Testing And Monitoring Of Fibre-Optic Networks

By Deepshikha Shukla

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An optical network monitoring system accurately detects and locates fibre degradation, alerting operators and managers with the details of faults. Measurement at desired intervals allows network operators to assess long-term fibre performance and efficient asset management.

Transmission of radio frequency (RF) or direct current (DC) over optical-fibre offers immunity to electrical interference. A fibre-optic network test involves optical channel monitoring, bit error rate testing, protocol testing, signal loss determination and other aspects of network deployment and service assurance. Optical time-domain reflectometers (OTDRs), bit error rate testers, optical spectrum analysers and optical power meters are some of the commonly-used tools.

Testing is done to evaluate the performance of fibre-optic components, cables and systems. Components such as fibres, splices, connectors, LED or laser sources, detectors and receivers are being developed to test and confirm performance specifications.

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Designers of fibre-optic cable plants and networks depend on these specifications to determine if the network will work for the desired application.

Measurement devices for optical networks

Smaller and portable microscopes are used to test fibre cables easily. There are two types of microscopes, namely, optical and video. An optical microscope incorporates both an objective and an eyepiece lens to view the end-face directly through the device. A video microscopes utilises an optical probe and a display for viewing the probe’s image.

A probe can reach hard-to-access places due to its small size. A screen allows images to be expanded for easier identification of contaminants and damage. Because the end-face is viewed on a screen instead of directly, a probe minimises the chance of a harmful laser light reaching a person’s eye.

Fibre monitoring helps assess fibre-optic performance metrics over time through the use of software tools and devices that comprise an integrated fibre monitoring and management system. These elements collectively facilitate the detection of faults, attenuation, degradation or security intrusions, and alarm the system administrator in real time when threats to the fibre-optic network integrity occur.

Fibre-optic cables are routinely tested after installation, spliced and terminated to test for continuity and polarity, checked for end-to-end insertion loss and then troubleshooting is done in case of any problems. Testing fibre-optics requires tools and instruments that must be chosen as per the components or cable plants being tested.

Fibre-optic testers like verification testers, certification testers and advanced OTDR testers perform basic inspection, troubleshooting and analysis of an existing fibre-optic cabling. Some fibre-optic cable troubleshooting tools have built-in results storage and automatic wavelength synchronisation that can save time and prevent errors.

Automated SYSTIMAX fibre calculator by Fluke Networks
Fig. 1: Automated SYSTIMAX fibre calculator by Fluke Networks

Transmitter and receiver power in a network are measured to check whether the system is operating properly or not. Measuring power requires only a power meter, but the meter must be set to the proper range and wavelengths, matching the source being used. Power measured by the meter must be compared to the specified power for the system to make sure it does not exceed the specified limit.

Oscilloscopes such as Keysight’s Infiniium UXR series are used for high-speed serial and optical networking test, measurement and development. These have analogue bandwidth in the range of 13GHz to 110GHz and 10-bit vertical resolution. This ensures signal integrity for the superior effective number of bits, leading to faster compliance testing and measurement.

Optical network monitoring system

An optical network monitoring system accurately detects and locates fibre degradation, alerting operators and managers with the details of faults. Measurement at desired intervals allows network operators to assess long-term fibre performance and efficient asset management. It is designed to be easily integrated with Web-enabled applications, multiple APIs and domains architecture within an organisation.

With increase in data rates, the system becomes more complex and many factors can impair system performance. This makes testing more complicated. For a network that can support high data rate applications as well as extended wavelength range operation for dense wavelength division multiplexing and coarse wavelength division multiplexing systems, it is important to carry out full fibre characterisation.

Passive optical network (PON) monitoring gives operators the ability to track, test and troubleshoot the entire fibre network. PON monitoring saves substantial time to repair and capital expenses during initial fibre plant construction, network operations, maintenance, fibre-to-the-home (FTTH) network deployments for single-family dwellings and multiple-density units, and any necessary field services.

