Control of slow light in a ring-out-ring structure

This paper proposes a ring-out-ring structure of coupled optical resonators to yield coupled-resonator-induced transparency （CRIT）. Considering the insertion loss of the coupler, it theoretically deduces the transmission and the effective phase shift. The influences of the insertion loss of the coupler on the transmittance, the effective phase shift, the group index and the CRIT linewidth are fully studied. We find that the increase in multiple m can effectively enhance the normal dispersion and the group index of the proposed structure. Moreover, the specific expression of the group index at resonance is theoretically deduced and discussed for the proposed structure with two rings. The result shows that the multiple m between the lengths of ring 1 and ring 2 can enhance the group index to m times that of the structure with two equal-sized rings at resonance. The control of slow light in the proposed structure is important for applications of highly sensitivity gyroscopes, optical delay lines and optical buffers, etc.