This PDF file contains the front matter associated with SPIE Proceedings Volume 9733, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

Direct diode laser systems gain importance in the fields of material processing and solid-state laser pumping. With increased output power, also the influence of strong optical feedback has to be considered. Uncontrolled optical feedback is known for its spectral and power fluctuation effects, as well as potential emitter damage. We found that even intended feedback by use of volume Bragg gratings (VBG) for spectral stabilization may result in emitter lifetime reduction. To provide stable and reliable laser systems design, guidelines and maximum feedback ratings have to be found. We present a model to estimate the optical feedback power coupled back into the laser diode waveguide. It includes several origins of optical feedback and wide range of optical elements. The failure thresholds of InGaAs and AlGaAs bars have been determined not only at standard operation mode but at various working points. The influence of several feedback levels to laser diode lifetime is investigated up to 4000h. The analysis of the semiconductor itself leads to a better understanding of the degradation process by defect spread. Facet microscopy, LBIC- and electroluminescence measurements deliver detailed information about semiconductor defects before and after aging tests. Laser diode protection systems can monitor optical feedback. With this improved understanding, the emergency shutdown threshold can be set low enough to ensure laser diode reliability but also high enough to provide better machine usability avoiding false alarms.

The degradation behaviors of 980-nm emitting nominally identical single-spatial mode lasers are studied during continuous wave long-term operation and during single-pulse stress tests. Both tests activate internal catastrophic optical damage as sudden degradation mechanism limiting the device lifetime. In case of high power stress-testing, the mechanism that initializes this effect is a spatial widening of the optical mode, resulting in increased absorption outside the waveguide. A similar disturbance to the optical mode is caused by defects that are generated during long-term operation. Thus two very different aging regimes eventually result in the same degradation scenario. We find that single-pulse stress-testing allows for activation of several degradation mechanisms in a device one after the other. Moreover, it becomes possible to distinguish between effects induced by gradual degradation and such that are independent on operation time. Thus stress-testing is considered a complementary tool, which might pave the way towards more economic device testing.

High power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well (QW) lasers are critical components for both telecommunications and potential space satellite communications systems. However, little has been reported on failure modes of state-of-the-art SM InGaAs-AlGaAs strained QW lasers although it is crucial to understand failure modes and underlying degradation mechanisms in developing these lasers that meet lifetime requirements for space satellite systems, where extremely high reliability of these lasers is required. Our present study addresses the aforementioned issues by performing long-term life tests under different test conditions followed by failure mode analysis (FMA) and physics of failure investigation. We performed long-term accelerated life-tests on state-of-the-art SM and MM InGaAs-AlGaAs strained QW lasers under ACC (automatic current control) mode. Our life-tests have accumulated over 25,000 test hours for SM lasers and over 35,000 test hours for MM lasers. FMA was performed on failed SM lasers using electron beam induced current (EBIC). This technique allowed us to identify failure types by observing dark line defects. All the SM failures we studied showed catastrophic and sudden degradation and all of these failures were bulk failures. Our group previously reported that bulk failure or COBD (catastrophic optical bulk damage) is the dominant failure mode of MM InGaAs-AlGaAs strained QW lasers. To the best of our knowledge, this is the first report demonstrating that the dominant failure mode of both SM and MM InGaAs-AlGaAs strained QW lasers is the bulk failure. Since degradation mechanisms responsible for COBD are still not well understood, we also employed other techniques including focused ion beam (FIB) processing and high-resolution TEM to further study dark line defects and dislocations in post-aged SM and MM lasers. Our long-term life test results and FMA results are reported.

Reliabilities of the 638-nm triple emitter broad area laser diode (BA-LD) with the window-mirror structure were studied. Methodology to estimate mean time to failure (MTTF) due to catastrophic optical mirror degradation (COMD) in reasonable aging duration was newly proposed. Power at which the LD failed due to COMD (P_COMD) was measured for the aged LDs under the several aging conditions. It was revealed that the P_COMD was proportional to logarithm of aging duration, and MTTF due to COMD (MTTF(COMD)) could be estimated by using this relation. MTTF(COMD) estimated by the methodology with the aging duration of approximately 2,000 hours was consistent with that estimated by the long term aging. By using this methodology, the MTTF of the BA-LD was estimated exceeding 100,000 hours under the output of 2.5 W, duty cycles of 30% .

