[Raman Signal Enhancement for Gas Detection Using a Hollow Core Optical Fiber].

Raman spectroscopy has been widely used for gas detection due to the advantages of simultaneous multiple species recognition, rapid analysis, and no sample preparation, etc. Low sensitivity is still a great limitation for Raman application. In this work a Raman system based on a hollow core optical fiber (HCOF) was built and the detection sensitivity for the gas was significantly improved. Also a comparison was carried out between the HCOF Raman system and back-scattering Raman system. The obtained results indicated that the HCOF Raman system could well enhance the signal while also for the background and noise. Using HCOF system, 60 folds signal enhancement was achieved with SNR improvement of 6 times for the N2 and O2 in air when comparing to the back-scattering system. While for the same signal intensity, with HCOF system the exposure time was well shortened to 1/60 and the noise was decreased to 1/2 than the back-scattering system.

[Discrimination of Crude Oil Samples Using Laser-Induced Time-Resolved Fluorescence Spectroscopy].

The Laser-induced fluorescence spectra combined with pattern recognition method has been widely applied in discrimination of different spilled oil, such as diesel, gasoline, and crude oil. However, traditional three-dimension fluorescence analysis method, which is not adapted to requirement of field detection, is limited to laboratory investigatio ns. The development of oil identification method for field detection is significant to quick response and operation of oil spill. In this paper, a new method based on laser-induced time-resolved fluorescence combined with support vector machine (SVM) model was introduced to discriminate crude oil samples. In this method, time-resolved spectra data was descended into two dimensions with selecting appropriate range in time and wavelength domains respectively to form a SVM data base. It is found that the classification accurate rate increased with an appropriate selection. With a selected range from 54 to 74 ns in time domain, the classification accurate rate has been increased from 83.3% (without selection) to 88.1%. With a selected wavelength range of 387.00~608.87 nm, the classification accurate rate of suspect oil was improved from 84% (without selection) to 100%. Since the detection delay of fluorescence lidar fluctuates due to wave and platform swing, the identification method with optimizing in both time and wavelength domains could offer a better flexibility for field applications. It is hoped that the developed method could provide some useful reference with data reduction for classification of suspect crude oil in the future development.

[Characterization of Time-Resolved Laser-Induced Fluorescence from Crude Oil Samples].

To evaluate the feasibility of laser induced time-resolved fluorescence technique for in-situ detection of underwater suspended oil spill, extensive investigations have been carried out with different densities of crude oil samples from six different wells of Shengli Oilfield in this work. It was found that the fluorescence emission durations of these crude oil samples were almost the same, the Gate Pulse Delay of DDG (Digital Delay Generator) in the ICCD started at 52ns and ended at 82ns with a width (FWHM) of 10 ns. It appears that the peak location and lifetime of fluorescence for different crude oil samples varied with their densities, and those with similar densities shared a similar lifespan with the closer peak locations of fluorescence. It is also observed that the peak of fluorescence remained the same location before reaching the maximum intensity, subsequently shift to longer wavelength as fluorescence attenuated from maximum intensity with a red shift among 17-30 nm varied with samples. This demonstrated that the decay rate of fluorescent components in the crude oils was different, and energy transfer between these components might exist. It is hoped that those obtained results and characteristics could be the useful information for identification of suspended spilled-oil underwater.

[Raman Signal Enhancement for Gas Detection Using a Near Concentric Cavity].

The detection of dissolved gases in seawater plays an important role in ocean observation and exploration. Raman spectroscopy has a great advantage in simultaneous multiple species detection and is thus regarded as a favorable choice for ocean application. However, its sensitivity remains insufficient, and a demand in enhancements is called! for before putting Raman spectroscopy to actual use in marine studies In this work, we developed a near-concentric cavity, in which laser beam could be trapped and reflected back and forth, for the purpose of intensifying Raman signals. The factors that would influence Raman signals were taken into account. The result show that the smaller angle between collection direction and optical axis of reflection mirror, the stronger the signal and signal to noise ratio (SNR) is. With a collection angle of 30 degrees, our Near-concentric Cavity System managed to raise the SNR to a figure about 16 times larger than that of common methods applying 90 degrees. Moreover, the alignment pattern in our system made it possible to excel concentric cavity with a 3 times larger SNR. Compared with the single-pass Raman signal, the signal intensity of our near-concentric cavity was up to 70 times enhanced. According to the obtained results of CO2 measurement, it can be seen that the new system provides a limit of detection(LOD) for CO2 about 0.19 mg x L(-1) using 3-σ criterion standard, and the LOD of 11.5 µg x L(-1) for CH4 was evaluated with the theoretical cross section values of CO2 and CH4.