Harmonic and phase distortion of analogue amplitude-modulated signals in bulk near travelling-wave semiconductor optical amplifiers

A theoretical model has been formulated using rate equations to analyze the response of a semiconductor optical amplifier to a single analogue amplitude-modulated input signal. The results are found to agree very closely with experimental results obtained from a bulk buried-heterostructure device, and show that signal distortion is significant even for input powers substantially below saturation. The level of distortion is strongly affected by both the saturation power of the device, and the Fabry-Perot resonance arising from residual facet reflectivity. On the basis of good agreement with experimental data, the theoretical model is used to assess a novel method of bias current feedback that reduces the effect of carrier density modulation by the modulation of the input signal, and greatly improves the harmonic performance of the device. Using this feedback method, the calculated value of signal phase distortion has been reduced by a factor of up to 40, and the harmonic distortion by up to 11 dB.