The spectral index method (SIM) for analysis of the rib waveguide in a
ir is improved by including the singular behaviour of the principal fi
eld component in the vicinity of the reentrant corners, thereby giving
the singular SIM (SSIM). This generally improves the value of the mod
al propagation constant as compared with more exact solutions. The new
approach is successfully extended further to the rib waveguide couple
r.