Fourier optics with linearly tapered waveguides: light trapping and focusing

TL;DR

This paper analyzes how tapered waveguides can trap and focus light, demonstrating an analytical solution that shows the spatial distribution of focused light as a Fourier transform of the input signal, with applications in optical nano-focusing.

Contribution

It provides an analytical solution for light trapping and focusing in linearly tapered waveguides using the slowly-varying envelope approximation, linking it to Fourier optics.

Findings

Analytical solution for linear taper waveguides

Focused light distribution as Fourier transform of input

Resembles optical lens behavior in paraxial approximation

Abstract

An optical pulse asymptotically reaching zero group velocity in tapered waveguides can ultimately stop at a certain position in the taper accompanied by a strong spatial compression. This phenomenon can be also observed in spatio-temporal systems where the pulse velocity asymptotically reaches the velocity of a tapered front. The first system is well known from tapered plasmonic waveguides where adiabatic nano-focusing of light is observed. Its counterpart in the spatio-temporal system is the optical push broom effect where a nonlinear front collects and compresses the signal. Here, we use the slowly-varying envelope approximation to describe such systems. We demonstrate an analytical solution for the linear taper and the piecewise linear dispersion and show that the solution in this case resembles that of an optical lens in paraxial approximation. In particular, the spatial distribution of the focused light represents the Fourier transform of the signal at the input.