On-chip positionable photonic waveguides for chip-to-chip optical interconnects

This paper reports on the progress related to a multichannel photonic alignment concept, aiming for sub-micrometer precision in the alignment of the waveguides of two photonic integrated circuits (PICs). The concept consists of two steps: chip-to-chip positioning and chip bonding provide a coarse alignment after which waveguide-to-waveguide positioning and fixing result in a fine alignment. For the waveguide-to-waveguide alignment, an alignment functionality is developed and integrated in one of the PICs, consisting of mechanically flexible waveguides and MEMS actuators. This paper reports on the fabrication and characterization of a mechanically flexible waveguide array that can be positioned by two out-of-plane actuators. Thermal actuators are integrated with mechanically flexible waveguide beams to enable positioning them with high precision. By adding a poly-Si pattern on top of SiO₂ beams, an out-of-plane bimorph actuator can be realized. An analytical model enables estimating the curvature and the deflection of a single bimorph beam. Acquiring a small initial deflection while having a large motion range of the actuator proves to have conflicting demands on the poly-Si/SiO₂ thickness ratio. In this paper, we show that suspended waveguide arrays with integrated alignment functionality have an initial deflection- they curl up- due to residual stress in the materials. The actuators can be operated using a driving voltage between 0V to 45V, corresponding to ∼50mW. Using higher voltages brings the risk of permanently changing the material properties of the heaters. The actuators can accomplish an out-of-plane crossbar translation up to 6.5 μm at ∼50mW as well as a rotation around the propagation direction of the light ranging from -0:1° to 0.1°. At a constant actuation power of ∼50mW, the crossbar shows a drift in vertical deflection of 0.16 μm over a time of 30 min.