Guiding light in bent waveguide superlattices with low crosstalk

Waveguide superlattices with a subwavelength pitch and low crosstalk can significantly increase the waveguide integration density and are beneficial for many chip-scale applications. Bending of such high-density waveguide superlattices is necessary for flexible signal routing. However, tight bending tends to induce high crosstalk between guided modes, as witnessed in multimode waveguide bends. Here we explore the mechanisms of light guiding and coupling in a subwavelength-pitch waveguide superlattice bend and analyze how bending further modifies the already “renormalized” parameters of superlattice modes via various physical effects. Particularly, bending can skew the phase mismatch in a waveguide superlattice, sometimes producing a near phase-matching condition and causing salient crosstalk spikes among non-first-nearest neighbors. Interestingly, a waveguide superlattice with less pristine phase mismatch may be more robust against such skew of phase mismatch and can suppress crosstalk spikes by ~10 dB. Bending with 5–15 μm radii and subwavelength pitches has been demonstrated with crosstalk lower than -19.5 dB. The scaling of the footprint of waveguide superlattice bending is analyzed, and significant footprint reduction can be achieved for chip-scale applications. The rationale for footprint reduction of superlattice bending under crosstalk constraint differs markedly from that of a single waveguide bend.