VCSEL arrays are the ideal light source for 3D imaging applications. The narrow emission spectrum and the ability for short pulses make them superior to LEDs. Combined with fast photodiodes or special camera chips spatial information can be obtained which is needed in diverse applications like camera autofocus, indoor navigation, 3D-object recognition, augmented reality or autonomously driving vehicles. Pulse operation at the ns scale and at low duty cycle can work with significantly higher current than traditionally used for VCSELs in continuous wave operation. With reduced thermal limitations at low average heat dissipation very high currents become feasible and tens of Watts output power have been realized with small VCSEL chips. The optical emission pattern of VCSELs can be tailored to the desired field of view using beam shaping elements. Such optical elements also enable laser safe class 1 products. A detailed analysis of the complete system and the operation mode is required to calculate the maximum permitted power for a safe system. The good VCSEL properties like robustness, stability over temperature and the potential for integrated solutions open a huge potential for VCSELs in new mass applications in the consumer and automotive markets.
VCSELs and VCSEL arrays are an ideal light source for time-of-flight based sensors. The narrow emission spectrum and the ability for short pulses make them superior to LEDs. Combined with fast photodiodes or special camera chips spatial 3D information can be obtained which is needed in diverse applications like camera autofocus, indoor navigation, 3Dobject recognition or even autonomously driving vehicles. VCSEL arrays are the way to tailor the output power. For pulse operation at low duty cycle average heat dissipation is no longer the upper limit to the operating point of VCSELs but over-pulsing becomes possible. Taking into account electrical boundary conditions and optimum conversion efficiency arrays can be designed for specific operating conditions. Measurements of arrays under short pulse operation are presented using a package with integrated driver.
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. <p> </p>New cooler concepts with integrated electrical and mechanical interfaces have been investigated and offer advantages for high power system design.<p> </p> 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. <p> </p>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. <p> </p>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.
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.
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.
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.
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
We present a model and results of simulations and experiments investigating the L-I characteristics of electrically pumped (EP-) VECSELs in the single- and multi-mode regime. In our model we use a mode expansion ansatz to treat the electromagnetic field inside the VECSEL cavity. The eigenmodes of the passive cavity are computed using the bidirectional beam propagation method (BDBPM) to solve the Helmholtz equation. The BDBPM allows us to account for the complex refractive index distribution within the semiconductor heterostucture, composed of approximately thousand interfaces along the optical axis in addition to lateral refractive index variations in oxide-confined devices as well as the macroscopic external cavity. We simulate the time evolution of the modal powers of several transverse modes and the spatial distribution of the inversion carriers in the quantum well plane. Therefore we solve an differential equation system composed of multimode rate equations and the carrier diffusion equation. With this ansatz we are able to identify cavity geometries suitable for single-mode operation assuming typical current profiles that are taken from photoluminescence measurements of the devices under investigation. Furthermore, we identify effects limiting the single-mode efficiency, such as poor gain and mode matching, reabsorption in unpumped regions of the quantum wells or enhanced carrier losses due to strong spatial hole burning. Critical parameters of the equations, such as optical losses, injection effciency, carrier recombination constants and gain parameters are obtained from experiments, microscopic models and literature. The simulation results are compared to experimental results from EP-VECSELs from Philips Technologie GmbH U-L-M Photonics.
In modelocked electrically pumped VECSELs (EP-VECSELs) the gain saturation strongly influences the pulse formation. Here we present a detailed gain characterization of EP-VECSELs as published the first time in . The spectral gain-distribution and the gain saturation behavior of two devices with different field-enhancement in the quantum-well gain layers are investigated. Comparing spectral bandwidth, small-signal gain and saturation fluence of the three devices, we chose the most suitable for modelocking experiments. Using a low-saturation fluence SESAM we have generated 9.5-ps-pulses with an average output-power of 7.6 mW at 1.4 GHz repetition-rate, which have been the
shortest pulses from an EP-VECSEL to date .
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
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.
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.
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 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.