With the advent of high power blue laser diodes in general and blue laser bars in particular new applications are emerging, utilizing this new technology. Possibly the biggest benefits compared to traditional high power diode laser wavelengths in the infrared spectral range are the improvements seen in copper welding applications, both in weld quality and overall process efficiency. A new generation of high power diode lasers with emission wavelengths near 450 nm is being developed at Coherent DILAS. These modules achieve high brightness levels combined with high power, suitable for materials processing applications. 500 W of optical output power from a 200 μm core fiber and 550 W from a 400 μm core fiber, each with an NA of 0.22, have been demonstrated. Modules are based on existing infrared product platforms already manufactured in high volume, allowing the usage of known-good processes and fully automated manufacturing equipment. At the same time, material costs are kept low, due to the large volume produced at other wavelengths. The main challenge in developing industrial grade laser modules in the blue spectral range is the required life time. While tremendous progress has been made in recent years, the chip material is still more sensitive to environmental factors compared to other high power diode laser bars. The issue is approached by Coherent DILAS in multiple ways with the goal of finding the best possible solution to minimize complexity in module design and operation, while meeting reliability requirements. Latest results, including life-test data under a variety of operating conditions, are presented.

More and more applications are using GaN laser diodes. Visible blue laser devices are well established light sources for converter based business projection of several thousand Lumens. Additional laser-based concepts like near-to-eye projection push device requirements above heretofore limits. In 2017, threshold currents of 10 and 20mA were reported for single mode blue and green laser, respectively. We will present a drastic reduction of laser threshold of green R&D laser samples by more than a factor of 2 down to 10mA. We also will discuss turn-on delay as a limiting factor for modulation speed and spatial resolution of flying spot projection. On the other side, new applications may occur in the near future. We will present research data on blue laser bars as a possible component for industrial applications like for materials processing. LIV characteristics are measured up to power levels of 107W. We observe power conversion efficiencies of 44% at 60W output power for our best samples.

The availability of high-power blue diode-lasers established a new class of laser sources for materials processing recently. With the significantly shorter wavelength compared to conventional laser sources for materials processing new applications are moving into the range of the feasible. There is a strong demand for welding applications with copper due to the change from internal combustion engines to electric drives, which even prompts laser manufacturers to find complex solutions to obtain a laser source in the wavelength range where copper shows higher absorption. With the appearance of high-power diode-laser bars in the blue wavelength range, proven optical concepts can be adapted for the setup of straightforward blue high-power diode-laser sources for materials processing. In context of the research project “BlauLas”, which is funded by the German Federal Ministry of Education and Research (BMBF) within the photonic initiative “EFFILAS” [1], Laserline, in cooperation with OSRAM, intends to realize a blue fiber-coupled cw diode-laser with a power exceeding 1 kW. Building on the results of the earlier presented 700 W fiber coupled laser source we present our new blue fiber coupled laser source with output powers surpassing 1 kW. A brief description of the optical concept and setup as well as an outlook on future strategies to increase output power and radiance of blue laser sources based on diode-laser bars are given. Additionally recently carried out application trials with this new powerful laser source are presented.

In this paper we report on further development of fiber coupled high-power diode lasers in the visible spectral range. New visible laser modules presented in this paper include the use of multi single emitter arrays @ 450 nm leading to a 120 W fiber coupled unit with a beam quality of 44 mm x mrad, as well as very compact modules with multi-W output power from 405 nm to 640 nm. However, as these lasers are based on single emitters, power scaling quickly leads to bulky laser units with a lot of optical components to be aligned. We also report on a new approach based on 450 nm diode laser bars, which dramatically reduces size and alignment effort. These activities were performed within the German government-funded project “BlauLas”: a maximum output power of 80 W per bar has been demonstrated @ 450 nm. We show results of a 200 μm NA0.22 fiber coupled 35 W source @ 450 nm, which has been reduced in size by a factor of 25 compared to standard single emitter approach. In addition, we will present a 200 μm NA0.22 fiber coupled laser unit with an output power of 135 W.

Today, infrared semiconductor lasers are used in a variety of applications in conjunction with a large range of different operating conditions. We report on improvements of different lasers, each tailored to the specific application. For cw laser bars, we report on efficiency improvements to further increase the output power beyond today's power limits for reliable operation with 250 W. For long term use under q-cw conditions, we show a very cost effective approach using a 1.5 mm cavity, capable to provide 500 W. For sensing applications we report on 200 mu m wide emitters providing 130 W of pulsed power, based on monolithically stacked laser structures.

