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.
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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. 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 lay down 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 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.
When fibers are manufactured within specified tolerances, there are still slight variations from one optical fiber to another. These variations can affect the performance of the splice even though the optical fibers are perfectly aligned when mated. The variations between two optical fibers that affect splice performance are referred to as intrinsic factors. One factor is the Numerical aperture mismatch this occurs when the NA of one optical fiber is different from the NA of the other optical fiber. If the NA of the transmitting fiber is larger than the NA of the receiving optical fiber a loss may occur. The exact loss from an NA mismatch is difficult to calculate.
Factors such as light source type, light source launch condition and optical fiber length, and bends in the optical fiber all affect the potential loss. Another factor could be a core diameter mismatch this occurs when there is a difference in the core diameters of the two opticasl fibers. A core diameter mismatch loss results when the core diameter of the transmitting optical fiber is greater than the core diameter of the receiving optical fiber. You can calculate the worst case loss percentage for a splice that joins different diameter fiber with a formula. You then can also use another formula to calculate the decibel loss. There is also noncercularity that is a intrinsic factor that causes loss. The noncircularity of the core will cause a loss when light from the core of the transmitting optical fiber enters the cladding of the receiving optical fiber. A way to make sure that connections are not lost when splicing is make sure the alignment of the ellipticities of the two cores.
Some of the extrinsic factors that affect optical fiber splice performance are factors related to the condition of the splice itself. One of the factors is lateral misalignment this occurs when the two optical fibers are offset. Lateral misalignment loss occurs when light from the core of the transmitting optical fiber enters the cladding of the receiving optical fiber. This can be avoided by paying close attention when connecting the two sections together. If the optical fibers in a splice meet each other at an angle a loss from angular misalignment may occur. The amount of loss depends the severity of the angular misalignment and the acceptance cones of the transmitting and receiving optical fibers. The best way to avoid this is to make sure each end is touching completely before spliced together.