A Dual-Polarisation Receiver for Multi-Beam Interferometry

天文学中的干涉测量学是一种利用一组望远镜来获得高空间分辨率和对恒星、星系、分子云、原行星盘和其他有趣的恒星物体进行极其详细的观测的技术。在毫米和亚毫米范围内，这些望远镜中的每一个通常都配备了外差接收器，将探测到的天文信号从几百GHz下转换到几GHz，以便标准电子学可以处理信号。为了获得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)，这些混合器需要冷却到超导材料的转变温度(通常为4K)以下。此外，为了恢复完整的信号强度，需要采用双极化检测方案，将输入信号的极化分为两个正交的极化状态，每个状态通过单独的SIS接收器链进行检测，导致接收器结构庞大和复杂。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(夏威夷)和阿塔卡马大毫米/亚毫米阵列(智利阿尔马)，每个望远镜只配备了一个像素接收器。然而，在每个望远镜的焦平面上安装更多的像素(即多光束干涉测量)将允许快速绘制扩展对象，如附近的星系，否则必须通过单个指向的马赛克来执行。这一点很重要，因为它将使天文学家能够更远更快地进行探测，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。双极化接收器也是B型宇宙微波背景(CMB)实验中的关键仪器，该实验旨在探索宇宙起源的理解，这是STFC计划中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极化接收器，允许在每个望远镜接收器中安装几个通道(像素)，而不会对现有望远镜的低温恒温器大小或冷却功率提出过高要求。这一设计的主要特点是，两个SIS混频器链将集成在一个芯片上，并位于单个模块中，使用新型的平面正交模换能器(OMT)来分离极化。该项目将与位于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，我们将与该中心在接收器设计和建造的所有方面进行合作。学生将开发工作在230 GHz左右的紧凑型双极化SIS接收器，以首次在外差接收器中展示这项开创性的技术。SIS混频芯片将在牛津的太赫兹探测器实验室进行测试，接收器也将在这里组装。完成后，接收器将被转移到CFA进行进一步集成，然后装运到SMA的焦平面上。SMA的焦平面上安装了一个干涉仪，该干涉仪包括8个可移动的直径6米的望远镜，安装在夏威夷的莫纳克亚。这个接收器也将构成未来构建多像元外差接收器阵列的基础，从而实现SMA的多光束干涉测量，一旦这项技术被证明是可行的，甚至ALMA。

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.