The FSR of the microwave photonic filter is given by FSR ¼ 1=DL, which should be carefully
designed for the subcarrier de-multiplexing and band rejection. The number of the optical source is
also an important parameter for the design of the microwave photonic filter. The frequency
responses of the multiple-optical-source-based microwave photonic filter with different wavelengths
(2 to 5) are shown in Fig. 2(a), with the same amplitude for every wavelength and even spacing.
One can see that the microwave photonic filter with N optical sources will have N-2 sidelobes, and a
microwave photonic filter with more taps has narrower passband bandwidth, which is shown as the
solid circles in Fig. 2(b). The frequency response of the microwave photonic filter has periodic
peaks and nonperiodic valleys, and by carefully designing the dispersive medium, the subcarrier
with desirable frequency can be located at the peak of the filter, and the undesirable one can be
placed at the valley frequency. It can also be seen that, as the number of taps increases, the filter’s
main to secondary sidelobe ratio (MSSR) increases slightly, which is shown as the solid rectangle in
Fig. 2(b). The relative amplitude of optical sources also has influence on the frequency response of
the microwave photonic filter. For the microwave photonic filter with two taps (a notch filter), the
notch depth decreases when the amplitude difference between the two wavelengths increases,
which is shown as the triangle in Fig. 2(b). In this way, the amplitudes of two optical sources should
be made equal to get a larger notch depth.