This PDF file contains the front matter associated with SPIE Proceedings Volume 9348 including the Title Page, Copyright information, Table of Contents, Introduction, and Conference Committee listing.

In this paper we present the next step on the roadmap “system scalability towards an output power above 100 kW”, first time presented in 2014 [1]. To take a step forward the optical power of the fiber-coupled diode laser has been increased beyond the power level 40kW. The power conversion efficiency exceeds 40%. The laser contains modules with 4 different wavelengths (960nm, 1020nm, 1040nm, 1060nm) there are two modules for each wavelength polarization multiplexed. After the slow-axis collimation these wavelengths are combined using dense wavelength coupling before focusing onto the fiber endface. The delivery-fiber is an uncoated fiber with a diameter of 2 mm and NA 0.22 corresponding a BPP of 220 mm mrad. In a stability test the laser delivered a constant maximum output power with less than ±0.5 % variation over 100h. Further results of the optical properties of the laser will be presented in this paper. This new laser is based on a turn-key industrial platform, allowing straight-forward integration into almost any industrial application, like welding or large area heat treatment. As application examples laser welding of thick sheet metal and pumping of an active fiber will be presented. The footprint of the complete system is 2.8 m² with a height below 1.8 m.

We report on continued progress in nLIGHT’s high power and high efficiency single emitter laser diodes from 915 nm to 980 nm range used for industrial and pumping applications. High performance has been demonstrated in nLIGHT’s diode laser technology in this spectral range with peak electrical-to-optical power conversion efficiency of ~65%. These diodes have been incorporated into nLIGHT’s fiber-coupled pump module, elementTM. We have reduced the slow-axis divergence of our brightest diodes by a half at the same operating power. This results primarily from suppression of higher-order lateral modes leading to lower beam-parameter-product at a given power compared to conventional broad area lasers. We have device designs that produce slow axis brightness of up to 4.3 W/mm-mrad which is 48% higher compared to our brightest broad area laser. This paper presents nLIGHT’s most recent improvement in slow-axis brightness resulting from reduced number of allowed modes in the slow-axis in a new broad area laser architecture called reduced-mode diodes (REM-diodes). We will detail the resulting power and brightness improvement along with preliminary reliability assessment of these diodes.

Recent advances in high power diode laser technologies have enabled advanced research on diode pumped alkali metal vapor lasers (DPALs). Due to their low quantum defect, DPALs offer the promise of scalability to very high average power levels while maintaining excellent beam quality. Research is being conducted on a variety of gain media species, requiring different pump wavelengths: near 852nm for cesium, 780nm for rubidium, 766nm for potassium, and 670nm for lithium atoms. The biggest challenge in pumping these materials efficiently is the narrow gain media absorption band of approximately 0.01nm. Typical high power diode lasers achieve spectral widths around 3nm (FWHM) in the near infrared spectrum. Gratings may be used internal or external to the cavity to reduce the spectral width to 0.5nm to 1nm for high power diode laser modules. Recently, experimental results have shown narrower line widths ranging from picometers (pm) at very low power levels to sub-100 picometers for water cooled stacks around 1kW of output power. The focus of this work is a further reduction in the spectral line width of high power diode laser bars emitting at 766nm, with full applicability to other wavelengths of interest. One factor limiting the reduction of the spectral line width is the optical absorption induced thermal gradient inside the volume Bragg grating (VBG). Simulated profiles and demonstrated techniques to minimize thermal gradients will be presented. To enable the next stage of DPAL research, a new series of fiber coupled modules is being introduced featuring greater than 400W from a 600μm core fiber of 0.22NA. The modules achieve a spectral width of <<0.1nm and wavelength tunability of +/- 0.15nm.

