is completely canceled out, and the ranging accuracy is no longer affected by the stability of the laser frequency difference.

[0072] The effectiveness of this invention can be further illustrated by the following experimental results.

[0073] I. Experimental Conditions

[0074] The laser wavelength is 1550nm, the signal carrier frequency is the fixed frequency difference of 1MHz applied by the signal generator, the number of full-phase FFT calculation points is 512, and the sampling frequency is set to 20MHz.

[0075] II. Experimental Content and Results

[0076] Experiment 1: Under the above experimental conditions, such as Figure 2 As shown, an experimental system for displacement measurement based on electro-optic modulation double-sideband was used to verify the accuracy of displacement measurement for uniformly moving targets. A displacement guide rail was used as the measurement reference, and a corner cube prism was fixed on the guide rail. The uniform motion of the corner cube prism was achieved by controlling the guide rail. The motion curve of the corner cube prism measured by this method was compared with the actual speed of the displacement guide rail, and the error was calculated to verify the system's accuracy in measuring the absolute distance to uniformly moving targets. Due to the limitations of the displacement range and maximum speed of the measurement guide rail, three different sets of experiments were designed, setting the corner cube prism's motion speed to 1 mm / s, 10 mm / s, and 100 mm / s, respectively. The motion curve of the corner cube prism was extracted by measuring its absolute distance.

[0077] Linear fitting was performed on the measured motion curve of the corner cube prism to obtain the measured speed of the guide rail. At a differential time of 1 second, the error between this speed and the applied ideal speed of the guide rail was within 1 μm / s. Further processing of the measurement results showed that the calculated displacement error was less than 1.5 μm, highly consistent with the calibration accuracy of 1.5 μm for the guide rail. This means that the accuracy of this method is better than the μm level. Comparing the displacement errors at three speeds, at a speed of 100 mm / s, a random walk phenomenon in the guide rail displacement was observed, which is related to the high-speed motion control mechanism of the guide rail. The experiment under different speed conditions preliminarily verified the high-speed displacement measurement capability of this method and its applicability for motion calibration measurement of high-precision guide rails across the entire measurement range.

[0078] Experiment 2: Using the displacement measurement system designed in this invention, the accuracy of the guide rail cannot be used to verify the measurement accuracy of this method, such as... Figure 3 As shown, a vibration experiment of a micro-displacement platform was designed. A corner cube prism was fixed on the micro-displacement platform, and a voltage was applied to the micro-displacement platform actuator through a signal generator to drive the platform's motion. The vibration amplitude was linearly related to the driving voltage. Six sets of experiments were designed to verify the measurement accuracy of this method when the target was vibrating. The six sets of experiments were set to have the corner cube prism vibrate in a sinusoidal manner with a driving frequency of 200Hz and driving voltages of 30mV, 40mV, 50mV, 60mV, 70mV, and 80mV, respectively.

[0079] like Figure 3 In (a), the dots represent the measured values ​​of the vibration amplitude of the corner cone prism under different driving voltages using this method. Based on the linear relationship between the driving voltage and the vibration amplitude, the measured values ​​are fitted once to obtain the displacement fitting curve. The fitting error is calculated based on the measured values ​​and the displacement fitting curve, and an error bar is drawn in conjunction with the root mean square error in the displacement measurement. Figure 3 (b) shows the displacement measurement results for 0.1s under a driving voltage of 60mV and 200Hz. Figure 3 (c) shows the standard sine fitting and cubic spline fitting of the measured values, along with a magnified view of the local area. Figure 3 In the figure, (d) represents the error between the measured value and the fitted curve.

[0080] The measured data corresponding to a 60mV, 200Hz driving voltage have a standard sinusoidal fitting amplitude of 60.021nm, a frequency of 199.896Hz, and a root mean square error of 1.22nm. Due to the hysteretic creep of the piezoelectric ceramic in the micro-displacement platform, the displacement of the corner cube prism itself is not a standard sine wave. Figure 3The (d) sinusoidal fitting error curve shows a consistent relationship between error and displacement. Since the fitting error also includes device error, the measurement error is less than 1.22 nm. Further cubic spline fitting of the measured values ​​yielded a fitting error of only 0.06 nm, which was used as the resolution indicator. Based on the sinusoidal fitting results at various voltages, the error bar for linear displacement fitting was obtained, indicating that the total displacement measurement error is less than 2 nm. Vibration experiments, under different vibration amplitude conditions, verified that the displacement measurement accuracy of this method is better than 3 nm, with a resolution in the sub-nanometer range.

