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Testing Fiber Attenuation
The most accurate way of measuring the fiber attenuation coefficient requires transmitting light of a known wavelength through the fiber and measuring the changes over distance. The conventional method, known as the cutback method, involves coupling fiber to the source and measuring the power out of the far end. The fiber is then cut near the source and power measured again.
For more accurate measurements, use mode conditioning on the fiber near the source. Multimode fiber needs careful conditioning with a mandrel wrap or other mode conditioner while singlemode fiber just needs one small loop (~2 inches or 50mm) to ensure the fiber has only one mode.
By knowing the power at the source and end of the fiber and the length of the fiber, its attenuation coefficient can be determined by calculating:
An alternative method of testing fiber, which may be easier in field measurements, involves using a fiber pigtail attached to the source for a launch cable. Then use a temporary mechanical splice on the other end to connect to the fiber to be tested. This is similar to the single-ended loss measurement of terminated cables, but uses the splice instead of connectors at the source end and a bare fiber adapter to connect the fiber to the power meter.
This method introduces more uncertainty in the measurement because of the loss of the splice coupled to the fiber under test, since it may not be easy to accurately calibrate the output power of the pigtail. The best method is to use a bare fiber adapter on the power meter to measure the output of the bare fiber, then attach the splice. Alternately, have the splice attached on the pigtail and couple a fiber to the pigtail with the splice and measure the power.
Remember that the splice requires a good cleave to get reliable measurements so a good fusion splicer cleaver is recommended. Also it may be necessary to renew the index-matching gel in the splice occasionally or replace the splice if it gets contaminated.
At the moment many OECD countries are concerned about the future of telecommunication networks as convergence and investment in next generation networks (NGN) begins. One of the important questions in this context is what role optical fibre networks will play. Fibre technology has been used in the backbones of telecommunications networks since the late 1970s. Developments in recent years have seen optical fibre communication technology being used closer to the end-user. Optical fibre technology is now the standard way to connect medium to large enterprises to telecommunications networks and to connect the last mile (referred to as the first mile) of cable and PSTN based TV, telephony and broadband networks to their backbones. From the core to the sub-loop, telecommunication networks are now based on fibre which, in this paper, will be referred to as fibre based networks. The term ëfibreí-based is therefore not limited to networks that roll out fibre to the ìlast inchî.
The paper is closely connected with ICCP papers on:
• Global opportunities for internet access developments (COM/DSTI/DCD(2007)3/FINAL), which goes into more detail on long haul, trans-oceanic networks.
• Public Rights of Way for Fibre Deployment to the Home (DSTI/ICCP/CISP(2007)5/FINAL) which goes much deeper into public rights of way.
• Convergence and Next Generation Networks (DSTI/ICCP/CISP(2007)2/FINAL).