Structure-induced asymmetry between counterpropagating modes in three-port couplers

The continuing controversy regarding the validity of assuming different photon lifetimes for counterpropagating modes in whistle-geometry ring resonators has led us to re-examine the commonly held view that coupling coefficients between any two ports in Y-junction couplers (regardless of their symmetry) do not depend on the direction of propagation. Rigorous three-dimensional finite-difference time-domain simulations show that the common assumptions about the operation o three-port couplers are sometimes grossly incorrect, as they arise from an oversimplified analytical model. An explanation is given why our results do not violate the Helmholtz reciprocity principle.