A micro-vibration measuring device and method based on elastic-optic phase heterodyne modulation

Abstract

The application belongs to the field of optical precision measurement, and particularly relates to a kind of microvibration measurement device and method of elastic light phase heterodyne modulation, light source emits detection beam and is divided into reference beam and measurement beam by unpolarized light splitting prism, the direction of polarization axis of reference arm polarizer is the first direction, the direction of polarization axis of measurement arm polarizer is the second direction perpendicular to the first direction, two beams of reflected light are combined after passing through unpolarized light splitting prism in turn through elastic light modulator and polarizer, elastic light modulator modulation fast axis is collinear with the first direction, the polarization axis of polarizer is 45 degrees with the first direction, photoelectric detector converts interference light signal into electrical signal, signal processing module carries out digital lock-in demodulation and extracts one frequency doubling and two frequency doubling signal amplitude, and host computer is based on the ratio of the two to retrieve microvibration displacement.The application does not need mechanical adjusting component, realizes high resolution, large dynamic range and high environmental adaptability of microvibration precision measurement through elastic light phase heterodyne modulation and ratio demodulation.

Description

Technical Field

[0001] This invention relates to the field of optical precision measurement, and in particular to a micro-vibration measurement device and method based on elastic-optical phase heterodyne modulation. Background Technology

[0002] High-precision non-contact vibration measurement is a core supporting technology in fields such as precision manufacturing, dynamic characterization of microelectromechanical systems (MEMS), structural health monitoring in aerospace, and performance testing of ultrasonic devices. With the rapid development of micro- and nano-devices towards miniaturization and high frequency, industrial and scientific research scenarios are placing higher demands on the displacement resolution, measurement bandwidth, long-term stability, and environmental adaptability of vibration measurements. Achieving picometer-level resolution non-contact stable measurement has become a key research direction in this field.

[0003] Currently, the mainstream technologies for measuring micro-vibrations include laser Doppler vibration measurement, fiber optic sensing vibration measurement, and laser interferometry. Laser Doppler vibration measurement, based on the optical Doppler effect, inverts vibration velocity and displacement by detecting the frequency shift of reflected / scattered light. Its displacement resolution can reach the picometer level, and the measurement bandwidth covers DC to tens of MHz. However, the system structure is complex, costly, and easily affected by changes in the reflectivity of the measured surface. Fiber optic sensing vibration measurement mainly uses fiber Bragg gratings or interferometric fiber structures, detecting vibration parameters through wavelength demodulation. It has strong anti-electromagnetic interference capabilities and can perform distributed measurements. However, its displacement resolution is only at the nanometer level and it is a contact measurement; the sensing unit must be fixed to the measured surface, making it difficult to meet the non-contact requirements of ultra-precision measurements. Laser interferometry, due to its non-contact, high precision, and high sensitivity advantages, has become the mainstream technology for high-precision measurement of micro-vibrations. Among them, the Michelson interferometer structure, with its simple optical path architecture, convenient adjustment, and high displacement resolution, is widely used in vibration metrology and precision testing scenarios, achieving sub-nanometer displacement resolution. It is currently the mainstream benchmark calibration scheme in the field of vibration metrology.

[0004] However, traditional Michelson interferometric vibration measurement technology still has the following inherent defects: First, it has poor environmental adaptability, and the optical path is sensitive to temperature gradients, airflow disturbances, and environmental vibrations, making the measurement stability easily affected by interference. Second, the introduction of mechanical adjustment components in traditional piezoelectric ceramic phase shifting or mechanical scanning methods results in nonlinear errors, slow response speeds, and long-term drift problems. Third, although existing elastic-optical modulation interferometric vibration measurement schemes have improved the system's anti-interference capability to some extent, they still have technical bottlenecks such as insufficient polarization control accuracy, low optical path modulation efficiency, and poor adaptability of demodulation algorithms. It is difficult to simultaneously achieve high-precision measurement of micro-vibrations with picometer-level ultra-high resolution, wide measurement frequency band, strong environmental adaptability, and high long-term stability, which restricts the widespread application of this type of technology in precision manufacturing site monitoring, dynamic testing of micro-nano devices, and other scenarios. Summary of the Invention

[0005] The purpose of this invention is to provide a micro-vibration measurement device and method based on elastic-optical phase heterodyne modulation, aiming to solve the problem that existing micro-vibration measurement technologies cannot simultaneously achieve picometer-level resolution, wide dynamic range, long-term stability, and environmental adaptability.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a micro-vibration measurement device based on elastic-optical phase heterodyne modulation, comprising: a light source for emitting a detection beam; a non-polarizing beam splitter prism disposed on the outgoing optical path of the light source for splitting the detection beam into a reference beam and a measurement beam, and combining the reflected reference beam and the measurement beam; a reference arm having a first polarizer and a first reflector sequentially disposed along the propagation direction of the reference beam, the polarization axis of the first polarizer being a first direction; a measurement arm having a second polarizer and a second reflector sequentially disposed along the propagation direction of the measurement beam, the second reflector being fixed to the micro-vibration object to be measured, the polarization axis of the second polarizer being a second direction, the second direction being perpendicular to the first direction; the reference beam returning to the non-polarizing beam splitter prism after reflection by the first reflector, the measurement beam returning to the non-polarizing beam splitter prism after reflection by the second reflector, the two reflected beams being combined into a single optical path by the non-polarizing beam splitter prism; and an elastic-optical modulator. In the output optical path of the non-polarizing beam splitter, a periodic phase modulation is applied to the combined beam. The fast modulation axis of the optical modulator is collinear with the first direction. A polarizer is disposed in the output optical path of the optical modulator, and the angle between the polarization axis of the polarizer and the first direction is 45°. A photodetector is disposed in the output optical path of the polarizer and is used to convert the received interference light signal into an electrical signal. A signal processing module is connected to the output end of the photodetector and the control end of the optical modulator, respectively. It is used to output a drive signal to the optical modulator and synchronously generate a reference signal, as well as to receive the electrical signal for digital phase-locked demodulation and extract the amplitude of the first harmonic signal and the amplitude of the second harmonic signal. A host computer is connected to the signal processing module and is used to invert the phase delay based on the ratio of the amplitude of the first harmonic signal to the amplitude of the second harmonic signal, and to calculate the displacement of the measured micro-vibration based on the phase delay. During the measurement process, the optical path and the modulation state remain statically stable.

