Research on seawater temperature and pressure sensor based on microfiber resonator

Aiming at the requirement of simultaneous measurement of seawater temperature and pressure, this paper presents a two-parameter sensing scheme of temperature and pressure using elliptical microfiber knot resonator (MKR). Its output spectrum consists of interference spectrum formed by modal interference within the microfiber and variant Vernier spectrum formed by resonance of different modes in the resonant ring region. The interference spectrum and variant Vernier spectrum were separated through Fast Fourier Transform (FFT) filtering. After analysis, pressure sensitivity of interference dip and variant Vernier envelope reached −46.68 pm/N and −63.46 pm/N, respectively, in the pressure range of 20 N ∼ 220 N. The maximal pressure sensitivity obtained by theoretical conversion is approximately −8.44 nm/MPa, and the pressure resolution is 2.37 × 10^-3 MPa, which corresponds to the water depth of 0.24 m. For interference dip and variant Vernier envelope, temperature sensitivities are −4.79 nm/℃ and −3.84 nm/℃ in 22℃∼38℃, respectively. This sensing structure shows good time stability, repeatability and hysteresis. The simultaneous demodulation of temperature and pressure is achieved by means of sensitivity matrix inversion. The sensing structure, which has a compact size, strong robustness, and high sensitivity, has a good application prospect for seawater temperature and pressure two-parameter sensing.