Phase locking in a two-element laser array: a test of the coupled-oscillator model.

The steady-state and transient dynamics of phase locking in a two-element Nd:YAG laser array have been studied. By creating two evanescent-coupled lasers in a Nd:YAG étalon using diode end pumping, the coupling strength between the laser elements in the array can be continuously varied by adjusting the positions of the pumping beams. This allows the observation of the phase-locking process over a wide range of coupling strength. We have found that the development of the phase-locked state is as fast as the onset of lasing without an evolutionary process. The instantaneous locking is also independent of the coupling strength once the coupling is strong enough to ensure phase locking. These phenomena disagree with the predictions based on the time-dependent coupled-mode theory of laser-array dynamics. Our experimental study and theoretical analysis have led to the conclusion that all predictions of optical instability in laser arrays need to be reexamined.