Local activation of surface and hybrid acoustic waves in optical microwires

Abstract

The rich dynamics of photon-phonon interactions in subwavelength dimension structures have generated immense scientific interest lately. Brillouin scattering self-cancellation [1] and surface Brillouin scattering [2] are among the newly discovered optoacoustic interactions in sub-wavelength silica microwires. Surface Brillouin scattering can be described as an inelastic light scattering generated by a surface acoustic wave (SAW) induced via electrostriction. Traveling at the boundary interface of dissimilar elastic stiffness regions, SAWs demonstrate clear advantages over bulk waves for their low speed and accessibility, permitting a wide range of applications, including surface studies, signal processing, time delay, etc.[3]. Unlike the simplified plane wave description of stimulated Brillouin scattering (SBS) in conventional optical fibers, the low dimension and hard mechanical boundary conditions of silica microwires confine elastic waves into eigenmodes. This feature gives rise to a variety of acoustic vibrations featuring different spatial distributions and speeds, among them hybrid shear-longitudinal acoustic waves (HAWs) and SAWs [2]. Being compatible with standard optical fiber, silica microwires simplify the explorations and manipulations of acoustic waves in an isolated mesoscopic phononics structure [4].The existence of multiple auxiliary elastic resonances in silica microwires has been experimentally verified by detecting the spontaneous Brillouin backscattering generated by thermally excited acoustic waves [2]. Although such measurements merely indicate the existence of multiple elastic resonances in the entire sample, applied techniques so far cannot resolve the exact longitudinal location of each resonance mode inside a microwire. Indeed, the existence of each acoustic mode highly depends on the local diameter of the microwire, and therefore different resonance frequencies are expected to be found at different positions of the tapered fiber section (e.g., in the transition region and waist). Here, we propose a method to stimulate and probe different classes of acoustic waves at specific locations inside an optical microwire, thus pinpointing precisely their respective locations and amplitude. We demonstrate that, by using a correlation-based distributed Brillouin technique [5][6][7], each Brillouin resonance frequency (mode) can be locally generated at any position over the tapered optical fiber with a spatial resolution of a few centimeters (or even millimeter scale). The technique employed here is based on the use of pump and probe light beams being phase-modulated with a given (same) phase pattern, so that an identical optical phase crossing will occur only at a specific microwire location (defining a so-called correlation peak). At the correlation peak, the locally generated acoustic waves (independently for each acoustic mode) have enough time to be efficiently activated, thus scattering pump light at the corresponding Brillouin resonant frequency [6,7]. This way, the phase-modulated Brilloui...

Phase-Coded and Noise-Based Brillouin Optical Correlation-Domain Analysis

“…In another recent study, phase-coded B-OCDA was used to monitor strain in composite material cylinders following impact [27,28]. Phase-coded B-OCDA also finds novel application in the mapping of opto-mechanical interactions along tapered glass micro-wires [29,30]. Optical fibers that are tapered down to micronscale diameter support a rich variety of bulk, surface, and hybrid guided acoustic modes [48,49].…”

“…The experimental uncertainty in the estimate of the local micro-wire diameter was only ±5 nm [29]. Very recently, the same groups applied phase-coded B-OCDA to the distributed analysis of SBS through multiple acoustic modes of a silica micro-fiber [30]. The diameter of the fiber was gradually tapered over several cm down to a minimum waist of 0.9 µm.…”

“…The spatial resolution of the analysis was 5 cm. Multiple surface and hybrid acoustic modes are observed [30]. [30].…”

On-chip correlation-based Brillouin sensing: design, experiment, and simulation

“…The higher spatial resolution offered by BOCDA opens up the opportunity to monitor and characterize smaller and more sensitive structures such as micro-fibers [35] and on-chip photonic waveguides [36]. SBS-response characterization becomes critical in micro-and nanoscale structures, where the geometrical non-uniformities along the waveguide result in a broadening of the SBS spectrum [37] and influence the applications which rely on the narrow-linewidth of the SBS process [4][5][6][7][8][9][10].…”

“…in micro-fibers [35]. Recently, sub-mm spatial resolution was achieved by employing the BOCDA technique in a chalcogenide photonic waveguide with an improved signal-to-noise ratio (SNR) compared to the optical fiber measurements [36].…”