Auxiliary function development for the LISA metrology system

authored by: Nils Christopher Brause
supervised by: Karsten Danzmann
Abstract: The Laser Interferometer Space Antenna (LISA) is a planned gravitational wave detector to be positioned in space. It consists of three spacecrafts that use Long Range Interferometry (LRI) to measure relative distance changes between them. An important component of LISA is the LISA Metrology System (LMS) which is responsible for the distance measurements as well as various auxiliary functions: The beatnote acquisition allows the LMS to lock to an incoming beatnote signal with an unknown frequency and amplitude. It measures both with a Fast FourierbTransform (FFT) and controls the starting frequencies and gains of the Digital Phase Locked Loops (DPLLs) accordingly. The laser locking algorithm is used to lock the frequency of one laser to the frequency of another laser. This is done by locking the difference frequency between two lasers to a constant target and thus enabling heterodyne interferometry. The amplitude of the incoming beatnote signal can vary greatly over time. To compensate for that, the Automatic Gain Control (AGC) functionality observes the amplitudes and reconfigures the gains of the DPLLs accordingly. In LISA the pointing will be measured using an advanced Differential Wavefront Sensing (DWS) scheme, which track the differential phases between the segments of a Quadrant Photo Diode (QPD) directly instead of calculating them from the measured phases of the segment DPLLs. This improves the Carrier to Noise Density Ratio (CNR) in the DPLLs by a factor of two. The absolute distance between the spacecrafts is also measured to enable Time-Delay Interferometry (TDI) in post-processing. This is done by sending a Pseudo-Random Noise (PRN) code via the laser link to a distant spacecraft, where it is correlated with a local copy of the same PRN code to determine the travel distance from the measured delay. Since only one of the three LISA spacecrafts has a radio link to earth, data has to be transferred between the three spacecrafts. This functionality is part of the Delay Locked Loop (DLL), by modulating the data onto the PRN code. In the course of this thesis, all the necessary auxiliary functions will be developed, thoroughly described and measured.
Organisation(s): QUEST-Leibniz Research School
Type: Doctoral thesis
No. of pages: 186
Publication date: 2018
Publication status: Published
Electronic version(s): https://doi.org/10.15488/3511 (Access: Open)
: Details in the research portal "Research@Leibniz University"

BibTeX

@phdthesis{1dc62d87d0674541b27e0c6d202f7ae8,
title = "Auxiliary function development for the LISA metrology system",
abstract = "The Laser Interferometer Space Antenna (LISA) is a planned gravitational wave detector to be positioned in space. It consists of three spacecrafts that use Long Range Interferometry (LRI) to measure relative distance changes between them. An important component of LISA is the LISA Metrology System (LMS) which is responsible for the distance measurements as well as various auxiliary functions: The beatnote acquisition allows the LMS to lock to an incoming beatnote signal with an unknown frequency and amplitude. It measures both with a Fast FourierbTransform (FFT) and controls the starting frequencies and gains of the Digital Phase Locked Loops (DPLLs) accordingly. The laser locking algorithm is used to lock the frequency of one laser to the frequency of another laser. This is done by locking the difference frequency between two lasers to a constant target and thus enabling heterodyne interferometry. The amplitude of the incoming beatnote signal can vary greatly over time. To compensate for that, the Automatic Gain Control (AGC) functionality observes the amplitudes and reconfigures the gains of the DPLLs accordingly. In LISA the pointing will be measured using an advanced Differential Wavefront Sensing (DWS) scheme, which track the differential phases between the segments of a Quadrant Photo Diode (QPD) directly instead of calculating them from the measured phases of the segment DPLLs. This improves the Carrier to Noise Density Ratio (CNR) in the DPLLs by a factor of two. The absolute distance between the spacecrafts is also measured to enable Time-Delay Interferometry (TDI) in post-processing. This is done by sending a Pseudo-Random Noise (PRN) code via the laser link to a distant spacecraft, where it is correlated with a local copy of the same PRN code to determine the travel distance from the measured delay. Since only one of the three LISA spacecrafts has a radio link to earth, data has to be transferred between the three spacecrafts. This functionality is part of the Delay Locked Loop (DLL), by modulating the data onto the PRN code. In the course of this thesis, all the necessary auxiliary functions will be developed, thoroughly described and measured.",
author = "Brause, {Nils Christopher}",
note = "Doctoral thesis",
year = "2018",
doi = "10.15488/3511",
language = "English",
publisher = "Leibniz Universit{\"a}t Hannover",
school = "Leibniz University Hannover",
}