Your mother was right--dirt is bad. Cleanliness can be next to impossible on the jobsite, but when it comes to fiber optics, it's mandatory. The problem is simply that fiber itself is small, about the size of a human hair. The light-carrying fiber core is even smaller, 62.5 or 50 microns for multimode or less than 10 microns for single-mode fiber.
Typical airborne dirt is larger than the diameter of single-mode fiber core and large in comparison to multimode fiber core. When dirt gets on the fiber core at a connector or splice, it absorbs the light passing through and causes high loss.
Working with fiber requires sensitivity to dirt and dust. Whether you are terminating, testing or setting up networks, you must be careful to keep everything as clean as possible. Now, we all know we can't always work in a "clean room" environment like semiconductor manufacturing facilities, but there are things we can do that will make life easier when working in typical situations.
Let's look at several steps in fiber optic installation and note what you can do to minimize the problem.
When splicing or installing connectors, keep your work area clean. Obviously, you want to avoid working in a heavy construction area if dust is everywhere. I know installers who have had to isolate themselves from the dust by bringing in one of those outdoor tents used in the winter to provide a warm work area. Try to avoid working under HVAC vents, as they continually blow dust. Find yourself a nice, quiet corner away from all activity and drafts.
Outdoors, it's common to do all your terminations in a splicing van. Airborne dirt is even worse outdoors, so the cables are left long and brought into a temperature-controlled van with air filtration. Not only does it keep the work area clean but extends the life of the fusion splicer, which is also sensitive to dirt. If a van is not available, a tent can be used. They even come with AC.
Before you start, clean your work surface carefully. Use an industrial-strength cleaner, preferably with an anti-static additive, to prevent attracting more dust. A black work surface is easier to keep clean and will help you find fiber scraps, too.
If you are polishing connectors, not only should you be concerned with the airborne dirt, but also the buildup on polishing film from grinding down the fiber and adhesive on the end of the ferrule. This dirt you can usually see. Polishing on top of the buildup will not render good results. When many polishing "tracks" become visible, it is time to switch to a new film.
Between each polishing step, use a lint-free pad dampened with 91 percent pure lab-grade isopropyl alcohol (not the rubbing stuff) to wipe polishing residue off the end of the connector ferrule. You can also use this same pad to clean the polishing films to extend their life.
As soon as the connector is polished, cleaned and inspected with a microscope, put a dust cap on it. Now, there is a reason these plastic covers are called "dust" caps--they are full of it. Their prime purpose is to protect the ferrule end from scratches, fingerprints, and accumulation of even more dust. But when you get ready to test the cable or attach it to a piece of network equipment, take off the cap and clean the connector ferrule carefully with another lint-free alcohol pad, just before you attach it to the test equipment.
It makes good sense to clean connectors before testing, as it will always make the loss lower. But most connectors use mating adapters, and installers rarely consider their part in the loss of mated connectors. Some of these mating adapters have plastic alignment sleeves. If used with a ceramic ferrule connector, you can see the ceramic ferrule becoming gray with only a few insertions. The connector is scraping off the plastic from the sleeve, covering the tip of the ferrule, causing loss. Throw those mating adapters away and use metal or ceramic ones rated for single-mode fibers.
Cleaning connector mating adapters or transceiver inputs and outputs is also important. Most people use "canned air" to blow them out--a big mistake. Canned air is really just a liquid propellant in a spray can. When you first spray it, it squirts out a lot of that liquid, which can dry and leave a bigger mess than when you started. If you go to a computer or office supply store, you can get a "blaster" that uses dry compressed CO2 that will blow out the dirt and leave no residue.
Cleanliness might be difficult to achieve, but careful attention to detail and developing good work habits will make working with fiber easy and yield better results. EC
HAYES is a VDV writer and trainer as well as president of the Fiber Optics Association. Find him at www.JimHayes.com.