Modal coupling in traveling-wave resonators

High-$Q$ traveling-wave-resonators can enter a regime in which even minute scattering amplitudes associated with either bulk or surface imperfections can drive the system into the so-called strong modal coupling regime. Resonators that enter this regime have their coupling properties radically altered and can mimic a narrowband reflector. We experimentally confirm recently predicted deviations from criticality in such strongly coupled systems. Observations of resonators that had $Q>{10}^8$ and modal coupling parameters as large as 30 were shown to reflect more than 94% of an incoming optical signal within a narrow bandwidth of 40 MHz.