[0007] The splitting ratio of the non-polarizing beam splitter is 1:1.

[0008] The photoelastic modulator includes a photoelastic crystal and an LC resonant high-voltage driving circuit. The driving port of the photoelastic crystal is connected to the output terminal of the LC resonant high-voltage driving circuit, and the control input terminal of the LC resonant high-voltage driving circuit is connected to the control output port of the signal processing module.

[0009] The signal processing module is a control and computing module with a field-programmable gate array (FPGA) as its core. An analog-to-digital conversion acquisition unit is set between the FPGA and the photodetector.

[0010] The photoelectric modulator integrates a temperature sensor, and the signal processing module adjusts the drive signal frequency of the photoelectric modulator based on the feedback signal from the temperature sensor.

[0011] The operating wavelength range of the light source is 170nm to 1550nm. A half-wave plate is also provided between the light source and the non-polarizing beam splitter. The half-wave plate is used to adjust the polarization direction of the detection beam so that the angle between the polarization direction of the beam incident on the polarizing beam splitter and the first direction is 45°.

[0012] Secondly, the present invention also provides a method for measuring micro-vibrations using elastic-optical phase heterodyne modulation, comprising: S1, splitting a detection beam emitted from a light source into a reference beam and a measurement beam via a non-polarizing beam splitter; modulating the reference beam into linearly polarized light in a first direction via a first polarizer and then incident on a first reflecting mirror; and modulating the measurement beam into linearly polarized light in a second direction via a second polarizer and then incident on a second reflecting mirror fixed to the micro-vibration object to be measured, wherein the second direction is perpendicular to the first direction; S2, returning the reference beam reflected by the first reflecting mirror and the measurement beam reflected by the second reflecting mirror to the non-polarizing beam splitter for beam combining into a single optical path, thereby combining the beams. The combined beam then passes sequentially through a photoelectric modulator and an analyzer. The fast modulation axis of the photoelectric modulator is collinear with the first direction, and the polarization axis of the analyzer is at an angle of 45° to the first direction. The photoelectric modulator applies periodic phase modulation to the combined beam. S3: The interference light signal is received by a photodetector and converted into an electrical signal. The intensity of the interference light signal is expanded according to a Bessel series of the first kind. Using the drive signal of the photoelectric modulator as a reference, the electrical signal is digitally demodulated using phase-locked loop (PLL) to extract the amplitudes of the first harmonic signal V1f and the second harmonic signal V2f. S4: Based on the amplitudes of the first harmonic signal V1f and the second harmonic signal V2f, the following formula is used... Calculate the ratio R using the formula Calculate the phase delay B using the formula. The displacement d of the measured micro-vibration is obtained by calculating the displacement d; where, Let λ be the phase modulation depth of the photoelastic modulator, and λ be the wavelength of the detection beam. and These are first-order and second-order Bessel functions of the first kind, respectively.

[0013] The digital phase-locked demodulation in step S3 specifically includes: outputting a drive signal to the elastic-optical modulator using a field-programmable gate array and synchronously generating a baseband reference signal with the same frequency and origin as the drive signal; performing multiplication and accumulation operations on the electrical signal after analog-to-digital conversion and the baseband reference signal to extract the amplitude of the first harmonic signal V1f and the amplitude of the second harmonic signal V2f.

[0014] Before step S1, a polarization calibration step is also included: calibrating the fast modulation axis direction of the elastic modulator to be collinear with the first direction.

[0015] In step S2, the periodic phase modulation applied by the photoelastic modulator is: , where ω is the modulation angular frequency of the photoelectric modulator.

[0016] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This application achieves the organic integration of photoelectric phase heterodyne modulation and Michelson interferometry through a specific polarization path configuration. The polarization axis of the reference arm polarizer is in the first direction, and the polarization axis of the measurement arm polarizer is in the second direction perpendicular to the first direction. Combined with an analyzer whose polarization axis forms a 45° angle with the first direction after beam combining, the two orthogonally polarized beams are projected onto the same direction by the analyzer to form interference. Simultaneously, the fast modulation axis of the photoelectric modulator is collinear with the first direction, ensuring that the linearly polarized light in the reference arm achieves maximum modulation efficiency. This polarization configuration makes the first and second harmonic components in the interference light intensity proportional to sin(2B) and cos(2B), respectively, laying the physical foundation for subsequent high-precision inversion of the phase delay B using harmonic ratios.

[0017] 2. This application employs the same non-polarizing beam splitter prism to simultaneously perform beam splitting and combining, and uses an all-solid-state elastic-optical modulator to replace traditional mechanical phase-shifting components, significantly improving the system's environmental adaptability and long-term stability. The reference beam and the measurement beam travel back and forth along a common optical path, experiencing almost identical environmental disturbances, resulting in strong common-mode suppression and effectively suppressing measurement errors introduced by temperature gradients and airflow disturbances. Simultaneously, all-solid-state modulation eliminates the inherent hysteresis nonlinearity, creep effect, and long-term drift problems of piezoelectric ceramics or mechanical scanning mechanisms. No mechanical adjustment is required during measurement, resulting in good long-term system stability, making it suitable for scenarios requiring continuous monitoring, such as precision manufacturing platforms.

