Oxide Aperture Research Articles

Effect of oxide aperture on the performance of 850 nm vertical-cavity surface-emitting lasers

An experimental study has been presented of the oxide-confined vertical-cavity surface-emitting lasers (VCSELs) operating in the 850 nm region of the electromagnetic spectrum. In this regard, various relevant VCSEL samples with numerous oxide aperture sizes have been fabricated and characterized. Thorough investigations of the electrical as well as optical characteristics of the fabricated samples have been performed, which include the overall device performance as a function of the oxidize aperture sizes. It is reported that the VCSELs with oxide aperture size <10 μm require low threshold currents (<1 mA). Further, the differential quantum efficiencies up to 28% corresponding to wall-plug efficiencies of up to 15% were measured for a number of these devices.

Low current density, inverted polarity, high-speed, top-emitting 850 nm vertical-cavity surface-emitting lasers

High-speed, oxide-confined, inverted polarity (n-up), polyimide-planarised 850 nm vertical-cavity surface-emitting lasers (VCSELs) were fabricated and characterised. The lasers exhibit a 23 dB frequency modulation bandwidth (f3dB) up to 15.2 GHz with a 10 mm oxide aperture diameter, at the lowest current density (Jbias) ever reported of 6.4 kA/cm 2 . The ratio f3dB 2 /Jbias ¼ 36.1 (GHz 2 /kA/cm 2 ) represents a 21% increase when compared with the highest pre- viously reported ratio. The threshold voltage and current were as low as 1.45 V and 0.9 mA, respectively, with a series resistance of 65 V. A rate-equation-based thermal VCSEL model was used to predict the device performance at different temperatures. Good agreements between measured and simulated DC characteristics were obtained.

The use of spatially ordered arrays of etched holes for fabrication of single-mode vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots

To suppress the generation of high-order modes in vertical-cavity surface-emitting lasers based on submonolayer InGaAs quantum dots, the method of formation of a spatially ordered array of etched holes in the upper distributed Bragg reflector was used. Single-mode vertical-cavity surface-emitting lasers for spectral region of 990 nm with current-aperture diameter of 20 μm, threshold current 0.9 mA, and maximum output power 3.8 mW at room temperature were demonstrated. Single-mode lasing with the coefficient of side-mode suppression in excess of 35 dB is retained in the entire range of pump currents. A decrease in the current oxide aperture to sizes that are close to those of the optical aperture brings about an increase in the external quantum efficiency; however, in this case, the transition to the multimode of lasing is observed at high pump currents.

Oxide aperture scaling effect on high-speed (>16 GHz) vertical-cavity surface-emitting lasers performance

High-speed, oxide-confined, polyimide-planarized 850 nm vertical-cavity surface-emitting lasers (VCSELs) with oxide aperture diameters of 9, 10, 12, 15, 20, and 30 μm have been fabricated and characterized. For a 9 μm oxide aperture diameter, the lasers exhibit a resonance frequency, a 3-dB modulation frequency, and a modulation current efficiency factor (MCEF) up to 12.4, 16.5 GHz, and 10.9 GHz/mA 1/2, respectively, at only 7.9 kA/cm 2. Threshold voltage and current were 1.45 V and 0.7 mA, respectively. It is demonstrated that increasing the resonance frequency with bias does not guarantee a higher modulation bandwidth. The influence of oxide aperture scaling effect on VCSEL performance is presented.

High-Temperature Dynamics, High-Speed Modulation, and Transmission Experiments Using 1.3- $\mu\hbox{m}$ InGaAs Single-Mode VCSELs

High-temperature dynamics, including small and large signal modulation response, of 1.28-μm GaAs-based vertical-cavity surface-emitting lasers (VCSELs) with highly strained InGaAs quantum wells have been investigated. The VCSELs are oxide-confined with a relatively large oxide aperture for high output power and have a surface relief for fundamental mode operation. An inverted surface relief is used to improve manufacturability and to suppress oxide modes that otherwise appear in VCSELs with a large detuning between the cavity resonance and the gain peak. The size dependence of the modulation bandwidth and bandwidth limitations are investigated. A VCSEL with an optimum combination of oxide aperture and surface relief diameters produces clear open eyes with an extinction ratio of > 6 dB at OC-48 and 10-GbE bit rates from 25 °C to 85 °C under constant drive conditions. The same VCSEL is also shown to be capable of error-free transmission (BER < 10-9) over 9 km of standard single-mode fiber under the same conditions.

