Cascaded Fiber Up-Taper Modal Interferometer and Its Application as Fiber Sensor

A cascaded fiber up-taper interferometer (CFI) is proposed and demonstrated analytically and experimentally. The sensor head of the CFI consists of two cascaded miniaturized fiber up-taper modal interferometers (MMIs). Sensing principle of the CFI is derived based on the transmission matrix method. Conclusions have been achieved that cascading MMIs can effectively improve its thermal sensitivity while strain sensitivity is reduced compared with that of the single MMI. Experimentally, thermal sensitivity of the CFI can reach 112.9 pm/°C, and almost two times as big as that of the single MMI. In contrast, its strain sensitivity is only -0.4 pm/με, which is only 2/5 as small as that of the single MMI.