1. Introduction . The spontaneous parametric down-conversion (SPDC) is a quantum phenomenon of generating two photons with angular frequency $\omega _1$and $\omega _2$by a pump light of angular frequency $\omega _1+\omega _2$[ 1 ]. It becomes a source of entangled photon pairs [ 2 ], so that the SPDC can be used in quantum information, such as the Bell inequality test experiment [ 3 , 4 ]. Compared with other entangled photon sources, such as the cascade emission fro...