  We developed a quantum theory for degenerate $\chi^{(3)}$ two-photon state
generated from optical fiber, and compared the theory predictions with an
experimental result which exhibits a Hong-Ou-Mandel dip visibility of around
94%. Excellent agreement between theory and experiment has been achieved, and
we attribute the missing 6% visibility mainly to spatial mode mismatch between
signal and idler photons at the beamsplitter.
