Optical fibers overcome the limits of Rayleigh backscattering

Fiber optic technology is considered to be ideal for distributed sensing applications. Optical fibers of more than 1 km length are actively used in fiber optic cables, therefore, they allow obtaining data about temperature, strain, and acoustics without need for electrical connections, herewith, optical fibers have resistance to harsh environmental conditions in different structures.

It should be noted that a very important thing about such fiber optic systems includes the fact that their operating principle is based on the backscattering of light propagating in the core of an optical fiber, generally in single-mode operation. Herewith, such a process as backscattering is applied with numerous types of sensor interrogators, containing optical frequency domain reflectometry and coherent OTDR employed in distributed acoustic sensing.

To be more precise, an acoustic sensor modifies the whole length of optical fibers into a distributed microphone. This fiber optic technology operates by launching highly coherent pulses into the optical fiber and compiling the backscattered light with very sensitive detectors. For instance, when acoustic waves pressure or increase the optical fiber, the backscattering signal of the fiber optic system demonstrates tiny changes thanks to the change in optical path length. 

Thus, the comparison between the backscatter and successive pulses makes it real to reconstruct the acoustic wave propagating along with the optical fiber. Usually, the inherent Rayleigh scattering (a universal signal that all optical fiber waveguides have) evokes the crucial backscattering signal. The thing is that spatially continuous Rayleigh scatter doesn’t need for additional processing of the fiber optic system, therefore, providing complete coverage over any waveguide length. Nevertheless, the Rayleigh backscattering signal in standard fiber optic cables has such a disadvantage as a very weal signal.

Nonetheless, the different technique includes the bare optical fiber to rise optical backscattering. It is possible to enlarge elastic scattering well above the native Rayleigh scattering by the impact of optical fibers to pulsed radiation, significantly, through the creation of periodic or quasi-periodic fiber Bragg gratings. This fiber optic system has been already tested and showed a more than 10 dB increase in optical backscattering over the native Rayleigh scattering of the optical fiber

Additionally, it has been claimed that it is possible to monitor the scattering level by changing the UV dosage on the fiber optic system, herein, it is possible to vary the dosage along the fiber length to reduce the attenuation of the optical fiber. “Long lengths of optical fiber can exhibit very low thresholds for various nonlinear effects, these effects are well known and place limits on applications in telecom and sensing that use long lengths of optical fiber.” Finally, fiber optic technology enables to increase of the enhancement along the length of the leading to efficient fiber transparency for optical scattering measurements.

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