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

Die Frequenzstabilität von Lichtquellen ist eine grundlegende Voraussetzung für die Erzeugung von Bose-Einstein-Kondensaten, die im Rahmen der MAIUS-2 und MAIUS-3 Höhenforschungsmissionen für quantenmechanische Tests der Universalität des freien Falls genutzt werden. Die Frequenzstabilisierung der verwendeten verstimmbaren Diodenlaser stellt aufgrund der Einschränkungen einer Höhenforschungsrakete sowie der extremen Umwelteinflüsse während des Fluges eine große Herausforderung dar. Insbesondere der vollständig autonome Betrieb sowie die Möglichkeit auch starke Abweichungen von der Sollfrequenz zu kompensieren, lässt sich mit bekannten Frequenzstabilisierungsmethoden unter Wahrung des begrenzten Leistungsbudget nur mit sehr hohem Aufwand realisieren. Hauptziel dieser Arbeit ist daher Umsetzung, Evaluation und experimentelle Demonstration eines neuartigen Ansatzes zur Frequenzstabilisierung von Diodenlasern. Dabei liegt der Fokus auf der Identifikation und Evaluation geeigneter Algorithmen sowie Signalverarbeitungsplattformen unter Berücksichtigung der Leistungsbeschränkung der Nutzlast einer Höhenforschungsrakete. Um die genannten Einschränkungen bekannter Verfahren zu kompensieren, wird eine vollständig digitale Frequenzstabilisierungsmethode vorgeschlagen. Im Kern beruht diese auf der Bestimmung der Laserfrequenz anhand eines kurzen, durch eine lineare Frequenzrampe erzeugten, Spektroskopiesignals, dessen Position im Gesamtspektrum mithilfe von Pattern-Matching-Algorithmen bestimmt wird. Für diese Anwendung werden verschiedene korrelationsbasierte Pattern-Matching-Algorithmen im Zeit- und Fourier-Bereich im Hinblick auf den verbleibenden Frequenzfehler sowie die nötige Ausführungszeit hin untersucht. Letztere muss möglichst gering bleiben, um einen hohen Regeltakt erreichen zu können. Alle betrachteten Algorithmen zeigen dabei eine prinzipielle Eignung, wobei insbesondere die Summe der Absoluten Differenzen (SAD) und die Summer der quadratischen Differenzen (SSD) bei der Evaluation als besonders gut geeignet identifiziert werden. Um ein möglichst kompaktes, leistungseffizientes System zu erhalten, wird ausgehend von diesen Ergebnissen, die Abbildung auf verschiedene, bereits im MAIUS-Projekt verwendete FPGAs und SoC- FPGAs untersucht. Neben der Beschreibung notwendiger Hardware-Module zur Signal-Generation und Extraktion wird dabei die Abbildung der Pattern-Matching-Algorithmen auf den Prozessor eines SoC-FPGAs, zwei Softcores sowie in dedizierte Hardware-Module betrachtet und detailliert evaluiert. Dabei ergibt sich ein Entwurfsraum, der sich über 5 Größenordnungen (60 μs bis 7 s) im Bezug auf die Ausführungszeit und 2 Größenordnungen (150 mW bis 3 W) im Bezug auf die Leistungsaufnahme erstreckt. Die geringste Verlustleistungsaufnahme bei hohen Regeltakten lässt sich mit der aufwendigen Abbildung der SAD in ein dediziertes, skalierbares Hardware-Modul erreichen. Dieses erlaubt abhängig von der Anzahl paralleler Kern-Module einen Regeltakt von bis zu 13 kHz. Mit diesem Modul wird anschließend ein vollständiges FPGA-basiertes Frequenzstabilisierungssystem aufgebaut. Dieses wird für die Demonstration und Evaluation der Pattern-Matching-basierten Laserfrequenzstabilisierungsmethode verwendet. Dabei wird bei der Analyse interner Fehlerwerte eine Frequenzstabilität von 15 MHz (±7,5 MHz) um die Mittenfrequenz von 384,231 THz über eine Beobachtungsdauer von mehr als 3 h erreicht. Dieser Wert wird durch eine Schwebungsmessung mit einem externen Referenzlaser bestätigt. Ausgehend von dem Regeltakt von 95 Hz des Demonstrationssystems ist zu erwarten, dass mit der vorgestellten Methode und mit einem optimierten optischen Aufbau eine noch deutlich höhere Frequenzstabilität im Bereich von bis zu 1 MHz realisierbar ist, wenn der maximal mögliche Regeltakt von bis zu 13 kHz des digitalen Systems ausgenutzt werden kann.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

