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PhD Defense: John Diehl

Wednesday, July 25, 2018
10:00 AM - 12:30 PM
Physics : 401
ADVISOR: Dr. Terrance Worchesky

TITLE: Residual Amplitude Modulation and Chromatic Dispersion Induced Distortion in Microwave Photonic Links

ABSTRACT:
Microwave Photonic Systems encode a radio-frequency signal of interest onto an optical carrier, thus taking advantage of the variety of advantages offered by the optics (low loss over distance, large operational bandwidth, immunity to electromagnetic interference, etc). They operate with a substantial dynamic range and bandwidth, requiring an understanding of parasitic effects typically ignored by telecommunications systems. The study of two such effects, chromatic dispersion induced second-order distortion and residual amplitude modulation, is contained in this work. A transfer-matrix approach is used to model various forms of photonic modulation techniques before being expanded for use in exploring the two topics of interest. The second-order distortion resulting from the presence of chromatic dispersion in the optical fiber is explored for phase modulation (with and without a Mach-Zehnder Interferometer demodulator) and intensity modulation (via an X- or Z-cut Mach-Zehnder Modulator). The model generated is compared to measured data at radio frequencies ranging from 3 GHz up to 75 GHz. Next, the unwanted amplitude modulation experienced when utilizing a Lithium Niobate phase modulator is theorized to be a result of a polarization misalignment at the front-end of the device. The model assumes a spatial separation of the ordinary and extraordinary modes result in a highly inefficient Mach-Zehnder Modulator. It also offers insight into a possible control mechanism in the form of a voltage shift of the input radio-frequency signal. Experimental verification of the proposed model is offered as well as confirmation of the control mechanisms ability to suppress the unwanted amplitude modulation by over 40 dB.