利用旋转双扰频器抑制光纤模态噪声
Fiber modal noise mitigation by a rotating double scrambler
摘要：
光纤模态噪声是高分辨率光谱学中的性能限制因素，无论是在实现高信噪比方面，还是在利用多模光纤馈电高分辨率光谱仪进行高精度径向速度测量时。传统上，通过搅动或“摇动”光纤来降低模态噪声。这样，在光纤中传播的光在许多不同的模式上重新分布。然而，在光纤仅具有有限数量的模式的情况下，例如在近红外波长处或在自适应光学辅助系统中，该方法变得非常低效。所需的强烈搅动会对光纤施加应力，并可能导致焦比下降，或者更糟，损坏光纤。作为一种替代方法，我们建议使用一个经典的光学双扰频器，一个设备，已经实现了许多高精度的径向速度光谱仪，以减轻模式噪声的影响，通过旋转扰频器的第一个光纤端在每次曝光。由于扰频器的第二光纤的旋转照明图案，被激发的模式连续变化。这导致在光纤出口处非常有效地平均模式图案，并大大降低模式噪声。在这方面的贡献，我们提出了一个原型设计和初步的实验室结果的旋转双扰频器。
英文摘要：
Fiber modal noise is a performance limiting factor in high-resolution spectroscopy, both with respect to achieving high signal-to-noise ratios or when targeting high-precision radial velocity measurements, with multi-mode fiber-fed high-resolution spectrographs. Traditionally, modal noise is reduced by agitating or "shaking" the fiber. This way, the light propagating in the fiber is redistributed over many different modes. However, in case of fibers with only a limited number of modes, e.g. at near-infrared wavelengths or in adaptive-optics assisted systems, this method becomes very inefficient. The strong agitation that would be needed stresses the fiber and could lead to focal ratio degradation, or worse, to damaging the fiber. As an alternative approach, we propose to make use of a classic optical double scrambler, a device that is already implemented in many high-precision radial-velocity spectrographs, to mitigate the effect of modal noise by rotating the scrambler's first fiber end during each exposure. Because of the rotating illumination pattern of the scrambler's second fiber, the modes that are excited vary continuously. This leads to very efficient averaging of the modal pattern at the fiber exit and to a strong reduction of modal noise. In this contribution, we present a prototype design and preliminary laboratory results of the rotating double scrambler.
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