Guided Acoustic Waves Brillouin Scattering applications in optical fibers.

Guided Acoustic Waves Brillouin Scattering (GAWBS) is a non-linear optical effect, in which two co-propagating optical waves that are detuned by specific frequency offsets stimulate guided acoustic waves of an optical fiber or waveguide. The frequency of the stimulated acoustic wave equals the detuning between the two optical frequencies, and its axial phase velocity matches the phase velocity of light in the waveguide. An additional optical wave that co-propagates in the same waveguide becomes affected by phase-matched photo-elastic perturbations and undergoes phase modulation at the acoustic wave frequency.

In standard optical fibers, the acoustic waveguide is the entire silica rod (usually of 125 µm diameter), whereas the optical waveguide is an 8 µm core on the axis of symmetry. This difference in dimensions reduces the efficiency of opto-mechanical coupling. At the same time, however, opto-mechanical interaction in fibers exhibit unique characteristics: while the stimulating optical waves never leave the core, the generated acoustic waves profiles span the entire cross-section of the fiber. In this seminar we will discuss the physics involved in the GAWBS process in optical fibers, and present two of its possible applications demonstrated by our research group in the last few years:

1. Distributed opto-mechanical sensing: GAWBS enables sensing of mechanical properties of an analyte outside the cladding of uncoated standard optical fiber. Recently, we were able to extend the capabilities of our sensing platform from a single-point measurement, to a distributed mechanical impedance sensing. First results distinguish between water and ethanol over 3km of standard single mode fiber with 100m resolution. Such measurements allow us to "listen", where we may not "look", as guided light never leaves the core of the fiber and does not "see" the medium under test. Measurements are also perfromed outside coated fibers.
2. A "phonon laser" over a multi-core fiber: Narrowband acoustic oscillations can be obtained through the introduction of feedback to the acoustic wave, in so-called "phonon lasers". In the current research of our group, stimulated emission of highly-coherent, guided acoustic waves is achieved based on inter-core, opto-mechanical cross-phase modulation in a commercial multi-core fiber, at room temperature. The fiber is connected within an opto-electronic cavity loop. Pump light in one core is driving acoustic waves via electrostriction, whereas an optical probe wave at a different physical core undergoes photo-elastic modulation by the stimulated acoustic waves. Single-frequency mechanical oscillations at hundreds of MHz frequencies are obtained. The linewidths of the acoustic waves oscillations are on the order of 100 Hz, orders of magnitude narrower than those of the pump and probe light sources. The results can pave the way towards practical phonon laser sources, for applications in sensing, metrology, information processing and more.