In present days lasers are used in many different applications like material processing, medical science, automotive, measurement and sensing applications, etc.. All of these applications require optics to make the emitted light usable. Moreover, there is a requirement for a certain beam profile to achieve the necessary characteristic. The applications are addressed by different laser types, but the optical solutions are influence by the major trends of miniaturization, improved imagine quality and cost reduction. The compactness of resent applications requires micro-optical solutions. The design, manufacturing and the evaluation of these solutions are still demanding. The presentation will look at different applications and derive the specifications for the micro-optics. The available manufacturing technologies will influence the achievable accuracy of the parts. Therefore, the talk will evaluate the effect of deviations on the performance of selected applications. The second part will focus on the selection of the appropriate manufacturing technology and the evaluation of the micro-optics. It can be demonstrated that the optimization of the process parameters during the manufacturing process improves the performance of the products.
We present a novel, high-power stack of 20% fill-factor, 976nm, laser-diode bars, each directly attached to an enhanced lateral-flow (ELF), copper-based, water-cooled heat-sink. The heat-sinks contain mounting screws that form a kinematic mount to minimize detrimental mechanical-stress on the diode bars while also providing beneficial, double-side cooling of the bars. A stack of 18-bars, emitting 2.54kW, was constructed to validate the technology. Using standard optics and a polarization multiplexer, a 320μm diameter, 0.3NA focus is achieved with a 6-bar stack that robustly couples 450W, with a ~67% coupling efficiency, from a passive, 400μm, 046NA doubleclad fiber.
For many innovative applications a significant improvement in the homogeneity of the laser beam is a critical requirement when using semiconductor lasers. There are several different methods for the homogenization of laser
radiation. Homogenization using micro-cylinder lens arrays is a considerably elegant and compact solution. In this case
the incident laser beam is separated into partial beams by one or more micro-lens arrays. These partial beams are then
overlaid in the homogenization plane by the downstream optics. Depending on the arrangement and geometry of the
micro-lenses, this enables homogeneously illuminated lines, rectangles or squares to be generated. The major advantage of this solution lies in the increased freedom of adjustment to account for the initial beam profile, as well as the extremely compact design. In addition to a comparison of different homogenization principles the paper describes new approaches of homogenization via micro-lens arrays and compares the impact on the array performance by different manufacturing approaches.
Solid-state lasers have been demonstrated as attractive drivers for laser-plasma interaction and have presently been
developed for various applications like inertial confinement fusion (ICF) , particle acceleration and intense X-ray
generation . Viable real world applications like power production at industrial scale will require high laser system
efficiency, repetition rate and lifetime which are only possible with semiconductor diode pumping. The paper describes the work conducted with two 20 kW diode laser sources pumping an ytterbium:YAG laser amplifier. The set-up acts as a small scale prototype for the DiPOLE project . This project aims to develop scalable gas cooled cryogenic multi-slab diode pumped solid state lasers capable of producing KJ pulse energy. A scale-down prototype is currently under development at the Central Laser Facility (CLF) designed to generate 10 J at 10 Hz. To secure an efficient pumping process the sources have to fulfill aside power requirement in the spectral and time domain, the claim for high homogenization and low divergence of the spatial and angular beam distribution as well as a minimization of losses within the optical path. The existing diode laser sources designed and built by INGENERIC deliver 20 kW pulsed power, concentrated on a plateau of FWHM dimension of 20 x 20 mm² with a homogeneity of more than 90 %. The center wavelength of 939.5 nm is controlled in a range of ± 0.1 nm. The time and area integrated spectrum of at least 76 % of the total energy is contained within a 6 nm wide wavelength band around the center wavelength. Repetition rates can be adjusted between 0.1 Hz up to 10 Hz with rise and fall times less than 50 μs and pulse durations from 0.2 ms to 1.2 ms. The paper describes the impact of different designs on the performance of pump sources and puts special emphasis on the influence of the optical components on efficiency and performance. In addition the influence of the measuring principle is discussed.
We developed a high brightness fiber coupled diode laser module based on single diode lasers providing more than 60
Watts output power from a 100 micron fiber at the optimum fiber laser pump wavelength of 976 nm. The advantage of
using multiple single emitters on a submount compared to using bars or mini bars is the direct fiber coupling by use of
optical stacking and the fact that no beam transformation is needed. We achieved best brightness with a high fill factor,
optical efficiency of more then 80% and wall-plug efficiency of more then 40%. The use of single emitters on a
submount also extends the life span due to reduced failure (x<sup>n</sup> vs. x) per device (n individual emitters vs. n emitters on a
bar (mini array)). Low drive current enables modulation.
