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Laser-assisted analytical instrumentation for biomedical diagnostics, in particular for the analysis of exhaled breath, is attracting increasing attention among researchers. The characteristics of laser spectroscopic techniques for breath monitoring are discussed and potential applications are outlined.

We present sub-Doppler heterodyne frequency measurements on 10 rovibrational transitions of carbonyl sulfide (OCS) between 2894 and 2910 cm−1. The measurements were made using a CO overtone laser which had limited tuneability through the generation of microwave sidebands in a CdTe crystal. With this technique the laser frequencies were shifted to the desired OCS transition frequencies. The transition frequencies could be measured with uncertainties less than 30 kHz (Δν/ν = 3 × 10−10) by frequency offset-locking the CO overtone laser to combination frequencies of two saturation-stabilized CO2laser standards. The measured transition frequencies of OCS were combined with previous sub-Doppler, Fourier transform, and microwave measurements to recalculate improved calibration tables for the 1001–0000, 111e1–011e0, and 111f1–011f0 bands. These tables are suitable for the calibration of infrared spectrometers in the 87 THz region (near 2900 cm−1).

Spectroscopic detection of ethane in the 3-µm wavelength region was performed by means of a cw optical parametric oscillator and cavity leak-out. We achieved a minimum detectable absorption coefficient of 1.6×10^-10 cm^-1/Hz, corresponding to an ethane detection limit of 6 parts per trillion /Hz. For 3-min integration time the detection limit was 0.5 parts per trillion. The levels are to our knowledge the best demonstrated so far. These frequency-tuning capabilities facilitated multigas analysis with simultaneous monitoring of ethane, methane, and water vapor in human breath.

A new scheme for coherently connecting optical frequencies in a 3:1 ratio has been demonstrated. To phase lock a Nd:YAG laser at 1064 nm with a CO overtone laser at 3192 nm, we generated their difference frequency in RbTiOAsO_4 (RTA) and beat it against the second harmonic of 3192 nm that was generated in AgGaSe_2.