Hi Namera basically
Extintion ratio
In all telecom. systems (for any propagating wave) the extinction ratio is the ratio between “1” and “0” levels (peak and valley). There is the extinction ratio for two polarization components which is defined in the same manner.
In general, this ratio depends on modulation voltage (half-wave voltage) which in its turn depends on the materials you use and your regime parameters.
FSR
Free spectral range (FSR) is the spacing in optical frequency or wavelength between two successive reflected or transmitted optical intensity maxima or minima of an interferometer or diffractive optical element