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Beam divergence and COD issues in double barrier separate confinement heterostructure laser diodes

Maląg A.

Bulletin of the Polish Academy of Sciences. Technical Sciences

2005

Vol. 53, nr 2

167-173

The double barrier separate confinement heterostructure (DBSCH) design aimed at reduction of vertical beam divergence and increase of catastrophic optical damage (COD) level for high power laser diodes (LDs) operation is presented. Insertion of thin, wide-gap barrier layers at the interfaces between waveguide and cladding layers of SCH gives an additional degree of freedom in design making possible more precise shaping of the optical field distribution in the laser cavity. By comparison with the large optical cavity (LOC) heterostructure design it has been shown that the low beam divergence emission, of DBSCH LDs can be attributed to the soft-profiled field distribution inside the cavity. This 'soft mode profile' seems to determine narrow laser beam emission rather than the field distribution width itself. The potential problem with the soft-profiled but relatively narrow (at half-maximum) mode distribution is a lower COD level. Widening of the mode profile by the heterostructure design corrections can increase it, but care must be taken to avoid excessive decrease of confinement factor (gamma). As a result it is shown that DBSCH design is possible, where the low be divergence and high COD level is achieved simultaneously. Wide stripe gain-guided LDs based on GaAsP / AIGaAs DBSCH SQW structures have been manufactured according to the design above. Gaussian-shaped narrow directional characteristics are in relatively good agreement with modelling predictions. Vertical beam divergences are 13-15° and 17-18° FWHM for design versions experimentally investigated. Threshold current densities of the order of 350-270 Acm-2 and slope efficiencies of 0.95 and 1.15 W / A have been recorded for these two versions, respectively. Optical power at the level of 1 W has been achieved. The version with lower beam divergence proves to be more durable. Higher optical power levels are to be obtained after heterostructure doping optimisation.