10-GHz 8.7-ps Pulse Generation From a Single-Mode Gain-Switched AlGaAs VCSEL at 850 nm

We have successfully generated a 10-GHz 8.7-ps pulse train at 850 nm by using a gain switching technique with a high-speed AlGaAs-based vertical-cavity surface-emitting laser (VCSEL). The VCSEL, which operates in a single transverse mode by employing an aluminum oxide aperture, has a modulation bandwidth of 6.3 GHz and a relaxation oscillation frequency at around 11.7 GHz. An error-free 10-Gb/s return-to-zero data modulation was demonstrated by using the generated 10-GHz pulse train.

Single-Mode InGaAs Photonic Crystal Vertical-Cavity Surface-Emitting Lasers Emitting at 1170 nm

Fabrication and performance of large-detuning strained, double-quantum-well, proton-implanted photonic crystal, vertical-cavity surface-emitting lasers is reported. The proton implant aperture is used to confine the current flow and the single-point defect photonic crystal is used to confine the optical mode, while the oxide aperture is introduced to reduce the leakage current. Single fundamental mode (side-mode suppression ratio ) continuous wave output power of has been achieved in the range at room temperature.

Suppression of Higher Order Transverse and Oxide Modes in 1.3-$\mu$m InGaAs VCSELs by an Inverted Surface Relief

It is shown, by a systematic variation of design parameters, that the use of an inverted surface relief is very effective for suppressing higher order transverse modes in oxide confined 1.3-mum InGaAs vertical-cavity surface-emitting lasers (VCSELs). Single-mode emission is achieved for a large variety of oxide aperture and surface relief diameters, with optimum designs, having a surface relief with a diameter half of that of the oxide aperture, producing 1.1-1.3 mW of single-mode power. It is also shown that the anti-phase layer employed to enable the use of an inverted surface relief is effective for suppressing oxide modes that otherwise appear in oxide confined VCSELs with a large detuning between the gain peak and the cavity resonance

Gigahertz modulation of GaAs-based bipolar cascade vertical cavity surface-emitting lasers

The high-frequency modulation characteristics of GaAs-based bipolar cascade vertical cavity surface-emitting lasers operating at 980 nm with GaAs tunnel junctions and p-doped Al0.98Ga0.02As oxide apertures have been measured. We achieve -3 dB laser output modulations of 6.5 GHz for two-stage and 9.4 GHz for three-stage devices in response to small-signal current injection at an operating temperature of -50 degrees C.

Optimization of oxide-confined vertical-cavity surface-emitting diode lasers

The performance of modern oxide-confined (OC) vertical-cavity surface-emitting diode lasers (VCSELs) is distinctly different than that of earlier VCSEL designs. As a result, hitherto known optimal VCSEL designs cannot be considered as the best ones any longer; they need new analysis to determine their optimal OC designs for their various applications. Therefore, in this paper, a comparative analysis of the room-temperature (RT) continuous-wave (CW) threshold operation of various designs of OC VCSELs is carried out using the 1.3 µm OC intra-cavity-contacted GaAs-based quantum-well (GaIn)(NAs)/GaAs VCSEL as a typical example. The analysis has been made with the aid of a comprehensive fully self-consistent three-dimensional optical-electrical-thermal-gain model. Depending on an application, different OC VCSEL designs have been found to be optimal. The lowest RT CW lasing threshold has been found to be achieved in the 1.5λ-cavity design with only one oxide aperture (for smaller active regions) or in the 3λ-cavity design with two oxide apertures (for larger active regions). On the other hand, to reach the single-fundamental-mode operation and/or longer lasing wavelengths, the 3λ-cavity OC VCSEL with one oxide aperture happened to be the best lasing design, especially for relatively large active regions.

