  We perform a high resolution Fourier Transform Spectroscopy of optical
sources in the few-photon regime based on the phenomenon of two-photon
interference in a beam splitter. From the heterodyne interferogram between test
and reference sources it is possible to obtain the spectrum of the test source
relative to that of the reference. The method proves to be a useful asset for
spectral characterization of faint optical sources below the range covered by
classical heterodyne beating techniques.
