光纤性能与长度的关系
The dependence of the properties of optical fibres on length
摘要：
我们调查的依赖于天文学中使用的光纤的长度，特别是焦比退化（FRD）的地方约束用于复用光谱的光纤光谱仪的性能。为此，我们提出了一个修改后的版本的FRD模型提出的Carrasco和Parry \cite{Carrasco1994}量化的散射缺陷的数量在光纤内使用一个单一的参数。该模型预测了实验中看到的许多趋势，例如，随着芯直径的增加，以及随着波长的增加，FRD减小。然而，该模型还预测了FRD与长度的强烈依赖性，这在实验中是看不到的。通过调整单纤维模型以包括第二纤维，我们可以量化由于端接方法引起的应力而导致的FRD的量。通过拟合模型的实验数据，我们发现，抛光的光纤会导致更多的应力被诱导在一个简单的劈裂技术相比，在光纤的末端。我们估计，由抛光引起的散射缺陷的数量大约是由解理产生的两倍。通过限制端部效应，该模型可用于估计非常长的光纤中的剩余长度依赖性，例如超大望远镜（ELT）所需的剩余长度依赖性，而无需进行昂贵的实验。我们还使用我们的数据来比较不同的光纤端接方法。
英文摘要：
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.
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