Cathodoluminescence (CL) analysis of high power laser diodes permits to reveal the main defects issued from the catastrophic optical degradation (COD). These defects are revealed as discontinuous dark lines along the ridge. The different levels of damage are analysed, and a thermomechanical model taking account of the thermal and mechanical properties of the laser structure is settled up. In this model the COD is described as a local temperature enhancement, which generates thermal stresses leading to the generation of dislocations, which are responsible for the degradation of the thermal conductivity of the of the active zone of the laser.

We report on our progress developing long wavelength high power laser diodes based on the InGaAsP/InP alloy system emitting in the range from 1400 to 2010 nm. Output power levels exceeding 50 Watts CW and 40% conversion efficiency were obtained at 1470 nm wavelength from 20% fill factor (FF) bars with 2 mm cavity length mounted on water cooled plates. Using these stackable plates we built a water cooled stack with 8 bars, successfully demonstrating 400 W at 1470 nm with good reliability. In all cases the maximum conversion efficiency was greater than 40% and the maximum power achievable was limited by thermal rollover. For lasers emitting in the range from 1930 to 2010 nm we achieved output power levels over 15 W and 20 % conversion efficiency from 20% FF bars with 2 mm cavity length on a conductively cooled platform. Life testing of the 1470 nm lasers bars over 14,000 hours under constant current mode has shown no significant degradation.

A novel, 9XX nm fiber-coupled module using arrays of highly reliable laser diode bars has been developed. The module is capable of multi-kW output power in a beam parameter product of 80 mm-mrad. The module incorporates a hard-soldered, isolated stack package compatible with tap-water cooling. Using extensive, accelerated multi-cell life-testing, with more than ten million device hours of test, we have demonstrated a MTTF for emitters of >500,000 hrs. In addition we have qualified the module in hard-pulse on-off cycling and stringent environmental tests. Finally we have demonstrated promising results for a next generation 9xx nm chip design currently in applications and qualification testing

High-brightness laser diode module over 300 W with 100 μm core/NA 0.22 fiber has been developed by integrating the several tens of optimally designed single emitter laser diodes in a newly designed package. We employed the Asymmetric Decoupled Confinement Heterostructure (ADCH) and the wide strip width to increase the durability for the catastrophic optical damage. High fiber-coupling efficiency was obtained with the uniquely designed micro-optical system. In addition, low thermal resistance made it possible to operate higher power. As a result, 300 W power was achieved without thermal rollover at 15.5 A with significantly high reliability. The high-brightness modules have a great advantage for high power fiber lasers such as 10 kW and beyond.

The demand for high brightness fiber coupled diode laser devices in the multi kW power region is mainly driven by industrial applications for materials processing, like brazing, cladding and metal welding, which require a beam quality better than 30 mm x mrad and power levels above 3kW. Reliability, modularity, and cost effectiveness are key factors for success in the market.

We have developed a scalable and modular diode laser architecture that fulfills these requirements through use of a simple beam shaping concept based on two dimensional stacking of tailored diode bars mounted on specially designed, tap water cooled heat sinks. The base element of the concept is a tailored diode laser bar with an epitaxial and lateral structure designed such that the desired beam quality in slow-axis direction can be realized without using sophisticated beam shaping optics. The optical design concept is based on fast-axis collimator (FAC) and slow-axis collimator (SAC) lenses followed by only one additional focusing optic for efficient coupling into a 400 μm fiber with a numerical aperture (NA) of 0.12.

To fulfill the requirements of scalability and modularity, four tailored bars are populated on a reduced size, tap water cooled heat sink. The diodes on these building blocks are collimated simply via FAC and SAC. The building blocks can be stacked vertically resulting in a two-dimensional diode stack, which enables a compact design of the laser source with minimum beam path length. For a single wavelength, up to eight of these building blocks, implying a total of 32 tailored bars, can be stacked into a submodule, polarization multiplexed, and coupled into a 400 μm, 0.12NA fiber. Scalability into the multi kW region is realized by wavelength combining of replaceable submodules in the spectral range from 900 – 1100 nm. We present results of a laser source based on this architecture with an output power of more than 4 kW and a beam quality of 25 mm x mrad.

