What is claimed is: 1. A booster amplifier that amplifies
optical signals on channels at different wavelengths in a
wavelength-division-multiplexing optical communications link,
wherein the booster amplifier has a gain spectrum, the booster
amplifier comprising: a plurality of variable optical
attenuators, each of which receives optical signals associated
with a different one of the channels; a multiplexer that
combines the optical signals for each of the channels onto a
single path; at least one gain stage that amplifies the optical
signals on the single path and provides corresponding amplified
output signals for the wavelength-division-multiplexing optical
communications link, wherein the gain stage includes at least
one pump for optically pumping the gain stage; and a control
unit that controls the plurality of variable optical attenuators
and the pump to control the gain spectrum. 2. The booster
amplifier defined in claim 1 wherein the variable optical
attenuators and multiplexer are part of the same dev