ECOC 1975 - ECOC 2013

180
In the short to medium distances, ie up to several hundred metres
applications at the present time a choice has to be made between single
fibre and bundle systems. A single fibre cable would contain several
single fibre go and return paths and some spare fibres, indeed spatial
multiplexing could be used. The essential is one fibre per communication
channel. Some of the factors affecting the choice are:-
the choice of source, laser or LED; the ease of making connectors and
joints; the possibility of data highway components; the ease of
making a robust cable; there is some bandwidth reduction in bundles
due to optical path length variations in fibres; although the bundle
provides redundancy a component of attenuation is due to breakages.
The use of lasers implies pulse analogue modulation.
In order to launch SUfficient power lasers are usually used for single
fibres but Burrus type high radiance LEDs can launch 1 mW of optical
power into multimode fibres and this is adequate for many applications.
Baseband modulation can then be used. Up to now single fibres with lasers
have suffered from limited life but this problem is being solved.
Moderately good joints with 3 or 4 dB loss can readily be made in fibre
bundles with the fluctuation due to fibre position being removed by giant
fibre mixing sections, these can be conveniently mounted in the bulkhead
portion of the connector. Etched and fused low loss joints can also be
made with more difficulty. The alignment tolerance is related to the
bundle diameter and can be obtained satisfactorily with a conventional
connector. With a single fibre the mechanical problem can be solved for
splices but demountable connectors are more difficult since the tolerance
is related to the fibre core diameter and is perhaps 5 or 10 microns. The
cable problem is one of great difficulty. With bundles a bare or perhaps
Imbricated bundle of fibres is encased in a tube which provides tensile
strength and crushing resistance. There is however little scope for
mechanical insulation of fibres. In the single fibre cable each fibre can
be individually protected or cushioned and further strength members
added but the redundancy in the fibre bundle is lost.
Airborne military applications of optical fibres centre around several
systems. There are general data, weapon control and selection and
communication systems. The EM! and good EMC features together with the
lightweight small volume and freedom or earthing problems are significant.
The increasing use of hybrid materials in aircraft construction renders
the interference problem more difficult. There may be point to point links
or databusses, for the latter the star system described by F Thiel can be
discussed but the vulnerability to damage and flexibility of the solution
must be examined, the latter partiCUlarly since aircraft may be used in
more than one role. Short link lengths are envisaged up to 30 metres
with perhaps 6 breaks and the data rates of up to 20 megabits or greater.
Since tolerable system losses are in the range 40 to 70 dB medium loss
fibre (100 dB/Km) is adequate but connector losses are important.
Techniques for installation and repair are needed and the fibre system
including cable must satisfy full military specification. This is an
area where the bundle versus single fibre choice must be made.
In Naval applications there are a similar set of applications with a
greater volume of data. EMI, small volume and freedom in routing cables
is important. Ranges can now be up to 150 metres but moderately low loss
fibre will still be satisfactory. Bandwidths of 60 MHz for analogue
signals may be required in transmitting radar data but microprocessors
will find greater use and there will be a need to interconnect perhaps