[0081] The experimental results above show that the method of the present invention designs a novel dual-frequency laser interferometer based on electro-optic modulation double-sideband. Vibration and velocity experiments verify that the accuracy of the rangefinder is better than 3nm and the resolution is in the sub-nanometer range. It solves the problems of nonlinear error caused by optical aliasing and velocity measurement limited by frequency difference in the existing dual-frequency laser heterodyne interferometry method.

[0082] The above experiments demonstrate the correctness and effectiveness of the method proposed in this invention.

[0083] The parts of this invention not described in detail are common knowledge to those skilled in the art.

[0084] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Obviously, those skilled in the art, after understanding the content and principle of the present invention, may make various modifications and changes in form and detail without departing from the principle and structure of the present invention. However, these modifications and changes based on the concept of the present invention are still within the scope of protection of the claims of the present invention.

Claims

1. A dual-frequency laser heterodyne interferometric displacement measurement system based on electro-optic modulation, characterized in that, include: Electro-optic modulation module, measurement optical path module, and photoelectric detection module; The electro-optic modulation module provides dual-frequency measurement light and reference light for the measurement optical path module and the photoelectric detection module, respectively. The measurement optical path module outputs dual-frequency measurement light carrying Doppler displacement information to the photoelectric detection module; The electro-optic modulation module consists of a signal generator, a frequency-stabilized narrow-linewidth laser, a Mach-Zehnder intensity modulator (MZM), an optical fiber coupler, and an optical fiber circulator. It is used to generate a frequency-stabilized laser and positive and negative double-sideband lasers as dual-frequency measurement light. The measurement optical path module consists of a collimator, a corner cube prism, a high-precision displacement guide rail, a signal generator, and a nanometer displacement stage. It is used to simulate different motion displacements of the target under test and generate corresponding Doppler frequency shifts. The photoelectric detection module consists of a 90° optical mixer, two balanced detectors (BPDs), a data acquisition card, and an FPGA. It is used to perform optical digital coherent detection of the input dual-frequency measurement light and the reference light, and calculate the nanometer displacement using a high-precision Ap-FFT displacement algorithm.

2. The system according to claim 1, characterized in that: The electro-optic modulation module includes a frequency-stabilized narrow-linewidth laser that generates a 1550nm fixed-wavelength frequency-stabilized laser and outputs it to an optical fiber coupler. The optical fiber coupler splits the frequency-stabilized laser into two beams: one beam is used as a reference beam and output to a 90° mixer in the photodetector module, and the other beam is output to an MZM modulator. A signal generator generates a fixed-frequency radio frequency signal as the dual-frequency difference and outputs it to the MZM modulator. The MZM modulator modulates the input fixed-frequency radio frequency signal with the frequency-stabilized laser to generate positive and negative double-sidebands as dual-frequency measurement light, which is output to an optical fiber circulator. The optical fiber circulator has three ports: port 1 receives the dual-frequency measurement light, which is then sent to a collimator in the measurement optical path module through port 2. Simultaneously, it receives measurement light carrying Doppler frequency shift information, and sends the Doppler measurement light to the 90° mixer in the photodetector module through port 3.

3. The system according to claim 1, characterized in that: The measurement optical path module includes a collimator connected to port 2 of the fiber optic circulator of the electro-optic modulation module, enabling coupling of the measurement light between the optical fiber and free space. Dual-frequency measurement light is incident on a corner prism, and the Doppler measurement light reflected back from the corner prism is received and output back to the fiber optic circulator. The corner prism is mounted on the target and reflects the Doppler measurement light parallel to the original optical path back to the collimator. The high-precision displacement guide rail is used to generate uniform displacement. The nano-displacement stage is used to generate nano-vibration displacement. The signal generator generates a sinusoidal voltage to drive the nano-displacement stage to generate displacement.

4. The system according to claim 1, characterized in that: The photoelectric detection module includes a 90° optical mixer that performs interference mixing on the received reference light and Doppler measurement light to generate four optical signals with phase differences of 0°, 90°, 180°, and 270°. The balanced detector is used for photoelectric conversion and DC elimination to generate two orthogonal electrical signals with phase differences of 0° and 90°. The acquisition card performs AD sampling to obtain the interference electrical signal. The FPGA integrates a high-precision Ap-FFT displacement algorithm to process the interference signal and obtain the displacement of the target object with nanometer precision.

