Broadband second-harmonic generation in step-chirped periodically poled lithium niobate waveguides

Periodically poled lithium niobate (PPLN) structures on a chip enable efficient second-order nonlinear optical effects, benefiting from the tight light confinement and the utilization of d₃₃. Here, we report a broadband second-harmonic (SH) generation in a step-chirped PPLN waveguide on X-cut lithium niobate on insulator (LNOI). The high fidelity of the poling period is demonstrated over the whole length of 7 mm using a non-destructive technique of piezoresponse force microscopy. The SH signal was continuously observed in the step-chirped PPLN waveguides while scanning the wavelength of the pump laser from 1550 nm to 1660 nm. The SH conversion efficiency was measured to be 9.6 % W^-1 cm^-2 at 1642 nm. This work will benefit wavelength conversions of light sources with wideband spectra.