High output power, high gain, and low noise are achieved in a two-stage optical amplifier, suitable for use as a repeater for a long haul lightwave communication system, in accordance with the principles of the invention, by employing a first amplifying stage having a signal gain sufficiently small to prevent self-saturation by amplified stimulated emission (ASE) that uses counter-propagating pump light to cause maximum inversion of the first stage amplifying medium. In an illustrative embodiment of the invention, EDFAs are used in each of two amplifying stages. The length of the EDFA in the first stage is short enough to ensure nearly complete inversion of the EDFA from pump light that counter-propagates with the signal. The counter-propagating pump light allows the invention to advantageously avoid the significant noise figure penalty from the input loss associated with co-propagating pump light. And, noise figure is improved becausecomplete inversion is achieved throughout the EDFA, and, at the input where the noise figure is most sensitive to inversion. The short length also eliminates self-saturation of the EDFA from ASE which degrades the noise figure. However, the length, and hence the gain, of the EDFA in the first stage is long enough to provide sufficient gain so that the noise figure of the two-stage amplifier, as a whole, is determined primarily by that of the first stage. A second EDFA in the second stage of the amplifier may then be configured using co-propagating or counter-propagating pump light for additional signal amplification to provide the required output power and gain for long haul lightwave systems. Other aspects of illustrative embodiments of the invention include the use of passive optical elements including filters, isolators, and attenuators. (FIG. 1)
HIGH POWER, HIGH GAIN, LOW NOISE, TWO-STAGE OPTICAL AMPLIFIERS