Pumping fiber lasers is the driving force for the development of high brightness, mid power, passively cooled, fiber
coupled diode lasers. We compare concepts for providing 50 W in a 100 micron fiber at the optimum fiber laser pump
wavelength of 976 nm. The set up is experimentally demonstrated and compared to the optical analysis.
Three basic diode laser concepts are included into this comparison: single emitters, high density emitter arrays and low
density emitter arrays on bars.
Low density stacking in the horizontal direction with increasing the filling factor by a microlens array is the first concept.
For this concept two diode bars with low filling factor are fast and slow axis collimated. Beam transformation, shaping
and focusing are similar to the second concept.
In the second concept a diode laser array with high filling factor is regarded. An 800 μm diode laser bar consists of an
array of four or five emitters. Two bars are polarization coupled and collimated with single lenses. Beam symmetrization
is performed by the well known step mirror. A simple anamorphotic optic enables beam shaping and fiber coupling.
The third one, single emitters, represents optical beam combining of laser diodes that are high density stacked in the
vertical direction. Five emitters are placed in an optical stack, each one collimated with its own lens. Two optical stacks
are polarization coupled and focused on the fiber end. The three concepts are compared in terms of power efficiency and
complexity, and the results of prototype systems are presented.
High optical output power in the multi-kW range from a fiber coupled diode laser can reach the beam quality of lamp
pumped solid state lasers. Direct diode laser application as deep penetration metal welding becomes feasible.
Polarization and wavelength multiplexing are established techniques to scale the optical power of diode lasers at almost
constant beam quality. By use of volume diffraction gratings in an external cavity laser it is possible to constrict the
spectral bandwidth of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to
the stabilization of the wavelength multiplexing of diode laser beams at small distance of the center wavelengths can be
The development of a fiber coupled diode laser is presented. The set up consists of twelve modules which serve for an
average optical power of 1.5 kW. Each module utilizes dense wavelength multiplexing of two diode laser bars with a
center wavelength spacing of 3 nm. The diode laser bars are wavelength stabilized at center wavelengths of 908 nm,
911 nm, 975 nm and 978 nm. The spectral bandwidth of all diode laser bars is within 1 nm in the full power range. Stable
operation at an average power of 136 W at 908/911 nm and 115 W at 975/978 nm with a wavelength shift less than
0.1 nm is achieved by the modules. Further coarse wavelength and polarization multiplexing and beam transformation
enable fiber coupling to a 600 &mgr;m fiber with a numerical aperture of 0.175 (95% power inclusion).
Volume holographic gratings (VHG) provide the capability of narrowing and stabilizing the wavelength of
semiconductor lasers by forming an external cavity laser (ECL). The standard configuration of these ECL's is to use a
collimating lens followed by the VHG to provide feedback to the resonator and lock the wavelength. In this
configuration both elements have to be carefully aligned with tolerances in the sub-µm and mrad range. The present
paper presents a fast-axis collimation lens (FAC) with integrated VHG for locking a laser diode bar. Besides the
advantage of having only a single element, the integrated element is also less sensitive to alignment tolerances with
respect to the locking due to the large divergence angle of the uncollimated array compared to a collimated array. Using
a standard AR coated array with 19 emitters an output power of 67.4 W was achieved. The spectral bandwidth was
within 1 nm over the whole power range. Due to high stability requirements in this application, glass was chosen as the
VHG material. Though the refractive index is low compared to standard FAC lenses, the design and manufacturing
process of the lens still guarantees a diffraction limited collimated beam.
At present methods of polarization and wavelength multiplexing with dielectric coatings are used to increase the brightness of diode lasers. The number of suitable diode laser wavelengths is limited by the temperature- and current-dependent spectral characteristics of the diode laser and the slope of dielectric edge-filters. By use of external volume diffraction gratings it is possible to constrict the emission spectra of diode lasers and to reduce the wavelength shift related to temperature or current injection. Due to the stabilization of the center wavelengths and the reduced bandwidth of the diode emission multiplexing of diode laser beams at small distance of the centre wavelengths can be realized.
The performance of wavelength stabilization by use of volume diffraction gratings adapted to AR-coated diode laser arrays in an external cavity as well as to standard high power diode laser arrays is discussed. Furthermore modules of two diode laser beams are combined by wavelength dependent diffraction of an adapted volume diffraction grating. The spectra of the diode lasers of the same wafer are stabilized with a centre wavelength spacing of about 3 nm and more than 95% of the optical output power of each beam within a spectral width less than 0.7 nm. An efficiency of more than 80% for multiplexing of two diode laser bars with good beam quality in fast axis of M<sup>2</sup> < 2 is achieved.