Fiber laser manufacturers demand high-brightness laser diode pumps delivering optical pump energy in both a compact fiber core and narrow angular content. A pump delivery fiber of a 105 μm core and 0.22 numerical aperture (NA) is typically used, where the fiber NA is under-filled to ease the launch of laser diode emission into the fiber and make the fiber tolerant to bending. At SCD, we have developed high-brightness NEON multi-emitter fiber-coupled pump modules that deliver 50 W output from a 105 μm, 0.15 NA fiber enabling low-NA power delivery to a customer’s fiber laser network. Brightness-enhanced single emitters are engineered with ultra-low divergence for compatibility with the low-NA delivery fiber, with the latest emitters delivering 14 W with 95% of the slow-axis energy contained within an NA of 0.09. The reduced slow-axis divergence is achieved with an optimized epitaxial design, where the peak optical intensity is reduced to both lessen filamentation within the laser cavity and reduce the power density on the output facet thus increasing the emitter reliability. The low mode filling of the fiber allows it to be coiled with diameters down to 70 mm at full operating power despite the small NA and further eliminates the need for mode-stripping at fiber combiners and splices downstream from our pump modules. 50W fiber pump products at 915, 950 and 975 nm wavelengths are presented, including a wavelengthstabilized version at 976 nm.

Continuous cost reduction, improved reliability and modular platform guide the design of our next generation heatsink and packaging process. Power scaling from a single device effectively lowers the cost, while electrical insulation of the heatsink, low junction temperature and hard solder enable high reliability. We report on the latest results for scaling the output power of bars for optical pumping and materials processing. The epitaxial design and geometric structures are specific for the application, while packaging with minimum thermal impedance, low stress and low smile are generic features. The isolated heatsink shows a thermal impedance of 0.2 K/W and the maximum output power is limited by the requirement of a junction temperature of less than 68oC for high reliability. Low contact impedance are addressed for drive currents of 300 A. For pumping applications, bars with a fill factor of 60% are deployed emitting more than 300 W of output power with an efficiency of about 55% and 8 bars are arranged in a compact pump module emitting 2 kW of collimated power suitable for pumping disk lasers. For direct applications we target coupling kilowatts of output powers into fibers of 100 μm diameter with 0.1 NA based on dense wavelength multiplexing. Low fill factor bars with large optical waveguide and specialized coating also emit 300 W.

Cutting of metal sheets is a key application in material processing and high power diode lasers gain further importance in this field due to their exceptionally good current to light conversion efficiency. Both, power scaling and process optimization are under investigation to improve the performance of this application. We report first results of a laser system combining these approaches. The presented diode laser power scaling is realized by means of an asymmetric, noncircular beam shape. The beam parameter product of the laser light is manipulated accordingly. In addition, the power scaling of this approach allows the generation of spot geometries which inherently support the interaction processes of the laser light with materials as metals. The setup is based on conventional, highly reliable and well tested principles and components, as they are passively cooled laser diode bars, coarse wavelengths coupling and fiber delivery. First results of material processing with stainless steel of 6 mm thickness are presented.

In this paper, laser modules based on newly developed tailored bars are presented. The modules allow efficient fiber coupling of more than 320 W into 10 mm-mrad or 160 W into 6 mm-mrad at one single wavelength. For further power scaling dense wavelength coupling concepts are presented which enable kW-class lasers with a beam quality of 10 mm-mrad.

For an economic production it is important to serve as many applications as possible while keeping the product variations minimal. We present our modular laser design, which is based on single emitters and various combining technics. In a first step we accept a reduction of the very high brightness of the single emitters by vertical stacking. Those emitters can be wavelength stabilized by an external resonator, providing the very same feedback to each of those laser diodes which leads to an output power of about 100W with BPP of <;3.5 mm*mrad (FA) and <5 mm*mrad (SA). Further power scaling is accomplished by polarization and wavelength multiplexing yielding high optical efficiencies of more than 80% and results in about 500 W launched into a 100 μm fiber with 0.15 NA. Subsequently those building blocks can be stacked also by the very same dense spectral combing technique up to multi kW Systems without further reduction of the BPP. These “500W building blocks” are consequently designed in a way that without any system change new wavelengths can be implemented by only exchanging parts but without change of the production process. This design principal offers the option to adapt the wavelength of those blocks to any applications, from UV, visible into the far IR. From laser pumping and scientific applications to materials processing such as cutting and welding of copper aluminum or steel and also medical application. Operating at wavelengths between 900 nm and 1100 nm, these systems are mainly used in cutting and welding, but the technology can also be adapted to other wavelength ranges, such as 793 nm and 1530 nm. Around 1.5 μm the diodes are already successfully used for resonant pumping of Erbium lasers.[1] Furthermore, the fully integrated electronic concept allows addressing further applications, as it is capable of very short μs pulses up to cw mode operation by simple software commands.

