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The gain dynamics in erbium-doped fibers (EDFs) with various numerical apertures (NAs) and cutoff wavelengths used in amplifiers employed in wavelength-division-multiplexing (WDM) networks is investigated by using the time-dependent amplifier model. The calculation shows that the transient response at a time immediately after some channels have been dropped depends on the cutoff wavelength, and the cutoff wavelength for the highest transient response is independent of NA. The calculation also shows that the transient response for a constant cutoff wavelength increases with an increase in NA. Experimental results for the transient response of EDFs with similar NAs and different cutoff wavelengths agree with the calculated results for various input signal powers, input signal power changes, and surviving channel wavelengths when there is no influence of neglection of amplified spontaneous emission and EDF background loss on the calculation.