Design und Evaluation von Hardware-Architekturen zur Stabilisierung verstimmbarer Diodenlaser unter Weltraumbedingungen

authored by: Christian Ulrich Spindeldreier
supervised by: Holger Christoph Blume
Abstract: Frequency stability of light sources is a fundamental requirement for the generation of Bose-Einstein condensates, which are used in quantum mechanical tests of the universality of free fall in the MAIUS-2 and MAIUS-3 sounding rocket missions. Frequency stabilization of the used tunable diode lasers is a major challenge due to the constraints of a sounding rocket as well as the extreme environmental conditions during flight. Especially the fully autonomous operation as well as the possibility to compensate even strong deviations from the nominal frequency can only be realized with very high effort using well-known frequency stabilization methods while keeping the restricted power budget. The main objective of this work is therefore the implementation, evaluation and experimental demonstration of a novel approach for frequency stabilization of diode lasers. The focus is on the identification and evaluation of suitable algorithms as well as signal processing platforms considering the power limitation of the payload of a sounding rocket. To compensate for the limitations of known methods, a fully digital frequency stabilization method is proposed. In essence, it is based on determining the laser frequency using a short spectroscopy signal generated by a linear frequency ramp, whose position in the overall spectrum is determined using pattern matching algorithms. For this application, different correlation based pattern matching algorithms in the time and Fourier domain are evaluated with respect to the remaining frequency error and the required execution time. The latter must remain as low as possible in order to achieve a high control frequency. All the algorithms considered show suitability in principle, with the sum of absolute differences (SAD) and the sum of squared differences (SSD) in particular being identified as well suited during the evaluation. Based on these results, the mapping to different FPGAs and SoC-FPGAs already used in the MAIUS project is examined in order to obtain a system that is as compact and energy-efficient as possible. Next to the description of the necessary hardware modules for signal generation and extraction, the mapping of the pattern matching algorithms to the processor of an SoC-FPGA, two soft cores or dedicated hardware modules is evaluated in detail. The results genreate a design space spanning 5 orders of magnitude (60 μs to 7 s) in terms of execution time and 2 orders of magnitude (150 mW to 3 W) in terms of power consumption. The lowest power dissipation in combination with the highest control frequencies can be achieved through the complex mapping of the SAD into a dedicated, scalable hardware module. Depending on the number of parallel core modules, this allows a control frequency of up to 13 kHz. This module is then used to build a complete FPGA based frequency stabilization system. It is used to demonstrate and evaluate the pattern matching based laser frequency stabilization method. Here, a frequency stability of 15 MHz (±7.5 MHz) around the center frequency of 384.231 THz is achieved over an observation period of more than 3 h when analyzing internal error values. This value is confirmed by a beat measurement with an external reference laser. Based on the control clock of 95 Hz of the demonstration system, the presented method is expected to achive significantly higher frequency stability when combining it with an optomized optical setup. If the maximum possible control clock of up to 13 kHz of the digital system can be exploited, it will presumable reach a frequency stability in the range of up to 1 MHz.
Organisation(s): Architectures and Systems Section
Type: Doctoral thesis
No. of pages: 178
Publication date: 2021
Publication status: Published
Electronic version(s): https://doi.org/10.15488/11020 (Access: Open)