Tracing part-per-billion line shifts with direct-frequency-comb Vernier spectroscopy

Accurate frequency measurements of molecular transitions around 2 ?m are performed by using a direct-frequency-comb spectroscopy approach that combines an Er+ frequency-comb oscillator at 1.5 ?m, a Tm-Ho fiber amplifier, and a Fabry-Perot-filter, high-resolution dispersive spectrometer optical multiplex-detection system. This apparatus has unique performances in terms of a wide dynamic range to integrate the intensity per comb mode, which allows one to measure molecular absorption profiles with high precision. Spectroscopic information about transition frequencies and linewidths is very accurately determined. Relative frequency uncertainties of the order of a few parts in 10-9 are achieved for rovibrational transitions of the CO2 molecule around 5100 cm-1. Moreover, tiny frequency shifts due to molecular collisions and interacting laser power using direct comb spectroscopy are investigated in a systematic way.