Experimental Demonstration of an Easy-to-Fabricate 1.3-μm Directly Modulated DFB Laser With Improved Beam Profile

The integration of a directly modulated laser with a spot-size-converter can greatly reduce packaging requirements thereby reducing costs and promoting applications of these devices. However, most of the reported spot-size-converters integrated directly modulated laser need complex fabrication process which will increase cost and may degrade modulation bandwidth. We report a 1.3-μm directly modulated laser with ridge waveguide distributed feedback region and buried waveguide spot-size-converter region to improve beam profile and increase coupling efficiency with low fabrication cost. The core layer with high refractive index is simply tapered into a 500-nm-wide waveguide before regrowth and after the regrowth process, the buried heterojunction spot-size-converter is formed. The ridge waveguide distributed feedback with conventional high reflection coating is replaced by active-distributed-reflector is applied to prevent the aluminium-containing multi quantum wells from the risk of oxidation and further reduce fabrication cost. The fabricated device realized a nearly circular beam profile and more than 70% improvement of power coupling into a 5-μm lensed-fiber compared to normal ridge waveguide lasers. The fabricated laser also demonstrated a 3-dB modulation bandwidth of ∼19 GHz with good single mode stability. Key parameters are extracted to prove that integration of this spot-size-converter does not degrade the modulation performance and the modulation bandwidth is limited by the material gain as the estimated D factor of the tested device is 2.32 GHz/mA close to that extracted from the material parameters.