Self-Aligned Microlens-Integrated Vertical-Cavity Surface-Emitting Lasers

A novel method for fabricating microlens-integrated vertical-cavity surface-emitting lasers (VCSELs) by selective oxidation of composition-graded digital alloy AlGaAs was developed. Due to the simultaneous formation of a microlens and oxide aperture via single-step oxidation, self-aligned microlens-integrated VCSELs could be fabricated using standard intracavity-contacted VCSEL processing without additional process steps. The output beam from the oxide-buried and oxide-removed AlGaAs microlens-integrated VCSELs was strongly focused to a beam radius of 3.7 and 3.5 mum at a distance of about 25 and 15mum, respectively, from the surface of the VCSEL

Single-mode submonolayer quantum-dot vertical-cavity surface-emitting lasers with high modulation bandwidth

Single-mode vertical-cavity surface-emitting lasers based on dense arrays of stacked submonolayer grown InGaAs quantum dots, emitting near 980nm, demonstrate a modulation bandwidth of 10.5GHz. A low threshold current of 170μA, high differential efficiency of 0.53W∕A, and high modulation current efficiency factor of 14GHz∕mA are realized from a 1μm oxide aperture single-mode device with a side mode suppression ratio of &amp;gt;40dB and peak output power of &amp;gt;1mW. The lasers are also suitable for high temperature operation.

Vertical-Cavity Surface-Emitting Lasers Based on Submonolayer InGaAs Quantum Dots

Molecular beam epitaxy-grown 0.98-mum vertical-cavity surface-emitting lasers (VCSELs) with a three-stack submonolayer (SML) InGaAs quantum-dot (QD) active region and fully doped AlxGa 1-xAs-GaAs DBRs was studied. Large-aperture VCSELs demonstrated internal optical losses less than 0.1% per one pass. Single-mode operation throughout the whole current range was observed for SML QD VCSELs with the tapered oxide apertures diameter less than 2 mum. Devices with 3-mum tapered-aperture showed high single-mode output power of 4 mW and external quantum efficiency of 68% at room temperature

12.5-Gbps Operation of 850-nm Vertical-Cavity Surface-Emitting Lasers With Reduced Parasitic Capacitance by BCB Planarization Technique

12.5-Gbps operation of 850-nm AlGaAs-based VCSEL fabricated using low-k benzocyclobutene (BCB) planarization technique is reported. The BCB has lowest dielectric constant of 2.65 among conventional passivation dielectrics including polyimide and thick passivation can be easily formed by simple planarization technique, resulting in very low parasitic capacitance especially at around the pad electrode. The thick BCB film buried around the epitaxial post structure reduces the parasitic capacitance down to 0.041 pF which is one third as low as that of the conventional SiN passivated VCSELs. The fabricated BCB-planarized VCSEL exhibits very high relaxation oscillation frequency of 12 GHz for the oxide aperture of 6.5 /spl mu/m, which corresponds to high modulation bandwidth f/sub 3dB/ of 16 GHz with relaxation oscillation frequency of 11 GHz for the oxide aperture of 8.5 /spl mu/m. These values are as comparably high as the reported highest values. Open eye-diagrams up to 12.5 Gbps are also confirmed implying that the presented VCSEL is applicable to the optical data network systems such as Giga-bit Ethernet at higher bit rates. Preliminary reliability test result shows stable optical output power at constant operating current at 100 /spl deg/C over 1000 h.

Separate-confinement-oxidation vertical-cavity surface-emitting laser structure

In the present paper, a comprehensive self-consistent three-dimensional model is used to analyze physical aspects of the operation of oxide-confined vertical-cavity surface-emitting diode lasers (VCSELs) and to optimize their structures. The impact of the built-in radial confining mechanisms created by oxide apertures, i.e., the influence of their diameters and localizations on radial confinements of both the current injection into VCSEL active regions and electromagnetic fields of successive cavity modes, has been investigated. Basically, there are two extreme cases: the index-guided (IG) VCSELs with the aperture localized at the antinode position of the optical standing wave, characterized by a very low lasing threshold but exhibiting rather poor mode selectivity, and the gain-guided (GG) VCSELs with the aperture shifted to the node position, ensuring usually the single-fundamental-mode operation, but at the expense of much higher lasing threshold. In the present paper, the separate-confinement-oxidation VCSEL structure has been proposed exhibiting simultaneously advantages of both the IG VCSELs (the low lasing threshold) and the GG ones (the single-fundamental-mode operation).

