Long span fiber amplifiers

We examine the theory of optical amplification in long span single-mode fibers which have low levels of erbium dopant. When pumped from the transmitter end at a suitable wavelength, lightly doped fibers can form a distributed amplifier, capable of overcoming transmission losses due to the intrinsic attenuation of the host glass at the signal wavelength. The entire communications link in effect becomes a fiber amplifier. We critically examine the potential of a long span erbium doped fiber amplifier and compare it to a system consisting of a short span power amplifier, followed by an undoped transmission fiber.