Spatial and spectral emission characteristics and efficiency of high-power diode laser (HPDL) based pump sources enable and define the performance of the fundamental solid state laser concepts like disk, fiber and slab lasers. HPDL are also established as a versatile tool for direct materials processing substituting other laser types like CO2 lasers and lamp pumped solid state lasers and are starting to substitute even some of the diode pumped solid state lasers. Both, pumping and direct applications will benefit from the further improvement of the brightness and control of the output spectrum of HPDL. While edge emitting diodes are already established, a new generation of vertical emitting diode lasers (VCSELs) made significant progress and provides easy scalable output power in the kW range. Beneficial properties are simplified beam shaping, flexible control of the temporal and spatial emission, compact design and low current operation. Other characteristics like efficiency and brightness of VCSELs are still lagging behind the edge emitter performance. Examples of direct applications like surface treatment, soldering, welding, additive manufacturing, cutting and their requirements on the HPDL performance are presented. Furthermore, an overview on process requirements and available as well as perspective performance of laser sources is derived.

In this paper we present nLIGHT’s most recent reliability assessment of both the released and newly developed high power, high brightness single emitter laser diodes for fiber laser pumps and material processing applications. We report on the latest updates of lifetests performed on released 18W-rated diode lasers which have been successfully incorporated into nLIGHT’s 210W 200μm/0.18NA element^TM pump module. A total of 371 units of 18W-rated single emitters at 915 nm, were assessed at 22A and 2 A at a junction temperature, Tj~70ºC. Cumulatively, these devices have accrued ~ 6.0 million equivalent device hours at module use conditions. The initial reliability analysis based on these lifetest results support <99% module reliability for 2-year of continuous operation. Industry leading dollars-per-watt element_TM e06, e12 and e18 packages based on these diode lasers are also presented. Two element_TM e18 packages have been lifetested for <5400 hours with only one device failure so far. We also report on the initial lifetest of the newly developed high brightness REM-diodes (Reduced Mode diodes) for new element_TM configuration. Preliminary highly accelerated lifetest on ~15 W REM-diodes show very low failure rate compared to the control diode lasers under the same conditions. The more optimized <15W REM-diodes have been lifetested for almost 4000h with no failures observed so far. Superior performance has already been demonstrated on the initialelement_TMe06, e12 and e18 packages with these new REM designs, supporting a 25% increase in power with a minimal degradation in NA. Module level reliability assessment is underway.

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%~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 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-power quasi-CW laser bars are of great interest as pump sources of solid-state lasers generating high-energy ultrashort pulses for high energy projects. These applications require a continuous improvement of the laser diodes for reliable optical output powers and simultaneously high electrical-to-optical power efficiencies. An overview is presented of recent progress at JENOPTIK in the development of commercial quasi-CW laser bars emitting around 880 nm and 940 nm optimized for peak performance. At first, performances of 1.5 mm long laser bars with 75% fill-factor are presented. Both, 880 nm and 940 nm laser bars deliver reliable power of 500 W with wall-plug-efficiencies (WPE) <55% within narrow beam divergence angles of 11° and 45° in slow-axis and fast-axis directions, respectively. The reliability tests at 500 W powers under application quasi- CW conditions are ongoing. Moreover, laser bars emitting at 880 nm tested under 100 μs current pulse duration deliver 1 kW output power at 0.9 kA with only a small degradation of the slope efficiency. The applications of 940 nm laser bars require longer optical pulses and higher repetition rates (1-2 ms, ~10 Hz). In order to achieve output powers at the level of 1 kW under such long pulse duration, heating of the laser active region has to be minimized. Power-voltage-current characteristics of 4 mm long cavity bars with 50% fill-factor based on an optimized laser structure for strong carrier confinement and low resistivity were measured. We report an output power of 0.8 kW at 0.8 A with <60% conversion efficiency (52% WPE). By increasing the fill-factor of the bars a further improvement of the WPE at high currents is expected.

