Square Knot Resonator-Based Compact Bending Sensor

In this letter, design and development of a square knot resonator (SKR) for sensing applications is proposed. The SKR is developed by carefully weaving a single-ended microfiber in and out of a double-ended microfiber. The dependence of the SKR's resonance wavelength on the circular path length and the effective index of microfiber is utilized to sense the bending. Two 4.5-cm-long and 16-μm-diameter tapered microfibers are used to construct an SKR having a diameter of 885 μm which has a free spectral range of 0.505 nm, and the Q-factor is more than 14500. Experimental results show that the bending sensitivity of the SKR is 3.04 nm/m^-1 with a maximum theoretical resolution of 3.29 × 10^-3m^-1. The temperature sensitivity of the SKR is 3.2 pm/°C.