Networks Europe Jul-Aug 2018 | Page 38

FIBRE OPTIC CABLING

Fig . 2

Distance ( m ) Channel Loss ( dB ) Distance ( m ) Channel Loss ( dB ) Distance ( m ) Channel Loss ( dB ) OM3 1000 4.5 300 2.6 100 1.9 OM4 1100 4.8 400 2.9 150 1.5

connector . Note also that the connector alignment comes primarily from the two Pins set to either side of the row ( s ) of fibre . High-grade connectors will include high precision Pins that are chamfered towards the tip for easier lead-in alignment .

What needs to be considered is that uninformed procurement or specification could lead to the cables ordered and installed compromising the application , and your vendor may have provided a product that ’ s acceptable within general fibre optic connector guidelines ; and so starts the who is culpable discussion .

A low loss connector Starting with the connector itself , the variations between a low and standard loss is down to a number of subtle but very important differences . The first is the size , tolerance , concentricity and eccentricity of the fibre capillaries . The fibre connector ferrules are measured in mm , whereas the hole is measured in microns and tolerances of hole size as little as 0.01μm , that is 0.00001mm , so small deviations can make a very large difference . The concentricity is how central the hole is in relation to the connector ferrule and eccentricity is how elliptical the hole may be .

So , considering single-mode core size is around 9μm , it should be unsurprising that minor offsets can bring about major losses in a system . Generally , 1.25 / 2.5mm connectors such as LC and SC are mated via an in-line adapter , where the alignment sleeve needs to be as accurately tolerance as the connectors . Generally , ceramic-sleeved adapters provide more accuracy than those containing metal or polymer ones though are mostly reserved for single-mode adapters . The cost difference is almost negligible and worth requesting ceramic-sleeved adapters for multimode applications .

When introducing MPO / MTP technology , the added challenge is to ensure alignment of all fibres within the ferrule . The spacing between each hole from F1 and F2 through to F12 must be equal and within tolerances . For 24f connectors , two rows require alignment and so brings the X-axis and the Y-axis into play . Then , for single-mode connectors , an 8 ° angle is shaped onto the end face of the

The termination process Once the specification on the connector has been selected , it must then be terminated , polished and tested correctly to ensure the losses are kept to a minimal . The mechanical fit to the cable is vital to ensure the connector does not slip , rotate or crush the fibre within the cable . Most connectors are held in place with a metal crimp ring and the fibre held in place by way of engineering epoxy . The fibre must be allowed to move within the cable so as the spring action on the connector can operate during insertion . If not , it ’ s possible and likely that the fibre will break within the connector or micro-bending which causes higher losses . So , the correct cable and connector match along with the correct crimp sets and tooling is imperative .

Once terminated and the epoxy is cured , removal of the excess fibre is required and controlled best by laser cleaving . The polishing stage comes next by way of ultra-fine lapping films on high precision polishers . On completion , end-face geometry checks ensure a convex or dome shape has been applied to the fibre – if not , high insertion losses and back reflection is created as the fibre itself must make physical contact with the opposing mated connector . Visual inspection to 400x magnification comes next and finally , optical performance testing to qualify the overall performance grade .

The smart option When specifying for your network , insisting on low loss or elite grade is the smart option when planning for future upgrades . Getting to know the cabling system at the component level , and working with partners who understand connectivity pitfalls and importantly have the experience and capability to provide best in class network systems . n

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