Quantum Dot Vertical-cavity Surface-emitting Lasers Research Articles

High-performance multi-junction cascade 1.3 µm quantum dot vertical cavity surface-emitting laser.

A high-performance 5-junction cascade quantum dot (QD) vertical cavity surface-emitting laser (VCSEL) with 1.3 µm wavelength was designed. The characteristics of the QD as active regions and tunnel junctions are combined to effectively increase output power. The photoelectric characteristics of single-junction, 3-junction cascade, and 5-junction cascade QD VCSELs are compared at continuous-wave conditions. Results indicate that the threshold current gradually decreases, and the output power and slope efficiency exponential increase with the increase of the number of active regions. The peak power conversion efficiency of 58.4% is achieved for the 5-junction cascade individual QD VCSEL emitter with 10 µm oxide aperture. The maximum slope efficiency of the device is 6.27 W/A, which is approximately six times than that of the single-junction QD VCSEL. The output power of the 5-junction cascade QD VCSEL reaches 188.13 mW at injection current 30 mA. High-performance multi-junction cascade 1.3-µm QD VCSEL provides data and theoretical support for the preparation of epitaxial materials.

Polarization-independent high contrast grating 1300 nm dot-in-a-well InAs quantum-dot VCSEL

High contrast grating (HCG) 1300 nm InAs/GaAs quantum dot (QD) vertical-cavity surface-emitting laser (VCSEL) has been introduced in this manuscript and the impact of device structure on the self-heating effect and device temperature has been investigated. A self-consistent model that couples the opto-electro-thermal model of the device to the optical gain model is used to calculate optical gain. Simulated results indicate that the proposed structure significantly lowers the hole-burning effect due to a decrease in temperature. So, in contrast to the conventional QD-VCSELs, the HCG QD-VCSEL exhibits better carrier confinement due to hole quantum efficiency of 54% as opposed to 45% for the conventional QD-VCSELs at their rollover current density. Also, results show an improvement of 66% in the maximum output power from the HCG QD-VCSEL compared to a conventional QD-VCSEL structure. In addition, the 3-dB modulation bandwidth has been boosted compared to conventional QD-VCSELs.

Modeling of Relative Intensity Noise in QD-VCSEL

In this paper, the dynamics and statics properties of a 1.3 m InAs/InGaAs Quantum-Dot (QD) vertical-cavity surface-emitting laser (VCSEL) have been investigated using a self-consistent coupled rate-thermal equations model. Results demonstrate that self-heating caused by current transition through the Bragg mirrors and active area of the laser produces temperature rise and deteriorates the performance of the laser where it is most apparent at higher temperatures. Although the minimum value of Relative intensity noise (RIN) and the turn-on delay and maximum value of the output power occurs at the same bias current, the maximum achievable 3-dB modulation bandwidth is obtained at lower bias currents. In addition, the larger aperture size and the greater number of QD layers modify the self-heating and enhance the efficiency and output characteristics of the laser where the rolled-off of the laser happens at higher bias currents.

Strain induced polarization chaos in a solitary VCSEL

Physical curiosity at the beginning, optical chaos is now attracting increasing interest in various technological areas such as detection and ranging or secure communications, to name but a few. However, the complexity of optical chaos generators still significantly hinders their development. In this context, the generation of chaotic polarization fluctuations in a single laser diode has proven to be a significant step forward, despite being observed solely for quantum-dot vertical-cavity surface-emitting lasers (VCSELs). Here, we demonstrate experimentally that a similar polarization dynamics can be consistently obtained in quantum-well VCSELs. Indeed, by introducing anisotropic strain in the laser cavity, we successfully triggered the desired chaotic dynamics. The simplicity of the proposed approach, based on low-cost and easily available components including off-the-shelf VCSELs, paves the way to the wide spread use of solitary VCSELs for chaos-based applications.

Low threshold continuous-wave lasing of yellow-green InGaN-QD vertical-cavity surface-emitting lasers.

