This guide, provided by Craig Buckingham from AFC Group, is aimed at those testing single-mode fibre that is used for long distance transmission links outside of the customer premises.

Polarisation Mode Dispersion (PMD) and Chromatic Dispersion (CD) become key limiting factors when trying to implement very high bitrate systems over extended distances – only by testing to these very specific parameters can system owners and operators be assured that their system will perform as expected.

Characterisation is a comprehensive suite of point-to-point physical layer optical tests that measures and determines the quality and potential transmission capability of a given optical fibre.

Fibre Link Characterisation (FLC) is the measurement of the fibre performance and the quality of any interconnections (splices, connectors…) between fibre sections, after the fibre link has been installed.

Network Characterisation (NC) provides the network baseline measurements before turning the transmission system on.

The Suite of Optical Tests
The recommended Fibre characterisation test suite is described in the ITU-T G.650.3 standard:

• Connector Inspection and cleaning.
• Insertion Loss at Multiple wavelengths (1310/1550/1625nm).
• Distance (fibre length).
• Connectors/splice loss.
• Connector Reflectance.
• Optical Return Loss (ORL).
• Polarisation Mode Dispersion (PMD).
• Chromatic Dispersion (CD).
• Attenuation Profile (AP) measurements.

Measurement units

• Loss is measured in decibel (dB) or dB per kilometre (dB/km).
• Splice and connector losses are measured in dB.
• Reflectance is measured in – dB. The greater the number the higher the reflection.
• Chromatic Dispersion (CD) is measured in picoseconds per nanometer (ps/nm) and ps/nm/km when normalized to the distance.
• Polarisation Mode Dispersion (PMD) is measured in picoseconds (ps) and picoseconds per square root kilometre (ps/√km) when normalised to the distance.

Connector Inspection
All connectors must be properly cleaned before use and cleanliness has to be verified.

Connectors are the only elements that can be easily disconnected and are subjected to dirt or scratches prior to reconnection, generating more than 80% of the link/system failures.

Inspection and cleaning procedure consist in verifying the connector surfaces (both sides of the connector), using a video inspection scope, and cleaning the connectors, with appropriate cleaning tools.

Insertion Loss (IL)
IL is the most important test to be performed, as each combination of transmitter/receiver has a power range limit.

IL Provides the most accurate end-to-end loss measurement of fibre optic link including end connectors.

An IL measurement requires a calibrated source and a power meter. The source sends a signal at a given power level, and the power meter reads the remaining power level at the far end of the link.

Optical Return Loss (ORL)
The optical return loss (ORL) represents the portion of light reflected back to the transmitter by the link components (connectors…) and the fibre itself.

It is the ratio between the transmitted power and the received power at the fibre origin.

The Optical Continuous Wave Reflectometry (OCWR) is the reference method for ORL testing (OTDR is an alternative one): A laser source and a power meter using the same test port are connected to the fibre under test.

OTDR
An Optical Time Domain Reflectometer (OTDR) operates as one-dimensional radar allowing for complete scan of the fibre from one end only. It provides the following information:

• Distance of fibre span.
• Total loss of fibre span.
• Location and loss measurement of all fusion splices.
• Location, loss, and reflectance measurement of all connector pairs and mechanical splices.
• Optical return loss of fibre span and total link.

The instrument sends out a laser light pulse and measures the round trip travel time of the laser to calculate distance. It also measures the power of the returned light (reflected or back scattered) to determine loss and reflectance. The loss and distance are then plotted on a graph known as a ‘trace’. In addition an OTDR can accurately pinpoint any artefact or ‘event’ on a fibre. The event can be characterised, measured and plotted.

Polarisation Mode Dispersion (PMD)
PMD introduces a differential time delay (mean Differential Group Delay) between signal components that are transmitted in two well-defined orthogonal polarisation states which may cause severe distortion in the optical receiver at the end of the fibre.

Chromatic Dispersion (CD)
Chromatic dispersion (CD) is the property that the effective Index of refraction of a medium (optical fibre) where propagates light, is colour-dependant, or varies as a function of wavelength., each wavelength travelling at different speeds in the medium

The CD of a given fibre is measured in ps/nm and represents the relative arrival delay (in ps) of two wavelength components separated by one nanometer (nm).

Attenuation Profile (AP)
Every fibre presents varying levels of attenuation across the transmission spectrum. The purpose of the AP measurement is to represent the attenuation as a function of the wavelength.

An AP test set consists of a broadband light source (transmitter) and a spectrum analyser (receiver). The broadband source sends light, with a given wavelength range, into the fibre under test, which is analysed by the spectrum analyser at the far end.

A reference measurement of the source and fibre jumpers is required prior to performing the measurements.

Measurement Report
Once each individual test has been performed, the fibre link is considered as characterised. The resulting data has to be analysed by the system engineers. The measurement report has to be produced and archived in the database.

Craig Buckingham RCDD has been involved with the ICT industry since beginning his career as Telecommunications Mechanic in the British Army 15 years ago. He has worked on all facets of the cabling industry including network configuration, cabling installation, design and consultation. Craig has been with AFC for 4 months now as their Technical Development Manager delivering technical Learning Sessions to Consultants and End Users alike.

Visit www.afcgroup.com.au.