Intermodal interferometer with low insertion loss and high extinction ratio composed of a slight offset point and a matching long period grating in two-mode photonic crystal fiber

An all-fiber modal interferometer based on a long period grating (LPG) inscribed in a two-mode photonic crystal fiber (PCF) and a slight core-offset spliced end is proposed and demonstrated. The LPG is fabricated to realize energy coupling from LP01 core mode to LP11 core mode, and the two core modes will interfere at the slight core-offset spliced end. We analyze the impact of energy coupling efficiencies of the LPG and the output spliced end on the extinction ratio of the interference fringes. With an appropriate energy coupling efficiency matching condition, our modal interferometer can realize lower insertion loss and high extinction ratio. Moreover, the sensitivities of our interferometer to strain and temperature are investigated, and the good stability of this device to external refractive index change is also demonstrated. As an all-fiber interferometer made of pure silica, this device has great potential applications in high temperature sensing fields, especially in harsh conditions. (C) 2015 Optical Society of America