5. A ranging method based on the system of claim 1, characterized in that, Includes the following steps: (1) The laser generates a frequency-stable 1550nm fixed-wavelength laser. The power of the beam is split into two beams by an optical fiber coupler. One beam is used as a reference beam and output to the 90° mixer in the photodetector module. The other beam is used as a measurement beam and output to the MZM modulator. (2) The signal generator produces a fixed frequency of Based on the actual measurement scenario's requirements for measurement speed, the minimum required RF modulation frequency and sampling frequency are calculated for the RF signal; and the dual-frequency difference is directly adjusted by adjusting the RF modulation frequency to achieve high-speed dynamic displacement measurement. (3) The radio frequency signal generated by the signal generator and the frequency-stabilized laser generated by the laser are subjected to carrier-suppressed double-sideband modulation at the Mach-Zehnder intensity modulator (MZM). The bias voltage is set at the minimum bias point to generate the dual-frequency measurement light. The positive and negative double-sided bands are transmitted to the measurement optical path module via an optical fiber circulator; (4) The measurement optical path module uses an integrated transceiver optical path. The corner cube prism is installed on the target to be measured. The motion of the target to be measured is simulated by a high-precision displacement guide rail and a nano-displacement stage. The corner cube prism transmits the incident dual-frequency measurement light. Parallel reflection back to the collimator yields Doppler measurement light superimposed with the Doppler frequency shift. This enables the coupling of measurement light between optical fiber and free space; (5) The Doppler measurement light and the reference light interfere and beat at the 90° mixer. The interference light is detected by two balanced detectors (BPDs) to obtain orthogonal interference signals containing displacement measurement information. And collect data using a data acquisition card; (6) The two orthogonal interference signals are sent into the FPGA, and the interference signals are processed by the high-precision Ap-FFT displacement algorithm to identify and obtain the phase difference corresponding to the target. and The implementation steps are as follows: (6.1) Construct the complex signal form of the two interference signals, perform windowing and superposition preprocessing on the original data, and obtain the full phase spectrum and phase spectrum after FFT; (6.2) Based on the spectral distribution, the center spectral lines of the upper and lower sidebands are identified. The frequency interval between the upper and lower sidebands is always equal to the frequency difference. The initial phase corresponding to the center spectral line of the sideband is approximately the phase at the data point. The phase invariance of the algorithm ensures the phase estimation accuracy of the center spectral line. (6.3) The phase of the upper and lower sidebands is obtained by solving point by point. The phase ambiguity problem causes the calculated phase to have periodic jumps. (6.4) By performing a phase unwrapping operation, the phase is restored to continuous phase. The phase of all data points is accumulated point by point to obtain the corresponding upper and lower sideband phase difference. and ; (7) Construct a double-sideband laser interferometric displacement measurement formula, substitute the phase difference between the upper and lower sidebands into the displacement measurement formula, and calculate the displacement change of the target. .

6. The method according to claim 5, characterized in that: Step (2) involves calculating the minimum required radio frequency modulation frequency and sampling frequency based on the actual measurement scenario's requirements for measurement speed. Specifically, the required radio frequency modulation frequency is... Greater than the Doppler frequency shift caused by the motion of the target being measured: ， in, The instantaneous velocity of the target being measured. Let be the refractive index of the medium. The wavelength of the frequency-stabilized laser; The frequency difference between the two frequencies is equal to the bandwidth of the two sidebands. The maximum measurement speed is limited by the sampling frequency. When the sampling rate is 1 Gsps, the theoretical maximum measurement speed is 100 meters per second, which meets the measurement speed requirements of different measurement scenarios.

7. The method according to claim 5, characterized in that: Step (4) describes the dual-frequency measurement light. According to the following formula: ， in, This represents the dual-frequency measurement light at time t. It represents the frequency of a narrow-linewidth laser, and exp represents an exponential function with the natural constant e as its base. To represent a complex number, Indicates time; The Doppler measurement light superimposed with Doppler frequency shift According to the following formula: ， in, The Doppler measurement light at time t represents the light emitted during the measurement. At the speed of light, The initial absolute distance difference between the propagation paths of the reference light and the measurement light before they meet; For the target being measured over time The amount of displacement change.

8. The method according to claim 5, characterized in that: The orthogonal interference signal in step (5) The details are as follows: ， The upper and lower sideband signals consist of a Doppler frequency shift term containing displacement information, a fixed carrier frequency offset term, and a fixed phase based on the initial absolute distance difference.

9. The method according to claim 5, characterized in that: The phase difference between the upper and lower sidebands mentioned in step (6) and The details are as follows: ， Among them, in the equations for the phase difference between the upper and lower sidebands, the radio frequency modulation frequency Errors can significantly affect the accuracy of displacement measurements, and this is related to the change in displacement. Treat them as unknown parameters for calculation and solution.

10. The method according to claim 5, characterized in that: The double-sideband laser interferometric displacement measurement formula described in step (7) is expressed as follows: ， in, The wavelength of the frequency-stabilized laser is given.

Images