Optical micro-knot resonator hydrophone

We present results of what are believed to be the first underwater acoustic sensor using an optical micro-knot resonator. The mechanism involves straining the micro-fibre loop of the knot via an appropriate encapsulation material. The micro-knot sensor exhibited a spectral full-width half-depth of 37.7pm with a Q-factor of 41100 after deconvolution with the source. The shift in wavelength at 1550nm was observed to be around 67pm RMS when exposed to underwater acoustic excitation at 40Hz demonstrating a normalized sensitivity of -288 dB re μPa(-1), or 5.83 fm/Pa.