A laser vibrometer calibration method using electro-optical modulation technology

Abstract

This invention discloses a method for calibrating a laser vibrometer using electro-optical modulation technology. The method includes: using an electro-optical modulator or a cascaded combination of electro-optical modulators to generate an equivalent vibration signal with a frequency range and measurement range that meets calibration requirements. The calibration frequency depends on the selected electro-optical modulator and can reach at least 24 MHz. The measurement range of the equivalent vibration signal can be expanded by increasing the electro-optical modulator's drive voltage or configuring the modulators in series. Furthermore, a software and hardware collaborative real-time signal demodulation system is used to establish a reference standard laser vibrometer that can effectively evaluate both amplitude and phase delay. This method achieves amplitude and phase delay calibration for the entire laser vibrometer, addressing the limitations of software demodulation methods for generating reference signals in phase delay calibration.

Description

Technical Field

[0001] The present invention relates to the technical field of laser vibrometers, and in particular to a laser vibrometer calibration method using electro-optical modulation technology. Background Art

[0002] A laser Doppler vibrometer (hereinafter referred to as a laser vibrometer) is an instrument that uses the laser Doppler effect and laser interferometry technology to perform non-contact measurements of an object's vibration velocity, acceleration, and displacement. It consists of a laser heterodyne interferometer and a Doppler signal demodulation system. It has the advantages of high spatial resolution, short measurement time, wide response bandwidth, and high velocity resolution. Therefore, it is widely used in many fields, including surface wave detection, acoustic and vibration measurement, modal analysis, structural flaw detection, and health care.

[0003] Polytec, a German company, is a major manufacturer of laser vibrometers, and its products are internationally leading. Conventional models offer bandwidths up to 1 MHz, while the UHF-120 laser vibrometer can reach bandwidths up to 1.2 GHz for specialized applications. In recent years, products from Optomet (Germany), Julight (Italy), Sunny Optical (Italy), the Beijing Great Wall Metrology and Testing Technology Institute of the Aviation Industry Corporation of China, and the Institute of Aerospace Information of the Chinese Academy of Sciences have also matured. As widely used testing equipment, laser vibrometers require metrological assurance for accurate and reliable measurement results. For calibration of laser vibrometers used for vibration and shock applications, comparison calibration with standard accelerometers or laser interferometers is typically performed within a frequency range of 0.4 Hz to 50 kHz. However, this method is limited by the high-frequency excitation capability of the vibration table, and the dynamic range of the excitation signal amplitude cannot meet the measurement capabilities of the laser vibrometer. In the frequency band from 50 kHz up to MHz, the synthetic Doppler signal method (electrical calibration) and the Doppler signal spectrum analysis method based on Bessel function fitting are used. These two methods only calibrate the Doppler signal processing system of the laser vibrometer, assuming that the optical path introduces no errors. However, in reality, the effect of the photodetector's frequency response on the Doppler signal modulation is a non-negligible error source in high-frequency applications. In the GHz frequency band, Polytec uses theoretical analysis and numerical simulation methods to estimate the maximum measurement uncertainty of laser vibrometers. Therefore, new excitation methods are needed to expand the calibration frequency range of the laser vibrometer.

[0004] In response to the above problems, the present invention provides a laser vibrometer calibration method using electro-optical modulation technology to achieve amplitude and phase delay calibration of the entire laser vibrometer. Summary of the Invention

[0005] The present invention provides a laser vibrometer calibration method using electro-optical modulation technology for realizing amplitude and phase delay calibration of the entire laser vibrometer.

[0006] The present invention provides a method for calibrating a laser vibrometer using electro-optical modulation technology. The method includes a data acquisition system, a signal generator, an electro-optical modulator driver, an electro-optical modulator, a reflector, a polarizer, a laser vibrometer to be measured, and a standard laser vibrometer. The method comprises the following steps:

[0007] Step 1: The signal generator generates a sinusoidal signal, which is driven by the voltage amplifier to drive the electro-optic modulator to work;

[0008] Step 2: The measurement beams from the laser vibrometer to be calibrated and the standard laser vibrometer are combined by a beam splitter, then incident on an electro-optical modulator through a polarizer. Their phases are modulated, and then they are incident on the electro-optical modulator again after passing through a reflector.

