Rayleigh backscatter mitigation by RF modulation in a 100-km remote fiber sensing system

Abstract

One of the main factors limiting high performance remote fiber sensing systems is the Rayleigh backscatter associated with a long length of optical delivery fiber. Rayleigh backscatter introduces amplitude and phase noise during interferometric signal extraction, resulting in degradation of system sensitivity. This noise source increases with the length of optical fiber used in the architecture, and thus traditionally sets the lower limit on signal strength and the total remote sensing distance. We present the latest results for a 100 km remote fiber dynamic strain sensing system, where a radio-frequency (RF) modulated laser is used to interrogate a fiber Fabry- Perot sensor. The signal extraction is derived interferometrically from the differential phase between the carrier and its RF sidebands. We demonstrate unprecedented remote sensitivity performance by complete mitigation of the debilitating effects associated with Rayleigh backscatter in the 100 km of optical delivery fiber. We show that optimization of the laser modulation depth, as well as fiber Fabry-Perot design both facilitate a large signal-to-noise ratio. This maximized signal-to-noise ratio enables the complete suppression of the noise associated with Rayleigh backscatter. The result is a long-distance remote fiber sensing system that is limited only by the laser frequency noise. This remote sensitivity is an important breakthrough for a range of applications, such as sea floor acoustic sensing arrays, deep sea hydrophone arrays, and remote surveillance arrays.