During the last 20 years the field of gravitational wave detection has moved from humble beginning to a vivid research field with several large instruments now taking data. High-precision interferometry is used to detect length changes of sub proton diameters in kilometer-long test ranges. New optics and new interferometer technologies are required to achieve the target sensitivity of these gravitational wave detectors. Numeric interferometer simulations have proven to be an essential tool for the development of new ideas for improving the laser interferometers at the heart of large-scale gravitational-wave detectors. Our group develops and maintains one of the main software tools in the field: Finesse.

The laser interferometers in current gravitational-wave detectors are the most sensitive interferometric length sensors ever built. Their sensitivity is limited by fundamental noise, such as the quantum fluctuations of the laser light itself. However, even fundamental noise can be overcome by new techniques in quantum optics. The most famous example is the use of so-called 'squeezed' light or, more generally, Quantum-Non-Demolition techniques. These techniques often rely on very high laser powers at which the interferometer behaviour changes due to radiation pressure effects on the optical components. This is a completely new regime of interferometry that we are only now are beginning to explore.

With this project we aim to develop new numerical algorithms to study the such systems, in particular their quantum behaviour, the dynamic opto-mechanical coupling and the limiting noise sources. This project will provide an interesting mix of theoretical modern optics, numerical modelling of physical system for the optical design and testing the results at large-scale gravitational wave observatories.

Modern gravitational-wave detectors, such as Advanced LIGO, use 4km long ‘arms’ to measure fluctuations in space-time caused by violent astrophysical processes. They are capable of measuring length changes of 10^-19m, and to reach this sensitivity they use such high laser power that the optical force dominates the response of the instrument. Even with 40kg mirrors, the quantum mechanical fluctuations of the radiation pressure will be the dominant source of low frequency motion. Our group will investigate how new interferometer configurations can use strong radiation pressure forces to improve performance and we will work with the LIGO community on how this knowledge can be applied to gravitational-wave detectors.

This experimental project will involve the development of a suspended interferometer dominated by radiation pressure. In parallel, the response of different interferometer configurations to both classical signals and quantum noise will be analysed. Construction and testing of the interferometer will be a multi-disciplinary effort, requiring quality mechanics, optics, and electronics. An integral component of learning the skills required for this work will be visiting LIGO to work with the full-scale detectors. With an operational interferometer, we will investigate how radiation pressure alters the classical response to signals, and how this will affect the quantum-noise limited response.

The detection of gravitational waves is currently one of the most challenging tasks in experimental physics. After several decades of development in interferometric detectors, the next generation is expected to reach the sensitivity required for a direct detection of gravitational waves. Some of the large laser interferometers (LIGO, GEO, VIRGO) have completed their data taking periods looking for gravitational waves, without achieving the long awaited first direct detection. Now the next generation of detectors with more than sufficient sensitivity is already under construction. The Advanced LIGO project (United States) is well underway and our group takes part in this exciting project.

At the same time we conduct lab-based research on new optical technologies which could be used to upgrade the interferometric detectors in the future. Past projects included the use of ring-shaped laser modes (see this and this paper), the experimental demonstration of diffractive optics or displacement noise free interferometry (see e.g. this article). This is a unique opportunity to study modern laser optics and to develop instruments or concepts that will be employed to improve the science reach of one of the most exciting international large-scale experiments.

Ground motion is a dominant noise source and the major cause of operational difficulty in gravitational wave detectors. Reducing the effect of ground motion is a major driver of cost and complexity in Advanced LIGO, and future detectors will need even better isolation at low frequencies. Since the best commercial inertial sensors in the world are already in use, something better must be developed. Our group will develop a research program aimed at applying the measurement technology of gravitational wave detection to inertial sensors. This will involve interferometric measurements of reference masses suspended with novel materials in precise configurations to overcome the current limitations of readout noise and thermal noise. This project will also investigate the fundamental limitations of inertial sensors, which are dominated at low-frequency by unknown anelastic mechanisms.

If suitable sensors are developed in a timely manner, there is the possibility of directly installing them at an Advanced LIGO detector and directly contributing to one of the most exciting science projects to date.