Knot Resonator in U-Shaped Microfiber With Multieffect for Seawater Temperature and Pressure Dual-Parameter Measurement

A probe-type microfiber resonator with multi-effect is proposed and its temperature-pressure dual-parameter sensing characteristics are investigated. The sensing probe consists of a microfiber knot resonator (MKR) and an adiabatic U-shaped taper. The sensing probe, which is called U-shaped MKR for short, can excite two different spectra which superimpose with each other in the output end. One part of the superimposed output spectrum is the interference spectrum, which results from mode interference in the U-shaped adiabatic fiber taper. The other part of the superimposed output spectrum is Vernier spectrum, which results from resonance of various modes within the knot resonator. The experimental tests on the temperature and pressure sensing performance show that the pressure sensitivities of the interference spectrum and Vernier envelope are as high as -0.19 nm/N and -0.44 nm/N, respectively, when pressure is between 20 N and 150 N. The maximum pressure sensitivity is converted to be around -58.69 nm/MPa. The minimum resolvable seawater depth is calculated to be only 3.4 cm. Temperature sensitivities of the two spectra are up to -5.33 nm/°C and -6.30 nm/°C, respectively, in the temperature range of 18°C to 50°C. Concurrent demodulation of temperature and pressure can be realized via a demodulation matrix which is constructed by sensitivities of both interference and Vernier spectra. The sensor exhibits excellent hysteresis, repeatability, and long-term stability, which has certain research significance for developing compact, robust and highly sensitive two-parameter sensors.