In this letter, a distributed optical fiber hydrophone (DOFH) based on Φ-OTDR is demonstrated and tested in the field. The specially designed sensitized optical cable with sensitivity up to −146 dB rad/µPa/m is introduced, and an array signal processing model for DOFH is constructed to analyze the equivalence and specificity of the distributed array of acoustic sensors. In the field test, a 104-meter-long optical cable and a Φ-OTDR system based on heterodyne coherent detection (Het Φ-OTDR) is utilized, and underwater acoustic signal spatial spectrum estimation, beamforming and motion trajectory tracking with high accuracy can be realized. As far as we know, this is the first report on the field trial of DOFH based on Φ-OTDR. The DOFH has the potential to achieve an array range of tens of kilometers, with elements spaced up to the meter level and flexible configuration, which has a broad application prospect for marine acoustic detection.

We demonstrate a compact fiber optic accelerometer (FOA) utilizing a push-pull spring-mass structure with a size of 25 mm × 25 mm × 25 mm, showcasing the excellent performance of strong sensitivity, minimal self-noise, and wide dynamic range. The presented sensing probe is a Michelson interferometer (MI) that employs the 3 × 3 coupler demodulation method to extract the information of vibration. Both arms of the MI are fiber coils wrapped around a movable push-pull folding spring structure, serving as the sensing arm for detection, which results in twice the sensitivity compared to a single-sensing arm accelerometer. The theoretical design and finite element analysis are validated experimentally, confirming that the proposed FOA demonstrates a mean sensitivity of 48.73 dB re rad/g over the frequency band of 5 Hz-1 kHz, closely matching the simulated results. It also achieves a large dynamic of 161.12 dB at 50 Hz. Noise evaluation experiment results reveal a low average noise level of 15.85 ng/Hz1/2 of the proposed sensor, indicating that it is one of the fiber optic interferometric accelerometers with the lowest noise reported to date.

We demonstrate a fiber-optic micro-electron-mechanical systems (MEMS) accelerometer based on a symmetric push-pull spring structure made by micromachining on a silicon substrate, which combines the advantages of high sensitivity, low self-noise, and large operation bandwidth compared with existing fiber-optic MEMS accelerometers. A balanced Michelson interferometer based on a 3×3 fiber coupler is employed as a displacement transduction system, and an improved elliptic fitting algorithm (EFA) is proposed for phase demodulation within this system, addressing the limitations of the traditional EFA under small-signal condition. Experimental results validate the theoretical design and finite-element analysis, demonstrating that the proposed sensor achieves an average sensitivity of 48.17 dB re rad/g within its operational bandwidth of 5–80 Hz and a resonant frequency of 139 Hz, both of which closely align with the simulated results. The sensor's transverse crosstalk is less than 1.53%. The results of the self-noise evaluation experiment verify that the proposed sensor attains a low average noise of 40 ng/Hz1/2, proving that it has nano-g resolution in acceleration detection applications.

Distributed Acoustic Sensing (DAS) offers numerous advantages, including resistance to electromagnetic interference, long-range dynamic monitoring, dense spatial sensing, and low deployment costs. We initially deployed a water–land DAS system at the Xinfengjiang (XFJ) Reservoir in Guangdong Province, China, to monitor earthquake events. Environmental noise analysis identified three distinct noise zones based on deployment conditions: periodic 18 Hz signals near surface-laid segments, attenuated low-frequency signals (<10 Hz) in the buried terrestrial sections, and elevated noise at transition zones due to water–cable interactions. The system successfully detected hundreds of teleseismic and regional earthquakes, including a Mw7.3 earthquake in Hualien and a local ML0.5 microseismic event. One year later, the DAS system was upgraded with two types of spiral sensor cables at the end of the submarine cable, extending the total length to 5.51 km. The results of detecting both active (transducer) and passive sources (cooperative vessels) highlight the potential of integrating DAS interrogators with spiral sensor cables for the accurate tracking of underwater moving targets. This field trial demonstrates that DAS technology holds promise for the integrated joint monitoring of underwater acoustics and seismic signals beneath lake or ocean bottoms.

Fiber interferometer based on low noise lasers are crucial to the micro-vibration sensing systems. Laser noise is a key factor impacting the detection resolution of such systems. To enhance the minimum measurable acceleration of the micro-vibration sensing system, a compact external cavity diode laser (ECDL) is designed and fabricated to reduce the frequency/phase noise and the relative intensity noise (RIN) of the laser source. The ECDL is composed of a gain chip and an external cavity based on two high-Q Fabry-Perot (FP) etalons with small FSR differences. The two FP etalons enable a narrowband filtering response that selects a single longitudinal mode in the laser cavity, resulting in a single-frequency laser output with low noise. In this way, the intrinsic linewidth of less than 400 Hz with the RIN of −153 dBc/Hz@10 MHz and the output power of 9.5 mW are realized. The ECDL is employed in the micro-vibration sensing system, the noise evaluation experiment results reveal that the overall noise floor is 21.47 ng/Hz^1/2 with an average sensitivity of 43.5 dB re rad/g.

A high performance and miniaturized micro-vibration sensing system is highly desirable for satellite payload platforms. In this paper, a compact micro-vibration sensing system assisted with silicon photonic integrated circuit is presented and experimentally demonstrated. A dedicated silicon photonic integrated circuit including IQ modulator and variable optical attenuators is designed and fabricated for high-sideband-suppression-ratio frequency shifting and high-extinction-ratio pulse generation as the carrier for signals detection. A push-pull fiber optic micro-vibration sensor is designed for micro-vibration sensing and an integrated coherent receiver is applied for demodulation of sensing signals. This integrated scheme achieves higher average sensitivity of 49.44 dB re rad/g with the sensor's size of 25 × 25 × 25 mm³ and lower noise level of -135 dB re g²/Hz.