Pollutant monitoring with fibre optics in the deep ultraviolet
This thesis reports on work carried out in the development of ultraviolet fibre-optic based absorption sensor systems, including those with the newly available ultraviolet improved silica fibres having low attenuation in the 200 nm to 250 nm wavelength region. Several approaches to optimize the optical design of such sensor systems, their sensitivity and stability are discussed. These fibre-optic sensor systems may be used for remote on-line and real-time analysis of process and water quality, enabling a separation of monitoring equipment from the sensor cell, which thus may be situated in a potentially hazardous environment. The effect of temperature variations on wavelength stability and dark output of inexpensive spectrometer modules, potentially useful for field applications, and the subsequent effect on the accuracy of absorption measurements, as well as the sensitivity of such spectrometer modules at wavelength below 250 nm, is investigated. Further, the performance of a remote fibre-optic sensor system, based on a reflectance cell with an optical pathlength of 1 cm, to measure nitrate concentrations in the wavelength region between 200 nm and 250 nm, is reported. Finally, to improve the sensitivity of such ultraviolet sensor systems, the performance of two fibre-coupled sensor cells with increased optical pathlengths has been investigated. The first sensor cell, based on an aluminium coated fused silica capillary cell, having an optical pathlength of 43 em, is demonstrated in the construction of a residual chlorine sensor. The second sensor cell, a capillary cell with an inner coating of Teflon AF, uses the low refractive index and the high transparency of Teflon AF in the ultraviolet to form a liquid-core waveguide (LeW). This sensor cell has an optical pathlength of 203 mm, extending the use of long pathlength cells to the 200 nm to 250 nm wavelength region. Its performance is illustrated when applied to monitoring low concentrations of nitrates, chlorine and acetylsalicylic acid.