Rayleigh-Scattering-Induced Noise in Analog RF-Photonic Links

Analog RF-photonic links hold the potential to increase the precision of time and frequency synchronization in commercial applications by orders of magnitude. However, current RF-photonic links that are used for synchronization must suppress optical-fiber-induced noise by using bi-directional active feedback schemes, in which light must travel through the optical fiber in both directions. These schemes are incompatible with most existing fiber-optic networks. Unless this noise can be suppressed using different methods, RF-photonic time and frequency synchronization will remain accessible only to the research community. As a first step towards identifying alternate means of suppressing the optical-fiber-induced noise, this dissertation presents an extensive experimental characterization and limited theoretical discussion of the dominant optical-intensity and RF-phase noise source in a laboratory setting, where environmental fluctuations are small. The experimental results indicate that the optical-fiber-induced RF-phase noise and optical-intensity noise are caused by the same physical mechanism. The experimental results demonstrate that this mechanism is related to the laser-phase noise but not the laser intensity noise. The bandwidth of the optical-fiber-induced noise depends on the optical-fiber length for lasers with low phase noise, while for lasers with high phase noise, the bandwidth is constant. I demonstrate that the optical-intensity and RF-phase noise can be mitigated by dithering the laser frequency. Based on these results, I hypothesize that interference from Rayleigh scattering is the underlying mechanism of the optical-intensity and RF-phase noise. Prior theoretical work, carried out with high phase noise lasers, predicts that the noise induced by this process will have a bandwidth that is proportional to the laser linewidth and that is constant with respect to the optical-fiber length, for lasers with high-phase noise, which is consistent with the experimental results. I derive a simple model that is valid for low-phase-noise lasers. I compare this model with the experimental results and find that it matches the optical-fiber-length-dependent bandwidth that is measured for low-phase-noise lasers.