Anomalous Optical Propagation and Potential Sensitivity Enhancement in a Micro-Coil Resonator Based on Microfiber

The resonance characteristics of the micro-coil resonator(MCR) with different turns and coupling conditions are presented, which show that the effective length of the MCR with small coupling coefficient at primary resonant is independent of the turns, and the resonance dips always appear in pairs corresponding to multiple critical-coupling because of the periodicity of coupling strength. However, complicated resonance is observed in an MCR with larger coupling coefficient and turns. The superluminal or subluminal propagation regime can be obtained respectively by adjusting the coupling condition, and the effect of loss on them in different coupling conditions is investigated. There is such a loss threshold in the critical-coupling and over-coupling conditions, near which the group refractive index abrupt changes between positive and negative extremum, and the threshold is increased by 10 5 orders of magnitude under over-coupling condition compared with critical-coupling. In view of the transmission near a resonant evolve not only with the detuning of transmission phase, but also drastically with the adjustment of coupling condition which will be modulated by the cladding refractive index and coupling length. Hence a potential sensitivity enhancement for biochemical and inertial sensing based on MCR is also investigated briefly.