Experimental Study Of Optical Fiber Vibration Doppler Effect Based On Frequency-shifted Interferometry

When the bending fiber is excited by external vibration,the length of the variable fiber changes due to the change of the axial strain,which leads to the change of the frequency of the propagating beam.This phenomenon is described as the Doppler Effect in the fiber.The circular optical fiber sensor produced based on this effect not only has the advantages of light weight and anti-electromagnetic interference of other optical fiber sensors,but also has no direction selectivity,which solves the problem of single direction of FBG sensor and Doppler laser sensor.In this paper,we proposed to use of circular fiber to make vibration sensor,and frequency shifted interference technology to demodulate Doppler shift of the light in the fiber rings,aiming to overcome the shortcoming of existing fiber sensors as well as apply frequency shift interferometry,a high precision and low demand for devices demodulation technology,to dynamic signal demodulation.(1)We described the development of Doppler Effect sensors,vibration sensors and fiber vibration sensors.Then typical Doppler sensors and the traditional vibration sensor were introduced.The advantages and disadvantages of the fiber vibration sensor and the various sensors were also mentioned.Then we introduced the development of frequency shift interference technology.(2)We studied the Doppler-effect in bending fiber and analyzed its theoretical calculation method.Then we presented the development of frequency-shifted interferometry,to which we attached the theoretical model and formula derivation of Sagnac loop interferometry and how to use it to demodulate Doppler frequency shift.(3)We fabricated the fiber loop vibration sensor,and get the theoretical Doppler frequency shifts of the sensor by measuring the strain on it.By using M-Z beat frequency interference system we proofed the existence of Doppler frequency shifts in vibrating fiber loops.(4)We built the experimental system to detector vibration signal by deducing the Doppler frequency shifts.The demodulation results show that the minimum detectable Doppler frequency shift is 15 kHz,corresponding to a vibration acceleration amplitude of 1m/s~2 at 300 Hz,and a sensitivity of 22.7 mV/g,when the power of laser source is set at 3mW.Linear relationship between vibration accelerations and Doppler frequency shifts at every vibration frequency was demonstrated.The dynamic range of the sensor,in terms of vibration acceleration amplitude,is at least 190 m/s~2 at 300Hz.The sensor detectability was confirmed in the frequency range from 100 Hz to3.6k Hz,limited by our excitation source.In this paper we used FSI to detect the Doppler frequency shifts in vibrating fiber sensor.Compared to traditional fiber Bragg grating(FBG)vibration sensor,the proposed sensor is easy to fabricate,low cost,and without directional dependency.The application of less costly FSI demodulation allows for custom tailoring of measurement sensitivity,and has superior signal stability due to its same-path interference nature.The measured Doppler frequency shift using the FSI system agrees well with the calculated one,derived from strain measurements.The results indicates that the system has a high sensitivity and wide frequency range,showing significant potentiality in scientific research and industrial application.