Low threshold continuous-wave (CW) lasing of current injected InGaN quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs) was achieved at room temperature. The VCSEL was fabricated by metal bonding technique on a copper substrate to improve the heat dissipation ability of the device. For the first time, lasing was obtained at yellow-green wavelength of 560.4 nm with a low threshold of 0.61 mA, corresponding to a current density of 0.78 kA/cm2. A high degree of polarization of 94% were measured. Despite the operation in the range of "green gap" of GaN-based devices, single longitudinal mode laser emission was clearly achieved due to the high quality of active region based on InGaN QDs and the excellent thermal design of the VCSELs.

Effect of doping on the polarization characteristics of spin-injected quantum dot VCSEL

The polarization characteristics of doped spin-injected quantum dot vertical cavity surface emitting laser (QD-VCSEL) have been studied using multi-population spin-flip coupled rate equation model. The stability conditions of spin-injected VCSEL are also investigated for different device parameters. Our analysis reveals that QD-VCSEL emitting at high lasing energies, close to the first excited state energy, demonstrates high output ellipticity and high ellipticity gain compared to QD-VCSEL emitting at the ground state energy. Also, we find that increasing the p-type doping concentration of the quantum dots increases the output ellipticity gain of the device and significantly alters the stability conditions of the VCSEL. Large instability is observed when the VCSEL is emitting near \(\sim \!\!70\hbox {meV}\) above the ground state energy and when the quantum dots are doped with \(\sim \!\!4\times 10^{10}\,\hbox {cm}^{-2}\) of p-type concentration. Moreover, we find that QD-VCSEL exhibits good stability when its emission energy is approximately 10–20 meV higher than the ground state energy and when the dots are doped with more than \(4\times 10^{10}\,\hbox {cm}^{-2}\) of p-type concentration.

Wavelength switching based on quantum-dot vertical-cavity surface-emitting laser

This study investigates, for the first time, static and dynamic wavelength switching characteristics of the 1.3 μm quantum-dot vertical-cavity surface-emitting laser (QD VCSEL). The free-running QD VCSEL with λ1 and λ2 state innately is injected by a laser source with λ1 state. When the injection power exceeds the threshold power, the dominant state of the QD VCSEL changes from λ2 to λ1 state. Results of this study demonstrate that the wavelength switching based on a 1.3 μm QD VCSEL has a simpler and more cost-effective configuration than those of previous systems.

Microscopic study of relaxation oscillations in quantum-dot VCSELs

We propose a theoretical model to investigate the switch-on dynamics of electrically pumped quantum dot vertical-cavity surface-emitting lasers. The model is based on the self-consistently combined quantum dot-wetting layer Maxwell–Bloch equations incorporating microscopically calculated Coulomb and phonon-assisted scattering processes between the quantum dot and the quantum dot-embedding wetting layer states. Our approach allows the calculation of the time delay before lasing as well as of the frequency and damping rate of the appearing relaxation oscillation. We study their dependence on the strength of the injection current density and on the number of QD-layers and DBR-layers by using the finite-difference time-domain method. The results show that switch-on delay time is in inversely proportion to the injection current density while the frequency and the damping rate of relaxation oscillation are proportional to the current density. If we increase the numbers of QD-layers and DBR-layers the delay time becomes shorter, and the frequency and the damping rate become larger.

Polarization switching and polarization mode hopping in quantum dot vertical-cavity surface-emitting lasers

We show experimentally that polarization mode hopping in quantum dot vertical cavity surface emitting lasers (VCSELs) takes place between nonorthogonal elliptically polarized modes. In contrast to quantum well VCSELs the average dwell time decreases with injection current. This decrease is by 8 orders of magnitude: from seconds to nanoseconds and is achieved without any modifications of the VCSEL internal anisotropies. The observed scaling happens in a range of currents as wide as 8 times the threshold value.

1.3-$\mu$m In(Ga)As Quantum-Dot VCSELs Fabricated by Dielectric-Free Approach With Surface-Relief Process

We present the 1.3-μ m In(Ga)As quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) fabricated by the dielectric-free (DF) approach with the surface-relief (SR) process. Compared with the conventional dielectric-dependent (DD) method, the lower differential resistance and improved output power have been achieved by the DF approach. With the same oxide aperture area, the differential resistance is reduced by 36.47% and output power is improved by 78.32% under continuous-wave operation; it is up to 3.42 mW under pulsed operation with oxide aperture diameter ~15 μm. The surface-relief technique is also applied, for the first time, in 1.3- μm QD VCSELs, and it effectively enhances the emission of the fundamental mode. The characteristic of small signal modulation response is also analyzed.

