Increased diffraction efficiencies of DBR gratings in diode lasers with adiabatic ridge waveguides

Abstract

The influence of the lateral layout on the diffraction efficiency of gratings in DBR lasers is presented. In this experimental study DBR ridge waveguide (RW) lasers with different ridge widths as well as straight and adiabatic waveguides are compared. The lasers are based on a vertical layer structure with an asymmetric super large optical cavity. DBR gratings of 3rd and 7th order are manufactured using electron beam lithography and dry etching. Their diffraction efficiencies are determined by measuring the optical output power emitted through the rear and front facets of unmounted devices. In comparison to a laser with a 30 μm ridge, the DBR diffraction efficiency in a laser with a 4 μm ridge is reduced by 46 percentage points. Implementing an adiabatic widening of the ridge width increases the diffraction efficiency from 35% to 72%. The latter is close to 81% achieved for the laser with 30 μm ridge. The new layout with enhanced DBR diffraction efficiency increases the optical output power of the narrow RW laser by a factor of 1.3. Similar results are obtained with 7th order gratings. All devices provide single-mode emission with spectral widths of 30 pm and side mode suppressions >60 dΒ. According to these results, implementing adiabatic waveguides is beneficial in terms of diffraction efficiency and performance of narrow RW lasers based on the applied vertical structure.