Integrated silica-on-silicon resonant coupler filter

The “resonant coupler” (RC) add-drop filter is a 2x2 device that diverts one of N input wavelengths into one output while the remaining N-l wavelengths couple into the second output. Specifically at off-resonance wavelengths, light is transmitted in the “bar state” from the input to the corresponding output. However at the resonant wavelength coupling occurs, and light is transmitted from the input to the “cross state” located at the opposite output: hence the name resonant coupler. The RC functions like Bomholdt et. al.’s ‘meander coupler’ll], however our device requires only one mask level and has more accurate control of the resonant wavelength. An example of a RC is shown in Fig. 1. expanded 50x in the vertical. The RC consists of a series of parallel waveguide couplers connected by waveguides of unequal arm lengths. In the case of only two couplers, the device reduces to a MZI. Resonance is achieved by making the path length difference the same for all pairs of arms. Furthermore, the coupling strengths can be tapered to substantially suppress the passband sidelobes. To separate the waveguide coupling from the path length difference we have introduced identical curved paths in both waveguides in to and out of the couplers such that the path length difference, while the waveguides are coupled, is negligible.