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

Abstract

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.

High Beam Quality in Broad Area Lasers via Suppression of Lateral Carrier Accumulation

“…However, an alternative explanation presents itself. In recent work, resonant anti-guiding structures have been shown to reduce BPP 0 without other negative effects, attributed to a reduction in the number of lateral modes supported for a given thermal profile [8]. However, even when the number of modes is reduced to 1, BPP 0 does not reduce to the diffraction limit [8].…”

“…In recent work, resonant anti-guiding structures have been shown to reduce BPP 0 without other negative effects, attributed to a reduction in the number of lateral modes supported for a given thermal profile [8]. However, even when the number of modes is reduced to 1, BPP 0 does not reduce to the diffraction limit [8]. Therefore, we propose instead that the offset in BPP 0 from the diffraction limit is a measure of both number of guided modes and of non-ideality in the system, such as a complex form of the thermal lens or material non-uniformity.…”

Recent progress in brightness scaling by coherent beam combining of tapered amplifiers for efficient high power frequency doubling

“…A low vertical divergence is beneficial to the use in a CBC setup, where tolerances in the alignment of the collimation lenses impact the achievable combining efficiency 14 . Furthermore, previous studies have shown that this amplifier design can also deliver high beam quality in slow axis with an improved power content in the central lobe 15,16 . A drawback is, however, its reduced electrical to optical efficiency 16 .…”

“…Furthermore, previous studies have shown that this amplifier design can also deliver high beam quality in slow axis with an improved power content in the central lobe 15,16 . A drawback is, however, its reduced electrical to optical efficiency 16 . We used 5 mm long devices (1 mm ridge + 4 mm taper) mounted p-side up on CuW heat spreaders and C-mounts, with separate electrical contacts in order to control the injection current of ridge (Irw) and tapered section (Itp) independently.…”

PROSPECTS OF LASER DIODES APPLICATIONS TO INERTIAL CONFINEMENT FUSION (Review)

“…Важный научный и технологический вклад в разработку систем спектрального суммирования был внесён в ходе выполнения исследований по Европейскому проекту BRIDLE (bright diode laser), в котором приняли участие практически все ведущие европейские промышленные компании и университеты [42]. Были созданы монолитные ЛЛД, каждый кластер которых имеет разную длину волны излучения с заданным периодом по ширине излучающей апертуры и представляет собой тандем в виде одномодового задающего генератора и усилителя (MOPA -Master oscillator -Power amplifier).…”