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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.

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.

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.

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.

Highly efficient single-spatial mode lasers with wattlevel output power in the 9xx-nm wavelength range are of great interest for a variety of applications. We experimentally investigate the influence of the mesa widths on the electrooptical properties of ridge-waveguide lasers, based on an extreme double asymmetric epitaxial structure. These properties are, for example, the conversion efficiency and the maximum power of single-lateral mode operation (kink power). At an output power of almost 1 W, single-lateral mode operation with the highest efficiency of 60% was achieved with a 10-μm ridge-width laser. The highest peak efficiency of 62% and the highest output power of 2.7 W with still 59% efficiency were yielded by a laser with a ridge width of 15 μm, at the expense of a slightly deteriorated beam quality due to the involvement of a higher order lateral mode. The kink power of the 10-μm-ridge laser can be further increased by decreasing the front facet reflectivity from 2.5% to 0.5%.

Laser light sources emitting in the deep ultraviolet wavelength range between 210 and 230 nm are of great interest for spectroscopic applications. Here, a compact DUV diode laser system emitting at a wavelength of 222.5 nm is presented. The system is based on frequency doubling of the laser radiation from a micro-integrated GaN external cavity diode laser module (μECDL) emitting at 445 nm. The μECDL has an optical pump power of 1.4 W with an emission bandwidth of 35 pm. Narrowband laser radiation in continouos wave operation with an output power of 160 μW at 222.5 nm is generated in a single-pass frequency doubling stage with a β-BaB 2 O 4 crystal. The results are suitable to address applications such as spectroscopic investigations of biological samples. The presented concept of a compact and efficient deep ultraviolet laser light source enables the realization of portable systems for which a small footprint and a low power consumption is essential.

Efficient, high brightness 1030 nm distributed Bragg reflector (DBR) tapered diode lasers with optimized lateral waveguide and grating designs are presented in this paper. The layout changes were carried out in order to enhance the diffraction efficiency of the manufactured DBR gratings and improve the spatial mode filtering for these lasers. In this study, lasers with straight waveguide and grating, straight waveguide and tapered grating, and tapered waveguide and straight grating are compared. The lasers provide an optical output power up to 15.5 W and narrowband emission over the whole power range. Within the studied parameter range, the highest central lobe output power of 10.1 W is obtained for a laser with a tapered grating. The layout comparison also shows that the influence of the applied ridge waveguide injection current density on the central lobe power is significantly different. Here, the lateral design with a tapered grating is the most robust layout. Compared to previous approaches for DBR tapered diode lasers, optimized waveguide and grating designs and their impact on laser performance may help to increase the application potential of such devices.

In this study, the authors present experimental results on 6 mm long monolithic passively colliding-pulse mode-locked (CPM) lasers. The AlGaAs-based devices with In x Ga1−x As y P1−y double-quantum-well (DQW) active regions emit between 860 and 880 nm. To optimise the laser performance and to shorten the pulses the optical gain is spectrally broadened to increase the number of longitudinal lasing modes by using a chirped DQW structure where the In-content of the QWs differs by Δx = 0.08. Lasers having the chirped DQW with lasers having a conventional, unchirped DQW active region are compared. The mode locking operation is investigated in dependence on gain current, absorber voltage and absorber length. According to the experimental results, the lasers with the chirped DQW exhibit an overall better performance and generate shorter pulses with higher average optical output powers over a wider range of parameters. For an optimum set of parameters, the generated pulses have a full width at half maximum of the autocorrelation function of 1.9 ps at a repetition frequency of 12.9 GHz.

A compact external cavity diode laser module with 1.4-W narrowband emission at 445 nm is presented. A commercially available broad-area GaN-based laser diode is used as gain medium and a volume Bragg grating is integrated for wavelength stabilization. The laser module is realized on a conduction cooled package with a footprint of 25 mm $\times $ 25 mm. The spectral width is smaller than 50 pm over the whole operating range. A suppression of the amplified spontaneous emission with more than 50 dB is measured. The laser module is suitable for subsequent nonlinear frequency conversion into the deep ultraviolet spectral range.