915nm high-power and high-reliability single emitter laser diodes based on Asymmetric Decoupled Confinement Heterostructure (ADCH) are demonstrated. Advantage of ADCH is that it can optimize active layer confinement () and confinement ratio of p- to n-doped layer (p/n), independently, to manage large effective spot size and low internal loss without any penalty in carrier confinement. 4mm-cavity, 100m wide stripe LDs with large effective spot size of 1.5m demonstrates record high Catastrophic-optical-damage (COD) free operation over 42W output. Accelerated aging tests are conducted for 325 devices in total with 1.8 million device hours. Mean time to failure of random failure mode is estimated to be 1.1 million hours for 12W at room temperature.

We report modeling and experimental results that demonstrate mechanisms limiting the output power of semiconductor lasers and a method experimentally yielding a dramatic increase of the maximum continuous wave output power. Unfolded cavity is used to achieve higher power and efficiency by improving the alignment between the carrier and photon density profiles in a long cavity device. This method offers reduced longitudinal spatial hole burning (LSHB) and lower photon density inside the laser cavity; therefore, it decreases possible LSHB and non-linear effects that could limit the output power of a semiconductor laser. We have demonstrated 29.5W output from 5.7mm long and 100um wide waveguide at 9xx nm using an unfolded cavity. A semiconductor laser with an unfolded cavity allows scaling of the output power by increasing the cavity length.

Substantial limitation of output power in AlGaInP based red broad area (BA) laser diode (LD) originates from an electron thermal overflow from an active layer to a p-cladding layer and fatal failure due to catastrophic optical mirror degradation during the LD operation. New red BA-LD was designed and fabricated. The LD chip had triple emitters in one chip with each stripe width of 60 um, and was assembled on Φ9.0 mm -TO package. The LD emitted exceeding 5.5 W at heat sink temperature of 25 °C and 3.8W at 45 °C under pulsed operation with frequency of 120Hz and duty of 30%, although the current product, which has a 40 um single emitter chip assembled on Φ5.6mm –TO, does 2.0 W at 25 °C. The lasing wavelength at 25 °C and 2.5W output was 638.6 nm. The preliminary aging test under the condition with the operation current of 3.56A, CW, auto-current-control mode (ACC), and the heat sink temperature of 20 °C (almost equal to the output of 3.5 W) indicated that the MTTF due to COMD was longer than 6,600 hours under CW, 22,000 hours under the pulse with duty of 30%.

We have continuously optimized high fill factor bar and packaging design to increase power and efficiency for thin disc laser system pump application. On the other hand, low fill factor bars packaged on the same direct copper bonded (DCB) cooling platform are used to build multi-kilowatt direct diode laser systems. We have also optimized the single emitter designs for fiber laser pump applications. In this paper, we will give an overview of our recent advances in high power high brightness laser bars and single emitters for pumping and direct diode application. We will present 300W bar development results for our next generation thin disk laser pump source. We will also show recent improvements on slow axis beam quality of low fill factor bar and its application on performance improvement of 4-5 kW TruDiode laser system with BPP of 30 mm*mrad from a 600 μm fiber. Performance and reliability results of single emitter for multiemitter fiber laser pump source will be presented as well.

In this study，the chip bonding processes for various chips from various chip suppliers around the world have been optimized to achieve reliable chip on sub-mount for high performance. These chip on sub-mounts, for examples, includes three types of bonding, 8xx nm-1.2W/10.0W Indium bonded lasers, 9xx nm 10W-20W AuSn bonded lasers and 1470 nm 6W Indium bonded lasers will be reported below. The MTTF@25℃ of 9xx nm chip on sub-mount (COS) is calculated to be more than 203,896 hours. These chips from various chip suppliers are packaged into many multiple single emitter laser modules, using similar packaging techniques from 2 emitters per module to up to 7 emitters per module. A reliability study including aging test is performed on those multiple single emitter laser modules. With research team’s 12 years’ experienced packaging design and techniques, precise optical and fiber alignment processes and superior chip bonding capability, we have achieved a total MTTF exceeding 177,710 hours of life time with 60% confidence level for those multiple single emitter laser modules. Furthermore, a separated reliability study on wavelength stabilized laser modules have shown this wavelength stabilized module packaging process is reliable as well.

