Envelope-Free Optical Vernier Method Enabled High-Sensitivity 2-D Bending Measurements Based on a Seven-Core Fiber Sensor

In this work, an envelope-fitting-free, peak-recognition-based optical Vernier method is proposed to demodulate fiber optic vector bend sensors for high-sensitivity bend measurement. The fiber bend sensor was made by inscribing a double-cascade Fabry-Perot (FP) interferometer in two edge cores of a seven-core fiber (SCF) using femtosecond laser machining. Utilizing the proposed peak-recognition optical Vernier method, a maximum bending sensitivity of 3270.2 pm/m ${}^{-{{1}}}$ has been achieved, which is approximately 26 times that of a single fiber cascaded FP cavity sensor. Since the peak-recognition optical Vernier method does not require fitting of the envelope modulation curve, the limit of detection is independent of figure of merit (FOM), which is usually inversely proportion to the sensor sensitivity, thus offering higher measurement accuracy and reliability.