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.

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. 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.

We make use of different numerical tools to design new interferometers, with a lot of the software being developed by us or within the gravitational wave community. In order to make these tools accessible for a wider audience, such as scientist in other fields, we have started to promote these tools on a web page (http://www.gwoptics.org). Existing numeric interferometer simulations have proven to be an essential tool for designing and commissioning large-scale laser interferometric gravitational-wave detectors. In order to provide the same service for advanced detectors for which the construction has been started recently, the numeric simulations must be extended to reflect the changes in the understanding of the used technologies.

This project will provide an interesting mix of theoretical modern optics, numerical modelling of physical system for the optical design and testing of real large-scale gravitational wave observatories.

The first direct detection of gravitational waves is expected to be accomplished during the next 10 years. Several international research groups are engaged in this hunt with several km-long interferometers. Soon after a first detection, the technology should be dedicated to a new kind of astronomy: gravitational wave astronomy.

Our group has been leading the optical design for a new detector, the Einstein Telescope (ET). Future, so-called third-generation, interferometric detectors such as ET will be able to produce a constant stream of data for astronomical analysis. To achieve this, they have to be optimised in their response function with respect to the most interesting sources, the inspirals of black holes and neutron stars. However, this calls for an enlargement the detector bandwidth to low frequencies where current detectors are limited by seismic noise. The best method to achieve a large bandwidth is the Xylophone approach of using several co-located interferometers tuned to different bands of the full spectrum. Further advantages can be achieved by using other co-located schemes, such as the Triple Michelson layout.

However, with a large number of co-located interferometers, the technical challenges increase, especially regarding the coupling of unwanted noise sources; the draft optical layout of the Einstein Telescope highlights the complexity of such systems. We will use digital signal processing schemes to study the feasibility of such systems and to optimise their performance.