Optical fiber sensors based on forward and backward stimulated Brillouin scattering

Stimulated Brillouin Scattering (SBS) is a non-linear interaction between two optical waves, commonly referred to as pump and signal, and a mediating acoustic wave. Since the early 90’s, extensive research in both academia and industry was dedicated to utilizing SBS interactions in remote, distributed sensing over standard optical fibers. Sensing is enabled by the dependency between the resonance frequency of the SBS interaction and physical attributes of the fiber, namely temperature and tensile strain. In the work done during the presented research, high resolution SBS temperature and strain sensing was extended to ranges of up to 8.8 km, with spatial resolution of down to 2 cm. Progress was enabled by adding several layers of radar-inspired coding techniques. The 440,000 points addressed mark the largest number of any Brillouin scattering-based sensor to date.

This seminar will focus on a new opto-mechanical fiber sensor of the analysis of surrounding media. Current approaches rely on spatial overlap between the optical mode and the substance under test, which precludes the use of standard fibers unless significant structural modifications are made. The proposed protocol addresses liquids outside the cladding of standard, unaltered 8/125 μm single-mode fibers. Measurements are based on forward stimulated Brillouin scattering by radial, guided acoustic modes of the fiber structure. The acoustic modes are stimulated by an optical pump pulse and probed by an optical signal wave, both confined to the core. The acoustic vibrations induce a non-reciprocal phase delay to the signal wave, which is monitored in a Sagnac interferometer loop configuration. The measured resonance frequencies and the excitation strengths of individual modes agree with the predictions of a corresponding, quantitative analysis. The acoustic reflectivity at the outer cladding boundary and the acoustic impedance of the surrounding medium are extracted from cavity lifetime measurements of multiple modes. The acoustic impedances of deionized water and ethanol are measured with better than 1% accuracy. The measurements successfully distinguish between aqueous solutions with 0, 4%, 8% and 12% concentrations of dissolved salt.