AbstractIn this paper, the modified step segment method (MSSM) is proposed as the highly accurate procedure in the analysis of the diffusion‐type optical waveguides and its accuracy is discussed theoretically. An optical directional coupler with exponential refractive index distribution is treated for which the numerically determined guided modes are compared with the analytical ones. As a result, it is found that the effective refractive index determined by MSSM agrees with the theoretical one and the error is less than 10−6.It also is understood that the convergence of the method is much better than that of the step segment method (SSM). Comparison of the present method with the linear segment one (LSM) indicates that the overflow can be avoided together with a considerable shortening of the CPU time. Therefore, it is concluded that the present method is very effective in the analysis of the diffusion‐type optical waveguides with very high acc