A WIDE-FSR WAVE-GUIDE DOUBLE-RING RESONATOR FOR OPTICAL FDM TRANSMISSION-SYSTEMS

A wide free-spectral-range (FSR) waveguide double-ring resonator (DRR) is investigated for use in optical frequency-division-multiplexing (OFDM) transmission systems. It consists of two ring resonators with different radii and three directional couplers. The FSR of a DRR is determined by the least common multiple of the FSR's of the two ring resonators. The DRR configuration can have an expanded FSR as compared with single-ring resonators. It is especially useful for optical waveguide type devices, because the FSR is limited by the minimum radius which is determined by the bending loss of the waveguide. In order to determine the parameters of the DRR, transmission equations of the DRR, which have different radii, are derived by using the scattering matrix. The dependence of the transmittance characteristics of the DRR on the amplitude coupling coefficients of directional couplers and on the propagation loss of the ring waveguides are studied. In addition, the design guidelines for OFDM transmission systems are described. The DRR has been fabricated in silica-based waveguides with an FSR of 37.2 GHz and a finesse of higher than 182 in the 1.55-mu-m wavelength region. The 37.2-GHz FSR is roughly six times wider than that of the single-ring resonator with an allowable bending radius of 5 mm and the finesse of higher than 182 is the highest reported finesse value for waveguide ring resonators.