The Any Light Particle Search experiment at DESY

The Any Light Particle Search (ALPS~II) is a light shining through a wall (LSW) experiment searching for axion-like elementary particles in the sub-eV mass range, which are motivated by astrophysics and cosmology and fulfill the requirements for being dark matter. ALPS~II aims to measure an axion-to-photon coupling of $2\times 10^{-11}\,\mathrm{GeV^{-1}}$, which is several orders of magnitude better than that of previous LSW experiments and will thus investigate a new parameter range. The increased performance is achieved by enhancing the magnetic field interaction length to 2 $\times$ 106\,m and by amplifying the signal in an optical cavity on each side of a light-tight barrier. The expected signal is in the order of 1 photon per day, which will be measured by photon detectors with very low dark count rates of $\mathcal O(10^{-6}\,\mathrm{Hz})$. This article gives a technical overview on the experiment design, previous and ongoing investigations and the current status with focus on the single photon detection

Compact Multifringe Interferometry with Subpicometer Precision

Deep-frequency-modulation interferometry combines optical minimalism with multifringe readout. However, precision is key for applications such as optical gradiometers for satellite geodesy or as dimensional sensors for ground-based gravity experiments. We present a single-component interferometer smaller than a cubic inch. Two of these are compared to each other to demonstrate tilt and displacement measurements with a precision of less than 20 nrad/Hz and 1 pm/Hz at frequencies below 1 Hz

Applying differential wave-front sensing and differential power sensing for simultaneous precise and wide-range test-mass rotation measurements

We propose to combine differential wave-front sensing (DWS) and differential power sensing (DPS) in a Mach-Zehnder type interferometer for measuring the rotational dynamics of a test-mass. Using the DWS method, a high sensitive measurement of 6 nrad Hz−1/2 in sub-Hz frequencies can be provided around the test-mass nominal position (±0.11 mrad), whereas the measurement of a wide rotation range (±5 mrad) is realized by the DPS method. The interferometer can be combined with deep frequency modulation (DFM) interferometry for measurement of the test-mass translational dynamics. The setup and the resulting interferometric signals are verified by simulations. An optimization algorithm is applied to find suitable positions of the lenses and the waist size of the input laser in order to determine the best trade of between the slope of DWS, dynamic range of DPS, and the interferometric contrast. Our simulation further allows to investigate the layout for robustness and design tolerances. We compare our device with a recent experimental realization of a DFM interferometer and find that a practical implementation of the interferometer proposed here has the potential to provide translational and rotational test-mass tracking with state-of-the-art sensitivity. The simple and compact design, and especially the capability of sensing the test-mass rotation in a wide range and simultaneously providing a high-precision measurement close to the test-mass nominal position makes the design especially suitable for example for employment in torsion pendulum setups. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Laser-Frequency Stabilization via a Quasimonolithic Mach-Zehnder Interferometer with Arms of Unequal Length and Balanced dc Readout

Low-frequency high-precision laser interferometry is subject to excess laser-frequency-noise coupling via arm-length differences which is commonly mitigated by locking the frequency to a stable reference system. This approach is crucial to achieve picometer-level sensitivities in the 0.1-mHz to 1-Hz regime, where laser-frequency noise is usually high and couples into the measurement phase via arm-length mismatches in the interferometers. Here we describe the results achieved by frequency stabilizing an external cavity diode laser to a quasimonolithic unequal arm-length Mach-Zehnder interferometer readout at midfringe via balanced detection. We find this stabilization scheme to be an elegant solution combining a minimal number of optical components, no additional laser modulations, and relatively low-frequency-noise levels. The Mach-Zehnder interferometer is designed and constructed to minimize the influence of thermal couplings and to reduce undesired stray light using the optical simulation tool ifocad. We achieve frequency-noise levels below 100  Hz/Hz at 1 Hz and are able to demonstrate the LISA frequency prestabilization requirement of 300  Hz/Hz down to frequencies of 100 mHz by beating the stabilized laser with an iodine-locked reference.DFG/SFB/112

