  The modulation efficiency of a tubular-lattice hollow-core fiber (HCF) by
means of flexural acoustic waves is investigated in detail for the first time.
The main acousto-optic properties of the HCF are evaluated employing 2D and 3D
models based on the finite element method. The induced coupling of the
fundamental and first higher-order modes is simulated in the wavelength range
from 743 to 1355 nm. Significant acoustic (amplitude, period, strain, energy)
and optical parameters (effective index, beat length, birefringence, coupling
coefficient) are analyzed. The simulations are compared to experimental
results, indicating higher modulation performance in HCFs compared to standard
optical fibers. In addition, useful insights into the design and fabrication of
all-fiber acousto-optic devices based on HCFs are provided, enabling potential
application in tunable spectral filters and mode-locked fiber lasers.