A multi-mode 9XXnm-wavelength laser diode was developed to optimize the divergence angle and reliable ex-facet power. Lasers diodes were assembled into a multi-emitter pump package that is fiber coupled via spatial and polarization multiplexing. The pump package has a 135μm diameter output fiber that leverages the same optical train and mechanical design qualified previously. Up to ~ 270W CW power at 22A is achieved at a case temperature ~ 30ºC. Power conversion efficiency is 60% (peak) that drops to 53% at 22A with little thermal roll over. Greater than 90% of the light is collected at < 0.12NA at 16A drive current that produces 3.0W/(mm-mr)² radiance from the output fiber.

There is an increasing demand for high-power, high-brightness diode lasers from 8xx nm to 9xx nm for applications such as fiber laser pumping, materials processing, solid-state laser pumping, and consumer electronics manufacturing. The kilowatt CW fiber laser pumping (915 nm - 976 nm), in particular, requires the diode lasers to have both high power and high brightness in order to achieve high-performance and reduced manufacturing costs. This paper presents continued progress in the development of high brightness fiber-coupled product platform, element^TM. Further brightness improvement and power-scaling have been enabled by both the rise in chip brightness as well as the increase in number of chips used to couple into a given numerical aperture. We have developed a new generation of high power broad area laser known as reduced-mode diode (REM-diode) which suppresses many of the higher order modes in the slow axis and reduces divergence up to two times at the same operating conditions. To date, we have achieved slow-axis brightness as high as 4.3 W/mm-mrad for devices with thermal resistance of ~2.5 C/W. As a result, we have achieved >75 watts from a 1×6 element^TMin the 9xx nm spectral range; and 177 watts of peak power from a 2×6 element^TM. We have also improved our optics for fiber-coupling which accommodates 7 emitters per polarization in the same numerical aperture. Using this configuration, we project 200 watts of peak power from a 2×7 element^TM with a reliable product at 176 W of power from 105 μm and 0.15 NA fiber. REM-diodes can also be wavelength stabilized using VBGs. The reliability of REM-diodes are equal or better than broad area lasers (BALs). We present current status on ongoing reliability assessment of chip-on-submount.

Due to their low quantum defect, diode pumped alkali metal vapor lasers (DPALs) offer the promise of scalability to very high average power levels while maintaining excellent beam quality. Research on DPALs has progressed to ever increasing power levels across multiple gain media species over the last years, necessitating pump power in the kW range. Each material requires a specific pump wavelength: near 852nm for cesium, 780nm for rubidium, 766nm for potassium, and 670nm for lithium atoms. The shorter pump wavelength below 800nm are outside the typical wavelength range for pump diodes developed for diode pumped solid state lasers (DPSS).

The biggest challenge in pumping these materials efficiently is the need for maintaining the narrow gain media absorption band of approximately 0.01nm while greatly increasing power. Typical high power diode lasers achieve spectral widths around 3nm (FWHM) in the near infrared spectrum, but optical gratings may be used internal or external to the cavity to reduce the spectral width. Recently, experimental results have shown yet narrower line widths ranging from picometers at very low power levels to sub-100 picometers for water cooled stacks around 1kW of output power.

The focus of this work is the development of a fiber-based pump system for potassium DPAL. The individual tasks are the development of high power 766nm chip material, a fiber-coupled module as a building block, and a scalable system design to address power requirements from hundreds of watts to tens of kilowatts. Results for a 3kW system achieving ~30GHz bandwidth at 766nm will be shown. Approaches for power-scaling and size reduction will be discussed.

Spectral beam combination expands the output power while keeps the beam quality of the combined beam almost the same as that of a single emitter. Spectral beam combination has been successfully achieved for high power fiber lasers, diode laser arrays and diode laser stacks. We have recently achieved the spectral beam combination of multiple single emitter diode lasers. Spatial beam combination and beam transformation are employed before beams from 25 single emitter diode lasers can be spectrally combined. An average output power about 220W, a spectral bandwidth less than 9 nm (95% energy), a beam quality similar to that of a single emitter and electro-optical conversion efficiency over 46% are achieved.

