Forward Brillouin Scattering Spectroscopy in Optical Fibers with Whispering‐Gallery Modes

Abstract Opto‐mechanical interactions in different photonic platforms as optical fibers and optical microresonators are raising great attention, and new exciting achievements have been reported in the last few years. Transverse acoustic mode resonances (TAMRs) in optical fibers –which can be excited optically via electrostriction and generate forward Brillouin scattering (FBS)– are being promoted as the physical mechanism for new fiber‐sensing concepts. Here, the study reports a novel approach to detect and characterize opto‐excited TAMRs of an optical fiber based on the interplay with optical surface wave resonances, i.e., optical whispering‐gallery mode (WGM) resonances. TAMRs induce perturbations in the geometry and the dielectric permittivity of the fiber over the entire cross‐section. It is shown that these perturbations couple the acoustic with the optical resonances and affect WGMs in a noticeable way. The study proposes and demonstrates the use of WGMs for probing opto‐excited TAMRs in optical fibers. This probing technique provides the narrowest linewidths ever reported for the TAMRs and demonstrates an optimum efficiency for the detection of low‐order TAMRs. The interplay between sensitivity, bandwidth, and Q factor of the WGM resonance is discussed.