|Ph.D Thesis||Department of Electrical Engineering|
|Supervisor:||Prof. Fischer Baruch|
|Full Thesis text|
Active and passive mode locking of lasers are the main methods for obtaining ultrashort light pulses. They are based on locking the phases of many axial (longitudinal) modes of a laser. While the mode locking has been traditionally viewed as a sort of kinematical mechanism, we have developed an approach that treats the many light mode system with noise as a statistical mechanics theory. An important ingredient of it arises from the understanding that noise, that takes the role of temperature, must be treated non-perurbatively. The outcome is statistical light-mode dynamics (SLD), a powerful theory for the discovery and study of points of sharp changes in the global structure of the optical waveform that have the significance of thermodynamic phase transitions. It was found, for example, that passive mode locking is nothing but a first order phase transition of the modes from random phase orientations to an ordered phase (pulses).
This thesis presents new findings in the SLD approach, showing how it leads us to basic understanding of the laser system and to new phenomena and observations, all verified experimentally. The basic achievements are:
1. Formation and annihilation of laser light pulses in cascaded first order phase transitions: The study of multi-pulse operation in passively mode locked (PML) lasers yields a thermodynamic-like phase diagram with boundaries representing cascaded first order phase transitions. They correspond to abrupt creation or annihilation of pulses.
2. Critical behavior of light in passively mode locked lasers: Light is shown to exhibit critical behavior in PML lasers with externally injected pulses. It is a first and unique example of critical phenomena in a one-dimensional many body light-mode system. The phase diagrams consist of regimes with continuous wave, driven “para-pulses”, and “spontaneous” pulses, separated by phase transition lines which terminate with critical or tricritical points.
3. Light-mode condensation in actively mode locked lasers: The study of active mode locking (AML) using SLD yields new results and predictions. The main result is that the formation of pulses exhibits in certain conditions a transition of the laser mode system to a pulse state that is similar to Bose-Einstein condensation.
4. Casmir-like light pulse interaction induced by amplified spontaneous noise in a laser cavity: A new mechanism for light pulse interaction in mode-locked lasers induced by amplified spontaneous noise is introduced. It is a light domain Casimir-like mechanism, in which the noisy background affects the pulse movement in the cavity.