In this paper, Rigorous Coupled Wave analysis is used to numerically evaluate the influence of emitter width, emitter pitch and focal length of transform lens on diffraction efficiency of the grating and spectral bandwidth.

To assess the chance of catastrophic optical mirror damage (COMD), the optical power in the internal cavity of a free running emitter and the optical power in the grating external cavity of a wavelength locked emitter are theoretically analyzed.

Advantages and disadvantages of spectral beam combination are concluded.

The modular concept of DirectPhotonics laser systems is a big advantage regarding its manufacturability, serviceability as well as reproducibility. By sticking to identical base components an economic production allows to serve as many applications as possible while keeping the product variations minimal. The modular laser design 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 several diodes in fast axis. This can be theoretically done until the combined fast axis beam quality is on a comparable level as the individual diodes slow axis beam quality without loosing overall beam performance after fiber coupling. Those stacked individual 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 100 W with BPP of <3.5 mm*mrad (FA) and <5 mm*mrad (SA). In the next steps, further power scaling is accomplished by polarization and wavelength multiplexing yielding high optical efficiencies of more than 80% and resulting in a building block module with about 500 W launched into a 100 μm fiber with 0.15 NA. Higher power levels can be achieved by stacking those building blocks using the very same dense spectral combing technique up to multi kW Systems without further reduction of the BPP.

The 500 W building blocks are consequently designed in a way that they feature a high flexibility with regard to their emitting wavelength bandwidth. Therefore, new wavelengths can be implemented by only exchanging parts and without any additional change of the production process. This design principal theoretically offers the option to adapt the wavelength of those blocks to any applications, from UV, visible into the far IR as long as there are any diodes commercially available. This opens numerous additional applications like laser pumping, scientific applications, materials processing such as cutting and welding of copper aluminum or steel and also medical application.

Typical operating at wavelengths in the 9XX nm range, these systems are designed for and mainly used in cutting and welding applications, but adapted wavelength ranges such as 793 nm and 1530 nm are also offered. Around 15XX nm the diodes are already successfully used for resonant pumping of Erbium lasers [1].

Furthermore, the fully integrated electronic concept allows addressing further applications, as due to short lead lengths it is capable of generating very short μs pulses up to cw mode operation by simple software commands.

We present a compact, modular and cross talk free approach for dense wavelength division multiplexing of high power diode lasers based on ultra-steep dielectric filters. The mini bars consist of 5 narrow stripe broad area emitters with a beam parameter product in the range of 2 mm mrad and a wavelength spacing of 2.5 nm between 2 adjacent emitters. Experimental results for fiber coupling (35 μm core diameter, NA < 0.2) of internally and externally stabilized diode lasers are presented. Optical losses are analyzed and alternative optical designs to overcome the current limitations of the setup are discussed.

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

Wavelength-stabilized high-brightness single emitters are commonly used in fiber-coupled laser diode modules for pumping Yb-doped lasers at 976 nm, and Nd-doped ones at 808 nm. We investigate the spectral behavior of single emitters under wavelength-selective feedback from a volume Bragg (or hologram) grating (VBG) in a multi-emitter module.

By integrating a full VBG model as a multi-layer thin film structure with commercial raytracing software, we simulated wavelength locking conditions as a function of beam divergence and angular alignment tolerances. Good correlation between the simulated VBG feedback strength and experimentally measured locking ranges, in both VBG misalignment angle and laser temperature, is demonstrated.

The challenges of assembling multi-emitter modules based on beam-stacked optical architectures are specifically addressed, where the wavelength locking conditions must be achieved simultaneously with high fiber coupling efficiency for each emitter in the module. It is shown that angular misorientation between fast and slow-axis collimating optics can have a dramatic effect on the spectral and power performance of the module.

We report the development of our NEON-S wavelength-stabilized fiber laser pump module, which uses a VBG to provide wavelength-selective optical feedback in the collimated portion of the beam. Powered by our purpose-developed high-brightness single emitters, the module delivers 47 W output at 11 A from an 0.15 NA fiber and a 0.3 nm linewidth at 976 nm. Preliminary wavelength-locking results at 808 nm are also presented.

