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How to raise fibre-to-waveguide coupling efficiency by co-designing a grating coupler and a microlens. Fields solved in Lumerical FDTD are passed to Zemax OpticStudio as ZBF, where POP is used to evaluate lens curvature and placement in a multi-scale analysis.
The key to efficient optical coupling between an optical fibre and a waveguide in a photonic integrated circuit (PIC) is co-designing the grating coupler and the microlens. This approach raises coupling efficiency dramatically and also improves tolerance to mode mismatch and to small misalignments. This article presents a design workflow built on multi-scale simulation using Ansys Lumerical and Zemax OpticStudio.
In optimal photonic device design, the combination of a grating coupler and a microlens is essential for spreading and collimating light effectively. In particular, when analysing how light behaves in structures larger than the wavelength scale, Physical Optics Propagation (POP) is applied.Lumerical FDTD The solver is used to analyse the electromagnetic field emitted by the grating coupler, and that data is exported in ZBF format. Importing it into Zemax OpticStudio makes it possible to analyse macro-scale propagation behaviour in detail.
Optimising the coupling efficiency between fibre and waveguide requires precise adjustment of the curvature and position of the microlens. For example, with 300 um between the optical fibre and the waveguide, simulation shows that a lens radius of curvature of 500 um gives the highest coupling efficiency. The results also show that adding a microlens is an effective way to improve tolerance to mismatch, which raises overall system performance.
Total system loss is analysed in detail by combining the Lumerical and Zemax results. Output coupling loss, for instance, is shown to be around 40%, and adding the microlens improves coupling efficiency dramatically. Input-direction coupling efficiency is analysed in the same way, which makes it possible to arrive at an optimal design that maximises overall system efficiency. Coupling design driven by multi-scale simulation in this way is the key to a step change in photonic integrated circuit performance.
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