Design of a compact silicon-based slot–waveguide crossing

A design scheme for silicon-based slot-waveguide crossing using a slot-to-strip mode converter (at each port) and a strip-multimode-waveguide (SMW) crossing is proposed. The guided modes of the input slot-waveguide are first efficiently transformed into that of the single-mode strip waveguide by using the mode converter, and then enter into the SMW, where fields converge at the center of the intersection due to the multimode interference effect. Consequently, the size of the input beam is much smaller than the width of the SMW at the crossing center, leading to the significant reduction of the crosstalk (CT) and radiation loss. The numerical results show that a slot-waveguide crossing operating at a wavelength of 1.55 μm with the insertion loss (IL), CT, and reflection (RT) of 0.086, -35.58, and -27.51 dB, respectively, is achieved. Moreover, the IL, CT, and RT as functions of the structural parameters together with the operating wavelength are analyzed in detail by using a finite-difference time domain method, and their fabrication tolerances are presented. In addition, the evolution of the injected fields along the propagation distance through the slot-waveguide crossing is also demonstrated.