Multiplexing technologies enable the development of high-brightness diode lasers for direct industrial applications. We present a High-Power Dense Wavelength Division Multiplexer (HP-DWDM) with an average channel spacing of 1.7 (1.5) nm and a subsequent external cavity mirror to provide feedback for frequency stabilization and multiplexing in one step. The "self-optimizing" multiplexing unit consists of four reflective Volume Bragg Gratings (VBGs) with 99% diffraction efficiency and seven dielectric mirrors to overlay the radiation of five input channels with an adjustable channel spacing of 1-2 nm. In detail, we focus on the analysis of the overall optical efficiency, the change of the beam parameter product and the spectral width. The performance is demonstrated using five 90 μm multimode 9xx single emitters with M²≤17. Because of the feedback the lateral (multimodal) spatial and angular intensity distribution changes strongly and the beam parameter product decreases by a factor of 1.2 to 1.9. Thereby the angular intensity distribution is more affected than the width of the beam waist. The spectral width per emitter decreases to 3-200 pm (FWHM) depending on the injection current and the reflectance of the feedback mirror (0.75%, 1.5%, 4%, 6% or 8%). The overall optical multiplexing efficiency ranges between 77% and 86%. With some modifications (e.g. enhanced AR-coatings) we expect 90-95%.

Both high-energy-class laser facilities and commercial high-energy pulsed laser sources require reliable optical pumps with the highest pulse power and electro-optical efficiency. Although commercial quasi-continuous wave (QCW) diode laser bars reach output powers of 300…500 W further improvements are urgently sought to lower the cost per Watt, improve system performance and reduce overall system complexity. Diode laser bars operating at temperatures of around 200 K show significant advances in performance, and are particularly attractive in systems that use cryogenically cooled solid state lasers. We present the latest results on 940 nm, passively cooled, 4 mm long QCW diode bars which operate under pulse conditions of 1.2 ms, 10 Hz at an output power of 1 kW with efficiency of 70% at 203 K: a two-fold increase in power compared to 300 K, without compromising efficiency. We discuss how custom low-temperature design of the vertical layers can mitigate the limiting factors such as series resistance while sustaining high power levels. We then focus on the remaining obstacles to higher efficiency and power, and use a detailed study of multiple vertical structures to demonstrate that the properties of the active region are a major performance limit. Specifically, one key limit to series resistance is transport in the layers around the active region and the differential internal efficiency is closely correlated to the threshold current. Tailoring the barriers around the active region and reducing transparency current density thus promise bars with increased performance at temperatures of 200 K as well as 300 K.

Laser stacks emitting short light pulses are ideally suited for medical and cosmetic applications. Developing enhanced, stable and reliable assembly processes, Jenoptik is reaching for higher energy densities and lower manufacturing costs. In this paper an improved technology for actively cooled QCW stacks is presented. Based on simulations and experimental data, the impacts on the laser stack performance are described and shown as power-current and thermal impedance plots. We show that the bar-to-bar pitch can be reduced from 1.7 mm to 1.2 mm without detrimental thermal effects for pulse durations up to 100 ms.

GaAs-based high-power broad-area diode lasers deliver optical output powers P_opt > 10W with efficiency > 60%. However, their application is limited due to poor in-plane beam parameter product BPP_lat=0.25×Θ_95%×w_95% (Θ_95% and w_95% 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 BPP_lat 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 BPP_lat 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 BPP_lat 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 BPP_lat, 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.

