Max Planck Institute for Gravitational Physics (Albert Einstein Institute)
Optical Simulations
The ultra-high precision of the LISA interferometry relies on many different factors and the suppression of a multitude of noise sources. Some of these noise sources can be studied with optical simulations.
Tilt-to-length (TTL) coupling, i.e. the coupling of angular and lateral jitter, shown here for the LISA test mass interferometer. Shown is how a jitter of the test mass relative to the optical bench (photodiode) affects the beam trace (red line) to the photodiode. The beam’s path length increases due to the angular or lateral jitter of the beam, which results in a phase change that is then interpreted as distance variation in the interferometer. This is tilt-to-length coupling. Lateral jitter coupling is likewise a TTL coupling effect because the coupling originates from a static tilt.
Marie-Sophie Hartig
Tilt-to-length (TTL) coupling, i.e. the coupling of angular and lateral jitter, shown here for the LISA test mass interferometer. Shown is how a jitter of the test mass relative to the optical bench (photodiode) affects the beam trace (red line) to the photodiode. The beam’s path length increases due to the angular or lateral jitter of the beam, which results in a phase change that is then interpreted as distance variation in the interferometer. This is tilt-to-length coupling. Lateral jitter coupling is likewise a TTL coupling effect because the coupling originates from a static tilt.
Marie-Sophie Hartig
The best-known example and our current work-focus is tilt-to-length coupling noise, i.e., the coupling of angular or lateral jitter into the interferometric phase readout. Another example studied in our simulations is stray light mitigation.
In our simulations, we fundamentally study tilt-to-length coupling noise to understand the underlying concepts (e.g. [1,2,3]). This allows us to define new noise mitigation strategies or refine existing ones. These strategies can be used in any precision interferometer, i.e. in our laboratory test setups, in LISA itself, in future geodesy missions like GRACE-FO, or others.
LISA optical bench (centre) with test mass (yellow cube), and a mock-representation of the telescope as seen in the IfoCAD visualization.
Megha Dave
LISA optical bench (centre) with test mass (yellow cube), and a mock-representation of the telescope as seen in the IfoCAD visualization.
Megha Dave
Directly applied for LISA, we study the magnitude of the coupling in each interferometer and its dependency on alignment and imperfections in the interferometers. For this, we have an implementation of key elements of the interferometry (i.e. the laser beams, optical bench, telescope and test mass) implemented in our in-house software library IfoCAD.
The software library IfoCAD is a key element in our work, which enables us to test how interferometer properties such as the alignment, beam characteristics, or motion in the system affect the interferometric signals. We continuously develop IfoCAD further and extend its applicability for more and more applications.
With the ever-increasing precision needed in simulations, we are often facing a lack of available simulation methods. We are, therefore, investigating the properties of optical simulation methods and how they can be extended for simulating with high precision non-Gaussian beams such as top-hat beams or generally clipped or aberrated beams and their propagation through optical setups.
Naturally, we compare our simulation results and findings with laboratory and space mission data. A key success in our work was the matching of our analytic equations and optical simulations with the LISA Pathfinder tilt-to-length coupling data.
In our current and future work, we focus on the LISA tilt-to-length coupling to prepare the fundamental understanding needed to suppress the noise ideally by design, alignment, and subtraction.
Literature: Simulations Methods
1.
Wanner, G.; Heinzel, G.; Kochkina, E.; Mahrdt, C.; Sheard, B.; Schuster, S.; Danzmann, K.: Methods for simulating the readout of lengths and angles in laser interferometers with Gaussian beams. Optics Communications 285 (24), pp. 4831 - 4839 (2012)
Kochkina, E.; Wanner, G.; Schmelzer, D.; Tröbs, M.; Heinzel, G.: Modeling of the general astigmatic Gaussian beam and its propagation through 3D optical systems. Applied Optics 52 (24), pp. 6030 - 6040 (2013)
Wanner, G.; Schuster, S.; Tröbs, M.; Heinzel, G.: A brief comparison of optical pathlength difference and various definitions for the interferometric phase. 10th International LISA Symposium (LISAX), Florida, USA, May 18, 2014 - May 23, 2014. Journal of Physics: Conference Series 610, 012043 , (2015)
Schuster, S.; Wanner, G.; Tröbs, M.; Heinzel, G.: Vanishing tilt-to-length coupling for a singular case in two-beam laser interferometers with Gaussian beams. Applied Optics 54 (5), pp. 1010 - 1014 (2015)
Wanner, G.; Karnesis, N.; LISA Pathfinder collaboration: Preliminary results on the suppression of sensing cross-talk in LISA Pathfinder. 11th International LISA Symposium, Zurich, Switzerland, September 05, 2016 - September 09, 2016. Journal of Physics: Conference Series 840, 012043, (2017)
Tröbs, M.; Schuster, S.; Lieser, M.; Zwetz, M.; Chwalla, M.; Danzmann, K.; Fernandez Barranco, G.; Fitzsimons, E. D.; Gerberding, O.; Heinzel, G.et al.; Killow, C. J.; Perreur-Lloyd, M.; Robertson, D. I.; Schwarze, T.; Wanner, G.; Ward, H.: Reducing tilt-to-length coupling for the LISA test mass interferometer. Classical and quantum gravity 35 (10), 105001 (2018)
Danzmann, K.; Heinzel, G.; Hewitson, M.; Reiche, J.; Tröbs, M.; Wanner, G.; Born, M.; Audley, H.; Karnesis, N.; Wittchen, A.et al.; Paczkowski, S.; Kaune, B.; Wissel, L.: LPF final report for the German contribution to the nominal mission. (2018)
The November of Science at the Max Planck Institute for Gravitational Physics (Albert Einstein Institute) and the Institute for Gravitational Physics of Leibniz Universität Hannover with seven exciting events
The gravitational-wave and experimental astrophysicist will establish a third department at the Max Planck Institute for Gravitational Physics in Hannover
