The rapid pace of advances in vertical-cavity surface- emitting lasers (VCSELs) has continued over the past couple of years. The widespread use of dielectric apertures formed primarily by lateral oxidation has provided much lower cavity losses, and this has enables a large decrease in device threshold as well as an increase in efficiency. The lowest optical losses have been obtained with thin or tapered oxide apertures. Within the past year, new strained- layer materials such as AlGaInAs have been incorporated to extend the benefits of strain to the 850 nm wavelength range. A record threshold of 290 (mu) A at 840 nm has been obtained. Devices have been designed for ultra-wide operating temperature ranges by using gain from different quantum levels at different temperatures. Submilliamp thresholds from 77 K to 373 K were demonstrated. The inclusion of low-loss dielectric apertures in wafer-bonded 1.55 micrometer InP/GaAs has yielded VCSELs with submilliamp thresholds for the first time. In addition, there has been considerable effort in making VCSEL arrays for parallel or free-space interconnect applications. Multiple wavelength arrays for even denser interconnects or wavelength addressing schemes have also been explored. In this paper we review some of this recent progress and point out issues still inhibiting further advances.

This manuscript reviews the present status of 'commercial- grade,' state-of-the-art planar, batch-fabricable, vertical- cavity surface-emitting lasers (VCSELs). Commercial-grade performance on all fronts for high-speed data communications is clearly established. In discussing the 'present,' we focus on the entrenched proton-implanted AlGaAs-based (emitting near 850 nm) technology. Renditions of this VCSEL design exist in commercial products and have enabled numerous application demonstrations. Our designs more than adequately meet producibility, performance, and robustness stipulations. Producibility milestones include greater than 99% device yield across 3-in-dia metal-organic vapor phase epitaxy (MOVPE)-grown wafers and wavelength operation across greater than 100-nm range. Progress in performance includes the elimination of the excessive voltage-drop that plagued VCSELs as recently as 2 to 3 years ago. Threshold voltages as low as V_th equals 1.53 V (and routinely less than 1.6 V) are now commonplace. Submilliamp threshold currents (I_th equals 0.68 mA) have even been demonstrated with this planar structure. Moreover, continuous wave (cw) power P_cw greater than 59 mW and respectable wall-plug efficiencies ((eta) _wp equals 28%) have been demonstrated. VCSEL robustness is evidenced by maximum cw lasing temperature T equals 200 degrees Celsius and temperature ranges of 10 K to 400 K and minus 55 degrees Celsius to 155 degrees Celsius on a single VCSEL. These characteristics should enable great advances in VCSEL-based technologies and beckon the notion that 'commercial-grade' VCSELs are viable in cryogenic and avionics/military environments. We also discuss what the future may hold in extensions of this platform to different wavelengths, increased integration, and advanced structures. This includes low-threshold, high- speed, single-mode VCSELs, hybrid VCSEL transceivers, and self-pulsating VCSELs.

We report high performance 850 nm VCSELs grown by OMVPE on both n-type and p-type GaAs substrates for low cost fiber optic data communication applications. These devices are intended for use in discrete and parallel array applications at data rates up to 1.5 Gbps per channel. Good epitaxial thickness uniformity during multi-wafer growth allows low cost manufacturing and reproducible device performance. Preliminary device reliability testing shows excellent stability in VCSEL performance under accelerated stress conditions.

We present the growth and characterization of vertical- cavity surface emitting lasers (VCSELs) from visible to near-infrared wavelength grown by metalorganic vapor phase epitaxy. Discussions on the growth issue of VCSEL materials include the control on growth rate and composition using an in situ normal-incidence reflectometer, optimization of ultra-high material uniformity, and comprehensive p- and n- type doping study in AlGaAs by CCl₄ and Si₂H₆ over the entire Al composition range. We also demonstrate our recent achievements of selectively oxidized VCSELs which include the first room-temperature continues-wave demonstration of all-AlGaAs 700-nm red VCSELs, high- performance n-side up 850-nm VCSELs, and low threshold current and low-threshold voltage 1.06 micrometer VCSELs using InGaAs/GaAsP strain-compensated quantum wells.

