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For an understanding of low temperature plasmas we need quantitative information on the species present - ions, atoms, radicals, electrons. We need their absolute concentrations and their energy distributions, preferably spatially resolved, and we need to measure these properties in a way which does not perturb the plasma. This talk will review some of our recent work in which we use absorption techniques to achieve some of these aims, by using narrow band diode lasers as the light sources and with sensitivities often enhanced by optical cavities used to increase the absorption path lengths. Examples will include observations of excited rare gas atom concentrations, measurement of the translational, rotational and vibrational distributions in metastable molecular nitrogen, and cavity enhancement of forbidden transitions in ground state atomic and molecular oxygen. Extension of diode laser wavelengths to the mid ir by using difference frequency generation methods will be illustrated by the spectroscopy of the decomposition products of hydrocarbon discharges. The emphasis will be on the use of compact, reliable, easily controlled laser systems for the quantitative understanding of the species present in low temperature plasmas. |