Browsing by Author "Livingstone, B."

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    Low-volume, fast response-time hollow silica waveguide gas cells for mid-IR spectroscopy
    (Optical Society of America, 2016-08-22) Francis, Daniel; Hodgkinson, Jane; Livingstone, B.; Black, Paul; Tatam, Ralph P.
    Hollow silica waveguides (HSWs) are used to produce long path length, low-volume gas cells, and are demonstrated with quantum cascade laser spectroscopy. Absorption measurements are made using the intrapulse technique, which allows measurements to be made across a single laser pulse. Simultaneous laser light and gas coupling is achieved through the modification of commercially available gas fittings with low dead volume. Three HSW gas cell configurations with different path lengths and internal diameters are analyzed and compared with a 30 m path length astigmatic Herriott cell. Limit of detection measurements are made for the gas cells using methane at a wavelength 7.82 μm. The lowest limit of detection was provided by HSW with a bore diameter of 1000 μm and a path length of 5 m and was measured to be 0.26 ppm, with a noise equivalent absorbance of 4.1×10 −4 . The long-term stability of the HSW and Herriott cells is compared through analysis of the Allan–Werle variance of data collected over a 24 h period. The response times of the HSW and Herriott cells are measured to be 0.8 s and 36 s, respectively.
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    Quantum cascade laser light propagation through hollow silica waveguides
    (Springer Verlag, 2015-04-03) Francis, D.; Hodgkinson, Jane; Livingstone, B.; Tatam, Ralph P.
    In this paper, the transmission characteristics of hollow silica waveguides with bore diameters of 300 and 1000 μm are investigated using a 7.8-μm quantum cascade laser system. We show that the bore diameter, coiling and launch conditions have an impact on the number of supported modes in the waveguide. Experimental verification of theoretical predictions is achieved using a thermal imaging camera to monitor output intensity distributions from waveguides under a range of conditions. The thermal imaging camera allowed for more detailed images than could be obtained with a conventionally used beam profiler. The results show that quasi-single-mode transmission is achievable under certain conditions although guided single-mode transmission in coiled waveguides requires a smaller bore diameter-to-wavelength ratio than is currently available. Assessment of mode population is made by investigating the spatial frequency content of images recorded at the waveguide output using Fourier transform techniques.