Distributed-Bragg-reflector (DBR) lasers with curved surface gratings produce stable, single-mode output, as demonstrated by M. Uemukai and collaborators at Osaka University (Osaka, Japan) and Chalmers University of Technology (Göteborg, Sweden). Output from the laser diverges at 6.2° (full angle) from a narrow active channel. The grating reflects this diverging beam back through the ridge structure to produce stable laser output around 967 nm with no mode hopping. The curved configuration provides a higher reflectivity than flat gratings demonstrated in previous devices.
In more-conventional devices, the DBR lasers are regrown over a grating. This regrowth process can be difficult when the compounds involved contain aluminum. The surface grating approach requires a single growth step, eliminating this complication. In addition, the design and fabrication techniques are compatible with constructing various different types of monolithic integrated circuits.
Device fabrication
The epitaxially grown devices consist of a single-quantum-well gradient-index separate confinement heterostructure (SQW-GRIN-SCH) sandwiched be tween cladding layers and topped by a contact layer. The structure contains a 6-nm-thick indium gallium arsenide quantum-well active region in a gradient-index layer; 0.1 µm above this lies a 10-nm-thick, high-index p-type gallium arsenide film that acts as an etch stop layer.
The p-type electrode material was applied to the upper cladding layer in stripes 2.5 µm wide by 600 µm long. Reactive ion etching (RIE) with hydrogen/methane gas removed the contact and cladding layers outside of this region to produce the narrow ridge structure that ensures lateral single-mode operation.
The researchers fabricated the gratings in a two-step RIE process. They first applied a protective layer of silicon oxide (SiO2) followed by a layer of photo resist. Curved DBR gratings with 100-µm interaction lengths and 440-nm periods were written into the resist by electron beam. Reactive ion etching with carbon tetrafluoride and hydrogen transferred the pattern into the SiO2, then a subsequent hydrogen/methane RIE process transferred the pattern through the high-index layer and cladding layer and 150 nm into the graded region.
An SiO2 insulating layer was deposited, and p-type contacts were applied over contact windows. The researchers removed the SiO2 from the grating to avoid degradation of reflectivity, then thinned the wafer to 150 µm, applied n-type contacts to the back side, and cleaved the wafer. For comparison, on the same wafer the researchers fabricated grating-free Fabry-Perot lasers with 2.5-µm-wide ridges.
Stable performance
The devices produced stable, single-mode output at 967.5 nm for a threshold current of 14.5 mA; slope efficiency was 0.27 W/A. Higher currents produced a peak power of 32.5 mW at 971.5 nm. Mode hopping occurred only when injection currents rose above 50 mA. In contrast, the Fabry-Perot devices exhibited mode hopping and multimode lasing under pulse operation; threshold current was 8.5 mA, with a slope efficiency of 0.31 W/A.