In this work, we recorded the retro-reflected and back-scattered light from a system of two components i.e, a single side AR coated SiO2 window and a mirror placed at tilt angle of 10◦ in the transmission of the window. Retro-reflection and back-scattering from components in an optical system can be detrimental for system performance such as the phase measurement errors, ghost images and laser induced damage in gravitational wave interferometry, optical communications, biomedical imaging and high power laser systems, respectively. Therefore, an accurate determination of the retro-reflected and back-scattered light in such systems is imperative for optimized system performance, particularly the systems where extreme phase sensitive measurements are of keen interest such as the gravitational wave detectors LIGO, Virgo, KAGRA, the future planned Einstein Telescope (ET) and Laser Interferometer Space Antenna (LISA). Using a balanced optical low coherence interferometer, we recorded and distinguished the contribution of light retro-reflected and back-scattered from the different optical surfaces of the two-component assembly. This work would pave the way for simultaneous characterization of the spectral properties of light retro-reflected and back-scattered by components in an optical system with the capability to accurately and effectively identify the impact of individual components as well as the global system performance.
Light scattered from optical components is a source of critical noise for interferometric gravitational wave detectors such as LIGO, Virgo, KAGRA, or the future space antenna LISA. Therefore it is important to accurately quantify the amount of light coherently back-reflected or back-scattered by optical components involved in the design of such instruments. As an example, for a good quality plane silica window (1 nm RMS roughness) illuminated by a Gaussian beam with a waist of 1 mm, the amount of light back-scattered by each interface is between -95 dB and -125 dB following the angle of incidence. This defines the stringent sensitivity requirements we have to satisfy for such applications. In our communication, we present the measurements performed on a Silver coated mirror using a low coherence interferometer implemented with balanced detection.
We measured very low light power backscattered by optical surfaces using low coherence interferometry. The backscattered light can indeed be a critical noise source for interferometric gravitational wave detectors such as LISA, the laser interferometer space antenna developed by ESA, with the support of NASA and European national space agencies. We present the preliminary measurement results of light backscattered from a silver mirror and a bare N-BK7 window 1 mm thick using a superluminescent diode centered at 1060 nm as broadband light source. The use of low coherence interferometry allows light backscattered by both sides of the N-BK7 window to be recorded independently with a noise floor of around -110 dB.
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