טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentAlon Schwartz
SubjectInvestigation of Arbitrary Noise Forms on Phase Transitions
in Passively Mode-Locked Lasers
DepartmentDepartment of Electrical Engineering
Supervisor Professor Emeritus Fischer Baruch
Full Thesis textFull thesis text - English Version


Abstract

In this thesis we apply the Statistical Light-mode dynamics (SLD) theory developed in our group that is based on statistical-mechanics, to the study of mode locked lasers in inhomogeneous noise structures. Passive mode locking is a phase transition phenomenon resulting from mode coupling in the laser, just as happens in many body interacting particle systems. In the laser system nonlinear long range interaction between modes is caused by saturable absorbers, causing passive mode locking and short pulse generation.

            In statistical physics there is a wide variety of methods and tools for analyzing many body systems with interaction. For mode locking in lasers it is natural to adopt those tools. Just as in statistical physics, the entropy and the free energy of the system are central quantities.

            It was previously shown that passive mode-locking is a first order phase transition, just as happens in thermodynamic systems (gas-liquid-solid) and magnetic spin systems. The modes can ‘freeze’ to form pulses, or vice versa, pulses can ‘melt’ into CW, when the noise that has the role of temperature decreases or increases correspondingly. The system thermodynamic phase is determined by the two opposite factors: the energy drives the system’s mode phases to order and the entropy to disorder.

            In this work we extend the statistical mechanics approach developed so far to study passive mode locked lasers with complex spatial noise structures in the laser cavity. The main results of our research are:

? We developed the SLD theory with structured noise. It includes a mathematical model that describes the dynamics of PML lasers with injected modulated (structured) white noise.

? We have found that pulses in passive mode-locking depend on the noise pattern in the laser cavity.

? We have found that pulses are formed at the noisiest regions (“hotspots”).

? The noise functions as a potential barrier that traps pulses.

? We conducted a series of experiments which verify the predictions of the theory.

? It is a first study of non uniform noise in a laser cavity and its effects on pulse formation in passively mode-locked lasers. We additionally report on unusual phenomena of pulse dynamics in non-uniform noise environment.