Optical fiber, like copper wire for communications, needs to be kept dry to maintain its performance. On wire, moisture causes high impedance and crosstalk. In fiber, it adds to the attenuation and may make fibers brittle and more susceptible to damage. As the fiber is manufactured, it is immediately coated with a 250-micron diameter acrylic coating that seals the glass fibers to protect them from moisture and handling damage, but that is only the first stage of protection.
Fiber optic cables can be installed in dry indoor environments or outdoors where aerial, buried and submarine cables are exposed to a broad range of temperatures, exposed to moisture or immersed in water. Compared to the outdoor varieties, indoor cables have it easy.
If the fiber is to be cabled in a “tight buffer” cable, such as simplex, zipcord or distribution cable for use indoors, it will be coated again with a 900-micron diameter soft plastic coating, generally called the buffer, that will further protect it from moisture. The buffer coating also makes the fiber much easier to handle because it provides stress-resistance and makes the fiber large and rugged enough to be easily handled for termination.
While tight-buffered fibers are not as rugged as jacketed cables, they can be terminated and placed in enclosures without further protection.
While the buffered fiber has two layers of protection, it is still not properly protected for the water immersion that occurs with direct burial or in conduit, which is prone to flooding. Even with jacketing, indoor cables use materials that are necessary for flame retardance, which unfortunately do not offer adequate water resistance for outside installation.
Most outdoor cables are of a loose-tube design, where up to 12 250-micron buffered fibers are encased in a separate “buffer tube” made from a water-resistant plastic. That tube is then flooded with a gel or dry powdered polymer that provides the water blockage necessary for fiber protection.
Some outdoor cables also add gel around the buffer tubes before adding strength members, jackets and even armoring as required by the installation environment. This way, every fiber has several layers of protection, so even if the cable jacket is breached and water gets into the cable, the fibers should be protected.
The water-blocking gel commonly used in cables makes termination and splicing much more difficult. As the cable is opened, all gel must be removed at each stage. When the fibers are exposed, they must be individually cleaned and dried for splicing—a time-consuming process.
If the fibers are directly terminated, each fiber needs to be slipped into a thin plastic tube called a “furcation tube,” part of a “breakout kit” that provides mechanical protection so the thin 250- micron fiber can be fitted with a connector safely. Those fibers not only need cleaning and drying, but also need dusting with a powder to smooth their fitting into the furcation tube.
Removing the gel is a slow, delicate process, especially on each of the fibers. Some cable makers recommend using wipes and isopropyl alcohol. Dedicated wipes with chemicals that dissolve the gel work even better in our experience. Some installers say that if you run out of the special wipes you can use bottled lemon juice to dissolve and remove the gel. Check with your cable makers before you try that.
A major advance in convenience was made several years ago when cable manufacturers realized that some water-absorbing polymers could also provide water protection. These polymers, similar to those used in simple consumer products such as disposable diapers, absorb massive quantities of water and form a water-block that is quite effective. Since it is included in the cable as either a powder or a tape, it is much easier to remove—just unwind the tape or dust off the powder and you are ready to terminate or splice.
Dry water-blocked cables are ideal for most campus installations, where you can even get double-jacketed tight-buffer cables for short building-to-building runs. Pull the cable into the building and remove the outdoor jacket, and you have an indoor-rated cable. Then strip the jacket, dust off the water-blocking material and you are ready to terminate. It is a real labor-saving cable for the installer who works primarily on indoor installations. Check the cable companies for their specific cable offerings. EC
HAYES is a VDV writer and trainer and the president of The Fiber Optic Association. Find him at www.JimHayes.com.