In principle this is a conventional two-tap delay line filter with a periodic notch response. You can see this in Fig. 1, where the signal is split by the PSD, delayed in the lower arm and summed with a coupler. The amount of time delay defines the free spectral range between the notches.

For the experimental setup and the measurement results the high birefringence fiber (Hi-Bi) generates the delay, because the propagation constants of the two orthogonally polarized modes are made quite different from each other.

Further information might be found here: Article Tunable single notch microwave photonic filter based on delay lines

Thanks Mr Stefan P for your care and guidance

I am highly obliged

Regards

This is a notch filter, different from bandpass filters with multiple pass-bands.