Laser bars, laser arrays, and single emitters are highly-desired light sources e.g. for direct material processing, pump sources for solid state and fiber lasers or medical applications. These sources require high output powers with optimal efficiency together with good reliability resulting in a long lifetime of the device. Desired wavelengths range from 760 nm in esthetic skin treatment over 915 nm, 940 nm and 976 nm to 1030 nm for direct material processing and pumping applications. In this publication we present our latest developments for the different application-defined wavelengths in continuouswave operation mode. At 760nm laser bars with 30 % filling factor and 1.5 mm resonator length show optical output powers around 90-100 W using an optimized design. For longer wavelengths between 915 nm and 1030 nm laser bars with 4 mm resonator length and 50 % filling factor show reliable output powers above 200 W. The efficiency reached lies above 60% and the slow axis divergence (95% power content) is below 7°. Further developments of bars tailored for 940 nm emission wavelength reach output powers of 350 W. Reliable single emitters for effective fiber coupling having emitter widths of 90 μm and 195 μm are presented. They emit optical powers of 12 W and 24 W, respectively, at emission wavelengths of 915 nm, 940 nm and 976 nm. Moreover, reliability tests of 90 μm-single emitters at a power level of 12W currently show a life time over 3500 h.

Laser diode manufacturers perform accelerated multi-cell lifetests to estimate lifetimes of lasers using an empirical model. Since state-of-the-art laser diodes typically require a long period of latency before they degrade, significant amount of stress is applied to the lasers to generate failures in relatively short test durations. A drawback of this approach is the lack of mean-time-to-failure data under intermediate and low stress conditions, leading to uncertainty in model parameters (especially optical power and current exponent) and potential overestimation of lifetimes at usage conditions. This approach is a concern especially for satellite communication systems where high reliability is required of lasers for long-term duration in the space environment. A number of groups have studied reliability and degradation processes in GaAs-based lasers, but none of these studies have yielded a reliability model based on the physics of failure. The lack of such a model is also a concern for space applications where complete understanding of degradation mechanisms is necessary. Our present study addresses the aforementioned issues by performing long-term lifetests under low stress conditions followed by failure mode analysis (FMA) and physics of failure investigation. We performed low-stress lifetests on both MBE- and MOCVD-grown broad-area InGaAs- AlGaAs strained QW lasers under ACC (automatic current control) mode to study low-stress degradation mechanisms. Our lifetests have accumulated over 36,000 test hours and FMA is performed on failures using our angle polishing technique followed by EL. This technique allows us to identify failure types by observing dark line defects through a window introduced in backside metal contacts. We also investigated degradation mechanisms in MOCVD-grown broad-area InGaAs-AlGaAs strained QW lasers using various FMA techniques. Since it is a challenge to control defect densities during the growth of laser structures, we chose to control defect densities by introducing extrinsic point defects to the laser via proton irradiation with different energies and fluences. These lasers were subsequently lifetested to study degradation processes in the lasers with different defect densities and also to study precursor signatures of failures - traps and non-radiative recombination centers (NRCs) in pre- and post-stressed lasers. Lastly, we employed focused ion beam (FIB), electron beam induced current (EBIC), and highresolution TEM (HR-TEM) techniques to further study dark line defects and dislocations in both post-aged and postproton irradiated lasers. We report on our long-term low-stress lifetest results and physics of failure investigation results.

We carried out a comprehensive study on single emitters with different antireflection (AR) coatings in the wavelength range between 780nm and 976nm, which have been exposed to optical feedback to investigate the reversible and irreversible impacts caused by back-reflected light. By observing the near-field pattern while varying the probe current, we got information about the influence on filamentation and on peak-power densities with and without external optical feedback. For GaAs-based laser diodes, the energy gap of GaAs makes a distinction at a wavelength of about 870nm. For shorter wavelengths, e.g. at 808nm, a substantial part of the feedback light is absorbed by the substrate and GaAs cap layers very close to the front facet leading to a significant heating of the outcoupling facet. For longer wavelengths, e.g. 976nm, this energy intrusion is not a local one at the front facet, but rather spreads along the whole cavity length.

