ABSTRACT
Ring resonator is a resonator that has two input and output ports. When a ring resonator is coupled to only one busline waveguide with input and output ports, the circuit is called “all-path filter.” Microring resonator is a waveguide type ring resonator circuit that has a very small radius in the order of several tens to hundreds micron of magnitude. Since the first proposal of optical waveguide ring resonator, it has been utilized as the cavity of ring laser and some sensor applications. However, the ring radius was in the order of millimeter, and so the free spectral range was too small to be used as the wavelength filter for dense wavelength-division-multiplexing systems. In 1997, an MIT group proposed the microring resonator for an add/drop filter, of which ring radius is in the order of several tens of the micron.
