  We experimentally observe the two-photon interference of multimode photon
pairs produced by an optical parametric oscillator far below threshold via a
michelson interferometer, which shows a multipeaked structure. We find that the
correlation function when the interferometer is unbalanced is clearly dependent
on the path difference and phase between two interfering beams, but the shape
of correlation function in balanced case is independent on the small path
difference and phase beside the height. All experimental results are well
agreed with the theoretical prediction.
