Spectroscopy with a coherent dual frequency comb interferometer at 3.4 μm

Abstract

A coherent dual fiber-comb spectrometer centered at 1.5 µm wavelengths is transferred to 3.4 µm by differencefrequency generation with a 1064 nm cw laser. It is shown that the residual linewidth between the comb teeth at 3.4 µm is resolution-limited to 200 mHz; such narrow linewidths can enable coherent dual-comb spectroscopy at high-precision and signal-to-noise ratio. We then discuss different interferometric configurations for coherent dual-comb spectroscopy. We find that a two-branch interferometric setup is appropriate to measure both the magnitude and phase spectrum of purely Doppler-broadened absorption lines. An initial measurement of methane lines in the υ 3 band P-branch with a resolution of 114 MHz is demonstrated.