Tunability of polarization-insensitive wavelength converters based on four-wave mixing in semiconductor optical amplifiers

Optically transparent wavelength converters, in which the output is a wavel ength-converted replica of the input, may be required to improve performanc e and ease management in future "mixed-mode" wavelength division multiplexe d 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-i nsensitive FWM in SOA's have been demonstrated, using two copolarized pumps , two orthogonal pumps, and polarization diversity. This paper presents a c omparison of their output signal-to-noise ratio, and hence their ultimate t unability. A simple analytical model for FWM in SOA's is used to predict th at when each scheme has the same inputs and produces the same frequency shi ft, the polarization-diversity scheme has the highest output signal-to-nois e ratio of the three schemes, and hence promises the greatest tunability.