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Within the earlier weblog we noticed that bi-directional (bi-dir) OTDR testing gives an a variety of benefits and allows you to take care of points arising from variations between fibers being spliced collectively (particularly distinction in Modal Subject Diameter – MFD) that end in false positives or false negatives on an OTDR take a look at end result and certification fails.
When to carry out bi-dir testing or if it’s obligatory within the first place is a tough determination to make; fortunately there are business pointers that assist make that call for you. Adhering to the related fiber take a look at requirements is important and the one method to assure {that a} fiber hyperlink is as much as specification and can carry out as anticipated. Requirements our bodies similar to IEC and ITU-T, lay out precisely what checks must be carried out and element how they need to be carried out to accurately characterise each side and component of a fiber hyperlink.
That is what the requirements say concerning right analysis of splice loss and finish connectors:
Rec. ITU-T G.650.3 (08/2017)
“In sensible engineering, unidirectional OTDR take a look at outcomes can be utilized to roughly choose the splice high quality, however correct splice loss measurement should be primarily based on the bidirectional OTDR take a look at.”
Rec. IEC 61280-4-2:2014
“In an effort to precisely measure the primary and final connection for bi-directional averaging, one ought to hold the launch and tail cords of their preliminary measurement positions. Thus, the launch wire of the primary course turns into the tail wire of the wrong way. It will make sure that an identical optical fibres are mated in order that the consequences of mode discipline mismatch between the take a look at cords and cabling may be averaged out.”
So, one of many causes for finishing up bi-dir OTDR testing is so that you could get true loss measurements for splices. That is achieved by averaging the loss measurements taken in each instructions (described in ITU-T G.650.3) and permits the removing of any extreme mismatches between fiber sections spliced alongside a hyperlink. And as you see the ITU-T group describes this as a “should”.
And don’t overlook the extremities, i.e., the connectors on the ends, are simply as more likely to have points because the splices and fiber sections in a hyperlink; some argue that they’re the extra vulnerable to points. The very last thing you need is for a “licensed” hyperlink to have issues at turn-up after which discover out it was resulting from insufficient or incorrect take a look at implementation and a scenario that might have been averted.
To an OTDR, an unmated connector seems to be like a cable finish or a fiber break and is a extremely reflective component. It merely displays many of the gentle that hits it and due to that on an OTDR hint it seems to be like a giant spike. With this blast of mirrored gentle, it’s unimaginable for an OTDR to let you know something about it aside from it’s an unterminated connector or a break.
To measure and qualify a connector it should be mated with one other connector in order that when an OTDR take a look at is carried out gentle passes by means of the connector finish face into the tip face of the receiving mated connector and receiving fiber, offering an optical linkage that enables losses to be measured for the tip connector.
A fiber hyperlink that has been bi-dir OTDR examined however with out the usage of a obtain fiber means you’ll be able to carry out bi-directional evaluation for the hyperlink splices however solely uni-dir for the tip connectors (regardless that you could have shot the fiber in each instructions). This implies you haven’t totally bi-directionally licensed the hyperlink and as such it’s a non-standard certification.
And that is what the IEC customary is speaking about when it mentions the usage of launch and tail cords (launch and obtain fibers): principally that you ought to be utilizing each with a view to carry out the bi-dir OTDR evaluation of the tip connectors. And, critically, these take a look at cords have to remain linked the place they’re — no switching of the launch obtain fibers linked to the fiber below take a look at. VIAVI works with many service suppliers and darkish fiber suppliers to use finest practices and get the fitting outcomes, which requires bi-dir testing for brand spanking new construct or community expansions.
The takeaways listed here are that correct splice loss measurement should be primarily based on bi-directional OTDR testing, and at all times use launch/obtain fibers to ship certification that features the tip connectors to be totally compliant with the requirements.
To be taught extra, check out our Bi-directional OTDR Testing web page.
Dou
glas Clague is at the moment options advertising supervisor for fiber optic discipline options at VIAVI. Doug has over 20 years of expertise in take a look at and measurement with a major deal with fiber optics and cable applied sciences, supporting the telecommunications business. Previous to VIAVI, Doug held positions as manufacturing engineer, options engineer and enterprise improvement supervisor. Doug has participated on quite a few business panels round fiber and cable expertise tendencies. He attended Brunel College in London and graduated with an honors diploma in electrical and digital engineering.