A high-power blue laser source was long-awaited for processing materials with low absorption in the near infrared (NIR) spectral range like copper or gold. Due to the huge progress of GaN-based semiconductors, the performance of blue diode-lasers has made a major step forward recently. With the availability of unprecedented power levels at cw-operating blue diode-lasers emitting at 450 nm, it was possible to set up a high-power diode-laser in the blue spectral range to address these conventional laser applications and probably beyond that to establish completely new utilizations for lasers. Within the scope of the research project “BlauLas”, funded within the German photonic initiative “EFFILAS” [8] by the German Federal Ministry of Education and Research (BMBF), Laserline in cooperation with OSRAM aims to realize a cw fiber-coupled diode-laser exceeding 1 kW blue laser power. In this paper the conceptual design and experimental results of a 700 W blue fiber-coupled diode-laser are presented. Initially a close look had to be taken on the mounting techniques of the semiconductors to serve the requirements of the GaN laser diodes. Early samples were used for extensive long term tests to investigate degradation processes. With first functional laser-modules we set up fiber-coupled laser-systems for further testing. Besides adaption of well-known optical concepts a main task within the development of the laser system was the selection and examination of suitable materials and assembling in order to minimize degradation and reach adequate lifetimes. We realized R&D blue lasersystems with lifetimes above 5,000 h, which enable first application experiments on processing of various materials as well as experiments on conversion to white-light.

Industrial material processing like cutting or welding of metals is rather energy efficient using direct diode or diode pumped solid state lasers. However, many applications cannot be addressed by established infrared laser technology due to fundamental material properties of the workpiece: For example materials like copper or gold have too low absorption in the near infrared wavelength range to be processed efficiently by use of existing high power laser systems. The huge interest to enable high power kW systems with more suitable wavelengths in the blue spectral range triggered the German funded research project 'BLAULAS': Therein the feasibility and capability of CW operating high power laser bars based on the GaN material system was investigated by Osram and Laserline. High performance bars were enabled by defeating fundamental challenges like material quality as well as the chip processes, both of which differ significantly from well-known IR laser bars. The research samples were assembled on actively cooled heat sinks with hard solder technology. For the first time an output power of 98W per bar at 60A drive current was achieved. Conversion efficiency as high as 46% at 50W output power was demonstrated.

Pump modules for fiber lasers and fiber-coupled direct diode laser systems require laser diodes with a high beam quality. While in fast axis direction diode lasers exhibit a nearly diffraction limited output beam, the maximum usable output power is usually limited by the slow axis divergence blooming at high power levels. Measures to improve the lateral beam quality are subject of extensive research. Among the many influencing factors are the chip temperature, thermal crosstalk between emitters, thermal lensing, lateral waveguiding and lateral mode structure.We present results on the improvements of the lateral beam divergence and brightness of gain-guided mini-bars for emission at 976 nm. For efficient fiber coupling into a 200 mu m fiber with NA 0.22, the upper limit of the lateral beam parameter product is 15.5 mm mrad. Within the last years, the power level at this beam quality has been improved from 44 W to 52 W for the chips in production, enabling more cost efficient pump modules and laser systems.Our work towards further improvements of the beam quality focuses on advanced chip designs featuring reduced thermal lensing and mode shaping. Recent R&D results will be presented, showing a further improvement of the beam quality by 15%. Also, results of a chip design with an improved lateral emitter design for highest brightness levels will be shown, yielding in a record high brightness saturation of 4.8 W/mm mrad.

We present the improvements in power conversion efficiency of high power multi-mode blue laser diodes. The improved device architecture shows a peak wall plug efficiency of ~44.2 % at room temperature. Furthermore, the device delivers an optical output power reaching 3.7 W at the peak efficiency. The signs of thermal roll-over at room temperature start appearing near 5 A of injection current for this particular device design. The device generates an optical output power of more than 7.5 W with overstressed current injection at room temperature. We have also evaluated the high temperature electro-optical performance. The roll-over point is shifted by approximately 1 A when the device is operating at 85°C. The optical output power reaches 4.8 W at thermal roll-over.

The range of applications of blue and green lasers is increasing from year to year. Driving factors are costs and performance. On one hand we study the characteristics of low power R&D c-plane laser structures with improved Gaussian vertical and horizontal beam profile: We present new best values for efficiencies of single mode green lasers of 10.8% at 517nm and new long wavelength data at 532nm with efficiency of 6.5%. Furthermore, we present a new R&D design of a blue single mode laser diode with a very low threshold of 8.5mA. On the other hand, recent R&D results on broad area multi-mode power designs are shown: Efficiencies of 43% at 4W optical output power are achieved. Lifetime tests as long as 10000h are presented. High reliability is reached by a new facet design.

