Johanna Kolb

VCSEL arrays with integrated optics

Systems with arrays of VCSELs can realize multi kilowatt output power. The inherent simplicity of VCSELs enables a performance and cost breakthrough in solutions for thermal processing and the pumping of solid state lasers. The use of an array of micro-optics i.e. one micro-lens per VCSEL enables multiple advantages: firstly it can function as a collimating lens in order to realize a brightness of an array which is similar to the brightness of a single VCSEL. Secondly the micro-lens can be part of an imaging system for tailored intensity distributions. Last but not least the microlens with moderate feedback into the VCSEL can help to select laser modes in order to increase brightness and mode stability. Wafer-level integrated micro-optics allow keeping the VCSEL advantage of realizing complete and operational lasers on wafer level including the micro-optics. This paper presents our approach to bond a 3” GaAs wafer with a micro-optics wafer of the same size. The type of glass used for the optics wafer has been selected to match the coefficient of thermal expansion of GaAs and is suitable for hot pressing of the lens structures. An alignment strategy with corresponding markers on both wafers is used to allow the alignment on a standard mask aligner thus realizing many thousand lens adjustments in a single process step. The technology can be combined with VCSEL wafers with thinned substrate as well as with complete substrate removal. The basic technology and illustrative prototype systems are described here.

Advanced characterization techniques for high power VCSELs

The performance of high power VCSELs in a specific application depends on the geometrical and thermal design as well as on the quality of the epitaxially grown material. Due to the relatively high heat load in densely packed high power arrays the temperature in the active zone and the DBR mirrors changes significantly with the applied current and the traditional characterization methods become less meaningful than for low power devices. This paper presents a method to measure temperature independent power curves with the help of short pulse techniques and data mapping at different heat sink temperatures. In addition the internal quantum efficiency, the transparency current and the gain coefficient are measured by a novel method which operates the VCSEL material as an edge emitter and applies a cut-back technique. The optical losses in the DBR mirrors are determined using external feedback. In summary all relevant parameters which determine the quality of an epitaxial design are measured independently and can be directly compared with modeling and help to optimize the high power VCSEL performance.

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.

VCSEL based sensors for distance and velocity

VCSEL based sensors can measure distance and velocity in three dimensional space and are already produced in high quantities for professional and consumer applications. Several physical principles are used: VCSELs are applied as infrared illumination for surveillance cameras. High power arrays combined with imaging optics provide a uniform illumination of scenes up to a distance of several hundred meters. Time-of-flight methods use a pulsed VCSEL as light source, either with strong single pulses at low duty cycle or with pulse trains. Because of the sensitivity to background light and the strong decrease of the signal with distance several Watts of laser power are needed at a distance of up to 100m. VCSEL arrays enable power scaling and can provide very short pulses at higher power density. Applications range from extended functions in a smartphone over industrial sensors up to automotive LIDAR for driver assistance and autonomous driving. Self-mixing interference works with coherent laser photons scattered back into the cavity. It is therefore insensitive to environmental light. The method is used to measure target velocity and distance with very high accuracy at distances up to one meter. Single-mode VCSELs with integrated photodiode and grating stabilized polarization enable very compact and cost effective products. Besides the well know application as computer input device new applications with even higher accuracy or for speed over ground measurement in automobiles and up to 250km/h are investigated. All measurement methods exploit the known VCSEL properties like robustness, stability over temperature and the potential for packages with integrated optics and electronics. This makes VCSEL sensors ideally suited for new mass applications in consumer and automotive markets.

Dynamics of the angular emission spectrum of large-area VCSELs

High power VCSELs can be realized by scaling up the active area of bottom-emitting devices. This results in a large Fresnel number of the laser cavity. The laser beam cannot be described with Gauss modes in a simple way anymore, but is best described in terms of tilted plane waves, called Fourier modes. The beam quality and mode spectra depending on the applied current and the temperature of the VCSEL are investigated. Two-dimensional measurements of the near and the far field are combined with power and spectral measurement to characterize the VCSEL. Polarization and Fourier filtering are used to examine the spatially-dependent emission in detail. A rich dynamic in the angular emission profile for large-area VCSELs is observed and can be explained by considering the residual reflections from the AR-coated substrate-air interface and thermal effects. The presented theoretical model simulates the dynamics of the angular emission. The calculated angular and spectral profiles match the experimental observations very well over the whole parameter range. The influence of the active area is studied for diameters of the oxide aperture from 20 up to 300 μm. For smaller diameters diffraction effects become more dominant, the Fresnel number is reduced and the emission spectrum gets closer to the Gauss mode description.

Temperature-dependent investigation of carrier transport, injection, and densities in AlGaAs-based multi-quantum-well active layers for vertical-cavity surface-emitting lasers

Abstract. The electro-optical efficiency of vertical-cavity surface-emitting lasers (VCSELs) strongly depends on the efficient carrier injection into the quantum wells (QWs) in the laser active region. Carrier injection degrades with increasing temperature, which limits VCSEL performance in high-power applications where self-heating imposes high-operating temperatures. In a numerical model, we investigate the transport of charge carriers in an 808-nm AlGaAs multi-quantum-well structure with special attention to the temperature dependence of carrier injection into the QWs. Experimental reference data were extracted from oxide-confined, top-emitting VCSELs. The transport simulations follow a drift-diffusion-model complemented by an energy-resolved carrier-capture model. The QW gain was calculated in the screened Hartree–Fock approximation. With the combination of the gain and transport model, we explain experimental reference data for the injection efficiency and threshold current. The degradation of the injection efficiency with increasing temperature is not only due to increased thermionic escape of carriers from the QWs, but also to state filling in the QWs initiated from higher threshold carrier densities. With a full opto-electro-thermal VCSEL model, we demonstrate how changes in VCSEL properties affecting the threshold carrier density, like mirror design or optical confinement, have consequences on the thermal behavior of the injection and the VCSEL performance.

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...

High power electrically pumped VECSELs and arrays

VECSELs are characterized by an outstanding brightness of 100kW/mm²/sr and a small spectral width. Electrical pumping and the potential to combine many emitters in arrays allow for highly integrated and easy to manufacture laser sources which can be scaled towards high power. This almost ideal value proposition is affected by the penalty in efficiency which reduces the output power from VCSELs towards multimode VECSELs and finally single mode VECSELs. The root causes for this lower efficiency are optical losses in the extended cavity, a mismatch of pump and mode profile and losses related to the oxide aperture which is used for current confinement. The reduction of losses requires a careful design of spatial doping distributions in the epitaxially grown layers as these losses have to be balanced against the requirement of low electrical resistance across the many hetero-interfaces in the DBR mirrors. The mismatch of pump and mode profile and the aperture related losses are addressed by an improved current injection enabled by a tailored electrical contact. In this paper optimized structures will be presented which enable a significant increase of efficiency and output power towards more than 150mW in a single mode and more than 300mW in multimode operation. The optical concept of the extended cavity can use a plane mirror in the simplest case thus facilitating the power scaling in arrays with many individual VECSEL apertures combined on a single chip.