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Experimental studies into the beam parameter product ( BPP ) of 940–980 nm GaAs-based high-power diode lasers are presented. Such lasers exhibit broadening far field and narrowing near field with increasing bias, with BPP increasing tenfold over the diffraction limit. First, spectrally-resolved beam profile measurements of lasers with monolithically-integrated gratings are presented, showing that a reproducible series of spatially-extended optical modes makes up the optical field. Then, changes to the device construction are presented, enabling effects limiting BPP to be inferred and addressed. Process- and package-induced effects can be minimized by design, while the effects of carrier, gain and temperature profiles dominate. Self-heating within the laser stripe raises the refractive index, forming a thermal lens, and the variation in curvature of this lens with bias and device construction directly affects BPP . Temperature non-uniformity along the resonator is also shown to strongly degrade BPP . Moreover, current spreading and the resulting lateral carrier accumulation (LCA) amplify high-order, high- BPP modes, thus degrading BPP for any given thermal lens. This LCA-induced degradation is shown to be suppressed by regrown lateral current-blocking structures. Finally, a flatter thermal lens and lower BPP can be achieved using thermal engineering, via changes to the epitaxial design or device layout.

Laser diode benches (LDBs) with lasing wavelength of 808 nm were developed, manufactured and endurance-tested for utilization as pump lasers for the Nd:YAG oscillator in the climate satellite MERLIN. The LDBs combine customized TE-polarized GaAs-based 5-mm minibars with fast axis collimation lenses to produce a reliable, space qualified component at optical power of 63 W per minibar. Accelerated life tests of 20 LDBs are presented. Except for a small increase in threshold current, no wear out in operating current or sudden failures were observed, and reliable operation over the full mission time of >4 gigashots was confirmed. The electro-optical characteristics remain in the specified ranges after exposure to the total operational load of the mission.

The importance of private lands conservation (PLC) for endangered species continues to grow as habitat is increasingly lost or fragmented. In Texas, nearly 98% of all property is privately owned, r...

In this letter we present new results on high power 970 nm ridge waveguide lasers based on an extreme double asymmetric epitaxial design. Due to an improved lateral-longitudinal design an M2-value below 1.1 and efficiency over 60% could be maintained at power levels slightly above 2 W. The lasers consist of a narrow mode-filter section at the back facet, a relatively broad section at the front facet for high power capability, and a tapered section in between with a very small taper-angle. We achieved the highest reported conversion efficiency for 1 W single lateral mode power to date of 67%, as well as the highest lateral (linear) brightness of 7.5 W/mm $\bullet$ mrad with 55% efficiency at 2.5 W. Results of first life time tests are presented which show the potential for commercial use.

The ascomycete fungus Ophidiomyces ophiodiicola (Oo) is the causative agent of ophidiomycosis (Snake Fungal Disease), which has been detected globally. However, surveillance efforts in the central U.S., specifically Texas, have been minimal. The threatened and rare Brazos water snake (Nerodia harteri harteri) is one of the most range restricted snakes in the U.S. and is sympatric with two wide-ranging congeners, N. erythrogaster transversa and N. rhombifer, in north central Texas; thus, providing an opportunity to test comparative host-pathogen dynamics in this system. To accomplish this, we surveyed a portion of the Brazos river drainage (~400 river km) over 29 months and tested 150 Nerodia spp. for the presence of Oo via quantitative PCR and recorded any potential signs of Oo infection. We found Oo was distributed across the entire range of N. h. harteri, Oo prevalence was 46 % overall, and there was a significant association between Oo occurrence and signs of infection in our sample. Models indicated adults had a higher probability of Oo infection than juveniles and subadults, and adult N. h. harteri had a higher probability of infection than adult N. rhombifer but not higher than adult N. e. transversa. High Oo prevalence estimates (94.4%) in adult N. h. harteri has implications for their conservation and management owing to their patchy distribution, comparatively low genetic diversity, and threats from anthropogenic habitat modification.

The phase noise caused by current fluctuations in single-pass semiconductor optical amplifiers is experimentally investigated. Using low-index quantum barrier designs in prototype devices is found to reduce phase noise by 18% compared to reference designs.

