Alexander Graham Bell's photophone of 1880 was a simple free space optical communication device that used the sun to illuminate a reflective acoustic diaphragm. A selenium photocell located 213 m (700 ft) away converted the acoustically modulated light beam back into sound. A variation of the photophone is presented here that uses naturally formed free space acousto-optic communications links to provide passive multichannel long range acoustic sensing. This system, called RAS (remote acoustic sensor), functions as a long range microphone with a demonstrated range in excess of 40 km (25 miles).

Date Published: 1 September 2006
PDF: 13 pages
Proc. SPIE 6304, Free-Space Laser Communications VI, 63040E (1 September 2006);

Published in SPIE Proceedings Vol. 6304:
Free-Space Laser Communications VI
Arun K. Majumdar; Christopher C. Davis, Editor(s)