Abstract

Stimulated Brillouin scattering (SBS) is typically the first optical nonlinearity encountered in lightwave systems in single-mode optical fibers. The repeaterless transmission span in such systems can be limited by SBS through a limitation on the maximum fiber injected power. However, SBS can normally be avoided in digital systems if the total fiber injected power is kept below 10 mW for bit rates above several hundred megabits for any modulation format (ASK, FSK, PSK) since the optical spectrum is broadened relative to the Brillouin linewidth.¹ In lightwave AM-VSB CATV systems with an external modulation, in contrast, the optical spectrum is essentially that of the unmodulated laser, so that the SBS threshold is at its theoretical minimum.² At the same time, these systems demand high fiber injected power in order to meet the typically required 55 dB of carrier-to-noise ratio (CNR).³ Therefore the power in such systems will exceed the SBS threshold even for modest system loss budgets. SBS leads to a limitation of the transmitted power and the addition of excess noise, which leads to degradation of the CNR.² In this paper we demonstrate a special modulation format that suppresses SBS, and we obtain a 5-dB increase in the allowed injected power without any degradation in the CNR.

© 1993 Optical Society of America