EoW November 2008

technical article

-60°C/+70°C/23°C two times while making loss measurements at 1,550nm after one hour at temperature through the cycles. Figure 4 shows typical results for samples of the new coating system versus samples of a typical commercial system. Both coating systems utilise coloured secondaries, but different formulations of secondary. The fibre specimens were chosen to match coating geometry, mode field diameter, and cut off wavelength. The two different coating systems both give good protection against the micro-bending stresses at 23°C. At -40°C the typical commercial primary is close to its T g but still provides good protection against micro-bending by stress relaxing in a reasonable time frame. Only a small added loss is seen at -40°C in the typical primary and none in the optimised primary fibre. At -60°C, the optimised primary is likewise close to its T g , with similar protection still provided, but the typical primary is now well below T g and the fibres show added loss. Desiring a more aggressive micro-bending environment, for the second method the IEC sandpaper drum test [7] was modified to provide a harsh micro-bending stress situation strong enough to affect single mode fibre even at room temperature. To do this, a 300mm diameter quartz drum was wrapped with adhesive backed, 40 grit sandpaper, creating a very rough surface around which a single layer of fibre was wound at 100 grams tension. Using matched fibre samples as with the basketweave/temperature cycling test, the 23°C attenuation was measured after winding. Then the drums were cycled to tempera- ture extremes, this time measuring attenuation at 1,550nm after one hour and again after four hours at temperature. The results are given in Figure 5 . The initial measurement at 23°C taken while the fibre was on the original spools shows similar loss of about 0.19 dB/km for these fibre specimens. After winding the drums, still at room temperature, the lower modulus of the optimised primary offers significantly better protection than the typical primary, with one third the added loss. Throughout the very demanding range of temperature and rough drum conditions, the optimised coating fibre shows much lower micro-bending response than the typical commercial system.

well beyond what is normally encountered in the field). After measuring the effect on attenuation at room temperature, the test structures can be temperature cycled to determine the additional loss induced by the temperature excursions. The first test is a basketweave/temperature cycling procedure. The sample fibre is wound at 50 grams tension on a 300mm diameter quartz cylinder with a 9mm ‘lay’. This creates numerous crossovers fibre-to-fibre in the course of winding 50 layers on the drum. The crossovers can cause added loss at room temperature if the fibre is sensitive enough, but normally little or no added loss is seen at this point. The drum with fibre on it is temperature cycled, in this experiment through -40°C/

With an eye to improving the low temperature protection against stress-induced micro-bending, the glass transition temperature is shifted more than 20°C lower than the typical coating described in Figure 2 . A much more rapid relaxation of stresses imposed during temperature excursions is to be expected. The results of tests designed to examine the micro-bending protection are shown in the next section. 3.2 Micro-bending sensitivity In order to establish a relative comparison of microbend sensitivity between the typical commercial primary coated fibre and fibre with the new coating system, two different methods were used for an evaluation. Both methods are designed to provide aggravated lateral stress conditions (where the second method actually goes

Figure 4 ▲ ▲ : Basketweave/temperature cycle results for typical commercial SM coating system (dashed) and the optimised coating system (solid)

Figure 5 ▲ ▲ : Sandpaper drum/temperature cycle results for typical commercial SM coating system (dashed) and the optimised coating system (solid)

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EuroWire – November 2008