Oxide-confined Vertical Cavity Research Articles

26.5625 Gbaud PAM-4 short-reach transmission at 75°C using 850 nm VCSELs with strategic oxide guiding layer positioning.

This article presents an all-epitaxy approach to reduce the root mean square spectral width (ΔλR M S) of 850 nm oxide-confined vertical cavity surface-emitting lasers (VCSELs) with a large aperture of 7 µm through strategic optimization of the oxide guiding layer within the epitaxy structure. At 75°C, the VCSEL demonstrates a ΔλR M S of ∼0.3 nm at a bias current of 7.5 mA. Furthermore, the VCSEL achieves successful transmission of 26.5625 Gbaud PAM-4 modulation over a short-reach (SR) OM4 fiber link while maintaining a TDECQ budget below the 4.5 dB specified by 50G IEEE Ethernet standards.

Serial data transmission at 224 Gbit/s applying directly modulated 850 and 910 nm VCSELs

High-speed discrete multitone serial data transmission over multi-mode fibre at gross data rates up to 224 Gbit/s achieved with directly modulated oxide-confined vertical cavity surface emitting lasers at 850 and 910 nm are presented.

(Invited) Light Management for Engineering Luminescence in Nanoscale Environments By Numerical Optimization

The spontaneous emission rate of fluorescencent species in general is affected by its environment. Geometrical structure and material composition of the environment can yield strongly increased rates. This effect is known as Purcell effect, and the magnitude of enhancement of the emission rate in a cavity is known as Purcell factor. The Purcell factor is proportional to the cavity mode's quality factor (Q factor) and inversely proportional to the modal volume. This implies that strongly resonant structures with small length scales (of the order of the wavelength of light) can greatly influence fluorescence. However, not only the emission rate can be controlled by a suitably chosen environment, but also the directional properties of the emitted light field can be engineered by placing the emitter in custom-made environments. In the regime of optical wave phenomena on nano-meter length scales (nano optics), Maxwell's equations for the 3D, vectorial electromagnetic field distribution can to be solved numerically in order to quantify the expected emission rate and far field radiation patterns. In order to obtain specific functionalities of optical devices, numerical optimization is performed. I.e., in numerical computations, structural parameters (layer thicknesses, structure dimensions, material composition, etc) are varied until a parameter set is reached which yields best performance for the specific functionality. This numerically optimized structure can then serve as design pattern for experimental realizations. We review our recent developments of finite-element method based and machine-learning inspired optimization algorithms [1]. We have applied these methods in order to optimize light extraction efficiency of quantum dots embedded in deterministically fabricated cavities [2] and in periodic nano-structures [3], as well as extraction efficiency from structured light emitting diodes (LED, OLED) [4]. We have also investigated possibilities to manipulate emission properties in chiral photonic structures [5], and to design vertical cavity lasers [6]. Further, in applications like light trapping in solar cells, for enhanced photovoltaic efficiency, the related problem of maximizing coupling of light of specific spectral properties into nanostructured solar cells can be treated [7]. In this contribution we will further present recent results on optimization of photon extraction efficiency from single photon sources for applications in quantum technology. This project has received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie grant agreement No 675745 (MSCA-ITN-EID NOLOSS) and from the EMPIR programme co-financed by the Participating States and from the European Union’s Horizon 2020 research and innovation programme under grant agreement number 14IND13 (PhotInd). We further acknowledge funding by Einstein Foundation Berlin (ECMath-SE6) and by Freie Universität Berlin (Dahlem Research School). [1] P.-I. Schneider, et al. Global optimization of complex optical structures using Bayesian optimization based on Gaussian processes. Proc. SPIE 10335, 103350O (2017).[2] M. Gschrey, et al. Highly indistinguishable photons from deterministic quantum-dot microlenses utilizing three-dimensional in situ electron-beam lithography. Nat. Commun. 6, 7662 (2015).[3] C. Barth, et al. Increased fluorescence of PbS quantum dots in photonic crystals by excitation enhancement. Appl. Phys. Lett. 111, 031111 (2017).[4] L. Zschiedrich, et al. Numerical analysis of nanostructures for enhanced light extraction from OLEDs. Proc. SPIE 8641, 86410B (2013).[5] P. Gutsche, et al. Locally enhanced and tunable optical chirality in helical metamaterials. Photonics 3, 60 (2016).[6] N.N. Ledentsov, et al. Direct Evidence of the Leaky Emission in Oxide-Confined Vertical Cavity Lasers. IEEE J. Quant. Electron. 52, 2400207 (2016).[7] K. Jäger, et al. Simulations of Sinusoidal Nanotextures for Coupling Light into c-Si Thin-Film Solar Cells. Opt. Express 24, A569 (2016).

Anti-waveguiding vertical-cavity surface-emitting laser at 850 nm: From concept to advances in high-speed data transmission.

