Transmission Attenuation Of A Laser Beam In Water And Its Effect On In-Situ Detection Using LIBS

With the rapid development of marine science, chemical sensors are becoming aresearch hotspot which has in-situ and long-term observation ability. Compared withthe common electrochemical sensors, the spectroscopic sensors have been apromising technology for the advantages of multi-element detection, rapid, flexibility,stand-off etc. Laser induced breakdown spectroscopy technology has theabove-mentioned advantages which can realize the trace detection of metal element inwater and has the capability of in-situ detection of ocean chemical environment.However, the seawater will not only change the characteristic of plasma but alsodecrease the intensity of laser energy and spectral signal of plasma, thus increase thedifficulty of in-situ detection in seawater. The content of this thesis is a part of the“quantitive analysis of metal elements in water and development of a LIBS-seasystem”. The research of attenuation of a laser beam, spectral signal, plasma spectralcharacteristics and mechanism of plasma process in liquid environment have beencarried out, with the hope of improving the in-situ analysis performance of LIBS.The thesis began with a brief background introduction of this work, after that was thedescription of laser induced plasma’s fundamental principle, optical properties ofseawater, a detailed review of current research and development situation of usingLIBS in aqueous solution and experimental apparatus. The author’s work mainlyincludes:1) Evaluating the effect of underwater transmission distance on in-situdetection in water through simulation and experiment.2) LIBS signal properties ofmetal elements in water had been investigated by side and back collection systems.The attenuation coefficient c of532nm and1064nm laser wavelength was obtainedby empirical formula, then three methods were adopted to simulate the laser energydecay using the attenuation coefficient. It showed that three methods had the sameresult under the condition of narrow laser beam. For532nm laser wavelength, transmission is greatly influenced by the concentration of particles in seawater due toscattering effect. But532nm wavelength is located in the “transmission window”which has less attenuation and can detect a long distance. For1064nm laserwavelength, its attenuation is mainly determined by the absorption of water whichlimits the transmission distance in water and suits to short distance detection.Meanwhile, the LIBS signal intensity of two wavelengths was investigated throughchanging the transmission distance of laser in water. It showed that LIBS signalintensity of1064nm laser decreased with the increase of detection range, but LIBSsignal intensity of532nm laser remained the same.A detection system was established to achieve the plasma characteristics of532nmand1064nm laser in aqueous solution. The characteristics of the laser induced plasmain water have been investigated with calcium chloride water solution samples and ithas been presented in chapter4. The lifetime of the laser induced plasma underwateris determined to be600ns for532nm laser pulses,1400ns for1064nm laser pulses bystudy the temporal characteristic. The optimal detection condition can be obtained byoptimal signal to background ratio at certain delay time. Compared with532nm laserpulses, the1064nm laser has stronger thermal ablation effect, plasma induced by1064nm laser pulses has higher electron density and longer lifetime, the electrondensity is about1017cm-3magnitude. From the plasma image taken by ICCD camera,we can see that plasma shape was linear structure along the direction of laser and“dynamic breakdown” phenomenon can be observed clearly. This phenomenon cancause poor plasma signal stability in aqueous solution. The “dynamic breakdown”phenomenon of1064nm laser is more obvious than532nm laser. Maybe the energy of1064nm is higher than532nm and the water absorption of1064nm is stronger whichhas strong thermal ablation effect. We can use short wavelength and lens focus andexpand laser beam diameter and increase the accumulation of laser pulses to improvethe stability of signal.Finally, a summary of the work was presented, and through improving experimentalmethod and device to study the LIBS in-situ detection ability based on the simulated marine environment.