Phase-locked arrays of vertical cavity surface-emitting semiconductor lasers

Abstract

Recently, several groups reported the observation of phase-locking in vertical cavity surface-emitting laser (VCSEL) arrays.1,2 The most efficient phase-locking was obtained using internal diffractive coupling, brought about by a tailored spatial mirror reflectivity modulation.1 Following a critical review of the various coupling methods available for VCSEL arrays, the reflectivity modulation method and its compatibility with the special features of VCSELs are analyzed. Exploiting the high efficiency of this coupling method, we were able to design and fabricate a number of array geometries tailored for various uses. High peak power (>300 mW) coherent beams were obtained from rectangular multielement arrays with optimized laser size, element spacing, and reflectivity modulation depth. Beams with a prescribed number of equal-intensity lobes (e.g., twenty lobes) were obtained using ring shaped arrays for the implementation of fanned-out laser sources. In both of these examples the two-dimensionality of the arrays played a major role: stabilizing the coherent output mode and adding modal design flexibility. The importance of both the array geometry and its dimensionality are discussed in detail. Theoretical analysis of the modal behavior using coupledmode theory for the weak coupling cases, and beam propagation methods for the strong coupling regime agrees well with measured array characteristics. Finally, the possibility of dynamically shaping the coherent modes by changing the current injection pattern to the array elements are discussed.