Study On High-speed Phase Demodulation Technique For Fabry-perot Sensors

At present, with the development of fiber sensing technology, optical fibersensor as an alternative sensor of traditional piezoelectric sensor, is also more andmore widely used in many areas, which impose the importance of research thedemodulation technology of optical fiber sensor. As in the measurement ofunderwater explosion shock wave, the traditional piezoelectric sensors cannot achieveprecise repetition shock wave signal accurately. While the optical fiber F-P cavitysensor with small size, high frequency and immune to electromagnetic interference isthe best choice. Demodulating the information which is modulated by the F-P cavityis realized by demodulating the cavity length of the F-P cavity sensors. In order torealize the cavity length demodulation of the F-P cavity sensor, various demodulationschemes have been researched and proposed, such as intensity demodulation method,the Fourier transform method, the fringe counting method and the peak trackingmethod and so on. Each cavity demodulation method has its own advantages anddisadvantages. However, in the shock wave measuring, these methods have theirlimitations. This paper focuses on the F-P cavity sensor demodulation technology formeasuring the transient signal.This paper introduces the F-P cavity sensing principle and the principle of threewavelength demodulation. This demodulation method is based on the two-beaminterference theory, using three different wavelengths of laser as the light source. Inorder to verify the theory, using the mathematics to simulate shock wave signal, basedon the three wavelength demodulation method to demodulate the modulated signals.The result shows that there is a good linearity between the modulated phase and thedemodulated phase.After analyzing the requirements of the three wavelength demodulation systemfor the light source and the photodetector, a suitable hardware is designed. As for thelight source section, this circuit includes following functions, there are automaticpower control to guarantee the power of the laser stability and automatic temperaturecontrol to keep the wavelength stability, and over-current protection and slow start toprevent the laser diode from being damage. All these are to ensure long-term stabilityof the laser work. And the photodetector section, the noise and the speed of response is a major influence that will affect the performance of the photoelectric detection.According to these two factors, a photovoltaic transimpedance amplifier circuitwas designed.At last, according the requirements of the system, set reasonable parameters,then, each separate component will meet the requirements of the overall performance.Source power fluctuation range is0.1mW, the fluctuation range of the wavelength ofthe light source only0.03nm. Which causes the phase noise is0.019rad, thecorresponding sound pressure is0.14MPa. For the shock wave to be measured withthe peak pressure of50MPa, the noise is in the acceptable range. Then choose theappropriate F-P cavity, build a sensing system for static calibration and dynamic test.Static calibration result is that the linear degree of linearity is0.9938and thesensitivity of the system is-0.1228MPa/rad. Dynamic test experiments show that amore accurate reproduction of the measured shock wave signal can be achievedthrough this demodulation technique. The measured shock wave rise time is2.7μs.The results showed that the three-wavelength demodulation technology meets F-Pcavity sensor measuring shock wave’s requirements well.