Non-linear optical phenomena are easily observable in high-power fibre-optic communication systems. The generation of new frequencies due to these phenomena may be used to design different applications. Four wave mixing, which is conventionally viewed as a deterrent to multi-channel wavelength-division-multiplexed propagation of signals in communication systems, can be used favourably to design optical sources with special characteristics such as multi-wavelength, frequency comb output and supercontinuum. These have applications in widely different fields like communication and medicine.