Pulsed operation of standard 980-nm emitting single-spatial-mode high power diode lasers at multi-watt power levels is studied. Primary emission, short wavelength infrared emission, as well as the spatio-temporal evolution of the near field are recorded. This approach allows for the determination of the operation parameters during which single-mode operation is maintained. This gives limits of safe operation far beyond the standard specifications as well as information about the relevant degradation mechanisms in this regime. Reference experiments with a set of long-term operated devices reveal gradual aging signatures and the starting points of the relevant aging processes become detectable. They are compared with those obtained from the devices operated under pulsed conditions.

The catastrophic optical damage (COD) of laser diodes consists of the sudden drop off of the optical power. COD is generally associated with a thermal runaway mechanism in which the active zone of the laser is molten in a positive feedback process. The full sequence of the degradation follows different phases: in the first phase, a weak zone of the laser is incubated and the temperature is locally increased there; when a critical temperature is reached the thermal runaway process takes place. Usually, the positive feedback leading to COD is circumscribed to the sequential enhancement of the optical absorption in a process driven by the increase of the temperature. However, the meaning of the critical temperature has not been unambiguously established. Herein, we will discuss about the critical temperature, and the physical mechanisms involved in this process. The influence of the progressive deterioration of the thermal conductivity of the laser structure as a result of the degradation during the laser operation will be addressed.

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.

We report on wavelength stabilized high-power diode laser systems with enhanced spectral brightness by means of Volume Holographic Gratings. High-power diode laser modules typically have a relatively broad spectral width of about 3 to 6 nm. In addition the center wavelength shifts by changing the temperature and the driving current, which is obstructive for pumping applications with small absorption bandwidths. Wavelength stabilization of high-power diode laser systems is an important method to increase the efficiency of diode pumped solid-state lasers. It also enables power scaling by dense wavelength multiplexing. To ensure a wide locking range and efficient wavelength stabilization the parameters of the Volume Holographic Grating and the parameters of the diode laser bar have to be adapted carefully. Important parameters are the reflectivity of the Volume Holographic Grating, the reflectivity of the diode laser bar as well as its angular and spectral emission characteristics. In this paper we present detailed data on wavelength stabilized diode laser systems with and without fiber coupling in the spectral range from 634 nm up to 1533 nm. The maximum output power of 2.7 kW was measured for a fiber coupled system (1000 μm, NA 0.22), which was stabilized at a wavelength of 969 nm with a spectral width of only 0.6 nm (90% value). Another example is a narrow line-width diode laser stack, which was stabilized at a wavelength of 1533 nm with a spectral bandwidth below 1 nm and an output power of 835 W.

An front facet-low reflection coated broad-area laser(BAL) diode with an emitter size of 50 μm x 1 μm and a chip length of 2000 μm is operated in the external cavity diode laser(ECDL). For wavelength stabilization and narrow spectral width, the diffraction grating is used in a Littrow configuration. At an injection current of 1.5 A, a output power of 0.65 W with a slop efficiency of 0.85 A/W, which is comparable to those of a solitary BAL diode, could be achieved with a spectral width of 120pm which is about 77 % narrower as compared to a solitary BAL diode. The peak wavelength stability below 10 pm was obtained in the wide range of output power up to 0.65 W.

This paper presents a high power laser diode method using a large core combined with mode control. The large core allows for high power, while the mode control in an external cavity maintains the beam quality. Analysis shows beam quality can be kept at the ideal diffraction limit for cores as large as 100 microns (transverse dimensions). Such size cores can support hundreds of watts. This means a new class of laser diodes with brightness levels several orders higher than current devices. Such capabilities are unheard of and will have an overwhelming effect on applications.

A new generation of diode-pumped high-energy-class solid-state laser facilities is in development that generate multijoule pulse energies at around 10 Hz. Currently deployed quasi-continuous-wave (QCW) diode lasers deliver average inpulse pump powers of around 300 W per bar. Increased power-per-bar helps to reduce the system size, complexity and cost per Joule and the increased pump brilliance also enables more efficient operation of the solid state laser itself. It has been shown in recent studies, that optimized QCW diode laser bars centered at 940…980 nm can operate with an average in-pulse power of > 1000 W per bar, triple that of commercial sources. When operated at pulsed condition of 1 ms, 10 Hz, this corresponds to > 1 J/bar. We review here the status of these high-energy-class pump sources, showing how the highest powers are enabled by using long resonators (4…6 mm) for improved cooling and robustly passivated output facets for high reliability. Results are presented for prototype passively-cooled single bar assemblies and monolithic stacked QCW arrays. We confirm that 1 J/bar is sustained for fast-axis collimated stacks with a bar pitch of 1.7 mm, with narrow lateral far field angle (< 12° with 95% power) and spectral width (< 12 nm with 95% power). Such stacks are anticipated to enable Joule/bar pump densities to be used near-term in commercial high power diode laser systems. Finally, we briefly summarize the latest status of research into bars with higher efficiencies, including studies into operation at sub-zero temperatures (-70°C), which also enables higher powers and narrower far field and spectra.

