We report the results of an investigation of the noise figure and conversion efficiency of erbium-doped fiber amplifiers for unsaturated, moderately saturated and heavily saturated operation. Quantum-limited 3 dB noise figures result only for small signal operation of low gain amplifiers. For high gain, partially saturated amplifiers, useful as in-line repeaters, bi- directional pumping results in the best combination of noise performance and conversion efficiency while co-propagation of signal and pump produces the best noise performance.
The dependence of the output saturation power on length is determined for an aluminosilicate erbium-doped fiber amplifier. For low gain amplifiers the output saturation power is found to decrease with increasing length. But for amplifiers with small signal gains above about 20 dB the output saturation power increases with length. The self-saturation by amplified spontaneous emission of such high gain amplifiers is responsible for the observation of increased saturation output powers.
Erbium-doped fiber amplifiers (EDFA’s) operate in the 1.5?m wavelength telecommunications window and have achieved high gain, high output power and near ideal noise performance. The feasibility of using semiconductor laser diodes to pump EDFA’s insures that EDFA’s are practical devices which will have great impact on optical communications as power boosters, optical repeaters and optical preamplifiers.
Characteristics of erbium-doped fiber amplifiers relevant for applications in optical communications are reviewed. Special attention is given to the relationship of saturation power to pump power the dependence of saturation characteristics on host glass erbium-doped fiber amplifiers with reduced pump requirements and a comparison of pumping at 980 nm and 1. 48 tm.
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