Introduction

One hundred years after its prediction by Albert Einstein on the basis of his Theory of General Relativity, the use of gravitational waves as carriers of information from cosmic phenomena is the essence of the emerging area of Gravitational Wave Astronomy, from which we expect revolutionary discoveries in Astrophysics, Cosmology and Fundamental Physics.

All gravitational wave detectors currently being operated are on-ground facilities implementing kilometric arm-length interferometry. These detectors have an unprecedented sensitive to sources in the kilohertz frequency band. It is however the low frequency range –in the millihertz– where the gravitational sky is richer. Opening this frequency window is precisely the main scientific objective of LISA, the future gravitational wave observatory in space.

Gravitational wave detectors are complex instruments that require high precision measurements. In particular, space borne observatories rely on measuring relative displacement between test masses in nominal free fall in space. The masses are kept inside the spacecraft by means of precision control loops that correct the spacecraft position. The LISA Pathfinder mission –launched in 2015– successfully demonstrated this technology for future observatories in space.

In the past, we have led the Spanish contribution to LISA Pathfinder with the development and in-flight operation of the data and diagnostics subsystem on-board the satellite consisting of 24 temperature sensors, 4 magnetometers, 1 radiation monitor and Data Management Unit, the instrument computer. We are currently applying this know-how to the future gravitational wave observatory, LISA, as well as other space missions that may benefit these technological developments.

Focus

In order to unveil this new window to the Universe, there are several challenges that need to be addressed. They basically arise from two main requirements: stability and precision. This research line aims to study and develop new metrology techniques and to push the current ones to the low frequency regime, i.e. the sub-millihertz band. In order to do so we develop ultrastable thermally controlled environments where to test key technologies and materials for gravitational waves detection at low frequencies and also for space applications facing similar technological challenges.

This includes investigation in aspects of sensor technology, optical metrology, analog signal conditioning circuit topologies, low-noise electronic components, analog-to-digital conversion techniques and digital signal processing.

In the following some of the aspects were we focus our research:

• Temperature sensing: Temperature perturbations contribute, among others, to the test mass free-fall in gravitational wave observatories in space. The main purpose of the temperature sensors on-board is to monitor the environment and characterize the thermal coupling to the experiment. We study control techniques to create ultrastable environment and develop ultrastable temperature sensing.

• Magnetic sensing: The dynamics of the free-falling masses inside a satellite show a dependence in magnetic field and magnetic field gradient perturbations. Among others, we investigate new magnetic sensors with dedicated noise reduction techniques at the sub-milli-Hertz frequency band.

• Particle counters: In order to measure the incident flux coming from the Galaxy and from the Sun, space-borne gravitational wave detectors need a dedicated particle counter. This instrument characterizes those particles interacting with the test mass which can potentially induce charge variations in the test mass and therefore introduce perturbations in the control loop of the free-falling test mass.

• In-flight critical software: As a part of our contribution to LISA Pathfinder, we developed the Data Management Unit –the LTP computer, a critical element of a space mission. In order to do so we develop tools for embedded software, FPGA control or software verification and testing. All these following aerospace standards.

• Data analysis techniques: our research also focus on the exploitation of the diagnostics data. In particular, in the implementation of data analysis techniques that can be used to characterise and disentangle the environment contribution. Among others, this task requires parameter estimation methods  --for instance, Monte Carlo Markov Chain algorithms, spectral and digital analysis. We applied these techniques during LISA Pathfinder in-flight operations and are currently developing the data analysis for LISA.

• Optical metrology: design and implementation of high precision laser metrology interferometers to study its behavior in the low-frequency regime and to develop the techniques that allow decoupling from the environment. We also study novel metrology techniques that enable more precise and stable sensors as, for instance, the usage of optomechanical resonators as precision references.

Selected publications

- Sub-femto-g free fall for space-based gravitational wave observatories: LISA pathfinder results
M. Armano et al.
Physical review letters 116 (23), 231101 (2016)

- Optimal design of calibration signals in space-borne gravitational wave detectors.
M. Nofrarias, N. Karnesis, F. Gibert, M. Armano, H. Audley, K. Danzmann, et al.
Physical Review D 93 (10), 102004 (2016)

- Thermo-elastic induced phase noise in the LISA Pathfinder spacecraft
F. Gibert, M. Nofrarias, N. Karnesis, L. Gesa, V. Martín, I. Mateos, A. Lobo, et al.
Classical and Quantum Gravity 32 (4), 045014 (2015)

- Towards a FPGA-controlled deep phase modulation interferometer
M. Teran, V. Martín, L. Gesa, I. Mateos, F. Gibert, N. Karnesis, et al.
Journal of Physics: Conference Series 610 (1), 012042 (2015)

- Low-frequency noise characterization of a magnetic field monitoring system using an anisotropic magnetoresistance
I. Mateos, J. Ramos-Castro, A. Lobo.
Sensors and Actuators A: Physical 235,57-63 (2015)

- On the role of radiation monitors on board LISA Pathfinder and future space interferometers
C. Grimani, C. Boatella, M. Chmeissani, M. Fabi, N. Finetti, M. Laurenza, et al.
Classical and Quantum Gravity 29 (10), 105001 (2012)

- Thermal diagnostics front-end electronics for LISA Pathfinder
J. Sanjuán, A. Lobo, M. Nofrarias, J. Ramos-Castro, PJ. Riu
Review of Scientific Instruments 78 (10), 104904 (2007)



Senior Institute members involved


M. Nofrarias, Ll. Gesa, V. Martin