Fig. 1. Poynting vector of the HE₁₁ mode of the microfiber in seawater. (a) Diameter is 400 nm; (b) diameter is 1000 nm^[43]

Fig. 2. Structure of the microfiber. (a) Structure and parameters of the SiO₂ microfiber; (b) refractive index profile; (c) variation curve of effective refractive index with fiber diameter^[51]

Fig. 3. Schematic diagram of the non-adiabatic tapering method. (a) Two-step drawing method; (b) multi-step intermittent drawing method; (c) optical fiber tapering machine based on plasma arc discharge^[61-62]

Fig. 4. Seawater temperature sensor based on MRR. (a) Microfiber Loop type ring resonator^[69]; (b) variation curve of the refractive index sensitivity of the MRR with the fiber diameter^[69]; (c) spectrum of the MRR under the peak shift sensing mechanism^[70]; (d) spectrum of the MRR under the intensity sensing mechanism^[72]; (e) schematic diagram of the two-point seawater temperature measurement system^[73]; (f) variation curve of the resonance wavelength with the seawater temperaturee^[73]

Fig. 5. Seawater salinity sensor based on MRR. (a) Experimental spectrum under the peak shift mechanism^[75]; (b) normalized transmission spectrum when elliptically polarized light is input^[76]; (c) variation curve of temperature sensitivity with the diameter of microfiber ring^[77]; (d) microfiber dual-arm MZI with resonator cavity^[78]; (e) transmission spectra under different salinities^[78]

Fig. 6. Experimental results of the MDC. (a) Schematic diagram of seawater temperature and salinity sensor^[82]; (b) experimental device of seawater temperature and salinity sensor^[83]; (c) transmission spectra of the coupler under different salinities^[83]; (d) transmission spectra of the coupler at different temperatures^[83]; (e) experimental device for seawater temperature, salinity and pressure sensor^[84]

Fig. 7. Structure of the MMZI single parameter sensor. (a) Schematic diagram of the MMZI sensor and the microscopic image of the sensing area^[85]; (b) sensitivity of the MMZI at 25 ℃^[86]; (c) transmission spectrum of sodium nitrate solution when the diameter of the microfiber is 3.46 μm^[87]

Fig. 8. Structure of the MMZI dual-parameter sensor. (a) Structure of the sensor^[87]; (b) typical transmission spectrum of the sensor^[87]; (c) transmission spectrum after high frequency filtering^[87]; (d) pressure distribution of the sensor when the pressure is 1 MPa^[88]; (e) transmission spectra under different pressures^[88]; (f) variation curve of interference peak wavelength with temperature^[88]

Fig. 9. Package of the microfiber sensor. (a) Schematic diagram of using probe to coat ethyl cellulose^[83]; (b) MMZI supported by U-shaped metal frame^[89]; (c) semi-encapsulated reinforcement structure ^[87]; (d) fully-encapsulated reinforcement structure^[88]

Fig. 10. Environmental adaptability of the microfiber sensor. (a) Long-term performance test results; (b) effect of vibration on the transmission spectrum; (c) effect of 30 Hz vibration on single-wavelength light intensity; (d) sea trial photos of disposable probes equipped with MMZI ^[89]

Fig. 11. Response time of different microfiber sensors. (a) Output response time of U-shaped metal frame package structure^[90]; (b) response time of C-shaped steel channel semi-encapsulated structure^[87]; (c) response time of C-shaped steel channel full package structure^[88]

Table 1. Performance parameters of three kinds of microfiber sensors