Fibre Performance Testing

A number of factors add up to the total losses in a fibre. Connectors poorly installed and stretched or excessively bent cables have the largest effects and will account for most of the losses.

Since the connections have to be both physical and optical there are a number of lesser factors that can affect the performance of the finished installation.

• Signal Attenuation
• Acceptance Angle
• Numerical Aperture (NA)
• Dispersion, Modal and Chromatic

Attenuation

Attenuation is a measure of the signal loss.

Attenuation in the fibre itself is usually extremely low. However every time you have to make a connection or splice significant losses will be introduced into the cable, reducing the effective length that you can run.

Attenuation is measured in decibels or decibels per kilometer (dB or dB/km). 3dB represents a 50% signal loss.

Acceptance Angle

In multimode fibre light can be introduced at a variety of angles and still be transmitted. The greater this angle the less perfectly aligned the connection has to be. The price you pay is in an increased level of Modal Dispersion, see the section below.

Numerical Aperture (NA)

Numerical Aperture is a decimal value between 0 and 1 that represents the ability of the fibre to accept light over a range of acceptance angles.

Lower values indicate that a fibre accepts light over a narrow range of angles. Single mode fibres have low NA values, so the light has to be very focused and enter at a very shallow angle, while multimode fibres have higher NA values and can accept less focused light from a wider range of angles.

This property explains why narrow band laser sources are commonly required on single mode fibres, whereas LED sources are acceptable for multimode fibres.

Modal and Chromatic Dispersion

In multimode fibre the multiple paths allow the signals to travel different distances and so degrade the signal in a way analogous to delay skew in twisted pair copper. This effect is known as Model Dispersion and gets worse the greater the difference in entrance angle of the light beams.

A similar effect, known as Chromatic Dispersion occurs when different frequencies travel at different speeds through a fibre again spreading and degrading the signal.

On short runs, such as those used on LANs these effects are likely to be small.

Tools

It's pretty clear from the above that you will need specialist tools to check the installation and performance of your cabling.

As a minimum you will need a light source and a power meter, reference cables and adapters. The light source will inject a known quantity of light at a specified frequency into the fibre and the power meter will measure the quantity and quality of the light received.

It is normal to test cables in both directions. Many installers find it useful to work in pairs and to supply each worker with a test kit so that both ends can be tested quickly without having to go back and forth swapping meter and source.

In addition it may be useful to have a Visual Fault Locator (VFL). If there are any breaks in the cable visible red light will spill out.

If you install long cable runs an Optical Time Domain Reflectometer (OTDR) may be useful. This device sends light pulses down the fibre and measures reflection throughput the entire length of the fibre. By measuring the time differences a plot is produced showing attenuation, length and losses at splices and connectors.

Bear in mind though that these are very expensive and a major investment, and not normally required for testing LANs. Investing in an OTDR is only really justified if you are involved in long cable runs, in a LAN environment you are just as likely to see reflections from connectors that will confuse the readings. Money spent on a good power meter and a dual light source is likely to be a wiser investment.

You will also need splicing and terminating tools, cleaning materials, polishers, and a microscope to check connections.

Remember that reference cables, plugs and connectors have a finite life and need to be replaced after a number of uses. Follow the manufacturer's recommendations or the readings may not be accurate.

Tests

Most of the testing you are likely to have to do will involve Local Area Networks (LANs). Obviously you do not need to perform any electrical tests, but you do need to test optical power and signal loss.

It is wise to test the cable on the reel for continuity. If the cable has been damaged you may as well find out now before you install it. A VFL is very quick way to perform this test, all you need is to check that light can travel the full length of the cable.

Power measurement

For a practical system you need enough power to make things work, but not so much that it overloads the receiver and causes problems.

You need a Power Meter and Source (of the correct frequency) and these need to be matched to known reference cables.

Optical power is measured in decibels (dB).

Multimode networks normally need to be tested at 850nm and 1300nm, a dual source is a wise investment as it allows you to run the two tests automatically saving time.

Single mode installations are usually tested at 1300nm. If you know the fibre will be used with Wavelength Division Multiplexing (WDM) you should also test at 1550nm.

Loss tests

The principle of measuring losses is simple, it's the difference between what you inject into the cable at the transmitter and what arrives at the receiver. Remember that every splice and connector (and usually to a much lesser extent the cable itself) all contribute to the losses.

For this test you need to inject a measured amount of light from a calibrated source into the cable and measure how much reaches the other end.

It is usual for the connector on the light Source to be fixed, but the power meter can accept different connectors. Note that reference cables do not last forever and need to be replaced after a number of uses.

Always keep the ends of reference cables protected to prevent dirt and scratches. Any damage to the ends of the cables will cause errors in the results.

Power loss is measured in dBm or microwatts.

Your tests should mimic as closely as possible the technology that will be used on the customer's LAN. So for example if LAN applications will be using LED sources then your tests should also use an LED source, if the LAN uses laser sources then you should use the same in your tests.

Typical values for losses are between 0.5 and 0.75 dB per connector, 0.2 to 0.3 dB per splice.

Cable losses for Multimode are 3 to 3.75dB/Km at 850nm, and 1 to 1.5dB/Km at 1300nm.

Single mode cable losses are 0.4dB/Km at 1310nm and 0.3dB/Km at 1550nm. Check these values against the specification supplied by the manufacturer.

In order to measure the losses you need establish a baseline reference. Do this by attaching a launch cable between the source and the power meter. This measurement is your baseline. Using the same launch cable connect it to the cable you wish to measure and connect the power meter to the other end. Take a second reading. By comparing it to the first reading you can ascertain the loss. You should measure at 850nm and 1300nm for multimode systems.

The maximum loss permitted for the network and application is called the Optical Loss Budget (OLB). Your total losses should be less than the permitted maximum. To work out the total losses for a cable you need to add all the losses in connectors splices etc. Use the formula:

OLB = (number of connectors x connector loss) + (number of splices x splice loss) + (cable length x cable coefficient in dB/Km)

With some power meters you can save time by setting a zero loss reference with the launch cable attached, then all you have to do is connect to the cable to be tested and measure the loss in dB directly.

You can also perform a test with a receive cable attached at the other end of the cable run. This called a double ended loss test.

The tests you have to do will depend on the Standard you require for your network.

• TIA/EIA-455-171A FOTP-171 Single ended loss test Standard
• TIA-526-14-A - OFSTP-14 Double ended loss test Standard
• TIA-526-7 - OFSTP-7 Single mode cable Standard

Results

Many manufacturers of test equipment imply that all you have to do is press the button and accept the results as shown. However you need to be somewhat more careful in the real world. Are you testing against the correct standard for the cable and network? Even if the standard is fine are the parameters set at the correct level? If you happen to select a set of parameters that are not appropriate you may get a pass, or indeed a failure, where it is not warranted.

Interpretation of the results requires you to know what has actually been tested, only then can you decide to accept the results or perform a retest.

You should always record the results, either on paper or in the meter itself. These should become a standard part of your installation process and kept as a record of the job both for yourself and for your customer. Remember the quicker you can present a finished report the sooner you can present your bill so the additional cost of a meter with built-in memory may be quickly recouped.