We developed a high brightness fiber coupled diode laser module providing more than 140W output power from a 105μm NA 0.15 fiber at the wavelength of 915nm.The high brightness module has an electrical to optical efficiency better than 45% and power enclosure more than 90% within NA 0.13. It is based on multi-single emitters using optical and polarization beam combining and fiber coupling technique. With the similar technology, over 100W of optical power into a 105μm NA 0.15 fiber at 976nm is also achieved which can be compatible with the volume Bragg gratings to receive narrow and stabilized spectral linewidth. The light within NA 0.12 is approximately 92%. The reliability test data of single and multiple single emitter laser module under high optical load are also presented and analyzed using a reliability model with an emitting aperture optimized for coupling into 105μm core fiber. The total MTTF shows exceeding 100,000 hours within 60% confidence level. The packaging processes and optical design are ready for commercial volume production.

Easy system design, compactness and a uniform power distribution define the basic advantages of high power VCSEL systems. Full addressability in space and time add new dimensions for optimization and enable “digital photonic production”. Many thermal processes benefit from the improved control i.e. heat is applied exactly where and when it is needed. The compact VCSEL systems can be integrated into most manufacturing equipment, replacing batch processes using large furnaces and reducing energy consumption. This paper will present how recent technological development of high power VCSEL systems will extend efficiency and flexibility of thermal processes and replace not only laser systems, lamps and furnaces but enable new ways of production. High power VCSEL systems are made from many VCSEL chips, each comprising thousands of low power VCSELs. Systems scalable in power from watts to multiple ten kilowatts and with various form factors utilize a common modular building block concept. Designs for reliable high power VCSEL arrays and systems can be developed and tested on each building block level and benefit from the low power density and excellent reliability of the VCSELs. Furthermore advanced assembly concepts aim to reduce the number of individual processes and components and make the whole system even more simple and reliable.

Distributed Bragg reflector (DBR) tapered lasers emitting near 1180 nm were developed. The integration of DBR surface gratings in an edge-emitting laser structure with a highly strained quantum well and a tapered laser geometry allows nearly diffraction limited emission into a single longitudinal mode with an optical output power of more than 2 W. The laser will allow direct second harmonic generation (SHG) in a single pass configuration and hence will enable the manufacturing of miniaturized laser modules near 590 nm for out-of-the-lab applications. An integration of a heater element at the DBR grating allows the tuning of the emission wavelength of more than 2 nm without the mechanical movement of gratings. This easy tuning simplifies the phase matching to a SHG crystal.

Copper-based micro-channel coolers (Cu-MCC) are the lowest thermal-resistance heat-sinks for high-power laserdiode (LD) bars. Presently, the resistivity, pH and oxygen content of the de-ionized water coolant, must be actively controlled to minimize cooler failure by corrosion and electro-corrosion. Additionally, the water must be constantly exposed to ultraviolet radiation to limit the growth of micro-organisms that may clog the micro-channels. In this study, we report the reliable, care-free operation of LD-bars attached to Cu-MCCs, using a solution of distilledwater and ethanol as the coolant. This coolant meets the storage requirements of Mil-Std 810G, e.g. exposure to a storage temperature as low as -51°C and no growth of micro-organisms during passive storage.

