Abstract
We use balanced optical low-coherence interferometry to measure the ultralow light power back-reflected and/or back-scattered by optical interfaces. Indeed, backscattered light from optical surfaces can be a critical source of noise in interferometric gravitational-wave detectors, such as the Laser Interferometer Gravitational-Wave Observatory (LIGO), Virgo, or the Laser Interferometer Space Antenna (LISA), and therefore, it is required to characterize, with high sensitivity, the scattering properties of elementary components involved in their optical design. The use of a broadband light source makes an easy identification of the scattering interface possible, while the implementation of balanced interferometric detection allows very low light levels to be detected, thanks to the simultaneous use of coherent gain and source intensity noise suppression. Experimental results are reported, first on bare glass optical windows (N-BK7 and S-LAH66) and then on a silver-coated mirror, that demonstrate the ability of this setup to quantify specular- or diffuse-reflectance coefficients as low as ${10}^{\ensuremath{-}11}$.
Highlights
Since the direct detection of gravitational waves (GWs) in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) [1], several projects for the development of dedicated GW antennas have been launched, either for terrestrial implementation (Einstein Telescope, ET) or for deployment in space (Laser Interferometer Space Antenna, LISA)
We show how the replacement of an unbalanced interferometric detection by a balanced one allows this dynamic range to be improved to 110 dB, by retaining all the advantages of low-coherence reflectometry
The pigtail consists of a Corning HI 1060 single-mode fiber, the output of which is located in the focal plane of an offaxis parabolic mirror reflective collimator (RC) with an effective focal length, f, of 7 mm (Thorlabs RC02APCP01)
Summary
Since the direct detection of gravitational waves (GWs) in 2015 by the Laser Interferometer Gravitational-Wave Observatory (LIGO) [1], several projects for the development of dedicated GW antennas have been launched, either for terrestrial implementation (Einstein Telescope, ET) or for deployment in space (Laser Interferometer Space Antenna, LISA) For all these interferometric instruments, scattered light is a critical issue [2,3,4,5,6,7,8,9,10] because, to a first approximation, it can induce phase-measurement errors that are directly influenced by the relative amplitude of these parasitic optical fields.
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