Low-Loss Silicon Directional Coupler with Arbitrary Coupling Ratios for Broadband Wavelength Operation Based on Bent Waveguides

TL;DR

This paper presents a broadband, low-loss silicon directional coupler with arbitrary coupling ratios, demonstrating high fabrication tolerance, minimal coupling variation, and suitability for mass production in integrated photonics.

Contribution

The design introduces a novel broadband, low-loss 2x2 splitter with arbitrary coupling ratios based on bent waveguides and rigorous coupled mode theory, achieving ultra low-loss and high fabrication tolerance.

Findings

Measured broadband coupling values of 0.4, 0.5, 0.6, 0.7

Coupling variation reduced from 0.391 to 0.051 over 80 nm

Maximum coupling variation of 0.112 across a 300 mm wafer

Abstract

We demonstrate a design for a high-performance $2 \times 2$ splitter meeting the essential requirements of broadband coupling, support for arbitrary coupling ratio, ultra low-loss, high fabrication tolerance, and a compact footprint. This is achieved based on a rigorous coupled mode theory analysis of the broadband response of the bent directional coupler (DC) and by demonstrating a full coupling model, with measured broadband values of 0.4, 0.5, 0.6, and 0.7. As a benchmark, we demonstrate a 0.5:0.5 splitter that significantly reduces coupling variation from 0.391 in the traditional DC to just 0.051 over an 80 nm wavelength span. This represents a remarkable 7.67 times reduction in coupling variation. Further, newly-invented low-loss bends were used in the proposed design leading to an ultra low-loss design with negligible excess loss ($\mathrm{0.003 \pm 0.013 \ dB}$). The proposed 0.5:0.5 silicon strip waveguide-based design is tolerant and shows consistently low coupling variation over a full 300 mm wafer showcasing a maximum cross coupling variation of 0.112 over 80 nm wavelength range, at the extreme edge of the wafer. Futhermore, we augmented the wafer mapping with a waveguide width fabrication tolerance study, confirming the tolerance of the device with a mere 0.061 maximum coupling variation with a waveguide width deviation of $\pm 20$ nm over 80 nm wavelength range. These specs make the proposed splitter an attractive component for practical applications with mass production.