High-resolution broadband (>100 cm^-1) infrared heterodyne spectro-radiometry using an external cavity quantum cascade laser

Broadband thermal infrared heterodyne spectro-radiometry using an external cavity quantum cascade laser as a tunable local oscillator has been performed over a frequency range of more than 100 cm^-1 at a central frequency of 1190 cm^-1. Heterodyne spectro-radiometry is demonstrated for two local oscillator tuning modes: broadband tuning for transmission and emission spectroscopy of broadband absorbers (Freon 12), and broadband frequency selection in combination with fine continuous frequency tuning for high-resolution (0.021 cm^-1) transmission spectroscopy (N ₂O). In each case concentration retrievals are performed and analyzed. The spectro-radiometer noise level is demonstrated to be twenty two and eight times the fundamental shot-noise limit in the two scanning modes respectively.