The shape of couped oxide-confined apertures governs the selective losses and dynamics of main optical supermodes in VCSELs and their polarization. (i) For the case of coupled apertures with broader (similar to 1.5 mu m) and shorter (similar to 2 mu m) bridge connecting the non-oxidized regions the VCSELs can demonstrate both co-polarized and cross-polarized lasing despite of the significant shape anisotropy of the coupled aperture. The loss discrimination among the polarized modes is weak. Strong antiphase intensity oscillations exist in the coupled apertures for symmetric (S) and antisymmetric (AS) supermodes are observed in streak camera studies in this case. Polarization switching and polarization hysteresises confirm low scattering losses in the coupled aperture system resulting in quasi-equal threshold currents and gains for the supermodes at different polarizations. The dephasing time is long, exceeding tens of nanoseconds. Upon current increase the splitting of the modes and the intensity oscillation frequency increase linearly with current reaching 50-70GHz. Once the device switches to self-injection locked (SIL) regime characterized by a single AS mode, the oscillation frequency decreases by similar to 25GHz and both apertures oscillate in phase. Upon further current increase the same linear slope of the frequency vs current is reestablished up to 60- 70GHz. Cross-polarized modes revealed below the onset of SIL regime may be responsible for the resonance frequency feature in the modulation response observed in circular polarized studies, even when the intensity of the cross-polarized mode is weak. (ii) For the coupled apertures connected by a longer (similar to 4 mu m) and narrower (similar to 1 mu m) bridge only co-polarized along the coupling axis S and AS modes are observed, indicating that scattering loss mechanisms become important and significantly and selectively affect threshold gains for differently polarized modes. No antiphase oscillations are observed for such chips below SIL threshold, and the mode splitting is reducing with current. In the SIL regime defined by the AS mode, in-phase oscillations evolve in both cavities at a frequency matching the reduced mode splitting similar to 20GHz at currents in the very vicinity of the onset of the SIL regime. The frequency of the first post-excitation oscillation rapidly increases above SIL threshold linearly with current reaching effective bandwidths similar to 100GHz. The following two oscillations proceed at lower frequencies. Further oscillations demonstrate further reduced frequencies and decay rapidly with time. Our data indicates that shape engineering can effectively control the mode scattering mechanisms. Consequently, one can design the intrinsic modulation response either by making it suitable for ultrahigh data transmission rates in digital format, or, as opposite, for generation of stable frequencies controlled by drive current. Cavity engineering becomes, however, challenging once ultimate control over the shape is required. Avoidance of oxide-confined apertures by applying purely metal apertures for optical confinement enables drastic extension of shape and loss engineering options while keeping low threshold, high differential efficiency and controlled S and AS mode splitting. We show that coupled apertures can be also generated by introducing etch pattern in the top dielectric cap layer, which effectively confines optical emission within similar to 1 mu m-scale areas and generates specific multispot near filed patterns for high order modes.

In this paper, we demonstrate a laser-based optical wireless communication (OWC) system employing a 940 nm single-mode (SM) vertical cavity surface emitting laser (VCSEL) and a multi-mode (MM) fiber-coupled receiver, achieving a record data rate beyond 70 Gb/s, while the optical transmit power is below 5 mW. The use of a high speed fiber-optic photoreceiver avoids limiting the communication bandwidth by the receiver, enabling ultra-high capacity and energy-efficient light fidelity (LiFi) links to unlock new applications. This work experimentally validates the feasibility of ultra-high speed indoor OWC systems using a single low-power and low-cost VCSEL for next-generation LiFi connectivity.

