Optically transparent wavelength converters, in which the output is a wavelength-converted replica of the input, may be required to improve performance and ease management in future "mixed-mode" wavelength division multiplexed networks. Four-wave mixing (FWM) in semiconductor optical amplifiers (SOA's) is an attractive optically transparent wavelength conversion technique because it allows pump tunability. So far, three schemes for polarization-insensitive FWM in SOA's have been demonstrated, using two copolarized pumps, two orthogonal pumps, and polarization diversity. This paper presents a comparison of their output signal-to-noise ratio, and hence their ultimate tunability. A simple analytical model for FWM in SOA's is used to predict that when each scheme has the same inputs and produces the same frequency shift, the polarization-diversity scheme has the highest output signal-to-noise ratio of the three schemes, and hence promises the greatest tunability.
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