Laser projectors integrated in portable devices offer a new platform for media display but put strong demands on the
laser sources in terms of efficiency, modulation band width, operating temperature range and device cost. Osram Opto
Semiconductors has developed and produces synthetic green lasers for projection applications on which the latest results
are reported. Based on vertical external cavity surface emitting laser (VECSEL) technology and second harmonic
generation an output power of >75mW has been achieved. The maximum output power is to a large extent limited by the
high thermal resistance of the monolithic VECSEL chip used. To overcome the thermal limitations a new thinfilm
VECSEL chip design is proposed where the epitaxial layers are transferred to a silicon carrier and processed on wafer
level, thus significantly lowering the thermal resistance and improving the maximum output power.
In this contribution the relevant technological aspects of LED-based lamps for solid state lighting are discussed. In
addition to general energy efficiency considerations improvements in LED chip technology and white light generation
Compact, stable and efficient green lasers are of great interest for many applications like mobile video projection,
sensing, distance measurement and instrumentation. Those applications require medium values of output power in the
50mW range, good wall-plug efficiency above 5 % and stable operation over a wide temperature range. In this paper we
present latest results from experimental investigations on ultra-compact green intracavity frequency doubled optically
pumped semiconductor InGaAs disk lasers. The green laser setup has been limited to a few micro optical and
semiconductor components built on a silicon backplane and fits within an envelope of less than 0.4 cc. An optical
frequency looking scheme in order to fix the fundamental wavelength over varying operating conditions like changing
output power and ambient temperature has been applied. The cavity has been optimized for fast modulation response and
high efficiency using quasi-phase matching non-linear material. Recent data from cw and high-frequency
characterization is presented.
Semiconductor disk lasers have attracted a lot of interest in the last few years due to high output power combined
with good beam quality and possible wavelength engineering. One of the disadvantages is the need for external
optical pumping by edge-emitting semiconductor lasers that increase packaging effort and cost. Therefore,
semiconductor disk lasers with monolithically integrated pump lasers would be of high interest. We report on
a novel design and experimental realization to monolithically integrate pump lasers with a semiconductor disk
laser in a one-step epitaxial design. By careful design of integrated pump lasers and stacking sequence, it is
possible to efficiently excite vertical emitter areas with different mesa sizes. First results are shown at 1060 nm
emission wavelength with high output power out of mesa diameters of 100 μm to 400 μm. The devices can be
conveniently characterized on a wafer level using dry-etched pump laser facets. In pulsed operation 1.7W out of
a 100 μm diameter mesa and 2.5W out of a 200 μm diameter mesa are demonstrated. Additionally, more than
0.6W in cw operation using a 400 μm structure were achieved. In summary, an innovative approach for truly
monolithic integration of a semiconductor disk laser with pump lasers has been pioneered.
Among enabling key components for mobile laser projection, the green laser plays an outstanding role: We present green
laser modules based on frequency doubled optically pumped semiconductor disk lasers. In these lasers with twofold
conversion, from 808nm over 1060nm to 530nm, active semiconductor components and second harmonic generation
have to be carefully optimized to realize good efficiency at moderate output powers. The concept was developed not
only to meet power and efficiency targets, but also to provide simple operation and control by pump diode current. The
latest concept targets green output powers of more than 50mW at wall plug efficiencies >7%, limiting total electrical
power consumption to less than 1W. Consequent use of micro optical components allows for a package volume of less
The market entrance of thinfilm based, substrate-less LEDs has stimulated the field of high-brightness LEDs. One of the most prominent advantages of thinfilm LEDs is the possibility to achieve a high light extraction efficiency independently of the chip area. This feature is particularly suitable for large-area, high-flux devices. In this paper, we report on high-power LEDs with a chip-area of 1 mm<sup>2</sup> for red and infrared emission. Mounted in packages with improved heat sinking and operated at a continuous-wave (cw) current of 800mA, the devices achieve an output power of 440 mW both for red (λ = 615 nm) and infrared (λ = 850 nm) wavelengths. Together with Osram's ThinGaN chips, a family of devices is available with very similar emission characteristics, performance and geometry, which allow the assembly of powerful light engines for a number of advanced applications.
In Thinfilm LEDs, the substrate absorption of the generated light is avoided by a metal reflector between the light emitting layer and the substrate. The light extraction can be further enhanced by buried microreflectors or surface texturing. We demonstrate that the combination of these technologies gives prospects equal or superior to all other known approaches in terms of luminous efficiency and luminance. At a peak wavelength of 617 nm, we have obtained a luminous efficiency of 95.7 lm/W at 20 mA. We further analyze the internal and light extration efficiencies of our LEDs using raytracing simulations as well as a theoretical model for the internal efficiency. This analysis shows quantitatively that the efficient light extraction from InGaAlP thinfilm LEDs becomes more and more difficult when approaching shorter wavelengths.
The concept of an AlGaInP thin-film light emitting diode includes a structure of semiconductor layers with low optical absorption on which a highly reflective mirror is applied. After bonding this wafer to a suitable carrier, the absorbing GaAs substrate is removed. Subsequently, electrical contacts and an efficient light scattering mechanism for rays propagating within the chip is provided. To achieve high efficiency operation it is crucial to optimize all functional parts of the device, such as the mirror, contacts, and active layer. Different mirrors consisting of combinations of dielectrics and metals have been tested. New chip designs have been evaluated to reduce the absorption at the ohmic contacts of the device. For efficient light scattering, the surface roughness of the at the emission window has to be optimized.
Using these structures, and a thin active layer consisting of five compressively strained quantum wells, an external quantum efficiency of 40% is demonstrated at 650 nm. Further improvement is expected.
Since the AlGaInP material system can provide only poor carrier confinement for active layers emitting in the yellow wavelength regime, the internal efficiency of these LEDs is comparably low. In order to reduce the problem of carrier leakage, a yellow active region usually consists of some hundred nanometers of active material. To circumvent the problem of this highly absorbing active layer, a separation of the light generation and the area of light extraction is suggested for yellow thin-film LEDs. First results are presented in this paper.
The combination of wafer soldering using metal layers and the introduction of buried micro-reflector structures has proven to be a promising approach to fabricate high brightness, substrate-less LEDs in the AlGaInP material system. In addition to the enhanced light output, the scalability of this approach has been predicted as a major advantage. In contrast to other approaches, larger area LEDs can be fabricated without altering the epitaxial structure and thickness of layers simply by offering a larger area for light generation. First samples of amber (λ = 615 nm) buried micro-reflector LEDs with side-length up to 1000 μm have been realized. Devices mounted in packages with improved heat sinks are capable of low voltage CW operation with currents as high as 600 mA (V<sub>fw</sub>≤ 2,8 V) without significant thermal flattening of the light-current characteristics. The maximum luminous flux achieved at these oeprating conditions is 46 lumen. Already these first experiments demonstrate the potential of the concept of buried micro-reflector LEDs not only for high-brightness but also for high-current operation. The results are among the best values of high-flux LEDs in this wavelength range.