[0018] 3. This application uses an FPGA as the core to achieve coordinated control of drive signal generation and digital phase-locked loop demodulation, and incorporates a temperature compensation closed loop to ensure the signal-to-noise ratio and demodulation accuracy of the measured signal. The FPGA outputs a drive signal to the LC resonant high-voltage drive circuit of the photoelastic modulator, and simultaneously generates a fundamental frequency reference signal with the same frequency and origin as the drive signal for digital phase-locked loop operation, eliminating frequency deviation and phase noise introduced by independent signal sources. At the same time, the FPGA adjusts the drive frequency in real time based on the feedback signal from the temperature sensor inside the photoelastic modulator, compensating for temperature drift of the crystal resonant frequency and ensuring long-term stability of the phase modulation depth δ0, enabling the system to maintain high-precision measurement capabilities even in complex industrial environments.

[0019] 4. This application, based on Bessel series expansion and ratio method demodulation algorithm, achieves high-precision inversion of micro-vibration displacement with resistance to light source fluctuations and a large dynamic range. By expanding the interference light intensity according to the first kind of Bessel series and extracting the first and second harmonic amplitudes using digital phase-locked loop, a ratio R is further constructed. This ratio calculation eliminates the influence of light intensity fluctuation terms, making the phase calculation results unaffected by light source power fluctuations and optical path energy attenuation. Finally, the micro-vibration displacement d is directly inverted using B=4πd / λ. The calculation process is simple, the response speed is fast, and the single data processing cycle can be as short as 1ms, realizing precise measurement of micro-vibrations that takes into account high resolution, wide dynamic range, and real-time response. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a micro-vibration measurement device with elastic-optical phase heterodyne modulation provided in an embodiment of this application. Detailed Implementation

[0021] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0022] This application provides a micro-vibration measurement device based on elastic-optical phase heterodyne modulation. For example, refer to... Figure 1 The micro-vibration measurement device based on photoelastic phase heterodyne modulation mainly includes a detection laser source 1, a half-wave plate 2, a non-polarizing beam splitter 3, a first polarizer 4, a first reflector 5, a second polarizer 6, a second reflector 7, a micro-vibration object to be measured 8, such as a precision manufacturing platform, a photoelastic modulator 9, an analyzer 10, a photodetector 11, a signal processing module 12, and a host computer 13. The signal processing module 12 preferably uses a field-programmable gate array (FPGA) as the core control and computing carrier.

[0023] Light source 1 is used to emit the detection beam. Light source 1 is selected based on its excellent monochromaticity, and its function is to provide a stable, coherent beam to the entire measurement system. Light source 1 can be of various types, such as a gas laser, solid-state laser, semiconductor laser, or fiber laser. The operating wavelength range of light source 1 is 170nm to 1550nm. This wavelength range covers a broad spectral region from deep ultraviolet to near-infrared. Within this wavelength range, different wavelengths of light sources can be selected according to actual measurement requirements. As one possible implementation, light source 1 uses a helium-neon (He-Ne) laser with a wavelength of 632.8nm. This wavelength light source has the characteristics of long coherence length and high frequency stability, which can meet the requirements of high-precision interferometric measurement.

[0024] A half-wave plate 2 is also disposed between the light source 1 and the unpolarized beam splitter 3. The detection beam emitted by the light source 1 first passes through the half-wave plate 2. The half-wave plate 2 is used to adjust the polarization direction of the incident light so that the polarization direction of the incident light after passing through the half-wave plate 2 matches the optical elements of the subsequent optical path. For example, the angle between the polarization direction of the beam incident on the unpolarized beam splitter 3 and the first direction is 45°. After adjustment by the half-wave plate 2, the Jones vector of the incident light can be expressed as:

[0025] in, The Jones vector represents the intensity of the light emitted from the reference arm and the measuring arm, indicating that the incident light is linearly polarized at a polarization angle of 45° to the horizontal.

[0026] A non-polarizing beam splitter prism 3 is disposed on the outgoing light path of the light source 1 to split the detection beam into a reference beam and a measurement beam, and to combine the reflected reference beam and the measurement beam. The non-polarizing beam splitter prism 3 is an optical element with polarization-independent beam splitting characteristics; the intensity distribution ratio of its transmitted and reflected light is unaffected by changes in the polarization angle and polarization state of the incident light, and will not experience a shift in the beam splitting ratio due to differences in the incident polarization state. In a preferred embodiment, the beam splitting ratio of the non-polarizing beam splitter prism 3 is 1:1, meaning the incident light is equally divided into a reference beam and a measurement beam with the same intensity. The non-polarizing beam splitter prism 3 simultaneously performs the dual functions of beam splitting and beam combining, splitting the detection beam into two paths on the incident light path and combining the two returning beams into a single optical path on the reflected light path. Using the same non-polarizing beam splitter prism to achieve beam splitting and beam combining simultaneously, the reference beam and the measurement beam are transmitted back and forth along a common optical path. The two beams experience almost the same environmental disturbances, such as temperature gradients and airflow disturbances, resulting in strong common-mode suppression capability and significantly improved system anti-environment interference capability.

[0027] For example, the Jones matrix of the non-polarizing beam splitter 3 is:

[0028] A first polarizer 4 and a first reflector 5 are sequentially arranged along the propagation direction of the reference beam in the reference arm. The polarization axis of the first polarizer 4 is the first direction. The first polarizer 4 is used to modulate the reference beam split from the unpolarized beam splitter 3 into linearly polarized light in the first direction. The first reflector 5 is fixedly arranged at the end of the reference arm and is used to reflect the linearly polarized light after passing through the first polarizer 4 back to the unpolarized beam splitter 3. The polarization axis direction of the first polarizer 4 in the reference arm, i.e., the first direction, is the reference reference for setting all subsequent polarization angles.

[0029] The Jones matrix of the first biasing device 4 is:

[0030] The Jones vector of the emitted light from the reference arm is:

[0031] The measuring arm is sequentially equipped with a second polarizer 6 and a second reflector 7 along the propagation direction of the measuring beam. The polarization axis of the second polarizer 6 is a second direction, which is perpendicular to the first direction, i.e., the angle between the second direction and the first direction is 90°. The second polarizer 6 is used to modulate the measuring beam split from the unpolarized beam splitter 3 into linearly polarized light in the second direction. The second reflector 7 is fixed to the micro-vibration object 8 to be measured and is used to reflect the linearly polarized light after passing through the second polarizer 6 back to the unpolarized beam splitter 3. When the micro-vibration object 8 to be measured undergoes micro-vibration, the second reflector 7 is displaced accordingly, causing a change in the optical path of the measuring beam, thereby introducing a phase delay in the measuring beam corresponding to the micro-vibration displacement.

