Proceedings Volume 3286

Vertical-Cavity Surface-Emitting Lasers II

Kent D. Choquette, Robert A. Morgan
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Proceedings Volume 3286

Vertical-Cavity Surface-Emitting Lasers II

Kent D. Choquette, Robert A. Morgan
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 20 April 1998
Contents: 7 Sessions, 25 Papers, 0 Presentations
Conference: Optoelectronics and High-Power Lasers and Applications 1998
Volume Number: 3286

Table of Contents

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Table of Contents

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  • New Epitaxial Structures
  • Emerging VCSEL Applications
  • Two-Dimensional VCSEL Arrays
  • Novel VCSEL Fabrication
  • Novel Laser Structures
  • Emission Modes and Polarization
  • VCSEL Performance and Analysis
  • New Epitaxial Structures
New Epitaxial Structures
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Ultralow-threshold 850-nm oxide-apertured vertical-cavity lasers using AlInGaAs/AlGaAs strained active layers
Jack Ko, Eric R. Hegblom, Yuliya A. Akulova, et al.
Recently 850-nm wavelength has been established as the standard for local area interconnects and data-link modules using GaAs/AlGaAs vertical cavity lasers (VCLs) have become commercially available. However, the lowest threshold current (Ith) up-to-date has been obtained from 980-nm VCLs using strained InGaAs quantum wells. In this presentation we report an ultralow CW, room temperature Ith of 156 (mu) A from a 2.8 micrometers diameter VCL with three AlInGaAs quantum wells in the active region. The AlInGaAs/AlGaAs quantum well active region is used to achieve laser emission near 850 nm while maintaining the benefits of strain in lasers. Previous studies have shown that strained AlInGaAs/AlGaAs in-plane lasers exhibit the same suppression to the propagation of dark-line defects as strained InGaAs lasers. Here we have performed a preliminary burn-in study on our devices to study the reliability in AlInGaAs. AlGaAs VCLs for the first time. We found that devices showed no degradation in either output power or threshold current after 30 hours of on-wafer testing at a constant current density of 22 kA/cm2 and junction temperature of 140 degrees C. We also measured devices at various stage temperatures and found that the lowest Ith, 110 (mu) A for the 2.8 micrometers diameter VCL, occurs near 230 Kelvin, where the quantum well gain peak and the cavity mode are aligned. In addition, we examined the behavior of the external differential efficiency as a function of device size and found that due to a thicker oxide aperture than intended, optical scattering losses start to dominate for devices smaller than 4 micrometers diameter.
Progress in GaInNAs/GaAs long-wavelength vertical-cavity surface-emitting lasers
Michael C. Larson, Masahiko Kondow, Takeshi Kitatani, et al.
GaInNAs is a novel laser diode active layer material which holds great promise for low-cost optical fiber transmission applications requiring emission wavelengths near 1.3 micrometers . GaInNAs permits the realization of a long-wavelength vertical-cavity laser grown directly on a GaAs substrate. Continuous-wave room-temperature photo-pumped laser oscillation has been demonstrated in vertical cavity laser designs employing single or multiple GaInNAs quantum wells, with lasing wavelengths as long as 1.256 micrometers . Electrically-injected devices have achieved pulsed operation at room temperature and above, with a minimum threshold current density of 3.1 kA/cm2, slope efficiency above 0.04 W/A, and output power above 5 mW for 45 micrometers -diameter devices. Threshold current has exhibited minimal dependence on temperature from 20 degrees C to 60 degrees C, and laser oscillation is observed for temperatures as high as 95 degrees C.
Emerging VCSEL Applications
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VCSEL applications in sensors and microsystems
Mial E. Warren, Richard Franklin Carson, William C. Sweatt, et al.
Vertical-cavity surface-emitting lasers (VCSELs) are uniquely suited to miniaturized free-space optical systems in which surface-mounting and hybrid assembly techniques can be used to combine different technologies together. Two examples are described of such microsystems that are being developed for sensing applications. The first example is an optical position sensing system for rotating parts. Progress on fabricating similar system by flip-chip bonding techniques is then discussed. The second examples is a chemical sensing/analysis system which uses a miniature fluorescence detection model that is based on surface- mounted VCSELs and diffractive optical elements. The detection modules is integrated with a capillary electrochromatography separation system and uses substrate- mode light propagation to focus the VCSEL beam on the capillary channel.
