Quantum Dot Lasers Research Articles

Reflection sensitivity of InAs/GaAs epitaxial quantum dot lasers under direct modulation

This paper reports on the reflection sensitivity under direct modulation operation of a 1.3 μm InAs/GaAs quantum dot laser that is epitaxially grown on silicon. The quantum dot laser exhibits a high tolerance to back reflections with low error transmission at 6 Gbps. This study paves the way for developing directly modulated isolator-free photonic integrated circuits based on quantum dot lasers.

The role of different types of dopants in 1.3 μm InAs/GaAs quantum-dot lasers

The performance of O-band InAs/GaAs quantum-dot (QD) lasers grown by molecular beam epitaxy with three different doping strategies in the active region are investigated for a temperature range of 17 °C–97 °C. The lasing performance indicates that the n-type doping technique reduced the threshold current density of InAs QD lasers across the full temperature range and narrowed the near field lasing spot. However, for short-cavity lasers, the n-type doped laser switches from ground-state to excited-state lasing at a lower temperature compared to undoped and p-type modulation-doped lasers. In contrast, the p-type modulation-doped lasers have a reduced threshold current density for higher temperatures and for shorter lasers with cavity lengths of 1 mm and below.

Spectral dispersion of the linewidth enhancement factor and four wave mixing conversion efficiency of an InAs/GaAs multimode quantum dot laser

The spectral dependence of the linewidth enhancement factor (αH-factor) of a multimode InAs/GaAs quantum dot laser is analyzed. Amplified spontaneous and high-frequency modulation methods are used to experimentally retrieve the αH-factor of each longitudinal mode below and above the threshold. A dispersion of the αH-factor is unlocked across the entire optical spectrum, which is further illustrated in the context of four wave mixing experiments. The results show that the induced conversion efficiency is increased at lasing wavelengths where the linewidth enhancement is lower. These results highlight the importance of carefully monitoring the linewidth enhancement factor in quantum dot lasers especially for frequency combs and mode-locking applications in future optical communication systems.

Mid-IR quantum dot cascade VCSEL: feasibility study and feature extraction.

Quantum cascade lasers with quantum dot (QD) active regions have been developed to overcome the drawbacks of quantum-well-based cascade lasers. Low threshold current and enhanced bandwidth characteristics of QD lasers can be combined with promising energy-efficient features of vertical cavity surface emitting lasers (VCSELs) through introducing the novel, to the best of our knowledge, concept of a QD cascade VCSEL (QDC-VCSEL). Design steps and predicted features of a QDC-VCSEL operating at 4.5 µm wavelength are discussed in this paper, where the cavity and active region are optimized for TE mode using a high contrast grating design. Laser characteristics are investigated by developing rate equations for the cascade scheme. Threshold and L-I characteristics are obtained at different temperatures. Also, the dependency of threshold current, slope efficiency, and modulation bandwidth on design parameters, mainly the carrier tunneling time, is studied. Results exhibit output peak powers of ∼2 and ∼0.5mW at 150 and 273 K, respectively. In addition, threshold current increment and modulation bandwidth reduction are observed with the increase in tunneling time.

Recent Progress of Quantum Dot Lasers Monolithically Integrated on Si Platform

With continuously growing global data traffic, silicon (Si)-based photonic integrated circuits have emerged as a promising solution for high-performance Intra-/Inter-chip optical communication. However, a lack of a Si-based light source remains to be solved due to the inefficient light-emitting property of Si. To tackle the absence of a native light source, integrating III-V lasers, which provide superior optical and electrical properties, has been extensively investigated. Remarkably, the use of quantum dots as an active medium in III-V lasers has attracted considerable interest because of various advantages, such as tolerance to crystalline defects, temperature insensitivity, low threshold current density and reduced reflection sensitivity. This paper reviews the recent progress of III-V quantum dot lasers monolithically integrated on the Si platform in terms of the different cavity types and sizes and discusses the future scope and application.

