Abstract

In long passive optical fibres excited by narrow linewidth sources, stimulated Brillouin scattering (SBS) is the first non-linear effect to reach threshold [1]. In optically amplifying fibre, the SBS characteristics are drastically modified, and the threshold power can reach μW levels [2]. This is particularly important in long lengths of fibre lightly doped with Erbium to provide distributed gain, as has been proposed for practical amplifier systems [3]. Above a certain threshold power, the electrostrictive phonon gain provided by interference of the forward pump and backward Stokes waves coherently amplifies the phonon population at the resonant frequency, resulting in a sudden stimulated surge in Stokes power. At sufficiently high pump powers, this process can continue, producing a cascade of successive Stokes orders. In passive fibre, the threshold power at which SBS becomes significant is governed by the temperature, the modal area and the pump laser linewidth [2]. In this paper, SBS in a 60 m long Erbium-doped fibre amplifier (EDFA) is explored for excitation by 80 nsec pulses at various different levels of EDFA gain. A cascade of first, second, third and fourth Stokes orders is observed; previous work reported only two Stokes orders under similar conditions [4].

© 1993 Optical Society of America