  We demonstrate a frequency multiplexed photon pair generation based on a
quadratic nonlinear optical waveguide inside a cavity which confines only
signal photons without confining idler photons and the pump light. We
monolithically constructed the photon pair generator by a periodically-poled
lithium niobate (PPLN) waveguide with a high reflective coating for the signal
photons around 1600 nm and with anti-refrective coatings for the idler photons
around 1520 nm and the pump light at 780 nm at the end faces of the PPLN
waveguide. We observed a comb-like photon pair generation with a mode spacing
of the free spectral range of the cavity. Unlike the conventional multiple
resonant photon pair generation experiments, the photon pair generation were
incessant within a range of 80 nm without missing teeth due to a mismatch of
the energy conservation and the cavity resonance condition of the photons,
resulting in over 1000-mode frequency multiplexed photon pairs in this range.