Oxide-confined vertical cavity surface emitting lasers (VCSELs) with anti-waveguiding AlAs-rich core presently attract a lot of attention. Anti-waveguiding cavity enables the maximum possible optical confinement of the VCSEL mode ("λ/2 design"), increases its oscillator strength and reduces dramatically the optical power accumulated in the VCSEL mesa regions outside the aperture. VCSEL designs are suggested that favor single transverse mode operation. Modeling including current-induced and absorption-induced overheating shows that the preference for the transverse fundamental mode persists up to 10 mA current at 5 µm aperture diameter. Error-free data transmission is realized up to 160 Gb/s in digital-multitone (DMT) format using single-mode anti-waveguiding VCSELs. The approach to single-mode anti-waveguiding VCSELs is extended over a broad spectral range realizing error-free high-speed data transmission at both 850 nm and 910 nm.

Oxide-confined leaky vertical-cavity surface-emitting lasers for single-mode operation

Oxide-confined leaky vertical-cavity surface-emitting lasers for single-mode operation

Direct Evidence of the Leaky Emission in Oxide-Confined Vertical Cavity Lasers

We report the observation of narrow tilted lobes in the far-field emission pattern of leaky oxide-confined vertical-cavity surface-emitting lasers (VCSELs). The VCSEL cavity is surrounded by two selectively oxidized aperture layers, which are intentionally designed to produce a high lateral leakage of the high-order transverse modes of the vertical cavity. The device operates in the fundamental transverse mode at oxide aperture diameters below $5~\mu \text{m}$ and currents up to 4 and 5 mA. At higher currents or larger aperture diameters, an additional high-order transverse optical mode evolves. This mode is revealed by the appearance of a shorter wavelength emission line in the electroluminescence spectra of the device, resulting in multi-mode lasing and in the related changes in the near- and far-field patterns. In the far-field pattern, the evolution of the high-order mode is revealed by the evolution of the two overlapping emission lobes at moderate (~5°) tilt angles with respect to the normal to the surface and by the appearance of a multi-spot or a ring pattern in the CCD camera images. Most importantly, the appearance of this high-order transverse mode is accompanied by an observation of much narrower lobes in the far-field pattern observed at significantly larger tilt angles (~35°). The tilt angle and the narrow angular width of these emission lobes in the far-field spectrum are in agreement with those calculated in the 3-D cold cavity modeling of the optical modes of the device. These narrow tilted lobes revealed in this paper are the fingerprints of the leakage effect in specially designed oxide-confined VCSELs with their intensity being proportional to the optical power leaving the aperture region in the direction parallel to the surface and propagating into the oxidized region. The effect can be applied for engineering of single-mode VCSELs, coherently-coupled 2-D VCSEL arrays, laterally integrated VCSEL-photodetector chips, and VCSELs integrated to slow light waveguides, for coupled optical gates for optical computers and other types of photonic integrated circuits.

The influence of quaternary electron blocking layer on the performance characteristics of intracavity-contacted oxide-confined InGaN-based vertical cavity surface emitting lasers

The effect of electron blocking layer (EBL) on the performance characteristics of InGaN-based vertical cavity surface emitting lasers (VCSELs) was numerically investigated using an integrated system engineering technical computer aided design (ISE TCAD) simulation program. Simulation results indicated that the performance characteristics of InGaN quantum well VCSEL were improved by the ternary [Formula: see text] EBL. Better performance was also obtained when [Formula: see text] EBL was replaced by a polarization-matched [Formula: see text] EBL having the same energy bandgap. The quaternary EBL enhances the output power and differential quantum efficiency (DQE) as well as reduces the threshold current compared with the ternary EBL. Enhancement in the value of the optical intensity was also observed in the VCSEL structure with quaternary EBL. Furthermore, the effect of Al composition of AlInGaN EBL on the performance of InGaN-based VCSEL structure that uses the quaternary AlInGaN EBL was studied. In mole fraction was 0.115, Al mole fraction changed from 0.260 to 0.290 by step 0.005, and optimum performance was achieved in 0.275 Al mole fraction of AlInGaN EBL.

Single-Mode Vertical Cavity Surface Emitting Laser via Oxide-Aperture-Engineering of Leakage of High-Order Transverse Modes

Properly designed oxide-confined vertical cavity surface emitting laser (VCSEL) allows leakage of the optical modes from the all-semiconductor core region to the selectively oxidized periphery if the orthogonality between the core mode and the modes on the periphery is broken by the oxidation-induced optical field redistribution. The leakage losses are stronger for high-order transverse modes, which have a higher field intensity close to the oxidized region. Single mode lasing in the fundamental mode can thus proceed up to large aperture diameters. The 850-nm GaAlAs thick oxide aperture VCSEL based on this concept is designed, modeled, and fabricated, showing single-mode lasing with the aperture diameters up to 5 μm. Side mode suppression ratio >20 dB is realized at the current density of ~10KA/Cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in devices with the series resistance of 90 Ω.

