Cladding mode loss effect on transmission characteristics of mechanically induced long-period fiber gratings

In the mechanically induced long-period fiber grating (MLPFG) fabricated by applying the periodical pressure on an optical fiber, the small bending occurs due to the periodical pressure, which influences the losses of the core and cladding modes. We propose the theoretical model of the MLPFG for estimating its transmittance based on the transfer matrix method and investigate the effect of the losses of the core and cladding modes in the MLPFG on the transmittance, theoretically and experimentally. We numerically clarify that the transmitted light spectrum of MLPFG shows only the main attenuation lobe, no side lobe, and the attenuation bandwidth is broadened as the cladding mode loss increases. We also clarify that the coupling coefficient, and the losses of the core and cladding modes in the fabricated MLPFG can be estimated from the measured transmitted spectrum based on our model. Moreover, we measure the transmittance of two types of MLPFGs that fabricated with a screw, weights, and metallic plates, and that fabricated with a screw and a heat-shrinkable tube. The measured spectra show good agreements with the calculated ones using our model. Our model will be useful for designing MLPFGs applied to the sensors.