Multiplexed fiber optic acoustic sensors in a 120 km loop using RF modulation

Abstract

The interrogation, via optical, fiber, of fiber Fabre Perot interferometers using laser based radio frequency modulation techniques, can provide ultra-sensitive acoustic sensing over very long distances. The benefits over other fiber optical acoustic sensing schemes include; immunity to laser polarization, coherence and intensity noise as well as reduced susceptibility to Rayleigh back scattering. Well defined error signals can be extracted at up to 120 km away. We report on the first multiplexed system, based on RF modulation interrogation techniques, in a 100 km fiber loop. We examine the achievable channel density as well as potential limits to strain sensitivity, such as inter-channel crosstalk, in a multiplexed RF modulated sensor system. The light-weight, small cross-section, intrinsic reliability, sensitivity and remote operation of the fiber sensor array based on RF techniques, enable new applications in hostile environments. The technique is free of electronics in the array part of the system, with all the electronic processing and control located remotely. There are no optical, amplifiers or pump lasers - the technique is entirely passive. With appropriate packaging, an array of either hydrophones or geophones may be created with applications in security and defense as well as in geological survey.