Ph.D. Dissertation Defense
Rayleigh-Scattering-Induced Noise in Analog RF-Photonic Links
James Cahill
3:00pm Tuesday, 7 April 2015, ITE 325b
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 active feedback schemes that 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 thesis 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 without active feedback 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. The literature predicts that the noise induced by this process will have a bandwidth that is proportional to the laser linewidth and constant with respect to the optical fiber length, for lasers with high-phase noise, which is consistent with the experimental results. I derive a simplified 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 measured for low-phase-noise lasers.
Committee: Drs. Gary Carter (Chair), Curtis Menyuk, Fow-sen Choa, Olukayode Okusaga, Weimin Zhou