Narrow-linewidth 1.5 μm quantum dot distributed feedback lasers for next generation coherent communication

InP-based quantum dot (QD) material with five QD layers in the active region was used to fabricate distributed feedback (DFB) lasers with a cavity length of ~1.5 mm. QDs enable tailoring of device properties, like gain, bandwidth and low linewidth enhancement factor (alpha-factor) that are favourable for applications in next generation coherent communication. Theoretical considerations taking into account the quasi zero-dimensional nature of the active material, clearly predict a strong reduction of the laser linewidth by appropriate dot design. Previous results on QD DFB lasers using 2 QD layers in the active region exhibited a linewidth of 110 kHz. New results based on 5 QD layers suggest considerably lower linewidth as was predicted by theory. First results suggest linewidths below 50 kHz.