The free spectral range (FSR) is the distance between adjacent peaks in the transferfunction.For a MZI the FSR is shown to be: \[FSR\ \left[m\right]\ =\ \frac{\lambda^2}{n_g\Delta L}\]
The Michelson interferometer has two passes so the FSR is below: \[FSR\ \left[m\right]\ =\ \frac{\lambda^2}{2n_g\Delta L}\]
The FSR for the different \(\Delta L\) values and interferometer types for all designs submitted for fabrication is shown below in table 1.