Abstract

Derivative spectroscopy, or wavelength modulation spectroscopy (WMS), consists in obtaining the concentration of a molecular absorbent by modulating the output wavelength of the optical source and detecting at one of the harmonics of the modulation frequency. The first and second harmonic signals are proportional to the first and second derivatives of the absorption feature, respectively, for small WM amplitude. The maximum gradient of the first and the absolute maximum of the second harmonic signal, which are located at the absorption line center, are directly proportional to the species concentration. [1-3]. In practice, the output power/current characteristic of the laser has a slight degree of nonlinearity which influences the form of the recovered signal. As a result, the detected derivative trace displays a non-constant baseline that degrades the absorption signal. Furthermore, such baseline does not remain unchanged over the operating period of the source, because it is influenced by environmental factors and aging. In this work we analyze the effect of the mentioned nonlinear laser characteristic on the detected harmonic signal, and we propose a simple post-detection processing method to capture the baseline from the demodulated signal in real time.

© 1998 Optical Society of America