High frequency in-core acousto-optic modulation of a suspended core optical fibre

Abstract

The confinement of high frequency acoustic waves inside a suspended core fibre (SCF) is numerically investigated for the first time. A 500 μm long acoustic cavity, based on a four-hole SCF, is designed, simulated and evaluated by using the finite element method. The cavity is acoustically excited in the frequency range of 50 - 56 MHz and the induced displacements are integrated along the fibre. A standard single mode fibre is simulated under the same conditions for comparison. The results show strong Lamb acoustic modes oscillating in the silica bridges and overlapping in the SCF core at the resonance of 52.84 MHz. The induced displacement achieves a maximum in the core centre decaying to an almost null value in the cladding. The acoustic wave concentration in the SCF core is 13 times higher compared to the standard fibre, indicating a promising solution to overcome the frequency limitation of the current all-fibre acousto-optic devices. The modulation efficiency is increased without reducing the fibre diameter, making the devices more stable, fast and suitable to modulate all-fibre lasers.