Epitaxial design development in 780 nm diode lasers for lower loss and lower bias-driven leakage enables increases in power (to 210 W) and conversion efficiency (to 57% at 60 W) in high repetition-rate (10 ms, 10 Hz), wide-aperture (1200 μm) devices ready for use in pumping high-energy-class thulium-doped solid-state lasers.

A root-cause analysis of thermal power saturation in broad area diode lasers is presented. Thermal power saturation limits in largest parts the optical power in continuous wave (CW) driven diode lasers, as junction temperature increases with bias. Accordingly, gain decreases and, hence, the carrier density in the quantum well increases (nonpinning) to compensate the low gain and the additional photon loss. A systematic variation of epitaxial designs was used to experimentally clarify the impact of modal gain on the temperature dependence of differential internal efficiency and optical loss. To account for this effect in simulation, a rate equation model was applied to determine temperature dependent slope efficiency. Longitudinal spatial hole burning (LSHB) causes an asymmetry of the carrier density profile, leading to high nonlinear carrier loss at the back facet, but also the average carrier density increases with temperature, enhancing the impact of LSHB. This enhancement effect leads to rapid degradation of internal differential efficiency with temperature and, hence, early power saturation. The extreme triple asymmetric vertical design is introduced, that enables devices to be realized that show minimized degradation of internal differential efficiency with temperature and therefore increased performance in CW operation.

Dense wavelength beam combining (DWBC) techniques can be used to image many diode lasers into a single beam with minimal degradation of beam quality M 2 95% (at 95% power) and conversion efficiency η E . Systems based on DWBC of broad area lasers can be scaled into the many-kW class for use in material processing applications [1] . Laterally single mode diode lasers with wavelength around λ = 970 nm that use advanced waveguide designs (with lateral mode filter and extremely asymmetric vertical design) offer a path to much higher brightness [2] . Such lasers achieve high power P opt and efficiency η E in continuous wave CW ( P opt = 1 W, η E = 65%, M 2 95% < 1.1) [3] and pulsed ( P opt = 18W, 3.3 ns 10 kHz, M 2 95% < 2.2) [4] operation, whilst remaining close to the diffraction limit. When external λ-stabilization is performed via a diffractive element (grating), the operating characteristics are strongly affected by the interaction between external optical configuration and internal diode laser design, seen in P opt , η E , spectral width Δ λ 95% (at 95% power), λ -tuning range and beam stability. The λ -tuning range and Δ λ 95% for each diode laser are especially important as this limits the number of emitters that can be combined and thus the maximum achievable power via DWBC.

Diode lasers are key sources of optical energy for industrial machine tools and continuous improvement in their performance is needed to support performance and cost scaling efforts. For example, disk lasers as used in laser tools for metal cutting are typically pumped in continuous wave (CW) mode by 1-cm wide diode laser bars with an operating wavelength around λ = 940 nm, and ongoing increases in optical power per bar P opt are needed. Specifically, power scaling is needed above the current CW commercial level of around P opt = 300 W per bar [1] . Bars with performance at P opt = 1 kW and above have been demonstrated in research studies for many years (e.g. [2] ), with most work focussing on bar optimization for pulsed operation (ns-ms) as recently reviewed in [3] .

We present progress in high power GaAs-based single-pass semiconductor tapered optical amplifiers and modules tailored for coherent beam combining (CBC) in master-oscillator power-amplifier configuration. Amplifier design is first studied, by varying device geometry and epitaxial structure in 976nm devices. Epitaxial structures with large vertical near field and low wave-guiding from the active region enable higher CBC efficiency. However, changes to in-plane geometry did not improve performance. Overall, CBC of tapered amplifiers is stable, reproducible and robust, motivating next the development of 1064nm CBC-ready stand-alone sub-modules. Design, construction and test results from the pilot-series fabrication of these amplifier modules are presented.

We present 940nm GaAs-based high-power broad-area diode lasers that use an enhanced self-aligned lateral structure "eSAS", implemented within an extreme-triple-asymmetric vertical structure with a thin p-side. In this structure, two-step epitaxial growth with intermediate selective etching is used to introduce current-blocking structures consisting of n-doped GaAs and InGaP layers outside the laser stripe, whose location, thicknesses and doping concentrations are precisely defined. These blocking structures confine current to the device center, thus reducing carrier losses in the edges and limiting the detrimental effects of lateral current spreading and carrier accumulation on beam quality, without compromising conversion efficiency, output power or polarization purity. We present results of eSAS single-emitters as well as bars with multiple emitters, in comparison to gain-guided reference devices. In addition, we demonstrate optimized blocking structures with improved current blocking, which are crucial for the realization of the eSAS structure.