Substantial improvements in the performance of optical interconnects based on multi-mode fibers are required to support emerging single-channel data transmission rates of 200 Gb/s and 400 Gb/s. Future optical components must combine very high modulation bandwidths—supporting signaling at 100 Gbaud and 200 Gbaud—with reduced spectral width to mitigate chromatic-dispersion-induced pulse broadening and increased brightness to further restrict flux-confining area in multi-mode fibers and thereby increase the effective modal bandwidth (EMB). A particularly promising route to improved performance within standard oxide-confined VCSEL technology is the introduction of multiple isolated or optically coupled oxide-confined apertures, which we refer to collectively as multi-aperture (MA) VCSEL arrays. We show that properly designed MA VCSELs exhibit narrow emission spectra, narrow far-field profiles and extended intrinsic modulation bandwidths, enabling longer-reach data transmission over both multi-mode (MMF) and single-mode fibers (SMF). One approach uses optically isolated apertures with lateral dimensions of approximately 2–3 µm arranged with a pitch of 10–12 µm or less. Such devices demonstrate relaxation oscillation frequencies of around 30 GHz in continuous-wave operation and intrinsic modulation bandwidths approaching 50 GHz. Compared with a conventional single-aperture VCSELs of equivalent oxide-confined area, MA designs can reduce the spectral width (root mean square values < 0.15 nm), lower series resistance (≈50 Ω) and limit junction overheating through more efficient multi-spot heat dissipation at the same total current. As each aperture lases in a single transverse mode, these devices exhibit narrow far-field patterns. In combination with well-defined spacing between emitting spots, they permit tailored restricted launch conditions in MMFs, enhancing effective modal bandwidth. In another MA approach, the apertures are optically coupled such that self-injection locking (SIL) leads to lasing in a single supermode. One may regard one of the supermodes as acting as a master mode controlling the other one. Streak-camera studies reveal post-pulse oscillations in the SIL regime at frequencies up to 100 GHz. MA VCSELs enable a favorable combination of wavelength chirp and chromatic dispersion, extending transmission distances over MMFs beyond those expected for zero-chirp sources and supporting transfer bandwidths up to 60 GHz over kilometer-length SMF links.

Vertical-cavity surface-emitting lasers ( VCSELs) are of utmost importance as key components for high-speed datacom, sensor and free-space applications. Therefore, for a successful further optimization of their performance understanding their behavior during operation is of crucial importance. A set of 850 nm VCSEL samples employing different doping of the active cavity zone are studied during operation by means of reverse current-voltage (IV) characteristics as well as photocurrent spectroscopy (PCS) under reverse bias. Reverse IV characteristics exhibits avalanche breakdown which enables an estimation of the electric field in the active region as a function of applied bias. Photocurrent spectroscopy is a powerful, nondestructive technique which measures essentially the convolution of the top mirror and intrinsic region absorption spectra and reveals quantum well transitions which redshift with reverse bias due to quantum-confined Stark effect (QCSE). The VCSELs are characterised before and after high current operation. VCSELs with a controlled doping of the active cavity region do not alter neither avalanche breakdown nor the QCSE shift of the quantum well transitions during operation. However, VCSELs without doping of the active cavity region show a systematic shift in breakdown voltage towards lower values, which is accompanied by an operation-induced redshift of quantum well transitions observed by PCS. These results indicate an increase of the built-in electric field in the active cavity zone after high current operation which is discussed in terms of conceivable processes such as dopant diffusion, impurity electromigration, burn-in of contacts and/or the activation of dopants during operation.

We report high frequency (20-100 GHz range) optical field intensity oscillations in laterally-coupled-cavity vertical-cavity surface-emitting lasers with several different techniques. The oscillation frequency is defined by the photon energy splitting of the coupled states. The resonance effect is stable in an extended current range and can enable modulation frequency resonances at higher frequencies as compared to the conventional relaxation oscillation frequency of the laser. This paves a way towards high-speed data transmission solutions at data rates beyond similar to 200 Gb/s with the advantage of better laser stability, as the resonance observed can reach high frequencies even at low current densities. A similar to 75 GHz intensity modulation between optical modes of a coupled-cavity VCSEL array was first reported by the authors in a two-aperture configuration in 2023 applying optical excitation [1]. Studies of 4- and 10-element coupled VCSEL arrays give further insight into the effects observed. New 3D numerical simulations and electrical modulation techniques have been applied to address the specific nature of the photon-photon resonance studies.

We present the first true voltage-mode VCSEL driver achieving 60Gbit/s at peak efficiency of 0.36pJ/bit with BER < 1e-12 improving the state-of-the-art by a factor of 2. Transmission experiments showcase error-free 56Gbit/s transmission up to 100m fiber. Advanced demonstration enables even 224Gbit/s when using SWDM at a total efficiency of 0.4pJ/bit.

