  A technique to measure the decoherence time of optical phonons in a solid is
presented. Phonons are excited with a pair of time delayed 80 fs, near infrared
pulses via spontaneous, transient Raman scattering. The fringe visibility of
the resulting Stokes pulse pair, as a function of time delay, is used to
measure the phonon dephasing time. The method avoids the need to use either
narrow band or few femtosecond pulses and is useful for low phonon excitations.
The dephasing time of phonons created in bulk diamond is measured to be tau =
6.8ps (1.56cm-1).
