For advanced gravitational wave detectors
The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves twice last year, suggesting that such detections could soon become routine. This took over a decade as it was hard to achieve the required sensitivity to detect these ripples in space-time, whose existence Albert Einstein predicted 100 years ago. Meanwhile, the European Space Agency (ESA) has started working on the Evolved Laser Interferometer Space Antenna (eLISA), a gravitational wave observatory to be launched in the 2030s. The LISA pathfinder spacecraft, a proof-of-concept mission was placed in orbit in December 2015 to demonstrate that the eLISA mission is possible. Back on the ground, researchers pieced together a facility to test technologies and materials that will be required in space-based gravitational wave observatories. Developed as part of the GRLOW (Gravitational wave detectors low-frequency technology test bed) project, the set-up is composed of a thermally controlled vacuum tank that allows to suppress environmental fluctuations. The proposed facility is able not only to screen the environmental fluctuations in the low-frequency regime but also to generate controlled perturbations. Inside the tank, an interferometer with picometre sensitivity allows the characterisation of materials – like carbon-fibre reinforced polymers and silicon carbide – in a measuring bandwidth relevant for space applications. The interferometer implementation is based on deep phase modulation, allowing continuous real-time tracking of free-falling mass, as required for gravitational wave detection. The GRLOW team built the required modulators from scratch to create a beatnote to be measured by a field-programmable gate array (FPGA)-based phasemeter. In addition, they developed the interferometer read-out system. The novel GRLOW approach involved the adoption of a system-on-chip implementation for the phasemeter. This would allow high-precision metrology without having to recompile the software each time an input parameter is modified. Importantly, researchers have shown with a table-top experiment that it is possible to achieve a sensitivity of 10 nm per Hertz square root. Lastly, data analysis techniques were developed to characterise the noise due to thermo-elastic distortion at very low frequencies in the LISA Pathfinder spacecraft. The new techniques are also applicable to the GRLOW set-up that will contribute to the start of the era of gravitational-wave astronomy, providing new information about the cosmos.
Keywords
Gravitational wave, LIGO, eLISA, LISA pathfinder, GRLOW, vacuum tank, interferometer