PON fibre monitoring uses very short pulse widths of light to receive backscatter and reflection signatures from the network. The shorter the pulse of light used, the higher the detection resolution becomes for probing the network.

Continuous trace monitoring rapidly identifies discrepancies from the reference trace and issues an alarm to dispatch the correct team to resolve the event, delivering maximum customer uptime. Reflectors can also be used to enhance visibility of optical network terminals under low-resolution or high-noise conditions.

The remote fibre monitoring system offered by M2 Optics ensures the performance and integrity of the optical fibre infrastructure. It can remotely monitor up to 16 individual fibre links per device with both dark and lit fibres. It is optimised for point-to-point fibre links up to 160 kilometres. It has integrated optical switching for continuous and/or on-demand link testing with an easy-to-deploy and easy-to-use setup. It can also be scaled up as the network grows.

OTDR display
Fig. 2: OTDR display (Credit: www.lanshack.com)

Optical time domain reflectometer

An OTDR utilises high-intensity laser light emitted at a pre-defined pulse interval through a connecting cable at one end of the fibre-optic cable run. It analyses the backscattered light returning to the source location. It is possible to take measurements over relatively long distances with a small amount of backscattered light using sensitive receivers and signal averaging.

Speed of the pulse can be calibrated as it passes down the fibre to create a display of the amount of backscattered light at any point in the fibre. Thus, an OTDR can measure time, calculate the pulse position in the fibre and correlate to what it sees in backscattered light with an actual location in the fibre.

Amount of power in the test pulse decreases in the cable plant under test, since it is attenuated as it passes along the fibre and suffers loss in connectors and splices. Thus, backscattered light reduces accordingly, producing a picture of the actual loss occurring in the fibre.

Slope of the fibre trace shows the attenuation coefficient (dB/km) of the fibre. Height of the peak indicates the amount of reflection at the connectors and splices. If peak has a flat top and tail on the far end, it indicates that the receiver is overloaded. Width of the peak signifies the distance resolution of the OTDR. An OTDR can also detect problems in the cable caused during installation. Avoid measuring loss in a section where the fibre looks non-linear at either end, especially near a reflective connector or splice.

Viavi Solutions’ OTU-5000 optical test unit combines OTDR and optical-switch technology to provide continuous monitoring of multiple fibres anywhere in the network. A single unit monitoring 72 fibres of 100km and more occupies only 1RU. It offers Web browser access, switch scalability up to 1080 ports, fault location detection and email notifications. It has dual power feeds, low power consumption and LAN-based firmware download features.

OTU-5000 optical test unit by Viavi Solutions
Fig. 3: OTU-5000 optical test unit by Viavi Solutions

Need for certification

Certification of fibre-optic cabling as per IEEE, TIA/EIA or ISO/IEC standards is necessary to ensure that the link will run the desired application. It includes two parts: basic test regimen and extended test regimen. Basic fibre optic cabling certification is performed with a power meter and light source or optical loss test set to ensure that the cable plant is within the loss budget before acceptance of the installation. And, extended fibre optic cabling certification and troubleshooting can be performed with an OTDR.

Insertion loss of an installed fibre-optic cable plant is the most important to measure. It is measured with a light source and power meter or optical loss test set, which is required by all international standards. Fibre-optic certification utilises an OTDR for loss testing to ensure the quality of individual components of the installed link. Fibre-optic cabling certification testing has become more important with an increase in network speeds, bandwidth demands, reduced distances and loss limitations.

Certification of fibre links requires the right testing equipment, and detailed knowledge of installation and application standards. A handheld fibre-optic certification tester, such as Fluke Networks’ CertiFiber Pro, can quickly and easily certify multi-mode and single-mode networks. It measures fibre length and optical loss on two fibres at two wavelengths. It can also compute optical loss budget, compare the results to the selected industry standards and provide an instant pass or fail indication. This fibre certifier’s test results can easily be saved and managed using such software as LinkWare (in case of CertiFiber Pro).


 

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