Eye-safe, diode-pumped all-solid-state lasers working in the 2 μm region are favourable for many applications including remote sensing, medicine, and gas sensing. Tm-doped crystals such as Tm:YAG and Tm:LuAG are among the commonly employed laser gain media for this purpose. These systems are usually pumped by laser diodes operating around 780 nm [1]. The corresponding stokes limit for laser slope efficiency is then about 39%. However, for highly-doped samples, the two-for-one cross-relaxation process enables laser efficiencies exceeding this limit, and a slope efficiency of 59% has been reported from a 785 nm Ti:Sapphire pumped Tm:YAG laser, indicating a cross-relaxation efficiency of around 1.5 (one 780 nm pump photon generating 1.5 laser photons around 2 μm) [2]. As an alternative pumping approach, Stoneman et al. pointed out the possibility to pump Tm-based lasers around 681 nm, due to the much stronger absorption line at this wavelength [3]. Compared to 785 nm pumping, using the stronger 681 nm line warrants utilization of shorter Tm-crystals, which minimizes self-absorption losses [3]. Moreover, 681 nm pumping could also facilitate usage of thinner crystals in thin-disk geometry and/or could decrease the required number of bounces on the thin-disk for sufficient absorption of the pump light [4]. Moreover, pumping at a wavelength with stronger absorption coefficient creates a more densely populated gain medium, which could increase the effectiveness of cross-relaxation process. As a drawback, the stokes limited slope efficiency reduces to around 34% for 681 nm pumping, and decreased absorption lengths might also induce stronger thermal effects. In their early work, Stoneman et al. used a Ti:Sapphire laser operating at 681 nm to pump a Tm:YAG crystal, but the obtained slope efficiency was only around 35%, indicating a weak cross-relaxation process [3].

Applied optics 58(11), 2973 - 2980 (2019). doi:10.1364/AO.58.002973, In this paper, we present highly efficient continuous-wave (cw) laser operation of Tm:YAG and Tm:LuAG lasers pumped by high-brightness red tapered diodes. The single-emitter tapered diode lasers (TDLs) provide up to 1 W of pump power around 680 nm. By adjusting the operation temperature of the TDL, the pump central wavelength could be matched to the strong absorption peak of Tm$^{3+}$ ions in this region ($^3 H_6 → ^3 F_3$ excitation). This absorption peak is around threefold stronger than the usually employed 785 nm transition ($^3 H_6 → ^3 H_4$). In the cw laser experiments, we have achieved slope efficiencies exceeding 55% at room temperature, which is far above the Stokes limited slope efficiency (34%), indicating presence of a strong two-for-one cross-relaxation process. Pumping with high-brightness tapered diode lasers further facilitated usage of smaller pump spots (enabling quite low lasing thresholds) and generation of near-diffraction limited output beam profiles from standard 𝑧-type cavities. To the best of our knowledge, this is the first report of diode pumping of Tm-doped solid-state lasers around 680 nm as well as the first usage of TDLs as pump sources in Tm-doped laser systems., Published by Optical Soc. of America, Washington, DC

A miniaturised red-emitting master-oscillator power-amplifier (MOPA) was developed. The MOPA emits a single longitudinal mode with a linewidth below 10 MHz and an optical output power of more than 300 mW at a wavelength of about 647 nm. The master-oscillator is a distributed-Bragg-reflector ridge-waveguide laser which provides the single mode with an output power of about 40 mW. The light is shaped by micro-lenses and guided through a miniaturised CdMnTe-Faraday isolator as well as a truncated tapered power amplifier; it is reshaped again afterwards to produce an almost circular, nearly diffraction-limited beam. The total length of the MOPA is below 50 mm, which fitted into a sealed butterfly package with a footprint of about 20 cm2.

de lasers are key components in material processing laser systems. While mostly used as pump sources for solid state or fiber lasers, direct diode laser systems using dense wavelength multiplexing have come on the market in recent years. These systems are realized with broad area lasers typically, resulting in beam quality inferior to disk or fiber lasers. We will present recent results of highly efficient ridge waveguide (RW) lasers, developed for dense-wavelength-beamcombining (DWBC) laser systems expecting beam qualities comparable to solid state laser systems and higher power conversion efficiencies (PCE). The newly developed RW lasers are based on vertical structures with an extreme double asymmetric large optical cavity. Besides a low vertical divergence these structures are suitable for RW-lasers with (10 μm) broad ridges, emitting in a single mode with a good beam quality. The large stripe width enables a lateral divergence below 10° (95 % power content) and a high PCE by a comparably low series resistance. We present results of single emitters and small test arrays under different external feedback conditions. Single emitters can be tuned from 950 nm to 975 nm and reach 1 W optical power with more than 55 % PCE and a beam quality of M2 < 2 over the full wavelength range. The spectral width is below 30 pm FWHM. 5 emitter arrays were stabilized using the same setup. Up to now we reached 3 W optical power, limited by power supply, with 5 narrow spectral lines.

