A review of recent advances in semiconductor laser based gas monitors

When first tunable diode lasers were developed in the mid-1960s they found immediate application as much needed tunable sources for high-resolution laser absorption spectroscopy commonly referred to as TDLAS (tunable diode laser absorption spectroscopy). Substantial improvements in sensitivity and detection speed have been achieved since then and an increasing number of laser based gas monitoring applications has been reported. Diode lasers in general continue to find application to research areas requiring very high resolution, accuracy and sensitivity. In this article the main features and applications of tunable diode laser absorption spectroscopy will be reviewed. The main characteristics of the currently available semiconductor diode lasers with respect to spectroscopic applications and sensitive detection techniques will be discussed. The focus will be on high frequency modulation schemes, which have been developed and utilized for a series of gas monitoring applications in the past. Recent approaches in sample modulation enhanced high frequency modulation spectroscopy developed to cope with limitations caused by signal instability will be addressed together with the future perspectives of TDLAS. (C) 1998 Elsevier Science B.V. All rights reserved.