Any detection made by the second generation detectors will offer an exciting glimpse of the new science that gravitational wave astronomy will provide. Once this window has been opened instruments with even greater sensitivity will be needed to enhance the view. Researchers have begun to consider what third generation instruments will look like. These instruments will certainly use non-classical states of light, such as squeezing, to increase sensitivity without increasing the circulating power beyond the megawatt level that is planned for second generation detectors.
Optics with extremely low optical loss are critical for achieving the exquisite sensitivity promised by second and third generation interferometers. Optical surfaces which scatter light can hurt the performance of advanced gravitational wave detectors in a number of ways: The losses can reduce the build-up of circulating power in the coupled Fabry-Perot arm cavities used to increase the sensitivity of the instrument over a conventional Michelson interferometer. The reduction in circulating power increases the high frequency quantum noise, which is one of fundamental limiting noise sources of second generation detectors. Losses are even more detrimental on the achievable performance if non-classical states of light are used. Squeezed light with a quantum noise improvement of 10 dB will be reduced to 8dB if it encounters a mere 5% total loss between the squeezed light generation and the photo-detector. The scattered light itself can also lead to increased noise in the interferometer if it is not controlled properly. If this stray light hits a vibrating object and is re-scattered into the interferometer it can impart the signature of the shaking object onto the interferometer readout thereby reducing sensitivity to true gravitational waves.
Padilla et al. have described the results from a new instrument that characterizes the properties of the light scattered from an optical surface. By mounting the laser, substrate and the beam dumps on a computer-controlled rotation stage they were easily able to image the scattered light at near normal incidence through to ninety degrees. The substrate was arranged so that the angle of incidence of the probe beam was near normal incidence.
The imaging technique enabled the separation of the light scattered by the optical coatings and the high quality substrates that they have been deposited on. High-quality fused silica is known to have extremely low scatter. The results of Padilla et al. have shown that anti-reflection coatings deposited using ion beam deposition can have scatter that is equal to or lower than that of the fused silica substrates. Now fused-silica will need to be further improved if scattering loss is to be reduced further.
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