[0032] The Jones matrix of the second bias generator 6 is:

[0033] The Jones matrix introduced by the micro-vibration object 8 under test is:

[0034] Where B is the phase delay introduced by the micro-vibration object 8 to be measured.

[0035] The Jones vector of the emitted light from the measuring arm is:

[0036] The reference beam is reflected by the first reflecting mirror 5 and returns to the unpolarized beam splitter prism 3. The measurement beam is reflected by the second reflecting mirror 7 and returns to the unpolarized beam splitter prism 3. The two reflected beams are combined into a single optical path by the unpolarized beam splitter prism 3. The Jones vector of the output beam after beam combining is:

[0037] The common-path round-trip transmission configuration ensures that the optical paths of the reference arm and the measurement arm overlap as much as possible, and the impact of environmental disturbances on the two beams is highly correlated, which is beneficial for achieving common-mode suppression and improving measurement stability.

[0038] An optical-elastic modulator 9 is positioned in the output optical path of the unpolarized beam splitter 3 to apply periodic phase modulation to the combined beam. The fast modulation axis of the optical-elastic modulator 9 is collinear with the first direction, meaning the fast modulation axis of the optical-elastic modulator 9 is aligned with the polarization axis of the first polarizer 4. This arrangement ensures that the linearly polarized light from the reference arm is incident along the fast axis of the optical-elastic modulator, achieving maximum phase modulation efficiency.

[0039] The piezoelectric modulator 9 utilizes the piezoelectric effect to achieve optical modulation: it converts the externally input driving electrical signal into periodically alternating mechanical stress and strain through a piezoelectric crystal, and applies this mechanical quantity to a crystal medium with piezoelectric properties, such as fused silica or lithium niobate, causing a periodic dynamic change in the refractive index inside the crystal, thereby modulating the polarization state, phase, and amplitude of the transmitted beam. The piezoelectric modulator 9 includes a piezoelectric crystal and an LC resonant high-voltage driving circuit. The piezoelectric crystal is the core element of the piezoelectric effect, and as a possible implementation, it is made of optical materials with good piezoelectric properties, such as fused silica or lithium niobate. The LC resonant high-voltage driving circuit consists of a resonant circuit composed of an inductor and a capacitor, and a high-voltage amplifier circuit. Its function is to provide a high-frequency high-voltage driving signal to the piezoelectric crystal, causing the piezoelectric crystal to operate in a mechanical resonance state, thereby achieving a large phase modulation depth with relatively low driving power. The driving port of the piezoelectric crystal is connected to the output terminal of the LC resonant high-voltage driving circuit, and the control input terminal of the LC resonant high-voltage driving circuit is connected to the control output port of the signal processing module 12. The FPGA outputs a modulation control signal, which, after being voltage and frequency regulated by an LC resonant high-voltage drive circuit, drives the photoelectroluminescent crystal to operate in a resonant state and adjusts its modulation depth. Simultaneously, the FPGA generates a fundamental frequency reference signal that is of the same frequency and origin as the modulation drive signal, used for phase synchronization in digital phase-locked loop (PLL) operations.

[0040] As one possible implementation, the resonant frequency of the photoelastic modulator 9 is 50 kHz. At this frequency, the photoelastic crystal can achieve a high quality factor (Q value) and high modulation efficiency. At the same time, this frequency is much higher than the frequency range of the micro-vibration to be measured, which is beneficial for separating the modulation signal from the vibration signal to be measured through subsequent digital phase-locked loop processing.

[0041] In some embodiments, the photoluminescent modulator 9 integrates a temperature sensor. The temperature sensor is used to monitor the operating temperature of the photoluminescent crystal in real time. The temperature sensor can be a thermistor, thermocouple, or an integrated temperature sensing chip, such as a digital temperature sensor. As one possible implementation, a negative temperature coefficient thermistor is used, which is small in size, has a fast response speed, and is inexpensive, making it suitable for integration into the photoluminescent modulator 9.

[0042] The signal processing module 12 adjusts the driving signal frequency of the photoelastic modulator 9 based on the feedback signal from the temperature sensor. The resonant frequency of the photoelastic crystal drifts with temperature changes; if the driving signal frequency deviates from the resonant frequency of the photoelastic crystal, the phase modulation depth of the photoelastic modulator 9 will decrease. The temperature will drop, affecting measurement accuracy. Through real-time feedback from the temperature sensor, the signal processing module 12 can dynamically adjust the output frequency of the LC resonant high-voltage drive circuit, ensuring that the photoelectric modulator 9 always operates at the resonant peak frequency. This compensates for the drift in the photoelectric crystal resonant frequency caused by temperature changes, ensuring the phase modulation depth. Long-term stability.

[0043] The Jones matrix of the light modulator 9 is:

[0044] in, It is the phase delay introduced by the elastic modulator 9. , The phase modulation depth of the optical modulator 9, ω is the modulation angular frequency of the photoelectric modulator 9.

[0045] The analyzer 10 is positioned in the output optical path of the photoelectric modulator 9. The polarization axis of the analyzer 10 forms a 45° angle with the first direction. The function of the analyzer 10 is to project two orthogonally polarized beams (the reference beam is linearly polarized in the first direction, and the measurement beam is linearly polarized in the second direction, with their polarization directions perpendicular to each other) onto the same polarization direction, so that the two beams satisfy the coherence condition and form interference. Without the analyzer 10, the two beams with perpendicular polarization directions cannot interfere, and the photodetector 11 will not be able to detect the interference signal.