GaAs based high power broad area lasers are the most efficient source of optical energy and are used in many industrial applications. Despite considerable improvement in power and efficiency in recent years, further improvement is needed due to the high demand from industry. We review here progress in vertical epitaxial layer design, showing how higher performance is enabled by migrating from asymmetric large optical cavity (ASLOC) designs to the newly developed extreme-triple-asymmetric (ETAS) vertical structure. Building on earlier studies at 940 nm, we focus on gain-guided lasers that have operating wavelength 970 nm, have 90 μm stripe width and 4 mm resonator length. We can emphasize the positive impact of epitaxial layer design, without need for advanced lateral structures. We show how design improvement increases conversion efficiency ηE at 12 W output power from 56% to 66%, whilst peak (saturation) power increases from Popt = 14 to 19 W in continuous wave (CW) mode for p-down single emitters on CuW carriers (thermal resistance 3 K/W). Progress in epitaxial design also leads to smaller lateral beam parameter product (BP Plat) at higher bias, leading to lateral brightness Popt/BPPlat < 3 W/mm × mrad. Specifically, in these most recent ETAS structures, by design BPPlat increases more slowly with self-heating, and this leads directly to lower BPPlat at high bias. We will also review options for further increased performance, include efforts to understand and improve BPPlat, which is also limited by a non-thermal ground level BPP0 (here ∼ 1 mm × mrad).

Laser diode minibars for QCW pumping Nd:YAG at 63 W were developed for the satellite MERLIN and subjected to accelerated life test. Evaluating 320 single emitters indicates a reliability of 99.99% during the mission lifetime.

European physical journal special topics 229(24), 3675 - 4284 (2020). doi:10.1140/epjst/e2020-000127-8, This report presents the conceptual design of a new European research infrastructure EuPRAXIA. The concept has been established over the last four years in a unique collaboration of 41 laboratories within a Horizon 2020 design study funded by the European Union. EuPRAXIA is the first European project that develops a dedicated particle accelerator research infrastructure based on novel plasma acceleration concepts and laser technology. It focuses on the development of electron accelerators and underlying technologies, their user communities, and the exploitation of existing accelerator infrastructures in Europe. EuPRAXIA has involved, amongst others, the international laser community and industry to build links and bridges with accelerator science — through realising synergies, identifying disruptive ideas, innovating, and fostering knowledge exchange. The Eu-PRAXIA project aims at the construction of an innovative electron accelerator using laser- and electron-beam-driven plasma wakefield acceleration that offers a significant reduction in size and possible savings in cost over current state-of-the-art radiofrequency-based accelerators. The foreseen electron energy range of one to five gigaelectronvolts (GeV) and its performance goals will enable versatile applications in various domains, e.g. as a compact free-electron laser (FEL), compact sources for medical imaging and positron generation, table-top test beams for particle detectors, as well as deeply penetrating X-ray and gamma-ray sources for material testing. EuPRAXIA is designed to be the required stepping stone to possible future plasma-based facilities, such as linear colliders at the high-energy physics (HEP) energy frontier. Consistent with a high-confidence approach, the project includes measures to retire risk by establishing scaled technology demonstrators. This report includes preliminary models for project implementation, cost and schedule that would allow operation of the full Eu-PRAXIA facility within 8—10 years., Published by Springer, Heidelberg

Experiment and simulation of high power diode lasers reveals a longitudinally-varying current density profile, due to spatial hole-burning. Current crowding at the front facet increases with cavity length and bias, and limits the achievable power.

