Fiber Strain And Temperature Sensors Based On Radio-frequency Demodulated Technique

Optical fiber sensors are well known for their advantages of small size,light weight,immunity to electromagnetism and its fast development in the past few decades.We can use it for high precision and sensitivity measurement over a long distance.It has been widely used in civil structures monitoring,such as bridges,dams and high rise buildings.Because of the high cost and low speed of optical spectrum interrogation,how to design a better performance optical fiber sensor is being studied by many researchers.Microwave photonics is an interdisciplinary field that links up microwave signal with optical signal.When an optical carrier modulated by microwave signals,it achieve a microwave photonics transmission system that can be immune to electromagnetism and have a large transmission capacity.Furthermore,combine optical fiber sensor techniques with microwave modulation technique we can obtain a system that not only possesses high sensitivity property of optic,but also a fast demodulation scheme.Compared with traditional optical fiber sensor,the scheme based on RF(radio-frequency)optical fiber sensors have a more stable structure.In this paper,we will present two types of optical fiber strain/temperature sensor system based on microwave modulation technique and optical fiber interference construction.(1)A optical fiber train/temperature sensor based on RF unbalanced Mach-Zehnder interferometer is proposed and demonstrated.We use a fiber Bragg grating as sensor unit,the induced differential chromatic dispersion transfers the wavelength shift of the FBG to the change of the RF phase difference between the two interferometric carriers,which ultimately leads to the variation of the RF signal intensity.A interrogation of strain/ temperature-turned FBG is accomplished and sensing sensitivity of 0.017MHz/??,0.00065dB/??,0.0891MHz/?C,0.00108 dB/?C are obtained,respectively.(2)A method based on photonic microwave single band-pass filter for measurement of temperature is proposed and demonstrated.We use one arm of Michelson interferometer as sensor element,and transfer the FSR of Michelson interference spectrum induced by temperature change to the central frequency of the microwave signal shift.The experimental measurements show that a high temperature sensing sensitivity of 15.35MHz/?C is achieved within the measurement range of 20-60?C,the fitness of liner is 0.9993,and we also discuss the factors that influence the sensitivity.In addition,we demonstrate a proof-of-concept experiment to analyze the performance of the microwave signal and propose several optimal schemes.