In order to be successful in our research on gas in scattering media absorption spectroscopy (GASMAS), we also develop systems for tunable diode laser absorption spectroscopy (TDLAS). One of our important achievement is the development of fully digital systems for wavelength modulation spectroscopy (WMS), decreasing cost, increasing flexibility and making systems more compact and easier to handle. Conventional, analogue signal generators and lock-in amplifiers are replaced by data-acquisition boards for normal personal computers. The diode laser is modulated using a digitial output, and the detector (photodiode) signal is collected by a AD-converter. Synchronisation of these two units ensures phase stability, allowing phase-stable data averaging and storage of raw data. Note that no information is lost when raw data is stored. The traditional approach is to store and average the output of a lock-in amplifier, thus storing only a single freqeuncy channel. When raw sensor data is available, all WMS overtones can be anlysed, and the dc level provides useful information for normalisation purposes.

Clinical system for non-invasive in situ monitoring of gases in the human paranasal sinuses
M. Lewander, Z. Guan, K. Svanberg, S. Svanberg, and T. Svensson
Optics Express 17, 10849-10863 (2009)

Pharmaceutical and biomedical applications of spectroscopy
in the photon migration regime
T. Svensson, PhD Thesis (2008)

VCSEL-based oxygen spectroscopy for structural characterisation of pharmaceutical solids
T. Svensson, M. Andersson, L. Rippe, S. Svanberg, S. Andersson-Engels, J. Johansson, and S. Folestad
Applied Physics B90, 345-354 (2008)

Flexible lock-in detection system based on synchronized computer plug-in boards applied in sensitive gas spectroscopy
M. Andersson, L. Persson, T. Svensson, and S. Svanberg
Review of Scientific Instruments78, 113107 (2007)