Fibre Optic Testing

Having had a premises fibre network installed and terminated within a commercial building, as the general contractor and/or building owner, you will want assurance that the cabling infrastructure has been properly installed. So, how do you tell a good fibre link from a bad one? Furthermore, what’s the definition of a good fibre? These questions are common in this industry. To help you answer these questions, BP Comms adhere to these standards.

Part 1 (General Requirements) of TIA/EIA-568-B.1 “Commercial Building Telecommunications Cabling Standard” rates the quality of a link based on several performance characteristics. This classification or definition of quality, based on the performance characteristics for horizontal, backbone, and centralized applications that are listed in the table below is widely accepted throughout the industry. BICSI has adopted this definition and references it in the organization’s Telecommunication Distribution Methods Manual (TDMM).

Cabling Application Link Loss Budget (dB) Wavelength (nm) Length (m)
Horizontal, MM ≤ 2.0 850 or 1300 ≤ 90
Backbone, MM ≤ 3.5/km + 0.75/conn + 0.3/spl 850 ≤ 2000
≤ 1.5/km + 0.75/conn + 0.3/spl 1300 ≤ 2000
Backbone, SM ≤ 1.0/km + 0.75/conn + 0.3/spl 1310 ≤ 3000
≤ 1.0/km + 0.75/conn + 0.3/spl 1550 ≤ 3000
Centralised, MM ≤ 3.3 850 or 1300 ≤ 300

Table 1

The fibre link test procedure consists of three steps:

  • Measure the optical link loss at the appropriate wavelength(s) as dictated by the application and fibre type.
  • Measure the link length.
  • Compare the results to the link loss budget, which represents the uppermost limit above which a fibre is considered no longer usable.

Test tools are different
Premises Local Area Network (LAN) networks differ from their long-haul Wide Area Network (WAN) cousins in several ways. First, multimode fibre dominates in premises applications. Second, premises link lengths are relatively short with few or no splices. Third, optical loss events are closely spaced and the number of terminated links to test is large. For example, it’s not unusual for the number of links to exceed 1,000, especially when the installation includes fibre in the horizontal to desktops. For these reasons, the tools you use to test LAN networks will be different from those you use to test WAN networks. But knowing which one to use and when can be a challenge if you don’t know what you’re looking for.

Attribute Visual Fault Locator Optical Loss Test Set OTDR Certification Test Set
Fibre Tracing Best Worse NA NA
Fault Location Better Worse Best Worse
End-to-End length NA NA Better Best
End-to-End link loss NA Best Better Best
Splice Loss NA NA Best NA
Reports NA Better Better Best
Cost Best Better Worse Better

Test tools for premises fibre testing
Whilst there is a choice of troubleshooting and testing tools available, certain types of fibre test equipment are better suited for premises testing.

Table 2 gives an overview of their strengths and weaknesses of four fibre testers for different applications..

Visual fault locator (VFL)
For visual verification and troubleshooting, a visual fibre tracer and a VFL are required. The latter uses a bright red laser that’s visible through some cable jackets to pinpoint the location of significant optical events like breaks in the fibre or loss caused by overly tight bends in the cable. However, the faults must occur where the cable is in the open and visual location is possible. You can also use VFLs to verify the integrity of mechanical splices and factory-polished/no-epoxy connectors during termination.

Optical loss test set (OLTS)
This test tool provides a quantitative optical loss measurement using a light source and an optical power meter. The tool directly measures loss by computing the difference between the optical power entering a fibre and the optical power exiting it. This test method can accurately predict actual network loss because it duplicates the transmission path and the wavelength of the active network. BP Comms use the advanced NOYES SMLP5-5 Multimode & Singlemode OLTS which includes a fully featured Optical Power Meter calibrated at four wavelengths & a stabilised Light Source containing a Class 1 laser source and Class 1 LED for both multimode and single-mode fibres. The meter is capable of storing results to speed testing and simplify reporting.

Optical time domain reflector (OTDR)
This advanced diagnostic tool for optical fibres allows the Fibre Engineer to take a snapshot of a fibre link. Like optical radar, the OTDR sends short pulses of light down one end of a fibre at a specified repetition rate. Light reflected back from fibre discontinuities and light continuously backscattered from the fibre itself travels back to the OTDR, where the instrument records the optical power and arrival time. The arrival time of the pulse from a given point in the fibre is related to its distance from the OTDR. With this information, the OTDR graphically displays returned power versus distance. OTDRs are well-equipped for troubleshooting problems because they allow you to visually locate reflective events like connections and fibre breaks and non-reflective events like splices and tight bends by studying the graphical “trace.” The power difference between two points on the trace is an estimate of optical loss.

OTDRA typical OTDR trace shows the general attenuation along a fibre and the reflections from various “”events’’. Eventually, either the end of the fibre or the OTDR’s inherent noise level prevents further reflections. There is one caveat here: the Fibre Engineer must understand OTDR behaviour when making measurements. It must be keep in mind that an OTDR is limited in its ability to resolve two closely spaced features by the width of the transmitted pulse. In other words, two closely spaced events may appear as one. OTDRs also have a dead-zone that limits their ability to locate events near the instrument, which can result in inaccurate length measurements, especially on short lengths found in the horizontal plant. Also, the reported end-to-end cable loss is an estimate based on the assumption that the fibre link’s backscatter level is homogenous over its length. This is often incorrect due to manufacturing variations or different fibres on each side of a feature. For this reason, an OTDR end-to-end link loss measurement often differs from that obtained by an OLTS.

BP Comms use the Anritsu 9076 series OTDRs for both Multimode & Singlemode fibre measurements.

Certification test set (CTS)
This test tool incorporates power meter technology for accuracy and standards-compliance with end-to-end length measurement capability and standards-based pass/fail analysis. A CTS consists of a main and remote unit, one for each end of the link. Each unit houses a power meter and a dual wavelength source. Most of the user interface is performed on the main unit, which also includes a nonvolatile memory and a PC communications port.

The CTS generally tests two fibres at two wavelengths in one operation. The tool measures the end-to-end length of each fibre using the zero dead-zone time-of-flight technique. It then compares each measurement against the user-selected standard for immediate pass/fail analysis.