Tuning dispersion by controlling phase variation in a ring resonator coupled with Mach-Zehnder interferometer

Full text: The group velocity of light pulses 𝑣_𝑔 can be controlled in dedicated media through material dispersion, in which the group velocity may be lower than the light velocity in vacuum 𝑐 (slow light) or larger than 𝑐 (fast light) or negative (backwards light). Otherwise, tuning dispersion to control the group velocity can also be attained through structural dispersion such as optical ring resonators which are very dispersive around its resonant frequency. In a ring resonator there are two methods to control 𝑣_𝑔 : one is tuning coupled coefficient between ring resonator and straight optical waveguide, the other is introducing active gain to change propagation loss in the resonator. However, the coupled coefficient is determinated by processing technology and cannot be changed after fabricating. And introducing active gain such as doping rare-earth ions Er³⁺ into optical waveguide material is a complicated process. What's more, tuning range of the group velocity of light is limited by the luminescence capability of rare-earth ions in the optical waveguide material. In this paper, we propose a ring resonator coupled with Mach-Zehnder Interferometer (MZI) structure in which the dispersion is tunable through two phase shift variation points in the MZI's arm and into the ring resonator respectively. In this way, the group velocity of light into the ring resonator can be controlled easily into the "slow light", "fast light" and "backwards light" region and in this structure we can obtain positive and negative group velocity in larger range than a single ring resonator coupled with a straight optical waveguide structure. Moreover, the capability of tuning dispersion of the cascade of MZI-coupled resonator structure is analysed and compared with cascade of ring resonator structure. (author)