Lateral leakage in symmetric SOI rib-type slot waveguides

We theoretically investigate the lateral leakage in fully symmetric horizontal rib-type slot waveguides caused by coupling between the TM-like slot mode and a TE slab mode. The leakage mechanism is described and the dependence on the geometry parameters is studied using the variational mode-matching method. In addition, the effective-index method to solve the transcendent eigenmode equation of rib-type slot waveguides is applied and compared with the aforementioned rigorous numerical method. We show that this semi-analytical approach yields results with sufficiently high accuracy. For design purposes the influence of structural deviations caused by the fabrication process is studied. We show that the usable range of the geometry parameters can be widely extended by exploiting leakage suppressing mechanisms.