Mode conversion effects take place when a dielectric optical guide is bent and its curvature varies along the guide. This paper considers and compares such conversion losses of singly and doubly clad slabs (adopted as analogs of corresponding fibers) for single mode transmission. It is shown that the conversion loss of a W-type slab is much smaller than that of a singly clad slab, provided that the beamwidths of both guides are equal. This property is the consequence of the wider mode spacing of the W-guide.
Shojiro Kawakami, "Mode conversion losses of randomly bent, singly and doubly clad waveguides for single mode transmission," Appl. Opt. 15, 2778-2784 (1976)