FEATURE FIBRE OPTIC NETWORKS
Accurate Testing of High-Speed Multimode Optical Networks An Eye for the Details By Robert Smith, Group Marketing Manager, FibreFab
The variability in test equipment launch casts doubt on the reliability of existing gigabit and 10 gigabit network certifications. We are concerned with 50/125 and 62.5/125 OM1, OM2, OM3 and OM4 optical fibres in high speed gigabit and 10 gigabit LANs. Current network standards such
Testing for multimode OTDR and Light Source and Power Meter (LSPM) Insertion Loss (IL) is often very inconsistent. There are three main reasons for this. OTDR optical loss measurements can vary significantly from one piece of equipment to
another, LSPM optical loss measurements can vary significantly from one set of equipment to another, and OTDR losses can vary significantly from losses measured by LSPM. In fact, differences of 50% or more between methods and equipment have been reported!
as the Gigabit Ethernet (1000BASE- SX and 10GBase –SR) 2002 specify tightened channel insertion losses over previous standards. Channel insertion losses in a network are now more critical than ever before. This means that the requirements for the testing of gigabit and 10 gigabit high speed LANs are becoming more critical as power margins become squeezed. Testing standards for high performance LANs are now specifying test equipment launch conditions. The variation in mode fill and Modal
Power Distribution (MPD), the degree to which power distribution across the fibre core optical power is distributed amongst the modes in a graded index multimode optical fibre core, have been identified as sources of inaccuracy and inconsistency. Different sources give different
degrees of core and mode filling. LEDs can give an overfilled launch which tends to lead to pessimistic ‘worse than expected’ test results, and underfilled launches such as those provided by VCSEL lasers can give overfilled ‘better than expected’ results. A pictorial representation of mode fill is given in Figure 1.
Our answer to this optical challenge
is the Optronics Encircled Flux Launch Controller (EFLC), a device that standardises the launch condition for current and future high speed LAN networks. The EFLC ensures that test equipment launches into 50/125 and 62.5/125 multimode optical fibres will fill the whole fibre core in a precise controlled manner without the problems associated with overfilled or underfilled launches.
Encircled Flux standards
Encircled Flux (EF) is a quantitative measure of the percentage of the total optical power radiating from the end of a multimode optical fibre as a function of fibre core radius. The IEEE specification specifies
the distribution of light energy across the core of a multimode fibre within concentric zones. For example, the 10 gigabit specification IEEE 802.3ae for 50/125 fibre specifies that less than 30% of the optical power must be radiated within a 4.5µm radius zone of the core, and that more than 86% of the optical power must be within a zone of 19µm radius. This is represented pictorially in the fibre end face Figure 2.
Figure 4 The Optronics EFLC allows a user
with an 850nm or 1300nm laser OTDR source or LED LSPM optical source to meet the Encircled Flux standard. This means that if you couple an existing OTDR or optical LSPM source to an Optronics EFLC you will achieve an Encircled Flux launch condition in accordance with the relevant international standards.
and 1300nm and 62.5/125 fibre at 850nm & 1300nm. The IEEE templates are applicable to near field (near fibre end) measurements. The method uses a video processing technique with data represented graphically. For channel losses, the IEC 61280-4-1 Ed. 2.0 and TIA-526-14-B “Multimode Cable Plant Attenuation Methods” technique is applicable using a channel loss template. The channel power loss distribution must fall within the control point markers as shown on the template in Figure 4.
[insert Figure 2] The compliant EF IEEE power
Figure 2
distribution is shown in Figure 3. Radiated power is shown in cumulative form on the Y axis with 1.0 representing 100% distributed power.
Figure 1 Figure 1 shows that neither the
‘overfilled’ launch condition that fills the core and cladding, nor the ‘underfilled’ launch condition where only the central portion of the fibre core is filled is ideal for testing accuracy or consistency.
18 NETCOMMS europe Volume I, Issue 6 2011 Encircled Flux Launch Controller Templates exist for 50/125 at 850nm
www.netcommseurope.com Figure 3
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