Trace gas detection using 1.3-microm InGaAsP diode laser transmitter modules.

The detection of small absorptions using a laser absorption spectrometer (LAS) based on InGaAsP diode laser transmitter modules has been investigated. The modules are normally employed in optical communication systems and as such operate at 1.3 microm and have single-mode-fibor pigtails to couple the light out. The minimum detectable absorption of the LAS wasfound tobe ~5 x 10(-5) with harmonic detection and ~1 x 10(-4) with sweep integration. The dominant noise source was caused by reflections off the cleaved end of the fiber pigtail. The strength and number of absorption lines in the 0.7-1.6-microm spectral region which are free from interference is considered for the major constituents of the atmosphere. It is found that there arc sufficient strong isolated lines for trace gas detection and monitoring purposes usinga LAS based on InGaAsP lasers and the reported minimum detectable absorption.