光纤中复杂现象的模拟
Simulation of complex phenomena in optical fibres
摘要：
光纤对于许多类型的高度复用和精密光谱学至关重要。新一代的多纤维仪器正在建设中的宇宙学的基本问题，如暗能量的性质调查的成功，需要准确的模拟的纤维系统，以实现其信号噪声的目标。尽管它们的结构简单，但纤维表现出意想不到的行为，包括光学扩展量（焦距比退化; FRD）和模态噪声的不守恒。此外，新的光纤几何形状（非圆形或锥形）已变得可用于改善加扰特性，加扰特性与模式噪声一起限制了精密光谱学中可实现的SNR。这些问题通常通过对候选纤维及其终端进行广泛测试来解决，但这些测试既困难又耗时。利用商业软件包可以通过射线追踪和波分析进行建模，但这些软件包无法解决更复杂的功能，特别是FRD。我们使用的相位跟踪射线跟踪方法提供一个实际的描述FRD来自我们以前的实验工作，圆形纤维，并将其应用到非标准的纤维。这使得加扰和FRD之间的关系首次被量化。我们发现，扰频主要影响近场图案的形状，但对重心的影响可以忽略不计。FRD有助于均匀化近场图案，但不会使其完全均匀。具有多边形横截面的纤维在不放大FRD的情况下改善了扰乱。椭圆形光纤与锥形相结合，可以提供有效的图像切片手段，以提高分辨率和吞吐量的乘积，但结果对照明的细节敏感。
英文摘要：
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 modellisation of the fibre system to achieve their signal-to-noise 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 homogenise the nearfield 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.
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