1030 nm DBR tapered diode lasers with different lateral layouts are presented. The layout comparison includes lasers with straight waveguide and grating, tapered waveguide and straight grating, and straight waveguide and tapered grating. The lasers provide narrowband emission and optical output powers up to 15 W. The highest diffraction-limited central lobe output power of 10.5 W is obtained for lasers with tapered gratings only. Small variations in central lobe output power with RW injection current density also indicate the robustness of that layout. For lasers with tapered waveguides, high RW injection current densities up to 150 A/mm2 have to be applied in order to obtain high central lobe output powers. Lasers with straight waveguide and grating operate best at low RW injection current densities, 50 A/mm2 applied in this study. Using the layout optimizations discussed in this study may help to increase the application potential of DBR tapered diode lasers.

International audience; 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.

Semiconductor lasers with integrated surface gratings are known to operate with narrow spectra as well as high power and efficiency. In this paper, we present a theoretical description of DFB lasers with 80th-order waveguide gratings, fabricated by etching narrow grooves into the epitaxial layer structure. The passive gratings are simulated by mode expansion and S-matrix algorithm yielding the reflection and transmission matrices in dependence on wavelength for given parameters of the grating, such as residual layer thickness and width of the grooves. The lasing condition is solved by constructing a round-trip operator and singular-value deposition for the calculation of lasing wavelength and threshold gain. A fast simulation of DFB lasers is performed within the framework of coupled-wave theory using adapted coupling coefficients. GaAs based ridge-waveguide and narrow-stripe broad-area lasers with integrated surface gratings are experimentally shown to emit output powers of 0.6 W into a single spectral line ( $\Delta \lambda _{{\rm FWHM}} pm) and $>$ 5 W into a spectrum with width $\Delta \lambda _{95\%}\approx 0.8$ nm, respectively.

We investigate passively mode-locked diode lasers with external cavity for ultrashort pulse generation. Our strategy to achieve ultrashort pulses is to generate strongly chirped pulses with a maximized bandwidth and to compress them externally. By managing intracavity dispersion with an evolutionary algorithm, we obtain pulse widths as short as 278 fs following this approach. We analyze the bandwidth of the optimized pulses in comparison to the available net gain bandwidth of the diode laser device to derive further strategies for achieving shorter pulses.

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- $\mu \text{m}$ 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 $\times $ mrad to be maintained to 7-W optical output, corresponding to a peak linear brightness of 3.5 W/mm $\times $ mrad, comparable with the highest reported values for 90- $\mu \text{m}$ stripe devices.

Two concepts of dual-wavelength 785-nm DBR ridge waveguide (RW) lasers, i.e. RW mini-arrays consisting of two DBR-RW lasers and Y-branch DBR-RW lasers, will be compared with respect to their usability as excitation light sources for shifted excitation Raman difference spectroscopy (SERDS). For both types of devices for each wavelength, output powers up to 215 mW were measured. A stable spectral distance between the laser emissions of the two resonator branches with the targeted value of 0.6 nm, i.e. 10 cm−1, is observed. In the case of the mini-array up to an output power of about 70 mW, the device shows single-mode operation. Although at higher power levels, mode hops and multi-mode operation occur, the emission width smaller than 0.15 nm still meets the requirements for Raman measurements of solids and liquids. Over the whole working range, the spectral distance between the two wavelengths is approximately constant with 0.62 nm. The near field shows two emission spots according to the dimension of the RW and their processed distance of 20 µm. The Y-branch laser shows single-mode operation up to 150 mW with a narrow spectral emission width. At higher powers also, multi-mode operation with an emission width of 0.15 nm occurs. The nearly diffraction-limited emission comes from one output aperture; the far-field emission shows a pronounced asymmetry between the two branches. Both types of devices fulfil the spectral requirements from Raman spectroscopy and SERDS up to 215 mW output power.

New pump module concepts had to be developed for space borne applications, because a simple transformation of terrestrial solutions to space requirements was often not useful. A planar approach has been chosen, which prevents inherent draw-backs of stacks.

