Ultrasound detection based on optical tapered-knot resonator sensor

In this work, we propose and experimentally demonstrated an optical ultrasound sensor based on tapered knot resonator (TKR) structure for ultrasound sensing. The sensor is fabricated by tapering a knotted single mode fiber (SMF), forming ring region and tapered region of TKR sensor. Higher order modes are excited and the mode interference is effectively enhanced. By simulating and analyzing transmission spectrum of TKR sensors with different diameter of ring region and different length of taper length, the diameter of ring region and the length of tapered region are optimized as 2 cm and 5 mm, respectively. We constructed the ultrasound detection experimental system, and the experimental results show that the TKR sensor performs high response characteristics for continuous ultrasound from 25 kHz to 305 kHz, and have a high signal-to noise ratio (the maximal calculated SNR is 37.89 dB). Moreover, the TKR sensor shows low temperature sensitivity of 6.64 pm/degrees C. The proposed TKR sensor has stable structure, wide ultrasound response range, and easily fabrication method, which shows potential application value in the field of high sensitivity ultrasound testing.