An overview is presented on the developments in semiconductor design and technology needed to reliably and reproducibly realize 1-cm GaAs-based diode laser bars with optical output in the kilowatt class, summarizing studies performed at the Ferdinand-Braun-Institut and elsewhere. In the more than ten years since the first demonstration in 2007, the operating conversion efficiency of kilowatt-class 1-cm laser bars has improved from 35% to over 65% and lateral far field has reduced from < 20° to below 9° (at 95% power) in quasi-continuous-wave testing. Greater than a kilowatt output is also demonstrated in continuous wave testing with ever-increasing efficiencies and brightness. In addition, when tailored for cryogenic operation, peak power increases to at least 2000 W and operating efficiency to above 70%. The highest performance is seen in the 940 nm to 980 nm wavelength range, with kilowatt operation also confirmed in the 780 nm to 900 nm range. We review the diode laser design and technological steps that were needed to enable steady performance progress, including both epitaxial design development (for low optical losses and reduced power saturation) and advances in 1-cm bar and packaging technology (for low stress, low electrical and thermal resistance). We also review status on efforts to increase power in other wavelength ranges, and discuss the path to further performance improvements.

Among the work done to improve laser performance by optimizing the efficiency of vertical laser structures and thermal management, the spectral quality of the emitted light is a key criterion for the usability of the device. Especially the exact adjustment of the emission wavelength, a small linewidth and a preferably small dependence of the wavelength on temperature changes are important for many applications. These requirements are met by implementing surface gratings into the devices. In the first part of the paper the theoretical tools which are used to simulate and optimize grating reflectivity are presented and discussed. 2D and 3D simulation models confirm possible high reflectivities (more than 90 %) of surface Bragg gratings, if the duty cycle is high ( 9 @ λ = 630 nm) are required. Electron beam lithography allows better design flexibility and larger etch depth tolerances, but requires a more complex etch mask preparation. With both techniques gratings up to a depth of 1.7 μm can be etched, which is sufficient to reach a high reflectivity for typical AlGaAs based laser structures. A single grating can be thermally tuned up to 7.5 nm. As an example for high power applications a 6 mm long distributed feedback broad laser with a stripe width of 30 μm as part of a spectral beam combining setup is presented. This device achieved a maximum output power of 6 W and a spectral width of 0.3 nm.

We present 1 kW-emitting diode-laser bars optimized for higher conversion efficiency and smaller far-field angle Θ95% power content), as needed, e.g., for solid-state laser pumping (wavelength λ= 940 nm). First, we review the latest high-efficiency designs, targeting reduced series resistance Rs and less power saturation and then discuss developments for high brightness via tailored chip-internal heat distribution. Recent results include conversion efficiency η of 66% and far-field width Θ 95%= 8.8° at 1 kW (thermal resistance Rth ~ 0.02 K/W), as well as 64% efficiency and 10.8° divergence at Rth ~ 0.05 K/W, equivalent to CW operation with advanced packaging.

Mid-infrared (MIR) solid state lasers based on thulium and holmium-doped crystals are of increasing interest in applications in medicine, material processing and particle physics. Thulium-doped lasers can be efficiently pumped at wavelengths around 780 nm and diode laser pumps with high conversion efficiency and high intensity are sought at this wavelength. Diode lasers integrated in laser stacks suitable for high duty cycle pumping are of particular interest for high energy class applications, especially when realizable without need for the additional cost and reliability hazard of microchannel cooling. However, high efficiency and reliable power is more challenging to realize at 780 nm than around 940…980 nm, due to limitations on the capability of the available semiconductor materials. Progress is therefore presented here in the design, realization and test of 780 nm pump sources suitable for high energy class pump applications, using GaAs-based TM-polarized diode lasers. We show how power per device can be increased from 4 W for conventional single emitters (90…100 μm) up to 60 W at high duty cycle (10%) and long pulse length (10 ms) for high brightness large aperture emitters (with 1200 μm aperture, equivalent to around 500 W per bar), at the cost of reduced operating efficiency (from 60 to 50%). We show progress in integrating these large aperture emitters into novel passively (macro-channel) edge-cooled stacks, that are then suitable for use in pumping high energy class Th:YAG laser systems.

A 780nm diode laser pump source is presented that uses passive side-cooling to enable Kilowatt-class output at >10% duty cycle (10ms, 10Hz). Beam quality is M 2 BPPmm-mrad) and initial reliability test extrapolates over GShot-class lifetime.

Figure 20.1 was not correct in the published article. The original article has been corrected. The published apologizes for the inconvenience.

