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Transverse mode dynamics in VCSELs: Spatiotemporal versus modal expansion descriptions

Mulet, Josep ; Balle, Salvador
Physical Review A 66, 053802 (1-8) (2002)

We discuss the range of validity of a modal description for the spatiotemporal dynamics of the optical field in vertical-cavity surface-emitting lasers (VCSELs). We focus on the secondary pulsations that appear during the turn-off transients when the injection current is modulated by a square-wave signal. We compare the results obtained with both a full spatiotemporal model and a modal expansion derived from this model. We find that the results obtained from the two descriptions agree for strong lateral guiding. However, for weak lateral guiding we find differences because the optical field profile changes significantly due to spatial changes in refractive index induced by the carrier density. The reason is that in the full spatiotemporal model a shrinkage of the mode profile occurs which leads to an enhancement of the secondary pulsations. This effect is not included in the modal expansion, and it determines the limits of validity of such an approach for gain-guided devices.