Beat Frequency Signal Demodulation Of Fiber Laser Sensing System

Optical fiber sensor has been widely used in many fields such as industrial production and safety monitoring.Compared with the electrical sensors,optical fiber sensors have the advantages of small size,high sensitivity,anti-electromagnetic interference and is easy to build a large-scale sensing network.Compared with the fiber Bragg grating sensor,the fiber laser sensor has narrower signal line width and higher signal-to-noise ratio,which leads to higher sensing accuracy and is suitable for the longdistance sensing system.However,the conventional optical demodulation method usually has slow processing speed,relatively low precision and high cost.In this paper,a fiber multi-longitudinal mode beat-frequency sensing demodulation system is studied,and some demodulation methods are proposed for the low-cost demodulation of highfrequency beat-frequency signals.A multi-longitudinal mode fiber laser beat frequency signal demodulation system based on band-pass sampling technique is proposed.To reduce the cost of high-frequency demodulation,the band-pass sampling technique is proposed which can replace the down-conversion process,and the high frequency band-pass signals can be sampled at a lower sampling rate.A straight cavity optical fiber laser strain sensing system is proposed.By using band-pass sampling technique,the beat frequency signal with the center frequency of 1.375 GHz is successfully sampled at a lower sampling rate,and the strain sensitivity of beat frequency signal is-0.36 k Hz/??.Since the absorption coefficient of the erbium-doped fiber decreases with the increase of temperature,a sensing configuration for simultaneous measurement of strain and temperature is proposed.As a result,the strain and temperature sensitivity of beat frequency signal is-0.49 k Hz/?? and 0 d B/??,respectively,and the corresponding strain and temperature sensitivity of laser power is-6.29 k Hz /? and-0.012 d B/?,respectively.This proposed system can measure strain and temperature simultaneously.Experimental results show that the beat frequency signal down-converted by the band-pass sampling technique has the same sensitivity as the original high-frequency signal.A multi-longitudinal mode fiber laser vibration sensing system based on software defined radio(SDR)is demonstrated.Using existing software platform to design a beat frequency signal demodulation system can greatly reduce the cost of hardware in the system.The block diagram of FM signal demodulation is constructed on the software radio platform,and the signal processing method including filtering,sampling and demodulation can be realized through the software,which greatly reduce the hardware cost and the complexity of the system.This proposed sensing system has high response to vibration signals from 10 Hz to 2 k Hz,which is expected to be used in hydrophone,road and bridge health monitoring and geological prospect.In this thesis,the demodulation methods of multi-longitudinal mode beat frequency signal in fiber laser sensing system is studied.Band-pass sampling and software demodulation of high frequency beat frequency signal are proposed.These methods can ensure the high sensitivity of the sensing system and reduce the cost of the whole system effectively,which are expected to be applied in fiber laser sensing system.