Expansion of spatial measurement range by use of vernier effect in fiber Bragg grating strain sensing system with synthesis of optical coherence function

Adopting a method named "synthesis of optical coherence function (SOCF)", we have proposed and developed an original technique to multiplex fiber Bragg grating (FBG) sensors with similar wavelength. When we synthesize delta-function-like optical coherence function, we can selectively obtain the information for one FBG at a position corresponding to the coherence peak. Therefore, by this technique, we can make use of an array of FBG\´s with the similar Bragg wavelength. However, to avoid the error signal by the coherence peak of higher order, we need to take delay line in the system whose fiber-length is equal or longer than the fiber-length of multiplexed FBG array, that is, spatial measurement range is limited to the half of the original one. In this paper, we propose the scheme by use of vernier effect to solve the issue on the spatial measurement range with the conventional coherence peak sweep method in FBG strain sensing system with SOCF.