The dependence of the properties of optical fibres on length

Abstract

We investigate the dependence on length of optical fibres used in astronomy, especially the focal ratio degradation (FRD) which places constraints on the performance of fibre-fed spectrographs used for multiplexed spectroscopy. To this end we present a modified version of the FRD model proposed by Carrasco and Parry \cite{Carrasco1994} to quantify the the number of scattering defects within an optical fibre using a single parameter. The model predicts many trends which are seen experimentally, for example, a decrease in FRD as core diameter increases, and also as wavelength increases. However the model also predicts a strong dependence on FRD with length that is not seen experimentally. By adapting the single fibre model to include a second fibre, we can quantify the amount of FRD due to stress caused by the method of termination. By fitting the model to experimental data we find that polishing the fibre causes more stress to be induced in the end of the fibre compared to a simple cleave technique. We estimate that the number of scattering defects caused by polishing is approximately double that produced by cleaving. By placing limits on the end-effect, the model can be used to estimate the residual-length dependence in very long fibres, such as those required for Extremely Large Telescopes (ELTs), without having to carry out costly experiments. We also use our data to compare different methods of fibre termination.Comment: 10 pages, 7 figure

Relative contributions of scattering, diffraction and modal diffusion to focal ratio degradation in optical fibres

“…Modern fibre designed for use in astronomical instrumentation has very few perturbations over long fibre lengths and therefore has a very weak FRD length dependency. The fibre end‐face preparation and external environment are likely to have a bigger impact on FRD than the fibre length; this supports the current literature (Poppett & Allington‐Smith 2010).…”

Modal noise prediction in fibre spectroscopy - I. Visibility and the coherent model

“…It is not straightforward to decide whether to base the model prediction on input focal ratio, output focal ratio or a mixture of both, because part of the FRD could occur at the each of the fibre endfaces (compare length effect, Poppett & Allington‐Smith (2010)), leading to a complex mode‐population dependence along the fibre. We chose modal noise to be a function of θ in as this seems to fit the coherent model best.…”

“…For instance it was shown by Baudrand & Walker (2001); Corbett (2006), that the SNR has no significant dependence on fibre length for L ≤ 100 m. This is possibly because the fibre does not terminate with a perfect quartz‐air intersection. This so called end‐effect (Poppett & Allington‐Smith 2010) may lead to a stronger mode mixing and thus less coherence between the modes, which would explain the diminished visibility. In the following L will therefore be treated as a free parameter.…”

The effect of core geometry on focal ratio degradation in optical fibres for astronomy

“…Consequently considerable effort has been expended in identifying and mitigating the various causes of FRD in fibre fed instruments (e.g. [2,3]), however the issue of fibre core geometry is a relatively unexplored area.…”