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