  It is shown that the copropagating three-wave-mixing parametric process, with
appropriate type-II extended phase matching and pumped with a short
second-harmonic pulse, can perform spectral phase conjugation and parametric
amplification, which shows a threshold behavior analogous to backward wave
oscillation. The process is also analyzed in the Heisenberg picture, which
predicts a spontaneous parametric down conversion rate in agreement with the
experimental result reported by Kuzucu et al. [Phys. Rev. Lett. 94, 083601
(2005)]. Applications in optical communications, signal processing, and quantum
information processing can be envisaged.
