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Reference Guide To Fiber Optics


Topic: Patchcords for Fiber Optic Cable Systems Table of Contents: The FOA Reference Guide To Fiber Optics


Patchcords in Fiber Optic Networks

     Patchcords are used to cross-connect installed cables and connect communications equipment to the cable plant. The choice of patchcords is very important to the performance of the network. Choose patchcords with these criteria:

  • Fiber type must match the type of fiber in the cable plant. For multimode cable, use only 50/125 patchcords with 50/125 fibers in cables and 62.5/125 patchcords with 62.5/125 fibers in cables. Although there are several types of each size fibers, matching the fiber type exactly is generally not required, e.g you can use OM3 patchcords on OM2 or OM4 cable plants. Singlemode fiber is mostly the same, but some long distance fibers have different mode field diameters and should use patchcords with matching fibers. Some patchcords are made from special bend-insensitive fibers. Although there has been some controversy about the compatibility of BI fibers to normal fibers, manufacturers seemed to have adapted their designs to ensure compatibility.
  • Choose patchcords with proper cable jacket color codes. This can help prevent mismatching fibers, especially multimode fibers. 
  • Choose patchcords with the correct matching connectors. Choose connector types that match the connectors on patch panels and equipment. Since many cable plants use one type of connector and the equipment another, e.g. STs on a multimode cable plant and LCs on the equipment, you may need to stock hybrid patch cables with different connectors on each end to make the connection. For singlemode networks, the connectors must also have the proper finished ferrule ends, either PC or ultra PC polish for most systems or angled PC (APC) connections for higher speed systems or those carrying AM video. Do not mix PC and APC connectors as that may damage them! The TIA 568 color code for connector bodies and/or boots is Beige for multimode fiber except aqua for laser-optimized fiber, Blue for singlemode fiber, and Green for APC (angled PC) connectors.



Patch panel with fiber optic patchcords

     The performance of a fiber optic network (and copper networks too) can be undermined the performance of the patchcords used for interconnects and connecting communications equipment. But it's easy to take patchcords for granted. They are inexpensive, often not tested properly if at all, and sometimes treated badly.
     Most users buy patchcords in bulk and store them in boxes until they are needed, when they are unpackaged and plugged in. Some users hang them on the sides of the equipment racks. That's not how they should be handled.
     When it comes to patchcords, there are three issues: quality, cleanliness and condition. You might assume that the patchcords you buy are good quality. They come in plastic bags usually with a label that identifies the fiber and connector type, length and test results. We’ve seen cheap imported patchcords with connectors with locking mechanisms that would not connect because they were improperly made and connectors with losses that tested at 1 dB although the label said 0.2 dB. Under closer examination, the label appeared to be from a copy machine not a printer, so I doubt the cable was even tested. While it's inconvenient and expensive, testing every patchcord is highly recommended per the standard test, FOTP-171. If you do not test them and have network problems, then look at them first when troubleshooting the system.

Testing Patchcords
FOTP-171
     For testing the loss of a patchcord, you only need an 850 nm LED light source for multimode cable or 1310 laser for singlemode, a fiber optic power meter and some reference patchcords. Just remember that the patchcords used for references in testing must be good for tests to be valid, so you test them as you would other patchcords, just more often.
     Testing patchcords is similar to testing any fiber optic cable. Use one reference patchcord to set a 0 dB reference. Connect a patchcord to test to the reference patchcord with a mating adapter. Connect the power meter to the other end of the patchcord and measure the loss. Since the length of the fiber is short, the loss contribution of the fiber is ignorable. And since one end of the cable is attached to the power meter, not another cable, you only measure the loss of the one connection between the reference cable and the cable under test, so you can test each connector individually.
     To complete the testing of the patchcord, reverse the cable you are testing to check the connector on the other end. Sometimes you will find one bad connector and can replace it to make the patchcord useful again. But often the cost of replacing the connector may be higher than replacing the patchcord itself.
     If your test equipment has different connectors than the patchcords you are testing, you will need hybrid reference cables with connectors compatible with the equipment on one end and the patchcord connectors on the other end. You will also need the correct connector adapters for your power meter.
     Obviously, all reference cables used for testing must have high quality connectors to get reliable test results. Use this same method to test your reference cables against each other and discard any with high losses, usually those with losses over 0.5 dB.

Cleaning Patchcords
     Cleanliness is an issue with brand new cables as well as used ones. All connectors should always have the polished ferrule covered by a “dust cap” to protect the end of the connector ferrule from damage and dirt. However, a user recently told me that after examining many connectors with a microscope, he understood why they were called dust caps, they were all full of dust. The dust cap should be used to protect the connector ferrule, but do not assume it keeps it clean. You must clean connectors before inserting them in mating adapters or active devices to ensure they are really clean. Here are directions for cleaning.

dirty fiber optic connector

Cleaning Removes Dirt That Causes Loss

     Those cables connecting equipment to the cable plant or hanging on the sides of racks are often mistreated, leaving kinks in the cable or scratches and scuffing on the connector ferrules. With the reference cables used for testing, the continual mating to other connectors, even when carefully cleaned before use, will cause scuffing that increases loss over time.
What this means is patchcords need continuous inspection and testing. It only takes a minute but can save you lots of troubleshooting time. Start by checking the connector ferrules with a fiber optic inspection microscope, preferably around 100 power, then test according to the procedure above. 

Handling Patchcords
     Patchcords are often subject to poor treatment. They are often hung off communications equipment or patch panels stressing the fiber at the back of the connector. When they are too long, they are bundled and hung in large piles on the side of equipment racks. Kinking is always a problem. Ideally, patchcords should be the right length, supported below the connection and carefully placed to prevent stress. Unfortunately, that is often not the case and network performance may suffer. Don't let your patchcords look like this:

Bad patch panel


Table of Contents: The FOA Reference Guide To Fiber Optics





 


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