Avoiding the “Bends” by Specifying the Correct Fiber by Tom Davis and David Mazzarese
Fiber has become an integral part of the world’s submarine cabling infrastructure over the last twenty years. These optical systems primarily support the international voice, data, and video networks of the world’s leading service providers, but there are numerous applications for optical connectivity that go far beyond the traditional undersea networks. Performance enhancements from fiber manufacturers, optical electronics manufacturers, and fiber cablers have enabled the market for undersea optical applications to grow beyond standard telecom usages.
These technology developments, coupled with cost reductions, have made optical fiber practical and beneficial for many undersea applications ranging from large private networks connecting offshore energy platforms, to remotely operated vehicles (ROVs) that can fit in a suitcase. The U.S. military has also been aggressive in the deployment of submarine fiber in command and control networks and surveillance systems, as well as in tactical weapons designs. Fiber’s small size, light weight, and high bandwidth characteristics are some of the driving factors in the growth of fiber
in this market. Another major benefit of optical cables over copper-based cables is they eliminate concerns about Electromagnetic Interference (EMI) and Radio Frequency Interference (RFI). This article addresses some of the current leading applications for fiber optic cables in various harsh undersea environments, along with key factors associated with fiber performance. Demands on Undersea Fiber As the title of this article implies, the performance of fiber is impacted by the amount of bending stress imposed on an undersea cable. This is not a great concern with traditional large transcontinental undersea cabling networks, because they use very robust cables that are protected by multiple layers of armor, with cable diameters approaching two inches. Today, however, more cable is being deployed in coastal areas where the shallow waters increase the chance that a cable will be snagged or pinched by boat anchors or commercial fishing rigs. Many of these cables serve less traditional undersea applications that require smaller, lighter, and more flexible designs. Such designs could subject the fiber to more stress than usual from the cable being bent, twisted, or pulled in an excessive manner. The consequences of these adverse cable conditions could eventually impact the performance of the fiber(s) within the cable by limiting the amount of light traveling through the fiber. The induced “macrobending” (too tight a bend radius) and “microbending” (pinching or squeezing the fiber) can increase the attenuation of the optical cable, resulting in a reduction of the optical power traveling down the cable. This in turn could cause the deterioration of the optical signal. Sensitivity to macrobending and microbending can limit the performance of an optical cable, especially 24