
waveguidewitha0.2-mm thick device layer with air cladding on top, a BOX layer of
0.75 mm is required to keep substrate leakage below 0.1 dB=cm for TE modes, while
the TM mode requires a 1.91-mm BOX layer thickness to achieve this loss. These
values must be additionally increased when lateral guiding is introduced.
Scattering losses can be evaluated by extending the treatment of Lacey and
Payne [21], which highlights the important role of both the RMS surface roughness
and the correlation length for perturbations. For example, for a symmetric slab
guide of thickness t and index n
1
embedded in a medium of index n
2
, one obtains
for a mode of propagation ...