An efficient, 1030 nm distributed Bragg reflector (DBR) tapered diode laser with 10.5 W central lobe output power is presented. The laser is based on a tapered DBR grating for enhanced spatial mode filtering and ion implantation along the mesa structure for the suppression of unwanted modes. The highest optical output power and electro-optical efficiency are 14 W and 50%, respectively. Intrinsic wavelength stabilization provides narrowband over the whole power range. Within the studied parameter range, the highest central lobe output power of 10.5 W corresponds to 75% of the emitted optical output power. A low degradation of the central lobe power content indicates the robustness of the developed design. The measured beam propagation ratios M2 are 1.2 (1/e2). These results may help to further increase the application potential for DBR tapered diode lasers.

Wavelength stabilized diode lasers, such as distributed Bragg reflector (DBR) lasers, are required for many different applications. A tool for manufacturing of DBR gratings is electron beam lithography, a flexible technology with high reproducibility. The resolution enables the realization of low-order gratings with high reflectivities. For a recent process of 1030 nm DBR lasers, numerical simulations for infinitely wide gratings predicted that diffraction efficiencies > 90% can be obtained (Fig. 1, left). However, measurements for processed ridge waveguide lasers showed a significant loss of output power through the anti-reflection coated rear facet [1]. Based on the ratio of output power measured at both facets, the experimentally obtained DBR diffraction efficiencies were only half the values obtained from the numerical simulations using the CAvity Modelling FRamework tool (CAMFR, http://camfr.sourceforge.net/). In comparison, high diffraction efficiencies were measured for broader DBR ridge waveguide lasers. For diode lasers, low DBR diffraction efficiencies may result in increased laser thresholds and reduced electro-optical efficiencies. In case of DBR tapered diode lasers low diffraction efficiencies may even cause a poor spatial mode filtering, significantly limiting the diffraction limited output power of such devices.

A dual-wavelength master oscillator (MO) power amplifier (PA) diode-laser system emitting at 785 nm suitable for shifted excitation Raman difference spectroscopy is presented. The laser system consists of a distributed Bragg reflector V-branch diode laser as a dual-wavelength MO and a ridge waveguide PA. The system reaches an optical output power of more than 750 mW at 25 °C. Optical spectra show wavelength stabilized single mode emission at 784.60 and 785.22 nm over the whole power range with a spectral width ≤10 pm (≤0.5 cm-1) and a spectral distance of 0.62 nm (≤10.1 cm -1).

We describe continuous-wave (cw) intracavity frequency-doubling experiments performed with a Cr:LiCAF laser. The Cr:LiCAF crystal is home-grown and had passive losses below 0.15% per cm. The laser is pumped by two recently-developed high-brightness tapered diodes, providing a total pump power of 2 W at 680 nm. The Cr:LiCAF laser generated up to 585 mW of cw output power around 800 nm with 43% slope efficiency at an absorbed pump power of 1.4 W. The low passive losses of the crystal enabled storage of up to 380 W of intracavity laser power using a 0.07% transmitting output coupler, demonstrating suitability of Cr:LiCAF gain media for intracavity nonlinear conversion experiments. By performing intracavity frequency doubling with a BBO crystal, cw second-harmonic powers as high as 265 mW around 400 nm have been realized with optical-to-optical conversion efficiencies as high as 13.3%. To our knowledge, these are the highest cw frequency-doubled laser powers and conversion efficiencies obtained from Cr:Colquiriites to date. Moreover, obtained efficiencies are superior compared to what have been achieved with similar Ti:Sapphire systems, due to lower passive losses of Cr:LiCAF crystal. These results demonstrate the appropriateness of Cr:LiCAF gain media as a high-power tunable cw radiation generator in 375–435 nm region.

GaAs-based narrow-stripe broad-area lasers with integrated surface gratings are shown to operate with high power and efficiency, low beam parameter product (BPP), and narrow spectra. These distributed feedback lasers are constructed using a surface-etched, 80th-order grating, leading to spectral width $\Delta\lambda_{95\%} (95% power) and wavelength ${\sim}{\rm970}~{\rm nm}$ . Optical output powers $({>}{\rm 6}~{\rm W})$ and power conversion efficiencies $({>}{50\%})$ comparable with devices without grating are enabled by a small coupling coefficient, $\kappa{\rm L}{\sim}{0.3}$ , consistent with coupled mode theory calculations. The lasers were constructed with a stripe width of 30 $\mu{\rm m}$ , enabling BPP ${ mrad up to 6-W output power, improved over devices without gratings, that show BPP ${>}{\rm 2.2}~{\rm mm}$ mrad. We attribute the improvement to spatial filtering of higher order lateral modes by the narrow surface grating. Such devices are attractive sources for use in spectral beam combination systems for material processing.

