High multimode CW powers have been achieved from vertical-cavity surface-emitting laser diodes (VCSEL's). However, due to the problems of thermal lensing and spatial hole burning, single mode powers have been limited to several mW. We show experimentally and with a simple model that, in weakly index guided VCSEL's, self focusing causes the fundamental mode width to decrease with increasing power in the mode, resulting in a rapid transition to multimode operation and limiting the maximum single fundamental mode power.
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