Real-time signal-processing concepts for trace-gas analysis by diode-laser spectroscopy

Tunable-diode-laser absorption spectroscopy fulfills the major requirements for trace-gas analysis: sensitivity, specifity, high detection speed, and the possibility of simultaneous in situ measurements. The well-known limitations for low-concentration measurements become more and more dominant at sub-part-per-billion levels, where sensitive spectrometers are often influenced by noise, drift effects, and changes in the spectral background structure. While many improvements in instrument development focus on optimizing electronics and optical components, much less effort has been put into postdetection signal processing and adaptive control. Therefore, a transputer-based platform has been developed which allows control of most relevant parameters of a diode-laser spectrometer. Fluctuations in the signal amplitude as well as drift and jitter effects in the frequency domain can cause a significant degradation of system performance and therefore determine the ultimate detection limit. A si gnal-processing concept with novel aspects for tunable-diode-laser spectroscopy is presented and discussed.