VCSEL arrays can play an important role in the increasing the data throughput of VCSEL-based optical interconnects both due to the need to increase the channel density and due to new emerging technologies like optical wireless. In this work we show the progress in the development of high-speed VCSEL arrays suitable for multicore fiber transmission leading to an increase of the total throughput through single fiber to 600 Gbps. We also discuss a novel type of compact VCSEL mini-arrays capable of high-speed modulation and coherent emission at the same time. Photon-photon resonance and coherent effects can help increase the resonant frequency and the bandwidth of the VCSELs and enable devices capable of 100 GHz operation.

We present a new hyperchromatic laser-based multifocal display. In the proposed design multiple full-colour virtual image planes can be displayed simultaneously at different depths in front of the observer through wavelength (de-)multiplexing. Each depth plane is displayed through its own combination of red, green and blue lasers at specific wavelengths. Hyperchromatic displays can be useful for augmented (AR) and mixed reality (MR) applications where real and virtual objects are located at different depths in front of the observer because they allow the human eye to focus on virtual objects and reduce the vergence-accommodation conflict (VAC). We present a laboratory demonstrator where the images generated by two red lasers (630 nm and 960 nm) were separated by more than 3 meters. We also discuss the applicability of the technology to the automotive head-up-display (HUD) systems and present an HUD system based on low-cost offthe-shelf components.

Vertical-cavity surface-emitting lasers (VCSELs) are of utmost importance as key components for high-speed datacom, sensor and free-space applications. Therefore, for a successful further optimization of their performance, understanding their aging behavior is of crucial importance. Photocurrent spectroscopy (PCS) is a powerful, nondestructive technique which can be used to analyze semiconductor materials. Applying it on VCSELs makes it a powerful tool to investigate these tiny devices. In this work, we present room temperature high-resolution PCS analyses of fresh vs. aged 850 nm VCSELs. These VCSELs are characterized before and after aging by means of PCS, which measures essentially the convolution of the top mirror and intrinsic region absorption spectra. Heavy hole and light hole quantum well transitions are revealed and the related quantum-confined Stark effect is studied. The VCSELs used in this study are mounted on a standard V-connector and were intentionally aged at extreme conditions to accelerate their degradation till reaching optical damage. It was found that in these VCSELs, a reduced PCS current is observed, which is possibly caused by nonradiative recombination centers generated by the aging-related processes. Moreover, we observe that aging of the devices at very high current densities results in the evolution of defect related states, which modify the IV-curve under reverse bias. Degraded devices also show a systematic shift in breakdown voltage towards lower values, indicating a possible shrinkage of the undoped region by impurity electromigration and diffusion. Interestingly, these changes are minimal in stable devices that were aged under normal conditions.

In this paper, current-dependent emission spectra and efficiency measured on the same AlGaInP red light-emitting diode (LED) pumped with the current pulses of very different durations are recorded. This enabled for the first time distinguishing between high-carrier concentration and self-heating effects on the efficiency decline at high current magnitudes. The electron leakage to the p-side of the LED structure, which is the major mechanism of the efficiency reduction, is found to rise substantially when the device self-heating starts to develop. As a result, in comparison to continuous-wave excitation, driving the LED with sub-microsecond current pulses allows suppressing the device self-heating and, eventually, increasing the operating current by an order of magnitude without noticeable efficiency losses. Based on the reduced ABC-model, neglecting Auger recombination, the light extraction efficiency, injection efficiency, and internal quantum efficiency of the LED are estimated, suggesting light extraction to be the most critical factor for the overall efficiency of the LED. The coupled spectral/power LED characterization using the variable-duration current pulse pumping is found to be an effective approach for analyzing mechanisms of the device operation.

We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over a wide range of correlated color temperature (CCT) to achieve warm white (CCT = 3600 K) to cool white (CCT = 13,000 K) emission by current modulation from 2.3 A/cm(2) to 12.9 A/cm(2). It is also demonstrated for the first time that a color rendering index (CRI) as high as 67 can be achieved with such a dichromatic source. The observed CCT and CRI tunability is associated with the spectral power evolution due to the pumping-induced carrier redistribution.

External quantum efficiency of industrial-grade green InGaN light-emitting diodes (LEDs) has been measured in a wide range of operating currents at various temperatures from 13 K to 300 K. Unlike blue LEDs, the efficiency as a function of current is found to have a multi-peak character, which could not be fitted by a simple ABC-model. This observation correlated with splitting of LED emission spectra into two peaks at certain currents. The characterization data are interpreted in terms of non-uniformity of the LED active region, which is tentatively attributed to extended defects like V-pits. We suggest a new approach to evaluation of temperature-dependent light extraction and internal quantum efficiencies taking into account the active region non-uniformity. As a result, the temperature dependence of light extraction and internal quantum efficiencies have been evaluated in the temperature range mentioned above and compared with those of blue LEDs.

