Dual-comb spectroscopy of methane based on a free-running Erbium-doped fiber laser.

Dual-comb spectroscopy has been developed into a high-precision technique that is capable of sensing many important species of samples, such as methane. Recent studies on single-cavity, dual-comb light sources further reduce the system complexity of such schemes. In contrast to the previous demonstrations around the lasing spectrum, this work significantly expands the spectral coverage of a dual-comb spectroscopy setup using one free-running laser to a region far beyond the laser's emission wavelengths. Nonlinear wavelength conversion based on soliton self-frequency shift is adopted to convert and tune the wavelengths of both dual-comb pulses to ~1650nm. It is shown that this process has introduced little additional intensity noise. The 2ν absorption band of methane from 1647 nm to 1663nm is measured with very good agreement with HITRAN, and the standard deviation of the residual is < ~0.006 after averaging ~1.96 seconds of data. Our results further elucidate the potential of dual-comb spectroscopy using one laser, and could pave the way for the development of low-cost, power-efficient, and compact dual-comb instrument targeting more spectral regions.