[0046] The Jones matrix of analyzer 10 is:

[0047] After beam combining, the output light passes sequentially through the optical modulator 9 and the analyzer 10. The Jones vector of the output light is:

[0048] Substituting the above Jones matrices, we can obtain the Jones vector of the output light as follows:

[0049] A photodetector 11 is disposed in the output optical path of the analyzer 10 to convert the received interference light signal into an electrical signal. The photodetector 11 can be a photodiode, avalanche photodiode, or photomultiplier tube, etc. As a preferred embodiment, the photodetector 11 uses a high-sensitivity, low-noise silicon-based PIN photodiode, which has high quantum efficiency and response speed in the visible light band, meeting the high-speed, high-precision detection requirements of micro-vibration interference signals. The photodetector 11 converts the received interference light intensity signal into a proportional analog voltage or current signal and outputs it to the signal processing module 12 for processing.

[0050] The light intensity passing through the photodetector (11) can be obtained using the light intensity formula:

[0051] Expanding the expression for light intensity using a Bessel series of the first kind, we get:

[0052] in, The light intensity emitted from the reference arm and the measuring arm. , , and These are the 0th, 1st, 2nd, and 3rd order Bessel functions of the first kind, respectively.

[0053] The signal processing module 12 is connected to the output terminal of the photodetector 11 and the control terminal of the photoelectro-optic modulator 9, respectively. It outputs a drive signal to the photoelectro-optic modulator 9 and synchronously generates a fundamental frequency reference signal, as well as receives electrical signals for digital phase-locked loop demodulation, extracting the amplitudes of the first and second harmonic signals. The signal processing module 12 is a control and computation module based on a field-programmable gate array (FPGA). An analog-to-digital converter (ADC) acquisition unit is provided between the FPGA and the photodetector 11. The ADC acquisition unit converts the analog electrical signal output by the photodetector 11 into a digital signal for the FPGA to perform digital phase-locked loop operations.

[0054] The signal processing module 12 uses an FPGA as its core control unit and has three functions: First, the FPGA outputs a drive input signal to the LC resonant high-voltage drive circuit of the optical modulator 9 to synchronously generate a fundamental frequency reference signal; Second, the FPGA performs a digital phase-locked loop operation on the reference signal and the acquired AC signal, and transmits the operation result to the host computer 13; Third, the FPGA is responsible for the acquisition control and data storage of DC signals, and uploads the DC data to the host computer 13 after completion.

[0055] The FPGA outputs a drive signal to the photoelectric modulator 9 and simultaneously generates a reference signal, ensuring that the reference signal and the modulation base frequency signal are strictly in the same frequency and from the same source. This effectively suppresses low-frequency environmental noise and DC drift, significantly improving the signal-to-noise ratio of the measurement signal. The analog-to-digital conversion acquisition unit between the FPGA and the photodetector 11 performs analog-to-digital conversion processing, providing digital input data for digital phase-locked loop (PLL) operations.

[0056] As one possible implementation, an FPGA integrates a digital lock-in amplifier module. The digital lock-in amplifier extracts the amplitude and phase of the signal component with the same frequency as the reference signal by multiplying and summing the acquired digital signal with a fundamental frequency reference signal of the same origin. Compared to analog lock-in amplifiers, digital lock-in amplifiers do not suffer from the temperature drift and 1 / f noise problems of analog devices, and have stronger suppression capabilities against low-frequency environmental noise and DC drift.

[0057] In some embodiments, the analog-to-digital conversion acquisition unit employs a high-speed, high-resolution successive approximation or Δ-Σ analog-to-digital converter with a sampling rate of not less than 10MHz and a resolution of not less than 16 bits to ensure accurate acquisition of the interference signal.

[0058] The host computer 13 is connected to the signal processing module 12 and is used to invert the phase delay based on the ratio of the amplitude of the first harmonic signal to the amplitude of the second harmonic signal, and to calculate the displacement of the measured micro-vibration based on the phase delay. The host computer 13 can be a terminal device with data calculation, storage, and display functions, such as a personal computer, industrial control computer, or embedded computer.

[0059] The host computer 13 is equipped with a data acquisition and processing module. This module is connected to the FPGA in the signal processing module 12 via a bidirectional data communication channel, such as a universal serial bus, Ethernet, or serial peripheral interface. It receives the first and second harmonic digital phase-locked loop amplitude data and DC signal buffer data uploaded by the FPGA in real time. It calculates the phase delay B through a preset ratio calculation algorithm, and then inversely calculates the displacement of the measured micro-vibration. The calculation results are automatically stored and the waveform is displayed. The host computer 13 can also configure and fine-tune various parameters of the output drive signal online and in real time through the supporting measurement and control software. In coordination with the LC resonant high-voltage drive circuit, it can realize real-time high-precision control of the modulation depth and modulation frequency of the elastic-optical modulator 9.

[0060] The measuring device provided in this application does not contain any mechanical adjustment components, and the optical path and modulation state remain statically stable during the measurement process. Traditional Michelson interferometric vibration measurement schemes rely on piezoelectric ceramics or mechanical phase shifters to achieve phase modulation, which has inherent problems such as hysteresis nonlinearity, creep effect, and long-term drift. This invention replaces mechanical modulation with all-solid-state modulation of the elastic-optical modulator 9, completely eliminating the nonlinear error and time drift problem introduced by mechanical means. The long-term stability of the system is significantly improved, making it suitable for scenarios requiring long-term continuous monitoring, such as precision manufacturing platforms.

[0061] As one possible implementation, the non-polarizing beam splitter prism 3 has a splitting ratio of 1:1, meaning the incident detection beam is equally divided into a reference beam and a measurement beam with the same intensity. Using a 1:1 splitting ratio ensures that the intensity of the reference beam and the measurement beam are equal, resulting in the interference signal at the photodetector 11 having the maximum modulation, which is beneficial for improving the signal-to-noise ratio and measurement accuracy.

[0062] It should be understood that the splitting ratio of the non-polarizing beam splitter 3 is not limited to 1:1. In other embodiments, depending on the actual measurement requirements and the power of the light source, the splitting ratio of the non-polarizing beam splitter 3 can also be other ratios, such as 60:40 or 40:60, as long as sufficient interference signal is obtained at the photodetector 11.