Diode lasers with 400- μm stripe widths measured in pulsed-current mode are shown to reach peak (saturation) powers of 60 W for 1-ms pulses (60 mJ) and 189 W for 300-ns pulses ( ~ 55 μJ). Use of laterally structured contact and etched index guides at the stripe edges enables these devices to sustain stable lateral near- and far-field profiles to the highest bias, with a lateral beam parameter product of . Optical calculations are presented for an example system where arrays of these single emitters are incoherently coupled into a fiber with 1-mm core and numerical aperture of 0.22, extrapolating 5 kW in fiber for 1 ms (5 J) and 18 kW in fiber for 300-ns pulses ( ~ 5 mJ). Such sources would be well suited for pumping Yb:YAG crystals for applications needing high beam quality, or as direct sources.

We demonstrate diode laser modules with high spectral radiance larger than 1 GW/cm2/sr/nm in the visible spectral range. These highly brilliant laser light sources enable the development of next-generation 3D displays. About 1W output power from small-sized modules was achieved at 635 nm by direct diode laser emission and at 530 nm using single pass second harmonic generation (SHG) of a highly brilliant near-infrared laser diode.

For a maximum fiber-coupled power, high power broad-area diode lasers must operate with a small lateral far-field divergence at high continuous wave (CW) powers. However, these structures are laterally multi-moded, with a low beam quality and wide emission angles. We present a new approach to suppress higher-order lateral modes based on an anti-guiding layer with a high refractive index inserted at the edges of the active stripe. Simulations of planar waveguides containing germanium layers reveal that strong mode-coupling effects occur. These are found to vary as a function of the thickness of the inserted layer. By embedding the germanium layer in the outer region of a ridge-waveguide, the mode coupling effects result in a reduction of the modal gain of higher-order lateral modes. The lateral far-field divergence of fabricated 90- μm stripe lasers emitting at 980 nm and containing such an anti-guiding layer is narrowed by 3° at a CW output power of 10 W.

In this letter, we report on monolithic distributed Bragg reflector ridge waveguide diode lasers. The lasers feature highly strained InGaAs quantum wells and fifth order surface gratings for a stabilized emission wavelength ~1120 nm. Output powers up to 1 W and a maximum conversion efficiency of ~34% were achieved in a spatial and spectral single-mode. In a preliminary reliability test, at 0.4 W a lifetime of could be demonstrated. Therefore, the laser sources should allow for an efficient non-linear frequency doubling to 560 nm.

First results on epitaxial-side (epi-down) mounting of terahertz quantum-cascade lasers (QCLs) on sapphire submounts using indium solder are presented. The single-plasmon ridge waveguide lasers emit in the range 3.1–3.3 THz. An epi-down mounting scheme for stressless assembly is realized, which provides enhanced heat dissipation at the corresponding operating temperature and high stability against thermal cycling. The thermal conductance of the devices is improved by 30%, and the maximum continuous-wave operating temperature is increased by 10 K for epi-down mounted lasers achieving output power levels ${>}{\rm 3}~{\rm mW}$ at 55 K.

A dual-wavelength monolithic Y-branch distributed Bragg reflection (DBR) diode laser at 671 nm is presented. The device is realized with deeply etched surface DBR gratings by one-step epitaxy. A maximum optical output power of 110 mW is obtained in cw-operation for each laser cavity. The emission wavelengths of the device are 670.5 nm and 671.0 nm with a spectral width of 13 pm (0.3 cm−1) and a mean spectral distance of 0.46 nm (10.2 cm−1) over the whole operating range. Together with a free running power stability of ± 1.1% this most compact diode laser is ideally suited as an excitation light source for portable shifted excitation Raman difference spectroscopy (SERDS).

