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The AM noise in operation of index-guided (GaAl)As lasers is found to be strikingly reduced by the antireflection (AR) coating of cavity facets. This effect is shown to result from the longitudinal multi-mode operation due to the reduced facet reflectivity. It is observed that the width of the envelope of a laser spectrum depends on the facet reflectivity as well as the laser output power. The AM noise in the low-frequency range of the reduced reflectivity lasers is shown to be nearly constant with respect to temperature change. In contrast, the AM noise in a laser without the AR facet coating is characterized by a number of spikes occurring over a wide temperature range.