Recent progress in distributed fiber optic sensors

Abstract

Rayleigh, Brillouin and Raman scatterings in fibers result from the interaction of photons with local material characteristic features like density, temperature and strain. For example an acoustic/mechanical wave generates a dynamic density variation; such a variation may be affected by local temperature, strain, vibration and birefringence. By detecting changes in the amplitude, frequency and phase of light scattered along a fiber, one can realize a distributed fiber sensor for measuring localized temperature, strain, vibration and birefringence over lengths ranging from meters to one hundred kilometers. Such a measurement can be made in the time domain or frequency domain to resolve location information. With coherent detection of the scattered light one can observe changes in birefringence and beat length for fibers and devices. The progress on state of the art technology for sensing performance, in terms of spatial resolution and limitations on sensing length is reviewed. These distributed sensors can be used for disaster prevention in the civil structural monitoring of pipelines, bridges, dams and railroads. A sensor with centimeter spatial resolution and high precision measurement of temperature, strain, vibration and birefringence can find applications in aerospace smart structures, material processing, and the characterization of optical materials and devices.

Keywords: Raman scattering; Rayleigh scattering; birefringence; brillouin scattering; distributed sensors; fiber optic sensors; optical frequency domain reflectrometer (OFDR); optical time domain reflectrometer (OTDR); strain; temperature; vibration.

Figure 1.

Typical spontaneous scattering spectrum from solid state matter.

Figure 2.

Schematic diagram for the spontaneous Rayleigh scattering process.

Figure 3.

Asymmetric Brillouin spectrum property of SMF28e, SMF28e+ and LEAF vs. position.

Figure 4.

Experimental setup of OFDR.

Figure 5.

(a) Differential Brillouin signal (intensity) [32]; and (b) Brillouin signal loss as a function of pulse width (here refers to the shorter pulse of the pulse pair) for 0.1 and 0.2 ns pulse width difference.

Figure 6.

Experimental results of the differential gain for the pulse width larger than phonon lifetime (a) and smaller than phone lifetime (b).