Estimating abundance is critical in many areas of ecology and conservation biology. Despite this, methodological limitations prevent population monitoring at large spatial scales. Automated acoustic methods, such as recording devices and sound recognition models, can determine occupancy and phenology but have not been utilized to estimate abundance in amphibians. Here we evaluate a method to estimate the number of breeding female Wood Frogs (Lithobates sylvaticus) on the basis of the relationship between egg mass abundance and the acoustic activity of males, using an automated recognizer to count and quantify their vocalizations. We collected egg mass abundance data and recorded acoustic activity at sites in New Brunswick, Canada and the northern United States in 2015 and 2016. Egg mass abundance at a site was best predicted by the maximum daily number of calls on the basis of a model selection approach and the predictive ability of models (root mean square error ± 16–19 egg masses or 59–70%). Th...

We report the first demonstration of 1.2 J-emission into long pulses ( $\ge 2$ ms) from passively cooled laser bars. These efficient high-fill-factor bars were developed for long-pulse pumping of Yb:CaF2-laser media at a power level of 600 W. The spatial brightness inferred (23 MW/cm $^{2}{\times } \text {sr}$ ) exceeds the state of the art. The bars are based on an extreme-double-asymmetric and an extremely-low-vertical-divergence epitaxial design for pumping at 940 and 975 nm, respectively. While the first design is optimized for high power-conversion efficiency $\eta _{\mathrm{ E}} > 60$ % (at 600 W), the latter exhibits low vertical divergence to enable low-loss external spectral stabilization at the zero-phonon line of Yb:CaF2 $(\eta _{\mathrm{ E}} = 50$ % at 600 W). The higher $\eta _{\mathrm{ E}}$ at 940 nm and lower thermal resistance due to its vertical structure improve the lateral farfield, increasing the brightness by nearly 50% over the 975 nm-structure. We varied the lateral layout comprising 186, 400, and 1200 $\mu \text{m}$ -stripes at 70% fill-factor. The smaller stripes revealed the highest brightness, while the degree of polarization increases by 5% using wider stripes.

For many species, the distribution and quantity of habitat is likely to be altered under climate change scenarios. Globally, temperatures are expected to increase, but changes in precipitation will be variable with some locations expected to receive more and other locations to receive less. The amount and timing of precipitation has a strong influence on wetland hydroperiod, which is a critical factor in determining wetland use by species of pond-breeding amphibians. Past research has focused on the direct effects of reductions in precipitation and shorter hydroperiods, such as larvae desiccation, but little attention has been given to the indirect consequences of increased precipitation and longer hydroperiods, such as changes in predation pressure. We used wood frogs (Lithobates sylvaticus), a species that typically breeds in ephemeral (temporary) wetlands, to test the hypothesis that increased precipitation and permanent wetlands would indirectly increase mortality in wood frog embryos through increases in the density of embryo predators, second-year green frog larvae (Lithobates clamitans), limnephilid and phryganeid caddisfly larvae and red-spotted newts (Notophthalmus viridescens). We conducted translocation experiments in 2014 and 2015. We monitored wood frog embryo hatch success and mortality from predation using open (predator access) and closed (predators excluded) cages in 16 wetlands in each year. We observed no difference in embryo hatch success between ephemeral and permanent wetlands, with water temperature, not pH, affecting hatch rates. Wood frog embryos in permanent wetlands with overwintered second-year green frog larvae had higher mortality from predation (0.80, 95% confidence intervals 0.65–0.91) than permanent wetlands (0.42, CI 0.26–0.60) and ephemeral wetlands (0.37, CI 0.31–0.45) that lacked predatory green frog larvae. Embryo mortality from predation increased with increasing green frog larvae density. Regression models showed that ponds with larvae densities >2.5 larvae m−2 resulted in 90% embryo mortality. Caddisfly larvae and red-spotted newt density had no effect, and pond water temperature had a weak negative effect on embryo mortality from predation. Under a wetter and warmer climate, springtime occupancy of predatory green frog larvae is likely to increase, resulting in a reduction in the amount of suitable wood frog breeding habitat. Our research suggests that indirect effects of climate change mediated through biotic interactions may play a critical role in the distribution of ephemeral wetland species.

