Jens Pollmann-Retsch

UHP lamp systems for projection applications

Projection systems have found widespread use in conference rooms and other professional applications during the last decade and are now entering the home TV market at a considerable pace. Projectors as small as about one litre are able to deliver several thousand screen lumens and are, with a system efficacy of over 10 lm W −1 , the most efficient display systems realized today. Short arc lamps are a key component for projection systems of the highest efficiency for small-size projection displays. The introduction of the ultra high performance (UHP) lamp system by Philips in 1995 can be identified as one of the key enablers of the commercial success of projection systems. The UHP lamp concept features outstanding arc luminance, a well suited spectrum, long life and excellent lumen maintenance. For the first time it combines a very high pressure mercury discharge lamp with extremely short and stable arc gap with a regenerative chemical cycle keeping the discharge walls free from blackening, leading to lifetimes of over 10 000 h. Since the introduction of the UHP lamp system, many important new technology improvements have been realized: burner designs for higher lamp power, advanced ignition systems, miniaturized electronic drivers and innovative reflector concepts. These achievements enabled the impressive increase of projector light output, a remarkable reduction in projector size and even higher optical efficiency in projection systems during the last years. In this paper the concept of the UHP lamp system is described, followed by a discussion of the technological evolution the UHP lamp has undergone so far. Last, but not least, the important improvements of the UHP lamp system including the electronic driver and the reflector are discussed. (Some figures in this article are in colour only in the electronic version)

High-power VCSEL systems and applications

Easy system design, compactness and a uniform power distribution define the basic advantages of high power VCSEL systems. Full addressability in space and time add new dimensions for optimization and enable “digital photonic production”. Many thermal processes benefit from the improved control i.e. heat is applied exactly where and when it is needed. The compact VCSEL systems can be integrated into most manufacturing equipment, replacing batch processes using large furnaces and reducing energy consumption. This paper will present how recent technological development of high power VCSEL systems will extend efficiency and flexibility of thermal processes and replace not only laser systems, lamps and furnaces but enable new ways of production. High power VCSEL systems are made from many VCSEL chips, each comprising thousands of low power VCSELs. Systems scalable in power from watts to multiple ten kilowatts and with various form factors utilize a common modular building block concept. Designs for reliable high power VCSEL arrays and systems can be developed and tested on each building block level and benefit from the low power density and excellent reliability of the VCSELs. Furthermore advanced assembly concepts aim to reduce the number of individual processes and components and make the whole system even more simple and reliable.

UHP lamps for projection systems: getting always brighter, smaller, and even more colorful

The past decade has seen a rapid development of projection systems. Projectors as small as only a few liters in size deliver several thousand screen lumens and are, with an efficacy of over 10 lm/W, the most efficient display systems realized today. This has been made possible by breakthroughs in lamp technology, particularly by the development of the UHP-lamp. This broadband light source with its outstanding brightness and lifetimes of over 10000 hours is ideal for projection applications. In this paper we want to describe three major technological trend lines in the development of UHP-lamps over the past decade: First, there is a trend towards brighter projectors, which is fostered by a brightness increase of the UHP-lamps. At the same time, projectors have seen a dramatic reduction in size, which has been made possible mostly by reducing lamp- and driver-size by even a factor of 10. This was only possible by the development of new ignition concepts as well as new optical designs of the reflector. And finally, UHP-lamps have seen quite some improvement in color rendering by using even higher pressures and shorter arc gaps. This allows for more colorful pictures and even more efficient projector designs.

Optimized VCSELs for high-power arrays

High-power VCSEL systems with multi kilowatt output power require a good electro-optical efficiency at the point of operation i.e. at elevated temperature. The large number of optimization parameters can be structured in a way that separates system and assembly considerations from the minimization of electrical and optical losses in the epitaxially grown structure. Temperature dependent functions for gain parameters, internal losses and injection efficiency are derived from a fit to experimental data. The empirical description takes into account diameter dependent effects like current spreading or temperature dependent ones like voltage drops over hetero-interfaces in the DBR mirrors. By evaluating experimental measurements of the light output and voltage characteristics over a large range of temperature and diameter, wafer-characteristic parameters are extracted allowing to predict the performance of VCSELs made from this material in any array and assembly configuration. This approach has several beneficial outcomes: Firstly, it gives a general description of a VCSEL independent of its geometry, mounting and detuning, secondly, insights into the structure and the underlying physics can be gained that lead to the improvement potential of the structure and thirdly the performance of the structure in arrays and modules can be predicted. Experimental results validate the approach and demonstrate the significantly improved VCSEL efficiency and the benefit in high power systems.

Design of high power VCSEL arrays

High power VCSEL arrays can be used as a versatile illumination and heating source. They are widely scalable in power and offer a robust and economic solution for many new applications with moderate brightness requirements. The design of high power VCSEL arrays requires a concurrent consideration of mechanical, thermal, optical and electrical aspects. Especially the heat dissipation from the loss regions in the VCSEL mesas into the surrounding materials and finally towards the heat sink is discussed in detail using analytical and finite element calculations. Basic VCSEL properties can be separated from the calculation of thermal resistivity and only the latter depends on the details of array design. Guidelines are derived for shape, size and pitch of the VCSEL mesas in an array and optimized designs are presented. The electro-optical efficiency of the VCSELs and the material properties determine the operation point. A specific VCSEL design with the shape of elongated rectangles is discussed in more depth. The theoretical predictions are confirmed by measurements on practical modules of top-emitting structures as well as of bottom-emitting structures.

