Wednesday, November 1, 2017

The #2 Pencil of Fiber Testing

Don't Take Fiber Launch Boxes For Granted

How often were we reminded to use only a #2 pencil when taking a standardized test? How ridiculous did it seem that instructors thought we would use anything but a #2 pencil? Not only would we bring our #2 pencils, but we would make sure we had at least one fully sharpened backup. Inevitably, the first circle would spell doom for a weak tip no matter how well you thought it was sharpened.

Many of us can still recite the litany of requirements for test taking. We've heard them so often, they're etched into our minds and our culture.

When I relate this to fiber testing, one question lingers in my mind. Where is our instructor to remind us the importance of fiber launch boxes? I am shocked by the stories I hear from the field where OTDR tests are performed with faulty or even without launch boxes. Two incidents I recently heard dealing with separate service providers encouraged me to write this blog.

The first situation caused quite a lot of havoc with a supplier who provided passive splitter cabinets for a large FTTP deployment. Tests were showing significant loss through splitter ports, but the results were inconsistent. Some ports tested fine while others were failing. Those results would not automatically cause you to question the launch cable you're testing with, but the end result proved that the launch cable used was faulty. Before the launch cable was called into question, additional replacement cabinets were shipped, many hours of resources utilized, and construction was delayed.

Another similar situation occurred with fiber access terminals on a FTTP project. Some hardened ports on the terminals were failing, but not every port or every terminal. It is reasonable to assume the product may be at fault due to the inconsistency of the tests. How could a launch cable be the issue when it is passing through the other ports? The connectors may still pass, but they may pass with higher insertion loss levels or back reflectance than they actually have. In this particular case, the launch cable used did not have the appropriate connector to mate with the hardened ports, therefore additional loss occurred due to air gaps.

These scenarios deal with inspecting and cleaning connectors as well as using the proper connections to ensure the best results. Let's take a look at the role of launch cables as well as highlight suppliers that make the extra effort to provide a superior launch box.

The launch cable (aka pulse suppressor) covers what is known as the dead zone of an Optical Time Domain Reflectometer (OTDR). Due to the high power pulse coming from the OTDR, the initial pulse plus a few meters is the length of cable where events are not seen--hence the dead zone. Using a launch cable and a receive cable allows you to see the first and last connections of the fiber under test. The image below shows an OTDR trace with launch and receive cables attached.

How long does a launch cable need to be? That is determined by the maximum pulse width of the OTDR, and the cable should be longer than the pulse dead zone used for the tests. For instance, a 1 μs pulse is approximately 100 m long. Therefore selecting a 150 m - 300 m pulse suppressor box or launch test cable would be appropriate.

Several OTDR manufacturers claim to have pulsewidths today as small as 3 nanoseconds, which is great for added visibility in shorter access network links. First of all, buyer beware if you source an OTDR in search of the short pulsewidth. Tests have shown that several equipment manufacturers are closer to 5ns when the gear is on the lowest setting. Second, it becomes even more critical to use a long launch cable to ensure you can accurately test the leading connector in the short run.

Nothing more than a jumper in a rugged enclosure, fiber test cords are made by a multitude of suppliers with a wide array of optical fiber types inside. Simply saying you would like a "Singlemode launch cable with an SC-APC connector" does not mean that you'll get a G.652.D singlemode OS2 fiber in the box. The popular mini launch coils might feature a bend-insensitive G.657.A1 singlemode fiber, while your fiber under test is G.652.x.

The case of mixed singlemode types is very common, but it is important to be aware of the TIA-526-14B standard (IEC 61280-4-1). This standard dictates that if you cannot validate the fiber characteristics between the test cords and fiber under test are the same, you must do a bi-directional fiber test. Optical Loss Test Sets accomplish this task, but it is less common for OTDR testing.

In order to test to this standard properly with an OTDR, the test cords may not move. It requires 3 test cables: a launch fiber, loopback fiber and a receive fiber. Bi-directional tests average the absolute values in each direction over a single strand or fiber pair.  This approach simplifies the test because the OTDR operator stays at one end of the fiber under test, and only moves the OTDR from the launch cable to the receive fiber.

Based on these requirements, it is only logical to require a high quality test cord that is rugged and reliable. It's important to make sure the test cords are supplied with the highest quality factory polished and terminated connectors. Those connections will be used more than any jumper in the network, so let's make sure they have low insertion loss, return loss and are prepared for the abuse.

Over the years, Fiber Plus International became a name that our customers associated with high quality launch cables. Service providers offering the highest speed and bandwidth service recognized the consistently low reflectance and insertion loss values. Fiber Plus' Dead Zone Eliminators are available in traditional pelican cases as well as small launch coils. Fiber Plus also manufactures bare fiber adapters that make testing fiber reels and non-terminated fiber a snap.

Another supplier of distinction for test cords is Megladon. Their patented Hardened Lens Contact (HLC) process changes the physical properties of the glass at the connector endface. HLC creates a higher performance connector endface as well as a scratch resistant surface that significantly increases the number of insertions per connector. Every assembly that Megladon manufactures features the HLC technology, so you can insist on the highest quality jumpers and test cords.

No matter how durable the endface of your test cords and jumpers may be, you still need to inspect and clean them. Comstar Supply carries a wide array of hybrid and dry cleaning solutions to keep your test cables performing consistently and reliably.

So the next time you break out the OTDR or loss test set, don't forget you're #2 pencil! Make sure they are sharpened (clean), and make sure you have enough of them to complete your test without having to leave your desk.

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