 or heat shrink tech- 15.7.2 Submarine optical cables niques. Intermediate repeaters on long optical cable routes can readily be A second example of an optical fibre cable for special application is accommodated in the same design of underground housings used one for submarine system use. These are now in service in both for metallic cable systems. It is only necessary to design an appro- shallow waters and in trans-oceanic links. Consideration has to be priate pressure tight cable termination and tail cable which is spliced given to the high strains during installation and in any recovery and to the main cable. With repeater spans up to 30km it is feasible to repair operations. In shallow waters additional protection must be power feed these repeaters over 0.9mm copper conductors in the provided against damage by anchors and trawling operations. cable. Although still practical at longer spacings, the voltage drop An example of cable manufactured by STC is shown in Figure in the cable becomes increasingly significant and local power 15.11 and the optical characteristics are given in Appendix 15.4 sources, such as solar power can be considered as an alternative. (Mazda, 1989). An interesting feature of this cable is the copper tube around the fibres which is the system power feeding conductor and also a hydrogen barrier. 15.9 CCITT standardised optical fibre Hydrogen causes adverse changes to the attenuation of optical fibre (Barnes, 1985). This is of no consequence on land line cable, Probably the most important contribution of the CCITT to the rapid but of much greater significance in a deep sea cable because of the development of fibre optic technology has been the achievement of high pressure. Loss mechanisms arise from interstitially dissolved internationally accepted definitions and reference test methods hydrogen, hydroxyl formation, and the formation of defect centres. (RTM). The hydrogen can arise by galvanic corrosion, metallic outgassing, The recommended fibre characteristics in G.651 and G.652 are poly