Suppressing ghost beams: Backlink options for LISA

In this article we discuss possible design options for the optical phase reference system, the so called backlink, between two moving optical benches in a LISA satellite. The candidates are based on two approaches: Fiber backlinks, with additional features like mode cleaning cavities and Faraday isolators, and free beam backlinks with angle compensation techniques. We will indicate dedicated ghost beam mitigation strategies for the design options and we will point out critical aspects in case of an implementation in LISA. © Published under licence by IOP Publishing Ltd.DFG/SFB/1128Deutsches Zentrum für Luft- und Raumfahrt (DLR)Bundesministerium für Wirtschaft und Technologie/50 OQ 0601NASA/NNX15AC48

Any light particle search: The ALPS II experiment at DESY

The ALPS II at DESY in Hamburg is a light-shining-through-a-wall (LSW) experiment searching for axion-like elementary particles in the sub eV mass range, which are motivated by astrophysics and cosmology. ALPS II aims at an axion-photon coupling sensitivity which is several orders of magnitude better than that of previous LSW experiments and will thus investigate a new parameter range. This is achieved by an increased magnetic field interaction length and by optical cavities on both sides of the wall, which further amplify the signal. In this Talk we will explain the working principle of LSW experiments and the uniqueness and technological challenges of the ALPS II experiment. Interim results of previous investigations will be presented as well as the current status. By converting axions to photons, we expect a rate of only 1 photon per day. The two sophisticated detector schemes, coherent heterodyne detection and a cryogenic single photon detector, are also presented and their implementation in the ALPS II experiment is discussed. ALPS II is aiming at a first data run in late 2021

Light-Shining-Through-A-Wall: The ALPS II experiment at DESY

The ALPS II at DESY in Hamburg is a light-shining-through-a-wall (LSW) experiment searching for axion-like elementary particles in the sub eV mass range, which are motivated by astrophysics and cosmology. ALPS II aims at an axion-photon coupling sensitivity which is several orders of magnitude better than that of previous LSW experiments and will thus investigate a new parameter range. This is achieved by an increased magnetic field interaction length and by optical cavities on both sides of the wall, which further amplify the signal. In this talk the working principle of LSW experiments and the uniqueness and technological challenges of the ALPS II experiment will be explained. Interim results of previous investigations will be presented as well as the current status. By converting axions to photons, we expect a rate of only 1 photon per day. The two sophisticated detector schemes, coherent heterodyne detection and a cryogenic single photon detector, are also presented and their implementation in the ALPS II experiment is discussed. ALPS II is aiming at a first data run in late 2021

