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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - InertialSensors (Interferometric inertial sensors for gravitational-wave detectors)

Teaser

Summary

The Advanced LIGO and Advanced Virgo detectors are the most sensitive length-measuring instruments ever built. Since the start of the InertialSensors project, the sensitivity of these detectors not only enabled the first ever observation of gravitational-waves, but the observation of 11 different events in our universe! The observations have provided further proof of Einsteinâ€™s general theory of relativity, and they have also changed the way we conduct astronomy.

The first, and thus far â€˜loudestâ€™, signal created a length change of 10^{-18}m in the Advanced LIGO detectors. To achieve this mind-boggling level of sensitivity, LIGO relies on several stages of a sophisticated and very effective vibration isolation system. However, despite using hundreds of state-of-the-art seismometers, the performance of LIGO is still partially limited by the sensors inside the active isolation system.

The aim of the InertialSensors project is to improve the performance of seismometers by using highly-sensitive interferometer readout. We propose to employ the core-technology of gravitational-wave detectors (interferometers) to create a new kind of seismometer, and then use that seismometer to improve the performance of the detector.
There are three major challenges in building seismometers:
1. Building very stable mechanics to softly suspend a â€˜reference massâ€™,
2. Precisely measure the position of the mass, and
3. Control the position of the mass so that it doesnâ€™t move too far.

We skipped the first challenge by using existing high-quality mechanics developed over the last three decades. For the second challenge, interferometers are excellent sensors that can have an extremely high resolution. To defeat the third challenge, we will use interferometers that are capable of measuring over a wide range, thus removing the need for active control of the reference mass.

Work performed

The project work was divided into three work packages:
Package 1, develop the low-noise interferometric readout: The group in Birmingham has significant expertise with interferometry, and in particular with devices compact enough for integration in a seismometer. With this advanced start, our first challenge was to consistently perform sensitive measurements at frequencies between 0.01 and 1Hz. We built a number of interferometers in several different sizes to allow us to systematically investigate the way in which external fluctuations, such as sound or vibration or temperature changes, influence the instrument. Our second interferometer design, pictured below, already produced excellent results, reaching the sensitivity required for new seismometers. More importantly, the results were repeatable, and we were able to move forward with confidence.

Package 2, integrate an interferometer with seismometer mechanics: The integration of an interferometer with the bulky, cylindrical mechanics of our chosen conventional seismometer required several design iterations. The first generation was deliberately over-sized and cumbersome, designed to allow us the easiest access to tune the optics. Despite its ungainly form, initial performance measurements were very promising. Without a vibration-isolation system, or multiple devices, we were unable to characterise the performance thoroughly, but at some frequencies the ground motion is small enough that we could see our instrumentâ€™s self-noise, which was substantially lower than that of the conventional readout. We used the large prototype to characterise how defects in the optics, particularly in the polarisation of the light, effect the output signal. This kind of test allows us to determine the required quality of the optics, and the maximum speed we can accurately track.

Package 3: construct a â€˜field-unitâ€™ capable of deployment at a gravitational-wave detector: Five different design iterations were required, solving a series of practical problems, before we had a solution that was simultaneously small, practical, functional, robust, and simple to align. The final â€˜deliverableâ€™ unit was not much larger than the base seismometer, with the interferometer bolted onto the side, and it is â€˜poweredâ€™ by an optical fibre carrying a small amount of infra-red laser-light. We tested the unit by placing it inside a box to shield it from air-currents and temperature fluctuations (standard practise with seismometers), and as with our first prototype, at frequencies where the ground vibration is low, we demonstrated performance close to the thermal-noise limit of the mechanics.

Final results

In the field of vibration isolation, the state of the art is the deployment of top-tier geophones and seismometers on an active platform, and using feedback loops to suppress the motion. Our aim is to improve the performance of the active sensors using interferometers. Throughout the course of the MSCA we have advanced the state of the art by:
- Developing new electronics that reach the readout noise limit of conventional coil-magnet geophones.
- Constructed compact interferometers capable of reaching the fundamental suspension thermal noise limit of conventional mechanics.
- Published a thorough review of the performance and designs of compact interferometers in the literature.
- Integrated several different compact interferometers with a conventional seismometer, and demonstrated superior performance.
The project will continue beyond the end of the action. More interferometric seismometers will be constructed, and by performing tests with them co-located, the underlying self-noise and performance will be quantified.

The near-term impact will be on the field of gravitational-wave detection, where interferometric sensors have the potential to improve the duty-cycle and sensitivity of the observatories. The resulting increase in the number of recorded gravitational-wave signals will help us to unlock some of the secrets of the history of our universe and how it has evolved.