Terahertz ranging and imaging of arbitrarily shaped objects at unknown distances with monolithic mode-locked laser diodes

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The ongoing development of THz technology, in particular compact and user-friendly THz systems, is increasingly opening up new fields of application. In addition to industrial quality control and communications engineering, THz systems will be increasingly used in user-defined scenarios in the future, such as in the field of art conservation and cultural heritage research. For example, THz imaging as a complementary tool to X-rays has already provided new insights into important works of art and ancient human remains. However, the THz systems used, which are based on fiber lasers, can only be used with great effort during on-site examinations. Furthermore, they are either only suitable for imaging planar surfaces or require complex mechanics such as a robotic arm. Here, the shape and position of the object must be known before the actual measurement process, so that currently only measurements in the laboratory are feasible.Within the scope of this project, both challenges are to be overcome. On the one hand, a compact THz system is to be developed, which is operated by a mode-locked laser diode. The high repetition rate of the laser diode allows the use of a short delay unit and the detection of THz pulses from any distance. The high stability of the mode-locked laser diodes is exploited to reconstruct the time axis of the THz signal with a high accuracy. This allows the use of simple and compact delay units without sacrificing precision. Remaining systematic errors of the delay unit are corrected by a genetic algorithm. A programmable optical filter is used to optimize the bandwidth and signal-to-noise ratio of the system. The ambiguity in the range measurement caused by the high repetition rate is resolved by two methods. One is to use the phase of the lock-in amplifier to determine the length of the THz free space path. Another approach relies on amplitude modulation of the optical signal feeding the THz optoelectronic antennas. A frequency sweep can also be used to determine the distance between the THz system and the target. In order to image objects and hidden objects of arbitrary shape without a priori knowledge, a divergent beam based method is adapted for THz systems with ultra-high repetition rate. Here, image reconstruction is realized in a post-processing step using Kirchhoff migration.As a result of this project, methods for a THz imaging system are developed, which in principle can be completely integrated. With the algorithms developed for this system, objects of arbitrary shape can be measured outside an optical laboratory.