Acousto-optic effect in optical fibre tapered structures for the design of filters

Acousto-optic all-fibre filters are a promising technology for dynamic gain equalisation of optical fibre amplifiers. In this paper we show that a controlled tapering of optical fibres can be regarded as a new degree of freedom for the design of acousto-optic filters. In particular, we investigate the coupling between the fundamental and several cladding modes, studying the evolution of the resonance conditions as the fibres are progressively tapered. Theoretical predictions are in agreement with experimental results for several tapered fibres. The application of this acousto-optic filter for gain equalisation of optical fibre amplifiers is demonstrated. The design consists of two cascaded multi-tapered fibre lengths driven by only one acoustic frequency. The spectral characteristics are determined by the taper profile. The amplified spontaneous emission of an erbium doped fibre amplifier is flattened to within 1 dB over a spectral range of 30 nm for different saturation levels of the amplifier. This method results in a simplified implementation of the acousto-optic filter in contrast to other multi-frequency design approaches. Optimum filter designs can be achieved by the combination of both degrees of freedom: taper control and multi-frequency tuning.