Chemical reactions are going on all around us and have an enormous impact on our lives. We rely on combustion reactions for most of our energy needs, atmospheric chemistry affects climate and human health, many industrial processes use complex chemical reactions and of course, life itself involves complex bio-chemical reactions, some of which result in emission of 'tell-tale' chemical species in breath. Controlling such reactions, or monitoring the products e.g. for safety or diagnostic purposes, often requires sensing several species simultaneously. This can present serious technical challenges. It is usually too time consuming, difficult or dangerous to take samples to analyse using conventional methods e.g. from inside furnaces for power generation or industrial processes, monitoring air pollution, detecting leaks or poisonous gases. It is better to detect the target species in situ using remote sensing. Optical methods for remote sensing use the absorption by gases of only certain precise wavelengths of light. These absorption lines give a "fingerprint" of each molecule. By varying the wavelength, or tuning, the wavelength emitted by a diode laser across one absorption line the absorbing species can be identified. Most diode lasers emit only one wavelength, tuneable over only a narrow range that is usually less than the separation between lines of different molecules. To detect more lines, or more species, we need a separate laser and detector for each line. This then becomes complicated and expensive. Some lasers however emit many wavelengths or modes, each such that an integer number of half-wavelengths fit into the length of the laser. These "modes" have a regular pattern covering a wide range and, by tuning over the wavelength interval between each mode, the whole range can be covered by a single laser. Such a multi-mode laser behaves like many separate lasers but with only one beam. So many lines or molecules can be detected when particular modes are absorbed. This "Multi-Mode Absorption Spectroscopy", MUMAS has recently been developed in Oxford and it allows many spectral lines or several molecules to be detected simultaneously using only one laser and one detector. The method has been shown to work using both diode lasers and specially made micro-lasers that emit light in the near-infrared where molecules absorb only weakly. Nonetheless, using MUMAS, several important species have been detected simultaneously, including CO2, CO and N2O. Small molecules and especially hydrocarbons like methane, CH4, absorb up to one thousand times more strongly in the mid-infrared. So using MUMAS in the mid-IR will be one thousand times more sensitive. This will allow molecules to be detected that are present in only a few parts per billion or to detect them in very small volumes.
The research proposed will take advantage of the very latest developments in quantum cascade lasers that emit mid-infrared light and can be used at ordinary temperatures. Previous versions of these lasers needed expensive and complicated cooling to very low temperature. The work will develop two sources of mid-IR light; one using these new quantum cascade lasers and the other using the micro-lasers developed previously with the wavelength shifted to the mid-IR using a specially modified crystal material called PPLN. Then, using these new lasers, MUMAS will be applied to detecting small concentrations of important molecules in industrial, environmental and medical diagnostics. New techniques will be developed to improve the speed and sensitivity of the measurements for applications in combustion engines, waste incineration, environmental sensing, industrial process monitoring, and possibly also medical diagnostics using breath analysis.
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