Self-heating effect in 1.3 μm p-doped InAs/GaAs quantum dot vertical cavity surface emitting lasers

The self-heating effect in 1.3 mu m p-doped InAs/GaAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) has been investigated using a self-consistent theoretical model. Good agreement is obtained between theoretical analysis and experimental results under pulsed operation. The results show that in p-doped QD VCSELs, the output power is significantly influenced by self-heating. About 60% of output power is limited by self-heating in a device with oxide aperture of 5x6 mu m(2). This value reduces to 55% and 48%, respectively, as the oxide aperture increases to 7x8 and 15x15 mu m(2). The temperature increase in the active region and injection efficiency of the QDs are calculated and discussed based on the different oxide aperture areas and duty cycle.

Polarization Characteristics of Quantum-Dot Vertical-Cavity Surface-Emitting Laser With Light Injection

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This investigation explores experimentally the optical characteristics of long-wavelength quantum-dot vertical-cavity surface-emitting lasers (QD VCSELs). The InAs QD VCSEL, fabricated on a GaAs substrate, is grown by molecular beam epitaxy with fully doped distributed Bragg reflectors. The optical characteristics of QD VCSEL without and with light injection are studied in detail. The QD VCSEL has the potential to be used in all-optical signal processing systems. </para>

High-Temperature Continuous-Wave Single-Mode Operation of 1.3-$\mu$m p-Doped InAs–GaAs Quantum-Dot VCSELs

In this letter, we have demonstrated continuous-wave single-mode operation of 1.3-mum InAs-GaAs quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) with p-type modulation-doped QD active region from 20degC to 60degC. The highest output power of 0.435 mW and lowest threshold current of 1.2 mA under single-mode operation are achieved. The temperature-dependent output characteristics of QD-VCSELs are investigated. Single-mode operation with a sidemode suppression ratio of 34 dB is observed at room temperature. The critical size of oxide aperture for single-mode operation is discussed.

Theoretical investigation of 1.3 μm dots-under-a-well and dots-in-a-well InAs/GaAs quantum dot vertical-cavity surface-emitting lasers

The threshold characteristic and output power of 1.3 μm quantum dot (QD) vertical-cavity surface-emitting laser (VCSEL) with dots-under-a-well and dots-in-a-well InAs/GaAs QD structures are investigated by using rate equation model and output power model. The influence of VCSEL and QD structures on the modal gain of VCSEL is analyzed. Threshold current density, quantum efficiency, and characteristic temperature are simulated for different QD structures. The dependence of output power of 1.3 μm QD VCSEL on the QD structure, threshold current, quantum efficiency, and oxide-aperture size is investigated in detail.

Polarization Switching in Quantum-Dot Vertical-Cavity Surface-Emitting Lasers

We demonstrate experimental evidence of polarization switching accompanied by polarization-mode hopping in quantum-dot vertical-cavity surface-emitting lasers. In our case, the polarization switching is associated with a change of linearly polarized light to elliptically polarized one, hence switching takes place between elliptically polarized states. Current-modulation measurements show that the polarization switching is of thermal origin.

Fabrication and modulation characteristics of 1.3 µm p-doped InAs quantum dot vertical cavity surface emitting lasers

We present the fabrication of 1.3 µm waveband p-doped InAs quantum dot (QD) vertical cavity surface emitting lasers (VCSELs) with an extremely simple process. The continuous-wave saturated output power of 1.1 mW with a lasing wavelength of 1280 nm is obtained at room temperature. The high-speed modulation characteristics of p-doped QD VCSELs of two different oxide aperture sizes are investigated and compared. The maximum 3 dB modulation bandwidth of 2.5 GHz can be achieved at a bias current of 7 mA for a p-doped QD VCSEL with an oxide aperture size of 10 µm in the small signal frequency response measurements. The crucial factors for the 3 dB bandwidth limitation are discussed according to the parameters' extraction from frequency response.

