Abstract: Taking the symmetric slab waveguide as an example,we show that radiation modes of a weakly guiding planar optical waveguide can be approximated by the free space modes,their field expression and normalization constants are simple and understandable physically and can be obtained directly without any calculation.By applying this approximation,the far field and radiation loss calculations caused by random wall imperfections have been significantly simplified.

Key words: slab waveguide, radiation modes