[0009] Step 3: The modulation phase is doubled. The measurement beam carrying the modulation phase information returns to the laser vibrometer to be calibrated and the standard laser vibrometer. The demodulated output equivalent velocity, displacement, and acceleration signals are synchronously collected by the data acquisition system. The demodulation results of the standard laser vibrometer are used as a reference signal to calibrate the amplitude response and phase delay of the laser vibrometer to be calibrated.

[0010] Furthermore, the standard laser vibrometer also includes an AWView host computer, an AD acquisition control module, an automatic gain control module, an FIR low-pass filter module for demodulation, an FIR low-pass filter module for output, and a CIC extraction module. The demodulation steps include the following:

[0011] Step 31: The AWView host computer controls the AD acquisition control module, automatic gain control module, FIR low-pass filter module, and CIC extraction module through the PCIe interface, so that FIR filters with different parameters and data extraction multiples can be selected according to specific application scenarios.

[0012] The present invention has the following advantages: by adding a standard laser vibrometer composed of a laser heterodyne interferometer and a software and hardware collaborative real-time signal demodulation system as a reference, the present invention achieves amplitude and phase delay calibration of the entire laser vibrometer. Specifically, the present invention includes: using an electro-optical modulator or a cascaded combination of electro-optical modulators to generate an equivalent vibration signal with a frequency range and a measuring range that meet calibration requirements. The calibration frequency depends on the selection of the electro-optical modulator and can reach at least 24MHz. The measuring range of the equivalent vibration signal can be expanded by increasing the driving voltage of the electro-optical modulator or configuring the electro-optical modulators in series. A technical solution for a software and hardware collaborative real-time signal demodulation system is proposed to establish a reference standard laser vibrometer that can effectively evaluate both amplitude and phase delay, thereby achieving amplitude and phase delay calibration of the entire laser vibrometer, and overcoming the limitations of the technical solution of generating reference signals through software demodulation in phase delay calibration. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Figure 1This is a schematic diagram of the calibration of the laser vibrometer based on the electro-optical modulation technology of the present invention;

[0014] Figure 2 This is a diagram of the software and hardware collaborative real-time signal demodulation system of the present invention. DETAILED DESCRIPTION

[0015] The present invention provides a laser vibrometer calibration method using electro-optical modulation technology, comprising a data acquisition system, a signal generator, an electro-optical modulator driver, an electro-optical modulator, a reflector, a polarizer, a laser vibrometer to be measured, and a standard laser vibrometer. The method comprises the following steps:

[0016] Step 1: The signal generator generates a sinusoidal signal, which is driven by the voltage amplifier to drive the electro-optic modulator to work;

[0017] Step 2: The measurement beams from the laser vibrometer to be calibrated and the standard laser vibrometer are combined by a beam splitter, then incident on an electro-optical modulator through a polarizer. Their phases are modulated, and then they are incident on the electro-optical modulator again after passing through a reflector.

[0018] Step 3: The modulation phase is doubled. The measurement beam carrying the modulation phase information returns to the laser vibrometer to be calibrated and the standard laser vibrometer. The demodulated output equivalent velocity, displacement, and acceleration signals are synchronously collected by the data acquisition system. The demodulation results of the standard laser vibrometer are used as a reference signal to calibrate the amplitude response and phase delay of the laser vibrometer to be calibrated.

[0019] Furthermore, the standard laser vibrometer also includes an AWView host computer, an AD acquisition control module, an automatic gain control module, an FIR low-pass filter module, an FIR low-pass filter module, and a CIC extraction module. The demodulation step includes the following:

[0020] Step 31: The AWView host computer controls the AD acquisition control module, automatic gain control module, FIR low-pass filter module for demodulation, FIR low-pass filter module for output, and CIC extraction module through the PCIe interface, so that FIR filters with different parameters and data extraction multiples can be selected according to specific application scenarios.

