Abstract
We propose a simple physical mechanism to explain observed instabilities in the dynamics of passively phased fiber amplifier arrays that arises from two properties: First that a weak phase disturbance of the output field of the array
is converted into a strong intensity disturbance through the mode-selective feedback mechanism. Second, that this
intensity fluctuation regenerates a phase fluctuation due to the nonlinear properties of the amplifying media. At
sufficiently high operating power levels this cyclic disturbance continues to grow upon each cavity round trip,
creating instability. This simple picture is supported by the results of a linear stability analysis of the set of
propagation and population rate equations, which are in good agreement with observed critical power levels. A third
level of quantitative confirmation was obtained by comparison to the results of numerical integration of the original
set of nonlinear equations. This predicted instability is entirely a property of passively phased arrays of more than
one element.