The effect of fibre core geometry on focal ratio degradation

Focal ratio degradation is the decrease in focal ratio (or equivalently the increase of numerical aperture) of a beam of light that has been transported through a multimode optical fibre. Multimode fibres are frequently used in astronomical instruments such as multi-object and integral field spectrographs due to the poor coupling efficiency inherent to single mode fibre use. Focal ratio degradation has potentially serious consequences for these instruments as it means that they require faster, more difficult to produce optics than they would otherwise and any focal ratio degradation above that allowed for in the design will result in loss of light. Focal ratio degradation is generally a small effect when the fibres are fed with light close close to the material numerical aperture of the fibre however in practice this is rarely done due to the technical difficulties and instead the fibres are used at slower focal ratios (lower numerical apertures) where focal ratio degradation is more significant. Consequently considerable effort has been expended in identifying and mitigating the various causes of focal ratio degradation in fibre fed instruments, however the issue of fibre core geometry is a relatively unexplored area.