Observing inside the coronagraphic regime with optimized single-mode nulling interferometry

The number of terrestrial exoplanets accessible to high-contrast coronagraphic imaging with large telescopes is limited by the smallest angular offset from bright stars at which coronagraphs can observe. However, it is possible to reach inside a telescope’s coronagraphic regime by employing nulling interferometry across a telescope’s pupil. Indeed, “cross-aperture” nulling interferometry can observe significantly closer to stars than typical coronagraphs, enabling observations even within the stellar diffraction core. Identifying an optimal nulling coronagraph, i.e., one with both a very small IWA and a high throughput for exoplanet light, would thus be of great interest. A systematic examination of available nulling options has therefore been carried out, which has led to three things. The first is a topological overview that unites both multi-aperture nulling interferometers and single-aperture phase coronagraphs into a common geometrical framework. The second is a new type of phase-mask coronagraph that has emerged from a gap in this framework, called here the “split-ring” coronagraph. The third is a clear identification of the optimal configuration for a nulling coronagraph, which turned out to be an aperture-plane phase knife, i.e., an achromatic -radian phase shift applied to half the telescope pupil prior to focusing the telescope’s point spread function (PSF) into a single-mode fiber. The theoretical peak efficiency of the phase-knife fiber coronagraph, 35.2% for a circular telescope aperture, is found to be almost twice that of the next most efficient case, the vortex fiber nuller, at 19.0%.利用大型望远镜开展高对比度日冕成像所能探测到的类地系外行星（terrestrial exoplanets）的数量，受限于日冕仪可观测亮恒星的最小角偏移量。不过，通过在望远镜光瞳上实施消零干涉测量，可突破望远镜日冕观测的极限范围。事实上，“孔径交叉消零干涉测量”可比常规日冕仪更近距离地观测恒星，甚至可在恒星衍射核心内开展观测。因此，研发兼具极小内工作角（Inner Working Angle, IWA）与高系外行星光透射率的最优消零日冕仪具有重要的科研价值。为此，研究人员对现有消零方案开展了系统研究，并取得三项核心成果：其一，构建拓扑概览，将多孔径消零干涉仪与单孔径相位日冕仪统一至通用几何框架中；其二，基于该框架的空白区域，提出新型相位掩模日冕仪，本文将其命名为“分环日冕仪”；其三，明确了消零日冕仪的最优配置——孔径平面相位刀，即在将望远镜点扩散函数（Point Spread Function, PSF）聚焦至单模光纤前，对一半望远镜光瞳施加消色差π弧度相移。研究发现，针对圆形望远镜孔径，相位刀光纤日冕仪的理论峰值效率为35.2%，几乎是次优方案涡旋光纤消零器（vortex fiber nuller）理论峰值效率（19.0%）的两倍。