Laser interferometry for LISA and satellite geodesy missions

The development and investigation of laser interferometry concepts for performing precise length measurements at frequencies below 1Hz is the main topic of this thesis. These concepts are quintessential for space-based measurements of gravitational waves or the Earth gravity field. For the Laser Interferometer Space Antenna (LISA) and future satellite geodesy missions, various interferometer types have been studied between 2014 and 2018 at the Albert Einstein Institute (AEI) in Hannover as a part of the work presented here. The first part of this thesis presents conceptual design studies of phase reference distribution systems (PRDSs) for LISA. The usage of Telescope Pointing is the baseline mechanism for the current LISA design and implies the need for a light-exchanging backlink connection between two rotating optical benches within one satellite. Different backlink implementations are presented and analyzed, the final choice however remains one of the last open questions for the LISA optical metrology. A test-bed for comparing three backlinks with each other in a single, so-called Three-Backlink interferometer (TBI) experiment, has been simulated and a detailed noise estimation, including a critical stray light analysis, is presented. A free-beam connection between two moving set-ups was established by which the full functionality of the experimental environment was validated. The design of the TBI has been completed and the experiment, consisting of two rotating quasi-monolithic optical benches, is currently under construction. The full experiment will enable to test the performance of LISA backlink candidates with a precision of 1 pm/√Hz in a relevant environment. The second part of this thesis describes alternative interferometer techniques for reducing the complexity of optical set-ups, while modern digital signal processing is applied for recovering the desired phase information. The simplifications in the optical part enables multi-channel operation and multi-degree of freedom readout, which is required for future gradiometers in satellites consisting of six or more test masses. An experiment simulating such a test mass readout with only a single optical component has been established. Interferometric readout noise levels of 1.0pm/√Hz at 100mHz were achieved by using deep frequency modulation interferometry (DFMI), a novel technique developed as part of this thesis.Die Entwicklung von Laserinterferometern für präzise Längenänderungsmessungen im 1mHz-Frequenzbereich ist das Kernthema dieser Arbeit. Diese finden Anwendung in der Detektion von Gravitationswellen und der Messung des Erdschwerefeldes aus dem Weltraum. Verschiedene Interferometerkonzepte wurden für die Laser Interferometer Space Antenna (LISA)- und zukünftige geodätische Missionen innerhalb der hier dargestellten Arbeit am Albert-Einstein-Institut (AEI) in Hannover zwischen 2014 und 2018 untersucht. Der erste Teil dieser Arbeit befasst sich mit einer Designstudie über unterschiedliche Phasenreferenzverteilungssysteme (PRDSs) für LISA. Das derzeitige Design sieht das sogenannte Telescope Pointing als Basis-Mechanismus vor, wodurch eine Backlink-Verbindung zwischen zwei rotierenden optischen Bänken innerhalb eines Satelliten benötigt wird. Die Laser werden hiermit zwischen den beiden Interferometern ausgetauscht. Eine konkrete Realisierung dieses Backlinks ist eine der letzten offenen Fragen für das optische Design von LISA. Das sogenannte Drei-Backlink Interferometer (TBI) wurde speziell entworfen und dient als Testumgebung, in welcher drei Backlinks in einem einzelnen Aufbau miteinander verglichen werden. Optische Simulationen und eine Vorhersage möglicher Rauschquellen werden in dieser Arbeit präsentiert. Eine Freistrahl-Verbindung zwischen zwei rotierenden Bänken wurde bereits untersucht und es konnte gezeigt werden, dass die experimentelle Infrastruktur voll funktionsfähig ist. Das Design des Drei-Backlink Experiments ist abgeschlossen und es wird derzeit konstruiert. Der zweite Teil dieser Arbeit beschreibt alternative Interferometertechniken um die Komplexität optischer Aufbauten zu reduzieren. Moderne digitale Verarbeitungssysteme werden benutzt, um die gewünschte Phaseninformation zurückzugewinnen. Eine Vereinfachung der Optik ermöglicht den Betrieb mehrerer Kanäle gleichzeitig und die Auslesung vieler Freiheitsgrade. Diese Techniken werden in zukünftigen Satelliten-Gradiometern benötigt, um die Bewegung mehrerer Testmassen zu bestimmen. Dies wurde in einem optischen Aufbau mit nur einer einzelnen optischen Komponente simuliert. Mit tiefen Laserfrequenzmodulationen (DFMI), einer im Rahmen dieser Arbeit entwickelten Methode, konnte eine Messgenauigkeit von unter 1.0 pm/√Hz bei 100mHz erreicht werden

Any light particle search at DESY

The Any Light Particle Search (ALPS II) is a light shining through a wall (LSW) experiment searching for axion-like elementary particles in the sub-eV mass range, which are motivated by astrophysics and cosmology and fulfill the requirements for being dark matter. ALPS II aims to measure an axion-to-photon coupling of $2\times 10^{-11}\textrm{ GeV}^{-1}$, which is several orders of magnitude better than that of previous LSW experiments and will thus investigate a new parameter range. The increased performance is achieved by enhancing the magnetic field interaction length to $2\times 10^{6}$ m and by amplifying the signal in an optical cavity on each side of a light-tight barrier. The expected signal is in the order of 1 photon per day, which will be measured by photon detectors with very low dark count rates of $\mathcal{O}(10^{-6}\textrm{ Hz})$. This article gives a technical overview on the experiment design, previous and ongoing investigations, and the current status with focus on the single photon detection

The Any Light Particle Search Experiment at DESY

The Any Light Particle Search (ALPS II) is a light shining through a wall (LSW) experiment searching for axion-like elementary particles in the sub-eV mass range, which are motivated by astrophysics and cosmology and fulfill the requirements for being dark matter. ALPS II aims to measure an axion-to-photon coupling of $2\times 10^{-11}\textrm{ GeV}^{-1}$, which is several orders of magnitude better than that of previous LSW experiments and will thus investigate a new parameter range. The increased performance is achieved by enhancing the magnetic field interaction length to $2\times 10^{6}$ m and by amplifying the signal in an optical cavity on each side of a light-tight barrier. The expected signal is in the order of 1 photon per day, which will be measured by photon detectors with very low dark count rates of $\mathcal{O}(10^{-6}\textrm{ Hz})$. This article gives a technical overview on the experiment design, previous and ongoing investigations, and the current status with focus on the single photon detection