Tilted Wave Lasers (TWLs) based on optically coupled thin active waveguide and thick passive waveguide offer an ultimate solution for thick–waveguide diode laser, preventing catastrophic optical mirror damage and thermal smile in laser bars, providing robust operation in external cavity modules thus enabling wavelength division multiplexing and further increase in brightness enabling direct applications of laser diodes in the mainstream material processing. We show that by proper engineering of the waveguide one can realize high performance laser diodes at different tilt angles of the vertical lobes. Two vertical lobes directed at various angles (namely, +/–27° or +/–9°) to the junction plane are experimentally realized by adjusting the compositions and the thicknesses of the active and the passive waveguide sections. The vertical far field of a TWL with the two +/–9° vertical beams allows above 95% of all the power to be concentrated within a vertical angle below 25°, the fact which is important for laser stack applications using conventional optical coupling schemes. The full width at half maximum of each beam of the value of 1.7° evidences diffraction– limited operation. The broad area (50 μm) TWL chips at the cavity length of 1.5 mm reveal a high differential efficiency ~90% and a current–source limited pulsed power >42W for as–cleaved TWL device. Thus the power per facet length in a laser bar in excess of 8.4 kW/cm can be realized. Further, an ultimate solution for the smallest tilt angle is that where the two vertical lobes merge forming a single lobe directed at the zero angle is proposed.

Paper reports the DBR-RWG surface grating design, the fabrication process, and the output characteristics of tapered DBR laser diodes for the applications, like for example LIDAR and range finding, that require eye-safe high-power single-mode coherent light sources. The fabricated regrowth-free DBR AlGaInAs/InP lasers exhibited a CW output power as high as 560 mW in single-mode operation at room temperature. At maximum output power the SMSR was 38 dB, proving the excellent behavior of the surface gratings. The tapered section enabled scaling the maximum CW power at room temperature from 125 mW to 560 mW, by increasing its length from 0.5 mm to 4.0 mm. The paper discusses the limitations and performance variation associated to the power scaling by using the tapered section length as a scaling parameter.

Single emitters emitting at a wavelength around 808 nm are highly-desired as pump sources of low power solid state lasers. The latest development of high-power single emitters having emitter apertures of 95 μm, 100 μm and 200 μm based on TM-polarized material are presented. The devices were characterized up to their maximum optical output powers of 15 W from 100 μm wide emitters and 22 W from 200 μm wide emitters. The operation point of these devices is set to 7 W and 10 W, respectively with wall-plug efficiencies of ~55%. Furthermore first results on 5- emitters bars operating at 40 W are presented.

The demand of high power diode lasers in the range of 910-980nm is regularly growing. This kind of device for many applications, such as fiber laser pumping [1], material processing [1], solid-state laser pumping [1], defense and medical/dental. The key role of this device lies in the efficiency (𝜂_𝐸) of converting input electrical power into output optical power. The high value of 𝜂_𝐸 allows high power level and reduces the need in heat dissipation. The requirement of wavelength stabilization with temperature is more obvious in the case of multimode 975nm diode lasers used for pumping Yb, Er and Yb/Er co-doped solid-state lasers, due to the narrow absorption line close to this wavelength. Such spectral width property (<1 nm), combined with wavelength thermal stabilization (0.07 𝑛𝑚 ∙ °𝐶⁻¹), provided by a uniform distributed feedback grating (DFB) introduced by etching and re-growth process techniques, is achievable in high power diode lasers using optical feedback. This paper reports on the development of the diode laser structure and the process techniques required to write the gratings taking into account of the thermal dissipation and optical performances. Performances are particularly determined in terms of experimental electro-optical characterizations. One of the main objectives is to determine the thermal resistance of the complete assembly to ensure the mastering of the diode laser temperature for operating condition. The classical approach to determine junction temperature is based on the infrared thermal camera, the spectral measurement and the pulse electrical method. In our case, we base our measurement on the spectral measurement but this approach is not well adapted to the high power diodes laser studied. We develop a new measurement based on the pulse electrical method and using the T3STER© equipment. This method is well known for electronic devices and LEDs but is weakly developed for the high power diodes laser. This crucial measurement compared to spectral one is critical for understand the thermal management of diode laser device and improve the structure based on design for reliability. To have a perfect relation between structure, and their modification, and temperature, FEM simulations are performed using COMSOL software. In this case, we can understand the impact of structure on the isothermal distribution and then reveal the sensitive zones in the diode laser. To validate the simulation, we compare the simulation results to the experimental one and develop an analytical model to determine the different contributions of the thermal heating. This paper reports on the development laser structure and the process techniques required to write the gratings. Performances are particularly characterized in terms of experimental electro-optical characterization and spectral response. The extraction of thermal resistance (R_th) is particularly difficult, because of the implicit low value (R_th ≈ 2𝐾/𝑊) and the multimodal nature of the diode laser. In such a context, thermal resistance has been measured using a dedicated equipment namely T3STER©. The results have been compared with those given by the well-known technique achieved from the spectrum of the diode laser (central wavelength variations vs temperature) that is more difficult to apply for multimodal diodes laser. The last section deals with thermal simulations based on finite elements method (FEM) modeling in order to estimate junction temperature . This study represent a significant part of the general Design for Reliability (DfR) effort carried out on such devices to produce efficient and reliable high power devices at the industrial level.

