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Fiber-Optic Technology

5. Fiber Geometry: A Key Factor in Splicing and System Performance

As greater volumes of fiber in higher fiber-count cables are installed, system engineers are becoming increasingly conscious of the impact of splicing on their systems. Splice yields and system losses have a profound impact on the quality of system performance and the cost of installation.

Glass geometry, the physical dimensions of an optical fiber, has been shown to be a primary contributor to splice loss and splice yield in the field. Early on, one company recognized the benefit provided by tightly controlled fiber geometry and has steadily invested in continuous improvement in this area. The manufacturing process helps engineers reduce systems costs and support the industry’s low maximum splice-loss requirement, typically at around 0.1 dB.

Fiber that exhibits tightly controlled geometry tolerances will not only be easier and faster to splice but will also reduce the need for testing by ensuring predictable, high-quality splice performance. This is particularly true when fibers are spliced by passive, mechanical, or fusion techniques for both single fibers and fiber ribbons. In addition, tight geometry tolerances lead to the additional benefit of flexibility in equipment choice.

The benefits of tighter geometry tolerances can be significant. In today’s fiber-intensive architectures, it is estimated that splicing and testing can account for more than 30 percent of the total labor costs of system installation.

Fiber Geometry Parameters

The three fiber geometry parameters that have the greatest impact on splicing performance include the following:

  • cladding diameter—the outside diameter of the cladding glass region.
  • core/clad concentricity (or core-to-cladding offset)—how well the core is centered in the cladding glass region
  • fiber curl—the amount of curvature over a fixed length of fiber

These parameters are determined and controlled during the fiber-manufacturing process. As fiber is cut and spliced according to system needs, it is important to be able to count on consistent geometry along the entire length of the fiber and between fibers and not to rely solely on measurements made.

Cladding Diameter

The cladding diameter tolerance controls the outer diameter of the fiber, with tighter tolerances ensuring that fibers are almost exactly the same size. During splicing, inconsistent cladding diameters can cause cores to misalign where the fibers join, leading to higher splice losses. The drawing process controls cladding diameter tolerance, and depending on the manufacturer’s skill level, can be very tightly controlled.

Core/Clad Concentricity

Tighter core/clad concentricity tolerances help ensure that the fiber core is centered in relation to the cladding. This reduces the chance of ending up with cores that do not match up precisely when two fibers are spliced together. A core that is precisely centered in the fiber yields lower-loss splices more often.

Core/clad concentricity is determined during the first stages of the manufacturing process, when the fiber design and resulting characteristics are created. During these laydown and consolidation processes, the dopant chemicals that make up the fiber must be deposited with precise control and symmetry to maintain consistent core/clad concentricity performance throughout the entire length of fiber.

Fiber Curl

Fiber curl is the inherent curvature along a specific length of optical fiber that is exhibited to some degree by all fibers. It is a result of thermal stresses that occur during the manufacturing process. Therefore, these factors must be rigorously monitored and controlled during fiber manufacture. Tighter fiber-curl tolerances reduce the possibility that fiber cores will be misaligned during splicing, thereby impacting splice loss.

Some mass fusion splicers use fixed v-grooves for fiber alignment, where the effect of fiber curl is most noticeable.

Figure 8. Cladding Diameter, Core/Clad Concentricity, and Fiber Curl

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