f and less tightly bound than the lower order In a single mode fibre the core size is about one tenth of that of a modes. Minor perturbations in the fibre geometry causes coupling graded index fibre (5 micron) and obviously the practical problems between modes and hence replacement of those vulnerable to loss. of injecting light into the fibre, jointing fibres and connector design Microbending loss occurs when a fibre is in contact with a rough are more difficult. However, solutions are now well established and surface that imposes many very small bends on the fibre. Bending single mode fibres have become the standard for virtually all new loss can be reduced by designing the fibre to have a small mode field telecommunication work. diameter, but this conflicts with the need for a large spot size to In a single mode fibre, part of the power is carried in the fibre reduce splicing and connector difficulties. cladding, and is still guided. The concept of mode field diameter is Standard fibre has a mode field diameter between nine and ten therefore more useful than core diameter and can be defined as the micrometres. 15/12 Telecommunication cables For a typical telecommunication fibre, a flaw only one micrometre deep will cause failure at about 1% strain. Flaws arise from a number of sources during fabrication, for instance: imperfec- tions in the substrate tube, scratches introduced by handling, discon- tinuities during deposition, stresses introduced during collapse or pulling, impurities implanted from the furnace during pulling and foreign matter trapped during coating. Tests on a number of fibre lengths will give a range of breaking strains depending on the largest flaw in each length tested, and the longer the sample lengths, the greater probability of finding larger flaws and hence lower strengths would be recorded. Results must be treated statistically, for example as the cumulative failure prob- abilities a function of strain (Figure 15.4). Below the critical strain at which a flaw will cause immediate fract