Reflectivity Measurement Of Fiber Bragg Grating By Cavity Ring-down Spectroscopy Technique

Fiber Bragg grating(FBG)was first invented by K.Hill et al in Canada in 1978.It is a kind of all fiber device,which is often used in filter and laser reflector,in fiber communication technology,optical fiber temperature sensor,infrared remote sensing technology.The applications of fiber lasers,microwave communication,bio-optics,optical information and its processing are widely used.A novel measurement method about the reflectivity of FBG based on cavity ring-down spectrum technique is proposed in this dissertation,and its relative key problems are studied in detail,The main research contents are as following:1?The transmission characteristic of FBG is studied by using mode-couple theory,the effects of grating length and coupling coefficient on its transmission characteristic.It's demonstrated that the reflectivity of FBG is proportional to grating length and coupling coefficient,reflection bandwide of FBG is inverse proportional to grating length.2?A novel linear system analysis method is proposed to study the response of FBG to different line-shape incident light waves,and a system analysis model is established.The response of FBG to Gaussian line-shape incident light wave is studied theoretically,and the responses of FBG to gaussian and Lorentz line-shape incident light wave are studied experimentally.The Gaussian line-shape light wave signal source comes from the spontaneous emission of Inp/InGaAsP multi-quantum well semiconductor,and the Lorentz line-shape light wave signal source comes from the spontaneous emission of Er-doped fiber.It' demonstrated that the influence of FBG on the incident optical signal occurs within the 3dB bandwidth of of FBG near its Bragg wavelength and is only related to its intrinsic transmission function,but the waveform of incident optical signal,the band-stop filter effect is also verified.In other words,all the light waves within the bandwide of FBG will be reflected back,and other light waves will pass through FBG smoothly.3?A new method to measure the reflectivity of FBG is proposed and verified experimentally,the measuring principle is presented,the function expression between cavity ringdown(CRD)time and the reflectivity of FBG is derived.Numerical results demonstrated that the output characteristic of FBG-FP cavity includes three phases:build-up,satbility and ringdown.The measurement of the reflectivity of the FBG is divided into three stages: 1)determine the mode(presented by wavelength);2)measure the ring-down time and determine the corresponding reflectivity of the FBG at the wavelength above;3)determine the reflectivity profile of the FBG.Synchronously changing the operation temperature of the two FBG,repeating stages 1)and 2)seventimes to obtain relative data,then,using fitting method,the reflectivity profile of the FBG is determined.Experimental result demonstrated that the reflectivity of the FBG is function of wavelength,its reflectivity at 1550.09 nm is 99.37%.Of course,the proposed measuring method can also be used for low reflectivity FBG.