A highly sensitive optical fiber curvature sensor based on cascaded fiber interferometers (CFIs) is demonstrated theoretically and experimentally. Theoretical results show that the scheme of cascaded fiber interferometers can be regarded as an equivalent fiber interferometer with longer interference length and narrower FSR. It serves as a curvature sensor with sensitivity up to 4.362 nm/m-1 within the measurement range of 0 to 1.134 m-1. Compared to the single fiber modal interferometer, the curvature sensitivity of the CFI is almost doubled. In addition, simultaneous curvature and temperature measurement is achieved using coefficient matrix method, eliminating the issue of temperature cross sensitivity.

A cascaded fiber up-taper interferometer (CFI) is proposed and demonstrated analytically and experimentally. The sensor head of the CFI is consisted by 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/μ $\epsilon$ , which is only 2/5 as small as that of the single MMI.