We used removal sampling (RS) and neutral red dye capture–mark–recapture (CMR) methods to estimate capture probabilities for larval Wood Frogs (Lithobates sylvaticus), Spring Peepers (Pseudacris crucifer), and first-year Green Frogs (Lithobates clamitans) in ponds in southern New Brunswick, Canada. We modeled capture probability as a function of environmental variables and tested whether marks were retained between surveys. We also performed simulations to understand the effect of survey effort, capture probability, and abundance on detection probabilities and the number of surveys needed to be confident of absence. Capture probabilities (P ± SD) were low and variable: Wood Frogs, P = 0.262 ± 0.128; Spring Peepers, P = 0.323 ± 0.241; and first-year Green Frogs, P = 0.159 ± 0.106. With the use of AICc, we determined a model with the proportion of non-Typha emergent vegetation to have a positive effect, whereas pond depth had a negative effort on capture probabilities in first-year Green Frog larva...

This chapter focuses primarily on progress in HPDLS that are grown on GaAs substrates and emit in the operating wavelength range λop = 900... 1000 nm, where the highest performance is reported, and the majority of studies are performed. Our discussion starts with a review of progress in broad area BA-HPDLs that reach the very highest powers and efficiencies, at the cost of degraded beam quality.

We present here a comparison of design approaches for improved continuous wave electro-optical performance in GaAs-based high power broad area diode lasers emitting at 970 nm. Diode lasers with $90 {\mu } \mathrm {m}$ stripe width based on established asymmetric large optical cavity designs achieve high peak conversion efficiency (68% at 5 W for 3 mm resonator length) but efficiency drops rapidly with power. In spite of halved electrical and thermal resistance, devices with longer resonators (6 mm) operate with only marginally improved efficiency and power (efficiency at 12 W increases from 57 to 59%, peak power increases from 13 to 14 W). In contrast, development in epitaxial design enables strong improvement in power and efficiency at high power. Specifically, recent progress in extreme double and triple asymmetric epitaxial designs have enabled devices with $90 {\mu } \mathrm {m}$ stripe width and 4 mm resonator length to improve their efficiency at 12 W output power from 56% to 66%, with their peak (saturation) power rising from 14 to 19 W.

Studies are presented on the anomalously high thermal boundary resistance observed in efficient high power diode lasers between the p-side contact and metal submount. This barrier is responsible for a thermal resistance of around 1 K/W (around 40% of total) and its elimination would significantly improve the beam quality and power in high power devices. First, we show that no barrier is observed on the n-side even up to high CW heat flux levels (to $\sim 2$ kW/cm$^{\mathbf {2}}$), confirming a p-side effect. Second, a 10-fold variation in the thermal conductivity of the submount material (from CuW to CVD diamond) is shown to have minimal impact on the measured thermal barrier, thermal resistance and curvature in the thermal lens, confirming a chip-internal effect. The measured variation in thermal lens was reproduced in simulation only when a boundary resistance was included. These results are further evidence that the thermal boundary resistance is related to material limits on heat transport out of the p-side of the devices.

Coherent beam combining (CBC) of high brightness tapered amplifiers (TPAs) is a promising approach for the development of high power semiconductor laser sources with excellent beam quality and offers a pathway to future direct diode high brightness system [1]. Operation of the amplifiers in quasi continuous wave (QCW) mode allows to overcome the thermal limitations in the amplifiers and to test their full power scaling potential. Additionally, high brightness QCW sources with high spectral purity are in demand as NIR pump sources for nonlinear frequency conversion to address medical applications of modulated high power visible laser sources. In this paper we discuss dynamic effects in pulsed high brightness tapered amplifiers and demonstrate their coherent superposition for brightness scaling. We use state-of the art TPAs with the so called ELOD2 epitaxial design, which allows us to maintain close-to-diffraction limited beam quality at high powers [2,3].

High power and efficiency enable new fields of applications for single spatial mode lasers, such as high brightness direct diode laser systems with dense wavelength multiplexing [1,2]. In previous work, we demonstrated that an efficient extreme asymmetric epitaxial structure enables highly efficient high-power single mode Ridge Waveguide (RW) lasers at 970 nm [3]. We will present results of a further optimized vertical and lateral design with increased single-spatial mode power of up to 2.5 W.

