Vertical couplers can serve as optical add/drop multiplexers. Consisting of two stacked proximal waveguides, they are simple in construction and can potentially be monolithically integrated with lasers, amplifiers, and photodetectors. Their parallel geometry has two disadvantages, however: uniform coupling produces high -9-dB sidelobes, and direct coupling to optical fibers is impossible. Scientists at the University of California (Santa Barbara and Santa Cruz, CA) have developed an alternate geometry that solves both these problems.
The device contains two vertically stacked waveguides that form an X with a crossing angle of 0.1° to 0.25° (see figure). It is constructed by fabricating each waveguide on a separate wafer, bonding the wafer faces together, and etching away one wafer substrate. Although both waveguides are of indium gallium arsenide phosphide, they have differing dimensions and quaternary compositions. The lower waveguide is 0.21 µm thick and has a bandgap at 1.4 µm, while the upper waveguide is 1 µm thick with a bandgap at 1.1 µm. Their propagation constants match at a particular wavelength, causing strong coupling only at that wavelength.
The crossed geometry naturally causes gradual coupling, reducing the sidelobe level to -26 dB. Coupling efficiency reaches 97% at a central wavelength of 1.56 µm and a 3-dB bandwidth of 6 nm. The divergence of the two waveguides allows fibers to be directly coupled to them. The device is polarization-dependent, with a 60-nm wavelength peak shift between orthogonal modes; coupling efficiency is strongly dependent on crossing angle. For more details, contact Bin Lin at email@example.com.