Distance on wavelengths between two consecutive peaks on graph called as "Free Spectral Range(FSR)" of interferometer and can be calculated mathematically using formula :
FSR = \(\delta\lambda = \frac{\lambda^2}{\delta L(n-\frac{dn}{d\lambda})} = \frac{\lambda^2}{\delta L n_g}\)
If FSR of interferometer can be obtained by optical analyser, than group index of waveguide can be derived from formula as :
\(n_g = \frac{\lambda^2}{\delta L*FSR}\) 
In already defined simulation configuration, FSR has been calculated as 5.3nm+-0.5nm from software internal functionality. Now, group index can be calculated as : 
\(n_g = \frac{1.55^2}{107.52^2 * 0.0053} \approx 3.92\) 
After reducing of path length difference nearly by half as 50\(\mu m\), simulation result is :