High power, efficient diode-laser bars with narrow far field angles are sought for many applications, for example in the 9xx nm-range for the pumping of Yb:YAG disc and slab lasers [1,2]. In previous work, broad-area (BA) diode-laser bars with 4 mm resonator length operated with high conversion efficiency η = 62% at operating power P op = 1 kW in quasi-continuous wave testing (200 μs, 10 Hz), by using low optical loss and low-resistivity vertical structures and high fill-factors (∼70%) [1]. Lateral far field (95% power) was Θ 95 % > 10° [2]. However, higher η and narrower Θ 95 % are needed for industrial application, and we seek improvements by altering the lateral bar structure for a fixed vertical design (from [1], wavelength λ = 930 nm, loss α i ≤ 0.4 cm−1).

Mid-infra-red solid state lasers based on thulium and holmium-doped crystals are of increasing interest in applications in medicine, material processing and particle physics. Thulium-doped lasers can be pumped at wavelengths around λ op = 780 nm and diode laser pumps with high conversion efficiency η E and high intensity are therefore sought at this wavelength [1]. However, high η E here is more challenging to realize than around λ op = 940…980 nm (for pumping Yb:YAG), due to the available semiconductor materials. High aluminium-content waveguide and cladding layers are needed (to suppress carrier leakage), whose low mobility leads to either high electrical resistance or requires high doping, leading to increased optical loss α i . In addition, the active region is realized using either compressively strained high-aluminum content (> 10%) InAlGaAs (which leads to challenges with facet passivation, limiting reliable power, and is prone to oxidize, limiting optical performance and lifetime) or tensile strained GaAsP (with high transparency current, limiting efficiency, and tendency towards rapid defect growth, limiting reliability). Commercial long resonator (L = 3 mm) GaAsP-based bars at λ op = 780 nm operate with peak η E ∼ 50% [2], significantly less than the η E > 65% achievable for λ op = 940 nm [3]. Although improvements in η E for GaAsP-based diode lasers with λ op = 780 nm are the subject of intense study, an alternative approach is to use longer wavelength devices at reduced operating temperatures, potentially enabling pumping at 780 nm with improved powers and efficiencies [3]. We therefore assess the performance of efficient 808 nm diode lasers at T HS = 200 K, testing their suitability as efficient 780 nm sources.

Over the last decades considerable efforts have been undertaken to increase output power, conversion efficiency and beam quality of GaAs based broad-area diode lasers by optimizing the epitaxial layer design as well as the lateral device structure. In this respect the reduction of current spreading is essential to meet future requirements for high power diode lasers. Lateral current spreading enhances the accumulation of carriers at the edges of the active region defined by the contact stripes which results in additional leakage current and lasing of higher-order lateral modes, reducing efficiency and beam quality. We address this issue by implementing a tailored deep implantation scheme as a current block, implanting O and Si, using two-step epitaxy. This work elucidates the effects of buried current apertures, fabricated by Si and O doping at different doses on the optoelectronic properties of broad area lasers. It will be shown how deep O- and Si-implantation significantly suppresses current spreading, leading to lower threshold currents and higher efficiency.

International audience; High brightness diode laser beam combining techniques are in demand for efficient high power nonlinear conversion. Coherent beam combining (CBC) is the only method that has the potential for brightness scaling by maintaining one single narrow spectral linewidth. CBC in a master oscillator power amplifier (MOPA) configuration using a small number of efficiently cooled tapered amplifiers is a promising approach for efficient brightness scaling in a simple architecture. We present the application of such a source based on CBC of three tapered amplifiers seeded by a DFB laser at λ = 976 nm for second harmonic generation (SHG). A maximum power of 2.1 W at 488 nm was generated by SHG in a MgO:PPLN bulk crystal limited by thermal effects. A clear benefit of the beam clean-up resulting from the CBC setup was documented leading to an improved nonlinear efficiency. As part of our ongoing studies into further brightness scaling in CBC architectures, we present an experimental analysis of the phase dynamics of tapered amplifiers in quasi continuous operation (QCW) at high currents. Furthermore, we are investigating different amplifier designs for improved beam quality at high powers and therefore improved combining efficiency.