Key applications for 780-830nm high power diode lasers include the pumping of various gas, solid state, and fiber laser media; medical and aesthetic applications including hair removal; direct diode materials processing; and computer-to-plate (CtP) printing. Many of these applications require high brightness fiber coupled beam delivery, in turn requiring high brightness optical output at the bar and chip level. Many require multiple bars per system, with aggregate powers on the order of kWs, placing a premium on high power and high power conversion efficiency. This paper presents Coherent’s recent advances in the production of high power, high brightness, high efficiency bars and chips at 780-830nm. Results are presented for bars and single emitters of various geometries. Performance data is presented demonstrating peak power conversion efficiencies of 63% in CW mode. Reliability data is presented demonstrating <50k hours lifetime for products including 60W 18% fill factor and 80W 28% fill factor conduction cooled bars, and <1e9 shots lifetime for 500W QCW bars.

Continuing improvements in the cost and power of laser diodes have been critical in launching the emerging fields of power over fiber (PoF), and laser power beaming. Laser power is transmitted either over fiber (for PoF), or through free space (power beaming), and is converted to electricity by photovoltaic cells designed to efficiently convert the laser light. MH GoPower’s vertical multi-junction (VMJ) PV cell, designed for high intensity photovoltaic applications, is fueling the emergence of this market, by enabling unparalleled photovoltaic receiver flexibility in voltage, cell size, and power output. Our research examined the use of the VMJ PV cell for laser power transmission applications. We fully characterized the performance of the VMJ PV cell under various laser conditions, including multiple near IR wavelengths and light intensities up to tens of watts per cm². Results indicated VMJ PV cell efficiency over 40% for 9xx nm wavelengths, at laser power densities near 30 W/cm². We also investigated the impact of the physical dimensions (length, width, and height) of the VMJ PV cell on its performance, showing similarly high performance across a wide range of cell dimensions. We then evaluated the VMJ PV cell performance within the power over fiber application, examining the cell’s effectiveness in receiver packages that deliver target voltage, intensity, and power levels. By designing and characterizing multiple receivers, we illustrated techniques for packaging the VMJ PV cell for achieving high performance (> 30%), high power (> 185 W), and target voltages for power over fiber applications.

High power VCSEL systems are a novel laser source used for thermal treatment in industrial manufacturing. These systems will be applied in many applications, which have not used a laser source before. This is enabled by the unique combination of efficiency, compactness and robustness. High power VCSEL system technology encompasses elements far beyond the VCSEL chip itself: i.e. heat sinks, bonding technology and integrated optics. This paper discusses the optimization of these components and processes specifically for building high-power laser systems with VCSEL arrays. New approaches help to eliminate components and process steps and make the system more robust and easier to manufacture.

New cooler concepts with integrated electrical and mechanical interfaces have been investigated and offer advantages for high power system design.

The bonding process of chips on sub-mounts and coolers has been studied extensively and for a variety of solder materials. High quality of the interfaces as well as good reliability under normal operation and thermal cycling have been realized. A viable alternative to soldering is silver sintering. The very positive results which have been achieved with a variety of technologies indicate the robustness of the VCSEL chips and their suitability for high power systems.

Beam shaping micro-optics can be integrated on the VCSEL chip in a wafer scale process by replication of lenses in a polymer layer. The performance of VCSEL arrays with integrated collimation lenses has been positively evaluated and the integrated chips are fully compatible with all further assembly steps.

The integrated high power systems make the application even easier and more robust. New examples in laser material processing and pumping of solid state lasers are presented.