Spectral index method applied to rib and strip-loaded directional couplers

The spectral index method is applied to both rib and strip-loaded directional couplers, both of which are widely used in optoelectronics. Analytical transcendental equations are constructed in terms of the polarised propagation constant β for the fundamental symmetric and antisymmetric modes. Excellent agreement with large scale mainframe programs is achieved for the calculated coupling lengths.