[0021] In this embodiment, the AD acquisition control module adopts TI's ADS42LB69.

[0022] In this embodiment, the automatic gain control module adopts TI's LMH6881, and uses the PL end of the Zynq MPSoC in the signal processing module to implement gain adjustment through the SPI interface, with a step size of 0.25 dB.

[0023] In this embodiment, the CIC extraction module adopts AMD's XCZU7EV-2FFVB1156I.

[0024] In this embodiment, Figure 1 As shown, an electro-optic modulator is used to simulate mechanical vibration to phase-modulate the laser vibrometer's measurement beam. This serves as a calibration signal, which is then assigned a value by a standard laser vibrometer and used as a reference signal to calibrate the amplitude and phase delay of the entire laser vibrometer. The standard laser vibrometer consists of a laser heterodyne interferometer with traceable process data and a real-time signal demodulation system coordinated by hardware and software.

[0025] In this embodiment, Figure 1 As shown in the figure, the role of the polarizer is to control the polarization state of the measuring beam so that the polarization direction of the measuring beam is consistent with the refractive index principal axis of the electro-optical crystal. In this configuration, the measuring beam only changes its phase but not its polarization state when passing through the electro-optical crystal, avoiding the influence of the birefringence effect and improving the stability of the equivalent Doppler signal generated by the electro-optical phase modulation method.

[0026] In this embodiment, Figure 1 As shown, the amplitude of the modulation phase introduced by the laser vibrometer when the light beam passes through the EO modulator twice is related to the driving voltage of the EO modulator. Therefore, the amplitude range of the modulation phase can be expanded by increasing the driving voltage of the EO modulator or connecting multiple EO modulators in series.

[0027] In this embodiment, Figure 1 As shown in the figure, a real-time signal demodulation system with software and hardware is used to realize the real-time demodulation of Doppler signals. The phase delay introduced thereby is stable and can be quantitatively evaluated, thus avoiding the limitations of the software demodulation method in phase delay calibration.

[0028] Although the specific embodiments of the present invention are described in detail in conjunction with the accompanying drawings, they should not be construed as limiting the scope of protection of the present invention. Various modifications and variations that can be made by those skilled in the art without creative work within the scope described in the claims are still within the scope of protection of the present invention.

Claims

1. A laser vibrometer calibration method using electro-optical modulation technology, comprising a data acquisition system, a signal generator, an electro-optical modulator driver, an electro-optical modulator, a reflector, a polarizer, a laser vibrometer to be calibrated, and a standard laser vibrometer, characterized in that: The specific steps of the method are as follows: Step 1: The signal generator generates a sinusoidal signal, which is driven by the voltage amplifier to drive the electro-optic modulator to work; Step 2: The measurement beams from the laser vibrometer to be calibrated and the standard laser vibrometer are combined by a beam splitter, then incident on an electro-optical modulator through a polarizer. Their phases are modulated, and then they are incident on the electro-optical modulator again after passing through a reflector. Step 3: The modulation phase is doubled, and the measurement beam carrying the modulation phase information returns to the laser vibrometer to be calibrated and the standard laser vibrometer. The equivalent velocity, displacement, and acceleration signals output by the demodulated output are synchronously collected by the data acquisition system. The demodulation results of the standard laser vibrometer are used as a reference signal to calibrate the amplitude response and phase delay of the laser vibrometer to be calibrated. The standard laser vibrometer is composed of a laser heterodyne interferometer with traceable process data and a real-time signal demodulation system coordinated by hardware and software. It also includes an AWView host computer, an AD acquisition control module, an automatic gain control module, an FIR low-pass filter module, an FIR low-pass filter module, and a CIC extraction module. The polarizer is used to align the polarization direction of the light beam with the main axis of the electro-optical crystal. The demodulation step includes the following: the AWView host computer controls the AD acquisition control module, the automatic gain control module, the FIR low-pass filter module for demodulation, the FIR low-pass filter module for output, and the CIC extraction module through the PCIe interface, so that FIR filters with different parameters and data extraction multiples can be selected according to specific application scenarios.

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