We fabricated a 302 mm diameter low-pass filter made of alumina that has an anti-reflection coating (ARC) made with laser-ablated sub-wavelength structures (SWS). The filter has been integrated into and is operating with the MUSTANG2 instrument, which is coupled to the Green Bank Telescope. The average transmittance of the filter in the MUSTANG2 operating band between 75 and 105 GHz is 98%. Reflective loss due to the ARC is 1%. The difference in transmission between the s- and p-polarization states is less than 1%. To within 1% accuracy we observe no variance in these results when transmission is measured in six independent filter spatial locations. The alumina filter replaced a prior MUSTANG2 Teflon filter. Data taken with the filter heat sunk to its nominal 40 K stage show performance consistent with expectations: a reduction of about 50% in filters-induced optical power load on the 300 mK stage, and in in-band optical loading on the detectors. It has taken less than 4 days to laser-ablate the SWS on both sides of the alumina disk. This is the first report of an alumina filter with SWS ARC deployed with an operating instrument, and the first demonstration of a large area fabrication of SWS with laser ablation.

中文翻译：

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大直径毫米波低通滤波器，由氧化铝制成，带有激光烧蚀减反射涂层

我们制造了一个直径为 302 毫米的氧化铝低通滤波器，该滤波器具有由激光烧蚀亚波长结构 (SWS) 制成的抗反射涂层 (ARC)。该过滤器已集成到 MUSTANG2 仪器中并与其一起使用，该仪器与 Green Bank 望远镜耦合。滤波器在 MUSTANG2 工作频段 75 至 105 GHz 之间的平均透射率为 98%。由 ARC 引起的反射损耗为 1%。s 和 p 偏振态之间的透射差异小于 1%。当在六个独立的过滤器空间位置测量传输时，我们观察到这些结果的精度在 1% 以内没有变化。氧化铝过滤器取代了之前的 MUSTANG2 Teflon 过滤器。将过滤器热沉到其标称 40 K 级时获取的数据显示性能与预期一致：300 mK 级上的滤波器引起的光功率负载减少了约 50%，检测器上的带内光负载减少了约 50%。用激光烧蚀氧化铝盘两侧的 SWS 只用了不到 4 天。这是第一份关于使用操作仪器部署 SWS ARC 的氧化铝过滤器的报告，也是使用激光烧蚀大面积制造 SWS 的首次演示。