Fibre Optic Cabling

Continuity Testing

A continuity check is one method for fibre optic testing and is done to ensure that the fibre has not been broken at any point, or to check that the fibre under test does indeed go from one location to the correct destination. Possibly the most frequently used method is to use a safe visible light source, particularly a safe high power visible red laser or LED. By injecting this light into one end of the fibre and checking that it is visible at the far end it is possible to determine that the fibre is indeed unbroken, or that the fibre under test does indeed go to the correct location. A decent light source will also highlight any fractured or kinked fibres with a visible glow, also fibres that are bent to tightly will also show light escaping. Visible light sources are sometimes not effective over long distances or with SingleMode cables, other methods for checking continuity under these circumstances is by using an infra red light source & power meter or an OTDR. A continuity check gives you a pass or fail result, either your fibre is unbroken between both ends (PASS) or it is broken between both ends in some way (FAIL). It is worth noting however that sometimes, where connectors etc get dirty, light may be visible but the performance of the cable may be degraded by the dirty connectors to such a degree that equipment may not run optimally over the fibres. Regular cleaning and checking of all fibre connectors is good practice, alternatively companies such as Abidale IT Solutions can provide a fibre optic system healthcheck.

Insertion Loss

Another method for fibre optic testing is insertion loss measurement. This measurement, typically gained using a stabilised light source and a power meter, will give an overall loss measurement of the fibre link including inherent fibre losses (per km), splices, kinks and bends. There are three methods or configurations for taking an insertion loss measurement that affect the readings you attain for losses associated with connectors.

Method 1 will measure losses associated with the connector pair at both ends of the link.

Method 2 will measure none of the terminating connector losses.

Method 3 will measure losses associated with one pair of connectors.

More detailed information on these three setups will be added to the site at a later date.

OTDR

The next method we will discuss for fibre optic testing involves the use of an OTDR; or Optical Time Domain Reflectometer. An OTDR is capable of measuring fibre lengths, losses, connector losses, splice losses and fibre defects. No device is required at the far end of the fibre to use an OTDR, results can be saved electronically to produce reports and enables accurate location of problems. On the downside OTDRs tend to be expensive, resolution is limited (due to length of light pulse for example) and the complexity of an OTDR means that for best results a user should undergo some form of training so that they may accurately interpret the results. The basic princIple of the OTDR is that it sends a pulse of light into the fibre, some of this light is reflected back and detected at the OTDR. The OTDR produces a trace on screen showing a line, or graph, when the light encounters certain anomalies - a splice for example - some light is reflected from the spliced joint, by measuring how much light is reflected the OTDR can determine the loss associated with each of these anomalies. A full description of the operation and capabilities of an OTDR is beyond the scope of this brief introduction to fibre testing, however we hope to publish a more informative and in depth article on OTDR in the near future. If you need your fibre systems testing using an OTDR but cannot justify the cost of purchase and the training of personnel then use a professional company such as Abidale IT Solutions who can carry out full system checks as required.

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