A microfiber knot resonator for ultrasonic underwater detection

Abstract

In this paper, a microfiber knot resonator (MKR) for detecting high frequency underwater acoustic signals is proposed. The micro-cavity structure based on the whispering gallery mode(WGM) has high sensitivity to the characteristic variables of the external environment, and the micronano fiber has the evanescent wave effect. Therefore, the MKR based on the WGM made by the micronano fiber can realize good underwater acoustic signal sensing. Polydimethylsiloxane (PDMS), which has low Young’s modulus and small refractive index difference with single mode fiber core, is selected as the packaging material. The PDMS films protect the structure of MKR while increasing the quality factor (Q-factor) of MKR. The Q-factor of the packaged MKR is up to $6.7\times 10^3$. The results show that the average sensitivity of MKR is −177 dB in the frequency range of 180 kHz-1 MHz, the sensitivity is −173.2 dB at 1 MHz, and the minimum detectable sound pressure(MDP) is 0.046 mPa$/\sqrt{\mathrm{Hz}}$. This paper shows the excellent potential of MKR in ultrasonic underwater detection, and the prospect of MKR in the field of underwater acoustic detection is expected to be further developed, and the application prospect of MKR in the integration of small size micro–nano sensors and sensor arrays.