Time- and frequency-domain characterization of the modulated ASE noise in SOA-MZI wavelength converters

Wavelength conversion in an semiconductor optical amplifier Mach-Zehnder interferometer (SOA-MZI) structure is based on cross-phase modulation (XPM) between modulated and continuous-wave input signals. It is shown that for counterpropagating input signals in an all-active SOA-MZI wavelength converter, the amplified spontaneous emission (ASE) noise is intensity modulated due to gain saturation in the conversion arm of the interferometer and the output combiner. Similarly, the waveform for the wavelength-converted signal is determined by both XPM and self-gain modulation. Time- and frequency-domain measurements are used to characterize the properties of the modulated ASE noise, wavelength-converted signal, and total signal. The operating point of the wavelength converter can be set to optimize the waveform quality for either the wavelength-converted signal or the total signal, in which case the other signal is distorted due to the modulated ASE noise. Although an optical filter is not required at the output of the wavelength converter for counterpropagating input signals, in a wavelength-division-multiplexing application the output signal will eventually be filtered and the influence of the modulated ASE noise must be considered.