Intensity noise induced by stimulated Brillouin scattering in optical fiber transmission systems

Stimulated Brillouin scattering (SBS), the main impairment in optical fiber communication systems, is a nonlinear process of the parametric coupling among a backscattered Stokes wave, pump wave, and electrostriction-induced traveling acoustic wave. SBS limits the maximum power that can be transmitted through an optical fiber, and also significantly increases intensity noise in both the backscattered and transmitted light.; The study of intensity noise induced by SBS in both backscattered and transmitted light is essential for understanding the SBS effect in fiber optical transmission systems. In this thesis, measurements of the average and spectral characteristics of the optical power transmitted and backscattered in a single-mode fiber transmission system are presented. The dependence of intensity spectra on the system parameters is discussed and compared to published models of the SBS process. We conclude that the intensity noise in the transmitted light is phase noise of the laser source, converted by SBS.; The phase to intensity conversion is attributed to modification of the pump optical spectrum by the frequency-dependent imaginary part of the Brillouin loss spectrum. A published theoretical model of a nonlinear phase shift at the optical carrier is extended to conversion of phase modulation to intensity modulation (PM-IM). The calculation agrees well with measurement of PM-IM conversion in a fiber transmission link. Compensation of PM-IM conversion by chromatic dispersion is predicted by the model and successfully observed in an experiment with a dispersion compensating fiber.; The model above does not predict the observed SBS-induced intensity noise in the transmitted light. By transmitting the SBS-degraded optical signal through a Brillouin amplifier, significant reduction of intensity noise (improvement of more than 6 dB of the signal-to-noise ratio (SNR) of the transmitted signal) is achieved. The dependence of the SNR output on the gain and the frequency detuning of the amplifier indicates that the nonlinear group delay at the carrier is the most reliable predictor of output intensity noise. Using a simple model of complex Brillouin gain, the nonlinear group delay is calculated and follows very well the measured SNR as the gain and the frequency detuning of the amplifier are varied.