Automated broad tuning of difference frequency sources for spectroscopic studies.

Transmission spectroscopy over large spectral ranges (>100 cm(-1)) generally requires a reference measurement to be taken separately from the sample scan. The ratio of the two measurements (i.e., the transmittance) is therefore susceptible to baseline changes that occur between the recording of the two spectra. The origins of relatively strong baseline changes (≫1%) of a difference-frequency-generation-based laser spectrometer (tuning range 2900-3144 cm(-1), 150 μW average power) were investigated and a method for minimizing them by improving reproducibility and reducing measurement time is presented. The new method was tested for a gas mixture and the sensitivity for broad absorption features was determined as 5×10(-3) minimum measurable absorbance for a total scan duration of 70 min.