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The exploitation and research of deep-sea hydrothermal vent has been an issue of great interest in ocean research in recent years. Laser-induced breakdown spectroscopy (LIBS) has great potential for ocean application due to the capabilities of stand-off, multiphase, and multielement analysis. In this work, a newly developed compact 4000 m rated LIBS system (LIBSea) is introduced with preliminary results of sea trials. The underwater system consists of an Nd:YAG single-pulsed laser operating at 1064 nm, an optical fiber spectrometer, an optics module, and an electronic controller module. The whole system is housed in an L800  mm×ϕ258  mm pressure housing with an optical window on the end cap. It was deployed on the remote operated vehicle Faxian on the research vessel Kexue, and in June 2015 was successfully applied for hydrothermal field measurements at the Manus area. The obtained results are shown that the LIBS system is capable of detecting elements Li, Na, K, Ca, and Mg in the hydrothermal area. Profiles of LIBS signals of elements K and Ca have also been obtained during the sea trial. The results show that the K emission line is gradually broadened with depth from sea surface to sea floor (1800 m or so); the K intensity shows a hump shape with maximum value at about 1050 m. The Ca emission line is rapidly broadened below 400 m and slowly narrowed to the sea floor; the Ca intensity shows no obvious change below 400 m and increases continuously to sea floor. A very interesting finding is that the small fluctuations of intensity profile curve of Ca show a degree of correlation with seawater temperature change. The sea trial results prove the performance of LIBSea. After further optimization, it is hoped to apply the LIBS system to the in situ mineral deposits and hydrothermal vent fluid detection in deep sea.

Recently, molecular emissions from the laser-induced plasma in ambient gas have gained increasing interest; however, very little is known about the case in water solutions. In this work, we investigated the spatiotemporal characteristics of molecular emissions, CaOH for instance, in underwater laser-induced breakdown spectroscopy (LIBS) by using time-resolved spectroscopy, spectral-resolved imaging, and shadowgraph techniques. It was shown that clear CaOH molecular bands can be observed in the spectrum at very early times after the laser pulse and presented a much longer lifetime and more homogeneous emission distribution compared with the Ca I and Ca II lines. Such unique characteristics of CaOH molecular emission inspired us to improve the performances of underwater LIBS by using the CaOH molecular bands instead of Ca I and Ca II lines. We demonstrated the excellent quantification results of CaOH with higher stability, less self-absorption, and reduced matrix effect. Meanwhile, the limit of detection (LOD) of Ca with the CaOH molecular band (2.46 ppm) is comparable to that with the atomic line of Ca I (2.07 ppm), and much lower than that with the ionic line of Ca II (13.81 ppm), indicating a good sensitivity of CaOH. This work gives not only some insights into the molecule formation mechanisms in underwater plasmas, but also provides new ideas to improve the analytical performances of underwater LIBS.

The detection of dissolved gases in seawater plays an important role in ocean observation and exploration. As a potential technique for oceanic applications, Raman spectroscopy has already proved its advantages in the simultaneous detection of multiple species during previous deep-sea explorations. Due to the low sensitivity of conventional Raman measurements, there have been many reports of Raman applications on direct seawater detection in high-concentration areas, but few on undersea dissolved gas detection. In this work, we have presented a highly sensitive Raman spectroscopy (HSRS) system with a special designed gas chamber for small amounts of underwater gas extraction. Systematic experiments have been carried out for system evaluation, and the results have shown that the Raman signals obtained by the innovation of a near-concentric cavity was about 21 times stronger than those of conventional side-scattering Raman measurements. Based on this system, we have achieved a low limit of detection of 2.32 and 0.44  μmol/L for CO₂ and CH₄, respectively, in the lab. A test-out experiment has also been accomplished with a gas-liquid separator coupled to the Raman system, and signals of O₂ and CO₂ were detected after 1 h of degasification. This system may show potential for gas detection in water, and further work would be done for the improvement of in situ detection.

Atmospheric line-of-sight (LOS) wind measurement by means of incoherent Cabannes-Mie lidar with three frequency analyzers with nearly the same maximum transmission of ~80% that could be fielded at different wavelengths is analytically considered. These frequency analyzers are (a) a double-edge Fabry-Perot interferometer (FPI) at 1064 nm (IR-FPI), (b) a double-edge Fabry-Perot interferometer at 355 nm (UV-FPI), and (c) an iodine vapor filter (IVF) at 532 nm with two different methods, using either one absorption edge, single edge (se-IVF), or both absorption edges, double edge (de-IVF). The effect of the backscattered aerosol mixing ratio, R(b), defined as the ratio of the aerosol volume backscatter coefficient to molecular volume backscatter coefficient, on LOS wind uncertainty is discussed. Assuming a known aerosol mixing ratio, R(b), and 100,000 photons owing to Cabannes scattering to the receiver, in shot-noise-limited detection without sky background, the LOS wind uncertainty of the UV-FPI in the aerosol-free air (R(b)=0), is lower by ~16% than that of de-IVF, which has the lowest uncertainty for R(b) between 0.02 and 0.08; for R(b)>0.08, the IR-FPI yielded the lowest wind uncertainty. The wind uncertainty for se-IVF is always higher than that of de-IVF, but by less than a factor of 2 under all aerosol conditions, if the split between the reference and measurement channels is optimized. The design flexibility, which allows the desensitization of either aerosol or molecular scattering, exists only with the FPI system, leading to the common practice of using IR-FPI for the planetary boundary layer and using UV-FPI for higher altitudes. Without this design flexibility, there is little choice but to use a single wavelength IVF system at 532 nm for all atmospheric altitudes.