The transmission system of this device consists of a diode pumped laser (wavelength 527nm, pulse repetition frequency 1kHz, pulse energy 200 µJ) and its receiver system uses an ICCD detector and a spectrometer for Raman and fluorescence signal measurement. The resulting device then has high sensitivity and is able to measure the relative spectral signal of sea water.
The experimental results, both in the laboratory and in situ, show that this ocean lidar system can detect the full spectrum of seawater simultaneously, including Mie scattering, Rayleigh scattering, Raman scattering, and fluorescence signal (Figs. 3, 5, 6). Therefore, data on the sea water’s optical properties, chlorophyll, DOM and temperature profile can be obtained at once. This ability of simultaneous measurement is very interesting for ocean optics. For instance, the ratios of elastic and Raman scattering, chlorophyll and Raman scattering, DOM and Raman scattering (Fig.7) are valuable only when the full spectrum is simultaneously measured.
This ocean lidar system is light in weight (20kg), compact in size (60x40x20 cm) and low in power cost (300W), so it can be installed on a ship, a remotely operated vehicle (ROV) or an autonomous underwater vehicle (AUV), and can work in wide space in the sea. Also, the rapid development of in situ laser Raman spectrometry (DORISS, deep-ocean Raman in situ spectrometer) provides new opportunities for remote sensing of chemical processes in the ocean. The lidar system proposed in this article is a promising prospect for the development of a compact DORISS.
Further improvements to this lidar system like, for instance, a laser pulse energy of 100mJ and a pulse repetition frequency of 50Hz, would allow the measurement depth of temperature profile to be not only 2 to 3m but about 10m or 20m, and of course, the detecting depth of Mie scattering, Rayleigh scattering, fluorescence signal and Raman scattering will also be deeper than in the system reported this paper. Increasing the detection depth will be an important improvement to this novel lidar device.
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