FREQUENCY-MODULATION MULTIPLEXING FOR SIMULTANEOUS DETECTION OF MULTIPLE GASES BY USE OF WAVELENGTH MODULATION SPECTROSCOPY WITH DIODE-LASERS

Modulation frequency multiplexing provides a straightforward method, a nalogous to television or radio broadcasting, for performing simultane ous detection of multiple gases by use of wavelength modulation spectr oscopy with diode lasers. When fiber-optic coupled lasers are used, ou r approach guarantees that all beams transit the same optical path and impinge on the same detector. Each laser is modulated at a different frequency and the detector output is processed by a set of lock-in amp lifiers, one for each laser, to measure the absorbance encountered by each laser. (C) 1998 Optical Society of America.