Design and fabrication of a circular digital variable optical attenuator

The second generation circular digital variable optical attenuator (CDVOA) with an effective area of 1500 μm diameter has been designed and fabricated based on SOI technology. C-band incoming Gaussian light can be reflected to an outgoing fiber from a shiny circular area, which is divided into sectors that can be individually tilted and addressed electrostatically to achieve variable light attenuation. Using a delay mask process, each movable component i) has an underlying ridge frame to maintain flatness, ii) is suspended by two micro beams at a bridge structure that connects to a handle where aluminum electrode is located underneath, and iii) is separated by wall structures at the handle area to reduce crosstalk from adjacent electrodes. Critical fabrication processes including the mirror and chip release are performed using a HF vapor phase etcher. Fluidic pressure and chip-dicing shocks are avoided. Initial results show that a mirror sector suspended by two 345 μm long beams with a cross-section of about 5×5 μm ² can be tilted to 2.8° at about 18 V driving voltage. Initial interferometric measurement gives estimated individual mirror flatness after metallic reflective coating to be about λ/15. The assembled chips are ready for further testing and characterization.