There is another dispersion effect, which is called as waveguide dispersion. It is dependent on both waveguide geometry and light source wavelength. Frequency sweep from 1.5\(\mu m\) to 1.6\(\mu m\) gives us below shown chart. Curve-fitting of neff values into a second order polynomial gives us the formula: 
\(n_{eff}=\ 2.44-1.13\left(\lambda-1.55\right)-0.04\left(\lambda-1.55\right)^2\)