Wavelength modulated multiheterodyne spectroscopy using Fabry-Pérot quantum cascade lasers.

Multiheterodyne spectroscopy implemented with semiconductor Fabry-Pérot lasers is a method for broadband (> 20 cm^-1), high spectral resolution (~1 MHz) and high time resolution (< 1 µs/spectrum) spectroscopy with no moving parts utilizing off-the-shelf laser sources. The laser stabilization approach demonstrated here enables continuous frequency tuning (at 12.5 Hz repetition rate) while allowing for multiheterodyne wavelength modulation spectroscopy (WMS). Spectroscopic detection of N₂O around 1185 cm^-1 is experimentally realized, which shows a direct absorption sensitivity limit of ~1.5⨯10^-3/√Hz fractional absorption per mode. This can be lowered using WMS down to 5⨯10^-4/√Hz per mode, limited by optical fringes. This approaches the range of sensitivities of standard single-mode laser based spectrometers, which demonstrates that the multiheterodyne method is well-suited for chemical sensing of spectrally broadened absorption features or for multi-species measurements.