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Proceedings Paper

Measurement by reflection analysis of optical attenuation through windows
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Paper Abstract

Free space optical communication systems deployed in office buildings are subject to transmission loss through windows. Window attenuation varies between 0.4 and more than 15 dB. Window attenuation values are required to calculate communications link power budget and availability. But direct measurement of window attenuation in high-rise buildings is difficult since it requires access to both sides of the window. In this paper, we present a method of measuring optical attenuation from the interior side of a window. This method is based on measuring back reflections of a laser beam propagating through a semi-transparent dielectric medium, thus eliminating the need for access to the exterior of a building. In this system, a laser beam is launched at 45 degree(s) to normal incidence in order for the user to discriminate between the various reflections from the dielectric interfaces within the medium. A photodetector is then moved through the plane of incidence and the intensities of reflections from interfaces within the medium are measured. A simple formula is used to calculate total transmission of the optical system based on the relative intensities of the incident light beam and all resulting reflections. In this approach, it is assumed that the reflectivities of the first and final interfaces are identical. The index of refraction for glass from one commercial fabricator varies little; hence the reflectivity of uncoated air-glass interfaces in a particular window is the same. The intensity of the reflection from the final interface is attenuated by the entire medium twice. By comparison of the incident, first, and final reflected intensity a transmitted intensity can be determined. The same equation is used for a medium with any number of dielectric interfaces. A measurement of optical loss through a window without access to both sides of the medium is now possible. This method has been demonstrated to be accurate (+/- 1dB) through various windows with optical losses of up to 12dB.

Paper Details

Date Published: 27 November 2001
PDF: 9 pages
Proc. SPIE 4530, Optical Wireless Communications IV, (27 November 2001); doi: 10.1117/12.449809
Show Author Affiliations
Eric C. Eisenberg, Terabeam Corp. (United States)
Jeff C. Adams, Terabeam Corp. (United States)
Carrie Sjaarda Cornish, Terabeam Corp. (United States)

Published in SPIE Proceedings Vol. 4530:
Optical Wireless Communications IV
Eric J. Korevaar, Editor(s)

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