In 2007, DILAS proposed the approach to tailor the output beam characteristics of laser diodes to match the required beam quality of a desired target fiber, thus, drastically simplifying the coupling optics to basically only fast and slow axis collimation lenses. Over the last years, we developed and improved this tailored bar (T-Bar) concept together with the tooling for fully automated mass production of fiber-coupled T-Bar modules for fiber laser pumping as well as for direct applications. We present results on the improvement of T-Bars tailored for optimized coupling into fibers with a diameter of 200 μm with NA 0.22 corresponding to a beam parameter product of 22 mm·mrad. Cost efficient coupling to this fiber requires a tailored beam parameter product smaller than 15.5 mm·mrad in slow axis direction corresponding to a slow axis beam divergence of 7° (full angle, 95% power content) for five 100 μm wide emitters. The improved T-Bars fulfil this requirement up to an output power of 52 W with a brightness of 3.1 W/mm·mrad and a power conversion efficiency achieving 69%. This progress in the T-Bar performance together with modifications in the module design led to the increase of the reliable output power from 135 W in 2009 to 360 W in 2017 for a T-Bar module with one baseplate. We will also give a review of the main development steps and further R and D improvements.

Shortwave infrared emission from 450 nm InGaN diode lasers is analyzed, and its physical origin is located by SWIR imaging of operating devices. Emission spectra taken in the 900-1700 nm range reveal three main contributions located at 900-1130 nm, 1130-1350 nm, and beyond 1350 nm. In concert with photoluminescence measurements at the substrate, these emission bands are identified as, first, genuine deep-level electroluminescence from the active region and deep-level defect-related emission from the substrate that is pumped by spontaneous 450 nm primary emission, second, pure deep-level defect emission, and third, Planck’s black-body radiation from the entire heated device and an additional deep-level defect contribution.

We report the record capacity of 10.7 Gb/s over 100-m step-index polymer optical fiber (SI-POF) with 1-mm core diameter over six WDM channels in visible wavelength range.

Surface morphology changes and transient reflectance changes at diode laser facets are monitored during the catastrophic optical damage (COD) process in a single pulse operation. Time-resolved micro-reflectance spectroscopy with a streak-camera (time resolution similar to 20 ns) allows us to observe the creation sequence of up to four distinct degradation seed points at a device facet within < 300 ns. The shape of the COD seeds is created within < 30-40 ns. Creation of non-planar facet areas by local melting represents the main mechanism behind the observed reflectivity changes. Subsequently the surface temperature decreases within the pulse which caused the COD.

In this paper we report recent developments on high power blue laser chips. Reduction of internal losses as well as optimized thermal management had been essential to increase optical output power. R&D samples with average performance of 3W optical output at junction temperatures of 130 degrees C are demonstrated. The chips are suitable for use in a novel multi chip housing: For the first time up to 20 blue laser chips have been packaged into one compact housing resulting in the first InGaN laser device with optical output >50W. The highly integrated package offers a unique small size. The outer dimensions of the package are 25.5mm x 35mm with an emitting surface of 16mm x 16.5mm. Therefore the complexity of optical alignment is dramatically reduced and only a single sheet multi lens array is required for beam collimation. Besides the unique technical performance the multi-die package offers significantly lower assembly costs because of the reduced complexity and assembly time. The butterfly package contains 4 bars with up to 5 multimode laser chips in series connection on each bar operating at 2.3A. The typical module wavelength is 450nm +/-10nm. At a case temperature of 50 degrees C the R&D samples achieve efficiencies of typ. 30% and an optical output power of 50W corresponding to an electrical power consumption of similar to 165W. This new technology can be used for high performance light engines of high brightness projectors.

We present improvements of the lateral beam divergence and brightness of gain-guided mini-bars for emission at 976nm at highest brightness levels. The beam characteristics of devices in production are tailored for optimized fiber coupling to fiber diameter 200μm and numerical aperture 0.22, corresponding to a beam parameter product of 22 mm mrad. Cost-efficient coupling to this fiber requires a beam parameter product tailored to 15 mm mrad in lateral direction. The corresponding devices feature 5 emitters with cavity length 4mm and a lateral electrical contact width of 100μm per emitter. These bars can be driven to 44W before exceeding the beam parameter product of 15 mm mrad, corresponding to a slow axis beam divergence of 7° and a linear brightness of 2.9 W/mm mrad. In this work, we demonstrate results of two new mini-bar designs. The first design is intended for cost-reduction of divergence limited coupling systems and improves the output power at the slow axis divergence limit of 7° from 44W to 52W. The second structure, designed for coupling to fibers with beam parameter products as low as 11 mm mrad, exhibits significantly higher beam quality and excellent maximum linear brightness of 4.5 W/mm mrad, albeit at lower output power. All the structures exhibit power conversion efficiencies approaching 70%.

Catastrophic optical damage (COD) in 450 nm emitting InGaN/GaN diode lasers is artificially provoked by applying single sub-microsecond current pulses of increasing amplitude. Studying a batch of devices in which COD does not represent the main degradation mode, we find that COD is a ‘hot’ process. It becomes re-ignited in subsequent pulses. During the process, the spatial filamentation changes abruptly and the outer appearance of the damage pattern is predominantly created within the initial pulse. The process can cause material ejection out of the front facet as shown by thermography.