Ultra High Repetition Rate Terahertz Time Domain Spectroscopy System for Ranging and Imaging

The dissertation deals with the design, improvement, and potential applications of terahertz time-domain spectroscopy (THz-TDS) systems with ultra-high repetition rates. In the first chapter, the terahertz frequency range is placed in a historical context, with significant contributions and achievements being presented. The relevance and scientific interest in this frequency range are emphasized. In addition, an overview of previous developments in the terahertz range is given. The second chapter deals with the theoretical principles required to understand how mode-locked laser diodes work. These are used in combination with an optical delay unit to generate and detect terahertz radiation. Furthermore, different types of laser diodes and their possible applications in terahertz technology are discussed. In addition, three methods for distance measurement and imaging with terahertz systems are presented, which are used in the further course of the work. The third chapter examines the characterization of two specific mode-locked laser diodes in order to identify stable operating points. The extremely high repetition rate results from the short cavity and thus the compact design of the laser source used. Thanks to their compact size and semiconductor-based manufacturing technology, these laser diodes offer the potential for cost-effective and miniaturized THzTDS systems. Since the properties of the laser diodes directly influence the generation and detection of the terahertz signals, the definition of stable and well-controllable operating points is of crucial importance. With these operating points, further THz-TDS system variants based on asynchronous optical sampling or optical sampling by electronic repetition rate detuning can be realized. The most significant system improvement is described in the fourth chapter: the integration of an interferometer. The same source that is used to generate and detect the terahertz radiation is also used to monitor the optical delay unit. This enables extremely precise sampling of the measurement signal and leads to a significantly increased measurement speed thanks to the use of faster delay units. In addition, the interferometer concept can also be transferred to conventional THz systems, which can also lead to potential improvements and cost reductions here. The fifth chapter examines applications in range measurement and imaging based on the system concepts described above. First, the terahertz system is presented as a correlation radar, which achieves accuracy in the micrometer range. Subsequently, the same system is combined with a delay unit, which enables measurement speeds of up to 700 measurements per second and used for two-dimensional and three-dimensional synthetic aperture imaging radar measurements. The results demonstrate the potential of this system in imaging radar technology. However, the system has one limitation: The high repetition rate leads to a small uniqueness range of 3 mm. To enable measurements outside this range, an additional modulation method is being investigated in which the optical signal is amplitude modulated with a harmonic signal in the gigahertz range using a Mach-Zehnder modulator. The analytical description of the expected measurement signal is discussed and compared with experimental results. The final chapter summarizes the results of the work and provides an outlook for future research.