Transmission limitations of CPFSK coherent lightwave systems due to stimulated Brillouin scattering in optical fiber

Transmission limitations imposed by stimulated Brillouin scattering (SBS) are investigated for repeaterless coherent lightwave systems, particularly for continuous phase-shift keying (CPFSK). The effects of SBS on the modulated signals in optical fibers are studied for CPFSK, amplitude shift keying (ASK), and phase shift keying (PSK). A generalized model based on the maximum power spectrum density (PSD) in the modulated signals is proposed for SBS analysis. This model is useful to estimate the Brillouin gain and its threshold to determine the optical power limitation of the fiber for any modulated signal with non-negligible linewidth. It is clarified theoretically that the Brillouin gain with CPFSK approaches that with ASK and PSK at higher bit rates if the modulation index is 1 and 0.65, respectively. The dependency of Brillouin gain on bit rate and linewidth of a laser diode transmitter are evaluated. SBS induced by CPFSK modulated signals was measured to clarify the modulation condition dependences and to verify the model