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In recent years, researchers have discovered the phenomena of slow light and superluminal light in many mediums and
structures. Coupled Resonator Induced Transparency (CRIT) is used to explain this phenomena and resonators which are
capable of inducing CRIT can be widely used in fields like optical sensor, feedback cavity of laser, optical filter and so
on. In coupled resonant cavity, mutually independent resonant states interact by weak coupling effect between different
cavities, thus changing the characteristics of the whole resonant system and generating CRIT. Generally, this
transparency can be produced in single mode fiber (SMF) coupled resonant cavity in both dual-ring and multi-ring
coupled structure. Based on these two fundamental structures, in this paper we put forward two new structures which can
also generate CRIT: multimode fiber (MMF) ring resonator and polarization maintaining fiber (PMF) ring resonator.
Substantially, we change tandem resonant cavities into parallel resonant cavities and they have the same resonant effect.
A section of PMF or MMF equals to dual-ring coupled structure or multi-ring structure respectively. Our proposal is
verified theoretically and experimentally. Both simulation and experimental results show that the PMF ring resonator can
induce more stable and symmetric transparency than the MMF ring resonator. This phenomenon can be explained from
three aspects: modal energy coupling efficiency, initial phase of coupling modes and modal polarization states.
Additionally, we connect several sections of PMFs and MMFs in serial with a rotation angle of 45°between each other's
ends. This improvement is equivalent to increase the number of parallel resonant cavities in an exponential extent and
the interference effect of light from different cavities can change the characteristics of the output transparency. We also
verify this structure theoretically and experimentally.
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