Any competent installer that is familiar with the equipment and techniques required for the testing of copper local area network cables should, with the aid of the right test equipment, have little difficulty in successfully testing fibre cables. Unlike certifying copper cables which require parameters such as NEXT and ELFEXT to be measured for all pair combinations, the certifying of optical fibre in a LAN premises environment is relatively simple.

With fibre, these parameters don't exist and the main consideration is assessing the loss (attenuation) over the link and ensuring that this loss does not exceed the maximum level specified by the cable manufacturer and network designer, or that defined by the standard the link is to be certified to.

Fibre optic test equipment is therefore designed around testing the fibre link to determine its loss. There are a number of different approaches to obtaining a reliable and accurate results and each method has its strengths and weaknesses, which have to be balanced against the all important price tag.

Visual Fault Locator (VFL)
One of the simplest and lowest cost test instruments is the Visual Fault Locator. For example, Mills Ltd offer a professional quality unit designed to cope with the demands of regular use at a buy price of just over £200. These portable instruments are ideal for troubleshooting continuity faults in both single mode and multi-mode fibres especially at fibre launch points or in OTDR dead zones - (see explanation of 'dead zones' in the OTDR section).

The VFL comprises a visual laser source which is precision coupled into an optical fibre. This type of tester is ideal for determining the correct distribution of fibres at each end of a fibre link, and for quickly identifying poor quality or damaged splices and damaged duplex cables. Damage is identified by the visible red light shining through the splice protector or the jacket of the duplex cord used.

VFL's are relatively inexpensive and simple to use. However, the installer cannot solely rely on the use of a VFL as an adequate means of fibre link testing since they do not measure how much light is reaching the far end of the link or how much is being reflected back to the source.

So whilst the VFL is ideal for cable identification and checking the soundness of local terminations, it needs to be used be used in conjunction with a testing method that will determine the loss characteristics.

Optical Light Sources and Power Meter Test Sets
These portable battery operated devices comprise a light source, which is connected to one end of the link under test and a power meter which is connected to the other. By measuring the actual amount of light that reaches the far end of the link, the installer can quickly determine the performance of a fibre link. Where the link loss budget is known, the results of the test can be compared with the specification to determine a pass or fail result.

The test set enables potential problems to be quickly identified for example, a high loss measurement compared to the maximum allowable loss model (Optical Link Budget).

A budget of around £500 will get you a professional test set that will operate at 850/1300 for multi mode use, suitable for either ST or SC connectors.

These testers provide the actual loss of optical power through a link and will provide a pass/fail result where the optical budget is specified. However, where testing is dependant on the loss related to the link length it is necessary to also accurately determine the link length.

These test kits are available with many options, for example memory capacity, documentation and reporting capabilities, inbuilt automating link budget calculation and an important parameter for the certification of any optical fibre, integrated cable length measurement.

Certification Test Kit
These are basically an optical light source and meter test set with integrated link length measurement for certification of installations. For example, at just under £2,500 for a complete kit with quad wavelength light source, these units have an integrated wizard that provides a Pass/Fail result against the chosen standard. Individual link details and test results can be stored and downloaded into a PC to produce certification results.

Whilst these certification kits are more expensive they are ideal for installers that are regularly installing new fibre links. They are versatile and can be used to certify single mode and multimode fibre links using ST or SC style connectors. They support most popular cabling standards to provide a complete testing solution.

Adapting your Category 6 tester for fibre use
As an alternative to a dedicated fibre test kit, some manufacturers offer adapters to enable installers to convert their existing copper cat 6 certification kits to test and certify optical fibre. Whilst this may appear to be an attractive option, the addition of the fibre adapter set to a copper tester can be more expensive overall since the installer may have to purchase two sets of optical adapters to accommodate both multimode and single mode optical fibres. Dedicated fibre certification kits on the other hand are available in quad wavelength versions suitable for both single and multimode fibres.

A further consideration is that the addition of a dedicated fibre tester to the installer's range of test equipment will allow copper and fibre testing to be carried out on site simultaneously, speeding testing and reducing installation costs.

