Fiber Optic Measurement Instruments for Field Use in Local Area Network Cabling

29. octobre 2015

Today, a growing number of fiber optic cables are deployed in corporate networks. These networks are typically challenged with the need to provide high bit rates or large distances. While copper cables often carry very high shield currents due to isolation and grounding issues, this drawback is completely eliminated by fiber optic cabling.

However, special measurement instruments are required to verify if and how well the fiber optic network works and to certify that the installed cabling is able to meet the high bandwidth demands of practical operation. Since measurement instruments for fiber certification are often complicated to use and far more expensive than comparable measurement equipment for copper cable testing, this has so far been weighed against the deployment of fiber optic cabling solutions.

Meanwhile, a choice of handheld , cost-efficient measurement instruments is available in the market for testing fiber optic cabling links in the field. Offerings range from a simple Visual Fault Locator (VFL), emitting a visible light to visually locate gross cable faults, loss test kits and certifiers for Tier 1 measurements to the „king‘s class“, the Optical Time Domain Reflectometers (OTDR) for Tier 2.

On top of this, inspection scopes, or still better, video microscopes are needed to examine the sensitive connector end face for dust particles and other contaminations before mating the connectors.

The following article explains the basic requirements for deploying fiber optic cabling in Local Area Networks (LAN) and provides an overview of the measurement equipment needed to certify these networks.

The way structured building cabling systems, including fiber cabling, have to be configured, is defined by the European EN 50173 cabling standard and its international equivalent, the ISO/IEC 11801standard. These standards devide cabling systems into three subsystems: Campus backbone cabling to interconnect buildings, building backbone cabling (riser cabling) to connect the floors in a building, and horizontal cabling that extends to the individual work areas. In today‘s networks, copper cables are typically used for horizontal cabling, while fiber optic cables are predominantly used for the building backbone and the campus backbone. The available choice is multimode and singlemode fiber cables. Both types of fibers have an outer diameter of 125µm, however, differ greatly in the diameter of the light-conducting fiber core. The core diameter of a singlemode fiber is 8 to 9µm. This is in the order of the length of a light wave (approx. 1µm), whereas multimode fibers have a core diameter of 50µm in Europe and 62.5µm in the USA.

As active multimode equipment is considerably less expensive, multimode fiber cables are preferred over singlemode fiber cables for building cabling . Singlemode fiber cables are the primary choice for MAN, WAN, and for longhaul transmissions, such as transatlantic transmissions.

The drawbacks of multimode fibers as compared to singlemode fibers are the maximum length and bandwidth restrictions. This is not - as may be supposed - due to a higher level of attenuation, but due to the inherent nature of transmission light in these fibers. Unlike the propagation of light in a singlemode fiber, light travels along multiple paths, called modes, in a multimode fiber. Since these paths do not have the same length, parts of the light pulse arrive with a delay at the other end. As a result, the pulse is spread. This effect is referred to as modal dispersion. Dispersion depends on the qualtiy of the fiber and on the length of the cabling run: The longer the fiber run, the higher the level of dispersion and the lower the bandwidth that can be achieved. The EN 50173 standard specifies four different types of multimode fibers (OM1-OM4). The advanced OM4 multimode fiber exhibits a significantly lower attenuation than OM3 multimode fibers, especially, at 850nm and 1300nm wavelengths, thus enabling distances of up to 150m in channels faster than 40GBit/s.

Whilst dispersion is less caused by the installation process of the fiber cable, but rather by the manufacturing process, the installation process has a direct impact on link loss, and hence, on transmission quality. Link loss principally results from the attenuation in the fiber optic cable itself (dB/km), from the number of splices and connectors.

For this reason, standards specify loss budgets. To ensure proper data transmission, attenuation needs to stay within the limits of the loss budget. Excessive bending of a fiber optic cable adds to attenuation or will even damage the fiber. Therefore, it is indispensible to maintain and not to exceed the allowable bend radius during the installation of fiber cabling. Other causes for high attenuation can be dirty or poor quality connectors or splices, which may, among others, produce strong reflections of light that will impair data transmission later on.

After completion of the fiber plant installation, and for troubleshooting as well, it is recommended to test the fiber optic links with an OTDR (Optical Time Domain Reflectometer) according to ISO/IEC 14763-3. A short laser pulse of typically a few nanoseconds is injected into the fiber, and the intensity of the backscattered light as a function of the respective propagation delay is recorded by the optical detector. An OTDR works very much like a radar system. Just that it does not detect any airplanes, but locates reflective events on the link and determines the attenuation they cause by the intensity of the backscattered light. The intensity of the diffusely backscattered light also allows to determine the attenuation of the fiber itself.

The above mentioned FiberXpert OTDR 5000, for instance, can be used for both, multimode and singlemode fiber optic cables. Multimode fibers are measured at 850/1300nm wavelengths and singlemode fibers at 1310/1550nm wavelengths. A typical test run only takes a few seconds, depending on the type of test selected - whether measurements are performed at a single or at multiple wavelengths - and which averaging method is chosen. The device automatically detects all events present on a fiber link. Poor connectors, exceeded minimum bend radii are automatically detected and marked. Using a very short laser pulse, FiberXpert OTDR 5000 features a particularly short dead zone and enables the resolution of closely spaced events – i.e. directly neighboring events.

In addition, FiberXpert OTDR 5000 is equipped with a power meter for a more detailed analysis of the losses. And in case of a broken fiber, the fiber break can be made visible with the integrated Visual Fault Locator. For this purpose, a VFL injects a laser beam into the cable at around 630nm wavelength (deep red) that is visible to the eye and that is well visible through the coating at the very location of the fiber break.

Fiber connector end faces are highly susceptible to contamination and should be inspected each time that connectors are mated. FiberXpert OTDR 5000 is equipped with a USB port suitable for connecting a video microsccope to view and inspect the fiber surface of patch cords and in patch panels directly on the screen of the FiberXpert OTDR 5000. Microscope images can be saved for documentation purposes along with all the other measurement values. 

The eXport evaluation software allows to document and analyze test results and reports that are downloaded from FiberXpert OTDR 5000 and from WireXpert using a single management application.

In conclusion: Fiber optic networks are at the heart of company-wide networking and need to be sufficiently tested before commissioning. Today, cost-efficient handheld measurement instruments are available that support the full range of field testing required. However, it is only the OTDR measurement in combination with a loss test and a connector endface inspection that will provide the network operator with the necessary proof of performance of his fiber optic installation.

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