Grating Interferometry

The search for gravitational waves has led to a new class of extremely sensitive laser interferometers. The first generation of large-scale laser-interferometric gravitational wave detectors [1, 2, 3, 4] is now in operation with the aim of accomplishing the first direct detection of gravitational waves. Simultaneously, new interferometer concepts are evaluated for future detectors.

Traditionally, partly transmissive mirrors are used in interferometers to split and combine coherent optical light fields. For high precision laser interferometers, such as for gravitational wave detection, non-transmissive reflection gratings (see figure above) offer a useful alternative way of splitting and combining light beams. The resulting all-reflective interferometers are beneficial because, firstly, they lower all thermal issues that are associated with absorbed laser power in optical substrates and, secondly, they may allow for opaque materials with favourable mechanical and thermal properties. With these two qualities all-reflective interferometer concepts have, in principle, great potential to become key technologies for enhancing the sensitivity of future generations of laser interferometric-gravitational wave detectors. However, in comparison to a standard reflective component (mirror or beam splitter) the diffractive nature of the gratings causes an additional coupling of geometry changes into alignment and phase noise.

Our group studies the proposed grating topologies and their use in third generation interferometers such as the Einstein Telescope [5], [6]. We focus our research on the alignment noise performance of these grating topologies. This is essential to determine which standard reflective component in a gravitational wave detector can be replaced with a non-transmissive grating optics.

The following figure shows a sketch of a Michelson and a linear Fabry-Perot interferometer with transmissive optical elements and possible all-reflective realizations of these devices based on diffraction gratings. Note that the Fabry-Perot interferometer can either be realised with a grating in first-order (resulting in two ports) or second-order Littrow mount (three ports).

The next steps in our grating studies are:

• Publish our latest results of a theoretical analysis of grating alignment noise couplings
• Setup a grating cavity and analyse the coupling of tilt and movement of the grating into the phase of the reflected light
• Help with the setup of a large grating interferometer in the Glasgow gravitational wave prototype in collaboration with the Glasgow University
• design a new grating interferometer topology which reduces the alignment noise effects

References:

• [1]S. Hild and the LIGO Scientific Collaboration. The status of GEO 600. Classical and Quantum Gravity, 23:643, October 2006.
• [2]F. Acernese and the VIRGO Collaboration. The Virgo status. Classical and Quantum Gravity, 23:635, October 2006.
• [3]D. Sigg. Commissioning of LIGO detectors. Classical and Quantum Gravity, 21:409, March 2004.
• [4]R. Takahashi and the TAMA Collaboration. Status of TAMA300. Classical and Quantum Gravity, 21:403, March 2004.
• [5]Lueck H et al 2007 Plans for E.T. (Einstein Telescope), presentation at the AMALDI 7 conference.
• [6]Design study proposal: Einstein gravitational wave Telescope, submitted to European Union. INFRA-2007-2.1-01 Design studies for research infrastructures in all S&T fields.