Brillouin Interaction between Two Optical Modes Selectively Excited in Weakly Guiding Multimode Optical Fibers

Abstract

A multimode optical fiber supports excitation and propagation of a pure single optical mode, i.e., the field pattern that satisfies the boundary conditions and does not change along the fiber. When two counterpropagating pure optical modes are excited, they could interact through the stimulated Brillouin scattering (SBS) process. Here, we present a simple theoretical formalism describing SBS interaction between two individual optical modes selectively excited in an acoustically isotropic multimode optical fiber. Employing a weakly guiding step-index fiber approach, we have built an analytical expression for the spatial distribution of the sound field amplitude in the fiber core and explored the features of SBS gain spectra, describing the interaction between modes of different orders. In this way, we give a clear insight into the sound propagation effects accompanying SBS in multimode optical fibers, and demonstrate their specific contributions to the SBS gain spectrum.

Brillouin interaction between single optical modes excited in multimode fibers

“…A single peak SBS gain spectrum shown in Fig. 3 (a) is similar to that for the SBS in a planar waveguide at small incident angles 42 , but the maximal SBS gain exceeds the SBS factor for a volume medium more than twice. This is due to nonuniform (bell-like) distribution of the pump power in the fiber core which reduces the effective fiber core area available for nonlinear interaction.…”

“…This is in contrast with the SBS in a planar waveguide that produces the sound amplitude which is always coinciding with the parent interference pattern. 42 Figure 3 shows the SBS gain spectra calculated for different interacting mode pairs (a-c) using Eq. 9 and radial distributions of the sound amplitude ( )…”

“…When the SBS involves interaction of high-order optical modes, the sound diffraction effect occurs due to the different manner of propagation in the optical fiber of optical and sound waves. The optical fiber supports the waveguide manner of propagation for optical waves only, whereas it remains a volume medium for sound waves (until we ignore its guiding and anti-guiding properties at sound frequencies) 42 . The optical eigenmodes in an optical fiber are expressed through special functions, while the sound eigenmodes remain to be plane waves.…”

“…In this way, we give a clear insight into the sound propagation effects accompanying SBS in multimode optical fibers and demonstrate their specific contributions to the SBS gain spectrum, particularly to the spectrum broadening and splitting in the case of highorder mode interaction. For better understanding of the explored mechanisms, the effects obtained for SBS in optical fiber are compared with similar effects obtained for SBS in a volume medium and planar waveguide 42 .…”

Enhancing the Distributed Acoustic Sensors’ (DAS) Performance by the Simple Noise Reduction Algorithms Sequential Application

“…Utilizing the Rayleigh backscattering of light propagating in an optical fiber, coherent optical reflectometry-CO-OTDR [4], phase-sensitive optical reflectometry-ϕ-OTDR [5][6][7][8][9], and optical frequency domain reflectometry-OFDR [10][11][12][13][14] have been proposed. In contrast to them, Brillouin and Raman reflectometry [15][16][17][18][19][20][21] use various types of inelastic scattering. Many of these technologies are adapted to perceive both static and dynamic external impacts on optical fiber, such as sound and other mechanical vibrations, temperature variations, and mechanical stress.…”