Simultaneous broadband nonlinear quadratic processes in lithium niobate on insulator waveguides

We analyze the spectral engineering capabilities for broadband second harmonic and difference frequency generation (SHG and DFG) in the telecom band afforded by dispersion-engineered waveguides in x-cut lithium-niobate-on-insulator (LNOI). Considering both silica- and air-clad 5 mm-long rib photonic wires in 5 mol% MgO-doped 600nm-thick LNOI, we identify in both cases working points for 258-433 nm-wide DFG acceptance bandwidths. Furthermore, air-clad waveguides are found to afford simultaneous broadband SHG (103 nm) and DFG (433 nm) conversion with expected efficiencies of ∼4700 % W^-1cm^-2, a particularly appealing feature for prospective χ(2) cascading devices. Finally, we study the tolerances of such working points to waveguide and poling parameters, identifying the most critical parameter to be the waveguide sidewall angle, affecting the SHG bandwidth (central wavelength) with a sensitivity of 95 nm (200 nm) for 1° deviation around the optimal working point.