Recent progress in Brillouin scattering based fiber sensors

Abstract

Brillouin scattering in optical fiber describes the interaction of an electro-magnetic field (photon) with a characteristic density variation of the fiber. When the electric field amplitude of an optical beam (so-called pump wave), and another wave is introduced at the downshifted Brillouin frequency (namely Stokes wave), the beating between the pump and Stokes waves creates a modified density change via the electrostriction effect, resulting in so-called the stimulated Brillouin scattering. The density variation is associated with a mechanical acoustic wave; and it may be affected by local temperature, strain, and vibration which induce changes in the fiber effective refractive index and sound velocity. Through the measurement of the static or dynamic changes in Brillouin frequency along the fiber one can realize a distributed fiber sensor for local temperature, strain and vibration over tens or hundreds of kilometers. This paper reviews the progress on improving sensing performance parameters like spatial resolution, sensing length limitation and simultaneous temperature and strain measurement. These kinds of sensors can be used in civil structural monitoring of pipelines, bridges, dams, and railroads for disaster prevention. Analogous to the static Bragg grating, one can write a moving Brillouin grating in fibers, with the lifetime of the acoustic wave. The length of the Brillouin grating can be controlled by the writing pulses at any position in fibers. Such gratings can be used to measure changes in birefringence, which is an important parameter in fiber communications. Applications for this kind of sensor can be found in aerospace, material processing and fine structures.

Keywords: Brillouin scattering; acoustic wave; birefringence; dynamic measurement; fiber optic sensors; polarization mode dispersion; strain; structural health monitoring; temperature.

Figure 1.

Schematic diagram for the Doppler effect.

Figure 2.

Small stress response in short and long fiber lengths.

Figure 3.

Time-domain waveforms and Brillouin loss spectra for various pulse widths [29] (Copyright © 1999 IEEE, Reprinted with permission).

Figure 4.

Brillouin loss at three different strains for 2 ns pulse (equivalent to 20 cm spatial resolution [30] (Copyright © 2005 OSA, Reprinted with permission).

Figure 5.

Definition of the REC for simulated Brillouin loss spectrum with the fiber length of 1,000 m, z = 0, Pump = 30 mW, Probe = 5 mW, pulse length = 20 m [31] (Copyright © 2005 IEEE, Reprinted with permission).

Figure 6.

Normalized Brillouin loss spectrum dip plotted as a function of the normalized Brillouin frequency shift. Simulation results (plain curve) and experimental data (diamond) at the middle of sensing length 40 m, Ppump = 5 mW, Pcw = 3 mW, pulse length of 20 cm, ER = 11 dB [31] (Copyright © 2005 IEEE, Reprinted with permission).

Figure 7.

Time-division-multiplexing for 100 km sensing length.

Figure 8.

Time traces of the heated 18 m fiber using FDM-BOTDA with differential Brillouin gain technique. [26] (permitted by Nova Scientific Pub).

Figure 9.

Schematic diagram of the ODPA-BOTDA [45] (Copyright © 2010 OSA, Reprinted with permission).

Figure 10.

Brillouin spectrum width of the BOTDA (20–60 ns pulse) and the ODPA-BOTDA sensor with 20/18 to 60/58n s pulse pair based on Equation (20) (a, left); time domain signal of ODPA-BOTDA vs. BOTDA (b, right) [45] (Copyright © 2010 OSA, Reprinted with permission).

Figure 11.

Brillouin grating generation and reading process.

Figure 12.

The intrinsic Brillouin grating spectral width as a function of length [71] (Copyright © 2010 OSA, Reprinted with permission).