Mapping the Stability and Dynamics of Optically Injected Dual State Quantum Dot Lasers

Optical injection is a key nonlinear laser configuration both for applications and fundamental studies. An important figure for understanding the optically injected laser system is the two parameter stability mapping of the dynamics found by examining the output of the injected laser under different combinations of the injection strength and detuning. We experimentally and theoretically generate this map for an optically injected quantum dot laser, biased to emit from the first excited state and optically injected near the ground state. Regions of different dynamical behaviours including phase-locking, excitability, and bursting regimes are identified. At the negatively detuned locking boundary, ground state dropouts and excited state pulses are observed near a hysteresis cycle for low injection strengths. Higher injection strengths reveal μs duration square wave trains where the intensities of the ground state and excited state operate in antiphase. A narrow region of extremely slow oscillations with periods of several tens of milliseconds is observed at the positively detuned boundary. Two competing optothermal couplings are introduced and are shown to reproduce the experimental results extremely well. In fact, the dynamics of the system are dominated by these optothermal effects and their interplay is central to reproducing detailed features of the stability map.

High-Efficiency Quantum Dot Lasers as Comb Sources for DWDM Applications

The trend of data center transceivers is to increase bandwidth while simultaneously decreasing their size, power consumption, and cost. While data center links have previously relied on vertical-cavity surface-emitting lasers (VCSELs) or in-plane lasers using coarse wavelength division multiplexing (WDM) to encode data, recently, dense WDM (DWDM) has moved to the forefront for next-generation links. Several approaches exist as light sources for DWDM links; DFB arrays, nonlinear microcombs, and semiconductor mode-locked lasers (MLLs). This paper focuses on quantum dot MLLs (QDMLLs), which currently offer the best efficiency, simplicity, and size. The efficiency of optical combs generated by QDMLLs is analyzed in depth in this study.

Monolithic Passive−Active Integration of Epitaxially Grown Quantum Dot Lasers on Silicon

Quantum Dot Lasers The cover art showcases a novel photonic integration platform that monolithically integrate lasers on silicon photonic chip. In this platform, the III–V material is grown in a patterned trench in the Si photonic wafer. The external cavity lasers can be fabricated in the III–V region with their light output coupled to silicon photonic waveguides to achieve ultimate passive-active integration. More information can be found in article 2100522 by Zeyu Zhang and co-workers.

Impact of Quantum Dots on III-Nitride Lasers: A Theoretical Calculation on Linewidth Enhancement Factors

III-nitride quantum dot (QD) lasers have potential applications in visible light regime, yet one of its most important dynamic parameters, the linewidth enhancement factor (LEF), has been rarely studied. We studied the LEF of InGaN QD lasers with a comprehensive theory model and found that the LEF values of InGaN QD lasers can be smaller and more robust against inhomogeneous broadening than those of InGaAs QD lasers. In addition, the LEF improvement from quantum well to QD in III-nitride semiconductors is much larger than that in III-arsenide. It is believed that the large confinement energy in III-nitrides and atomic-like density of states of QDs contribute to these advantages of III-nitride QD lasers. Our results not only reveal the advantages of using III-nitride QD for high spectral purity, low-chirp laser applications, but also provide a general guideline for designing QD lasers with lower LEF values in various semiconductor materials.

Investigation into the InAs/GaAs quantum dot material epitaxially grown on silicon for O band lasers

InAs/GaAs quantum dot (QD) lasers were grown on silicon substrates using a thin Ge buffer and three-step growth method in the molecular beam epitaxy (MBE) system. In addition, strained superlattices were used to prevent threading dislocations from propagating to the active region of the laser. The as-grown material quality was characterized by the transmission electron microscope, scanning electron microscope, X-ray diffraction, atomic force microscope, and photoluminescence spectroscopy. The results show that a high-quality GaAs buffer with few dislocations was obtained by the growth scheme we developed. A broad-area edge-emitting laser was also fabricated. The O-band laser exhibited a threshold current density of 540 A/cm2 at room temperature under continuous wave conditions. This work demonstrates the potential of large-scale and low-cost manufacturing of the O-band InAs/GaAs quantum dot lasers on silicon substrates.

Origin of the Diffusion-Related Optical Degradation of 1.3 μm Inas QD-LDs Epitaxially Grown on Silicon Substrate

This paper investigates the origin of the diffusion process responsible for the optical degradation of InAs quantum dot (QD) laser diodes epitaxially grown on silicon. By means of a series of constant-current stress experiments carried out at different temperatures, we were able to quantitatively evaluate the temperature acceleration of the degradation process. In addition, the presence of temperature thresholds above which the degradation rate drastically increases was ascribed to the onset of a recombination-enhanced degradation process, which is favored at high temperatures. Finally, the comparison of the experimentally determined diffusion coefficients with prior scientific reports suggests that degradation is related to the recombination-enhanced diffusion of Be, used here as p-type dopant, or of the lattice defects limiting Be diffusion. The original results of this work provide new insight on the microscopic origin of the gradual optical degradation of quantum-dot lasers, which will find wide application in silicon photonics.

