ABSTRACT
We propose a fast and non-destructive method to characterize the absolute diameter and uniformity of micrometer-scale fiber tapers using a pump and probe forward Brillouin scattering setup. The fundamental torsional-radial acoustic mode supported by the wire is excited using a pulsed pump laser and oscillates at a frequency that is inversely proportional to the taper waist diameter. This standing time-varying torsional-radial wave induces polarization modulation on a probe signal, whose spectrum structure reveals the sample diameter and its non-uniformity. By comparing our results with measurements using scanning-electron microscopy, a relative deviation of 1% or less was demonstrated, and diameter non-uniformity of less than 0.5% could be detected.
ABSTRACT
The interaction between light and acoustic phonons is strongly modified in sub-wavelength confinement, and has led to the demonstration and control of Brillouin scattering in photonic structures such as nano-scale optical waveguides and cavities. Besides the small optical mode volume, two physical mechanisms come into play simultaneously: a volume effect caused by the strain-induced refractive index perturbation (known as photo-elasticity), and a surface effect caused by the shift of the optical boundaries due to mechanical vibrations. As a result, proper material and structure engineering allows one to control each contribution individually. Here, we experimentally demonstrate the perfect cancellation of Brillouin scattering arising from Rayleigh acoustic waves by engineering a silica nanowire with exactly opposing photo-elastic and moving-boundary effects. This demonstration provides clear experimental evidence that the interplay between the two mechanisms is a promising tool to precisely control the photon-phonon interaction, enhancing or suppressing it.
