Advanced Interferometric Gravitational Wave Detection

The first direct detection of gravitational waves is expected to be accomplished during the next 10 years. Several research groups world wide are engaged in this hunt with up to ten detectors currently operating: four resonant bar detectors and six large interferometers. Soon after a first detection, the technology should be dedicated to a new kind of astronomy: gravitational wave astronomy.

The measurement band for ground-based detection is limited by man-made and natural gravity-gradient noise below 1 Hz. Current detectors reach a strain sensitivity of h ~ 10^-21. The peak sensitivity is obtained at frequencies of >100 Hz. For lower Fourier frequencies several technical noise sources and seismic noise are the limiting factors. At higher frequencies the limit is set by the shotnoise of the detected light. Current and future ground-based gravitational-wave detectors will be limited over a wide band by either thermal noise of the test-masses (mirrors) or quantum noise of the optical readout. To overcome these noise sources we need to implement advanced techniques of various kinds. Our group is working on analysing advanced optical techniques for their theoretical features and practical feasibility.

Improving the Interferometric Readout

All interferometric detectors today are based on the Michelson interferometer. The name `advanced interferometry' covers every extension of the classic topologies of laser interferometer types. In our case the term `interferometer topology' refers to the optical layout, i.e. the positioning and interplay of optical components. The topology defines the principle properties (for example, the sensitivity) of the instrument. In addition, the `interferometer configuration' describes the additional, necessary setup for creating a real interferometer. This includes

• converting the optical phase into an electrical signal
• all control systems for achieving and maintaining a stable operation}
• additional changes to the setup to reduce noise (e.g. through scattered light)

The first advanced interferometers used Power Recycling: a partially transmissive mirror in the interferometer input allows to resonantly enhance the light stored inside the interferometer and thus effectively increases the laser power. Future generation of interferometric gravitational wave detectors might employ completely different topologies.

• The laser interferometers in current gravitational-wave detectors are so precise that their sensitivity is limited by the quantum fluctuations of the laser light itself in a wide frequency range. These fluctuations are not a fundamental limit as such because they can in principle 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.
• The thermal noise of the test masses describe all fluctuations of the mirror surface or the changes of the index of refraction of a mirror substrate which are a consequence of the non-zero temperature of the mirror. Any mechanical internal or external (e.g. through the suspension) loss channel corresponds to a mechanical fluctuation. The thermal noise can be reduced by cooling the test masses or by designing the interferometer such that the thermal noise couples less into the output signal. Currently three approaches are under investigation:
  • All Reflective Interferometer: Gratings can be used instead of mirrors so that the laser beam will not pass through any substrate material. This reduces the noise due to thermal fluctuation of the refractive index. It also allows a wider range of (non-transparent) materials to be used for the test masses.
  • Coating-free Mirrors: A large fraction of the thermal noise is in fact not originating in the mirror substrates but in the optical coatings. New topologies that make use of internal total reflection are currently studied as they might allow to achieve low loss optical systems while using only very thin coatings.
  • Non Gaussian Beams: By using laser beams with a flat-top spatial profile, the influence of thermal-noise induced surface distortion of the test mass. The surface deviations with a smaller size (or spatial frequency) than the beam diameter are averaged out and do not appear in the interferometer signal.

The Next Generation of Interferometric Detectors

The Advanced LIGO project (United States) is well underway and our group takes part in this exciting project by designing and building one part of the new mirror suspension system. The Italian-French VIRGO collaboration has just started the design process for its next generation detector. Here, our group contributes to the design of the main optical layout.

The gravitational wave community is quickly growing and counts several hundred researchers in Europe alone. During the next years a design study team will be formed to organise the required research and development. The design study phase is a very intellectually challenging time as very different physical phenomena have to be analysed and understood, as for example, the seismic noise created by surface waves in the earth's crust or the behaviour of massive objects spiralling into a black hole.