Light efficient parallel interconnect using integrated planar free-space optics and vertical-cavity surface-emitting laser diodes
Planar optics is an approach to monolithically integrate free-space optical system. Diffractive microoptical elements are etched into a transparent substrate which serves as a medium for light propagation as well as a board for optoelectronic or electronic components. Mirrors and imaging optics are sued to keep the light traveling within the substrate along a zigzag path. Arrays of VCSEL devices can be integrated on the substrate by means of flip-chip bonding. Among the interesting applications of that technology are optical interconnections for VLSI systems where the optics can provide a large number of parallel channels. A critical issue of the practical realization is the light efficiency of the optical interconnect. Here, we propose a planar-optical implementation of the interconnect using analog grey-scale lithography resulting in a light- efficiency optical system.
Combined photonics and MEMS function demonstration
Olga Blum-Spahn, Mial E. Warren, Hong Q. Hou, et al.
We discuss tow examples of integration of micro- electromechanical system (MEMs) and a photonic device. In the first instance, a MEMs locking device pin is driven by a voltage generated by photovoltaic cells connected in series, which are driven by a laser. In the second case, a VCSEL emitting at 1.06 micrometers is packaged together with a metallized MEMs shutter. By appropriate alignment to the opening in the shutter, the VCSEL is turned on and off by the movement of the Si chopper wheel.
Two-Dimensional VCSEL Arrays
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Oxide-confined 2D VCSEL arrays for high-density inter/intra-chip interconnects
Roger King, Rainer Michalzik, Christian Jung, et al.
We have designed and fabricated 4 X 8 vertical-cavity surface-emitting laser (VCSEL) arrays intended to be used as transmitters in short-distance parallel optical interconnects. In order to meet the requirements of 2D, high-speed optical links, each of the 32 laser diodes is supplied with two individual top contacts. The metallization scheme allows flip-chip mounting of the array modules junction-side down on silicon complementary metal oxide semiconductor (CMOS) chips. The optical and electrical characteristics across the arrays with device pitch of 250 micrometers are quite homogeneous. Arrays with 3 micrometers , 6 micrometers and 10 micrometers active diameter lasers have been investigated. The small devices show threshold currents of 600 (mu) A, single-mode output powers as high as 3 mW and maximum wavelength deviations of only 3 nm. The driving characteristics of all arrays are fully compatible to advanced 3.3 V CMOS technology. Using these arrays, we have measured small-signal modulation bandwidths exceeding 10 GHz and transmitted pseudo random data at 8 Gbit/s channel over 500 m graded index multimode fiber. This corresponds to a data transmission rate of 256 Gbit/s per array of 1 X 2 mm2 footprint area.
Uniformity and performance of selectively oxidized VCSEL arrays
Kent M. Geib, Kent D. Choquette, Hong Q. Hou, et al.
We report the uniformity characteristics of low threshold of 1060 nm and high power 850 nm 8 X 8 individually addressable oxide-confined VCSEL arrays. Uniformity of lasing thresholds and operating characteristics are described, as well as thermal issues for 2D laser arrays.
VCSEL arrays with low threshold currents and stable linear polarization
Markus Hauser, Gerhard Boehm, Marcus Arzberger, et al.
8 X 8 arrays of vertical-cavity surface-emitting lasers (VCSELs) with lateral current injection via the doped GaAs cladding layers have been fabricated. To obtain low series resistances ohmic p-contacts were made to a p-doped GaAs cladding layer above the active MQW-region of the double mesa VCSEL-diodes. Lateral current confinement is made by introducing a 50 nm thick n-type GaAs current blocking layer into the p-doped cladding. The laser diodes are structured by selective removal of this layer in the cavity and subsequent growth of the top Bragg mirror in a second MBE sequence. The epitaxial overgrowth resulted in layers with unchanged structural and optical quality, when compared with layers grown in a single epitaxial run on planar growth surfaces. The overgrowth of the 50 nm steps leads to about 1 micrometers wide facets in the (011) direction. These VCSELs have threshold currents of 470 (mu) A and a maximum output power of 5 mW for diodes with blocking layer diameters of 6 micrometers and 14 micrometers , respectively. The external differential quantum and power conversion efficiencies amount to 46 percent and 16 percent, respectively. The external differential quantum and power conversion in single mode emission at all current levels, and show a stable linear polarization along the (011) crystal axis, due to the facetting mentioned above. The on-off ratio between the orthogonal directions is better than 20 dB. Internal losses and quantum efficiencies have been determined varying the device diameter and number of top DBR pairs. Device characteristics are very homogeneous not only within one array but also over almost the whole two-inch wafer, e.g. threshold currents differ by less than 10 percent.
