Self-Smoothed Gradient Index Waveguides for Low-Loss, High-Density Photonic Integrated Circuits

In this paper, we report a novel self-smoothed gradient index cladding waveguide for low-loss, high-density photonic integrated circuits (PICs). In conventional PIC waveguides, there is a fundamental trade-off between propagation loss and bending loss. High-index-contrast waveguides can provide strong mode confinement and small bending radius, but they are highly sensitive to sidewall roughness and therefore exhibit increased scattering loss. In contrast, low-index-contrast waveguides can reduce propagation loss, but they require a much larger bending radius and are not suitable for dense photonic integration. To overcome this limitation, we propose a self-smoothed double-cladding waveguide architecture composed of a high-index core, a gradient index first cladding layer, and a low-index second cladding layer. The first cladding layer can be formed by advanced conformal coating processes, such as non-uniformly cycled atomic layer deposition and gradient index dip coating, which provide both a gradual refractive index transition and a self-smoothing effect on the rough sidewall. We perform quantitative analyses of the propagation loss, bending loss, and mode field distribution of the proposed structure. The numerical results confirm that the propagation loss can be reduced by approximately two orders of magnitude while maintaining low bending loss and mode confinement comparable to that of a conventional rib-shaped waveguide. We also experimentally verify the self-smoothing effect by using atomic layer deposition (ALD)-grown non-uniformly cycled nanolaminates and coating a rough sapphire bar with a high-refractive-index polymer, which significantly reduces the measured surface roughness and improves optical transparency. These results confirm that the proposed self-smoothed gradient index cladding waveguide can be an effective platform for realizing low-loss, high-density PICs.