Analysis and demonstration of coupling control in polymer microring resonators using photobleaching

We describe postfabrication trimming of coupling in both laterally and vertically coupled polymer microring resonators (MRRs), using photobleaching. For both cases, a tapered directional-coupler-based simple analytical model is developed to simulate the change in coupling due to a bleaching-induced decrease in refractive index. A tightly focused laser beam spot (a few kilowatts per square centimeter) is used to precisely bleach the coupling region alone. Coupling control is achieved for (1) high-Q passive rings by bleaching the vertically coupled chromophore-doped bus waveguide, and for (2) laterally coupled electro-optic ring modulators, by bleaching both the ring and the waveguide in the coupling region. The power coupling ratio (PCR) of an undercoupled high-Q MRR filter is reduced by 0.54 percentage points for the TE mode, causing the MRR finesse to increase from a value of 72 to 108. For a ring modulator, the PCR was increased by 3.5 percentage points for the TM mode, causing a 6 dB increase in extinction ratio, to achieve a final value of nearly 25 dB. Phase/group-delay characterization confirmed that the ring was trimmed toward critical coupling.