Low-loss and compact

Opt. Express 23, 21334–21342 (2015)

Credit: OSA

Silicon nitride (Si₃N₄) has become a popular material for waveguides in photonic integrated circuits (PICs) because of its low optical propagation losses. Research is now investigating how to implement multilayer, 3D PICs based on Si₃N₄. To be practical, a Si₃N₄-based 3D optical chip should have low interlayer vertical coupling loss and low waveguide crossing loss. However, there is a trade-off between such losses — a larger gap reduces crossing loss whereas a smaller gap improves vertical transmission. Now, Kuanping Shang and colleagues from University of California, Davis in the USA report a design to help address this problem based on waveguides with a 200-nm-thick Si₃N₄ core and a 700 nm interlayer gap. The designed 3D platform offers 0.01 dB coupling loss from a proposed vertical coupler and multilayer crossing loss of 0.167 dB at a crossing angle of 90°. A small interlayer crosstalk of only −52 dB is measured. The researchers further demonstrate a dual-layer 27 × 32 port multilayer star coupler composed of multiple connected couplers, which is useful for 3D PICs. They say that their platform is compatible with active thermal tuning and can be used to make compact photonic devices such as arrayed waveguide gratings.