Shang-Qian WU¹, Tetsuya KIMISHIMA¹, Yotsumi YOSHII², Hiroaki KUZE² and Nobuo TAKEUCHI²

¹Tsukuba Laboratories, Nippon Sanso Corporation, 10 Ohkubo, Tsukuba, Ibaraki 300-2611, Japan
²Center for Environmental Remote Sensing, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan

(Received April 18, 2002; Accepted June 5, 2002)

In wavelength modulation spectroscopy using diode lasers, detection limit of molecular absorption is often imposed by spurious fringe noises that arise from slight reflections among optical components. In an attempt to reduce this fringe effect, we propose and demonstrate the use of a one-dimensional focal plane array. With the help of a multiplexer, a single lock-in amplifier is employed to obtain multiple-channel signals from wavelength-modulated output of an array detector. It is found that the intensity change of the molecular absorption signal is nearly proportional to the transversal beam intensity distribution, whereas that of fringe noise is much less conspicuous. These features, as well as the phase difference among the channels, contribute to separate the signal and fringe noise components from the multiplexed spectra, leading to the overall improvement in the signal–to–noise ratio.

Key words: diode laser, molecular spectra, wavelength modulation, focal plane array, fringe noise