A fully transparent wavelength converter based on four wave mixing in a semiconductor optical amplifier (SOA) fiber ring laser is theoretically analyzed by using a lumped model.We find that an optimal noise figure could be achieved by adding an external gain in the ring cavity under certain limitation limitation conditions.Such a wavelengthconverter has a wavelength shiftof 13.8 nm rather than 6.9 nm with a BER below 10 -9 .