  In recent works, the idea of time-reversed laser oscillation has been
proposed and demonstrated to realize a two-channel coherent perfect absorber
[Y.D. Chong et al., Phys. Rev. Lett. 105, 053901 (2010); W.Wan et al., Science
331, 889 (2011)]. Here the time reversal of optical parametric oscillation in a
nonlinear ?(2) medium is considered and shown to realize a coherent perfect
absorber for colored incident signal and idler fields. A detailed analysis is
presented for the time-reversed process of mirrorless optical parametric
oscillation in the full nonlinear regime
