Effect of inter-level relaxation and cavity length on double-state lasing performance of quantum dot lasers

Double-state lasing phenomena are easily observed in self-assembled quantum dot (QD) lasers. The effect of inter-level relaxation rate and cavity length on the double-state lasing performance of QD lasers is investigated on the basis of a rate equation model. Calculated results show that, for a certain cavity length, the ground state (GS) lasing threshold current increases almost linearly with the inter-level relaxation lifetime. However, as the relaxation rate becomes slower, the ratio of excited state (ES) lasing threshold current over the GS one decreases, showing an evident exponential behavior. A relatively feasible method to estimate the inter-level relaxation lifetime, which is difficult to measure directly, is provided. In addition, fast inter-level relaxation is favorable for the GS single-mode lasing, and leads to lower wetting layer (WL) carrier occupation probability and higher QD GS capture efficiency and external differential quantum efficiency. Besides, the double-state lasing effect strongly depends on the cavity length.     

Influence of a single quantum dot state on the characteristics of a microdisk laser

We report a quantum dot microcavity laser with a cw sub-microW lasing threshold, where a significant reduction of the lasing threshold is observed when a single quantum dot (QD) state is aligned with a cavity mode. The quality factor exceeds 15,000 before the system lases. When no QD states are resonant, below threshold the cavity mode initially degrades with increasing pump power, after which saturation occurs and then the cavity mode recovers. We associate the initial cavity mode spoiling with QD state broadening that occurs with increasing pump power.     

Lasing spectra of 1.55 μm InAs/InP quantum dot lasers: theoretical analysis and comparison with the experiments

In this paper, a theoretical model is used to investigate the lasing spectrum properties of InAs/InP (113)B quantum dot (QD) lasers emitting at 1.55 μm. The numerical model used is based on a multi-population rate equation (MPRE) analysis. It takes into account the effect of the competition between the inhomogeneous broadening (due to the QD size dispersion) and the homogenous broadening as well as a nonlinear gain variation associated to a multimode laser emission. The double laser emission and the temperature dependence of lasing spectra of self-assembled InAs/InP quantum dot lasers is studied both experimentally and theoretically.     

Stability of Optically Injected Two‐State Quantum‐Dot Lasers

Simultaneous two‐state lasing is a unique property of semiconductor quantum‐dot (QD) lasers. This not only changes steady‐state characteristics of the laser device but also its dynamic response to perturbations. In this paper we investigate the dynamic stability of QD lasers in an external optical injection setup. Compared to conventional single‐state laser devices, we find a strong suppression of dynamical instabilities in two‐state lasers. Furthermore, depending on the frequency and intensity of the injected light, pronounced areas of bistability between both lasing frequencies appear, which can be employed for fast optical switching in all‐optical photonic computing applications. These results emphasize the suitability of QD semiconductor lasers in future integrated optoelectronic systems where a high level of stability is required.     

Numerical analysis of the effects of carrier dynamics on the switch-on and gain-switching of quantum dot lasers

A systematic analysis of the influence of the capture, inter-level relaxation and exciton dephasing time constants on the dynamic behavior of quantum dot Fabry-Perot semiconductor lasers is done taking into account the lasing from the ground and excited states. The simulation results show that the carrier time constants studied influence significantly the static characteristic of the laser, its switch-on response and the pulses generated by gain-switching.     

High Power 980 nm InGaAs/AlGaAs Strained Quantum Well Lasers

1lntroductionCarhan,aIVcolutnnelement,hasmanyadvantagesinGaAsAlGaAsmaterials,suchasIowdiffudricoefficient,relativelowactivateenergyabout26meV,highincmptiOnconcentratboandhighm0bilityduetothelowcomensaterate.SocarbonhasbeenwidelyusedinGaAsAlGaAsheter0unctionbipoartransistors(HBT),modulationdoPingfieldeffecttransistors(m),tunneldiodes,iInPurityinducedlayerdisorderinglaserdiodeS,anddistributedBraggreflectors(DBRs)intheverticalcavitysurfaceedrittinglasers(VCSEL).Ingeneral,therearesever…     

High Power 980 nm InGaAs/AlGaAs Strained Quantum Well Lasers

By low-pressure metalorganic chemical vapor deposition (LP-MOCVD) system,InGaAs/AlGaAs graded-index separate-confinement heterostructure strained quantum well lasers are grown with carbon doped the upper cladding layer and the capping layer. Carbon tetracholride (CCl4) is used as the carbon source. 100 μm oxide stripe lasers are fabricated,and the laser output power per facet (uncoated) reaches 1.2 W with 2A injection current under the room temperature continuous wave (CW) operation. The threshold current density is 150 A/cm2 with 1000 μm cavity length. The slope efficiency per facet reaches 0.53W/A,and the total external differential quantum efficiency is above 85%. The relations between the threshold current densities,the differential quantum efficiency and the cavity length are studied.     

