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Core ellipticity

Generally speaking, core ellipticity (the term ‘core circularity’ usually refers to the inverse concept as core ellipticity) refers to how much the core geometry deviates from being completely circular. While a moderate core ellipticity guides perfectly well and does not cause any problems when using fibers, an excessive ellipticity can originate high losses when splicing to more circular core fibers, as the cores will not fully overlap.

An optical fiber is often thought of as a truly symmetric cylindrical waveguide with perfectly circular co-axial layers of glass. However, any real fiber will always show some degree of irregularity to this co-axial structure and some level of core-ellipticity will be seen. This is commonly seen amongst highly birefringent fibers where there are asymmetric stresses on the core which often cause it to be noticeably elliptical. This rarely affects the performance of the fiber, and in fact some designs specifically have an elliptical core to create birefringence, known as form/shape birefringence.
Stress applied across the core of fibers to encourage stress-birefringence can lead to a slightly elliptical core, and this is the case of many designs including some Panda or Bow-Tie HiBi PM fibers.  When both stress-based and form birefringence are present, the core ellipticity will induce form birefringence that will counter-balance the stress birefringence (note the orientation of fast and slow axes in the diagrams). However, the form birefringence is not generally significant in comparison to the stress birefringence, and the fiber design is considered to cope with this effect.

Although in general core ellipticity is not a significant issue when splicing fibers and there is little impact if both fibers are slightly elliptical, Fibercore does have strict specifications for all fibers in terms of maximum ellipticity that is practical. When splicing to other highly circular fibers, such as telecoms type PM fiber with moderate birefringence, our Telecoms PM Fiber is the preferred choice of fiber as it has been developed to offer a very circular core.


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