Linewidth of an externally modulated laser subjected to Rayleigh backscattering from a fiber

It is found that the linewidth of a semiconductor laser that is externally phase modulated and connected to a fiber with Rayleigh backscattering is broadened. At low modulation rates the linewidth can be several orders of magnitude greater than the free-running linewidth, but it converges to the free-running linewidth at sufficiently high modulation rates. As a result, the isolation needed to attain a specific linewidth can be calculated.>