Reflectivity measurement of fiber Bragg grating by cavity ring-down spectroscopy technique

A novel method to measure the reflectivity of fiber Bragg grating (FBG) is proposed and demonstrated experimentally. A kind of fiber Bragg grating FP(FBG-FP) cavity is constructed, which includes a couple of identical uniform FBG as the cavity mirrors linked by a piece of Er-doped fiber. The response of FBG-FP cavity to pulsed laser injection is demonstrated theoretically and experimentally, and the effect of the reflectivity of FBG to the output of FBG-FP cavity is discussed. The reflectivity of FBG is achieved by measuring the transmission loss of FBG-FP cavity with cavity ring-down spectroscopy technique. A general relationship between the reflectivity of FBG and ring-down time of FBG-FP cavity is derived. It's demonstrated that the output loss of FBG-FP cavity increases as the reflectivity of FBG decreases, the stable light intensity in the cavity decreases fastly, and the ring-down time of the FBG-FP cavity decreases, the operating wavelength of FBG is a function of its temperature. The results demonstrate a new concept of the measurement of reflectivity of FBG and the technical feasibility.