Four-wave mixing (FWM) occurs when two or more frequencies of light propagate through an optical fibre together. Providing a condition known as phase matching is satisfied, light is generated at new frequencies using optical power from the original signals. Generation of light through four-wave mixing has serious implications for the rapidly expanding telecommunications field of wavelength division multiplexing (WDM). The generation of new frequencies from two or three input signals is shown schematically in the diagram below. The equation that defines the frequency of a FWM product is also shown.

WDM systems send data through an optical fibre using a number of channels (typically 16 or 32) - each with its own designated frequency. If two or more channels interact with each other through four-wave mixing, optical power will be generated at new frequencies at the cost of reduced power in the original channels. This power loss makes it more difficult to correctly detect the digital data in these channels - making errors more likely and reducing the information bandwidth of the system.

However, four-wave mixing is not just a detrimental effect. Possible applications for FWM in fibre include:

• High-speed, all-optical frequency conversion
• Dispersion compensation in long-haul optical fibre links
• Distributed measurements of dispersion and nonlinear parameters in fibres
• Millimetre wave generation for microcellular mobile communications

A report on four-wave mixing will soon be available that explains the theory behind the effect and why it is becoming so important to the optical fibre telecommunications industry. Possible applications of the effect in the field of measurements are also examined. The report is entitled "A Report on Four-Wave Mixing in Optical Fibre and its Metrological Applications".