Novel VCSEL Fabrication
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Integration of LEDs and VCSELs using fluidic self-assembly
Joseph J. Talghader
In this paper the integration of LEDs and VCSELs with silicon electronics using fluidic self-assembly (FSA) is reviewed. FSA is a micromechanical technique in which devices fabricated in one substrate can be integrated with systems on a target substrate. The devices to be integrated are etched into shapes or 'blocks' that match receptor sites or 'holes' in the target substrate. After etching, the blocks are suspended in a slurry. This slurry is placed in a recirculation pump along with the target substrate. The pump moves the slurry over the target substrate where the blocks fall into the holes or are swept off the substrate to be circulated again. FSA has been demonstrated with yields > 99 percent for devices of 30 micrometers and larger. Smaller devices have yet to be tested. A five mask process has been developed that successfully integrates GaAs/AlGaAs LEDs and VCSELs on silicon. The critical steps are: 1) etching the receptor sites in the silicon, 2) isolating the sites from the remainder of the substrate, and 3) bonding/contacting the blocks after assembly. The latter bonding step involves thin film metal multilayers on the block and in the receptor that form a hard solder after a low temperature bake step. The remaining steps of planarizing and interconnecting the devices are similar to those used for VLSI.
Vertical-cavity surface-emitting lasers fabricated using a sealing of AlAs against wet oxidation
Dae-Ho Lim, Gye Mo Yang, Jong Hee Kim, et al.
We have studied the process of sealing on exposed. AlAs and the wet oxidation behavior of AlAs to prevent further wet oxidation. A critical processing step consists of the formation of an oxide surface barrier by the first set oxidation for a short time at 390-430 degrees C in the stream environment of previously room-ambient exposed AlAs surface. During this brief wet oxidation,a dense oxide barrier with a thickness of approximately 1 micrometers is formed, which further blocks diffusing oxygen species during the second wet oxidation. The oxide surface barriers of approximately 1 micrometers thickness formed at 408 degrees C and 410 degrees C have shown the best effectiveness of sealing against further wet oxidation. The effectiveness of the sealing is demonstrated through its use as a mask against wet oxidation in the fabrication of oxide-confirmed vertical-cavity surface-emitting lasers.
Dependence of the lateral oxidation rate of an AlAs layer used as a current aperture in vertical-cavity surface-emitting lasers on different physical parameters
Bikash Koley, Mario Dagenais, Ruxiang Jin, et al.
The wet oxidation kinetics of an AlAs layer used as a current aperture in selectively oxidized vertical-cavity- surface-emitting-lasers (VCSELs) is investigated in details. The process is modeled as a diffusion-reaction process. A strong dependence of the oxidation rate on the temperature, at which the wet oxidation is being carried out, is observed. The temperature dependence of the oxidation process is explained theoretically by considering equivalent reaction activation energies for the oxidation reaction. Also for oxidation over a long time interval, variation of the oxidation rate with the variation of the radius of the etched mesa of the VCSEL is observed. A theory has been developed considering the 3D diffusion of the oxidant modules is an already oxidized cylindrical AlAs region and the reaction of the diffusion of the oxidant molecules in an already oxidized cylindrical AlAs region and the reaction of the oxidant molecule at the oxidized-unoxidized AlAs interface. Relevant material parameters, that are independent of the size and geometry of the etched VCSELs, are extracted from the experimental results. Using them in the theoretical model, the rate equation of the lateral oxidation process is obtained. The theory predicts the dependence of the oxidation process on the size of the VCSEL, the AlAs layer thickness, as well as on the physical properties of the AlAs layer. The theoretical predictions have been verified by a number of experiments with reproducible results.
Real-time-controlled epitaxy of vertical-cavity surface-emitting lasers (VCSELs) using a standard pyrometer
Gerhard Boehm, Markus Hauser, Markus Sexl, et al.
For the reproducible growth of VCSELs by MBE a growth rate control of better than +/- 0.3 percent is necessary. To achieve this high accuracy, a real-time monitoring of the growth is essential. In our work on 0.98 micrometers VCSELs the standard pyrometer read-out is used as a simple and inexpensive tool to maintain the center wavelength of the Bragg reflectors as well as the cavity resonance precisely within +/- 2 nm. During growth the pyrometer signal is oscillating mainly for layers with different refractive indices. The correlation of the phase information and the period of the oscillation with ex-situ reflection measurements obtained in a series of calibration samples provides precise information on the actual growth rate. Practically this is done by fitting real-time the minima and maxima of the oscillating temperature. The obtained information can be used to adjust the growth rates by varying the source temperatures. For the regrowth of patterned laser structures this technique proved successful as well.
