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 .