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 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. Early on, one company recognized the benefits provided by tightly controlled fiber geometry and has steadily invested in continuous improvement in this area. Its tightly controlled manufacturing process helps engineers reduce systems costs and yet remain within the industry's low maximum splice-loss requirement.

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 are 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 needs dictated by each individual system, it is important to be able to count on consistent geometry along the entire length of the fiber and not to rely solely on measurements made only at the end of the fiber.

Cladding Diameter

Cladding diameter tolerances control 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 be misaligned where the fibers join, leading to higher losses.

Cladding diameter tolerances are controlled by the drawing rate. Some manufacturers are able to control the tolerance of the cladding to a level of 125.0 ±1.0 µm. Once the cladding diameter tolerance is tightened to this level, core/clad concentricity becomes the single largest geometry contributor to splice loss.

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.