Abstract: An optical amplifier suitable for use in optical communication systems such as for telecommunications or cable television provides, for purposes of wavelength division multiplexed communications, the ability to amplify optical signals in both the C band, L band, and S band using conventional EDFA amplifiers. The S band, normally not amplifiable by EDFAs, here is amplified by converting the S band signal to a C band optical signal, providing optical amplification by an EDFA, then reconverting the amplified C band optical signal back to the S band in the same or a subsequent amplifier stage. The wavelength converters are, for instance, periodically poled lithium niobate devices using three-wave mixing, and employing difference frequency generation techniques.