Fiber-Optic Strain Sensors Based on Linearly Chirped Tapered Fiber Bragg Gratings With Tailored Intrinsic Chirp

In this paper, spectrally tailored tapered chirped fiber Bragg gratings (TCFBGs) are considered for use as strain sensors. Both gratings were written in fused tapered optical fiber using linearly chirped fiber Bragg gratings in co-directional and counter-directional chirp configurations. Theoretical and numerical analysis as well as experimental verification of the influence of applied strain on spectral width of both TCFBG structures were carried out. The results show that TCFBGs exhibit monotonic strain response over the wide operating range of the applied force. Compared with standard tapered FBG written using uniform phase mask, in the case of co-directionally written TCFBG with substantially larger grating chirp the monotonic operating range can be easily broadened toward the higher strain values (even above the applied force at which the optical fiber breaks). In turn, the intrinsic chirp of the counter-directionally written TCFBG can be tailored in such a way as to ensure that the monotonicity of its strain response is always satisfied when tensile force is applied. This is due to the spectral broadening of the reflected spectrum when strain increases.