EuroWire May 2020

Technical Article

operational temperature range is from -60°С to +85°С, for which a filled gel rated for -60°С is used.

OPGW cable with 48 Ultra-Low Loss Fibres

Mean value ~ 0,165 dB/km Max value ~ 0,168 dB/km

Temperature cycling test of OPGW cable with ultra-low loss fibres

3 Measurements set-up 3.1 Temperature cycling test

Attenuation dB/km

Temperature °C

Attenuation at 1550nm(dB/km)

▲ ▲ Figure 3 : Temperature cycling test of OPGW cable with ultra-low loss fibres

▲ ▲ Figure 4 : Attenuation distribution at 1,550nm of deployed cables with ultra-low loss fibres for three 150 km spans. Splice losses are included

For temperature measurements, we used a climatic chamber (shown in Figure 2 ) which can accommodate more than 1 km of OPGW cable. Thermal stability of the chamber was less than 1°C. Such a good thermal stability was made possible by appropriate choice of time of relaxation at each measurement point. Temperature range was from -40°C to +85°C for each cycle, as it was required by the customer and the application area environment. Three cycles have been carried out in this test. In our set-up, 48 ultra-low loss fibres were combined in two bundles. Fibres from each bundle were spliced together to provide better accuracy for OTDR measurements of attenuation changes during the test, and each bundle consisted of more than 20 different fibre samples. This set-up was used to accurately reflect real long-haul terrestrial cable deployment conditions that typically contain a large number of spliced cable sections. The two bundles were deliberately designed to represent the upper end of ultra-low loss distribution (first bundle average attenuation 0.169 dB/km at 1,550nm) and the typical attenuation portion (second bundle average attenuation 0.163 dB/ km at 1,550nm). This cable test procedure was carried out in full compliance with IEC 60794-1-22 Method F1. 3.2 OTDR measurements of the deployed cable OPGW cable deployed in the field represented a fusion-spliced concatenation of a number of cable lengths varying from 6 to 8 km, and the total cable length was approximately 500 km. Several OTDR devices with dynamic range of 45 dB were used for the measurements. ▼ ▼ Figure 5 : Attenuation distribution at 1,550nm for 500 km of deployed OPGW cables with ultra-low loss fibres. Splice losses are included

Reference attenuation dB/km

Max attenuation dB/km

Set of fibres

Delta dB/km Fibre specification* dB/km

1 2

0.169 0.163

0.174 0.168

0.005 0.005

<0.05

<0.05 *Note: fibre itself exceeds specification for wider temperature range (-60°C to +85°C) and in cable ▲ ▲ Table 1 : Attenuation increase at 1,550nm during temperature cycling test at temperature ranging from -40°C to +85°C

To achieve the desired accuracy and spatial resolution the transmission line was divided in three sections of approximately 150 km, and each section was measured separately from both ends by OTDR. Attenuation measurements of the installed OPGW cable in the field were done twice. The first measurement was carried out straight after the cable deployment. Subsequent OTDR measurements were performed approximately one year after the installation. 4 Results Figure 3 shows attenuation changes at 1,550nm for two fibre bundles during the temperature cycling test for all three cycles. During this test, no significant differences in attenuation performance were observed between the two fibre bundles. The maximum attenuation increase for both fibre sets during three cycles did not exceed 0.005 dB/ km, which is well within the desired limit of 0.01 dB/km (as noted earlier) and is far from the specification limit of 0.05 dB/ km [4] .

We believe that this measurement result indicates that the chosen OPGW cable design is suitable for ultra-long haul applications with strong optical budget limitations. Since temperature cycle test represents a good proxy for long-term cable performance in the field, we expect similar attenuation variations during cable lifetime. The summary of the data during the temperature cycle test is shown in Table 1 . Figure 4 shows the results of OTDR measurements of three spans of installed and spliced OPGW cable. Despite the fact that the cable was installed by several construction companies, and deployment, splicing and measurements were done by different technicians, we observed similar attenuation distribution for 48 cabled fibres for all three spans. The mean attenuation at 1,550nm wavelength was 0.165 dB/km, but only a few of the 48 fibres showed the maximum attenuation close to 0.168 dB/km. Then we calculated the attenuation distribution of installed cable for the entire 500 km transmission line by taking the weighted average of three previously measured cable sections. This data is presented in Figure 5 . One can see that the distribution becomes tighter as the number of concatenated lengths increases. The mean attenuation among all 48 fibres remains the same – around 0.165 dB/km – but the maximum attenuation decreased to 0.167 dB/km. Taking into account that ultra-low loss fibres with the mean attenuation of about 0.163 dB/km were used in OPGW cable

▼ ▼ Figure 6 : Attenuation change after one year of exploitation of OPGW cable with ultra-low loss fibres

500 OPGW cable line with 48 Ultra-Low Loss Fibres

Attenuation at1550nm(dB/km)

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May 2020