GAIN SATURATION IN 3-LEVEL AND 4-LEVEL FIBER AMPLIFIERS

The results of a theoretical study of three- and four-level amplifiers using Er3+ and Nd3+-doped silica fibers are discussed. The analytical expression for the gain factor in case of gain saturation and arbitrary dopant distribution across the fiber waveguide are given. It is shown that in a three-level amplifier (Er3+) constancy of the gain factor with increasing signal power is provided by the accompanying increase in the pump power absorption. In a four-level amplifier (Nd3+) the flatness of the amplitude characteristic of gain can be achieved by a proper choice of the dopant distribution profile, for instance, in the shape of a ring.