Where does OTDR testing fit?

In the last two tips of this series we have been discussing fiber transmission loss testing using a light source and a power meter. Another way of measuring fiber loss uses what is called an Optical Time Domain Reflectometer. An OTDR sends pulses of light into the fiber at one end and measures the reflections back from the fiber at the same end. The principle advantage of an OTDR is that since it measures the time taken for the light to reflect back, the OTDR can give the distance to the various sources of loss in the fiber. (For more information on OTDRs, please visit our web site). An OTDR is very useful for troubleshooting a fiber system, since it can locate a break or a high loss in the fiber.

So how useful is OTDR testing for a newly installed system? That depends. For a multi-kilometer link, with several splices or connectors in line, OTDR testing provides a very useful record of the initial state of the fiber system. If there is a problem later, you can take a new OTDR trace and compare it to the original trace. This can also be useful for monitoring the health of the fiber link and potentially detecting problems before they become major losses. Thus for any link that is not simply a single piece of fiber with connectors directly attached to the ends, OTDR testing is very useful.

But what if you have a short length with just one connector on each end? What useful information does an OTDR trace provide? Well, it will give you the length of the cable, although that is usually marked on the outside of the cable jacket in any case. It will give you the loss per kilometer of the fiber, but in most short distance systems that loss is not significant enough to be a concern. The most important point to know about the OTDR testing is that it will NOT give you a measurement of the connector loss at each end unless extra steps are taken. Let me explain.

The OTDR is a relative reading instrument - that is, the fiber loss is relative to the initial fiber reflected signal. If there is a connector in the middle of the fiber under test, the drop in signal represented by the connector loss can be measured. However, the initial drop at the front panel connector is not measured. Thus, in order to measure the initial connector loss properly, an extra jumper cable which normally needs to be at least about twenty meters long must be connected between the OTDR and the fiber system under test. Similarly, at the far end of the cable, unless there is a piece of fiber connected after the system under test then connector loss at the far end will not be measured. If these extra steps are not taken, then the only useful information that OTDR provides is the length of the cable.

Transmission loss testing is the most accurate way of measuring the actual loss of the fiber system using the method that is most similar to the way fiber will be used. Thus, transmission loss testing is mandatory for every fiber system. OTDR testing can provide useful information on complex systems, but for short distance link systems the OTDR test does not provide much useful information. In most cases you can save yourself the extra cost of OTDR testing by eliminating it from your testing procedure.

Note to installers: if your customer asks for OTDR testing, you may want to gently inquire as to what information they are looking for. Again here is an opportunity to demonstrate that you know what you are talking about. Explain to them the advantages and limitations of OTDR testing. And as for the loss testing, clearly diagram how you carried out the test so the customer can properly interpret the test results.

Next time: "Fiber cable types constructions - advantages and disadvantages".