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.
In this paper we present a simple approach to achieving nanosecond pulses from a directly q-switched high-power resonator based on extra-large mode area (XLMA) fibers with a beam quality factor M<sup>2</sup> < 15. An average output power of > 500 W has been demonstrated for repetition frequencies between 50-100 kHz. The resonator consists of a single fiber q-switched with soldered Pockels-cells which exhibit a very high contrast ratio leading to output pulses down to about 10 ns and peak powers up to > 250 kW at 1064 nm wavelength. <p> </p>By using this design instead of a fiber MOPA setup, a cost-effective and less complex system could be implemented.
In surface processing applications the correlation of laser power to processing speed demands a further enhancement of the performance of short-pulsed laser sources with respect to the investment costs. The frequently applied concept of master oscillator power amplifier relies on a complex structure, parts of which are highly sensitive to back reflected amplified radiation. Aiming for a simpler, robust source using only a single ytterbium doped XLMA fiber in a q-switched resonator appears as promising design approach eliminating the need for subsequent amplification. This concept requires a high power-tolerant resonator which is provided by the multikilowatt laser platform of Laserline including directly water-cooled active fiber thermal management. <p> </p>Laserline GmbH and Fraunhofer Institute for Laser Technology joined their forces<sup>1</sup> to upgrade standard high power laser sources for short-pulsed operation exceeding 1 kW of average power. Therefor a compact, modular qswitch has been developed. <p> </p>In this paper the implementation of a polarization independent q-switch into an off-the-shelf multi-kilowatt diodepumped continuous wave fiber source is shown. In this early step of implementation we demonstrated more than 1000 W of average power at pulse lengths below 50 ns FWHM and 7.5 mJ pulse energy. The M<sup>2</sup> corresponds to 9.5. Reliability of the system is demonstrated based on measurements including temperature and stability records. We investigated the variation possibilities concerning pulse parameters and shape as well as upcoming challenges in power up-scaling.
Proc. SPIE. 10514, High-Power Diode Laser Technology XVI
KEYWORDS: Semiconductors, High power lasers, Copper, Materials processing, Laser applications, Semiconductor lasers, High power diode lasers, Fiber coupled lasers, Laser systems engineering, High power fiber coupled lasers
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. <p> </p>Within the scope of the research project “BlauLas”, funded within the German photonic initiative “EFFILAS”  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. <p> </p>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.
XLMA fibers based on Yb-doped bulk silica possess an excellent refractive index and doping level homogeneity . To
achieve the highest optical-to-optical efficiency and long-term operation without degradation we simulated the effect of the brightness conversion factor of different core dopant compositions of such XLMA fibers. We also investigated the beam quality of a multi-kW single XLMA fiber laser system and its long-term stability. The current state-of-the-art
XLMA single fiber laser has 5 kW maximum output power and a degradation rate of about 0.5 % / 500 h at 4 kW
measured over a period of 1700 h. Several application tests demonstrate the excellent performance of the XLMA fiber
Fiber laser based brightness converters enable diode laser beam sources to access a superior beam quality of better than 10 mm × mrad in combination with multi kW output power. A design of a fiber laser that is based on a single active optical converter fiber that is pumped by a direct diode is presented. Due to the high transfer efficiency of such brightness converters an electrical/optical efficiency > 25% can be achieved. The current status with an output power > 4 kW in combination with a beam quality of < 5 mm × mrad will be described. The principal design of such diode laser based fiber brightness converters will be presented and building blocks of such lasers will be outlined. As an application example laser welding will be presented of both the fiber converter laser and direct diode laser using optical light guides with identical core diameters on both lasers for comparison. Additionally, fibers with a core diameter of 200μm will be used on the fiber converter laser to perform remote welding. The weld results will be compared regarding welding depth and surface quality of the weld samples to determine the optimum power/brightness levels for different aluminum and steel materials.
The quality of Yb-doped fused bulk silica produced by sintering of Yb-doped fused silica granulates has improved
greatly in the past five years [1 - 4]. In particular, the refractive index and doping level homogeneity of such materials
are excellent and we achieved excellent background fiber attenuation of the active core material down to about 20 dB/km
at 1200 nm. The improvement of the Yb-doped fused bulk silica has enabled the development of multi-kW fiber laser
systems based on a single extra large multimode laser fiber (XLMA fiber).
When a single active fiber is used in combination with the XLMA multimode fiber of 1200 μm diameter simple and
robust high power fiber laser setups without complex fiber coupling and fiber combiner systems become possible. In this
papper, we will discuss in detail the development of the core material based on Yb-doped bulk silica and the
characterization of Yb-doped fibers with different core compositions.
We will also report on the excellent performance of a 4 kW fiber laser based on a single XLMA-fiber and show the first
experimental welding results of steel sheets achieved with such a laser.