Ph.D Thesis

Ph.D StudentRosen Amir
SubjectDynamics of Multiple Light Pulses and Modes in Lasers
in the Presence of Noise
DepartmentDepartment of Electrical and Computer Engineering
Supervisor PROFESSOR EMERITUS Baruch Fischer
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Mode locked lasers that generate ultra short light pulses are key elements in many scientific and technological systems.

The combination of many modes with interaction between them when nonlinear elements like a saturable absorber are present in the cavity, as well as the noise in the system, inevitably generated by the amplifier, makes it a unique ground for applying statistical-mechanics techniques.

It provides a powerful approach that explains long standing questions and understanding of mode locking such as associating it as a thermodynamic-like first order phase transition and predicts other features such as critical behavior when external synchronized pulses are injected into the laser cavity.

In the first part of the work, we present a dramatic experimental verification of critical behavior in a mode locked laser for the first time. Various critical exponents, the universal fingerprint which characterizes the behavior of the system around the critical point, were also measured yielding excellent agreement with the theory.

In the second part of the work, lasers emitting multiple pulses subjected to injection with a pulse rate that can be at some (same or other) harmonic of the basic cavity frequency are theoretically and numerically analyzed. Rich thermodynamic-like phase diagrams with multiple phase transitions, multiple critical points and higher order phenomena are predicted.

When we advance towards the case where the number of pulses in the cavity is large as well, we ascend a layer in the hierarchy, regarding the pulses as the elementary particles in the many body system instead of the light modes. In the third part of the work, we present an analysis of the many-pulse system with statistical mechanic means. Bunching properties of the pulses are theoretically predicted and then compared with the experiment yielding excellent agreement. A surprising "order induced by noise" phenomenon was experimentally observed and theoretically explained by the gain depletion repulsion mechanism.

In the last part of the work, a regime in which the pulses become ordered into long quasi crystals is experimentally demonstrated. The role of the gain depletion effect is also studied in this regime.

The research provides basic and necessary means for the analysis and the understanding of the principles of mode locked lasers. The work also yields a new statistical-mechanics paradigm that enables experimental study in a light system that is also purely one-dimensional - a rare opportunity in real physical systems.