Broadband frequency conversion and “area law” in tapered waveguides

Abstract

We propose to use a tapered waveguide to achieve optical frequency conversions on a photonic chip. We show that in tapered waveguides, the frequency conversion has a much broader bandwidth, which is proportional to the waveguide width difference as Δλ ∝ 1.7δw. More importantly, the conversion efficiency within the bandwidth is almost constant, which is favorable for ultrashort pulses. This simple but efficient design not only enables the conversion bandwidth to be engineered, but also is tolerant to fabrication error. We demonstrate an “area law” for the frequency conversion process that the integral of the conversion efficiency within the conversion bandwidth does not change against the waveguide lateral shape. This can be used as a general guideline for integrated nonlinear optical device designs. With our approach, high-efficiency and wavefront-keeping conversion for short pulses becomes possible on a photonic chip, which allows the applications for frequency comb and scalable on-chip information processing.