Fibres for optical communications

Abstract

The feasibility of operating a non-simultaneous, bidirectional, unrepeatered communications link over long lengths of optical fibre at a data rate of 1Mb/s is discussed. The link is intended for communicating between a stationary local terminal and a mobile remote terminal. A design study of the link has shown that the optical fibre is the most critical system component and that precise characterisation of the fibre is essential.

A 1Mb/s optical fibre data link has been constructed to test the validity of the design study, and has been successfully operated over a distance of 5 km with an error-rate of <10^-9. The optical receiver utilizes a silicon PIN photodiode feeding a low-noise preamplifier while the transmitter incorporates either a high-radiance LED or a cw semiconductor injection laser, generating half-width return-to-zero optical pulses at wavelengths of 0.9 µm and 0.85 µm respectively. Recently a distance of 10.5km has been achieved with a 25dB signal/noise ratio for the LED source. An analogue system incorporating a PIN detector and LED source has also been demonstrated. The signal/noise ratio is 21dB over 7km of low-loss fibre.

A new method has been invented, the near-field scanning technique, for providing a simple and rapid method for obtaining reproducible and detailed index-profile measurements. The index profile is determined by observation of the light intensity variation across the output face of a short length of fibre illuminated by an incoherent source. The profile obtained in this way may be suitable for many applications; however for accurate determinations it is necessary to take into account the presence of tunneling leaky modes which contribute additional power to the inferred refractive-index profile and cause a length-dependent error. A normalised correction factor is developed which may be applied to the measured intensity profile once the fibre length, core diameter and numerical aperture are known. The technique has been extensively investigated, both theoretically and experimentally, and the resolution appears to be good when scanning typical multimode fibres. It has allowed for the first time, observation of imperfections in the index profile right down to individual cvd layers.

A novel technique is also reported for the determination of the profile dispersion parameter P, by direct measurement on the fibre itself. The method is based on the fact that measurement of the wavelength dependence of the numerical aperture yields the difference in dispersion between the core and cladding glasses from which P may be calculated. Very little sample preparation is required and accurate experimental data has been obtained over the wavelength range 0.35 to 1.9 µm for germanosilicate, phosphosilicate and borosilicate glasses, whilst the range 0.35 to 1.2 µm has been covered for fluoro-silicate glass. For germanosilicate glass, P is found to be independent of both dopant concentration and thermal history. The results show that the exploitation of the longer-wavelength region will require either the design of fibres specifically for 1.3 µm operation, or the development of fibres which exhibit a low pulse dispersion over an extended spectral range.

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