Silver-coated 45 radiated tilted fiber grating based interferometer and its sensing applications

We proposed a silver-coated 45 radiated tilted fiber grating (45 RTFG) based interferometer, in which the 45 RTFG is working as a beam splitter and a coupler, and the light transmitting in the fiber core is radiated out and resonant in the cladding radial cavity formed by the fully coated silver film, which forms multiple beam interference. We have theoretically and experimentally investigated the proposed fiber interferometer in terms of polarization-dependent transmission spectra, the free spectral range (FSR), spectral visibility and sensing characteristics. Due to the single polarization radiation feature of 45 RTFG, only S-polarized light could form the multiple beam interference and P-polarized light would pass through the grating with low loss. And the FSR of the proposed fiber interferometer is around 6.8 nm, which is determined by the 125 m cladding diameter. The theoretical results show that the spectral visibility of interference is proportional to the coupling efficient of 45 RTFG and when the coupling efficient reaches 0.92, the spectral visibility is the maximum. In the experiment, the fiber interferometer formed by the 45 RTFG with 11 dB polarization extinction ratio has the spectral visibility of 26 dB. Moreover, we have also investigated the temperature, strain and twist characteristics of the proposed interferometer. The experimental results show that the transmission peak has a red shift with increasing temperature, with a sensitivity of 10 0.1 pm/C; The strain causes a blue shift for the transmission peak with -0.42 0.01 pm/ sensitivity, and the twist sensitivity reaches 22.408 0.2 dB/rad. The proposed silver-coated 45 RTFG based interferometer has an in-fiber and compact structure, which can be potentially used for optical fiber sensing.