Recent developments in the applications of mid-infrared lasers, LEDs and other solid state sources to gas detection

Although mid infrared research into sources and detectors has made considerable progress in recent years, requirements for gas sensing purposes for source power and the detectivity of diode detectors - particularly in combination - remain to be convincingly demonstrated in an industrial context. Published results are often confusing in that they apply to a variety of pulse lengths and duty cycles. We suggest a standardised approach in terms of an average cw power output. Parameters such as radiance, area, drive current and electrical power are also important. We discuss the relative merits of lasers and LEDs, i.e. the use of line or band absorption in gas sensing. We report recent advances in the use of immersion optics leading to detectors with peak D*similar to5x10(9) cmHz(1)/W-2(-1) at 5.4mum, LEDs with outputs improved by a factor of 5 and an LED/Diode/White cell gas sensing demonstration giving 2ppm sensitivity for NO2 at room temperature with an electrical power requirement of only 0.25mw. Further consideration addresses sensitivity of gas sensing, path length and volume, time constant and temperature stability. Latest results are assessed on the basis of the above and combined with some market indications.