A Dual-Species Atom Interferometer Payload for Operation on Sounding Rockets

We report on the design and the construction of a sounding rocket payload capable of performing atom interferometry with Bose-Einstein condensates of 41 K and 87 Rb. The apparatus is designed to be launched in two consecutive missions with a VSB-30 sounding rocket and is qualified to withstand the expected vibrational loads of 1.8 g root-mean-square in a frequency range between 20–2000 Hz and the expected static loads during ascent and re-entry of 25 g. We present a modular design of the scientific payload comprising a physics package, a laser system, an electronics system and a battery module. A dedicated on-board software provides a largely automated process of predefined experiments. To operate the payload safely in laboratory and flight mode, a thermal control system and ground support equipment has been implemented and will be presented. The payload presented here represents a cornerstone for future applications of matter wave interferometry with ultracold atoms on satellites

The Bose-Einstein Condensate and Cold Atom Laboratory

Microgravity eases several constraints limiting experiments with ultracold andcondensed atoms on ground. It enables extended times of flight withoutsuspension and eliminates the gravitational sag for trapped atoms. Theseadvantages motivated numerous initiatives to adapt and operate experimentalsetups on microgravity platforms. We describe the design of the payload,motivations for design choices, and capabilities of the Bose-Einstein Condensateand Cold Atom Laboratory (BECCAL), a NASA-DLR collaboration. BECCALbuilds on the heritage of previous devices operated in microgravity, featuresrubidium and potassium, multiple options for magnetic and optical trapping,different methods for coherent manipulation, and will offer new perspectives forexperiments on quantum optics, atom optics, and atom interferometry in theunique microgravity environment on board the International Space Station

FPGA based laser frequency stabilization using FM spectroscopy

Frequency stabilized light sources with narrow linewidth are mandatory for atom interferometry based experiments. For compact experiment designs used on space platforms, tunable DFB diode lasers are often used. These lasers combine low energy consumption with small sizes, but lack long-term frequency stability. This paper presents an FPGA based laser frequency stabilization system for highly variable target frequencies using frequency modulated Rb-spectroscopy achieving latencies below 100 μs. The system consists of a DFB laser, a Rb-spectroscopy cell, a laser current controller and an FPGA board with an analog-digital conversion board. The digital part of the frequency stabilization system is a SoC mapped on an FPGA. The SoC consists of a processor, enabling user interaction via network connection, and the dedicated frequency stabilization module. This module consists of a demodulation stage, digital filters, a frequency estimator and a controller. To estimate the frequency, small ramps of the laser frequency are generated using a high-speed DAC connected to the laser current controller. The absorption spectroscopy output of this beam is sampled using a photodiode and a high-speed ADC. After signal conditioning with digital filters, the frequency estimator extracts the present mid-frequency of the laser applying pattern matching with a prerecorded reference spectrum. The frequency controller adjusts the mean laser current based on this estimation. The performance as well as the accuracy of the proposed laser stabilization system and its FPGA resource and power consumption are evaluated

QUANTUM GASES ABOARD THE ISS - CAPABILITIES OF THE BECCAL FACILITY

BECCAL (Bose-Einstein-Condensate - Cold Atom Laboratory) is an experiment designed to be housed on the International Space Station (ISS) within a bilateral collaboration between DLR and NASA. The payload's design and operation are based on the previous quantum experiments under microgravity, QUANTUS (drop tower), MAIUS (sounding rocket), and CAL (NASA operated ISS experiment). The scienti�c capabilities, outlined here, cover a wide range of cold atom manipulation and observation. Additionally, the payload strives to pave the road for future microgravity missions housing cold atom ensembles

A Dual-Species Atom Interferometer Payload for Operation on Sounding Rockets

We report on the design and the construction of a sounding rocket payload capable of performing atom interferometry with Bose-Einstein condensates of K and Rb. The apparatus is designed to be launched in two consecutive missions with a VSB-30 sounding rocket and is qualified to withstand the expected vibrational loads of 1.8 g root-mean-square in a frequency range between 20-2000 Hz and the expected static loads during ascent and re-entry of 25 g. We present a modular design of the scientific payload comprising a physics package, a laser system, an electronics system and a battery module. A dedicated on-board software provides a largely automated process of predefined experiments. To operate the payload safely in laboratory and flight mode, a thermal control system and ground support equipment has been implemented and will be presented. The payload presented here represents a cornerstone for future applications of matter wave interferometry with ultracold atoms on satellites