  We theoretically and experimentally demonstrate coherence phenomena in
optical parametric amplification inside a cavity. The mode splitting in
transmission spectra of phase-sensitive optical parametric amplifier is
observed. Especially, we show a very narrow dip and peak, which are the shape
of $\delta $ function, appear in the transmission profile. The origin of the
coherence phenomenon in this system is the interference between the harmonic
pump field and the subharmonic seed field in cooperation with dissipation of
the cavity.