Reduced linewidth enhancement factor due to excited state transition of quantum dot lasers

The carrier induced refractive index change and linewidth enhancement factor α due to ground-state (GS) and excited-state (ES) transitions have been compared by measuring the optical gain spectra from an InAs/GaAs quantum dot (QD) laser structure. It is shown that the ES transition exhibits a reduced α-factor compared to the value due to the GS transition. This result can be explained by the α-factor due to the ES transition having a smaller increase from the non-resonant carriers in the combined state of the wetting layer and InGaAs strain reducing layer than the α-factor increase due to the GS transition, since the relaxation time for carriers from the combined state of the wetting layer and InGaAs strain reducing layer to the ES is shorter than to the GS. The result reported here shows another advantage of using ES QD lasers for optical communication, in addition to their higher modulation speed.     

External cavity quantum dot tunable laser through 1.55 μm

The optical performance of InAs/InGaAsP quantum dot (QD) lasers grown on (1 0 0) InP was studied for three different material structures. The most efficient QD laser structure, having a threshold current of 107 mA and an external differential quantum efficiency of 9.4% at room temperature, was used to form the active region of a grating-coupled external cavity tunable laser. A tuning range of 110 nm was demonstrated, which was mainly limited by the mirror and internal losses of the uncoated laser diode. Rapid state-filling of the QDs was also demonstrated by observing the evolution of the spectra with increasing injected current.     

InAs–GaAs self-assembled quantum dot lasers: physical processes and device characteristics

The gain characteristics of InAs–GaAs self-assembled quantum dot lasers are studied using two complementary techniques. The modal gain is derived from a measurement of the normal incidence, inter-band photoconductivity. For a device containing a single layer of dots the maximum modal gain of the ground state transition is found to be insufficient for lasing action. As a consequence lasing occurs for excited state transitions, which have a larger oscillator strength, with the precise transition being dependent upon the device cavity length. The second technique uses the Hakki–Paoli method to determine the spectral and current dependence of the gain. A quasi-periodic modulation of the below threshold gain is observed. This modulation is shown to be responsible for the form of the lasing spectra, which consist of groups of lasing modes separated by non-lasing spectral regions. Possible mechanisms for this behaviour are discussed.     

Radiation characteristics of injection lasers based on vertically coupled quantum dots

We have studied injection lasers based on InGaAs/GaAs vertically coupled quantum dots (QD) grown by molecular beam epitaxy. The threshold current density decreases by one order of magnitude down to 90 A cm⁻²(300 K) with an increase of the number of QD stacks (N) up to 10. ForN≥ 3 lasing occurs via the QD ground state up to room temperature. Differential efficiency increases withNup to 50%. No change in range of high temperature stability of threshold current density (J_th) was observed, while the characteristic temperature (T₀) measured at 300 K increases from 60 to 120 K. Using InGaAs-AlGaAs QD with higher localization energy allowed us to decreaseJ_thdown to 60 A cm⁻²and to increase the differential efficiency up to 70%.     

Gain recovery dynamics and photon-driven transport in quantum cascade lasers

Quantum cascade lasers are semiconductor devices based on the interplay of perpendicular transport through the heterostructure and the intracavity lasing field. We employ femtosecond time-resolved pump-probe measurements to investigate the nature of the transport through the laser structure via the dynamics of the gain. The gain recovery is determined by the time-dependent transport of electrons through both the active regions and the superlattice regions connecting them. As the laser approaches and exceeds threshold, the component of the gain recovery due to the nonzero lifetime of the upper lasing state in the active region shows a dramatic reduction due to the onset of quantum stimulated emission; the drift of the electrons is thus driven by the cavity photon density. The gain recovery is qualitatively different from that in conventional lasers due to the superlattice transport in the cascade.     

基于钠硼铝硅酸盐玻璃的PbSe量子点红外单模光纤激光

根据实验制备的钠硼铝硅酸盐PbSe量子点玻璃及其透射电子显微镜(TEM)图、吸收谱和发射谱,计算机数值模拟了以PbSe量子点作为激活增益介质的红外单模光纤激光。应用遗传算法,通过数值求解粒子数速率方程和激光谐振腔振荡方程,优化计算了量子点光纤激光器(QDFL)的最佳抽运波长、光纤长度、掺杂浓度及出射镜反射率。结果表明:饱和抽运功率为2 W,在1676nm激光波长处,QDFL最大输出功率可达1.36 W,抽运效率达68%。与通常的掺稀土离子(Yb3+、Er3+)的光纤激光器相比,QDFL具有抽运效率高、激励阈值低、掺杂密度可调、光纤饱和长度短等特点。由于量子点辐射波长的尺寸依赖特性,容易形成多波长激射或波长可调的新型激光器。     

InGaAsP/InGaP/GaAs单量子阱激光器工作特性

利用低压-金属有机化学汽相沉积(LP-MOCVD)方法研制出InGaAsP/InGaP/GaAs单量子阱大功率激光器并分析了阈值电流密度、特征温度和外微分量子效率与腔长的关系.     

