“…Extrinsic methods equalize the spectrum by making use of a filtering device. Several approaches have been studied to fabricate gain equalization filters, including thin film filters [96,97], long-period gratings [98,99], liquid crystals [100], FBG [101][102][103][104], silica wave-guide structure [22,23,105], Mach-Zehnder based filters [77,106], and acousto-optical filters [107][108][109]. Among these different technologies for spectrum equalization, the lattice filter has the advantage including the possibility of implementation in all-fiber structure, the available systematic design approach, and the potential of simultaneous multi-port spectrum equalizations.…”