[0063] This application also provides a method for measuring micro-vibrations using elastic-optical phase heterodyne modulation, including: S1. The detection beam emitted by the light source is split into a reference beam and a measurement beam by a non-polarizing beam splitter. The reference beam is modulated into linearly polarized light in the first direction by the first polarizer and then incident on the first reflecting mirror. The measurement beam is modulated into linearly polarized light in the second direction by the second polarizer and then incident on the second reflecting mirror fixed on the micro-vibration object to be measured. The second direction is perpendicular to the first direction.

[0064] The detection beam emitted by light source 1 first passes through half-wave plate 2, and after its polarization direction is adjusted, it is incident on unpolarized beam splitter prism 3. Unpolarized beam splitter prism 3 splits the incident light into two beams: one transmitted beam serves as a reference beam entering the reference arm, and the other reflected beam serves as a measurement beam entering the measurement arm. The reference beam is converted into linearly polarized light in the first direction after passing through the first polarizer 4, and then illuminates the first reflecting mirror 5. The measurement beam is converted into linearly polarized light in the second direction after passing through the second polarizer 6, and then illuminates the second reflecting mirror 7 fixed on the micro-vibration object 8 to be measured.

[0065] Step S1 splits the detection beam into two linearly polarized beams with mutually perpendicular polarization directions, laying the foundation for subsequent interferometric measurements. The mutually perpendicular polarization directions of the reference beam and the measurement beam ensure that the two beams do not interfere directly after being combined, but rather require projection through the analyzer 10 to form interference. This provides an independent control space for the phase modulation of the optical modulator 9.

[0066] S2. The reference beam reflected by the first mirror and the measurement beam reflected by the second mirror are returned to the non-polarizing beam splitter and combined into the same optical path. The combined beam passes through the elastic modulator and the analyzer in sequence. The fast modulation axis of the elastic modulator is collinear with the first direction. The polarization axis of the analyzer is at an angle of 45° with the first direction. The elastic modulator applies periodic phase modulation to the combined beam.

[0067] In this step, the reference beam, after being reflected by the first mirror 5, returns along its original path and passes again through the first polarizer 4 and the unpolarized beam splitter 3. The measurement beam, after being reflected by the second mirror 7, returns along its original path and passes again through the second polarizer 6 and the unpolarized beam splitter 3. The two returning beams are combined at the unpolarized beam splitter 3 to form a single optical path.

[0068] The combined beam first passes through the optical-elastic modulator 9. The fast modulation axis of the optical-elastic modulator 9 is collinear with the first direction, i.e., consistent with the polarization direction of the reference beam. This ensures that the reference beam is incident along the fast axis of the optical-elastic modulator, achieving maximum phase modulation efficiency. The optical-elastic modulator 9 applies periodic phase modulation to the combined beam, and this modulation is... .

[0069] Phase modulation depth This is the maximum amplitude of the phase change applied to the light beam by the photoelastic modulator 9, and its magnitude depends on factors such as the material properties, geometry, driving voltage amplitude, and operating temperature of the photoelastic crystal. In some embodiments, the phase modulation depth... When π is set, the first-order Bessel function is... and second-order Bessel function They all have large values, which is beneficial for obtaining higher harmonic and second harmonic signal amplitudes.

[0070] The modulation angular frequency ω = 2πf, where f is the modulation frequency of the photoelastic modulator 9. In a preferred embodiment, the modulation frequency of the photoelastic modulator 9 is 50 kHz, corresponding to a modulation angular frequency ω = 2π × 50000 rad / s. This modulation frequency is much higher than the frequency range of the micro-vibration to be measured, allowing the first, second, and third harmonic signals to be separated from the vibration signal to be measured in the frequency domain, facilitating the accurate extraction of the amplitude of each harmonic component using digital phase-locked loop (PLL) technology.

[0071] sinusoidal phase modulation This allows the Bessel series expansion of the interference light intensity to have a concise mathematical form, with the first harmonic term containing... The second harmonic term includes The ratio of the first harmonic to the second harmonic can be easily eliminated. and By finding the common factors, we can finally obtain the concise relation R = tan(2B).

[0072] The combined beam passes through the optical modulator 9 and then through the analyzer 10. The polarization axis of the analyzer 10 makes an angle of 45° with the first direction, meaning that the polarization axis of the analyzer 10 lies on the angle bisector of the first and second directions. After passing through the analyzer 10, the two beams of light with mutually perpendicular polarization directions are projected onto the same polarization direction, satisfying the coherence condition and forming interference.

[0073] The function of step S2 is to apply high-frequency phase modulation to the beam combining beam through the photoelastic modulator 9, and modulate the micro vibration signal (low-frequency signal) to be measured onto the high-frequency carrier (photoelastic modulation frequency and its harmonics), so that the weak signal can be extracted from the strong background noise through digital phase-locked loop technology.

[0074] S3. The interference light signal is received by the photodetector and converted into an electrical signal. The intensity of the interference light signal is expanded according to the first type of Bessel series. The electrical signal is digitally phase-locked demodulated with reference to the driving signal of the elastic-optical modulator, and the amplitude of the first harmonic signal V1f and the amplitude of the second harmonic signal V2f are extracted.

[0075] In this step, the photodetector 11 converts the received interference light intensity signal into an electrical signal. The expression for the interference light intensity is:

[0076] As can be seen from the above expression, the interference light intensity includes a DC term, a fundamental frequency term (ω term), a second harmonic term (2ω term), and a third harmonic term (3ω term). The amplitude of the fundamental frequency term is proportional to... The magnitude of the second harmonic term is proportional to Digital phase-locked loop (PLL) technology can be used to extract the amplitudes of the fundamental frequency term and the second harmonic term separately.

