Quantum cascade lasers (QCLs) in the IR range possess tuneable wavelengths, rendering the whole interferometer redundant for proteins that have a reaction cycle, that is, that can be excited several times. In addition, QCLs are much brighter than conventional blackbody radiators, which further increases the signal-to-noise ratio.
Read the full article here: FTIR Spectroscopy: A Comprehensive Biological Investigator.