If you are a happy owner of an OTDR, chances dictate you are aware that these little beauties require annual calibrations. These annual calibrations are generally required by job sites and ensure your equipment is testing according to factory specifications. Did you know that the standard annual calibration service may not be enough?
For so many OTDR users out there these units are being used and abused daily. What is the most abused part of these units? The connector… Now in a perfect world most professionals highly suggest you utilize a pulse box (or pulse suppressor/launch reel). Not only do these items allow you to test the first and last connector more accurately but they save the wear and tear on your OTDR ports. With that being said, of course we know many do not use these pulse boxes and plug directly into the fibers they are testing. Now there is one major thing we strive to teach our customers, and that is the number one enemy to the fiber optic industry is dirt/ dust/grime/debris.
This Happens Far More Than We Would Like
It’s shocking but literally specs of dust have brought down entire networks, cell towers, company buildings…you name it. Improperly handling fiber ends and connectors are generally where all the issues in the fiber optic world stem from. We have seen techs take a connector, breath on it and wipe it on their work pants. Then they plug that into a potentially million dollar system… it is in these moments that the fiber gods cringe… Now this happens far more than we would like to see, but it is what it is. But when this happens to OTDR ports it generally causes damage to the internal connector on the unit. That is something you cannot repair in the field. The bad news is, if you cause enough damage, your unit is down, including your current project and possibly income. Many of you during your career have probably seen an error message on your unit stating something along the lines of “High reflectance at OTDR port”. This is bad news. Generally this means either the internal ferrule inside the connector is extremely dirty or in the worst case, damaged. Plugging in dirty connectors is generally the main issue, however you can actually plug in the wrong style as well. If you have UPC style port but plug in an APC connector, this will probably damage the ferrule as well. Always watch out for color schemes. Generally blue is UPC and green is the APC style. Do Not mix them!
Below I want to show you some examples of different images of ferrule end faces. Including clean and proper, and the several most common issues you will see.
There Is More to This Most People Think
Now I would like to talk to you about what it takes to repair a connector (ferrule) on an OTDR. There is more to it than most think.
First the unit needs to be carefully and professionally opened up exposing the internal components. There you will find fiber which can be either jacketed or bare. The fiber connects the connector (ferrule) to other components deeper inside the unit and then to the board. We will not get too technical here but give more or less a general overview. Now if the ferrule is damaged and needs to be replaced, the connector is cut off and a new connector and ferrule is spliced on. If it is not entirely ruined, the end face can be repaired by beingre-polished. Now you may have sent in a unit for this service in the past and wondered why you are also charged for a calibration on top of the repair. The simple answer is: when you change the physical properties of either the end face or the length of fiber inside the unit you need to ensure the unit still runs at factory spec. You can think of a high end sports car, you wouldn’t just slap in high performance aftermarket parts and drive down the road. You need to tune it and make sure everything is running as it should.
Once the connector or ferrule is repaired or replaced a reputable calibration lab (such as FIS) will then test it using an interferometer. An interferometer is a device that can provide a profile of the end of the connector. It can verify the end radius and its conformance to standards. It can also profile the polished fiber end, show how well it fits the profile of the connector ferrule, protrusion of the fiber from the ferrule, quality of the polish and find the center of the radius of the curvature of the end of the polished fiber.
Then your unit is carefully put back together and then a general calibration is performed. Below is a good example of both the internal structure of an OTDR along with a sample image of an interferometer test.
I hope this information today provided you a general overview of connector/ferrule service along with why you always want to ensure your equipment and connectors are clean and well kept.
Image of Internal Structure & Components of an OTDR
Image of End Face Shown in an Interferometer Test (without measurements)