Ultrabroadband tunable difference frequency generation in a standardized thin-film lithium niobate platform

Thin-film lithium niobate (TFLN) on an insulator is a promising platform for nonlinear photonic integrated circuits (PICs) due to its strong light confinement, high second-order nonlinearity, and flexible quasi-phase-matching for three-wave mixing processes via periodic polling. Among the three-wave mixing processes of interest, difference frequency generation (DFG) can produce long-wave infrared (IR) light from readily available near IR inputs. While broadband DFG is well studied for mid-IR frequencies, achieving broadband idler generation within the telecom window (near C-band) and the short-wave infrared (near 2 micron) is more challenging due to stringent dispersion profile requirements, especially when using standardized TFLN thicknesses. In this paper, we investigate various standard waveguide designs to pinpoint favorable conditions for broadband DFG operation covering several telecom bands. Our simulations identify viable designs with a possible 3-dB conversion efficiency bandwidth (CE-BW) of 300 nm and our measurements show idler generation from 1418 nm to 1740 nm, limited by our available sources, experimentally confirming our design approach. Furthermore, temperature tuning allows a further shift of the idler towards the mid-IR, up to 1819 nm. We also achieve a stretched wavelength range of idler generation by leveraging the longitudinal variation of the waveguide in addition to poling. Finally, our numerical simulations show the possibility of extending the CE-BW up to 780 nm while focusing on waveguide cross-sections that are available for fabrication within a foundry. Our work provides a methodology that bridges the deviations between fabricated and designed cross-sections, paving a way for standardized broadband DFG building blocks.