Why excess fiber length (EFL) matters?

Optical Fiber Protection

Before exploring Excess Fiber Length (EFL), it’s important to restate the relevance of FIMT (Fiber in Metal Tube). Many optical fiber applications require the protection of a stainless-steel tube to guard the fibers against external influences and extreme weather conditions. Methods of protecting the optical fiber have evolved alongside the innovations of the optical fiber itself, unlocking potential across various applications and industries. Powerful examples are found in the Oil & Gas Industry in several upstream, midstream, and downstream applications, the impact of each environment featuring heavily in our fiber optic cable design considerations.

For example, downhole applications in upstream facilities must be resilient against high temperature and heavy use. The Harsh Environment Cable (HEC) is a FIMT that protects the optical fiber from severe conditions, allowing the fiber to perform Distributed Temperature Sensing (DTS) and Distributed Acoustic Sensing (DAS) functions across the Oil & Gas Industry. The ability to implement fiber optic technology in the industry increases the efficiency of the work and increases the project profit margin.

Excess Fiber Length (EFL)

The example above is just one illustration of why optical fibers require protection against external influences. When referring to such protection, one abbreviation stands out as being amongst the most important to consider.

For example, If the metal tubing undergoes exposure to a continuous temperature of 150°C (302°F), the metal expands at a rate of approximately 16 x 10-6/K.

The fiber, in contrast, undergoes a negligent degree of linear expansion. If it were that the fiber length inside the tube was precisely the length of the metal tube, the tension within the fiber would reach such a degree to render the fiber useless for signal transmission because of the extreme stress. If these stresses are allowed to become too great, they may even result in the breakage of the fiber. To countervail this effect, the required extra length of fiber to be placed inside the metallic tube is calculated and ensured during manufacture to compensate for the thermal expansion of the tube.


Calculating the EFL

Correctly calculating the EFL is fundamental to guaranteeing peak optical performance. A loose tube design ensures less strain on the optical fiber, resulting in the lowest optical attenuation losses, which is critical for data transmission and fiber optic sensing applications.

As we know how the steel behaves and the application’s temperature range, preparing the required “EFL” is possible to ensure safe and dependable operation in the field. In some instances, however, fiber type and tube dimension may present limitations on how much the fiber can operate within a fiber-bearing tube.

Another critical factor in the determination of EFL is adherence to the maximum bend radius of the optical fiber, which, if exceeded, could lead to unacceptable attenuation, i.e., the guiding of optical signals outside of the fiber.

In Conclusion

High fiber-density cables are becoming more in demand alongside the market for 5G and high-performing networks. As a result, the fiber optic cable design considerations are prone to change. Just as the exterior of FIMT suits the environment, the makeup of the optical fiber changes to suit the cable’s function.

NBG can produce FIMT with an extremely high fiber count. We treat every project as a standalone project to deliver a custom solution that best fits the project’s needs. We are one of the leading companies worldwide in ensuring the required excess fiber length values in producing FIMT products. Contact us for more information.

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