We report an in-situ method for accurate flux monitoring in gas-source molecular-beam epitaxy (GSMBE) growth of vertical-cavity surface-emitting lasers (VCSELs). The values of the effusion cell fluxes are determined using a single reflectivity spectrum made on the incomplete structure. The flux values used for the growth of the first part of the device are extracted from the reflectivity spectrum using a simulated annealing algorithm. The knowledge of the flux values allows us to calculate the thickness and the composition of the layers used for the growth of the first part of the device. Before completing the structure, corrections are made on the last part of the device in order to adjust the cavity resonance wavelength of the final device at the desired wavelength. Using this method we have obtained a cavity resonance wavelength controlled to 0.15%.

The microstructure and interface properties of Al_xGa_{1- x}As materials that have been laterally oxidized in wet N₂ for several compositions (x equals 0.80, 0.82 . . . 1.00) and temperatures (360 degrees Celsius to 450 degrees Celsius) have been studied. The micro-structure is found to be relatively insensitive to composition and oxidation temperature. The oxidation forms an amorphous solid solution (Al_xGa_1-x)₂O₃ that transforms to polycrystalline, (gamma) -(Al_xGa_1-x)₂O₃ under electron beam exposure in the electron microscope. Evidence suggests a small fraction of crystalline (Al_xGa_{1- x})₂O₃ is formed via post oxidation annealing of the oxide. The level of hydrogen present in the oxidized layers is 1.1 multiplied by 10²¹ cm^-3, which is too low for the amorphous phase observed to be a hydroxide rather than an oxide. The amount of As in the layer is reduced to less than 2 atm%, and no As precipitates are observed. The (Al_xGa_1-x)₂O₃/GaAs interface is abrupt, but prolonged oxidation will cause the GaAs to oxidize at the internal interfaces. The reaction front between the oxidized and the unoxidized Al_xGa_1-xAs has a 10 to 20 nm-wide amorphous zone that shows a different contrast than the remainder of the amorphous oxide and is stable under electron irradiation.

Time resolved photoluminescence at 295 degrees K has been used to characterize carrier recombination in a single 80 angstrom In_0.20Ga_0.80As quantum well before and after wet thermal oxidation of a 300 angstrom Al_0.96Ga_0.04As layer which is separated from the quantum well by 100 angstrom GaAs and a 225 angstrom Al_0.75Ga_0.25As barrier layer. Both of these layers are repeated on the other side of the quantum well and all together are typical of a half wave cavity spacer section used in low threshold microcavity VCSELs. Before oxidation the radiative lifetime is 12 ns. After steam oxidation for 5 minutes at 420 degrees Celsius the lifetime and intensity of the photoluminescence remains unchanged. An oxidation time of 10 minutes at the same temperature reduces the radiative lifetime to less than 1 ns and decreases the photoluminescence intensity by a factor of five. In addition, the lifetime and intensity of the photoluminescence remain the same as in the unoxidized case when the Al_0.96Ga_0.04As layer is etched off in a 1:1 HCl solution, possibly indicating that surface recombination at the Al_0.75Ga_0.25As barrier is not responsible for the shorter lifetimes in the oxidized samples. Furthermore, secondary ion mass spectrometry data on steam oxidized and unoxidized samples shows the presence of a significant oxygen concentration in the quantum well for oxidized samples that had sub nanosecond lifetimes and no oxygen in the quantum wells for samples that were not steam oxidized and displayed 12 ns lifetimes.

To insert high performance oxide-confined vertical-cavity surface-emitting lasers (VCSELs) into the manufacturing arena, we have examined the critical parameters that must be controlled to establish a repeatable and uniform wet thermal oxidation process for AlGaAs. These parameters include the AlAs mole fraction, the sample temperature, the carrier gas flow and the bubbler water temperature. Knowledge of these critical parameters has enabled the compilation of oxidation rate data for AlGaAs which exhibits an Arrhenius rate dependence. The compositionally dependent activation energies for Al_xGa_1-xAs layers of x equals 1.00, 0.98 and 0.92 are found to be 1.24, 1.75, and 1.88 eV, respectively.

In this paper, we measure the size dependent optical scattering and electrical losses in etched-post and dielectrically apertured vertical-cavity lasers (VCLs). We show that reduced optical scattering losses are responsible for the dramatic improvement in device scaling seen with the use of the oxide-defined apertures. Furthermore, we experimentally show how to reduce this optical scattering loss through the use of thin apertures. We find that the electrical losses (due to current leakage around the active region and carrier diffusion in the active region) in the structures are minimized by reducing the doping near the active region, minimizing the current leakage. Finally, based on the experimental results, theoretical design curves for VCL scaling are calculated.