Nanostructured Current-Confined Single Quantum Dot Light-Emitting Diode at 1300 nm

A novel light-emitting-diode structure is demonstrated, which relies on nanoscale current injection through an oxide aperture to achieve selective excitation of single InAs/GaAs quantum dots. Low-temperature electroluminescence spectra evidence discrete narrow lines around 1300 nm (line width approximately 75 microeV) at ultralow currents, which are assigned to the emission from single excitons and multiexcitons. This approach, which enables the fabrication of efficient nanoscale active devices at 1300 nm, can provide single-photon-emitting diodes for fiber-based quantum cryptography.

Low thermal resistance high-speed top-emitting 980-nm VCSELs

Increasing copper plated heatsink radii from 0 to 4 mum greater than the mesa in vertical-cavity surface-emitting lasers (VCSELs) reduced the measured thermal resistance for a range of device sizes to values 50% lower than previously reported over a range of device sizes. For a 9-mum diameter oxide aperture, the larger heatsink increases output power and bandwidth by 131% and 40%, respectively. The lasers exhibit a 3-dB modulation frequency bandwidth up to 9.8 GHz at 10.5 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The functional dependence of thermal resistance on oxide aperture diameter indicates the importance of lateral heat flow to mesa sidewalls

Design and Evaluation of Fundamental-Mode and Polarization-Stabilized VCSELs With a Subwavelength Surface Grating

We demonstrate 850-nm oxide-confined vertical-cavity surface-emitting lasers (VCSELs) with a locally etched subwavelength surface grating that are single-mode and polarization stable from threshold up to thermal roll-over, reaching /spl sim/4 mW of output power. The side-mode suppression ratio (SMSR) is >30 dB and the orthogonal polarization suppression ratio (OPSR) is /spl sim/20 dB. Moreover, no distortion of the far-field beam profile is observed as a result of the surface grating. Our numerical calculations show that a carefully designed VCSEL can have a high simultaneous mode and polarization selectivity without a significant increase in loss for the favored fundamental mode with polarization state perpendicular to the grating lines. This indicates characteristics such as threshold current and resonance frequency will not be notably degraded. The calculations also show a low sensitivity to variations in grating etch depth and duty cycle, which relaxes fabrication tolerances. In our experimental parametric study, where the oxide aperture diameter, surface grating diameter, and grating duty cycle were varied, the combined mode and polarization selection was investigated. For an optimum combination of oxide aperture and surface grating diameters of 4.5 and 2.5 /spl mu/m, respectively, the device is found to be single-mode and polarization stable for a broad range of grating duty cycles, from 55% to 75%, with only a small variation in other laser performances, which is in line with theory.

Physical Analysis of a Possibility to Reach the 1.30-μm Emission from the GaAs-Based VCSELs with the InGaAs/GaAs Quantum-Well Active Regions and the Intentionally Detuned Optical Cavities

Physical aspects of an operation of the GaAs-based InGaAs/GaAs quantum-well (QW) VCSELs with the intentionally detuned optical cavities have been considered in the present paper using the comprehensive three-dimensional self-consistent optical–electrical–thermal-gain simulation. In GaAs-based structures, very good DBR resonator mirrors and a very efficient methods to confine radially both the current spreading and the electromagnetic field with the aid of oxide apertures may be applied. It has been found using the above simulation that even currently available immature technology enables manufacturing the above devices emitting radiation of wavelengths over 1.20 μm. In particular, while the room-temperature 1.30-μm lasing emission is still beyond possibilities of the InGaAs/GaAs QW VCSELs, these structures may offer analogous 1.25-μm emission, especially for the high-power and/or high-temperature operation.

Single mode photonic crystal vertical cavity lasers

We report the accuracy of the photonic crystal model in describing the characteristics of vertical cavity surface-emitting lasers with lateral optical confinement consisting of a periodic array of etched circular holes. Experiments were carried out to compare predictions of the photonic crystal model to observed modal device characteristics, and the oxide aperture size was optimized to give maximum output power and lower threshold. The role of loss in improving modal properties was also investigated. Optimized lasers exhibit submilliamp threshold current and operate in the fundamental lateral mode for all currents.