Novel Laser Structures
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VCSELs with a novel integrated quantum-well absorber
Sui F. Lim, Janice A. Hudgings, Gabriel S. Li, et al.
The high reflectivity of VCSEL mirrors renders the device very sensitive to small changes int he reflectivity or absorption within the laser cavity. Therefore, the integration of a quantum-well absorber within the VCSEL enables compact, monolithic devices with multiple functions. As a result, the VCSEL exhibits a variety of useful characteristics under different configurations. In this work, we demonstrate the use of a quantum-well absorber within one of the VCSEL mirror stacks. With strategic design and simple biasing circuitry, we have shown experimentally (1) a VCSEL with an integrated quantum-well photodetector, (2) a self-pulsating VCSEL with a controllable quantum-well saturable absorber, and (3) a novel technique for VCSEL modulation where the quantum-well absorption is modulated, thereby modulating the laser light output. The theory, design criteria, experimental results, and potential applications for these devices are discussed.
Coupled-resonator vertical-cavity laser
Kent D. Choquette, Weng W. Chow, Hong Q. Hou, et al.
The monolithic integration of coupled resonators within a vertical cavity laser opens up new possibilities due to the unique ability to tailor the interaction between the cavities. We report the first electrically injected coupled resonator vertical-cavity laser diode and demonstrate novel characteristics arising form the cavity coupling, including methods for external modulation of the laser. A coupled mode theory is used model the output modulation of the coupled resonator vertical cavity laser.
Room-temperature cw operation of InGaAsP/InP microdisk lasers
Damien Courtney, Peter O'Brien, Peter M. W. Skovgaard, et al.
Microcavity semiconductor lasers are interesting for fundamentals of controlled spontaneous emission, thresholdless lasing, quantum physical interactions between photons, excitons and cavity polaritons. They also promise novel engineering applications such as low noise oscillators, filters and optoelectronic switches. Microdisk lasers which support whispering gallery modes are especially useful in that they provide 2D wavelength scale confinement. Novel InGaAsP/InP microdisks supported on glass substrates have recently been fabricated in our laboratories and have been pumped optically, resulting in the first CW room temperature lasing in these devices.
Emission Modes and Polarization
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Transverse modes, patterns, and polarization behavior in VCSELs
John Gerard McInerney, Hua Li, Yi-Guang Zhao
Vertical cavity surface-emitting semiconductor lasers provide solutions for many engineering applications and fundamental scientific investigations. Knowledge of the transverse field and polarization properties is often essential, and in many cases it is highly desirable to select a single predetermined transverse mode and polarization state. Here we review recent research in characterizing, modeling and controlling transverse modes and polarization effects in vertical cavity lasers.
Optical versus electrical pumping in VCSELs
We present a detailed device characterization of a series of optically pumped VCSELs, emphasizing in particular a comparison with electrically pumped devices. We conclude that fundamental device parameters such as threshold pump power, input-output efficiency and polarization behavior do not depend on the pumping scheme.
Polarization selection in external-cavity birefringent vertical-cavity surface-emitting lasers
Angel Valle, Luis Pesquera, Jaume Dellunde, et al.
Calculations are reported of the effect of weak optical feedback on the polarization properties of birefringent vertical-cavity surface emitting lasers (VCSELs) in an external cavity configuration. Attention is focused on the competition between two orthogonal polarization of the fundamental transverse mode of the weakly index VCSEL. Ohmic heating effects on polarization selection and sensitivity of external cavity VCSELs are analyzed. We show that control of the emission polarization selection and sensitivity of external cavity VCSELs are analyzed. We show that control of the emission polarization can be exercised even for very small external reflectivities by appropriate choice of optical feedback delays. The range of currents in which a particular polarization is selected narrows as thermal effects become important. A controlled displacement of the external reflector with respect to the laser can maintain the polarization over a wide current range in the presence of thermal effects. The polarization selectivity is shown to be dependent upon the strength of optical feedback. Polarization is also shown to be highly sensitive to small changes in optical feedback delay and to very small unwanted reflections.
Bias-induced polarization properties in vertical-cavity surface-emitting lasers
Byueng-Su Yoo, Hye Yong Chu, Hyo-Hoon Park, et al.
