Supra-terahertz technology for atmospheric and lower thermosphere and lower thermosphere

在过去十年中，卫星遥感测量地球中间层和低热层(MLT)组成的进展增加了我们对大气组成的了解。尽管如此，以前的卫星飞行任务没有直接对关键大气物种进行全球测量，这些物种，特别是原子氧和羟基自由基，是在多太赫兹(THz)光谱范围(3-5太赫兹)运行的低地球轨道飞行任务的目标。因此，向欧空局提出了一种低成本的高层大气探测仪(LOCAUS)，它将能够探测到50至400公里高度之间的各种重要物种(O、O3、OH、NO、CO、H2O和HO2)。外差辐射测量提供了一种光谱分辨率，非常适合于表征来自MLT的发射特征。在过去的二十年里，这项技术已经在亚太赫兹频率下通过一些太空飞行任务得到了证明和验证。然而，从未尝试在空间环境中进行3太赫兹(超太赫兹)以上的工作，因此没有对一些可能对气候变化和相关的空间气象影响产生影响的重要大气物种进行测量。即使是在空中平台上操作的系统也很少见，需要完全不适合太空飞行的大型仪器。因此，需要开发能够从空间进行全球大气测量的紧凑型、高灵敏度、超太赫兹外差系统。为了实现这一目标，需要开发外差混频探测器及其本振(LO)技术。用于对地观测的首选外差混频装置是肖特基势垒二极管。尽管它是一种著名的半导体器件，但它还没有以超过3THz的平面形式展示出来，需要解决与制造和电路嵌入相关的挑战，才能实现这一技术进步。此外，向混频二极管提供LO功率及其耦合，虽然在亚太赫兹频率已经存在技术障碍，但在超太赫兹范围内是一个特别难解决的问题。幸运的是，量子级联激光器(QCL)半导体器件的出现提供了一种小型化、低功率、超太赫兹LO源的前景，作为频率下转换过程的一部分，该光源具有足够的输出功率来‘泵浦’混频二极管。此外，电磁模拟软件现在可以分析和优化QCL和肖特基二极管器件及其各自的电气嵌入电路，并采用先进的新微制造技术进行相应的制造。然而，在实现超太赫兹MLT遥测仪器之前，还需要技术开发。例如，QCL和肖特基器件的性能优化、与公共(波导)封装的物理集成以及频率稳定是必要的。因此，我们提出了一个概念验证开发计划，目标是将关键组件技术(QCL和肖特基二极管)演示到最低技术准备水平TRL 3。在该计划中，我们将通过逐步开发的方法显著推进核心外差技术，目标是在公共波导安装中集成和测试QCL和肖特基二极管。还将考虑TRL 4及以后的科学应用和未来技术发展。欧空局已接受轨道概念，认为这一概念需要进一步评估，作为未来在轨演示的前奏。通过国家空间研究中心的这一概念验证方案，太赫兹频谱仪的技术进步将使朝着这一重要科学目标取得的进展有一个阶段性的变化，并为联合王国未来与欧空局的在轨方案做好准备。

Advances in satellite remote-sensing measurements of the constituents of the Earth's mesosphere and lower thermosphere (MLT) have increased our knowledge of atmospheric composition over the last decade. Nonetheless, global measurements of key atmospheric species have not been made directly by previous satellite missions and these species, particularly atomic oxygen and the hydroxyl radical (OH), are targets for a low Earth orbit mission operating in the multi-terahertz (THz) spectral range (3 - 5 THz). A LOw Cost Upper-Atmosphere sounder (LOCUS) has therefore been proposed to ESA, which would be able to detect a broad range of important species (O, O3, OH, NO, CO, H2O and HO2) between altitudes of 50 and 400 km.Heterodyne radiometry provides a spectral resolution that is well suited to characterising emission signatures originating from the MLT. The technique has been demonstrated and proven at sub-terahertz frequencies through a number of space flight missions over the past two decades. However, operation above 3 THz (supra-terahertz) has never been attempted from a space environment, and measurements of a number of important atmospheric species that have potential impact on climate change and related space weather effects have therefore not been made. Even systems operated from an airborne platform are rare and require large instruments that are completely unsuitable for space flight. There is therefore a need to develop compact, high-sensitivity, supra-terahertz heterodyne systems capable of undertaking global atmospheric measurements from space. To achieve this goal, technical development of the heterodyne mixer detector and its local oscillator (LO) is required.The preferred heterodyne mixing device for Earth observation is the Schottky barrier diode. Although a well-known semiconductor device, it has not been demonstrated in a planar form beyond ~3 THz and challenges related to fabrication and circuit embedding need to be solved to allow this technical evolution. Additionally, the provision of LO power and its coupling to the mixer diode, whilst already presenting a technical barrier at sub-terahertz frequencies, is a particularly difficult problem to resolve in the supra-terahertz range. Fortunately, the advent of the quantum cascade laser (QCL) semiconductor device provides the prospect of a miniaturized, low power, supra-terahertz LO source with sufficient output power to 'pump' the mixer diode as a part of the frequency down-conversion process. Additionally, electromagnetic simulation software now permits the analysis and optimisation of QCL and Schottky diode devices and their respective electrical embedding circuits, with new and advanced micro-fabrication techniques allowing corresponding manufacture. However, technical development is required before a supra-terahertz MLT remote sounding instrument can be realised. For instance, QCL and Schottky device performance optimisation, physical integration into a common (waveguide) package, and frequency stabilisation are necessary.We therefore propose a proof-of-concept development programme with an objective of demonstrating key component technologies (QCL and Schottky diode) to a minimum technical readiness level of TRL 3. Within this programme we will significantly advance core heterodyne technologies through a stepwise development approach, and with a goal of integrating and testing a QCL and Schottky diode in a common waveguide mount. Consideration will also be given to the scientific application and future technical development towards TRL 4 and beyond.ESA has accepted the LOCUS concept as one requiring further evaluation as a prelude to a future in-orbit demonstration. Technical advancement of a terahertz frequency spectrometer through this NERC Proof of Concept Programme would provide a step-change in the progress towards this important scientific objective, as well as positioning the UK ideally for future in-orbit programmes with ESA.