Direct electro-optic modulation of lightwaves in optical fibers

In this dissertation, feasibility of direct electro-optic modulation of lightwaves in glass and plastic optical fibers is investigated. He-Ne laser lightwaves guided through an optical fiber are modulated when a short section of the optical fiber is subjected to an audio frequency modulating voltage applied to a pair of electrodes that sandwich the fiber section. The laser light emerging from the fiber output endface and impinging on the photodetector is intensity or amplitude modulated. The photodetector position was varied in order to probe the emerging light beam at various locations. Significant temporal variations in the amplitude, waveform, and phase of the detected output signal are observed under seemingly constant operating conditions. These variations are observed in the time domain representation and in the spectrum of the detected output signal. Similar temporal variations are observed in the spectrum of the photodetected signal at various detector locations. Variations in the amplitude, waveform, and phase of the detected output signal are observed as slight movements of the photodetector position are made before the temporal variations occur. Also, within each fixed detector position, similar variations in the condition of the detected output signal are observed allowing for the temporal variation to set in. A theoretical model is presented to explain the obtained results in this work. The author assumes that when a section of an optical fiber is subjected to a modulating electric field, propagating modes in the fiber are phase modulated to different degrees since each mode in the fiber has different phase constant. These phase modulated modes exit the fiber output endface and combine at the photodetector. This results in intensity modulation of the laser light beam. The experimental results and observations presented in this dissertation clearly indicate that the direct electro-optic modulation of lightwaves in plastic and glass fibers, used in this work, is feasible