Demonstration of a 17 × 25 Gb/s Heterogeneous III-V/Si DWDM Transmitter based on (De-) Interleaved Quantum Dot Optical Frequency Combs

We discuss the design and demonstration of a space and dense wavelength division multiplexed heterogeneous III-V/Si transmitter based on a single multi-wavelength quantum dot laser source and ultra-power-efficient metal-oxide-semiconductor capacitor (MOSCAP) (de-)interleaver. This paper begins by introducing a transceiver architecture capable of > 1 Tb/s transmission with < 1.5 pJ/bit power consumption, followed by a detailed discussion of the heterogeneous laser source and (de-)interleaver. The O-band quantum dot laser, based on a compound cavity design, has a FSR ∼ 64 GHz with a 1σ variation of ∼ 1.08 GHz and a measured relative intensity noise (RIN) of ∼ -144 dB/Hz for the largest comb peak. The single-ring-assisted asymmetric Mach-Zehnder interferometer (1-RAMZI) MOSCAP (de-)interleaver exhibit cross-talk (XT) levels down to -27 dB for tuning powers of 10.0 nW. Finally, to the best of our knowledge, we have demonstrated for the first time, a simultaneous wavelength and space division multiplexed transmitter fabricated on a heterogeneous III-V-on-silicon platform. Experiments show (de-)interleaved 17 optical comb lines, each modulated at 25 Gb/s non-return-to-zero (NRZ) for an aggregate bandwidth of 425 Gb/s.

Dot States Distribution, Inhomogeneous and Linewidth Broadenings in Quantum Dot Lasers

Dot States Distribution, Inhomogeneous and Linewidth Broadenings in Quantum Dot Lasers

Crack propagation in low dislocation density quantum dot lasers epitaxially grown on Si

Low threading dislocation density in epitaxial lasers on Si is required for high performance and robust devices for silicon photonic integrated circuits. However, as the threading dislocation density is further reduced, a point is reached where it is energetically favorable for cracking to occur in the layers after cooldown to room temperature due to the thermal expansion coefficient mismatch between the film and the substrate. This can be solved in most cases by increasing the optical confinement and reducing the total layer thickness. We combine models of dislocation motion (controlling plastic relaxation) and thin film channel cracking to describe the impact of dislocation density and cooling rate, which addresses a well-known and previously unsolved problem in heteroepitaxial growth for optoelectronic and electronic devices. Agreement between predictions and experiments illustrates that the model is effective in identifying critical dislocation densities, film thickness, and cooling rates that avoid cracking. We believe that this work is the first attempt to merge the fracture mechanism and dislocation-mediated plastic relaxation in semiconductor films to solve a practical problem in optoelectronic materials.

Influence of potential well thickness on the carrier transport characteristics of InGaAs quantum dot laser diodes.

The characteristics of InGaAs quantum dot laser diodes have been investigated by inserting InGaAs wells of different thicknesses in the active zone. The simulation results indicated that the maximum output power of the InGaAs quantum dot laser diode with a 0 nm-thick InGaAs well is 0.72 W at an injection current of 1.1 A. However, the quantum dot laser diode with a 12 nm-thick InGaAs well has a maximum electro-optical conversion efficiency of 40.6%. The properties of carrier transport in the active zone were analyzed via the potential difference of the band offset among potential barrier, potential well, and waveguide layer. It is observed that electron leakage increases owing to the inserted InGaAs well. Auger recombination not only increases the carrier injection efficiency but also is acts as the key factor of carrier loss in the active zone. Moreover, the inserted InGaAs well can improve carrier transport in the active zone and increase the carrier utilization efficiency. The power-loss mechanism of the quantum dot laser diode was elucidated from the optical and carrier losses. It is found that optical loss is the key factor that affects the output power.

