ABSTRACT
Freeform dielectric waveguides connect optical chips made of different materials in fully integrated photonic devices. With a spatial extent in the order of 100 µm, they constitute a computational challenge and make Maxwell full-wave solvers unhandy for the accelerated design. Therefore, it is of utmost importance to have tools that permit the fast prediction of waveguide loss to enable the rapid optimization of waveguide trajectories. Previously developed methods relied on the assumption that only a single mode propagates in the waveguide. However, the propagation of higher-order modes is not just unavoidable due to the geometry of the waveguides, but also, sometimes, beneficial as it increases the number of channels to transmit information. In this contribution, we present an approximate method for the fast calculation of transmission that accommodates the presence of higher-order waveguide modes, and assess its liability by describing light propagation through selected devices.
ABSTRACT
Multi-photon lithography allows us to complement planar photonic integrated circuits (PIC) by in-situ 3D-printed freeform waveguide structures. However, design and optimization of such freeform waveguides using time-domain Maxwell's equations solvers often requires comparatively large computational volumes, within which the structure of interest only occupies a small fraction, thus leading to poor computational efficiency. In this paper, we present a solver-independent transformation-optics-(TO-) based technique that allows to greatly reduce the computational effort related to modeling of 3D freeform waveguides. The concept relies on transforming freeform waveguides with curved trajectories into equivalent waveguide structures with modified material properties but geometrically straight trajectories, that can be efficiently fit into rather small cuboid-shaped computational volumes. We demonstrate the viability of the technique and benchmark its performance using a series of different freeform waveguides, achieving a reduction of the simulation time by a factor of 3-6 with a significant potential for further improvement. We also fabricate and experimentally test the simulated waveguides by 3D-printing on a silicon photonic chip, and we find good agreement between the simulated and the measured transmission at λ = 1550â nm.