An erbium doped fiber amplifier has a shape of the gain spectrum which is independent of the operating conditions by using a gain medium with this property. The gain medium can be an erbium-doped fiber with a wavelength-independent gain swing over a range of wavelengths (especially a low Al.sup.3+ -content aluminogermanosilicate EDF). As an alternative, the gain medium can consist of at least two pieces of EDFs, so that changes of the shape of the gain spectra (induced by changes of the operating conditions) in the different types of EDFs effectively offset each other. Within an overlapping wavelength range, the gain spectrum can then be further modified in a power-independent way (i.e.--independent of operating conditions) so as to make the gain spectrum flat (i.e.--gain wavelength-independent) by means of one or several optical filter(s) placed before and/or after the gain medium, and/or distributed along the gain medium, or by means of a "filtering EDF", specially selected in a constant population inversion region of the EDFA. The gain spectrum thus obtained is both flat and independent of operating conditions so that a dynamic gain flatness which is independent of input signal power, operating gain, and population inversion is realized. As a desired consequence of the dynamic gain flatness, the gain of different channels in a WDM system will always be equal, irrespective of the operating conditions. Furthermore, AC gain tilt will be eliminated for analog AM CATV systems.