We present a 940 nm quasi-continuous wave semiconductor laser designed as a building block for high-power fiber coupled pump modules. The laser comprises a $400~\mathrm{\mu} \mathrm{m} $ narrow-stripe array mounted on an aluminum nitride substrate using hard solder. The chip has been optimized for high optical power and low lateral far-field angles. Two vertical and six lateral structure variations have been investigated to determine the best achievable performance. Operating at 1 ms pulse width and a repetition rate of 10 Hz, the laser device reaches a maximum pulse power of 86 W from a $400~\mathrm{\mu} \mathrm{m} $ aperture and more than 62% maximum conversion efficiency. Low lateral far-field angles (95% power enclosed) of 11.5° and 13.5°, depending on the epitaxial design, enable efficient multimode fiber coupling. The potential for highly reliable applications has been demonstrated.

We have realized a laser platform based on GaAs diode lasers that allows for an operation in mobile exper-imental setups in harsh environments, such as on sounding rockets. The platform comes in two versions: a master-oscillator-power-amplifier and an extended cavity diode laser. Our very robust micro-optical bench has a footprint of 80 x 25 mm2. It strictly omits any movable parts. Master-oscillator-power-amplifier systems based on distributed feedback master oscillators for 767 nm and 780 nm narrow linewidth emission have been implemented by now. A continuous wave optical output power of > 1 W with a power conversion efficiency of > 25% could be achieved. The continuous tuning range of these lasers is on the order of 100 GHz, the linewidth at 10 μs is about 1 MHz. For applications demanding a narrower linewidth we have developed an extended cavity diode laser that achieves a linewidth of 100 kHz at 10 μs. These lasers achieve a continuous spectral tuning range of about 50 GHz and an continuous wave optical power up to 30 mW. The modules have been successfully vibration tested up to 29 gRMS along all three axes and passed 1500 g shocks, again along all 3 axes. Both, master-oscillator-power-amplifiers and extended cavity diode lasers, have been employed in sounding rocket experiments.

A 1030 nm distributed Bragg reflector (DBR) tapered diode laser with nearly diffraction-limited emission is presented. The laser provides an optical output power of 12.7 W with an electro-optical efficiency >40 %. At 10.5 W of optical output power, a central lobe power content of 8.1 W and a nearly diffraction-limited beam propagation ratio of M 2 = 1.1 (1/e 2) are obtained. The corresponding brightness is 700 MW cm−2 sr−1. Compared to previous approaches, intrinsic wavelength stabilization is obtained by a third-order DBR grating manufactured using more reproducible electron-beam lithography. A narrowband emission is measured over the whole power range with a spectral bandwidth of about 17 pm at 12.5 W. Based on the measured electro-optical, spectral and spatial properties, the laser is suitable for applications requiring narrowband, high-power emission with high spatial quality.

Ultrashort pulse generation with semiconductor lasers poses a promising alternative to currently available femtosecond laser sources like solid state and fiber lasers. Semiconductor devices can be produced inexpensively, are energy efficient and their wavelength can be designed by band gap engineering. Furthermore they feature a tunable repetition rate. Yet pulse duration and peak power of those devices limit their potential for applications so far. However, recent research demonstrated a reduction of the pulse width from 534 fs (full width half maximum) to 216 fs by shaping the spectrally resolved spectral phase and amplitude inside the cavity. The utilized system consisted of a mode-locked edge emitting semiconductor laser diode, a spatial light modulator inside the external cavity to carry out the pulse shaping and an evolutionary algorithm to optimize the phase and amplitude. Here we present the results of separate phase and amplitude shaping as well as their interaction if optimized together at the same time. Furthermore we demonstrate the flexibility of the phase and amplitude shaping with respect to each other. Thus we expect of our system to enable adaptation to a resonator external dispersion.

Nearly diffraction-limited emission from a distributed Bragg reflector (DBR) tapered diode laser is presented. Intrinsic wavelength stabilization is achieved with a 3rd order DBR grating manufactured by electron beam lithography. At a heatsink temperature of 15°C an optical output power of 12.7 W with an electro-optical efficiency > 40% is obtained. The corresponding emission wavelength is 1030.57 nm and spectral bandwidths of 0.02 nm are measured over the whole power range. At 10.5 W of optical power 8.1 W are contained in the central lobe. The measured beam propagation ratio and brightness are 1.1 (1/e 2 ) and 700 MWcm -2 sr -1 , respectively. With these parameters, the laser is suitable for applications such as non-linear frequency conversion.

