Tolerant silicon photonic microcircuits

Research Area: Design and Modeling for Integrated Photonics, Silicon photonics for telecom, datacom and interconnect , Large-scale Photonic Integration

Main Researcher: Sarvagya Dwivedi

In the past decade, silicon photonics has rapidly become one of the most promising photonic integration technologies.
The main reason is due to silicon has a very high refractive index, enabling optical waveguides with a very high refractive index contrast. The consequence of this is that waveguide cross sections can be shrunk to sub-micrometer dimensions, and the waveguides can be bent with only a few micrometer bend radius. Therefore, photonic circuits in silicon can be orders of magnitude smaller than their predecessors made in low-contrast glass. However, some of the strengths of silicon photonics also introduce weaknesses. The high refractive index contrast that allows the small waveguides and sharp bends also makes the waveguides terribly sensitive to any variation in the cross section. While good process control already allows nanometer-level accuracy, this is often insufficient for wavelength selective functions, such as channel drop filters in a wavelength-division multiplexing (WDM) system. Pre-fabrication or post-fabrication trimming of the components can be used to correct the effects of the last nanometer fabrication offsets. Also, silicon has a strong thermo-optic coefficient, meaning that its refractive index changes as a function of temperature. This is beneficial if it is used for tuning circuit elements, but that also implies additional power consumption. A passive silicon photonic circuit will therefore need temperature stabilization, or a method is needed to compensate for the thermo-optic effect of silicon. One method for this is the introduction of materials with opposite thermo-optic coefficients, such as polymers, but this introduced additional process complexity and it is not always desirable to incorporate such materials in a CMOS-like process flow.

Compact all- silicon athermal filter

We propose to address the tolerances not at the technology level, but rather through smart design solutions.For instance an a-thermal filter can be constructed by combining two waveguide geometries with a different thermo-optic coefficient, and this is achieved by just changing the width of the waveguide or polarization ( shown in the figure) , and thus the confinement of light in silicon.
The key challenge that we will address in this project is the poor tolerance of silicon photonic devices to variability, induced by both the manufacturing process and operational behavior. The research is on going for more complex circuits.

Other people involved:

• Herbert D'heer
• Sarvagya Dwivedi
• Thijs Spuesens
• Yufei Xing
• Wim Bogaerts
• Alaa Elshazly

Related Research Projects

• FWO: G013815N-ScaleModel (Scalable behavioral models for large-scale silicon photonics)