Patent ID: 6891662

Claim:
An apparatus comprising: a first optical amplifier comprising a first fiber doped with a rare earth element and provided with first pumping light so that signal light is amplified as the signal light travels through the first fiber, the amplified signal light being output from an output end of the first fiber; and a second optical amplifier comprising a second fiber doped with a rare earth element and positioned so that the signal light output from the output end of the first fiber travels through the second fiber, the second fiber being provided with second pumping light so that the signal light is amplified as the signal light travels through the second fiber, wherein the second pumping light is forward pumping light, and the first fiber has a length so that a value is positive at the output end of the first fiber, said value being a difference obtained by subtracting a light power at the output end of signal light at a first wavelength in the case where the signal light at the first wavelength and a signal light at a second wavelength are multiplexed together and transmitted through the first and second fibers while a power of the first and second pumping light is constant, from a light power at the output end of the signal light at the first wavelength while the first wavelength, but not the second wavelength, is transmitted through the first and second fibers.