Development And Feasibility Investigation Of Cavity Ringdown Systems For Trace Dissolved Gas Detection

Air-sea gas exchange is one of the ways that the ocean and atmosphere are linked,the ocean serves as both a source and sink of the atmospheric gases of environmental andclimatic importance. Understanding the variation and distribution of dissolved gases inthe ocean requires extensive observation using vast amounts of scientific instruments andsensors. A newly developed laser absorption spectrometry, cavity ringdown spectroscopy(CRDS), has the potential to develop an unattended, online gas sensor for continuousmeasurements due to its high-sensitivity, high-selectivity, fast response, and absolutequantification ability in particular. However, two issues, miniaturization and stabilization,need to be considered in the case of under-water deployment. With the hope ofdeveloping an under-water sensor using CRDS, and as a pilot study of applying CRDSinto marine survey, two cavity ringdown experimental systems, which were featured withcompact alignment and stability, were built up and tested in this dissertation toinvestigation the feasibility of cavity ringdown systems for trace dissolved gas detectionin water.This dissertation begins with an background introduction of dissolved gas detection,and an overview of on-line gas sensors for under-water deployment. Chapter2offers abrief introduction of CRDS principle using a photon bullet model, followed by a detailedreview on the development and progress of CRDS in various research fields, especially ininstrumentation development. The bulk of author’s contribution, which was to performthe experiments within the general group efforts, are described in chapter3,4, and5.Chapter3addresses the experimental investigation for quantitative analysis of tracegases using pulsed laser CRDS (pulsed-CRDS). The integration level of the opticalplatform, which was mounted on a10mm thick,250mm×600mm aluminum plate, wasimproved by employing miniature equipments. According to the laser output wavelengthof266nm, two substances with different absorption cross sections, acetone and dimethyldisulfide (DMDS), were selected to prepare simulative gas samples and theirconcentrations were determined using a background subtraction method. A rapid response time of a few seconds, ringdown baseline stability better than0.5%, linear response to gassamples of various concentrations, were obtained with this pulsed-CRDS system. A limitof detection less than0.4ppm (parts per million) for methane measurement was estimatedbased on the obtained results.For the need of higher sensitivity and availability of laser source, the design andrealization of a continuous wave CRDS (CW-CRDS) experimental system is given inchapter4. A distributed feedback (DFB) laser diode (LD) was used as the light source inthis system, and a LD current modulation scheme was employed to realize cavityexcitation and shutoff. On one hand, the wavelength of the LD was modulated so the laserbeam could be able to excite the ringdown cavity once its frequency came into resonancewith a cavity mode. On the other hand, laser current was cut off to shut the laser outputoff, so ringdown decay could be observed. As a consequence, the CW-CRDS system wassimplified by using current modulation method rather than using optical switch andpiezoelectric transducer to achieve cavity excitation and shutoff.Based on the CW-CRDS system, a series of experiments were carried out in chapter5. The R(4) line of methane in2ν3overtone band around1651nm was studied.Preliminary results showed that the CW-CRDS system had a stability better than0.5%and a theoretical detection limit of42ppb. Also, with fixed resolution of the detector andanalog to digital converter, it was observed that the selection of fitting window andcumulative number of ringdown waveform could affect the acquisition of experimentparameter—cavity ringdown time τ. The ringdown spectrum of the R(4) line of methanein2ν3overtone band around1651nm was studied, and the methane concentration in thelaboratory air was determined to be2.06ppm using the background subtraction method.Finally, a summary of the work and some suggestions for possible futuredevelopment are given in chapter6.