GaAs-based broad-area diode lasers are needed with improved lateral beam parameter product (BPPlat) at high power. An experimental study of the factors limiting BPPlat is therefore presented, using extreme double-asymmetric (EDAS) vertical structures emitting at 910 nm. Continuous wave, pulsed and polarization-resolved measurements are presented and compared to thermal simulation. The importance of thermal and packaging-induced effects is determined by comparing junction -up and -down devices. Process factors are clarified by comparing diodes with and without index-guiding trenches. We show that in all cases studied, BPPlat is limited by a non-thermal BPP ground-level and a thermal BPP, which depends linearly on self-heating. Measurements as a function of pulse width confirm that self-heating rather than bias-level dominates. Diodes without trenches show low BPP ground-level, and a thermal BPP which depends strongly on mounting, due to changes in the temperature profile. The additional lateral guiding in diodes with trenches strongly increases the BPP ground-level, but optically isolates the stripe from the device edges, suppressing the influence of the thermal profile, leading to a BPP-slope that is low and independent of mounting. Trenches are also shown to initiate strain fields that cause parasitic TM-polarized emission with large BPPlat, whose influence on total BPPlat remains small, provided the overall polarization purity is >95%.
High power 9xx-nm broad-area lasers with improved beam quality are required for many applications, but the physical limitations remain unclear, especially the relative importance of free-carrier and self-heating effects. Experimental data are, therefore, presented on a series of diagnostic lasers where the lateral carrier profile at the edges of the electrical contacts has been modified via implantation in order to assess its influence on beam quality. We show that carrier accumulation at the edges of the (90-µm wide) contacts can be eliminated and that as a consequence, near and far field are narrowed and the rate of increase of beam parameter product (BPP) with self-heating reduces by 35%. Overall, the suppression of lateral carrier accumulation allows BPP < 2 mm × mrad to be maintained to 7-W optical output, corresponding to a peak linear brightness of 3.5 W/mm × mrad, comparable with the highest reported values for 90-µm stripe devices.Index Terms-Broad area laser, deep implantation, lateral carrier profile, proton bombardment, slow axis beam quality.
Broad area lasers emitting near 940 nm are fabricated using a process based on two-step epitaxy. The n-side of the layer structure and the active layer are grown during the first epitaxial step, the p-side during the second. Between the first and the second step a shallow etching is used to remove the active layer from the two sides and at the two facets. This simple approach allows the creation of buried mesa lasers with non-absorbing mirrors, resulting in a reduced lateral current leakage, lower threshold current and higher efficiency, plus an increased robustness with respect to catastrophic optical damage.
High power broad-area diode lasers are the most efficient source of optical energy, but cannot directly address many applications due to their high lateral beam parameter product BPP = 0.25 × Θ L 95% × W 95% (Θ L 95% and W 95% are emission angle and aperture at 95% power content), with BPP > 3 mm×mrad for W 95%~9 0µm. We review here progress within the BRIDLE project, that is developing diode lasers with BPP < 2 mm×mrad for use in direct metal cutting systems, where the highest efficiencies and powers are required. Two device concepts are compared: narrow-stripe broad-area (NBA) and tapered lasers (TPL), both with monolithically integrated gratings. NBAs use W 95% ~ 30 µm to cut-off higher order lateral modes and reduce BPP. TPLs monolithically combine a single mode region at the rear facet with a tapered amplifier, restricting the device to one lateral mode for lowest BPP. TPLs fabricated using ELoD (Extremely Low Divergence) epitaxial designs are shown to operate with BPP below 2mm×mrad, but at cost of low efficiency (<35%, due to high threshold current). In contrast, NBAs operate with BPP < 2 mm×mrad, but maintain efficiency >50% to output of > 7 W, so are currently the preferred design. In studies to further reduce BPP, lateral resonant anti-guiding structures have also been assessed. Optimized anti-guiding designs are shown to reduce BPP by 1 mm×mrad in conventional 90 µm stripe BA-lasers, without power penalty. In contrast, no BPP improvement is observed in NBA lasers, even though their spectrum indicates they are restricted to single mode operation. Mode filtering alone is therefore not sufficient, and further measures will be needed for reduced BPP.
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