Blue high-power semiconductor lasers have increased greatly in performance over the recent decade enabling new application fields from high brightness projection up to materials processing beyond 1000W output power systems. Base for best system performance is optimal chip design and reliability of the semiconductor device. In this paper chip design optimization of blue high-power semiconductor laser bars will be shown: In contrast to IR laser bars with high lateral emitter fill factors beyond 50%, optimum design with maximum output power and efficiency for GaN laser bars is currently at very low fill factors in the range of 10%. Laser bar designs ranging from 5% fill factor up to 12.5% fill factor were fabricated and investigated. Additionally, two different emitter pitches with 200μm and 400μm were compared. The design with an emitter width of 30μm and a pitch of 400μm resulted in overall best performance. Additionally, lifetime investigations of single emitters in TO-packages will be discussed. The laser diodes were tested up to 5000h duration at different conditions in operating temperatures ranging from 64°C to 96°C and output power up to 3.5W. Dominating degradation mechanism is wear-out which is accelerated by optical output power and additional thermal activation. Extrapolation of the test results in combination with an acceleration model points towards a median lifetime of up to 65.000h for 25°C operation.
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” , 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.
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.
Except for their primary emission, diode lasers frequently show emissions at lower photon energies. We present a study in which we record and analyze emission images of (In,Ga,Al)N-based 450 nm emitting diode lasers. Imaging is realized in the spectral ranges of two broad secondary emission bands, which are peaking in the yellow region at 580 nm (VIS) and in the infrared at 875 nm (IR). Both bands have their principal origin in the active region of the device. The VIS emission spectrum looks like the well-known yellow GaN-emission, but comes exclusively from the active region. It is very likely an electroluminescence that involves trapping of non-equilibrium carriers into defects located in the active region, followed by radiative recombination under emission of VIS photons. The IR emission involves also emission from the active region, but significant contributions are also observed in the substrate. The latter contribution could be generated by absorption of spontaneous primary emissions there. Moreover, we modelled emission images by raytracing. This allows the determination of absorption coefficients and refractive indexes of the active region, the unpumped epitaxial layer, and the substrate. The VIS signal from the active region proved to be proportional to the nonequilibrium carrier concentration. This makes it potentially interesting for analytical purposes, e.g., the imaging of carrier concentration profiles.
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.