An all optical fiber CTD sensor used in seawater based on an OMC is demonstrated. And the OMC is fabricated by fusing and tapering two twisted conventional communication fibers based on the improved flame-brushing method. By monitoring the sensing dips, in situ measurement of salinity, temperature and depth (pressure) in seawater can be realized respectively. Especially, the water depth (pressure) sensing function of the OMC is demonstrated for the first time, and the pressure sensitivity is 50 times higher than a bare fiber Bragg grating (with a wide pressure measurement range from 0 MPa to 25 MPa). The highest sensitivities of salinity, temperature and depth are 1596 pm/%, 2326 pm/°C, and 169 pm/Mpa, respectively.These are high enough for the requirements of marine dynamic environmental monitoring and underwater target detection of fiber-optic hydrophone system. The all optical fiber CTD sensor demonstrated here show advantages of easy fabrication, compact structure, high sensitivity, low cost, and compatibility with the optical fiber system, which may find applications in multifunction optical sensors in ocean detections and other applications.

In order to meet the needs of phase generated carrier (PGC) demodulation technology for interferometric fiber optic hydrophones, we proposed an optical microfiber all-optical phase modulator (OMAOPM) based on the photo-induced thermal phase shift effect, which can be used as a phase carrier generation component, so as to make the modulation efficiency and working bandwidth of this type of modulator satisfy the requirements of underwater acoustic signal demodulation applications. We analyzed the modulation principle of this modulator and optimized the structural parameters of the optical microfiber (OM) when the waist length and waist diameter of OM are 15 mm and 1.4 μm, respectively. The modulation amplitude of the modulator can reach 1 rad, which can meet the requirements of sensing applications. On this basis, the fiber optical hydrophone PGC-Atan demodulation system was constructed, and the simulated underwater acoustic signal test demodulation research was carried out. Experimental results showed that the system can demodulate underwater acoustic signals below 1 kHz.

A distributed fiber optic hydrophone (DFOH) is a new type of fiber optic hydrophone (FOH) with adjustable structure. The dependence of the directivity of a DFOH on array structure is theoretically and experimentally studied. The directivity function of a sensing channel and that of a DFOH are derived. Based on the directivity function, the simulations are performed. Finally, the theoretical analysis is demonstrated by the experiments performed on Qingyang lake, and the results reveal that the longer sensing channel length guarantees the lower first-order side lobe and the narrower main lobe. As the channel length increased from 1 to 3, the main lobe width and first-order side lobe height decreased by 4.9° and 6 dB, respectively. In addition, channel spacing is irrelevant to directivity as the spacing is shorter than the wavelength. As the channel spacing increased from 0 to 1, the variations of the main lobe width and first-order side lobe height are lower than 0.5° and 0.94 dB, respectively. This study would provide guidance for the structure design of a distributed fiber optic hydrophone in signal processing.

A non-intrusive mattress based on an optical-fiber Michelson interferometer is designed for daily sleep monitoring. The optical phase signal of the optical-fiber Michelson interferometer caused by the heartbeat and respiration is demodulated by the phase-generated carrier (PGC) method. The physiological signals and vital indicators including heart rate (HR), respiration rate (RR), and signal energy (SE) are extracted from the optical phase by algorithmic processing. A series of experiments are conducted to confirm the feasibility of the mattress for sleep monitoring. The mattress not only can achieve HR and RR counting, but also can record the waveform of the sleep-induced signal accurately. The body states can also be distinguished by the SE. In an all-night sleep monitoring experiment, the HR measured by the mattress is compared with the HR measured by a commercial smart band, showing a maximum error of 6 bpm (beat per minute). The designed mattress based on an optical-fiber Michelson interferometer shows good performance and great potential in non-intrusive sleep monitoring.

Phase consistency among hydrophones in fiber-optic hydrophone (FOH) arrays is crucial for effective beamforming. In this study, we investigate the photoelectric detection phase characteristics of FOHs based on the 3 × 3 coupler demodulation technique. We develop a theoretical model combining the 3 × 3 coupler demodulation algorithm with coupled-mode theory to analyze acoustic signal responses. Our model reveals that phase shifts from coupler-to-photodetector and coupler-to-sensing-arm connections arise from different mechanisms, and both are capable of causing π rad phase inversions in demodulated signals. We demonstrate that distinct connection configurations can be classified into groups yielding identical polarity outcomes, and that the input port selection for incident light does not affect output signal phase polarity. Experimental results validate these theoretical predictions. This work establishes critical hardware-level prerequisites for phase polarity consistency in FOH arrays, complementing existing calibration techniques and enhancing array performance in underwater target detection and localization.