The development of practical traveling-wave optical amplifiers is expected to simplify wavelength-division-multiplexed systems since a single device can then amplify all channels simultaneously. Although this is an important potential application the interaction between multiple signals at different wavelengths in these devices is not currently well understood. One such interaction is four-wave mixing in which three tones interact through the third- order susceptibility to produce a fourth tone or intermodulation product. In a many-channel system, many such products may be generated within each channel, and system performance suffers. For tone separations less than the reciprocal of the carrier spontaneous lifetime, 1/τs, it is expected that the dominant third-order nonlinearity results from the periodic variation in the carrier density1 at the beat frequencies between pairs of tones δf=f1−f2. These fluctuations in carrier density cause gain and refractive-index fluctuations at δf that result in the generation of optical-frequency intermodulation products. We measured this intermodulation distortion (IMD), for the case in which two of the input tones are degenerate, by amplifying two tones with optical frequencies f1 and f2 and observing the tone generated at 2f2 − f1.
© 1988 Optical Society of AmericaPDF Article