Optical cavity boosts broadband FTS sensitivity

An optical cavity-enhanced phase-shift setup was demonstrated by combining Fourier-transform spectroscopy (FTS) and incoherent broadband cavity-enhanced absorption spectroscopy (IBB-CEAS) techniques. The Fourier-transform spectrometer was a Bruker IFS 125 HR with a dielectrically coated calcium fluoride beamsplitter and the detector was an indium gallium arsenide (InGaAs) photodiode. High-resolution spectra of the evacuated optical cavity and of the cavity filled with 26.7 mbar of pure CO₂ were taken in the region between 5800 and 7000 cm^-1. The results show that the Doppler-width of CO₂ in the near-infrared region is 0.013 ^-1 and the spectrum is obtained from the average of 200 interferometer scans corresponding to an actuation time of 90 mins. The CO₂ as the calibration gas and the HITRAN database as reference for the line strengths allows an effective determination of reflectivity over a spectral range of more than 100 nm.