Nonlinear distortion due to optical amplifiers in subcarrier-multiplexed lightwave communications systems

We show that an optically amplified, subcarrier-modulated lightwave signal suffers both a compression of the optical modulation indices and second-order nonlinear distortion (NLD). Expressions are derived which show that the NLD can seriously degrade performance, unless the lowest subcarrier frequency is larger than several gigahertz. System implications of these effects are discussed.