טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentGordon Ariel
SubjectStatistical Mechanics Approach and Order-Disorder Phenomena
in Nonlinear Many-Mode Optical Systems
DepartmentDepartment of Electrical Engineering
Supervisor Professor Emeritus Baruch Fischer


Abstract

In this work we introduce a new fundamental approach in laser physics, based on statistical-mechanics. The approach is employed to improve the understanding of known laser phenomena and to predict new ones. In particular, we present a solution to the long-standing question of the passive mode locking threshold for pulsation, and identify it as a first order noise-induced phase transition. The theoretical predictions are corroborated in a series of experiments.


The motivation for using statistical mechanics is that many mode lasers are systems of many interacting degrees of freedom on one hand, and exhibit order-disorder behavior (such as mode locking) on the other hand. Mathematically the statistical-mechanics description is established by an exact solution of a Fokker-Planck equation. The solution turns out to be in many important cases the Gibbs distribution, only that the energy and temperature are replaced by a generalized Hamiltonian and noise power respectively.


Once the Gibbs distribution is found, powerful techniques from equilibrium statistical mechanics, are employed to obtain results on mode locked lasers, such as the above mentioned phase transition and many more.


Entropy considerations and noise are shown to play a crucial role in shaping the behavior of mode locked lasers. In particular we show that noise of very small power can dictate to a passively mode locked laser whether to produce pulses or no. We have shown that nonlinearities introduce a new power scale to the laser, much smaller than the operating power, to which noise should be compared.


The theoretical results are corroborated in a series of experiments.