Linewidth Enhancement Factor Impact on Optical Feedback Mutual Coupled Quantum Dot Lasers

In this article, we numerically study and analyse the roles of linewidth enhancement factor (α) in the dynamic operation of the mutual regime of the transmitter and receiver quantum dot laser lasers supported by optical feedback. A set model of adequate rate equations describing the overall dynamics in a quantum dot system subjected to optical feedback were solved numerically. The results reveal a clear chaotic regime between the receiver and the transmitter lasers at α = 3, which is incredibly advantageous for secure optical communications and encoding decoding data transmission. Moreover, at the other value of linewidth enhancement factors, namely 2, 2.5, 3.5 and 4, the optical regime works in high synchronisation with either periodic or steady state forms.

Laser Characteristic and Strain Distribution Dependence on Embedding Layer Thickness of Quantum Dots Laser Diodes Grown on InP(311)B Substrate

Herein, 14 layer‐stacked quantum dots laser diodes (QD‐LDs) are fabricated with different thicknesses of embedding layers grown on InP(311)B substrate, and the lowest threshold current (Ith) of 21.6 mA is demonstrated in an as‐cleaved ridge‐structured QD‐LD with 13 nm‐thick embedding layers. Ith has the minimum value when the thickness is 13 nm. The factors unique to QDs grown on an InP(311)B substrate are assumed as the overlap integral of the wavefunction of electrons and holes and the decrease in gain owing to the formation of a miniband. According to the simulation results of the local strain profile in the embedding and QD layers, the strain in the QD and embedding layer is higher in the case of 7 nm thickness than in the case of 20 nm thickness. As a result of the large strain, because the thickness of the embedding layer decreases, a large internal electric field tends to be generated, causing the wavefunctions to become largely separated and localized. Therefore, the overlap integral of the wavefunction becomes smaller, and because of the decrease in the radiation recombination probability, the internal quantum efficiency also decreases, causing the characteristic of Ith on the thickness of the embedding layer.

Random number generation with arrays of coupled quantumdot micropillar lasers

The paper investigates the results of random number generation with arrays of coupled quantum-dot micropillar lasers. The micropillars array laser generation is modeled based on the rate equations for quantum dot lasers. The numerical simulation of the dynamics for the arrays of coupled quantum-dot micropillar lasers is carried out utilizing the semi-implicit Euler method, implemented in Julia programming language. The algorithm of random bit sequence generation consists of the following steps: sampling the values of the total field intensity for coupled micropillar lasers; normalizing and discretizing the obtained values per resolution of the 8-bit analog-to-digital converter; extracting the four least significant bits from the bit representation of the sampled values; concatenating the bit values in a single sequence. The possibility of the bit sequences generation having an equiprobable distribution of zeros and ones with a performance of up to 400 Gbit/s was shown utilizing a random number generator based on an array of coupled quantum-dot micropillar lasers for sequences with a length of 14285716 bits at a sampling rate of 100 gigasamples per second and four least significant bits extraction. The resulting bit sequences successfully passed 14 NIST 800-22 statistical tests for the p-value equal to 0.01. The proposed method can be applied to develop random number generators based on larger arrays of coupled quantum-dot micropillar lasers. The results can be utilized in the experimental implementation of random number generators based on arrays of coupled quantum-dot micropillar lasers.

Low temperature Fermi-Dirac distribution in InAsP quantum dot lasers

Abstract We examine the probability of carriers in a 770 nm InAsP quantum dot (QD) laser by analyzing Fermi-Dirac distribution using the segmented contact method. This study compares the results of InAsP QD materials with the standard 720 nm InP QD laser that was developed by Cardiff University. Both samples were grown under the same conditions on a compatible wafer. InAsP QD showed non-uniformity in the dot size with a higher strain field, as evidenced by transmission electron microscopy images, as well as a deeper dot confinement by approximately 100 meV, as confirmed by Photovoltage Spectroscopy. This study revealed a breakdown of the Fermi-Dirac distribution at 150 K for InAsP QD materials. This could make InAsP a capable applicant for the ultrafast source generation.

Internal optical loss and light-current characteristic in injection lasers

Possibility of tailoring the light-current characteristic (LCC) shape in quantum dot (QD) lasers by varying uniformity of QDs is discussed. Making the QD ensemble less uniform results in roll-over in the LCC. The second branch in the LCC appears with making the QD ensemble even less uniform.