Distortion characteristics of intensity modulated optical signals in semiconductor optical amplifiers are investigated. The intermodulation distortions (IMD's) due to the signal-induced carrier density modulation in semiconductor optical amplifiers for a subcarrier multiplexed (SCM) signal are calculated based on the model proposed by H. E. Lassen. As a result of the calculation it is found that the effect of the residual facet reflectivity of th optical amplifier on IMD is significant. At facet reflectivity of 0.1% the worst case carrier-to-second orther IMD ratio is 3 dB lower than that corresponding to 0.01% reflectivity for the case under consideration. The decrease in the worst case carrier-to-third order IMD ratio is 10 dB for the same change in the reflectivity. It is also confirmed by calculations that the IMD's decrease at subcarrier frequencies higher than the reciprocal of the carrier lifetime. In the case of two-stage cascaded optical amplifiers, it is found that the increase of IMD's due to the second amplifier is larger for the third-order IMD compared to the second-order one.
In the experiment employing an AlGaAs optical amplifer with a mean facet reflectivity of 3 % the ratio of carrier-to-second order harmonic distortion as a function of amplified output power has been measured. The result of the measurement confirms qualitatively the theoretical prediction
