Frequency-correlated photon-pairs tunable across the telecom spectrum via cascaded SHG ＋ SPDC in PPLN

We explore the cascaded second-order (χ(2)) nonlinear process of second-harmonic generation (SHG) + spontaneous parametric down-conversion (SPDC) at telecom wavelengths in a type-0 phase-matched Zn-diffused 5% MgO-doped periodically poled lithium niobate (PPLN) ridge waveguide. We simulate the waveguide modes using finite element method (FEM) to determine the signal and idler peak wavelengths produced via the cascading effect at a particular temperature and pump wavelength. We also derive an analytical expression for the effective χ(3) conversion efficiency (equivalently, the signal/idler photon flux) and experimentally verify that the cascaded process produces discrete temperature-tunable signal/idler sidebands (around a ∼1550-nm pump) that can be continuously tuned across the E-, S-, L-, and U-bands of the telecom spectrum. Such frequency-correlated photon pairs produced in a fiber-coupled integrated setup could have applications in quantum computing and communication.