Advanced modulation schemes for suppression of stimulated Brillouin scattering in optical fibre amplifiers

High power single-frequency lasers have attracted much attention lately due to their potential for application in phased-array lasers, coherent LIDAR, gravitational wave detection, coherent beam combining and many more. Optical fibre amplifiers operating in master-oscillator power-amplifier (MOPA) configuration are very well suited for power scaling of these lasers. A severe limitation on power scaling of these lasers is imposed by the stimulated Brillouin scattering (SBS) in optical fibre amplifiers. Since SBS has potentially the lowest threshold of all the nonlinearities in optical fibres, it is critical that it be addressed. The central idea of this thesis is to use advanced modulation schemes for suppressing SBS in optical fibre amplifiers. These modulation schemes are aimed at phase modulating either Brillouin Stokes wave using cross phase modulation from the intensity modulated pump or the laser wave through pure phase modulation by using an electro-optic modulator. In both cases the spectral linewidth of the respective lightwaves is broadened due to phase modulation. We explore optimized modulation formats for optical spectral control and suppression of SBS. In the former case, we experimentally achieve 5 dB SBS threshold enhancement in Raman and rare-earth doped optical fibre amplifier. In the latter, we use periodic optimized phase modulation formats to achieve SBS threshold enhancement, theoretically. It is also demonstrated that these periodic arbitrary wave-forms perform better compared to the random waveforms like white Gaussian noise. Furthermore, we investigate Brillouin gain efficiency in optical fibres for lengths that are generally employed in high power fibre amplifiers. The optical fibres show a lower- than-expected threshold at these lengths which is attributed to the wave mixing effects. Simulations based on the bidirectional waves are carried out to support these experimental results.