Impact of gain compression on modulation response and dynamic properties of three state lasing InGaAs/GaAs quantum dot lasers

In this paper, we have theoretically studied dynamics of a semiconductor quantum dot (QD) laser for enhancing its small signal and large signal modulation as a function of compression gain. We have considered InGaAs/GaAs QD laser rate equations and solved this equation system numerically. We have revealed that a diminution in compression gain leads to an improvement in frequency bandwidth for this three state lasing system. We also have calculated turn on delay and output power that obviously indicates the effect of compression gain on relaxation oscillations.     

Effect of inhomogeneous broadening of submonolayer quantum dots on the generation of vertical-cavity laser with saturable absorber

The experimental dependences for a laser based on submonolayer quantum dots (QDs) without an absorber were used to calculate the energy characteristics of a semiconductor laser based on QDs and quantum wells with a saturable absorber under bistable lasing conditions; the data obtained make it possible to implement a monolithic version of this laser. All of the mechanisms of carrier relaxation and QD filling, which are important for experiment, are taken into account. The effect of inhomogeneous QD broadening over resonant frequency is considered. The laser mode shift that is caused by an increase in the pump current is found to limit the lasing power. It is shown that the choice of active and passive layers with inhomogeneously broadened QDs makes it possible to obtain bistable lasing in the entire range of available pump currents.     

半导体量子点激光器研究进展

首先简要地回顾了半导体激光器发展的历史和量子点激光器所特有的优异性能,进而介绍半导体量子点及其三维量子点阵列的制备技术,然后分别讨论了量子点激光器（能带）结构设计思想,实现基态激射时所必须具备的条件和近年来国内外半导体量子点器的研究进展。最后分析讨论了量子点激光器研制中存在的问题和发展趋势。     

Micro-cavity semiconductor lasers with controlled spontaneous emission

The principle and applications of quantum electrodynamics in microcavity semi-conductor lasers are reviewed. The coupling efficiency of spontaneous emission into a lasing mode and the spontaneous lifetime are modified by various microcavity structures. As a consequence of the increased coupling efficiency, those microcavity semi-conductor lasers are expected to feature a low threshold current, high quantum efficiency and broad modulation bandwidth. One remarkable result of the increased coupling efficiency is lasing without inversion. The other is intensity squeezing at any pump rate.     

Global rate equation description of a laser

The global integro-differential rate equations describing a multimode laser are analyzed. Expressions for the relaxation oscillation frequencies and their damping rates in the single-mode and two-mode regimes are obtained without specifying either the cavity geometry or the longitudinal pump profile. On the same level of generality, we prove the existence of universal relations relating the peaks of the power spectra in the two-mode regime. For a Fabry-Perot with arbitrary longitudinal pump profile, series expansions of all the physical functions are derived in powers of the pump moments. These moments are averages of the pump profile over cavity modes at linear combinations of the lasing frequencies and their harmonics. These results apply to end-pumped and/or partially filled lasers. For a single mode Fabry-Perot laser, we prove that the contribution to the steady state intensity from the lasing mode varies from 75% close to the lasing threshold to zero at high intensity. The remainder comes from the harmonics of the lasing mode. Analyzing the steady state single mode intensity equation in terms of the pump gratings, we prove that close to the lasing threshold only the space average of the pump and its grating oscillating at twice the lasing wave number do not vanish. This provides a hint towards the justification of the usual modal rate equations which retain only these two functions in the dynamical evolution of a laser. For a Fabry-Perot with constant pump profile, an exact expression for the upper boundary of the stable single mode regime is derived. In that two-mode regime, we prove that there is a critical value of the pump at which the ratio of the two relaxation oscillation frequencies is 2, leading to an internal resonance.     

Double-state controllable optical switching through three tunnel-coupled quantum dots inside waveguide coupled photonic crystal microcavity

We study optical transmission properties of a combined system which is composed of a photonic crystal (PC) microcavity with low quality factor Q, a triple quantum dot (QD) embedded in cavity and two parallel waveguides. We demonstrate that low coupling strength (i.e., the weak coupling regime) between a cavity and a dot, by means of electron tunnel-induced coupling, can lead to a type of double-state controllable optical switching under the experimentally available parameter conditions.