Abstract

Recent advances in frequency-modulation (FM) spectroscopy¹ allow quantum-limited absorption measurements on unmodulated species with visible absorption lines. This method opens up the possibility of absorption detection for applications such as atmospheric remote sensing and combustion studies. There are two impediments to extending FM to ultraviolet transitions: Most phase modulators are ineffective in the ultraviolet, and even with adequate modulators, direct generation of cw ultraviolet light is problematic. Here I point out that both of these problems may be overcome by using an actively mode-locked visible dye laser as a source of quasi-cw light, which may then be phase modulated and subsequently doubled with high efficiency. Tran et al.² have demonstrated that a frequency-modulated laser remains frequency modulated after doubling. In addition, a mode-locked pulse train may be used for FM spectroscopy as long as the modulation frequency and spectroscopic feature width are much greater than the bandwidth of the pulse train. In that case each locked mode will contribute coherently to the FM signal. This state of affairs holds for suitably adapted sync-pumped dye lasers and many molecules of interest.

© 1985 Optical Society of America