Nonreciprocal optical functionalities like optical isolators and circulators are key components for the suppression of unwanted optical feedback in lasers and are also widely used for light routing in fiber-based measurement systems such as optical coherence tomography. Therefore, they are important building blocks in integrated optics, which promises further miniaturization and cost reduction of optical elements for telecom, datacom, and sensing applications. In this work, we experimentally demonstrate a four-port polarization independent optical circulator on a polymer-based hybrid integration platform. The circulator consists of polymer waveguides and two thin-film polarization beam splitters (PBSs) inserted into waveguides via etched slots. Crystalline, pre-magnetized bulk Faraday rotators (FRs) and half-wave plates (HWPs) are inserted into free-space sections, formed by pairs of waveguide butt-coupled GRIN lenses. For a first demonstrator, on-chip losses down to 5 dB and optical isolations up to 24 dB were measured, depending on the different input and output constellations, as well as the polarization. By applying an external magnetic field opposite to the magnetization of the faraday rotators, it is possible to repole the magneto-optic material, leading to reversely circulating light inside the device. This enables optical switching between ports in form of a latching switch, which maintains its state after removing the external magnetic field.
3D photonic integration introduces a new degree of freedom in the design of photonic integrated circuits (PICs) compared to standard 2D-like structures. Novel applications such as large-scale optical switching matrices, e.g. for top-of- rack cross connect switches in data centers, benefit from the additional design flexibility due to their waveguide crossing-free architecture and compact footprint. In this work, a novel 3D 4×4 multi-mode interference coupler (MMI) based on HHI’s polymer-based photonic integration platform PolyBoard is presented. The fabrication process of the PolyBoard platform allows for the realization of vertically stacked polymer waveguide layers. Cascading two of the presented 3D 4×4 MMIs will form the building block of future large-scale 3D switching matrices. The 3D 4×4 MMI structure comprises two waveguide layers separated by a distance of 7.2 μm, with two input and two output waveguides in each layer, and a multimode interference (MMI) section in between. The vertical MMI section serves as the interconnection between the different waveguide layers and distributes the incoming light from each input waveguide across the four output ports of the 4×4 MMI. Design rules and fabrication methodology of these novel structures are presented in detail. Preliminary measurements demonstrate the proof-of-concept indicating an insertion loss below 9.3 dB, including fiber-chip coupling loss and the 6 dB intrinsic loss.
Recent developments in versatile polymer-based technologies and hybrid integration processes offer a flexible and cost-efficient alternative for creating very complex photonic components and integrated circuits. The fast and efficient test, optimization and verification of new ideas requires an automated and reproducible simulation and design process supporting flexible layout-driven and layout-aware schematic-driven methodologies. Targeting very complex designs, even small fabrication tolerances of one building block could make a huge difference on the performance and manufacturability of the whole structure. To reduce risk of failure and to make performance predictions by virtual prototyping reliable, the simulation model of each single building block needs to be working correctly based not only on the appropriate mathematical and physical equations, but also on adequate information provided by the foundry where the final structure will be manufactured.
The PolyPhotonics Berlin consortium targets to address these design challenges and establish a new versatile integration platform combining polymer with Indium-Phosphide and thin-film filter based technologies for numerous photonics applications in the global communications and sensing market. In this paper we will present our methodologies for modelling and prototyping optical elements including hybrid coupling techniques, and compare them with exemplary characterization data obtained from measurements of fabricated devices and test structures. We will demonstrate how the seamless integration between photonic circuit and foundry knowledge enable the rapid virtual prototyping of complex photonic components and integrated circuits.