An optical source 10 comprising an optical output 12, a pump optical source 14, an optical splitter arranged to receive an optical signal from the pump optical source and to split the optical signal into a pump signal and a seed pump signal. A seed signal forming apparatus 18 is arranged to receive the seed pump signal at the pump wavelength and to transform the seed pump signal into a seed signal at a seed wavelength. A first microstructured optical fiber (MSF1) 20 is arranged to receive the pump signal and the seed signal. MSF1 is arranged to cause the pump signal to undergo four-wave mixing seeded by the seed signal on transmission through MSF1 such that a first optical signal at a signal wavelength and second optical signal at an idler wavelength are generated. One of the signal wavelength and the idler wavelength are the seed wavelength and one of the first and second optical signals are provided to the optical output.