Suppression of stimulated Brillouin scattering in analog CATV transmission systems

This thesis presents a systematic investigation of stimulated Brillouin scattering (SBS) in analog fiber-optic cable-television (CATV) transmission systems. To this end, the author has designed and built a high-bandwidth time-resolved chirp measurement system used to characterize the CATV transmitter. The complex average electric field and corresponding optical spectrum can be obtained using this system. From this, the relationship between SBS suppression and CATV transmitter characteristics is examined. We show that conventional SBS suppression theory breaks down for the high levels commonly used in optical CATV systems. A more suitable approach based on transmitter power and chirp measurement and detailed spectral analysis is demonstrated for two 1550nm distributed feedback (DFB) lasers. This new measurement technique can determine the level of SBS suppression in optical fiber over a much wider measurement range compared to conventional techniques. This makes it ideal for the development and characterization of enhanced SBS suppression schemes.; Using this approach, we investigated a variety of two-tone suppression schemes using a combination of direct laser and external phase dithering. Two-tone dithering is shown to be more effective at suppressing SBS than single-tone dithering. The best suppression scheme presently feasible is pure two-tone phase dithering at a frequency ratio of 3:2, generating an SBS threshold of 21.4dBm and requiring an RF bandwidth of only 3.9GHz. This level of suppression is 3.4dB higher than what has been previously reported. Applying this suppression technique to an 80-channel CATV system we show that self-phase modulation (SPM) becomes the dominant nonlinearity. The interaction of SPM with fiber dispersion degrades the CSO distortion to unacceptable levels. We discuss ways of overcoming this.