Supra-terahertz technology for atmospheric observations of the mesosphere and lower thermosphere

在过去十年中，卫星遥感测量地球中间层和低热层成分方面的进展增加了我们对大气成分的了解。尽管如此，以前的卫星飞行任务并没有直接对关键的大气物质进行全球测量，这些物质，特别是原子氧和羟基自由基，是在多太赫兹光谱范围（3 - 5太赫兹）内运行的低地球轨道使命飞行任务的目标。因此，已向欧空局提出了一种低成本高层大气探测器，该探测器将能够探测50至400公里高度之间的各种重要物质（O、O3、OH、NO、CO、H2O和HO 2）。在过去二十年中，该技术已通过多次空间飞行任务在亚太赫兹频率上得到演示和证明。然而，从未尝试过在空间环境中进行3太赫兹以上的操作，因此尚未对可能影响气候变化和相关空间气象效应的一些重要大气物质进行测量。即使是从空中平台操作的系统也很少见，需要大型仪器，完全不适合太空飞行。因此，需要开发能够从太空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，需要外差混频器检测器及其本振（LO）的技术发展。对地观测的首选外差混频装置是肖特基势垒二极管。虽然是一种众所周知的半导体器件，但它尚未以超过~3 THz的平面形式展示，并且需要解决与制造和电路嵌入相关的挑战以实现这种技术发展。此外，LO功率的提供及其与混频器二极管的耦合虽然在亚太赫兹频率处已经存在技术障碍，但在超太赫兹范围内是一个特别难以解决的问题。幸运的是，量子级联激光器（QCL）半导体器件的出现提供了小型化、低功率、超太赫兹LO源的前景，该超太赫兹LO源具有足够的输出功率来“泵浦”混频器二极管作为下变频过程的一部分。此外，电磁仿真软件现在允许QCL和肖特基二极管器件及其各自的电气嵌入电路的分析和优化，新的和先进的微制造技术允许相应的制造。然而，在实现超太赫兹MLT远程探测仪器之前，需要进行技术开发。例如，QCL和肖特基器件性能优化、物理集成到通用（波导）封装中以及频率稳定都是必要的。因此，我们提出了一个概念验证开发计划，目的是将关键组件技术（QCL和肖特基二极管）演示到TRL 3的最低技术准备水平。在该计划中，我们将通过逐步开发的方法显着推进核心外差技术，并在一个共同的波导安装集成和测试QCL和肖特基二极管的目标。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LOCUS概念，认为这是一个需要进一步评价的概念，作为未来在轨演示的前奏。通过该NERC概念验证方案，太赫兹频谱仪的技术进步将为实现这一重要科学目标的进展提供一个步骤性变化，并使英国成为欧空局未来在轨方案的理想选择。

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.