SENSITIVE TRACE GAS-DETECTION WITH NEAR-INFRARED LASER-DIODES AND AN INTEGRATING SPHERE

We describe a simple spectrometer for sensitive trace gas detection in the atmosphere. A communication laser diode is used as a light source , and a commercial integrating sphere is used as a multipass absorptio n cell. We developed a theoretical formulation of the relative absorpt ion of the optical power by trace gases in the sphere and applied it t o two kinds of experimental result: one that is concerned with a struc tureless broad absorption band of butane with the use of a 1.2-mu m mu ltimode laser diode, and one that is related to the study of an isolat ed and sharp rovibrational line of water vapor in air at atmospheric p ressure with the use of an 830-nm single-mode laser diode. With equiva lent path lengths of several meters obtained with a 10-cm-i.d. integra ting sphere we can demonstrate the usefulness of such a device as a br oadband multipass cell for the measurement of small absorptions. (C) 1 996 Optical Society of America.