Wavelength converter and methods are based on cross-gain modulation. The wavelength converter has two semiconductor optical amplifiers to perform double cross-gain modulations and double inversions. The first semiconductor optical amplifier performs a first cross-gain modulation on a first continuous wave light using the input light as a modulating light to generate an inverted first-stage cross-gain modulated light. The second semiconductor optical amplifier performs a second cross-gain modulation on a second continuous wave light using the first-stage cross-gain modulated light as a modulating light to generate a double-inverted output light. The output light is non-inverted with respect to the input light.