A Dual-Polarisation Receiver for Multi-Beam Interferometry

天文学中的干涉测量是一种利用一系列望远镜获得高空间分辨率和极其详细的观测恒星、星系、分子云、原行星盘和其他有趣的恒星物体的技术。在毫米和亚毫米范围内，这些望远镜通常配备一个外差接收器，将探测到的天文信号从几百千兆赫降至几千兆赫，以便信号可以通过标准电子设备进行处理。为了实现探测极弱天文信号的高灵敏度，这些接收器需要使用超导量子混合器（SIS，超导-绝缘体-超导探测器），需要冷却到超导材料的转变温度以下（通常为4K）。此外，为了恢复完整的信号强度，双极化检测方案需要将输入信号的极化分解为两个正交的极化状态，每个偏振状态都通过单独的SIS接收链进行检测，导致接收机结构庞大而复杂。由于这种复杂性，世界上几乎所有主要的毫米/亚毫米设施，如亚毫米阵列（SMA，夏威夷）和阿塔卡马大型毫米/亚毫米阵列（ALMA，智利），每个望远镜都只配备了一个像素的接收器。然而，在每台望远镜的焦平面上安装更多的像素（即多光束干涉测量法）将允许对扩展对象（如附近星系）进行快速映射，否则就必须通过拼接单个指向来完成。这很重要，因为它将使天文学家能够更快、更远地探索，提高我们对行星、恒星和星系形成的理解，这是STFC概述的关键研究主题之一。双偏振接收器也是b模宇宙微波背景（CMB）实验的关键仪器，该实验旨在了解宇宙的起源，这是STFC计划的另一个主要研究领域。该项目的主要目标是建立一个紧凑的双偏振接收器，允许在每个望远镜接收器中安装多个通道（像素），而不会对现有望远镜的低温恒温器尺寸或冷却能力有过多的要求。本设计的主要特点是将两个SIS混频器链集成在单个芯片上并位于单个块中，使用新型平面正交换能器（OMT）来分离极化。该项目将与美国马萨诸塞州剑桥市的哈佛-史密森天体物理中心（CfA）合作进行，我们将与他们在接收器设计和建造的各个方面进行合作。该学生将开发工作在230 GHz左右的紧凑型双偏振SIS接收器，首次在外差接收器上展示这项开创性技术。SIS混频器芯片将在牛津大学的太赫兹探测器实验室进行测试，接收器也将在这里组装。在完成后，接收器将被转移到CfA进行进一步整合，然后将其安装在SMA的焦平面上，SMA是一个由8个可移动的6米直径望远镜组成的干涉仪，位于夏威夷的莫纳克亚山。该接收机也将成为未来构建多像素外差接收机阵列的基石，一旦该技术被证明是可行的，就可以实现SMA甚至ALMA的多波束干涉测量。

Interferometry in astronomy is a technique that employs an array of telescopes to obtain high spatial resolution and extremely detailed observation of stars, galaxies, molecular clouds, proto-planetary disk and other interesting stellar objects. In millimetre (mm) and sub-mm range, each of these telescopes is generally equipped with a heterodyne receiver that down-coverts the detected astronomical signal from several hundred GHz to a few GHz, so that the signal can be processed by standard electronics. To achieve high sensitivity for detecting extremely weak astronomical signals, these receivers require the employment of superconducting quantum mixers (SIS, Superconductor-Insulator-Superconductor detector) that need to be cooled below the transition temperature of the superconducting material (typically 4K). In addition, to recover the full signal strength, dual-polarization detection scheme is required to split the polarization of the incoming signal into two orthogonal polarization states, where each is detected via a separate SIS receiver chain, resulting in bulky and complicated receiver architecture. Due to this complexity, almost all the major mm/sub-mm facilities in the world, such as the Sub-Millimetre Array (SMA, Hawai'i) and the Atacama Large Millimetre/Sub-Millimetre Array (ALMA, Chile), have only one-pixel receiver equipped in each of the telescope. Installing more pixels however at the focal plane of each telescope (namely multiple-beam interferometry) will allow fast mapping of extended object such as nearby galaxies, that otherwise would have to be carried out with mosaicking of individual pointing. This is important as it will allow the astronomers to probe further and faster, improving our understanding of the planet, star and galaxies formation, one of the key research themes outlined by STFC. Dual polarisation receiver is also the key instrument in the B-mode Cosmic Microwave Background (CMB) experiment in search for the understanding of the origin of the Universe, another major research area within the STFC programme.The main objective of this project is to build a compact dual-polarization receiver that will allow the installation of several channels (pixels) in each telescope receiver without excessive requirements on the cryostat size or cooling power of existing telescopes. The key feature of this design is that the two SIS mixer chains will be integrated on a single chip and located in a single block, using a novel planar orthomode transducer (OMT) to split the polarization.The project will be carried out in collaboration with the Harvard-Smithsonian Centre for Astrophysics (CfA) at Cambridge, Massachusetts, US, with whom we will collaborate in all aspects of the receiver design and construction. The student will develop the compact dual-polarization SIS receiver operating around 230 GHz to demonstrate this pioneering technology for the first time in heterodyne receivers. The SIS mixer chip will be tested in the THz Detectors Laboratory at Oxford and the receiver will also be assembled here. Whence completed, the receiver will be transferred to CfA for further integration before shipping for installation in the focal plane of the SMA, an interferometer comprising eight movable 6-meter diameter telescope sited on the Mauna Kea of Hawai'i. This receiver will also constitute a building block for the future construction of multi-pixel heterodyne receiver array and hence realizing multiple-beam interferometry for the SMA, and even ALMA once this technology is proven to be feasible.