Waveguide

Notes

Sidewall roughness from lithography and etch dominates loss, which is why photonic lines care about line-edge quality more than critical dimension.

Overview

A waveguide, in photonics, is a structure that confines light and guides it along a chosen path, usually by surrounding a core of higher refractive index with a cladding of lower index. On a photonic chip, waveguides play the role that metal wires play in electronic circuits, carrying light between lasers, modulators, filters and detectors. Optical fibre is the best-known waveguide; this page concerns the patterned waveguides built on chips.

How it works

Light travelling in the core strikes the boundary with the cladding at a shallow angle and is reflected back by total internal reflection, so it stays trapped as long as the path bends gently enough. In silicon photonics the core is a strip of silicon on a silicon-on-insulator wafer, surrounded by silica. Because silicon's refractive index (about 3.5) is far higher than silica's (about 1.44), the light is tightly confined and the core can be only a few hundred nanometres across, which allows dense circuits and tight bends. Silicon nitride cores, with lower index contrast, are another common choice and are valued for very low loss.

Waveguides come in several shapes. A strip or channel waveguide confines light in both directions across the guide; a rib waveguide leaves a thin slab of silicon on either side of a raised ridge; and slab waveguides confine light in one direction only. Designers usually keep waveguides single-mode, carrying one guided pattern of light, so that signals do not smear out.

The main performance limits are propagation loss and sensitivity to fabrication. Imperfections such as rough etched sidewalls scatter light, tight bends radiate it, and small changes in width or thickness shift how light behaves, which affects filters and interferometers built from waveguides. The cross-section also makes behaviour depend on polarisation.

Examples

• A silicon strip waveguide routing light between a modulator and a germanium photodetector on a data centre transceiver chip.
• A silicon nitride waveguide used for a low-loss delay line or filter in a photonic circuit.
• Waveguide-based Mach-Zehnder interferometers that split, delay and recombine light to modulate it.
• A multi-project wafer run through AIM Photonics, where designers use the institute's design kit to lay out waveguides.

Why it matters

Every photonic integrated circuit is built from waveguides, so their loss, size and uniformity set how large and efficient a circuit can be. Photonic device designers, process engineers who etch and clad them, and test engineers measuring loss on wafers all work with them, and foundries and institutes supply design kits that include them.

Related terms

• PIC
• Optical transceiver
• Grating coupler
• Microring resonator

Where it appears on the Hub

Used on 12 pages of the Hub. In each group, the pages that mention it most come first.