Study on a polarization-entangled photon-pair source based on niobium-doped potassium titanyl phosphate

We investigate the high-purity polarization-entangled photon-pair generation in niobium-doped potassium titanyl phosphate (Nb:KTP). The technique is based on Type II spontaneous parametric down-conversion (SPDC). Two kinds of Nb:KTPs with Nb-doping concentrations of 3.4 mol% and 7.5 mol% will be considered in each periodically poled and non-poled case. The properties of Type II SPDC, including the range of the beam propagation direction (or the spectral range of photons) and the corresponding effective nonlinearities and beam walk-offs, will be discussed in detail both theoretically and numerically. This is followed by a joint spectral analysis is followed to quantify the heralded-state spectral purities of the photon pairs generated by Type II SPDC. The results show that photon-pairs can be generated with high purities of 0.995–0.997 with proper pump filtering. We will also show that the spectral positions of photon pairs can be chosen from a wide range, including 1520.0 nm, 1582.4 nm, 1811.6–1837.4 nm, and 1998.6–2026.8 nm.