Split-beam Fourier filter and its application in a gain-flattened EDFA

Erbium-doped fiber amplifiers (EDFAs) used in long wavelength-division-multiplexed (WDM) systems will require flat gain over the wavelength range of the WDM channels,1 A recent theoretical study2 showed that a Mach-Zehnder filter characteristic with appropriate parameters gives good gain equalization in the 1540-1560-nm wavelength range. In this paper we describe a novel filter, called the split-beam Fourier filter, that has a Mach-Zehnder characteristic with an easily designed wavelength period that offers simplicity and extremely low excess loss as well as tunability of the center wavelength and extinction.