Efficient light coupling into a photonic crystal waveguide with flatband slow mode

TL;DR

This paper presents a highly efficient method for coupling light into a photonic crystal waveguide's slow mode using a specially designed coupler with high transmission efficiency and minimal reflection, suitable for integrated photonics.

Contribution

The authors introduce a novel coupler design that achieves over 95% coupling efficiency into flatband slow modes with detailed analysis of mode conversion and parasitic reflections.

Findings

Coupling efficiency exceeds 95% over an 8 nm wavelength range.

Mode conversion efficiency within the coupler is over 99.7%.

Parasitic reflectance is below 0.6% across the operational bandwidth.

Abstract

We design an efficient coupler to transmit light from a strip waveguide into the flatband slow mode of a photonic crystal waveguide with ring-shaped holes. The coupler is a section of a photonic crystal waveguide with a higher group velocity, obtained by different ring dimensions. We demonstrate coupling efficiency in excess of 95% over the 8 nm wavelength range where the photonic crystal waveguide exhibits a quasi constant group velocity vg = c/37. An analysis based on the small Fabry-P\'erot resonances in the simulated transmission spectra is introduced and used for studying the effect of the coupler length and for evaluating the coupling efficiency in different parts of the coupler. The mode conversion efficiency within the coupler is more than 99.7% over the wavelength range of interest. The parasitic reflectance in the coupler, which depends on the propagation constant mismatch between the slow mode and the coupler mode, is lower than 0.6% within this wavelength range.