Minimum drop-loss design of microphotonic microring-resonator channel add-drop filters

Microring-resonator filters have important applications as filtering elements in microphotonic circuits. In this paper, we address the question of optimum design of resonator-based add-drop filters in the presence of finite losses, and show that symmetric coupling provides the optimum design. This conclusion contravenes previous work on this subject, and the oft-cited critically coupled resonator case. While the minimum bandwidth of a resonant filter is ultimately limited by intrinsic losses, i.e. the intrinsic Q, we show that the symmetric design can approach twice as narrow a linewidth as a critically coupled design for the same losses, in principle. We present a coupledmode theory (CMT) model, and a complete electromagnetic device design example based on finite-difference time-domain field simulations which validates our conclusions.