Mechanically flexible waveguide arrays for optical chip-to-chip coupling

This paper reports on the progress related to a multichannel photonic alignment concept, which aims to achieve submicrometer alignment of the waveguides of two photonic integrated circuits (PICs). The concept consists of two steps: chip-to-chip positioning and fixing provide a coarse alignment after which waveguide-to-waveguide positioning and fixing result in a fine alignment. For the waveguide-to-waveguide alignment, mechanically flexible waveguides are used. Positioning of the waveguides is performed by integrated MEMS actuators. The flexible waveguides and the actuators are both integrated in one of the PICs. This paper reports on the fabrication and the mechanical characterization of the suspended waveguide structures. The flexible waveguide array is created in a PIC which is based on TriPleX technology, i.e. a silicon nitride (Si3N4) core encapsulated in a silicon dioxide (SiO2) cladding. The realized flexible waveguide structures consist of parallel cantilevered waveguide beams and a crossbar that connects the free ends of the waveguide beams. The fabrication of suspended structures consisting of a thick, i.e. 15 mu m, TriPleX layer stack is challenged by the compressive mean stress in the SiO2. We have developed a fabrication method for the reliable release of flexible TriPleX structures, resulting in a 96% yield of cantilever beams. The realized suspended waveguide arrays have a natural out-of-plane deformation, which is studied using white light interferometry. Suspended waveguide beams reveal a downward slope at the base of the beams close to 0.5 . In addition to this slope, the beams have a concave upward profile. The constant curvature over the length of the waveguide beams is measured to range from 0.2 mu m to 0.8 mu m. The pro files measured over the length of the crossbars do not seem to follow a circular curvature. The variation in deflection within crossbars is measured to be smaller than 0.2 mu m.