Ensuring accurate results
Whether the installer uses a dedicated set or an adapter for a cat 6 copper tester an important factor for delivering accurate results will be ensuring the equipment is well maintained, ie in calibration, and that launch leads have been referenced correctly. Reference point errors can be introduced by the installer referencing the launch leads at either end through a coupler, then using them to hook up to the fibre under test. The result of this is that one connection becomes invisible to the tester and the reported result will be incorrect.

The accuracy of test results from fibre optic light source and power meter kits can be improved by winding the launch lead around a suitable mandrel. This is necessary because most fibre optic light sources use a LED to launch light into the fibre, however the LED is very large in relation to the core of the fibre and light can be transmitted along the outer core and even the cladding of the cable. Because the length of the fibre used in the launch leads is usually very short this spurious light can reach the power meter giving an inaccurate reference point. By wrapping the fibre launch lead five times around a 22mm (for bare fibres) or 25mm (for jacketed fibres) mandrel, the unwanted light is removed and the reference light is concentrated in the fibre core, improving the accuracy of the results recorded.

In order to maintain complete accuracy, it is important that equipment is recalibrated annually and it should be returned to your dealer or agent for this purpose.

OTDRs
Light sources and power meters with inbuilt length capability and automated optical link budget (OLB) calculation are an excellent method of certifying new installation and verifying the capability of existing fibres, however what happens when the tester identifies a problem?

In the event of a damaged or broken fibre, the installer needs to be able to locate the problem area quickly and accurately. While VFLs and Light source/ Power meter kits will clearly identify that the link has a problem, they are ineffective at identifying the actual location of the fault.

OTDRs work like a radar, they send a light pulse down the link and measure the power of the reflected signal. By starting a clock at the same time as the pulse is sent, and determining the speed at which light will travel down the fibre (this is a known value based on the Grade of Refractive Index) or GRI, the OTDR will identify the exact location of the break that is causing the reflection. Other events which cause reflectance include bad connectors, splices or excessive bending of the fibre, all of which can be detected, measured and characterised by the OTDR.

Modern OTDRs are compact hand held devices with built in display which shows the trace result. By examining the peaks on the trace, the installer can identify any issues along the length of the cable. By determining the point at which a problem occurs, the installer can examine cables for damage or re-terminate any suspect connectors.

When choosing an OTDR it is important to select a unit with the right specification for the type of networks being installed. Typically local area networks are much shorter in length than telecoms links and an OTDR for LAN testing will launch a lower power light pulse into the cable. Even so, the OTDR will be unable to measure attenuation on very short links and cannot differentiate between closely spaced events such as connectors in patch panels. This part of the fibre link where the reflective event cannot be seen by the OTDR is called the 'dead zone'. Take care when comparing specifications of OTDRs. Don't be swayed by claims of a very high bandwidth as this would be achieved by using very long pulses which, in turn, lead to very long dead zones.

OTDRs are the most costly of the testers typically around £5000 for a well specified unit. Certification using an OTDR can be a little more complex as generally the OTDR does not automate the process of calculating the OLB and this task falls to the operator. The user first must know the standard that they are required to certify against, and know the limits for that standard at each required wavelength. After this manual calculation is made, the actual loss is then subtracted from the OLB enabling a "PASS" or "FAIL" result to be determined.

In this instance, certification documentation becomes another manual process as the OTDR will typically only report the trace result, link length overall loss and event loss.

Conclusion
The quality of a fibre network installation is determined by comparing the actual link loss with the budget provided by the cable manufacturer, network designer or standards such as ISO11801. For any link to "PASS" and thus achieve certification, the measured loss must be within the loss budget.

In summary, the low cost VFL is a useful quick check tool that is ideal for identifying cables and checking continuity. It is also ideal for detecting basic problems in fibre links such as poor connectors and splices at termination points and patch panels.

The OTDR is the most expensive fibre tester and is the best tool for accurate fault finding and characterising the cables signature. Whilst it can be used for the certification of links it does require more input from the operator.

For most installers, the simplest, quickest and most accurate method of certification for newly installed fibre networks is the optical loss test set with integrated link length measurement. These units are versatile and accurate. They can store link results, make calculations and download test results to PC's to provide a record and enable certificates to be printed.

This article was originally published in NCN magazine.


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