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Research on fiber optic interferometric vibration measurement system based on internal modulation
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摘要:
研制了一种基于内调制的光纤干涉振动测量系统。对分布式反馈激光器 (DFB)进行电流内调制，产生正弦频率调制单频激光。经光纤环形器与光纤微探头输出测量光对振动源进行干涉测量，返回的测量光与光纤微探头自反射的参考光进行干涉获得相位生成载波 (PGC)干涉信号。基于可编程逻辑门阵列 (FPGA)数字信号处理平台设计了PGC干涉信号解调算法，通过5参数椭圆拟合提取附加光强调制等因素引入的误差系数，对相位非线性误差进行补偿，实现振动位移的高精度测量，并采用快速傅里叶变换 (FFT)算法对振动位移频谱进行分析。进行了理论分析并搭建了振动测量系统，开展了干涉信号解调实验、位移测量实验和振动测量实验。实验结果表明，该系统的振动频率范围覆盖1142 Hz；在10 µm的位移步进实验中，测量结果的平均偏差为0.173 µm；振动频率的分辨力为1.221 Hz，谐波失真率小于1.36%；有望应用于精密振动测量领域。
Abstract:A fiber microprobe vibration measurement system based on internal modulation has been developed. A sinusoidal phase modulated laser source is generated by modulating the current of the distributed feedback laser (DFB) with a sinusoidal signal. After passing through a fiber circulator and a fiber microprobe, the output laser beam is used to measure the displacement of a vibration source. The returned laser beam interferes with the reference laser reflected by the fiber microprobe to generate a phase generated carrier (PGC) interference signal. A real-time PGC signal processing algorithm is designed through a programmable logic gate array (FPGA) digital computing platform. A five-parameter ellipse fitting method is unitized to extract the error items introduced by additional intensity modulation and other factors and compensate for the phase nonlinear error. The fast Fourier transform (FFT) algorithm is unitized to analyze the vibration displacement. Theoretical analysis was conducted and a vibration measurement system was built. A series of experiments were conducted, including PGC signal demodulation, displacement measurement, and vibration measurement. The experimental results show that the vibration frequency range of the system covers 1142 Hz. In the 10 µm step displacement experiment, the average deviation measured is 0.173 µm. The resolution of vibration measurement is 1.221 Hz, and the harmonic distortion is less than 1.36%. The measurement system is expected to be applied in the field of precise vibration measurement.
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Key words:
- vibration measurement /
- internal modulation /
- microprobes /
- phase generated carrier /
- ellipse fitting
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Overview: Vibration measurement is significant and widely used in the fields of spacecraft performance evaluation, material damage detection, machinery diagnostics, and acoustic sensing. Specifically, the vibration measurement based on a sinusoidal phase modulation laser interferometer has become a lasting research focus because of the advantages of strong anti-electromagnetic interference, high sensitivity, and large dynamic range. Sinusoidal phase modulation interferometers based on external modulation use adding piezoelectric ceramics or electro-optic modulators to the reference arm of the optical path. Introducing additional components will increase the hardware cost of the measurement system and result in a longer reference arm, which may introduce drift errors. Differently, sinusoidal phase modulation interferometers based on internal modulation directly modulate the laser current sinusoidally. The advantage is that no additional modulation devices need to be introduced. Besides, the collimator, spectroscope, and reference mirror in the measurement optical path can be integrated into a single fiber microprobe, improving practicality. However, it inevitably has periodic nonlinear errors caused by the phase modulation depth, carrier phase delay, and additional intensity modulation, which limits the accuracy of vibration measurement.