Vertical cavity surface emitting lasers (VCSELs) have been the subject of intense research in recent years. The compact nature of the devices means that heat generated within is not as readily dissipated as with more conventional stripe geometry lasers. Advances in the design of distributed Bragg reflector (DBR) cavity mirrors and intracavity contact schemes have reduced the threshold voltage from greater than 10 V to little more than the lasing photon potential, in some cases. However, thermal management is still a limiting factor for high power or high efficiency output from VCSELs By analyzing a variety of devices we have devised a simple but powerful model to explain the current-light response of VCSELs which is strongly dependant on the temperature rise in the active layer. Effects of the relative position of the cavity resonance and gain spectrum are also discussed.

We present the primary laser printing system performance issues which are the driving forces for multiple beam laser printing. Although edge emitting semiconductor lasers have allowed progress in this area, vertical cavity lasers have substantial advantage in the longer term. We further present needs and issues with VCSEL performance which must be addressed for the application of VCSELs to high performance laser printing. Finally, we explore advanced print architectures which would be enabled by the VCSEL device.

Applications for serial and parallel fiber optic data links are reviewed along with the barriers to widespread commercial adoption. An alternative migration path from copper to optical media, enabled by VCSEL technology, is investigated including initial performance results.

Analog optical vector matrix processors (AOVMP) have been implemented over the past three decades utilizing a variety of methodologies. Most of these methodologies were dependent on external modulation of the laser source. Photonic Systems Incorporated has furthered the development of the AOVMP by using a 64 channel analog modulated vertical cavity surface emitting laser diode (VCSEL). The novel analog modulation of the VCSEL is performed by linearizing the output of the VCSEL to 8 bits using real-time 12 bit look-up tables. VCSEL analog modulation characteristics and linearization techniques are discussed along with AOVMP performance.

Traditionally, large core (greater than 100 micron) step index multimode optical fiber has occupied a reactively small niche of applications in data communications. While the large diameter of this type of fiber makes it easy to align to optoelectronic devices, its bandwidth*distance (BW*D) product is low due to modal dispersion between the large number of modes supported by a fully filled fiber. Recently, interest has been renewed in using an underfilled launch to excite 62.5 micron core graded index multimode fiber as a way to improve its bandwidth performance. With the proper launch conditions, this same effect has been measured in large core fiber. A vertical cavity surface emitting laser (VCSEL) is used to provide a low numerical aperture launch into a large core fiber, which has a relatively large numerical aperture. The laser thus underfills the modes of the fiber, and a bandwidth enhancement for the fiber is obtained. Results of experiments performed on step and graded index large core multimode fibers using a direct VCSEL launch are presented. In addition, these relaxed alignment tolerance fibers allow the utilization of very low cost cabling and connectorization procedures for parallel optical fiber cables. Data, including skew, bandwidth, and insertion loss, are presented on these cables.

Data are presented on half-wave cavity vertical cavity surface emitting lasers using both oxide confinement and high contrast upper and lower distributed Bragg reflectors. Six pairs of MgF/ZnSe distributed Bragg reflectors make up the top mirror and 11 pairs of Al_xO_y/GaAs DBRs make up the lower DBR. The oxide layer thickness in the Al_xO_y/GaAs DBRs is less than a quarter-wavelength in order to reduce strain on the quantum well active region. The lasing characteristics of device sizes ranging from 7 micrometer to 1 micrometer are analyzed through spectral data, far field radiation patterns and lasing threshold.

Wafer-bonded AlAs/GaAs mirrors and AlGaInAs strain- compensated multiple quantum well active layers have been applied into 1.3 micrometer vertical-cavity surface-emitting lasers (VCSELs). Double-bonded 1.3 micrometer VCSELs have operated at room temperature pulsed conditions with a high output power of 4.6 mW, a high characteristic temperature of 132 K, and a large side-mode suppression-ratio of 42 dB. A novel more practical approach for 1.3 micrometer VCSELs have been proposed and demonstrated a very low room temperature pulsed threshold current density of 1.13 kA/cm² and a very low threshold current of 2 mA. Further improvement focusing on practical approaches for long wavelength VCSELs is underway.

In this work we report on 64 degree celsius continuous-wave operation of a 1.5 micrometer vertical cavity laser. This laser consists of two fused Al(Ga)As/GaAs mirrors with a strain-compensated InGaAsP/InP active region. Selective lateral oxidation is used for current confinement. Minimum room temperature threshold current is as low as 0.8 mA, and maximum cw output power is as high as 1 mW at 15 degrees Celsius. Pulsed operation is achieved up to 100 degrees Celsius.