780 nm Oxide-Confined VCSEL With 13.5 Gb/s Error-Free Data Transmission

A 780 nm emission wavelength (far-red) oxide-confined vertical cavity surface-emitting laser with a threshold current of 0.6 mA at 20 °C is demonstrated. The device shows an 11.2 GHz modulation bandwidth when biased at I=8 mA (I/ITH=13.3), and it achieves the error-free data transmission at 13.5 Gb/s.

Cold-cavity measurement of optical loss from oxide-confined vertical-cavity surface-emitting lasers

Microcavity laser design and performance optimization requires a quantitative knowledge of the cavity optical losses. A generalized method using sub-threshold spectral measurements matched to model calculations is demonstrated to determine optical loss in microcavity lasers. Cold-cavity spectral characteristics are used to extract the size-dependent optical loss for small diameter oxide-confined vertical-cavity surface emitting lasers. For oxide aperture diameters less than 4 μm, the oxide scattering loss can be greater than 10 cm−1, similar to the typical values of free carrier absorption loss.

850 nm Oxide-VCSEL With Low Relative Intensity Noise and 40 Gb/s Error Free Data Transmission

We have designed and fabricated a high speed 850 nm oxide-confined vertical cavity surface emitting laser with an oxide aperture dimension of ~ 4 μm and a threshold current ITH=0.53 mA at room temperature (20 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">°</sup> C). It demonstrates a modulation bandwidth of 21.2 GHz, and achieves a laser relative intensity noise reaching standard quantum limit 2hν/P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> =-154.3 dB/Hz at high bias I/I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">TH</sub> =10. Furthermore, error-free data transmission at 40 Gb/s is obtained at I=6.5 mA which corresponds to an energy/data efficiency of 431 fJ/bit.

Improved performances of 850nm vertical cavity surface emitting lasers utilizing the self-planar mesa structure

We presented the self-planar mesa structure for improving the performances of 850-nm oxide-confined vertical cavity surface emitting lasers (VCSELs). By employing the self-planar mesa, the maximum output power was increased from 8 mW to more than 11 mW, and the maximum wall-plug efficiency was improved from 26% to 36% for the VCSEL with an oxide aperture size of 13 mu m at 15 degrees C. The thermal resistance was decreased from 1.16 degrees C/mW to 0.94 degrees C/mW. Thermal simulations about different mesa structures were carried out. And the enhanced lateral heat dissipation and current diffusion within VCSELs was considered to be the main reason for improved performances. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.

Relative intensity noise in high speed microcavity laser

We have fabricated a high speed single mode microcavity laser of the form of oxide-confined vertical cavity surface emitting laser (VCSEL) and achieved an ultralow threshold current (ITH = 0.13 mA at 20 °C) with lasing wavelength at 837 nm. The optical spectrum of the microcavity VCSEL exhibits a mode spacing of 3.1 nm, which is corresponding to an optical modal cavity dimension of 2.5 μm. The device exhibits an enhanced modulation bandwidth of 22.6 GHz and a thermal noise limited laser intensity noise (electrical power spectral density of laser intensity noise below the thermal noise floor −174 dBm/Hz) as a consequence of low power laser operation and reduced mode competition in the microcavity.

Oxide-confined mid-infrared VCSELs

The first oxide-confined GaSb-based vertical-cavity surface emitting laser operating in the mid-infrared is demonstrated. Laser operation at -20°C in continuous wave is achieved with a device emitting around 2.3 µm. The influence of the lateral confinement on laser performances is presented and discussed.

Study on influence of oxide aperture shape on modal characteristics of VCSELs

The effects of oxide aperture shape whose size is small, on the mode characteristics of external cavity oxide-confined photonic crystal vertical cavity surface emitting laser (VCSEL) is studied extensively based on the finite-difference time-domain algorithm. A new three-dimensional model of optical field in photonic crystal VCSEL is built and the influence of the variation in aperture shape on the characteristics of far-field and spectrum is pointed out. It is found that the modal characteristics of PhC-VCSEL are influenced by the shape of oxide aperture, the influence on spectrum feature is more apparent. It can be explained from the distribution of modal field by the fact that the symmetrical characteristic of prismatic oxide aperture is not identical to that of high-order mode. But the influence is less apparent while the depth of photonic crystal hole is deeper. The reason for this matter is illustrated. The results can be used to guide the fabrication of external cavity oxide-confined photonic crystal VCSELs.

