Fiber optic cable is the primary medium for high-speed backbones in building and campus area networks. Some forward-looking networks even provide fiber to the desktop, or at least fiber to the work area. This article identifies the types of test equipment that may be used to certify fiber optic cables, and a fiber test set that is often useful for quality control and fault location.

Measuring Loss Using an OLTS – Always Required

Read TIA-568B from cover to cover and you will find it requires you to make only one type of measurement, insertion loss, to certify a fiber optic cable. It should be noted that TIA-568B actually refers to the terminated fibers inside a cable as link segments (connection to connection) or links, which may comprise two link segments jumpered together at a cross-connect.

Also known as “dB loss”, “attenuation” or simply “loss”, insertion loss is singled out by the TIA because it can be impacted by poor installation practices. For example, a poorly polished connector can cause high loss, or a cable pulled around a corner that does not meet minimum bend radius specs may exhibit high loss, especially on single-mode fibers at 1550 nm.

TIA-568B specifies that you must measure loss using an optical power meter and the proper light source to certify an optical fiber cable. Multimode fiber loss measurements must be made using an LED source. Single-mode fiber loss measurements require a laser source.

Light sources and optical power meters are available as low-cost, stand-alone units (Figure 1), or they may be integrated into “smart” optical loss test sets (Figure 2), which offer additional features such as dual-fiber testing, length measurement and pass/fail analysis. Either type of tester will provide accurate standards-compatible results, providing you use proper reference setting and connector cleaning procedures.

TIA-568B also specifies maximum lengths for horizontal and backbone optical fiber cables, which you must verify as part of the certification process. However, TIA-568B does not require that you opticallymeasure cable length. You may, for example, use a tape measure or simply refer to length markings on the cable itself.

Bottom line: to certify an optical fiber cable you will need either an optical power meter and compatible light source(s), or an OLTS main-remote pair. Other test equipment may be very useful as discussed next, but nothing else is required.

OTDR – Optional But Often Useful

TIA-568B also contains component specifications including maximum loss values for connections, splices, and optical fiber segments. So in addition to end-to-end loss, many cable installers feel they should measure the loss of each connection and splice, and check cables for “macro-bends” and other defects, for quality assurance purposes. In premises network applications, the type of fiber tester normally used for these functions is an optical time domain reflectometer or OTDR.

OTDRs operate like radar. They generate short pulses of light and then sample the light backscattered by fiber segments and reflected by connections and other events. This allows the user (or OTDR event table software) to estimate the loss slope or “attenuation” of fiber segments and the insertion loss of individual connections and splices.

The setup used to measure the loss of an optical fiber cable with an OTDR is shown in Figure 3. To measure the loss of the first and last connection in each fiber link or link segment under test you must use a launch and receive cable respectively. In premises network applications, launch and receive cables, also known as “launch reels,” “pulse suppressors,” or “test cables,” typically provide 100 or more feet of fiber in a ring shaped or rectangular case, terminated by jumpers.

To illustrate the advantages of an OTDR, consider a 100 m (328 ft) backbone cable with the following component loss values:

Equipment room connection (1.2 dB)

Splice (0.1 dB)

Fiber (0.1 dB)

Telecom closet connection (0.3 dB)

According to TIA-568B, the maximum acceptable loss for this cable is 1.5 dB of connection loss (0.75 dB for each connection) plus 0.3 dB of splice loss and 0.1 dB of fiber loss (1 dB / km) for a total of 1.9 dB. Since this cable has an overall (end-to-end) insertion loss of 1.7 dB, it probably would be certified by an OLTS measurement. But as shown in Figure 3, an OTDR trace of this cable would reveal that the telecom closet connection has a loss of 1.2 dB, which exceeds the TIA-568B specified maximum of 0.75 dB. An OTDR can indicate and localize problems that would often be missed by an OLTS or optical power meter / light source kit.

And even in cases where an OLTS can detect a fault, for example the “infinite” loss caused by an open connection or fiber break, it cannot tell you where the fault is located. An OTDR trace, on the other hand, will locate such events, as illustrated in Figure 5.

With these many advantages you may ask is it necessary to measure cable loss using an OLTS (or power meter and light source) if you have already tested a fiber cable using an OTDR. The answer is a definite yes, for two reasons. First, the currently applicable standard (TIA 568B) requires this. Second, OTDR measurements can slightly under estimate loss, especially on multimode fibers, which may make you happy but will not leave your customer confident that all the cables you just installed meet TIA requirements.

Bottom line: No standard requires that you use an OTDR to certify fiber optic cables. In fact OTDR tests cannot be substituted for OLTS loss measurements. But OTDR tests in many cases offer additional information that can help you detect and pro-actively fix problems often missed by OLTS tests. And on some jobs you may be required to test all optical fiber cables using an OTDR by a quality-conscious customer who has received advice from a detail-oriented consultant.

Final Note – Tier 1 vs. Tier 2

You may have heard or read about Tier 1 and Tier 2 testing. These concepts are defined in another TIA document, TSB 140, written to clarify the requirements in several TIA standards including TIA-568B. Basically, loss measurements made using an OLTS or power meter/light source kit, and length verification by any means, are Tier 1 or required tests. OTDR tests are considered Tier 2 or optional.

All you need to remember is that post-installation loss testing with an OLTS main-remote pair, or an optical power meter and light source kit, is required to certify a fiber optic cable. Whether or not you also do OTDR testing is up to your schedule, budget, and often your customer.