 rather wide absorption bands of overtones of the 2800nm fun- mode strippers. damental which can typically be seen around 1400nm, 1250nm, A single mode guide will only be single mode above a certain 970nm and 750nm. In modern fibres the water content can be wavelength and below this value second order modes will also be reduced to less than 1 part in 108. propagated. The cut-off wavelength is usually in the range 1100- 2. Scattering loss. Rayleigh scattering of the light occurs within 1280nm. the molecules of glass material itself. This loss is independent Reference has already been made to dispersion shifted fibres in of the light intensity but varies inversely with the fourth power the earlier section on material dispersion. This involves increasing of wavelength. Hence the advantage of operating at longer the dopant concentrations to shift the naturally occurring zero value wavelengths. These loss mechanisms result in three operating at 1270nm in silica fibre to the lower loss window of 1550nm. transmission windows in the loss spectrum of silica optical However, this also serves to increase the loss at 1550nm. The effect fibre in the region of 800nm, 1300nm and 1500nm. Historically can be mitigated by changing the refractive index profile of the fibre 850nm was the first to be commercially exploited and 1550nm from a rectangular to a triangular shape. is the most recent. However, interest continues in the use of the Single mode fibre provides a low loss and high bandwidth trans- 850nm window for short distance applications because the mission bearer which is economic over a wide range of digital bit electro-optic devices are considerably cheaper. Most commer- rates using either laser or LED sources, and thus provides an cial activity is directed at 1300nm with 1550nm limited to operating administration with a "future-proof" investment. applications where very long spans are warranted, such as The low values of pulse dispersion make system planning a submerged cable systems. relatively straightforward task up t