[0077] The specific process of digital phase-locked demodulation is as follows: the field programmable gate array (FPGA) outputs a drive signal to the optical modulator (9) and synchronously generates a base frequency reference signal with the same frequency and origin as the drive signal. The electrical signal is converted from analog to digital and then multiplied and accumulated with the base frequency reference signal. Specifically, the acquired digital signal is multiplied with the base frequency reference signal (frequency ω) and the second harmonic reference signal (frequency 2ω) respectively, and then after low-pass filtering or accumulation averaging, the amplitude of the base frequency signal V1f and the amplitude of the second harmonic signal V2f can be extracted.

[0078] The purpose of step S3 is to accurately extract the amplitude values ​​of the first and second harmonic signals carrying the micro-vibration information to be measured from the interference signal containing noise, so as to provide accurate data input for subsequent phase calculation.

[0079] For example, the digital phase-locked demodulation in step S3 specifically includes: outputting a drive signal to the optical modulator 9 using a field-programmable gate array and synchronously generating a baseband reference signal with the same frequency and origin as the drive signal. After analog-to-digital conversion, the electrical signal is multiplied and accumulated with the baseband reference signal to extract the amplitude of the first harmonic signal V1f and the amplitude of the second harmonic signal V2f.

[0080] Specifically, the FPGA integrates a direct digital synthesizer (DDS) module to generate precise sine or square wave drive signals. This drive signal is amplified by an LC resonant high-voltage drive circuit to drive the photoelectric modulator 9. Simultaneously, the FPGA uses the same DDS phase accumulator to generate reference signals (including a fundamental frequency reference signal and a second harmonic reference signal) that are in phase and frequency with the drive signal.

[0081] After the analog-to-digital conversion acquisition unit converts the analog electrical signal output by the photodetector 11 into a digital signal, the FPGA performs digital phase-locked loop (PLL) operation on the digital signal. The core of the PLL operation is phase-sensitive detection: the digital signal is multiplied by the fundamental frequency reference signal and the second harmonic reference signal respectively, and then the product results are accumulated and averaged to extract the amplitude of the signal component with the same frequency as the reference signal.

[0082] The same frequency and same source setting ensures that there is no frequency deviation or phase drift between the reference signal and the modulation base frequency signal, eliminating the phase noise and frequency mismatch problems introduced by independent signal sources in traditional lock-in amplifiers, and significantly improving the accuracy of the first / second harmonic amplitude extracted by digital lock-in.

[0083] S4. Based on the amplitude V1f of the first harmonic signal and the amplitude V2f of the second harmonic signal, according to the formula... Calculate the ratio R using the formula Calculate the phase delay B using the formula. The displacement d of the measured micro-vibration is obtained by calculating the displacement d; where, Let λ be the phase modulation depth of the photoelastic modulator, and λ be the wavelength of the detection beam. and These are first-order and second-order Bessel functions of the first kind, respectively.

[0084] Specifically, the amplitudes of the first, second, and third harmonic signals obtained through digital phase-locked loop in the AC term are as follows:

[0085]

[0086]

[0087] To obtain the phase delay B introduced by the micro-vibration, a ratio R is defined:

[0088] Substituting the expressions for V1f and V2f into the above equation, we get: The phase delay B introduced by the micro-vibration can be obtained using the inverse trigonometric function:

[0089] Since micro-vibrations cause changes in optical path difference, leading to alterations in phase difference, this can be calculated using the conversion relationship between phase difference and optical path difference. The relationship between the phase delay B introduced by micro-vibrations and the optical path difference L is as follows:

[0090] Where λ is the wavelength of the incident light, and L is the optical path difference between the reference arm and the measuring arm.

[0091] The device provided in this application embodiment is a Michelson interferometer. When the incident light passes through the measuring arm, the micro-vibration object 8 under test causes a micro-vibration change d, corresponding to a change of twice the optical path difference, that is: Where d represents the change in micro-vibration caused by the micro-vibration object 8 under test. Substitution In the middle, one can obtain By transforming the above formula, we can obtain the change in micro-vibration d caused by the micro-vibration object 8 under test:

[0092] Step S4 eliminates the light intensity fluctuation term through ratio calculation. This ensures that the phase calculation results are not affected by fluctuations in light source power and attenuation of optical path energy, enabling high-precision micro-vibration displacement inversion.

[0093] As one possible implementation, a polarization calibration step is included before step S1: calibrating the fast modulation axis direction of the optical modulator 9 to be collinear with the first direction. For example, specifically: before the formal measurement, the azimuth angle of the optical modulator 9 is rotated to precisely align the fast modulation axis direction of the optical modulator 9 with the polarization axis direction (i.e., the first direction) of the first polarizer 4. Calibration can be achieved by retaining only the reference arm beam in the optical path (blocking the measurement arm beam), and observing the signal amplitude output by the photodetector 11. When the fast modulation axis direction of the optical modulator 9 is collinear with the first direction, the modulation efficiency of the optical modulator 9 on the reference beam is maximized, and the amplitude of the modulated signal output by the photodetector 11 reaches its maximum value.

[0094] The polarization calibration step ensures that the linearly polarized light from the reference arm is incident along the fast axis of the electro-optic modulator 9, thereby maximizing phase modulation efficiency. It is also a necessary condition for the mathematical relationship between the diagonal element phase factor of the modulation matrix of the electro-optic modulator 9 and the measured phase B to hold true in the subsequent derivation of the Jones matrix.

[0095] In the description of this specification, specific features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments or examples.

