High-resolution Frequency Scanning System For Optical Fiber Sensing

A high-resolution frequency scanning system has been proposed and demonstrated in this paper after investigating and discussing the working principles of optical spectral measurement method.Moreover,owing to small frequency resolution of the microwave source and high bandwidth of electro-optic modulator,the resolution of this system is largely improved.Meanwhile,fiber Bragg grating Fabry-Perot interferometer?FBG-FPI?and a sensitivity enhanced fiber rings have been experimented in this paper.The resolution,stability performance and the measurement range of this system have been analyzed at last.The paper mainly includes the following three parts?1.A high-resolution sensing system is proposed and demonstrated,which has high potential for dynamic signal sensing.The frequency scanning system consists of a frequency-stabilized laser,a microwave generator,an electro-optic modulator?EOM?,a photodetector and a data acquisition?DAQ?.Moreover,because of small frequency resolution of the microwave source and high bandwidth of electro-optic modulator,the resolution of this system is largely improved.Meanwhile,the measurement of ultrahigh bandwidth?less than 1 pm?can be expected.2.Owing to the ultra-narrow transmission bands of the sensor head and the small resolution of the microwave scanning demodulation system,the system can realize a high resolution for strain,curvature and lateral force sensing.The sensor head is a Fabry-Perot interferometer?FBG-FPI?formed by two cascaded uniform FBGs.And the sensitivities of strain,curvature and lateral force sensing are 9.696 MHz/??,4346.97 MHz/m-1 and-120.02 MHz/g respectively and the system can realize a high resolution for strain,curvature and lateral force sensing,up to n?,10^-3 m^-1 and?g respectively.3.A sensitivity enhanced fiber-optic strain sensing system with Vernier effect based on microwave frequency scanning is proposed and experimentally demonstrated.The strain sensor consists of two cascaded fiber rings which are composed of just standard single-mode fibers.Since the two fiber rings are set to have almost same free spectra ranges,Vernier effect is generated in the transmission spectrum.The experimental results show that the strain sensitivity has been improved over 20 times higher than that of a single fiber ring.Finally,the stabilization of the proposed system was tested and the detection limit can be obtained with the value of ?14 n?Owing to the high speed and small frequency resolution of the proposed microwave frequency scanning in our system,this system has high potential for tiny variation of dynamic signal sensing.