ECOC 1975 - ECOC 2013

PRACTICAL DESIGN REQUIREMENTS FOR OPTICAL FIBRE TRANSllISSION SYSTEMS
R \v Berry and R C Hooper
INTRODUCTION
153
Advances in the various technologies contributing to designs of optical
fibre transmission systems have been very rapid, and show every sign of
continuing apace. In these circumstances, there are some difficulties in
producing realistic estimates of eventual system costs, particularly when
items such as testing and maintenance are to be included. The construction
of laboratory demonstration systems helps to some extent, in that
such systems indicate probable trends in development; but it is only by
working towards the design and installation of practicable field trial
systems that the remaining intangibles can be revealed and working
solutions found.
8.448 Mbit/s SYSTEM DESIGN
An initial 8.1+1+8 Mbit/s laboratory system 1 was constructed in June 1974,
using high-radiance light emitting diodes (l.e.d.), silicon avalanche
photo-diodes 2 (a.p.d.) and same high loss unprotected fibre with a
numerical aperture (n.a.) of 0.5. The system was originally designed
to meet a repeater section error-rate of 1 .6 in 10 10 (equivalent to 2
in 107 for a system of 2500 km length, with a nominal repeater spacing
of 2 km), and this requirement was met with an optical path loss of
37 dB per section.
The demonstration highlighted features which required significant
improvement for practical system use:- (a) compatible voltage supplies
and simple power feed arrangements, (b) an overall reduction in repeater
power consumption, (c) improved fibre-to-device couplings, (d) improved
fibre-to-fibre joints, (e) improved repeater input stage design, (f) a
compact repeater module design, (g) improved stability of the a.p.d. gain
against ambient temperature changes and (h) fibre of specified loss and
dispersion, with good dimensional tolerances, and \nth adequate protection
against mechanical damage.
Compatibility of voltage supplies has required the introduction of a
reliable dc-to-dc converter to provide bias for the a.p.d.; the version
in present use is coupled with a temperature compensation circuit, which
also provides voltage regulation, for the a.p.d. The complete unit takes
1.7 mA from a 6v supply, and can provide up to 90V bias for the a.p.d.
Firm proposals for power-feed arrangements must await the outcome of
further work, but we expect tO,achieve a 12V repeater, \dth a current
consumption (excluding supervisory circuits) of less than 100 mAo
A demountable fibre-to-device coupler using lenses has been developed.
The coupler has been shown to be adequately stable against mechanical
The authors are with the Post Office Research Department,
Dollis Hill, London.