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.

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.

GaAs-based diode lasers that deliver high powers at high efficiencies within a narrow, stable spectral window are in strong demand for applications including solid state laser pumping and direct material processing. Wavelength stabilization can be integrated in the diode laser using surface patterning or etch and regrowth techniques. We review recent performance, focusing on diode lasers with distributed feedback with lasing wavelength of 975 nm. We show that DFB lasers operate with conversion efficiency that is 4…8 %-points (1.07x to 1.17×) lower than reference devices, review performance limits and note potential approaches to address these limits.

Monolithic spectral stabilization is demonstrated in narrow-stripe broad-area lasers (NBA) with high power (5W), conversion efficiency (50%) and high brightness, by using optimized high-order surface-etched DFB gratings. However, surface etched gratings introduce a high index contrast into the semiconductor, leading to the scattering losses increasing rapidly with groove etch depth, limiting efficiency and yield. We therefore review progress in the exploitation of novel, non-uniform grating configurations for improved performance. Devices with non-uniform gratings whose groove etch depth decreases toward the front facet (apodized grating) are shown to operate with enhanced spectrally stable power (6W) compared to devices with uniform gratings.

We describe the coherent beam combining of three high-power tapered laser amplifiers seeded by a DFB laser at λ = 976 nm, and demonstrate a combined power of 12.9 W in a close to diffraction-limited beam at a combining efficiency of > 65%. We use individual devices on C-mounts in a multi-arm interferometer. The phase pistons of each beam are matched by automatically controlling the ridge current of the amplifiers. Increased power per element was achieved by making use of a simplified, efficiently cooled single emitter-based optical system.

Coherent beam combining (CBC) aims at increasing the spatial brightness of lasers. It consists in maintaining a constant phase relationship between different emitters, in order to combine them constructively in one single beam. We have investigated the CBC of an array of five individually-addressable high-power tapered laser diodes at λ = 976 nm, in two architectures: the first one utilizes the self-organization of the lasers in an interferometric extended-cavity, which ensures their mutual coherence; the second one relies on the injection of the emitters by a single-frequency laser diode. In both cases, the coherent combining of the phase-locked beams is ensured on the front side of the array by a transmission diffractive grating with 98% efficiency. The passive phase-locking of the laser bar is obtained up to 5 A (per emitter). An optimization algorithm is implemented to find the proper currents in the five ridge sections that ensured the maximum combined power on the front side. Under these conditions we achieve a maximum combined power of 7.5 W. In the active MOPA configuration, we can increase the currents in the tapered sections up to 6 A and get a combined power of 11.5 W, corresponding to a combining efficiency of 76%. It is limited by the beam quality of the tapered emitters and by fast phase fluctuations between emitters. Still, these results confirm the potential of CBC approaches with tapered lasers to provide a high-power and high-brightness beam, and compare with the current state-of-the-art with laser diodes.

We present results of GaAs-based distributed-feedback (DFB) broad-area (BA) lasers with integrated surface-etched gratings. Lasers with longitudinally varying (apodized) Bragg gratings are compared to lasers with uniform gratings. DFB-BA lasers with apodized gratings show improved conversion efficiency and output power compared to those with uniform gratings. DFB-BA lasers with narrow contact stripe width W = 30 μm and resonator length L = 6 mm show a peak conversion efficiency of 60% and maintain efficiency >50% to an output power of >6 W. The spectral FWHM linewidth is below 0.5 nm. Furthermore DFB lasers with implemented apodized gratings are less sensitive to fabrication tolerances.

We describe a new coherent beam combining architecture based on passive phase locking of emitters in an extended cavity on the rear facet and their coherent combination on the front facet. This rear-side technique provides strong optical feedback for phase locking while maintaining a high electrical-to-optical efficiency. Two high-brightness high-power tapered laser diodes are coherently combined using a Michelson-based cavity. The combining efficiency is above 82% and results in an output power of 6.7 W in a nearly diffraction-limited beam with an M4σ2≤1.2. A semi-active automatic adjustment of the current enhances the long-term stability of the combination, while the short-term stability is passively ensured by the extended cavity. This new laser configuration exhibits the simplicity of passive self-organizing architectures while providing a power conversion efficiency of 27% that is comparable to master oscillator power amplifier architectures.

We demonstrate a rear-side phase-locking architecture with two high-brightness diode lasers. This technique is based on the passive phase-locking of emitters in an external cavity on their rear facet, and their coherent combination on the front facet. Two high-brightness high-power tapered laser diodes are coherently combined using a Michelson-based cavity. The combining efficiency is above 80% and results in an output power of 6.7 W in a nearly diffraction-limited beam. The rear-side architecture is then used with a laser bar of 5 tapered emitters using an interferometric extended cavity, based on a diffractive optical element. We describe the experimental evaluation of the diffractive optical element, and the phase-locked operation of the laser bar.

Narrow-stripe (30 μm aperture) broad area lasers with monolithically integrated DFB-surface-gratings deliver 5 W optical output per emitter with 50% conversion efficiency, spectral width ≤ 1 nm from 970…980 nm, and beam parameter product < 2 mm×mrad.

