Aspects of laser absorption spectroscopy in the mid-infrared and visible

Rights statement
Awarding institution
  • University of Strathclyde
Date of award
  • 2016
Thesis identifier
  • T14581
Person Identifier (Local)
  • 200959262
Qualification Level
Qualification Name
Department, School or Faculty
  • Laser absorption spectroscopy can be used to identify and quantify gas analytes. The sponsor company’s present systems operate in the mid-infrared using room temperature pulsed quantum cascade lasers (pulsed-QCL’s). These systems use the noise reduction / sensitivity enhancing technique of sweep integration (SI). In this work, an extension of measurement capabilities is sought in two ways. Firstly, sensitivity enhancement is pursued. The noise reduction technique of wavelength modulation spectroscopy (WMS) is applied using a room temperature continuous wave (cw) QCL spectrometer. Secondly, molecular oxygen is added to the list of measurable analytes. This molecule’s near-infrared and visible transitions are addressed with a wavenumber prototype semiconductor diode laser.The sensitivities of the SI and WMS techniques are compared for the cw-QCL spectrometer, and compared to the SI sensitivity of a typical company pulsed-QCL system. New analysis and modeling software was written to facilitate the thesis work and to carry it forward. A thorough analysis of a pulsed-QCL CT3000 analyzer is undertaken to minimize a reduction in capability - should an oxygen measuring laser replace one of its pulsed-QCL’s.The experimental work was constrained by time and budget - particularly with regard to the cw-QCL spectrometer’s AC-coupled detection. Using AC-coupled detection had cost and integration advantages, but posed a number of problems - including electronic incompatibility issues. Nevertheless, the outlook is positive, and a modest sensitivity improvement was found for WMS over sweep integration (0.017 absorbance units (a.u.) in 102.4s compared to 0.080 a.u. in 51ms). Both sensitivities are some way behind the present sweep integration performance of the company’s pulsed spectrometers (0.004 a.u. in 10ms). However, the sensitivities are comparable to earlier stages of development. In the case of oxygen spectroscopy, the prototype diode laser’s thermal stability was an issue, but several spectral regions were found to be suitable for single or multimode spectroscopy.
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