Deep ultraviolet (DUV) lasers emitting below 300 nm are of great interest for many applications, for instance in medical diagnostics or for detecting biological agents. Established DUV lasers, e.g. gas lasers or frequency quadrupled solid-state lasers, are relatively bulky and have high power consumptions. A compact and reliable laser diode based system emitting in the DUV could help to address applications in environments where a portable and robust light source with low power consumption is needed. In this work, a compact DUV laser system based on single-pass frequency doubling of highpower GaN diode laser emission is presented. A commercially available high-power GaN laser diode from OSRAM Opto Semiconductors serves as a pump source. The laser diode is spectrally stabilized in an external cavity diode laser (ECDL) setup in Littrow configuration. The ECDL system reaches a maximum optical output power of 700 mW, maintaining narrowband emission below 60 pm (FWHM) at 445 nm over the entire operating range. By direct single pass frequency doubling in a BBO crystal with a length of 7.5 mm a maximum DUV output power of 16 μW at a wavelength of 222.5 nm is generated. The presented concept enables compact and efficient diode laser based light sources emitting in the DUV spectral range that are potentially suitable for in situ applications where a small footprint and low power consumption is essential.

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.

Tailored diode laser single emitters with long (6 mm) resonators and wide (1.2 mm) emission apertures that operate with 940 nm emission wavelength were assembled in novel edge-cooled vertically stacked arrays, and used to construct a compact and highly efficient fiber coupled pump source for Yb:YAG pulsed high-energy class solid-state lasers. The novel configuration is shown to allow repetition rates of 200 Hz at 1 ms pulse duration, at an output power of 130 W per single emitter. The emission of two stacked arrays was then optically combined to realize pump modules that deliver 6 kW peak power (pulse energy 6 J) from a 1.9 mm core diameter fiber, with wall plug efficiency of 50%. This represents a significant improvement in terms of duty cycle and electro-optical efficiency over conventional sources. The pump module has been successfully tested at the Max Born Institute, Berlin during trials for pumping of disk lasers.

In this work, we investigate experimentally and theoretically limitations of highly efficient high-power single-pass second-harmonic generation process in a quasi-phase matched magnesium oxide-doped lithium niobate ridge waveguide due to structural imperfections, optical absorption and resulting heat generation. With the distributed Bragg reflector diode laser emitting at 1,061 nm, applied in the presented bench-top experiment, a coupling efficiency into the ridge waveguide nonlinear device of 86 % is realized. An internal normalized conversion efficiency of 375 % (Wcm $${}^2$$ ) $${}^{-1}$$ is determined. A maximum second-harmonic power of 386 mW is reached at a corresponding opto-optical and electro-optical conversion efficiency of 38 and 9.5 %, respectively. A saturation of the conversion efficiency at a high power level as well as its dependency on the nonlinear device structure inhomogeneity are confirmed experimentally. With a theoretical model incorporating geometrical inhomogeneity, linear and nonlinear absorption and resulting heat load the second-harmonic generation in a ridge waveguide can be simulated realistically. Increased values of nonlinear absorption coefficients, higher than until now reported, lead to a very good agreement between theoretical and experimental results. Moreover, according to the simulation, structural inhomogeneities can enhance the SHG conversion efficiency at high output powers.

The cascading of nonlinear crystals has been established as a simple method to greatly increase the conversion efficiency of single-pass second-harmonic generation compared to a single-crystal scheme. Here, we show for the first time that the technique can be extended to sum frequency generation, despite differences in the phase relations of the involved fields. An unprecedented 5.5 W of continuous-wave diffraction-limited green light is generated from the single-pass sum frequency mixing of two diode lasers in two periodically poled nonlinear crystals (conversion efficiency 50%). The technique is generally applicable and can be applied to any combination of fundamental wavelengths and nonlinear crystals.

High-power distributed-feedback (DFB) lasers for the wavelength range near 940 nm (i.e. about 10,600 cm−1) were used for line-broadening measurements of individual rotational-vibrational absorption lines of water vapour at atmospheric pressure using a minimalist set-up. The laser has a maximum output power larger than 500 mW. Over the whole power range from threshold to maximum power, it operates in single mode operation with a tuning range of 4.7 nm, i.e. 50 cm−1, at 20°C. With an emission line-width ≤2 MHz (0.66×10−4 cm−1), the device is well suited for high-resolution spectroscopy.