The future generation of modern illumination should not only be cheap and highly efficient, but also demonstrate high quality of light, light which allows better color differentiation and fidelity. Here we are presenting a novel approach to create a white solid-state light source providing ultimate color rendition necessary for a number of applications. The proposed semi-hybrid device combines a monolithic blue-cyan light emitting diode (MBC LED) with a green-red phosphor mixture. It has shown a superior color rendering index (CRI), 98.6, at correlated color temperature of around 3400 K. The MBC LED epi-structure did not suffer from the efficiency reduction typical for monolithic multi-color emitters and was implemented in the two most popular chip designs: "epi-up" and "flip-chip". Redistribution of the blue and cyan band amplitudes in the whitelight emission spectrum, using the operating current, is found to be an effective tool for fine tuning the color characteristics.

Efficiency of commercial 620 nm AlGaInP Golden Dragon-cased high-power LEDs has been studied under extremely high pump current density up to 4.5 kA/cm(2) and pulse duration from microsecond down to sub-nanosecond range. To understand the nature of LED efficiency decrease with current, pulse width variation is used. Analysis of the pulse-duration dependence of the LED efficiency and emission spectrum suggests the active region overheating to be the major factor controlling the LED efficiency reduction at CW and sub-microsecond pumping. The overheating can be effectively avoided by the use of sub-nanosecond current pulses. A direct correlation between the onset of the efficiency decrease and LED overheating is demonstrated.

Two blue (450 nm) light–emitting diodes (LED), which only differ in top p-GaN layer growth conditions, were comparatively investigated. I-V, C-V, TLM, Electroluminescence (EL) and Photoluminescence (PL) techniques were applied to clarify a correlation between MOCVD carrier gas and internal properties. The A-structure grown in the pure N2 environment demonstrated better parameters than the B-structure grown in the N2/H2 (1:1) gas mixture. The mixed growth atmosphere leaded to an increase of sheet resistances of p-GaN layer. EL and PL measurements confirmed the advantage of the pure N2 utilization, and C(VR) measurement pointed the increase of static charge concentration near the p-GaN interface in the B structure.

Sapphire substrates are widely used for the manufacturing of blue and green LEDs and diode lasers. Dicing of sapphire wafers with deposited GaN device structures is therefore an important technological challenge. During the last ten years the dicing efficiency has been investigated for a wide range of wavelengths from DUV (157nm) to UV (355nm); pulse widths (in nanosecond regime); beam shaping; and power levels [1]. Traditionally, some industrial sapphire dicers use nanosecond pulses with a wavelength of 355 nm. In this configuration, the laser cutting is performed through the back side of the sapphire wafer. Furthermore, nanosecond pulses with a wavelength of 266 nm are used for cutting from the front (the side with the structure) of the wafer. A few attempts to dice with a green laser, also in Q-switched regime, emitting pulses with a width of a few tenths of a nanosecond have been reported in the literature [1, 2]. Recently the first Q-switched infrared pulsed lasers generating ten picosecond pulses at a wavelength of 1060/532 nm were presented on the market.

In this work carrier lifetime investigations by applying photoluminescence frequency domain technique to investigate blue and green light emitting diode (LED) structures, thus representing wide spectra for the InGaN LEDs were performed. Similar measurements have been carried out in GaN at extremely low excited carrier densities [1], as low as 1 mW/cm 2 by using UV LED as an excitation source. For these measurements we applied laser diode (LD), to excite resonantly the MQW's in the sample investigated with modulated light. This allowed us to cover a broad range of excitation power density (1 – 500 mW/cm 2 ). To analyze the activation mechanisms measurements down to 10 K temperatures were performed. The non-equilibrium charge carrier lifetime dynamics was investigated by applying a model with a superposition of exponential and stretch exponential decay depended on the sample and on the measurement condition. The uniqueness of this method is the possibility to study the transient processes in structures under very low to low non-equilibrium charge carrier densities at an un-saturated recombination channel condition.