In the European project MORPHIC we develop a platform for programmable silicon photonic circuits enabled by waveguide-integrated micro-electro-mechanical systems (MEMS). MEMS can add compact, and low-power phase shifters and couplers to an established silicon photonics platform with high-speed modulators and detectors. This MEMS technology is used for a new class of programmable photonic circuits, that can be reconfigured using electronics and software, consisting of large interconnected meshes of phase shifters and couplers. MORPHIC is also developing the packaging and driver electronics interfacing schemes for such large circuits, creating a supply chain for rapid prototyping new photonic chip concepts. These will be demonstrated in different applications, such as switching, beamforming and microwave photonics.
We present the design of a non-volatile, bistable silicon photonic MEMS switch. The switch architecture builds on our previously demonstrated silicon photonic MEMS switch unit cell, using vertically movable adiabatic couplers. We here propose to exploit compressive stress in the movable polysilicon waveguides in a controlled manner, to intentionally displace the movable waveguides out of plane upon release. We design the waveguide suspensions to achieve close alignment with the fixed bus waveguide in the ON state, and positioning of the movable waveguide far from the fixed waveguide in the OFF state. Both ON and OFF positions are stable mechanically, without the need for maintaining an actuation voltage. In order to actuate the movable waveguide, we design vertical comb drive actuators that allow to commutate between both stable ON and OFF positions. Finite Element simulations predict electrostatic switch actuation with less than 30 V for compressive stress typically accessible in deposited polysilicon thin films. We validate the bistability mechanism by comparison with a representative experimental demonstrator. The demonstrator consists of a structured 100 nm poly-Si layer, deposited by chemical vapor deposition onto a thermally oxidized (1 μm) silicon wafer, exhibiting a compressive intrinsic stress of 275 MPa. Upon direct writing laser based photolithography, etching and final HF vapor release, the suspended structures bend into either stable position, and we measure a total buckling amplitude of 800 nm, sufficient to entirely de-couple the waveguides optically in the OFF state.
We present a design for an on-chip MEMS-actuated Variable Optical Attenuator (VOA) based on Silicon Photonic MEMS technology. The VOA consists of 30 individual mechanically movable MEMS cantilevers, suspended over an integrated, 1 μm wide bus waveguide, each terminating with two optical attenuation bars. By exploiting the pull-in instability, electrostatic actuation allows to move the individual cantilevers into proximity of the waveguide, leading to scattering of the evanescent field and thus attenuation of the remaining optical power in the waveguide. Electrodes are placed below the cantilevers for electrostatic actuation. Mechanical stoppers are used to avoid contact between the cantilevers and the electrodes and to keep the bars at a precisely defined distance of 60 nm away from the bus waveguide. The attenuator provides nearly zero insertion loss in OFF state, while in ON state, the attenuation range is defined by the number of actuated digital attenuation cantilevers and can be adjusted in discrete increments of only 1.2 dB. Owing to the small size, fast microsecond scale response time can be achieved, and electrostatic MEMS actuation allows for broadband and low-power operation. Our design exhibits a compact footprint of 30 μm × 45 μm, attenuation from 0 dB to 36 dB, while keeping return loss below 27 dB. To the best of our knowledge, this is the first presentation of a design of a VOA in Silicon Photonic MEMS technology.