In this paper, a fiber microprobe vibration measurement system based on internal modulation has been developed to solve the problems mentioned above. A sinusoidal phase modulation laser source is generated by modulating the current of a distributed feedback laser (DFB) with a sinusoidal signal. After passing through a fiber circulator and a microprobe, the output laser beam is used to measure the displacement of the vibration source. The returned laser beam interferes with the reference laser reflected by the microprobe to generate a phase generated carrier (PGC) interference signal. A real-time interference signal processing algorithm is designed based on a programmable logic gate array (FPGA) digital computing platform. The error items introduced by additional intensity modulation and other factors are extracted through five-parameter ellipse fitting to compensate for the phase nonlinear error, and achieve high-precision measurement of vibration displacement. The fast Fourier transform (FFT) algorithm is used to analyze the vibration displacement. Theoretical analysis was conducted and a vibration measurement verification system was built. Interference signal demodulation experiments, step displacement measurement experiments, and vibration measurement experiments were carried out. The experimental results show that the vibration frequency range of the system covers 1142 Hz. In the 10-µm step displacement experiment, the average deviation measured is 0.173 µm. The resolution of vibration measurement is 1.221 Hz, and the harmonic distortion is less than 1.36%. The measurement system is expected to be applied in the field of precision vibration measurement.
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图 1 基于内调制的光纤微探头式振动测量方法原理框图。 (a)光路； (b)信号处理
Figure 1. Schematics of vibration measurement by a fiber microprobe with internal modulation. (a) Optical part; (b) Signal processing
图 2 非线性误差仿真结果。 (a)李萨如图； (b)相位解调误差
Figure 2. Results of nonlinear errors from simulation. (a) Lissajous figures; (b) Phase demodulation error
图 3 基于椭圆拟合的信号处理原理示意图
Figure 3. Schematic diagram of signal processing based on ellipse fitting
图 4 基于内调制的微探头式振动测量系统
Figure 4. Microprobe vibration measurement system based on internal modulation
图 5 半仿真测量结果。 (a) 0.5 kHz振动测量结果； (b) 0.5 kHz振动频谱； (c) 0.8 kHz振动测量结果； (d) 0.8 kHz振动频谱； (e) 1.0 kHz振动测量结果； (f) 1.0 kHz振动频谱
Figure 5. Measurement results of semi-simulation. (a) Vibration measurement results of 0.5 kHz; (b) Vibration spectrum of 0.5 kHz; (c) Vibration measurement results of 0.8 kHz; (d) Vibration spectrum of 0.8 kHz; (e) Vibration measurement results of 1.0 kHz; (f) Vibration spectrum of 1.0 kHz
图 6 相位解调中的李萨如图。 (a)原始李萨如图； (b)三步补偿的李萨如图
Figure 6. Lissajous figures in phase demodulation. (a) Initial Lissajous figure; (b) Lissajous figures after compensation of each step
图 7 各阶段相位解调结果。 (a)初始相位解调结果； (b)第一步补偿； (c)第二步补偿； (d)第三步补偿
Figure 7. Phase demodulation results after each step. (a) Initial phase demodulation results; (b) First compensation; (c) Second compensation; (d) Third compensation
图 10 振动测量结果。 (a) 25 Hz振动测量结果； (b) 25 Hz振动频谱； (c) 50 Hz振动测量结果； (d) 50 Hz振动频谱； (e) 600 Hz振动测量结果； (f) 600 Hz振动频谱
Figure 10. Results of vibration test. (a) Vibration measurement results of 25 Hz; (b) Vibration spectrum of 25 Hz; (c) Vibration measurement results of 50 Hz; (d) Vibration spectrum of 50 Hz; (e) Vibration measurement results of 600 Hz; (f) Vibration spectrum of 600 Hz
图 11 不同振动频率下振幅和总谐波失真测量结果
Figure 11. Results of amplitude and THD at different vibration frequencies
表 1 信号处理系统测试中的主要参数
Table 1. Main parameters in signal processing system testing
参数 数值 参数 数值 测量光强度I0 1 相位调制深度C 2.63 rad 光强调制系数m 0.1 空气折射率n 1.00032 调制频率fc 20 kHz 激光波长λ0 1550 nm 载波相位延迟φc 30 rad 振动幅度l 1 µm 光强调制相位延迟φm 18 rad 振动频率fv 0.5，0.8，1.0 kHz 
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