VCSEL arrays expanding the range of high-power laser systems and applications

Thermal treatment may be by far the most frequent process used in manufacturing, but only at a few places lasers could make an inroad. For thermal treatment homogeneous illumination of large areas at a lower brightness, and accurate temporal as well as spatial control of the power is required. This is complicated for conventional high-power lasers, while VCSEL arrays inherently have these capabilities.Because of their fast switching capability and low power dissipation, vertical-cavity surface emitting laser-diodes (VCSELs) have been widely used for datacom and sensing applications. By forming large-area arrays with hundreds of VCSELs per mm2, their use can be further expanded to high-power applications. In this way power densities of several W/mm2 are achieved, making VCEL arrays an ideal solution for many heating applications, ranging from melting and welding of plastics and laminates to curing, drying and sintering of coatings.A turn-key system concept has been developed allowing fast and easy configuring systems to the specifications of the applications. The compact and robust system can be built directly into the manufacturing equipment, thus making expensive fibers and homogenizing optics superfluous. These systems are now finding their first inroads into industrial applications and have been designed-in into commercially available production machines.Thermal treatment may be by far the most frequent process used in manufacturing, but only at a few places lasers could make an inroad. For thermal treatment homogeneous illumination of large areas at a lower brightness, and accurate temporal as well as spatial control of the power is required. This is complicated for conventional high-power lasers, while VCSEL arrays inherently have these capabilities.Because of their fast switching capability and low power dissipation, vertical-cavity surface emitting laser-diodes (VCSELs) have been widely used for datacom and sensing applications. By forming large-area arrays with hundreds of VCSELs per mm2, their use can be further expanded to high-power applications. In this way power densities of several W/mm2 are achieved, making VCEL arrays an ideal solution for many heating applications, ranging from melting and welding of plastics and laminates to curing, drying and sintering of coatings.A turn-key system concept has been developed allowing fast and easy configur...

16.1: UHP Lamps with Increased Efficiency

Shorter arcs and higher gas pressures increase the collection efficiency and produce a spectrum which is ideal for video projection. Taking into account the physical lamp efficiency ideal arc lengths are given. New UHP products will realise 30% more light on the screen.

Integrated high power VCSEL systems

High power VCSEL systems are a novel laser source used for thermal treatment in industrial manufacturing. These systems will be applied in many applications, which have not used a laser source before. This is enabled by the unique combination of efficiency, compactness and robustness. High power VCSEL system technology encompasses elements far beyond the VCSEL chip itself: i.e. heat sinks, bonding technology and integrated optics. This paper discusses the optimization of these components and processes specifically for building high-power laser systems with VCSEL arrays. New approaches help to eliminate components and process steps and make the system more robust and easier to manufacture. New cooler concepts with integrated electrical and mechanical interfaces have been investigated and offer advantages for high power system design. The bonding process of chips on sub-mounts and coolers has been studied extensively and for a variety of solder materials. High quality of the interfaces as well as good reliability under normal operation and thermal cycling have been realized. A viable alternative to soldering is silver sintering. The very positive results which have been achieved with a variety of technologies indicate the robustness of the VCSEL chips and their suitability for high power systems. Beam shaping micro-optics can be integrated on the VCSEL chip in a wafer scale process by replication of lenses in a polymer layer. The performance of VCSEL arrays with integrated collimation lenses has been positively evaluated and the integrated chips are fully compatible with all further assembly steps. The integrated high power systems make the application even easier and more robust. New examples in laser material processing and pumping of solid state lasers are presented.

Vertical-cavity surface emitting laser-diodes arrays expanding the range of high-power laser systems and applications

Thermal treatment may be by far the most frequent process used in manufacturing, but only at a few places lasers could make an inroad. For thermal treatment, homogeneous illumination of large areas at a lower brightness, and accurate temporal as well as spatial control of the power is required. This is complicated for conventional high-power lasers, while vertical-cavity surface emitting laser-diode (VCSEL) arrays inherently have these capabilities. Because of their fast switching capability and low power dissipation, VCSELs have been widely used for datacom and sensing applications. By forming large-area arrays with hundreds of VCSELs per mm2, their use can be further expanded to high-power applications. In this way, power densities of several W/mm2 are achieved, making the VCEL arrays an ideal solution for many heating applications, ranging from melting and welding of plastics and laminates to curing, drying, and sintering of coatings. A turn-key system concept has been developed allowing fast and easy ...

Advanced UHP lamps for projection systems

Projection systems for large screens have made tremendous progress during the last years, both in terms of performance and size reduction. Improved UHP lamp systems made a major contribution to enable the new generation of projectors. The arc gap is reduced to 1 mm only and allows a high collection efficiency in the projector. At the same time the lamp wattage was increased. In this way, todays projectors can create high-quality XGA pictures with more than 3000 screen lumens using one single 200 W UHP-lamp. Such a projector reaches an efficiency of more than 10 screen lumens per watt electrical input power. The volume of lamp and driver has been reduced by one order of magnitude during the last six years. This was possible by recent progress that has been achieved on the ignition of the lamp. By using a UV-enhancer cavity in the lamp seal and an additional antenna the ignition voltage could be reduced from 20 kV to below 5 kV. This allows more compact drivers and is ideal for miniaturizing projectors. A new optical concept allows for extremely compact reflector systems: A dichroic coating applied to one half of the UHP burner focuses all light into one hemisphere. Additionally 20-30% more light can be collected in systems with high optical demands. Making use of both the reduced ignition voltage and the new optical concept a reduction of the volume of lamp and driver by a factor of 10 has been realized.