How does bending effect a Fiber Patchcord?
Fiber optic cables by their nature and the way they are manufactured are designed to be enduring the stress applied to them during their installation and maintenance, however as they are made of glass and are highly fragile it is highly advisable by the manufacturers to lower this stress to a minimum. Bending the fiber cables and the amount of quality loss depends on the type of the cable, if it’s Single- mode or Multi- mode cable, their design, their core diameter and their transmission wavelength. Usually longer wavelengths are more sensitive to stress and bending losses.
The process of bending or pulling loss starts inside the cable as the optical signal within the cable is not guided through the core of the fiber, instead a big part of the light itself is lost and bouncing in the walls and the cladding in the cable thus creating a high loss in optical light. Bending would most probably permanently damage the fiber cable by causing cracks in it. This would compromise the quality of the signal and the integrity of the data transmission. This is easily put to the test with the help of a visible laser put in the fiber itself and bending it at a certain point. The light loss will be visible where the cable is being bent.
During the last couple of years manufacturers and the Fiber Optic Association started developing a new type of cables that are more durable and can withstand higher stress and bending. This was firstly developed for the Single- mode fibers and after a couple of years for the Multi- mode fibers. The way they were testing the bending and the endurance of the cables was with the help of a piece of wood and bending the cable around it in front of a wide audience.
The bending of the fiber optic cables is measured by the bend radius. Only in the last couple of years this bending radius has been industry standardized by the Fiber Optic Association. In contrary before it was standardized the bending radius have been governed by the cable manufacturers. The new standard defined by the ANSI/TIA/EIA-568B.3 named "Optical Fiber Cabling Components Standard" sets exact performance specifications concentrated on the minimum bend radius and the maximum pulling tensions for 50/125 micron and 62.5/125 micron fiber optic cables. With the new standard introduced the manufacturers have the obligation to specify the minimum bending radius to which the cable could be safely bent during the installation. Most commonly the minimum bend radius of 1.6mm and 3.0mm fiber cables is around 3.5cm and the minimum bend radius for patch cable is around ten times the cable diameter. If referring to the manufacturer’s recommended bend radius is not possible, the general guideline for cable bending is no more than 20 times the diameter of the cable itself.
There are two types of bending radius: micro bends and macro bends. As the name suggests macro bends are larger than micro bends. Even though the two terms are very similar there is a significant difference in differentiating them. Macro bends are usually the bends that would be visible by the naked eye and micro bends are small microscopic deviations along the fiber itself.
However, it doesn’t take much for a micro bend to happen as it could be also caused by the fiber coating squeezing the cable because of very low temperatures. There is a standardized micro bend test procedure defined by the Fiber Optics Association named “FOTP-68 Optical Fiber Micro bend Test Procedure”. One way of developing and manufacturing more micro bend enduring fiber optic cables is by applying several layers of primary coating which would eventually protect the fibers of being bent.
Macro bends on the other hand, as aforementioned, is tested by wrapping the fiber cable around a specific material of a specified diameter. The standardized macro bend testing defined by the Fiber Optics Association is called “FOTP-62 IEC 60793-1-47 Measurement Methods and Test Procedures – Macro bending Loss”.
Another aspect of the bend radius that would affect the fiber cable performance is the path of the patch cable. This should be clearly defined by the manufacturer. If this is not properly done it would cause increased congestion in the termination panel possibly violating the band radius threshold. The patch cable should be easily accessed, easier to be maintained at all points of its path. Because the patch cables are commonly kept together with cable ties, manufacturers advise these cable ties be used with caution. Tightening the cable ties with an installation tool is harmful to the fiber optic cables and could very easily cause a full fiber breakage. Manufacturers advise the cable ties to be hand tightened but in the same time to leave them loose enough to be moved along the cable by hand.
The patch cable path should be well-defined and reduce the risk of stressing the cable. This way the patch cable path would be easier and quicker to be accessed by the engineer for maintenance works. The reduced fiber twists would ensure the optic light in the cable travels in the core of the cable thus minimizing the escape through the walls and the coating of the cable.
As the proper fiber management would affect the network’s reliability, performance and the cost, a well-defined cable paths could ensure a safe ground for future maintaining and network upgrading.
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