From here, the peaks are found using the MATLAB "findpeaks" routine.  The distance between peaks determines the FSR, which in turn can be used to find the group index.  Figure \ref{528835} shows the plot of the MZI function that has been fit to the measured data.  It can be determined the the group index of this MZI is 4.21.
Figures \ref{764326}-\ref{152499} show plots from other MZI devices, with \(\Delta\)L from 30 to 400 nm.