M. Winterfeldt

Experimental investigation of factors limiting slow axis beam quality in 9xx nm high power broad area diode lasers

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 ...

Assessing the influence of the vertical epitaxial layer design on the lateral beam quality of high-power broad area diode lasers

GaAs-based high-power broad-area diode lasers deliver optical output powers Popt > 10W with efficiency > 60%. However, their application is limited due to poor in-plane beam parameter product BPPlat=0.25×Θ95%×w95% (Θ95% and w95% are emission angle and aperture, 95% power content). We present experimental investigations on λ = 9xx nm broad area lasers that aim to identify regulating factors of the BPPlat connected to the epitaxial layer design. First, we assess the thermal lens of vertical designs with varying asymmetry, using thermal camera images to determine its strength. Under study are an extreme-double-asymmetric (EDAS) vertical structure and a reference (i.e. more symmetric) design. The lateral thermal profiles clearly show that BPPlat increase is correlated to the bowing of the thermal lens. The latter is derived out of a quadratic temperature fit in the active region beneath the current injection of the laser device and depends on the details of the epitaxial layers. Second, we test the benefit of low modal gain factor Γg0, predicted to improve BPPlat via a suppression of filamentation. EDAS-based lasers with single quantum well (SQW) and double quantum well (DQW) active regions were compared, with 2.5x reduced Γg0, for 2.2x reduced filament gain. However, no difference is seen in measured BPPlat, giving evidence that filamentary processes are no longer a limit. In contrast, devices with lower Γg0 demonstrate an up to twofold reduced near field modulation depth, potentially enabling higher facet loads and increased device facet reliability, when operated near to the COD limit.

Development of high-power diode lasers with beam parameter product below 2 mm×mrad within the BRIDLE project

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 × ΘL 95%× W95% (ΘL95% and W95% are emission angle and aperture at 95% power content), with BPP > 3 mm×mrad for W95%~90μm. 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 W95% ~ 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.

Novel approaches to increasing the brightness of broad area lasers

Progress in studies to increase the lateral brightness Blat of broad area lasers is reviewed. Blat=Pout/BPPlat is maximized by developing designs and technology for lowest lateral beam parameter product, BPPlat, at highest optical output power Pout. This can be achieved by limiting the number of guided lateral modes and by improving the beam quality of low-order lateral modes. Important effects to address include process and packaging induced wave-guiding, lateral carrier accumulation and the thermal lens profile. A careful selection of vertical design is also shown to be important, as are advanced techniques to filter out higher order modes.

Coherent combining of high brightness tapered lasers in Master Oscillator Power Amplifier configuration

Improved diode laser beam combining techniques are in strong demand for applications in material processing. Coherent beam combining (CBC) is the only combining approach that has the potential to maintain or even improve all laser properties, and thus has high potential for future systems. As part of our ongoing studies into CBC of diode lasers, we present recent progress in the coherent superposition of high-power single-pass tapered laser amplifiers. The amplifiers are seeded by a DFB laser at λ = 976 nm, where the seed is injected into a laterally single-mode ridge-waveguide input section. The phase pistons on each beam are actively controlled by varying the current in the ridge section of each amplifier, using a sequential hill-climbing algorithm, resulting in a combined beam with power fluctuations of below 1%. The currents into the tapered sections of the amplifiers are separately controlled, and remain constant. In contrast to our previous studies, we favour a limited number of individual high-power amplifiers, in order to preserve a high extracted power per emitter in a simple, low-loss coupling arrangement. Specifically, a multi-arm interferometer architecture with only three devices is used, constructed using 6 mm-long tapered amplifiers, mounted junction up on C-mounts, to allow separate contact to single mode and amplifier sections. A maximum coherently combined power of 12.9 W is demonstrated in a nearly diffraction-limited beam, corresponding to a 65% combining efficiency, with power mainly limited by the intrinsic beam quality of the amplifiers. Further increased combined power is currently sought.

Chip-carrier thermal barrier and its impact on lateral thermal lens profile and beam parameter product in high power broad area lasers

High power broad area diode lasers with high optical power density in a small focus spot are in strong commercial demand. For this purpose, the beam quality, quantified via the beam parameter product (BPP), has to be improved. Previous studies have shown that the BPP is strongly affected by current-induced heating and the associated thermal lens formed within the laser stripe. However, the chip structure and module-assembly related factors that regulate the size and the shape of the thermal lens are not well known. An experimental infrared thermographic technique is used to quantify the thermal lens profile in diode lasers operating at an emission wavelength of 910 nm, and the results are compared with finite element method simulations. The analysis indicates that the measured thermal profiles can best be explained when a thermal barrier is introduced between the chip and the carrier, which is shown to have a substantial impact on the BPP and the thermal resistance. Comparable results are observed in furt...

Assessment of factors regulating the thermal lens profile and lateral brightness in high power diode lasers

The lateral beam parameter product, BPPlat, and resulting lateral brightness of GaAs-based high-power broad-area diode lasers is strongly influenced by the thermal lens profile. We present latest progress in efforts using FEM simulation to interpret how variation in chip construction influences the thermal lens profile, itself determined experimentally using thermography (thermal camera). Important factors are shown to include the vertical (epitaxial) structure, the properties of the submount and the transition between chip and submount, whose behavior is shown to be consistent with the presence of a significant thermal barrier.