Influence of the bus waveguide on the linear and nonlinear response of a taiji microresonator

We study the linear and nonlinear response of a unidirectional reflector where breaking Lorentz reciprocity is observed. The device under test consists racetrack microresonator, with an embedded S-shaped waveguide, coupled to external bus waveguide (BW). This geometry known as "taiji" microresonator (TJMR), allows selectively couple counter-propagating modes depending on propagation direction incident light and, at level, leads effective reciprocity. Here, we show that full description needs consider also role BW, which introduces (i) Fabry-Perot oscillations (FPOs) due reflections its facets, (ii) asymmetric losses, depend actual position TJMR. At sufficiently low powers loss does not affect behavior, but FP interference fringes can cancel effect waveguide. However, high input power, both FPOs contribute redistribution energy between clockwise counterclockwise within strongly modifies response, giving rise counter-intuitive features where, BW properties determine violation in particular, difference transmittance two directions excitation. experimental results are explained by using analytical model based transfer matrix approach, numerical finite-element exploiting intuitive diagrams.