Leaky Modes Analysis in Very Deeply Etched Semiconductor Ridge Waveguides

Leaky modes in very deeply etched semiconductor ridge waveguides have been analyzed by the compact 2-D finite-difference time-domain method combined with the uniaxial anisotropic perfectly matched layer absorption boundary condition and the Pade?? approximation transform technique. Results show that instead of continuously dropping, the loss of these leaky modes tends to saturate when the etching depth under the waveguide core is very deep. Mode coupling between these leaky modes and the modes in the lower cladding slab waveguide has been used to explain this saturation behavior. It is also found that transverse-electric TE₂₀ modes can have a lower loss than TE₁₀ modes at specific ridge width in such deep ridge waveguides.