LIGO / Virgo Collaboration
Gravitational waves are oscillating perturbations in the space-time metric that propagate at the speed of light and are produced by some of the most catastrophic and energetic events in the Universe, such as the collapse of Supernovae, the coalescence of neutron stars, the collision between two black holes, and so on. One of the main results of General Relativity states that mass and energy produce a curvature of four-dimensional space-time, and matter moves in response to this curvature. Einstein's mathematics showed that massive accelerating objects would warp the space-time structure in such a way that "waves" of distorted space-time would radiate from the source.
Figure 1 - Virgo instrument (Cascina, Pisa, Italy)
Gravitational waves have been very recently detected thanks to very long base interferometric detectors like LIGO, Virgo and KAGRA. These interferometers are based on a Michelson interferometer yet much more sophisticated.
Interferometers are very interesting tools used in many fields of science and engineering, basing their mode of operation on merging two sources of light to create an interference pattern which can be measured and analyzed as it contains information about the object or phenomenon being studied.
Figure 2 Michelson Interferometer. The basic Michelson configuration has been used in 1887 in the Michelson and Morley Experiment
In a basic Michelson interferometer (Figure 1), a laser beam passes through a beamsplitter which splits a single beam into two identical beams as shown in. At the end of each arm, a mirror reflects each beam back to the beam splitter where the two beams merge back into a single beam. In merging, the light waves from the two beams interfere with each other before traveling to a photodetector, which measures the resulting beam's brightness. If the two beams travel exactly the same before recombining, the photodetector will either see a beam as bright as the pre-split beam or nothing at all, depending on how the mirrors are set up. Gravitational waves interferometers are set up so that nothing reaches the photodetector as long as the arms don't change their lengths (dark fringe operation mode).
As the differential arm length perturbations are extremely small (10-21 m), maximum precision in the whole optical set-up is required, and all possible noise sources must be minimized. To minimize this noise arms are in vacuum, to avoid air molecules and dust scattering, yet diffraction due to the finite size of the end-mirrors and other optical components may cause a small amount of stray light.
Figure 3 – Scattered light issue
One of the main issues to deal with, concerns the scattered light noise, depicted in Figure 3. Though very high-quality mirrors are used, dust is filtered out and high vacuum made, a finite amount (few ppm) of the input laser power ends up outside the interferometer mode through scattering or reflection from the various optics. The resulting scattered light propagates in the steel pipe and interacts with the pipe itself and with all the various objects fixed to the pipe, which are in a state of vibration (seismic activity, thermal and acoustical coupling), acquiring a spurious phase. A second scattering process on a mirror surface may send a finite amount of the incoming scattered light into the stored wave with which it interferes, transmitting its noisy phase modulation acquired from the vibrating walls. Baffles suppress the flux of scattered light propagation. Among the most serious scattered light noise source: backscatter off bare walls near each mirror, backscatter off baffles far from each mirror and backscatter off objects at the far end of the beam tube.
To prevent stray light from reaching the steel pipe’s wall, baffles are placed all along the cavity to intercept scattered rays. Yet, baffles themselves are source of diffraction when light impinges on their rim and, as any source of noise must be controlled and mitigated, an accurate study on baffles diffraction is mandatory.
Figure 4 - Possible hig-performance baffle geometry
The aim is to study the impact of the diffracted field on the main laser beam and to propose some solutions to mitigate this effect, in terms of baffles’ geometries, materials and arrangement.