We demonstrate group index matching (GIM) as an effective design strategy for building a widely tunable spontaneous parametric downconversion (SPDC) source in a singly poled lithium niobate on insulator (LNOI) waveguide. Considering SPDC processes in which the interacting photons satisfy either asymmetric or symmetric GIM (AGIM and SGIM) conditions in a strongly dispersive LNOI waveguide, we demonstrate a significantly enhanced pump tuning range ( \(\sim\) 536 nm for AGIM and \(\sim\) 625 nm for SGIM) in the same device. This allows for a single device to function as a tunable photon pair source offering a wide range of downconverted wavelengths with distinct spectral characteristics. Through our simulations, we demonstrate photon pairs covering broad wavelength ranges (for AGIM, signal and idler can span 763–2500 nm and 1547–2498 nm, respectively; for SGIM, signal covers 820–2499 nm and idler can span 1715–2494 nm). Within this range, both spectrally correlated and uncorrelated photon pairs can be generated, supporting diverse quantum applications. Numerical simulations performed using MATLAB and Ansys Lumerical’s finite-difference eigenmode (FDE) solver validate these results. Further, we analyze the influence of waveguide cross-sections on group index engineering capabilities and their effect on the output quantum light. The reported work highlights the significance of dispersion properties of the non-linear waveguides and their role in building tunable, multipurpose quantum light sources which can cater to various applications such as linear optical quantum computing, quantum communication and quantum metrology, without either changing the waveguide geometry or its poling period.
