  We demonstrate a novel technique of coherent Raman spectroscopy with a
femtosecond laser. We apply to a molecular sample a sequence of pairs of
ultrashort excitation and probe pulses, with a linearly increasing time delay
between the two pulses from one pair to the next. We measure, as a function of
the delay, the intensity modulation in the signal resulting from the
differential detection of the Stokes and anti-Stokes radiations generated at
the sample. The Fourier transform of such time-domain signal reveals the
spectrum of the excited vibrational Raman transitions. The experimental
proof-of-principle demonstrates high resolution, broad spectral span and
suppression of the non-resonant background, as well as sensitivity enhancement
due to the differential detection.
