Optical sensor for combustion, industrial process and environmental applications

Monitoring of combustion and other industrial processes often needs rapid measurement of several species or parameters. In some cases it is too time consuming, difficult or dangerous to take samples to analyse e.g. inside furnaces for power generation or industrial processes, monitoring air pollution, detecting leaks or poisonous gases. Remote sensing methods use the fact that gases absorb only certain precise wavelengths of light, absorption lines, giving a fingerprint of each molecule. A diode laser can be used to measure the absorption as the wavelength is varied, or tuned, across one absorption line. Most diode lasers emit only one wavelength which can be tuned over only a narrow range that is usually much less than the separation between lines of different molecules. To detect more lines or more species simultaneously we need a separate laser and detector for each line. This then becomes complicated and expensive. Some lasers however emit many wavelengths, each such that an integer number of half-wavelengths fit into the length of the laser. These modes give a regular pattern of spectral lines covering a wide range and, by tuning over the space 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 the great advantage that many spectral lines or several molecules can be detected simultaneously using only one laser and one detector. This could make a compact, cheap and reliable remote multi-species sensing system for monitoring and active control applications. The research proposed will develop micro-cavity lasers to emit multi-mode laser light in the infra-red. A very short (micro) cavity has a large separation between the wavelengths of the modes so can distinguish separate absorption lines and cover a wide range at the same time. Since most molecules absorb infra-red wavelengths it is important to make these lasers emit in this spectral range. Secondly the research will address the major problem that trace gases, at low concentrations, absorb only a tiny fraction of the light. So it can be hard to tell the difference between a small dip in the intensity caused by absorption - the signal, and a dip due to the random fluctuations of the laser intensity or the detector - noise. One way to improve the signal-to-noise ratio is to make the signal vary at a regular frequency by modulating it in some way. Since the noise is spread out over all frequencies, measuring in a small range around the modulation frequency eliminates most of the noise but keeps most of the signal. This can be done by modulating the wavelength of the laser over a small range at high frequency while, at a slower rate, scanning the centre wavelength across the spectrum. This wavelength modulation spectroscopy WMS allows gases at levels of one part per million to be detected. The primary research will test the technique of infra-red MUMAS by detecting simultaneously CO and CO2. The ratio of these two gases is one indicator of the completeness of combustion e.g. in coal or gas-fired power stations and automobile engines. Rapid monitoring of the combustion process is essential for active control to optimize performance for fuel economy and reduced emissions. This will be a test bed for applications in other industrial process control. The next stage will detect other species important in exhaust gases, waste incineration, atmospheric monitoring, e.g. H2O, NH3 CH4 and other hydrocarbons. The MUMAS fingerprint also allows temperature and pressure to be measured remotely. MUMAS has potential for a new type of sensor in engineering and control applications such as combustion engines, waste incineration, atmospheric and industrial process monitoring, and possibly also medical diagnostics using breath analysis.