Abstract

Erbium-doped fiber amplifiers (EDFAs) have recently been extensively studied for applications in long-distance transmission^{[1] [2]} and subscriber loop distribution systems^[3][4][5]. The noise figure of the fiber amplifier is a parameter that must be considered in these systems to optimize overall system performance. Noise figures of an EDFA pumped at 1480 nm and 980 nm were reported to be near 5 dB^[6], and 3 dB^[7], respectively. However, these numbers were measured when the amplifiers were operated in the linear region. An EDFA may be operated in the gain saturation region when it is used as a power amplifier in the subscriber loop^{[3] [5]}, or when it is one of the latter stages of many cascaded amplifiers in a transmission system. In the latter case, the amplifier gain saturates because of the accumulated amplified spontaneous emission (ASE) noise, which may be due to the finite passband of interstage optical filters, or because there is not enough inter-stage loss to compensate the amplifier gain. Under the above conditions, questions arise as to whether the amplifier noise figure increases while the gain is saturated, and what are the parameters that control this noise figure degradation. The subject of this paper is to examine these questions.

© 1990 Optical Society of America