Optical fibres in telecommunication systems now carry more channels and higher optical powers than ever before. Systems are operating in which the fibre carries such a high optical power density that signals can modify the transmission properties of the fibre. An optical channel can then affect how it and other channels propagate through the fibre - leading to nonlinear effects. By the term nonlinear, we mean that the optical signal leaving the fibre at a given wavelength no longer increases linearly with the input power at that wavelength. Nonlinearity in optical fibre essentially leads to the conversion of power from one wavelength to another. The system implications of this wavelength conversion depend on the type of channel or channels used to carry the data.

Nonlinear effects have become much more important since the development of the optical fibre amplifier. These amplifiers can boost the power in a number of channels at different wavelengths simultaneously rather than having a separate repeater for each channel. This allows many more channels to be multiplexed into a single fibre than was economically viable with optoelectronic repeaters. Although the individual power in each channel may be below that needed to produce nonlinearities, the combined effects of all channels can quickly become significant. The combination of high total optical power and a large number of channels at closely spaced wavelengths is ideal for many kinds of nonlinear effects, including:

• Cross-Phase Modulation (XPM)
• Four-Wave Mixing (FWM)
• Stimulated Brillouin Scattering (SBS)
• Stimulated Raman Scattering (SRS)

As part of the BERR’s support for industry through the National Measurement System infrastructure, NPL is developing a range of measurement services to provide National Standards traceability for optical fibre nonlinearity. This work is being carried out by NPL in partnership with PK Technology Ltd. and The Femtosecond Optics Group, Imperial College.

Accurate determination of nonlinear fibre parameters will become increasingly important in the future to aid design of soliton systems and also to ensure that the detrimental effects of nonlinearities can be avoided. Specifically, services are currently being developed to measure:

• Nonlinear Coefficient
• Effective Area
• SBS and SRS Thresholds

A number of reports on nonlinear effects in fibres have been prepared and can be downloaded via the above links. For more information on the development of nonlinear fibre measurements at NPL, or to request a paper copy of any of the reports, please contact Andrew Deadman