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Performance of high-concentration Er/sup 3+/-Yb/sup 3+/-codoped phosphate fiber amplifiers in a copumping configuration is presented. Gain, noise figure, and output signal saturation power of fiber amplifiers with different lengths are reported. From a 3.6-cm-long fiber, 18-dB internal gain, i.e., 5 dB/cm, for small signal input at 1535 nm, was achieved. To the best of our knowledge, this is the highest gain per unit length for erbium-doped fiber amplifiers ever reported.