Rotational Fourier-Transform Mid-IR Spectroscopy with a Quantum Cascade Laser Frequency Comb

Real-time spectroscopy with simultaneous high speed, high spectral resolution, and broad bandwidth is of vital importance for studying non-repetitive reactions and process monitoring [1]. The general purpose instrument for infrared spectroscopy has been Fourier-Transform infrared (FTIR) spectroscopy, due to its flexibility brought by extremely broad-band globar sources. While state-of-the-art rapid-scan FTIR can reach time resolutions of 100's or even 10's of ms, these instruments suffer from an inevitable tradeoff between speed and spectral resolution. This trade- off is caused by the linear translation of the scanning mirror which makes simultaneous high spectral and temporal resolution impossible. In previous research works [2]–[4] an attempt has been made to resolve this issue. However, the solution resulted in co-linear incoming and out- going beams, presenting feedback into the light source, and an undesired dependence of the optical delay along the beam diameter, meaning sources with low coherence and very long path lengths could not be used. In this work we overcome the above mentioned issues, and demonstrate an FT spectrometer whose operational principle is based on continuous rotational, rather than linear, motion of the scanning mirror, decoupling the spectral resolution from the temporal one (see Fig. 1(a)). Fig 1(a) shows a schematic of the developed rotational delay line system, which consists out of a pair of static retroreflectors and a rotational retroreflector.