Study On Methods Of The Dynamic Demodulation For Optical Fiber Fabry-Perot Sensors

The mechanical effects of the shock wave, which is generated by underwater explosion, have a wide range of applications in civilian and military. Therefore, the measurement of various parameters changes very important. The traditional sensors used to measure the underwater explosion shock wave were piezoelectric ceramics and piezoelectric polymers. The sensitivity of the piezoelectric ceramic sensor is high, but its own impedance mismatch with the underwater acoustic medium impedance and the frequency response curve is not flat, which cannot reproduce the shock wave signal accurately; the frequency response curve of piezoelectric polymer is very flat, but the sensitivity is low. Both of the sensors are very poor to resistant the electromagnetic interference. The optical fiber sensor can solve these problems. The fiber itself is not sensitive to the electromagnetic and the volume of the probe is very small, which can achieve precise measurements. At the same time, the Optical fiber material can subject to the explosion impact of underwater shock wave. The Fiber F-P sensor has ultra-small size and is easy to get high frequency response and high spatial resolution, which is the most appropriate solution for the measurements of underwater shock wave.This paper studies the dynamic demodulation algorithm based on fiber F-P sensors, the work as follows.Firstly, this paper provides a three-wavelength light excitation, arbitrary phase step passive homodyne demodulation method, which is based on fiber F-P sensor, used to measure underwater shock wave signal. This dynamic demodulation method is based on the two-beam interference theory, using three different wavelengths of the light source and arbitrary phase step of output to realize the phase demodulation of the measured shock wave signal. It is so suitable for low precision fiber optic F-P sensor’s dynamic demodulation that we can achieve more accurate reproduction of the measured shock wave signal.Secondly, this paper provides a four-wavelength light excitation, arbitrary phase step passive homodyne demodulation method, which is based on fiber F-P sensor, used to measure underwater shock wave signal. This dynamic demodulation method is based on the three-beam interference theory, using four different wavelengths of the light source. The fiber optic F-P cavity based on a three-beam interference produced without coating, and its characteristics are determined only by the optical fiber itself. It simplifies the production process of low-precision F-P sensor, reduce costs, and can measure the underwater shock wave signal.Thirdly, both of two algorithms are proved by the theoretical analysis, software simulation and calibration experiments. The three-wavelength light source excitation, arbitrary phase step passive homodyne demodulation algorithm is correct, the software simulation results are consistent with the theory, and the data obtained in the experimental calibration analysis is consistent with the simulation results; achieve to measure the underwater shock wave signal. The four-wavelength light source excitation, arbitrary phase step passive homodyne demodulation algorithm is correct, the software simulation results are consistent with the theory, but the static test calibration results need to further modify the parameter settings in order to continue the dynamic calibration experiments.