Boris Vodonos

This work presents an experimental study of the noise effect on pulse formation in passively mode locked lasers. We follow a novel statistical mechanics approach to mode locking, recently introduced in our group, and demonstrate many of its theoretical predictions.

The statistical mechanics approach emerges as natural tool for studying such a nonlinear many-mode laser system with its inherent noise. This approach is especially powerful for passively mode locked lasers, where generation of short optical pulses is identified as a first-order noise and power dependent phase transition and thus the solution is given to the long-standing question on the origin of power threshold for pulsation self-starting.

Directly controlling the level of the noise in the laser cavity by injection of amplified spontaneous emission from an external noise source we observe the first-order noise-dependent phase transition in the time domain, as well as an abrupt jump in the measured RF power of the detected laser output. The RF power is a natural order parameter distinguishing between pulsed operation and quasi continuous-wave regime.

Our detailed experimental study of multiple pulse formation yields a thermodynamic-like ``phase diagram" with boundaries representing cascaded first-order phase transitions that correspond to abrupt creation and annihilation of pulses and a quantized RF power behavior as system parameters (noise and/or pumping levels) are varied. The theory and the experiment are in excellent agreement, both qualitatively and quantitatively.

Another part of our experimental study is addressed to the self-starting stochastic behavior of passive mode locking. Our quantitative study confirms that pulsation self-starting is a Poisson process that results from an entropic noise-activated switching barrier.