We report on the bias-induced polarization changes of optically pumped top-emitting vertical-cavity surface- emitting lasers (VCSELs). Under optical pumping with zero bias voltage, tow orthogonal polarization states along (110) and 110) crystallographic axes are simultaneously observed with small differences in the relative partitioning. The partitioning of the optically pumped output power between the linear polarization states is shown to be strongly dependent on the applied electrical bias. With increased forward bias, the emission polarized to the dominant polarization direction of the electrically pumped output of the VCSEL is enhanced and becomes a dominant polarization states, suggesting that the polarization characteristics of the device under electrical pumping are closely related to the applied electrical current.
Off-axis emission from native-oxide-confined VCSELs
We analyze the near and far field intensity distributions of large aperture native oxide confined AlGaAs/GaAs VCSELs. We demonstrate single mode highly divergent emission when the cavity is red shifted with respect to the quantum well gain peak. As the gain peak is detuned red of the cavity, the VCSEL operates in multiple less divergent modes.
VCSEL Performance and Analysis
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High-power VCSELs: modeling and experimental characterization
Rainer Michalzik, Martin Grabherr, Karl Joachim Ebeling
Broad-area oxide confined vertical cavity surface emitting lasers are investigated theoretically and experimentally as high-power laser sources. A self-consistent laser model comprising current and temperature distribution, carrier diffusion, and a simplified optical submodel is employed to explain measured output characteristics. Top and bottom emitting devices of various diameters are designed and fabricated and the scaling laws for various laser parameters are extracted. A comparison between both emission schemes is provided. Maximum output powers of 180 mW and 350 mW obtained from both top emitters and heat-sink mounted bottom emitters of 150 micrometers and 200 micrometers active diameter, respectively, represent the state-of-the-art. Even higher output powers at improved conversion efficiencies are suggested to be obtained from densely spaced 2D arrays with properly applied heat-sinking.
Full vectorial eigenmode analysis of VCSELs: threshold gain values and modal frequencies
Dariusz W. Burak, Rolf H. Binder
A complete theoretical approach to the electromagnetic properties of vertical-cavity surface-emitting lasers (VCSELs) is presented. The Maxwell's equations are solved by using the local mode expansion technique. The transverse components of the electromagnetic field in each layer of the cavity are expressed in terms of complete set of orthogonal local modes. Matching these components at each boundary yields the vectorial transform matrix of the structure. The cavity eigenmodes are found from the condition of vanishing in-coming amplitudes. Results for the characterization of laser modes include modal frequencies, threshold gains and eigenmode light-field vector patterns. The influence of scattering losses on threshold gain values as a function of VCSEL radius, model order and number of dielectric layers are discussed.
Optical optimization of microcavity VCSELs
Michael J. Noble, John P. Loehr, James A. Lott
We present a full vector, finite element analysis of oxide apertured VCSELs, focusing on the optical properties required for low threshold design. We examine several versions of an 870 nm oxide DBR, oxide aperture VCSEL design to gain insight into the physical processes determining diffractive loss. Our results suggest the diffraction may be modeled as a coupling loss to the parasitic mode continuum. In this approach, the loss is determined from two competing factors: (1) the lasing mode penetration into the radial cladding region, and (2) the relative alignment of the eignenmode and parasitic mode wavevectors. We also find the characteristic blueshift resulting from the transverse optical confinement.
Vertical-cavity surface-emitting laser: theoretical analysis and experimental investigation on white amplitude noise
Philippe Signoret, Bernard Pierre Orsal, M. Van De Bergh
The amplitude noise of a VCSEL emitting in the 840 nm range has been studied in a medium frequency range. The optical noise spectral density is similar to the optical nose of a 'normal' edge emitting laser. However, the value of the white electrical nose spectral density is two decades higher than the shotnoise reference level; over theoretical approach allow us to think that the electrical noise is dominated, above threshold, by the noise of the multiple mirror junctions.
New Epitaxial Structures
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VCSELs in '98: what we have and what we can expect
Larry A. Coldren, Eric R. Hegblom, Yuliya A. Akulova, et al.
Recent results from the authors group are summarized as a general indicator of the current state of the art in vertical-cavity surface-emitting lasers (VCSELs). These include results from engineered-aperture VCSELs with high wall-plug efficiencies at low powers, high-efficiency bottom-emitting cryo-VCSELs with wavelengths < 900 nm, low-threshold AlGaInAs VCSELs emitting at 850 nm, and arrays of VCSELs used in parallel free-space links as well as WDM arrays butt-coupled to multimode fiber. Analysis indicates that size-dependent losses limit the scaling of VCSELs below 5 micrometers in diameter unless special engineered apertures and/or short cavities are used. The analysis also shows that lateral carrier confinement is necessary to obtain efficient devices below 2 micrometers in diameter.