Scaling of the threshold current density in apertured vertical cavity lasers is limited by scattering losses, current spreading, and carrier diffusion. We consider the contributions of all three effects, but focus on current spreading. We analyze a vertical cavity laser (VCL) with low scattering losses so the scaling of the threshold current density is dominated by current spreading under the aperture. We show that a simple analytic estimate (appropriate for circular geometry) for the increase in threshold matches experimental data extremely well without any fitting parameters. One can also conveniently apply the estimate of current spreading to VCLs with double apertures or multiple layers of different resistivities. We also show that current spreading should only negligibly reduce the differential efficiency as implied by experiment.

For many years, the operation of long-wavelength (1.3 micrometer and 1.55 micrometer) vertical-cavity surface- emitting lasers (LW-VCSELs) was restricted to low temperatures. Continuous-wave lasing above room-temperature (up to 64 degrees Celsius) has been achieved only recently. The strong temperature sensitivity of the lasing threshold is well known from their edge-emitting counterparts, but LW- VCSELs exhibit principal differences. Focusing on the so far most successful concept of wafer-fused LW-VCSELs, the physical mechanisms are analyzed that affect their temperature sensitivity. The analysis includes optical gain, carrier losses, optical losses, and self-heating. Photon absorption within the valence bands and auger recombination are found to limit high-temperature lasing. Scaling down the active area by lateral oxidation, VCSEL self-heating can be reduced despite a rising thermal resistance. Based on an excellent agreement with measurements at lower temperatures, numerical VCSEL simulation is employed to investigate laser operation up to 120 degrees Celsius. With minimization of threshold current and absorption losses and with proper adjustment of the gain peak wavelength, high-temperature continuous-wave lasing is predicted that is less temperature sensitive than in edge-emitting lasers.

We examine the threshold characteristics of selectively oxidized VCSELs as a function of the number, thickness, and placement of the buried oxide apertures. The threshold current density for small area VCSELs is shown to increase with the number of oxide apertures in the cavity due to increased optical loss, while the threshold current density for broad area VCSELs decreases with increasing number of apertures due to more uniform current injection. Reductions of the threshold gain and optical loss are achieved for small area VCSELs using thin oxide apertures which are displaced longitudinally away from the optical cavity. We show that the optical loss can be sufficiency reduced to allow lasing in VCSELs with aperture area as small as 0.25 micrometer².

A novel integration method is described that relies on the thermal oxidation of AlAs to form a buried current blocking layer. This integration technology, called thermal oxidation isolation (TOI), is an extension of recent work involving oxidized VCSELs. However, in addition to incorporating a conventional thermal oxide current aperture to define VCSEL active regions, a buried oxide layer is also used to provide inter-device isolation. As a demonstration of this concept, a GaAs MESFET and resonant cavity LED are integrated and characterized. The buried oxide layer is situated under the FET channel such that the transistor is effectively stacked on top of the LED. The oxide layer is also used to form a current aperture in the LED and directs current flow vertically through this device. Solid-source MBE is used to grow the device layers on a p-type GaAs substrate. The epitaxial structure consists of a p-type bottom mirror consisting of 24.5 pairs of alternating AlAs and GaAs quarter-wave layers, an undoped one-wave active region containing 3 multiplied by 80 angstrom InGaAs quantum wells and a single n-type AlAs/GaAs top mirror period. The fabrication sequence, described in some detail, is straightforward. A wet etch is used to define one mesa for the LED and a second for the MESFET. The top AlAs layer, exposed at the mesa periphery by this etch, is oxidized at 410 degrees. Celsius in a steam ambient to form the current- guiding regions. A conventional MESFET fabrication sequence is then used to complete the transistor and form the LED cathode (which is connected to the FET drain). A back contact is then deposited to form the LED anode. In all, five mask levels are used to fabricate the integrated FET/LED (or VCSEL) structure. Functionality of these prototype devices is demonstrated by dc and modulation measurements. The MESFET gate length and width are 3 micrometer and 100 micrometer, respectively. The transistor operated in the depletion mode with a typical Idss of 8 mA and a maximum transconductance of 35 mS/mm. The LED wavelength is about 990 nm and has output power in the (mu) W range when driven by the MESFET.