We demonstrate a novel type of distributed optical fiber acoustic sensor, with the ability to detect and retrieve actual temporal waveforms of multiple vibration events that occur simultaneously at different positions along the fiber. The system is realized via a dual-pulse phase-sensitive optical time-domain reflectometry, and the actual waveform is retrieved by heterodyne phase demodulation. Experimental results show that the system has a background noise level as low as 8.91×10^-4 rad/√Hz with a demodulation signal-to-noise ratio of 49.17 dB at 1 kHz, and can achieve a dynamic range of ∼60  dB at 1 kHz (0.1 to 104 rad) for phase demodulation, as well as a detection frequency range from 20 Hz to 25 kHz.

We report the first distributed optical fibre trace-gas detection system based on photothermal interferometry (PTI) in a hollow-core photonic bandgap fibre (HC-PBF). Absorption of a modulated pump propagating in the gas-filled HC-PBF generates distributed phase modulation along the fibre, which is detected by a dual-pulse heterodyne phase-sensitive optical time-domain reflectometry (OTDR) system. Quasi-distributed sensing experiment with two 28-meter-long HC-PBF sensing sections connected by single-mode transmission fibres demonstrated a limit of detection (LOD) of ∼10 ppb acetylene with a pump power level of 55 mW and an effective noise bandwidth (ENBW) of 0.01 Hz, corresponding to a normalized detection limit of 5.5ppb⋅W/Hz. Distributed sensing experiment over a 200-meter-long sensing cable made of serially connected HC-PBFs demonstrated a LOD of ∼ 5 ppm with 62.5 mW peak pump power and 11.8 Hz ENBW, or a normalized detection limit of 312ppb⋅W/Hz. The spatial resolution of the current distributed detection system is limited to ∼ 30 m, but it is possible to reduce down to 1 meter or smaller by optimizing the phase detection system.

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The air micro-and nanobubbles on a silicon wafer surface, generated by ethanol−water exchange method in THF solution, are found with anomalous small contact angles on the gas side due to the pinning effect. As the pinning effect is only with the limited region of a bubble and varies with bubble size, the difference in contact angle between the microbubbles and nanobubbles is recognized. With a high-resolution atomic force microscopy, in situ direct observations of THF hydrate nucleation are performed in the presence of air micro-and nanobubbles in solution. On the basis of the observations, the sizes of the hydrate crystallites along the bubble edge are much larger than those in nonbubble regions, which can be explained by the lower nucleation barrier at the contact line region as to the classical nucleation theory. The growth of hydrate crystals at the bubble contact line experiences the competition for THF molecules, probably through Oswald ripening process, resulting in the spaced distribution of THF hydrate crystallites along the bubble edge.

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