Comb-locked deep-ultraviolet laser system for precision mercury spectroscopy

Doppler-broadening gas thermometry is extended to the deep-ultraviolet domain using a comb-locked laser frequency chain, expressly developed for precision spectroscopy of mercury vapors at the wavelength of 253.7 nm, in coincidence with the (6s2)S01→(6s6p)P13 intercombination line. The system is based on a double-stage second-harmonic generation process of an external-cavity diode laser at 1014.8 nm, which is frequency locked to a self-referenced optical-frequency-comb synthesizer by means of an efficient nonlinear frequency-mixing scheme. An absolute frequency axis in the UV region is produced by scanning the comb repetition rate, with the entire frequency chain following the reference comb tooth. The complete characterization of the phase noise of the comb-locked near-infrared laser demonstrates line-emission narrowing down to the limit imposed by the coherence of the comb teeth. After a proper processing of the phase noise, the Gaussian and Lorentzian components of the width of the UV radiation are determined. This is useful to estimate the instrumental perturbation to the probed mercury line for the aims of thermodynamic temperature metrology. In this regard, the preliminary spectroscopic results are quite promising, despite the difficulties arising from the short wavelength of operation.