Fast, real-time spectrometer based on a pulsed quantum-cascade laser

We describe a mid-infrared spectrometer that is based on the combination of a multiple-pass absorption cell and a submicrosecond pulsed quantum-cascade laser. The spectrometer is capable of both making sensitive measurements and providing a real-time display of the spectral fingerprint of molecular vapors. For a cell with a path length of 9.6 m, dilution measurements made of the v_9 band transitions of 1,1-difluoroethylene indicate a sensitivity of 500 parts in 10^9, corresponding to a fractional absorbance of 4 × 10^-4 .