Theoretical investigation of static and dynamic characteristics of vertical-cavity surface-emitting lasers with incorporated two-dimensional photonic crystals

A dynamical model of oxide-confined Vertical-Cavity Surface-Emitting Lasers (VCSELs) with two-dimensional photonic crystals (PCs) incorporated within them so called PC-VCSELs is presented and used to optimise designs for high-power single-mode operation. Three PC-VCSEL designs are considered: (I) with holes in the top DBRs, (II) with PC holes situated between their DBRs and (III) with PC holes etched through the entire VCSEL. A simulated design for a PC-VCSEL of type (I) with holes of d = 2 μm diameter, a = 4 μm lattice constant (d/a = 0.5) and 2.2 μm depth was found to improve the single mode behaviour but not enough to establish single mode behaviour for large apertures. The modulation behaviour was not degraded by the PC. Simulations of type (II) and (III) PC-VCSELs, with the same parameters, have shown multimode operation and degraded modulation properties. Simulations of PC-VCSELs of type (III) with holes of d = 0.2 μm diameter and a = 0.4 μm lattice constant (d/a = 0.5) have shown improved modulation properties and enhanced single mode power for small apertures. In simulation, PC-VCSELs incorporating multiple PC-defects have shown order of magnitude increases in the single mode output power. However, the modulation properties of these VCSELs show degradation due to gain saturation and hopping of the optical modes localized within the PC defects.

Low threshold current density, low resistance oxide-confined VCSEL fabricated by a dielectric-free approach

We present the fabrication process and experimental results of 850-nm oxide-confined vertical cavity surface emitting lasers (VCSELs) fabricated by using dielectric-free approach. The threshold current of 0.4 mA, which corresponds to the threshold current density of 0.5 kA/cm2, differential resistance of 76 Ω, and maximum output power of more than 5 mW are achieved for the dielectric-free VCSEL with a square oxide aperture size of 9 μm at room temperature (RT). L–I–V characteristics of the dielectric-free VCSEL are compared with those of conventional VCSEL with the similar aperture size, which indicates the way to realize low-cost, low-power consumption VCSELs with extremely simple process. Preliminary study of the temperature-dependent L–I characteristics and modulation response of the dielectric-free VCSEL are also presented.

Current self-distribution effects in oxide-confined vertical-cavity surface-emitting lasers

We develop a self-consistent electro-optical model for oxide-confined Vertical-Cavity Surface-Emitting Lasers, including optical polarization and spin-flip dynamics. When subject to large signal modulation, current self-distribution effects reduce the amount of spatial-hole burning in the device, leading to a reduced overshoot during transients and an increased damping of the relaxation oscillations.

The influence of injection current on transverse mode characteristics of vertical-cavity surface-emitting lasers

The influence of transverse mode were researched with the injection current changing via experiments conducted with 980nm oxide-confined vertical cavity surface emitting lasers (VCSELs). Based on the spatio-temporal rate equation, the injection parameter dependence of the transverse mode characteristics of weak-index guiding VCSELs were theoretically researched by integrating the spatially dependent part. The result of the experiment indicated that the higher-order modes begin to emerge and exhibit strong competition with the injection current increased when the oxidized aperture was kept unchanged, and the distribution of the carrier transfers from the center to the edge. Furthermore, the main order becomes weaker because of the spatial hole burning. Compared with the appearance of higher-order modes with different oxidized aperture, the small injection aperture exhibits better single mode behavior.

Lattice Temperature Model and Temperature Effects in Oxide-Confined VCSEL’s

A lattice temperature model is derived for oxide-confined vertical cavity surface emitting lasers (VCSELs) based on carrier transport and the conservation of energy. Peltier heat is caused by the bandedge and quasi-Fermi level discontinuities at a heterojunction. However, considering the device size, Peltier heat needs to be distributed and is not just generated at the interface, otherwise, an anomolous spike in temperature will occur. We have developed a novel treatment to model the Peltier heat at a heterojunction by use of a Monte Carlo simulation. Peltier heat is found to be a major heat contributor, and it results in a rapid and high temperature rise in the separate confinement heterostructure (SCH) region of the laser diode. We have also shown that the carrier thermal conductivities for materials with high mobilities must be included at high carrier densities because they contribute to additional spreading of the thermal energy. Subsequently, this lattice temperature model is coupled self-consistently to electronic and optical solvers to form a complete simulator for VCSELs. Self-heating causes a fast temperature rise when the VCSEL is operated under continuous wave conditions, causing resonant wavelength changes and an eventual thermal rollover. The resonant wavelength shift has been shown to be caused mainly by the heating of the distributed Bragg reflectors even though the peak temperature occurs within the SCH region. Possible physical factors causing the thermal rollover have also been examined with our complete simulator. The Auger recombination process is found to be one of the main factors causing the thermal rollover in 980 nm oxide-confined VCSELs while the photon lifetime is a factor in determining the position of the thermal rollover. We have also achieved a very good match between our simulated results and experimental data.