[0096] The above are merely specific embodiments of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A micro-vibration measuring device based on elastic-optical phase heterodyne modulation, characterized in that, include: A light source (1) is used to emit a detection beam; a non-polarizing beam splitter (3) is disposed on the outgoing light path of the light source (1) to split the detection beam into a reference beam and a measurement beam, and to combine the reference beam and the measurement beam after reflection; a reference arm is provided with a first polarizer (4) and a first reflector (5) in sequence along the propagation direction of the reference beam, wherein the polarization axis of the first polarizer (4) is the first direction; a measurement arm is provided with a second polarizer (6) and a second reflector (7) in sequence along the propagation direction of the measurement beam. The second reflector (7) is fixed on the micro-vibration object (8) to be measured. The polarization axis of the second polarizer (6) is the second direction, which is perpendicular to the first direction. The reference beam is reflected by the first reflector (5) and returns to the non-polarizing beam splitter (3). The measurement beam is reflected by the second reflector (7) and returns to the non-polarizing beam splitter (3). The two reflected beams are combined into the same optical path by the non-polarizing beam splitter (3). The elastic modulator (9) is set on the non-polarizing beam splitter (3). The output optical path of the photoelectric modulator (9) is used to apply periodic phase modulation to the combined beam. The fast modulation axis of the photoelectric modulator (9) is collinear with the first direction. The polarizer (10) is disposed on the output optical path of the photoelectric modulator (9). The polarization axis of the polarizer (10) is at an angle of 45° with the first direction. The photodetector (11) is disposed on the output optical path of the polarizer (10) and is used to convert the received interference light signal into an electrical signal. The signal processing module (12) is connected to the output optical path of the photodetector (11). The output end and the control end of the optical modulator (9) are respectively connected to output a drive signal to the optical modulator (9) and generate a reference signal synchronously, and receive the electrical signal to perform digital phase-locked demodulation and extract the amplitude of the first harmonic signal and the amplitude of the second harmonic signal; the host computer (13) is connected to the signal processing module (12) and is used to invert the phase delay based on the ratio of the amplitude of the first harmonic signal to the amplitude of the second harmonic signal, and calculate the displacement of the measured micro-vibration based on the phase delay; the optical path and the modulation state remain statically stable during the measurement process.

2. The micro-vibration measuring device with elastic-optical phase heterodyne modulation according to claim 1, characterized in that, The non-polarizing beam splitter (3) has a beam splitting ratio of 1:

1.

3. The micro-vibration measuring device with elastic-optical phase heterodyne modulation according to claim 1, characterized in that, The photoelastic modulator (9) includes a photoelastic crystal and an LC resonant high-voltage driving circuit. The driving port of the photoelastic crystal is connected to the output terminal of the LC resonant high-voltage driving circuit, and the control input terminal of the LC resonant high-voltage driving circuit is connected to the control output port of the signal processing module (12).

4. The micro-vibration measuring device with elastic-optical phase heterodyne modulation according to claim 1, characterized in that, The signal processing module (12) is a control and operation module with a field-programmable gate array as its core. An analog-to-digital conversion acquisition unit is provided between the field-programmable gate array and the photodetector (11).

5. The micro-vibration measuring device with elastic-optical phase heterodyne modulation according to claim 1, characterized in that, The elastic modulator (9) integrates a temperature sensor, and the signal processing module (12) adjusts the driving signal frequency of the elastic modulator (9) according to the feedback signal of the temperature sensor.

6. The micro-vibration measuring device with elastic-optical phase heterodyne modulation according to claim 1, characterized in that, The operating wavelength range of the light source (1) is 170nm to 1550nm; a half-wave plate (2) is also provided between the light source (1) and the unpolarized beam splitter (3). The half-wave plate (2) is used to adjust the polarization direction of the detection beam so that the angle between the polarization direction of the beam incident on the unpolarized beam splitter (3) and the first direction is 45°.

7. A method for measuring micro-vibrations using elastic-optical phase heterodyne modulation, characterized in that, include: S1. The detection beam emitted by the light source (1) is split into a reference beam and a measurement beam by the non-polarizing beam splitter (3). The reference beam is modulated into linearly polarized light in the first direction by the first polarizer (4) and then incident on the first reflector (5). The measurement beam is modulated into linearly polarized light in the second direction by the second polarizer (6) and then incident on the second reflector (7) fixed on the micro-vibration object (8) to be measured. The second direction is perpendicular to the first direction. S2. The reference beam reflected by the first reflector (5) and the measurement beam reflected by the second reflector (7) are returned to the non-polarizing beam splitter (3) and combined into the same optical path. The combined beam is then passed sequentially through the elastic modulator (9) and the second reflector (7). Analyzer (10), the fast modulation axis of the optical modulator (9) is collinear with the first direction, the polarization axis of the analyzer (10) is at an angle of 45° with the first direction, the optical modulator (9) applies periodic phase modulation to the beam after beam combining; S3, receive the interference light signal through the photodetector (11) and convert it into an electrical signal, expand the light intensity of the interference light signal according to the first type of Bessel series, and perform digital phase-locked demodulation on the electrical signal with the driving signal of the optical modulator (9) as a reference, and extract the amplitude of the first harmonic signal V1f and the amplitude of the second harmonic signal V2f; S4, according to the amplitude of the first harmonic signal V1f and the amplitude of the second harmonic signal V2f, according to the formula Calculate the ratio R using the formula Calculate the phase delay B using the formula. The displacement d of the measured micro-vibration is obtained by calculating the displacement d; where, λ is the phase modulation depth of the photoelastic modulator (9), and λ is the wavelength of the detection beam. and These are first-order and second-order Bessel functions of the first kind, respectively.

8. The micro-vibration measurement method with elastic-optical phase heterodyne modulation according to claim 7, characterized in that, The digital phase-locked demodulation in step S3 specifically includes: outputting a drive signal to the elastic-optical modulator (9) using a field-programmable gate array and synchronously generating a base frequency reference signal that is in the same frequency and source as the drive signal; multiplying and accumulating the electrical signal with the base frequency reference signal after analog-to-digital conversion; and extracting the amplitude of the first harmonic signal V1f and the amplitude of the second harmonic signal V2f.

9. The micro-vibration measurement method according to claim 7, characterized in that, Before step S1, a polarization calibration step is also included: calibrating the fast modulation axis direction of the elastic modulator (9) to be collinear with the first direction.

10. The micro-vibration measurement method with elastic-optical phase heterodyne modulation according to claim 7, characterized in that, In step S2, the periodic phase modulation applied by the elastic modulator (9) is as follows: , where ω is the modulation angular frequency of the photoelastic modulator (9).

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