We report on the realization of distributed Bragg reflector laser diodes emitting near a wavelength of 1064 nm in a single longitudinal mode with high output power and kHz-level intrinsic spectral linewidth. Linewidth measurements are performed with a heterodyne measurement setup in which both lasers are weakly locked to each other in order to reject slow frequency drifts. The linewidth is analyzed in dependence of the output power. The FWHM linewidth is almost independent from the output power and corresponds to approximately 180 kHz whereas the intrinsic linewidth decreases with a 1/P out dependence. The minimum intrinsic linewidth is 2 kHz at an output power of 180 mW. The measured intrinsic linewidth is in good correspondence with a theoretical estimation.

Tapered diode lasers in the red spectral range with nearly diffraction-limited output are prospective light sources for display applications and analytic methods such as fluorescence microscopy. The authors have studied the influence of the number of quantum wells and the lateral design of 660 nm high-power tapered diode lasers on the output power and the beam quality. From these investigations the authors have developed an optimised design and achieved a record output power of 1.5 W with a beam quality close to the diffraction limit ( M 2 (1/ e 2 )=1.5). The authors also demonstrate operation over 4500=h at power levels between 0.5 and 1.0=W.

We present a novel concept of a finite-aperture tapered unstable resonator laser based on two distributed Bragg reflectors at the rear and front facets with different lateral widths. An analysis of the passive resonator, as well as a spatiotemporal simulation of the active device, is presented. A variation of the ratio between the widths of both Bragg reflectors changes the losses of the lateral modes differently, leading to certain geometries with good modal discrimination and beam quality. The principle of operation is demonstrated by fabrication and experimental characterization of a first-generation of device.

Using a tunable external cavity tapered diode laser (ECDL) pumped quasi-three-level Nd:YAG laser, a fivefold reduction in threshold and twofold increase in slope efficiency is demonstrated when compared to a traditional broad area diode laser pump source. A TEM00 power of 800 mW with 65% slope efficiency is obtained, the highest reported TEM00 power from any 946 nm Nd:YAG laser pumped by a single emitter diode laser pump source. A quantum efficiency of 0.85 has been estimated from experimental data using a simple quasi-three-level model. The reported value is in good agreement with published values, suggesting that the model is adequate. Improvement of the 946 nm laser due to the ECDL's narrow spectrum proves to be less significant when compared to its spatial quality, inferring a broad spectrum tapered diode laser pump source may be most practical. Experimental confirmation of such setup is given.

In this paper, we investigate the modal behavior and the spatial coherence properties of a gain-guided laser array emitting an optical output power of more than 50 W in quasi-continuous-wave operation at a wavelength of 1070 nm and above. The lateral near- and far-field intensity profiles and the Wigner distribution function were measured from low to high output power. The array modes were calculated by solving the waveguide equation taking into account the power-dependent temperature distribution and were also experimentally determined by a modal decomposition of the cross spectral density. The analysis revealed that, at a low power, a single array mode lases, whereas at high power, multiple single stripe modes dominate the lasing due to the thermally induced index rise under the stripes.

A review about second harmonic generation using edge emitting diode lasers and nonlinear crystals to obtain laser radiation in the blue-green spectral range is presented. Therefore, pump laser radiation with high brightness and narrow bandwidth is necessary. Thus, this review gives an overview of the advances made with distributed feedback and Bragg reflector lasers, tapered lasers and amplifiers as well as external cavity diode lasers and master oscillator power amplifier schemes to achieve high brilliance emission. Since periodically poled materials have enabled high second harmonic conversion efficiencies with low and moderate pump powers, the review is focused on frequency doubling using those materials. The most commonly used materials, their properties and limitations are discussed briefly. Single pass and resonant SHG setups with waveguide and bulk nonlinear crystals are discussed and an emphasis on building compact and integrated devices is made.

Many physical effects can potentially limit the peak achievable output power of single emitter broad area diode lasers under high current, pulse-pumped operation conditions. Although previous studies have shown reliable operation to high pump levels (240 A, 300 ns, and 1 kHz), power was found to saturate. We present here results of a systematic study to unambiguously determine the sources of this power saturation. A combination of detailed measurements and finite element device simulation were used for the diagnosis. We find that the measured power saturation is dominated by electron leakage caused by band bending at high bias due to the low mobility of the p-type waveguide. However, the power saturation is only fully reproduced when longitudinal spatial hole-burning is included. Higher powers are expected to be achieved if higher energy barriers and lower confinement factors are used to mitigate leakage and longitudinal hole-burning, respectively.