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This paper describes the investigation of the waveguide properties of a silicon-on-insulator (SOI)-based two-dimensional photonic-crystal slab. It is found that coupling between transverse-electric (TE)-like defect modes and transverse-magnetic (TM)-like slab modes occurs in some frequency range due to structural asymmetries in the vertical direction. This TE-TM coupling, together with the smaller refractive-index contrast between the slab and dielectric cladding, results in propagation losses for a line-defect waveguide in an SOI structure. The paper also presents optimization methods for obtaining a wide lossless propagation band using such a line defect.