Temperature Characteristics of 1.3-$\mu$m p-Doped InAs–GaAs Quantum-Dot Vertical-Cavity Surface-Emitting Lasers

In this paper, we present results from room-temperature continuous-wave operation of 1.3-mum p-doped InAs-GaAs quantum-dot (QD) vertical-cavity surface-emitting lasers (VCSELs) with high <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> of ~510 K and low threshold current density of ~65 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> per QD layer. The highest output power from the device is over 0.74 mW. The temperature characteristics of the devices are investigated. It is demonstrated that deterioration in QD VCSEL performance due to self-heating results from the temperature sensitivity of QD emission, instead of mismatch between the gain wavelength and cavity modes. The real temperature at the QD VCSEL active region above threshold is estimated from the shift in lasing wavelength, which is in good agreement with calculations based on a self-consistent rate equation and thermal conduction model. The analysis shows that enhancing the carrier confinement in the QD wetting layer contributes to improving the saturated output power of the QD VCSEL.

Structure optimisation of modern GaAs-based InGaAs/GaAs quantum-dot VCSELs for optical fibre communication

AbstractRoom-temperature (RT) continuous-wave (CW) performance of modern 1300-nm oxide-confined In(Ga)As/GaAs quantum-dot (QD) vertical-cavity surface-emitting diode lasers (VCSELs) taking advantage of many QD sheets is investigated using our comprehensive self-consistent simulation model to suggest their optimal design. Obviously, quantum dots should be as uniform as possible and as dense as possible to ensure high enough optical gain. Besides, our simulation reveals that efficient and uniform current injection into VCSEL active regions necessary to enhance excitation of the desired fundamental LP01 mode is accomplished in the VCSEL configuration with the broad-area bottom contact and the ring upper one as well as with the oxide aperture localized within the first period of the upper p-type DBR. The doping of the DBR mirrors is chosen as a compromise between their high enough electrical conductivity and low enough free-carrier absorption. The oxide aperture is additionally introducing the radial optical waveguiding. Moreover, our analysis has been concluded that VCSEL active regions should be composed of at least 9 QD sheets to acquire efficient RT CW operation. Furthermore, rather longer optical cavities are recommended in this case because localization of QD sheets should be adjusted to the anti-node positions of the optical cavity standing wave.

Dynamic Characteristics and Linewidth Enhancement Factor of Quantum-Dot Vertical-Cavity Surface-Emitting Lasers

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This study explores the relative intensity noise characteristics of quantum-dot vertical-cavity surface-emitting lasers (QD VCSELs). The resonance frequency and eye diagram are presented. The linewidth enhancement factor (<formula formulatype="inline"><tex Notation="TeX">$\alpha$</tex></formula> factor) of QD VCSEL is also investigated experimentally. The values of <formula formulatype="inline"><tex Notation="TeX">$\alpha$ </tex></formula> factor were measured to be between 0.48 and 0.60. Moreover, a photonic RF phase shifter is examined using the QD VCSEL. A phase shifter with a total phase shift of 2<formula formulatype="inline"><tex Notation="TeX">$\pi$</tex> </formula> was demonstrated. These investigations and demonstrations will be useful in the field of QD VCSEL. </para>

Self-Consistent Analysis of Carrier Confinement and Output Power in 1.3-$\mu{\hbox {m}}$ InAs–GaAs Quantum-Dot VCSELs

A self-consistent model comprising rate equations and thermal conduction equation is used to analyze the influence of self-heating on the carrier occupation, quantum efficiency, and output power of 1.3- $\mu{\hbox {m}}$ InAs–GaAs quantum dot (QD) vertical-cavity surface-emitting lasers (VCSELs). The simulation results show that the poor hole confinement in QDs is due to the thin wetting layer, and increase in QD density and layer number can significantly improve the self-heating effect and quantum efficiency of the device. The output power of the QD VCSEL is mainly determined by the quantum efficiency. High output power can be achieved by the high number of QD layers and QD density. However, there exists an optimized number of QD layers ( $\sim$ 15) to achieve the highest output power.