Simulation of complex phenomena in optical fibres
Simulation of complex phenomena in optical fibres
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光纤中复杂现象的模拟
DOI：
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发表时间：
2012
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通讯作者：
U. Lemke
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作者：
J. Allington;G. Murray;U. Lemke
光纤对于许多类型的高度多路复用和高精度光谱学来说是必不可少的。正在建造的新一代多纤维仪器要成功地研究宇宙学中的基本问题，如暗能量的性质，就需要对纤维系统进行准确的建模，以实现它们的信噪比目标。尽管结构简单，但纤维表现出意想不到的行为，包括拉伸率不守恒(焦比退化，FRD)和模式噪声。此外，新的光纤几何结构(非圆形或锥形)已可用于改善扰乱特性，与模式噪声一起，限制了精密光谱分析中可实现的SNR。这些问题通常是通过对候选纤维及其端接进行广泛的测试来解决的，但这些测试既困难又耗时。使用商业软件包可以通过光线跟踪和波分析进行建模，但这些软件不能解决更复杂的特征，特别是FRD。
我们使用一种相位跟踪光线跟踪方法来提供来自我们先前在圆形光纤上的实验工作的FRD的实际描述，并将其应用于非标准光纤。这使得加扰和FRD之间的关系能够第一次被量化。我们发现，扰动主要影响近场模式的形状，而对重心的影响可以忽略不计。FRD有助于使近场图案均匀，但不能使其完全均匀。具有多边形截面的纤维在不放大FRD的情况下改善了扰乱。椭圆形纤维与锥化相结合，可以提供一种有效的图像分层方法，以提高分辨率和吞吐量的乘积，但结果对光照细节敏感。我们还研究了接近极限数值孔径的光纤的性能，因为这可能会影响一些主聚焦光纤仪器的信噪比的一致性。
Optical fibres are essential for many types of highly multiplexed and precision spectroscopy. The success of the new generation of multifibre instruments under construction to investigate fundamental problems in cosmology, such as the nature of dark energy, requires accurate modellization of the fibre system to achieve their signal-to-noise ratio (SNR) goals. Despite their simple construction, fibres exhibit unexpected behaviour including non-conservation of etendue (focal ratio degradation, FRD) and modal noise. Furthermore, new fibre geometries (non-circular or tapered) have become available to improve the scrambling properties that, together with modal noise, limit the achievable SNR in precision spectroscopy. These issues have often been addressed by extensive tests on candidate fibres and their terminations, but these are difficult and time-consuming. Modelling by ray tracing and wave analysis is possible with commercial software packages, but these do not address the more complex features, in particular FRD.
We use a phase-tracking ray-tracing method to provide a practical description of FRD derived from our previous experimental work on circular fibres and apply it to non-standard fibres. This allows the relationship between scrambling and FRD to be quantified for the first time. We find that scrambling primarily affects the shape of the near-field pattern but has negligible effect on the barycentre. FRD helps to homogenize the near-field pattern but does not make it completely uniform. Fibres with polygonal cross-section improve scrambling without amplifying the FRD. Elliptical fibres, in conjunction with tapering, may offer an efficient means of image slicing to improve the product of resolving power and throughput, but the result is sensitive to the details of illumination. We also investigated the performance of fibres close to the limiting numerical aperture since this may affect the uniformity of the SNR for some prime focus fibre instrumentation.