GAIN AND NOISE IN RARE-EARTH-DOPED OPTICAL FIBERS

Two pumping schemes are considered for optical amplifiers based on rare-earth-doped optical fibers in which pump light is fed either along with the signal light (copropagating) or through the opposite end of the fiber (counterpropagating), including the effects of the excited-state absorption (ESA) process. A rate-equation analysis shows that when ESA is absent the signal gain is the same in both pumping schemes, even under gain saturation, whereas when ESA is present the saturated signal gain is larger in the counterpropagating scheme. Independently of ESA the optical signal-to-noise ratio at the amplifier output is the same in both pumping schemes if the dopant behaves as a four-level laser system, and it is larger in the copropagating scheme for a three-level dopant.