Sensing trace emissions and the proportion of oxygen (O2) in exhaust gas are important for monitoring the efficiency of power-generation plants and combustion equipment. Rather than taking the standard optical-sensing approach of pairing a tunable IR laser with absorption spectroscopy, a group at the Indian Institute of Technology (Chennai, India) is using a superluminescent diode (SLD) as the source. The advantage is a simple, stable configuration, although at the cost of some sensitivity.
Two different SLDs were used to sense O2 via its 760 nm absorption lines, and ammonia (NH3) via its 1530 nm absorption lines. The light was coupled via optical fiber to a spectrum analyzer for O2 and an optical spectrum analyzer for NH3. Over an 8 m absorption path, O2 concentrations were observed over a range from 5 to 150 kPa. Concentrations of NH3 over a 2 m path at 0, 13.5, and 20.3 kPa were observed. The detection limit for NH3 was 2 kPa, but with a stronger absorption line could be lowered to 26.8 Pa (300 parts per million), or even further with a longer path length. The broadband nature of the source could enable simultaneous detection of more than one gas. Contact Nilesh Vasa at [email protected].