Tensor characteristics of forward Brillouin sensors in bare and coated fibers

Abstract

Forward Brillouin scattering fiber sensors can detect and analyze media outside the cladding of standard fibers, where guided light does not reach. Nearly all such sensors reported to-date have relied on the radially symmetric guided acoustic modes of the fiber. Wave motion in these modes is strictly dilatational. However, forward Brillouin scattering also takes place through torsional–radial guided acoustic modes of the fiber. Torsional–radial modes exhibit more complex tensor characteristics, and they consist of both dilatational and shear wave contributions. In this work, we show that forward Brillouin sensing through torsional–radial acoustic modes is qualitatively different from processes based on the radial ones. While dilatational wave components may dissipate toward liquids outside the fiber cladding, shear waves do not. Consequently, the effect of outside liquids varies among torsional–radial modes. Those modes that are dominated by their dilatational components undergo faster decay rates, whereas other modes with large shear contributions decay at much slower rates in the same liquid. The difference in decay rates may reach a factor of seven. Experimental observations are well supported by the analysis. The differences among modes are also found with liquid outside specific coating layers. Large changes in decay rates are observed when a phase transition between solid and liquid occurs outside the cladding boundary. The monitoring of multiple mode categories provides more complete assessment of outside media and enhances the capabilities of forward Brillouin scattering fiber sensors.