TMOKE Coupled Waveguide System: A New Approach for Non-Reciprocal Transmission

Optical transmission non-reciprocity requires a propagation medium that simultaneously offers time and spatial symmetry breaking [1]. Several physical properties have been explored to realize such symmetry breakings in guiding structures, like magneto-optics (MO) [2], nonlinear optics [3], time modulation [4] or optomechanics [5]. Transverse Magneto-Optical Kerr Effect (TMOKE) has appeared as one of the most relevant non-reciprocal MO effect in waveguided configurations since it doesn't modify light polarization. However even in case of efficient MO materials, TMOKE remains a perturbative effect, i.e. a weak effect, which has to be enhanced using some complex or large footprint components. As a consequence, the trade-off between non-reciprocity performance, insertion losses, and operation bandwidth is difficult to achieve [2]. A recently proposed alternative solution is to exploit TMOKE as a perturbation of a coupled waveguides system [6]. In weak coupling regime, the TE coupled modes, i.e. the TE supermodes, can be expressed as a linear combination of two individual waveguide modes. By inserting MO material between both waveguides in TMOKE configuration, the induced perturbation in the coupled system leads to asynchronous coupling. In plasmonic slot waveguides, this results in the asymmetrization of the coupled modes [6], i.e. an asymmetric and opposite repartition of the energy carried by each supermode components. In consequence supermodes beating induces non-reciprocal transmission. In this paper, we will focus on the TMOKE-supermodes beating along propagation (Fig. 1). We will first propose an analytical model to describe the behaviour of the non-reciprocal beating. This model applies as well on TMOKE-plasmonic slot waveguides as on TMOKE-dielectric coupled system. Second, we will show that specific opto-geometrical parameters enable the design of performing non-reciprocal devices: we will present several isolator or circulator architectures based on such TMOKE coupled waveguides. As an example, the simulated device that includes SOI waveguides and Ce:YIG MO material in Fig. 1 has a footprint of ~550 μm and its 20 dB-isolation bandwidth is as large as 35 nm at a wavelength of 1.55μm.