We have devised an oxide- and regrowth-free approach for current confinement in vertical-cavity surface-emitting lasers (VCSELs) which uses the photons in the cavity to optically define a current path through the device. For this purpose, an optical switch, implemented as a phototransistor (PT), is epitaxially integrated into the cavity. The PT layers become locally conductive where the highest photon density is reached in the resonator and establish the current aperture. By introducing additional photons with an external laser beam into the resonator, we show here that this current aperture can be manipulated. We demonstrate the possibility of redefining the location of the current aperture and discuss the consequences on the light-current characteristics of the devices.
We have devised a novel oxide-free and regrowth-free approach for optically controlled current confinement in vertical-cavity surface-emitting lasers (VCSELs). This is realized with a monolithically integrated phototransistor (PT), which is configured as an optical switch and embedded between the two Bragg reflectors. We have fabricated functional PT-VCSELs by one-step epitaxial growth plus metal deposition with different top contact sizes. We present light–current–voltage characteristics of the lasers as well as a simple theoretical model explaining the occurrence of a distinct turn-on point and clarify epitaxial design requirements to reach strong optically controlled current confinement.
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.
We are implementing an electro–thermal simulation tool to optimize the characteristics of GaAs-based vertical- cavity surface-emitting lasers (VCSELs). For this purpose it turned out to be necessary to revisit basic material parameters. In this paper we elaborate on the composition, carrier density, and temperature dependencies of the electron mobility of AlxGa1−xAs semiconductors. We present the principles of the pragmatic quasi-three- dimensional (q3D) device model and show selected results.
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.
In comparison to widely used InGaAs Quantum Wells (QW) in high speed VCSELs operating at 25 Gbps and beyond, we present an investigation on the use of GaAs QWs, which have proven their ability to serve reliably in 10 Gbps and 14 Gbps VCSEL products and allow for an evolutionary extension of data rates based on mature technology. As data centers continuously increase in size, the demand for longer reach optical links within these data centers is addressed by the proposal of using small spectral width single-mode VCSELs that offer the potential of significantly reduced chromatic dispersion along optical fibers of several 100 m length. Performance and modeling parameters of single-mode VCSELs are being compared to those of typical multi-mode VCSELs built from identical epitaxy and process technology.
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.
Many VCSEL based applications require optical feedback of the emitted light. E.g. light output monitor functions in
transceivers are used to compensate for thermally induced power variation, power degradation, or even breakdown of
pixels if logic for redundancy is available. In this case integrated photodiodes offer less complex assembly compared to
widely used hybrid solutions, e.g. known in LC-TOSA assemblies. Especially for chip-on-board (COB) assembly and
array configurations, integrated monitor diodes offer a simple and compact power monitoring possibility. For 850 nm
VCSELs the integrated photodiodes can be placed between substrate and bottom-DBR, on top of the top-DBR, or
inbetween the layer sequence of one DBR. Integrated intra-cavity photodiodes offer superior characteristics in terms of
reduced sensitivity for spontaneously emitted light [1] and thus are very well suited for power monitoring or even endof-
life (EOL) detection. We present an advanced device design for an intra-cavity photodiode and according
performance data in comparison with competing approaches.
Over the past 3 years laser based tracking systems for optical PC mice have outnumbered the traditional VCSEL market
datacom by far. Whereas VCSEL for datacom in the 850 nm regime emit in multipe transverse modes, all laser based
tracking systems demand for single-mode operation which require advanced manufacturing technology. Next generation
tracking systems even require single-polarization characteristics in order to avoid unwanted movement of the pointer
due to polarization flips. High volume manufacturing and optimized production methods are crucial for achieving the
addressed technical and commercial targets of this consumer market. The resulting ideal laser source which emits
single-mode and single-polarization at low cost is also a promising platform for further applications like tuneable diode
laser absorption spectroscopy (TDLAS) or miniature atomic clocks when adapted to the according wavelengths.
KEYWORDS: Vertical cavity surface emitting lasers, Modulation, Eye, Multimode fibers, Signal generators, Near field optics, Near field, Silicon, Transmitters, Oxidation
We present flip-chip attached high-speed VCSELs in 2-D arrays with record-high intra-cell packing densities. The advances of VCSEL array technology toward improved thermal performance and more efficient fabrication are reviewed, and the introduction of self-aligned features to these devices is pointed out. The structure of close-spaced wedge-shaped VCSELs is discussed and their static and dynamic characteristics are presented including an examination of the modal structure by near-field measurements. The lasers flip-chip bonded to a silicon-based test platform exhibit 3-dB and 10-dB bandwidths of 7.7 GHz and 9.8 GHz, respectively. Open 12.5 Gbit/s two-level eye patterns are demonstrated.
We discuss the uses of high packing densities for the increase of the total amount of data throughput an array can deliver in the course of its life. One such approach is to provide up to two backup VCSELs per fiber channel that can extend the lifetimes of parallel transmitters through redundancy of light sources. Another is to increase the information density by using multiple VCSELs per 50 μm core diameter multimode fiber to generate more complex signals. A novel scheme using three butt-coupled VCSELs per fiber for the generation of four-level signals in the optical domain is proposed. First experiments are demonstrated using two VCSELs butt-coupled to the same standard glass fiber, each modulated with two-level signals to produce four-level signals at the photoreceiver. A four-level direct modulation of one VCSEL within a triple of devices produced first 20.6 Gbit/s (10.3 Gsymbols/s) four-level eyes, leaving two VCSELs as backup sources.
We report on the design, fabrication and test results of monolithically integrated transceiver chips consisting of GaAs metal-semiconductor-metal photodiodes and 850nm wavelength vertical-cavity surface-emitting lasers. These chips are well suited for low-cost and compact bidirectional optical interconnection at Gbit/s data rates in mobile systems and industrial or home networks employing large core size multimode fibers.
We report on the design and experimental results of monolithically integrated optoelectronic devices containing distributed feedback (DFB) laser, electroabsorption modulator (EAM), and semiconductor optical amplifier (SOA). Common InGaAlAs multiple quantum well (MQW) layers are used in all device sections. The incorporation of local lateral metal gratings in the DFB section enables device fabrication by single-step epitaxial growth. The emission wavelength is λ=1.3 micrometer. More than 2 mW single-mode fiber-coupled output power as well as 10 dB/2 V static extinction ratio have been achieved. Modulation experiments clearly show 10 Gbit/s capability.
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