Flexible spectrally tunable radiation sources for multi modal spectroscopic analysis

该项目的长期愿景是实现一种可调谐的光源，用于紫外光、近红外和太赫兹范围内的同时多模式传感器应用。我们在应用阶段的目标是在单片可调谐二极管激光器的基础上开发一种同时用于太赫兹和拉曼光谱的传感器。太赫兹和拉曼光谱处理不同的激发，相应的附加信息对于几个应用是有趣的。作为应用实例，我们的目标是用太赫兹光谱检测相关废气，用拉曼光谱检测烟尘颗粒。这种传感器的许多其他可能应用之一是控制包装食品：即使通过包装，食品的质量也可以通过拉曼光谱进行分析，而太赫兹光谱将能够分析包装内的气氛(例如水分含量)。如果有可能实现一种双色辐射源，其中两个波长的光谱距离可以从0到6.4 nm电调谐，则可以通过不同的频率产生来覆盖光谱相关的范围von 0到3THz(例如，对于在排气过程中相关的气体N2O、NO、CO、NO2、SO2和H2O)。随着相应的光谱距离达到7 nm，人们还可以将宽广的拉曼特征(例如非晶态碳)与荧光或环境光等干扰背景区分开来。因此，我们的项目目标是实现一种基于半导体激光器的单片集成辐射源，能够实现波长间隔高达7 nm的可调双色操作。在此激光光源的基础上，我们的目标是实现一种多模式传感器，利用太赫兹光谱分析相关废气，利用拉曼光谱分析碳烟颗粒。这一发展的一个基础是提高对激光设备横向设计(波导曲率、臂的耦合)的理解，以及对太赫兹产生和拉曼光谱对这种光源的要求的理解。我们选择830 nm作为多色半导体激光器的中心波长。一方面，这是一个很好的拉曼光谱波长，它减少了可能出现的荧光，并且可以使用所需的滤光片。使用硅基探测器可以通过光谱指纹检测拉曼光谱以进行物质识别。另一方面，该波长也非常适合于LT-GaAs光导天线的THz产生，因为它与LT-GaAs的吸收光谱很好地匹配。

The long-term vision of this project is to realise a tunable light source for simultaneous multi modal sensor applications in the UV, in the near Infrared, and in the THz-range. Our goal for the application period is to develop a sensor for simultaneous THz- and Raman spectroscopy on the basis of monolithically tunable diode lasers. THz- and Raman spectroscopy address different excitations and the corresponding additional information is interesting for several applications. As an application example, we aim to detect relevant exhaust gases via THz-spectroscopy and soot particles via Raman spectroscopy. One out of many other possible applications of such a sensor is to control packaged food: even through the packaging the quality of the food could be analysed by Raman spectroscopy, and THz spectroscopy would enable to analyse the atmosphere within the package (e.g. water content). If it was possible to realise a two-colour radiation source in which the spectral distance of the two wavelengths could be tuned electrically from 0 to 6.4 nm, one could cover the spectroscopically relevant range von 0 to 3 THz (e.g. for the gases N2O, NO, CO, NO2, SO2 and H2O which are relevant in exhaust processes) by difference frequency generation. With the corresponding spectral distances up to 7nm one could also separate broad Raman features (e.g. of amorphous carbon) from disturbing background like fluorescence or ambient light. Therefore, our project goal is to realise a diode-laser based monolithically integrated radiation source that enables a tunable two-colour operation with up to 7 nm wavelength separation. On the basis of this laser source, we aim to realize a multi-modal sensor which analyses relevant exhaust gases by THz spectroscopy and soot particles by Raman spectroscopy. A basis for this development is to improve the understanding of the laser devices with regard to their lateral design (curvature of waveguides, coupling of arms) and the understanding of the requirements for this light source given by the THz generation and Raman spectroscopy. We chose 830 nm as the center wavelength for the multi-colour diode laser. On the one hand, this is a well established wavelength for Raman spectroscopy which reduces potentially appearing fluorescence, and for which required filter elements are available. A detection of the Raman-spectra for substance identification through spectral fingerprints is possible with Silicon based detectors. On the other hand, this wavelength is also well suited for THz generation with LT-GaAs based photoconductive antennas since it fits well to the absorption spectrum of LT-GaAs.