**Xi-Rong Su 1, Yi-Wen Huang 1, Tong Xiang 1, Yuan-Hua Li 1,2 and Xian-Feng Chen 1,\***


Received: 11 October 2019; Accepted: 10 November 2019; Published: 13 November 2019 -

**Abstract:** In this work, the cascaded second-order spontaneous parametric down-conversion (SPDC) is considered to produce pure state photon triplets in periodically poled lithium niobite (PPLN) doped with 5% MgO. A set of parameters are optimized through calculating the Schmidt number of two-photon states generated by each down-conversion process with different pump durations and crystal lengths. We use a Gaussian filter in part and obtain three photons with 100% purity in spectrum. We provide a feasible and unprecedented scheme to manipulate the spectrum purity of photon triplets in the communication band (C-band).

**Keywords:** pure state; cascaded spontaneous parametric down-conversion (SPDC); numerical simulation
