Ultra-low-loss optical delay line on a silicon chip

TLDR

Optical fibre delivers long, non‑resonant time delays and is coiled to reduce footprint, yet its high attenuation makes on‑chip waveguide integration difficult, despite the potential for shock‑resistant, system‑on‑chip functionality.

Abstract

Light propagation through an optical fibre causes a long, non-resonant (true) time delay used in numerous applications. In contrast to how it is deployed in optical communication systems, fibre is coiled in these applications to reduce footprint. This is a configuration better suited for a chip-based waveguide that would improve shock resistance, and afford the possibility of integration for system-on-a-chip functionality. However, integrated waveguide attenuation rates lag far behind the corresponding rates of optical fibre, featuring attenuation many orders larger. Here we demonstrate a monolithic waveguide as long as 27 m (39 m optical path length), and featuring broadband loss rate values of (0.08±0.01) dB m−1 measured over 7 m by optical backscatter. Resonator measurements show a further reduction of loss to 0.037 dB m−1, close to that of optical fibres when first considered a viable technology. Scaling this waveguide to integrated spans exceeding 250 m and attenuation rates below 0.01 dB m−1 is discussed. Fibre-optic waveguides are used to provide timing delays for different sensing and signal processing applications, but their transfer to on-chip platforms is a challenge. Here low-loss delay lines based on whispering-gallery spiral waveguides up to 27 m long are produced, presenting a scalable alternative.