Photonic crystal slab edge directional coupler for deflection sensing

The design, fabrication, and transmission measurements of a photonic crystal slab edge directional coupler (PCDC) for submicron deflection sensing is presented. The dielectric modes between two structurally isolated photonic crystal edges allows for a directional coupler to be formed with low insertion loss and reduced coupling distances. The output transmission from the coupler can be related to the distance between neighbouring PC edges, thereby allowing it to be used as an intensity-based optomechanical sensor. PCDC sensors were fabricated by selectively etching the buried oxide (BOX) of surface micromachined silicon-on-insulator wafer. Based on transmission measurements, the sensitivity to horizontal separation between the edges of a fabricated PCDC of length 24.3 µm was evaluated to be 1.6 %/nm at 1495 nm. The transmission sensitivity to vertical separation between the PCDC edges of length 12.6 µm was calculated to be 0.25 %/nm, when the PCDC edges were initially displaced vertically by a distance of 300 nm. The PCDC sensors demonstrated here are compatible with broadband sources and do not depend on BOX thickness, reducing the probability of stiction.