In this work, coupling of radiation generated by a distributed Bragg reflector (DBR) tapered diode laser around 1064 nm into a single-mode-fiber (SMF) within a butterfly module with a footprint < 10 cm² is demonstrated. The module comprises temperature stabilizing components, a brightness maintaining micro optical assembly mounted with submicrometer precision and a standard FC/APC output connector. The aim of the introduced concept is to improve the beam quality and to eliminate the current dependent beam astigmatism, characteristic for tapered diode lasers and amplifiers, and, thus, provide an efficient, multi-Watt laser light source characterized by a narrow-band spectrum and a stigmatic, nearly Gaussian laser beam independent of the operating point. A maximum power ex SMF of 2.5 W at a coupling efficiency of 57 % is reached in the presented butterfly module.

In this paper, we present hybrid assembly technology to maximize coupling efficiency for spatially combined laser systems. High quality components, such as center-turned focusing units, as well as suitable assembly strategies are necessary to obtain highest possible output ratios. Alignment strategies are challenging tasks due to their complexity and sensitivity. Especially in low-volume production fully automated systems are economically at a disadvantage, as operator experience is often expensive. However reproducibility and quality of automatically assembled systems can be superior. Therefore automated and manual assembly techniques are combined to obtain high coupling efficiency while preserving maximum flexibility. The paper will describe necessary equipment and software to enable hybrid assembly processes. Micromanipulator technology with high step-resolution and six degrees of freedom provide a large number of possible evaluation points. Automated algorithms are necess ary to speed-up data gathering and alignment to efficiently utilize available granularity for manual assembly processes. Furthermore, an engineering environment is presented to enable rapid prototyping of automation tasks with simultaneous data ev aluation. Integration with simulation environments, e.g. Zemax, allows the verification of assembly strategies in advance. Data driven decision making ensures constant high quality, documents the assembly process and is a basis for further improvement. The hybrid assembly technology has been applied on several applications for efficiencies above 80% and will be discussed in this paper. High level coupling efficiency has been achieved with minimized assembly as a result of semi-automated alignment. This paper will focus on hybrid automation for optimizing and attaching turning mirrors and collimation lenses.

In this work, we present measurements of efficiency-optimized 940 nm diode laser bars with long resonators that are constructed with robustly passivated output facets at the Ferdinand-Braun-Institut (FBH). The measurements were performed at room temperature on a test bench developed at HiLASE Centre, as a function of operating condition. The single-diode bars generated < 1.0 kW when tested with 1 ms pulses at 1-10Hz operating frequency, corresponding to < 1 J per pulse. The maximum electrical-to-optical efficiency was < 60 %, with operating efficiency at 1 kW of < 50%, limited by the ~ 200 μΩ resistance of the bar packaging. In addition, slow axis divergence at 1 kW was below 6° FWHM and spectral width at 1 kW was below 7 nm FWHM, as needed for pumping Yb-doped solid state amplifier crystals.

High-power lasers are useful instruments suitable for applications in various fields; the most common industrial applications include cutting and welding. We propose a new application of high-power laser diodes as in-bulk heating source for food industry. Current heating processes use surface heating with different approaches to make the heat distribution more uniform and the process more efficient. High-power lasers can in theory provide in-bulk heating which can sufficiently increase the uniformity of heat distribution thus making the process more efficient. We chose two media (vegetable fat and glucose) for feasibility experiments. First, we checked if the media have necessary absorption coefficients on the wavelengths of commercially available laser diodes (940-980 nm). This was done using spectrophotometer at 700-1100 nm which provided the dependences of transmission from the wavelength. The results indicate that vegetable fat has noticeable transmission dip around 925 nm and glucose has sufficient dip at 990 nm. Then, after the feasibility check, we did numerical simulation of the heat distribution in bulk using finite elements method. Based on the results, optimal laser wavelength and illuminator configuration were selected. Finally, we carried out several pilot experiments with high-power diodes heating the chosen media.

We investigated a novel design concept of index-guided tapered LDs with linearly effective-refractive-index variation to make a quality beam in 808 nm for intermediate power LDs between a few decades of mW to ~ W. In this concept, the tapered width at each position in the propagation direction varies linearly depending on change in effective-refractive-index not geometry. We use GaAsP/InGaP/InGaAlP quantum well LD structure of 808 nm and standard LD fabrication processes to test. To design a detail structure, we use the effective-refractive-index method and transfer matrix method. The tapered ridge LD with linear effective-refractive-index variation shows more stable in beam quality but needs more study to optimize the structure.