Ph.D Thesis

Ph.D StudentZeitouny Abraham
SubjectTheoretical and Experimental Study of Fiber Lasers for
Optical Communication and Sampling of RF Signals
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROF. Moshe Horowitz


Fiber lasers can produce short pulses at high rates with a very low timing jitter. Therefore, such lasers are attractive sources for optical communication systems and for optical sampling systems. One of the problems encountered in fiber lasers is pulse dropout and regeneration. Under proper working conditions dropped-out pulses should regenerate. 

The regeneration of pulses that are dropped in the laser was studied in this work. A new theory was introduced, based on soliton perturbation theory, which enables to determine the optimal region of operation of the laser. The noise around the pulse was found to be stable due to the electro-optic modulator and the intracacvity  filter, that transfer part of the energy of the continuous noise to the stable modes of the pulse. In a time slot where a pulse is absent, this mechanism does not exist and the noise may become unstable. The theoretical results were carefully compared to the results of a comprehensive numerical simulation. In order to verify the theoretical predictions, an actively-harmonically mode-locked fiber laser was built. By using an electrical pulse source and an electronic circuit, we were able to drop only a few pulses out of the pulse train and follow their dynamic behavior for durations of up to 30~ms with a resolution on the order of a few picoseconds. The results of this experiment enabled, for the first time, to measure directly the dynamics of pulse recovery from dropout. When the laser power is high enough, the generation of a pulse pair in a single time slot was experimentally observed, for the first time in actively mode-locked fiber lasers. The experimental results were in a good agreement with a comprehensive numerical simulation of the laser.

Two applications of fiber lasers for generating and sampling microwave signals were demonstrated. By using a fiber laser and two linearly chirped Bragg gratings we produced linearly chirped microwave pulses. The bandwidth of the microwave pulses was much larger that that obtainable by electronic systems. Sampling of narrow-band RF signals with high carrier frequency was performed by implementing the method of under-sampling. The source for the optical pulses was a CW laser and two electro-absorption modulators. This type of source is more simple, compact and stable, compared to a fiber laser.