Back Reflection Images courtesy of Atlantic-Cable.com
Lumps and Bumps One of the major design challenges for a modern DWDM fibre optic submarine cable system is ensuring that end to end transmission performance is the same for each wavelength. The attenuation of fibre differs slightly with each wavelength as does the gain of optical amplifiers and, due to manufacturing tolerances, no amplifier or piece of fibre is exactly the same as another. These small differences are cumulative along a system and so it becomes essential to equalise the discrepancies, either at the Terminal Station or, for trans-oceanic systems, in the submerged plant. This need for equalisation is not a modern requirement, during the Telephone Era (1950 -1986), wideband equalisation was standard practice and it was a far more complex task than today’s optical equalisation.
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To illustrate this, we will consider perhaps the most successful, in terms of sales, submarine telephone cable system ever and
by Stewart Ash
how its manufactures went about system equalisation. The system in question was the STC Submarine Systems (now AlcatelLucent Submarine Networks) 14Mhz System. The system design was based on a 1.47” (37.34mm) diameter, coaxial cable with a characteristic impedance of 54.4Ω; repeaters, to compensate for the signal loss in the cable, were spaced at regular intervals along the cable. Each repeater contained power separation filters to separate the transmission signals from the DC power and directional filters to divide the transmission path into Low Band (0.3 – 6.00MHz) and High Band (8.00 – 13.7MHz). The attenuation, or power loss, of a coaxial cable is proportional to the frequency of the signal transmitted; the 14MHz system was designed such that, for each repeater span, the cable had an attenuation of 45dB at the highest operating frequency of 13.7MHz, under sea bed conditions. This resulted in a nominal repeater spacing of 6.5nm (12.06km). Under sea bed conditions is an
important statement because, unlike fibre optic cable, the attenuation of coaxial cable changes with temperature and hydrostatic pressure. This meant that cable sections were manufactured to a specific length in order to have the correct attenuation in a particular location on the sea bed at a specific water depth and sea bottom temperature. The repeater amplifiers were designed to compensate for the loss of the cable plus the loss of the repeater filters. Of course, it was impossible to make the gain shape of the amplifiers exactly match these losses and, due to manufacturing tolerances, each repeater’s overall performance was unique. This meant that cumulative, frequency dependent differences developed, section by section, along the system. The repeater also introduced a secondary transmission problem. In order to joint the 1.47” (37.34mm) cable to the repeater, the cable had to be tapered down to a 0.310 (7.87mm) diameter tail cable that had a braided outer screen. This