Progress in studies to increase the lateral brightness B-lat of broad area lasers is reviewed. B-lat=P-out/BPPlat is maximized by developing designs and technology for lowest lateral beam parameter product, BPPlat, at highest optical output power P-out. 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

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.

We demonstrate a rear-side phase-locking architecture with two high-brightness diode lasers. This technique is based on the passive phase-locking of emitters in an external cavity on their rear facet, and their coherent combination on the front facet. Two high-brightness high-power tapered laser diodes are coherently combined using a Michelson-based cavity. The combining efficiency is above 80% and results in an output power of 6.5 W in a nearly diffraction-limited beam.

We describe a new coherent beam combining architecture based on passive phase-locking of two laser diodes in a Michelson external cavity on their rear facet, and their coherent combination on the front facet. As a proof-of-principle, two ridge lasers have been coherently combined with >90 % efficiency. The phase-locking range, and the resistance of the external cavity to perturbations have been thoroughly investigated. The combined power has been stabilized over more than 15 min with an optical feedback as well as with an automatic adjustment of the driving currents. Furthermore, two high-brightness high-power tapered laser diodes have been coherently combined in a similar arrangement; the combining efficiency is 70% and results in an output power of 4 W. We believe that this new configuration combines the simplicity of passive self-organizing architectures with the optical efficiency of master-oscillator power-amplifier ones.

Low-fill-factor laser bars composed of five narrow-stripe broad-area lasers (30 μm × 6000 μm) with monolithic spectral stabilization are presented. Each laser on the bar emits at a unique wavelength from 970 to 980 nm with a channel spacing of 2.5 nm. Narrow spectral line width c1 nm per emitter is obtained by implementing 40th Bragg order distributed feedback surface gratings with varied grating periods. The design and fabrication of these laser bars is reviewed, with optimal grating etch depth determined using simulations that combine 2D-simulation of the reflectivity and optical losses of a 40th-order surface grating with coupled mode theory. The selected etch depth delivers a grating coupling coefficient of κL ~ 0.2, which is shown to be sufficient to suppress lasing in Fabry-Perot modes for narrow line operation across all wavelength channels. However, additional optical scattering losses of the gratings are found to limit output power and efficiency. The final realized minibars deliver 25-W continuous wave output power with a conversion efficiency >40% and operate with a beam parameter product of ~1.6 mm mrad per single emitter, twofold improved in comparison with typical 90-μm wide broad area laser. Such laser minibars are an attractive source for brilliant dense spectral beam combining.

We propose a newCBC architecture for the passive phase-locking of a diode laser array that is designed to ensure a stable high-power operation and a high electrical-to-optical efficiency. It uses a common laser external cavity on the back side of the emitters for phase locking, while coherent beam superposition of the phase-locked beams is realized on the front side.

A review is presented on how diagnostic studies, technological and design improvements and novel device configurations have enabled a more than fourfold improvement in lateral brightness in high power diode lasers, which demonstrate > 4W/mm-mrad.

We present a design study of high power narrow stripe broad area diode lasers at 9xx-nm with contact stripe widths of 20 μm, 30 μm and 50 μm. The devices are deeply implanted with helium (He + ) at the edges of the electrical contact, to reduce lateral current spreading and lateral carrier accumulation. All devices operate with a lateral beam parameter product (BPP) below 2 mm x mrad, but differ strongly in linear brightness and maximal output power. The linear brightness can reach up to 5.6 W/mm x mrad from a 20 μm wide stripe at an optical output power of 4 W. However, at higher output power the beam quality degrades strongly for 20 μm wide stripes, making stripe width of 30 μm or 50 μm more beneficial for high brightness at P ⩾ 5 W.

Laser systems based on spectral beam combining (SBC) of broad-area (BA) diode lasers are promising tools for material processing applications. However, the system brightness is limited by the in-plane beam param- eter product, BPP, of the BA lasers, which operate with a BPP of < 3mm-mrad. The EU project BRIDLE (www.bridle.eu) is developing novel diode laser sources for such systems, and several technological advances are sought. For increased system brightness and optimal ber-coupling the diode lasers should operate with reduced BPP and vertical far eld angle (95% power content), μV 95. The resulting diode lasers are fabricated as mini- bars for reduced assembly costs. Gratings are integrated into the mini-bar, with each laser stripe emitting at a different wavelength. In this way, each emitter can be directed into a single bre via low-cost dielectric filters. Distributed-feedback narrow-stripe broad-area (DFB-NBA) lasers are promising candidates for these SBC sys- tems. We review here the design process and performance achieved, showing that DFB-NBA lasers with stripe width, W = 30 μm, successfully cut of higher-order lateral modes, improving BPP. Uniform, surface-etched, 80th-order Bragg gratings are used, with weak gratings essential for high e ciency. To date, such DFB-NBA sources operate with < 50% effciency at output power, Pout < 6 W, with BPP < 1.8 mm-mrad and offV 95 36 . The emission wavelength is about 970 nm and the spectral width is < 0.7 nm (95% power). The BPP is half that of a DFB-BA lasers with W = 90 um. We conclude with a review of options for further performance improvements.