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Gravitational waves were originally predicted by Albert Einstein in 1916 as a direct implication of the theory of General Relativity. In this theory, concentrations of mass or energy warp space-time and changes in the position or shape of these objects produce a distortion that propagates through an otherwise flat space-time at the speed of light, the so called gravitational wave.

Several gravitational wave detectors have been built based either in acoustic resonators or in long arm length interferometers. However, no detection has been claimed to the date due to the extreme weakness of the gravitational interaction, which imposes high sensitive requirements in the sensitivity of the detectors. A second difficulty arises due to the fact that these facilities operates on ground and, because of seismic noise, they are limited at low frequencies (below 1 Hz) where the gravitational wave sky is expected to be rich in emitting sources.

Recently, the European Space Agency has selected the ‘Gravitational Universe’ as the scientific theme of a large class mission to be launched in the 2030 decade, while the LISA Pathfinder mission is currently in the launch pad with scheduled launch on Dec. 2nd 2015. The latter represents a milestone in the field of gravitational wave detection in space since it will test for the first time the required technologies to detect gravitational wave in space, i.e in the milliHertz range.

The current project GRLOW (Gravitational Wave Detectors Low Frequency Test Bed) aims to set up a facility to test technologies at very low-frequencies by means of high precision metrology with the scientific purpose of exploring the low frequency measurement window and developing the measurement techniques that will be required in future space-based gravitational wave missions. These techniques will apply as well to other scientific space mission aiming at high precision metrology measurements, as the GRACE (Gravity Recovery and Climate Experiment) follow-on and the STE-QUEST (Space-Time Explorer and QUantum Equivalence Principle Space Test) missions. The proposed test bed can thus be considered as a transversal test facility for space-related technologies.

The set-up implemented within the project is composed by a very high-precision thermally controlled vacuum tank which allows to suppress environment fluctuations in the low frequency regime, i.e. down to 0.1 mHz. Inside the tank, an interferometer with picometer sensitivity will allow the characterization of materials used in space applications and opto-electronics equipment in a high stability environment.

Our interferometer implementation is based in the deep phase modulation scheme. In this framework, the hardware complexity of other interferometer set-ups using, for instance, heterodyning techniques is substituted by post-processing analysis. Typically, the implementation of an heterodyne interferometer may require two acoustic-optic modulators to create a beatnote to be measured in a phasemeter. In our approach we use instead a piezo tube where we coil around a 5m polarizing-maintaining optical fiber.

We have built and characterised ourselves this modulator evaluating different parameters of the design, as for instance the optical fiber length or different options to attach the fiber to the piezo (tape, glue, etc.). We finally got to a design that is currently providing good results, with a measured efficiency of 250nm per applied volt.

In parallel we developed the interferometer read-out system from scratch. We started with Si photodiodes. The transimpedance amplifier design was originally a purely resistive element from where we evolved to designs including an operational amplifiers with a high measuring bandwidth. That way, we improved our measuring bandwidth from the original 50kHz to 14MHz in the operational-amplifier based design. In a final phase of the project, we have substituted the original Si photodiodes by InGaAs quadrant photodiodes which will allow the determination not only of length changes, but also attittude variations by means of the differential wave-front sensing technique.

The phasemeter and post-processing of the data is an important part of our experiment where we have devoted several efforts. We have developed the software infrastructure that will allow a FPGA-based phasemeter configurable in real-time thanks to the System On Chip (SoC) approach. The following components have been synthesized in a Xilinx FPGA: a Gaisler LEON 3 Soft-Core CPU, a 4DS FMC116 ADC wrapped in a custom made component that communicates directly with the CPU using AMBA technology bus, and a custom embedded RTEMS Application running on SoC, that is in charge of acquiring, processing and transmitting data to Host PC Application through ethernet TCP/IP, system monitoring and configuration managing. In parallel, the Host PC Application manages the user interface to customize the system and data persistence.

The implementation of the SoC approach in FPGA-based phasemeter is a novel proposal from GRLOW. It has an important potential impact in the area of high precision metrology since the state-of-the-art implementations do not have an embedded CPU in the FPGA, forcing them to recompile the application software each time any parameter of the system has to be modified.

We have shown that with a table-top experiment we were able to achieve a sensitivity sensitivity 10 nm/sqrt(Hz) at 1 Hz. Although latest results are still to be published, the integration of the interferometer in a Carbon-Reinforced-Fiber Polymer (CFRP) bench have reduced the dependence on environment fluctuations. This, together with the integration of the experiment in vacuum has increased substantially the sensitivity of the experiment reaching now the sub-nanometer resolution.

In order to perform high-precision measurement, the interferometer must be located in a vacuum controlled environment. We have designed a vacuum chamber for this particular purpose. The chamber is a cylinder with a diameter of 550mm and 500mm height. It has feedthroughs to allow electrical connections and optical fibers to perform interferometer measurements inside.

Although the chamber provides a first layer of insulation against temperature fluctuations of the environment, our experiment requires further isolation to prevent from thermal fluctuations at very low frequencies. For that reason we have designed a low frequency thermal passive insulator that allows to decouple the inner experiment from the outside temperature variations. It consists of concentric cylindrical elements made of stainless steel polished to mirror quality. Each cylinder is supported by means of low temperature conductivity materials. By means of this structure, we reduce heat transfer by conductivity and radiation and our results have shown that the temperature sensitivity is below 10^(-5) K/sqrt(Hz) down to 7 mHz. We are currently implementing the active control that will allow to reduce the temperature dependence of the experiment inside below the milliHertz range.

With the thermally stabilised vacuum chamber, we have proceed with the last phase of GRLOW, which is the characterisation of materials and optoelectronics elements in the low frequency regime. With that aim we have stablished a collaboration with the German Space Agency (DLR) to test in our set-up steering mirrors for the GRACE follow-on mission, a geodesy mission to map the Earth gravitational field. The steering mirrors are a key optic elements which needs to be tested to high precision and stability, given the stringent requirements of the mission. These elements have been considered as a possibility for the gravitational wave space mission, LISA, which proofs the wide applicability and interest of the research pushed forward by GRLOW.

It is worth mentioning that he have been actively developing data analysis techniques that are relevant for the GRLOW aims. We have developed a technique to subtract the phase noise temperature contribution in laser interferometer in the very low frequency range, i.e. down to 0.1 mHz. This analysis was performed on the LISA pathfinder optical bench but it is equally applicable to the GRLOW set-up. We have also characterised the noise due to thermo-elastic distortion at very low frequencies in the LISA Pathfinder spacecraft which, again, is strongly related with the research done in our GRLOW facility.

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