disclosed is a gain - controllable wideband optical fiber amplifier for amplifying both c - band and l - band optical signals , which comprises : a first amplifying section configured to be pumped in at least one direction , amplify both c - band and l - band optical signals and output an amplified spontaneous emission ; an optical attenuator for attenuating the power of the spontaneous emission ; and a second amplifying section pumped by the attenuated spontaneous emission to secondarily amplify the amplified l - band optical signals .