Stephan Gronenborn

Modular VCSEL solution for uniform line illumination in the kW range

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 use of VCSEL arrays for high power laser diode applications enables multiple benefits: Full wafer level production of VCSELs including the combination with micro-optics; assembly technologies allowing large synergy with LED assembly thus profiting from the rapid development in solid state lighting; an outstanding reliability and a modular approach on all levels. A high power VCSEL array module for a very uniform line illumination is described in detail which offers >150W/cm optical output and enables less than 1% non-uniformities per mm along the line. The applied optical principle of near field imaging and massively superposing many thousand VCSELs by arrays of micro-lenses gives perfect control over the intensity distribution and is inherently robust. A specific array of parallelogram shaped VCSELs has been developed in combination with an appropriate micro-lens design and an alignment strategy. The concept uses parallel and serial connection of VCSEL arrays on sub-mounts on water coolers in order to realize a good combination of moderate operating currents and reliability. Lines of any desired length can be built from modules of 1cm length because this optical concept allows large mounting tolerances between individual modules. Therefore the concept is scalable for a wide range of applications. A demonstrator system with an optical output of 3.5kW and a line length of 20cm has been realized.

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.

Gain characterization and passive modelocking of electrically pumped VECSELs

Linear and nonlinear gain characterization of electrically pumped vertical external cavity surface emitting lasers (EP-VECSELs) is presented with spectrally resolved measurements of the gain and with gain saturation measurements of two EP-VECSEL samples with different field enhancement in the quantum-well gain layers. The spectral bandwidth, small-signal gain and saturation fluence of the devices are compared. Using the sample with the larger bandwidth, we have demonstrated the shortest pulses generated from a passively modelocked EP-VECSEL to date. With a low-saturation-fluence SESAM for passive modelocking we have achieved 9.5-ps pulses with 7.6 mW average output power at a repetition rate of 1.4 GHz. With a higher output coupler transmission the pulse duration was increased to 31 ps with an average output power of 13.6 mW. The pulses were chirped mainly due to the group delay dispersion (GDD) introduced by the intermediate DBR, which compensates the optical loss in the structure.

High power VCSEL systems for tailored intensity distributions

Arrays of high power VCSELs offer a unique opportunity to create a target intensity distribution which is tailored to the needs of a specific application. The concept presented here images the near field of the VCSEL onto the target. This is achieved by a combination of micro-lenses and field lenses in order to superimpose many VCSELs in an array. The optical system can be simple and the freedom to realize a wide variety of different intensity distributions with one and the same optics is large. The total power can be scaled by using arrays of VCSELs and due to the superposition of many emitters the illumination pattern has low speckle and is robust against single emitter failures.

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.

Absorber and gain chip optimization to improve performance from a passively modelocked electrically pumped vertical external cavity surface emitting laser

We present an electrically pumped vertical-external-cavity surface-emitting laser (EP-VECSEL) modelocked with a semiconductor saturable absorber mirror (SESAM) with significantly improved performance. In different cavity configurations, we present the shortest pulses (2.5 ps), highest average output power (53.2 mW), highest repetition rate (18.2 GHz), and highest peak power (4.7 W) to date. The simple and low-cost concept of EP-VECSELs is very attractive for mass-market applications such as optical communication and clocking. The improvements result from an optimized gain chip from Philips Technologie GmbH and a SESAM, specifically designed for EP-VECSELs. For the gain chip, we found a better trade-off between electrical and optical losses with an optimized doping scheme in the substrate to increase the average output power. Furthermore, the devices bottom contact diameter (60 μm) is smaller than the oxide aperture diameter (100 μm), which favors electro-optical conversion into a TEM00 mode. Compared to ...

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.

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.