Self-consistent calculation of mode spectra and modulation responsein wurtzite GaN quantum-well lasers

The photon rate-equation formalism is used to evaluate the multimode photon density, the output lasing power, and the modulation frequency response in pseudomorphically strained wurtzite GaN quantum-well lasers. The formalism is based on a self-consistent methodology that couples an envelope function (or k·p) Hamiltonian with Poisson's equation. From this approach, we consider (1) the band structure under the influence of large piezoelectric fields and with many-body effect; and (2) the stimulated and spontaneous emissions for each Fabry-Perot mode. Our calculations predict a threshold current density of approximately 1 kA/cm² and an intrinsic 3-dB modulation bandwidth of 11.7 GHz at 40-mW output power for a 50-Å pseudomorphically strained GaN-Al_0.2Ga_0.8N single-quantum-well (SQW) laser. Our estimation of the threshold current density represents the theoretical limit and is compatible with recent experimental results in InGaN multiple-quantum-well (MQW) lasers     

Optical gain and gain suppression of quantum-well lasers withvalence band mixing

The effects of valence band mixing on the nonlinear gains of quantum-well lasers are studied theoretically for the first time. The analysis is based on the multiband effective-mass theory and the density matrix formalism with intraband relaxation taken into account. The gain and the gain-suppression coefficient of a quantum-well laser are calculated from the complex optical susceptibility obtained by the density matrix formulation with the theoretical dipole moments obtained from the multiband effective-mass theory. The calculated gain spectrum shows that there are differences (both in peak amplitude and spectral shape) between this model with valence band mixing and the conventional parabolic band model. The shape of the gain spectrum calculated by the new model becomes more symmetric due to intraband relaxation together with nonparabolic energy dispersions. Optical intensity in the GaAs active region is estimated by solving rate equations for the stationary states with nonlinear gain suppression     

Theoretical calculation of lasing spectra of quantum-dot lasers:effect of homogeneous broadening of optical gain

A theoretical calculation is presented for the effect of homogeneous broadening of optical gain on lasing spectra of quantum-dot lasers. Based on a coupled set of rate equations considering both the size distribution of quantum dots and a series of longitudinal cavity modes, we show that dots with different energies start lasing independently due to their spatial localization when the gain spectrum is a delta-like function, and that the dot ensemble contributes to a narrow-line lasing collectively under large homogeneous broadening. The result explains quite excellently the experimental lasing spectra found in self-assembled InGaAs-GaAs quantum-dot lasers     

Electronic band structures and optical gain spectra of strainedwurtzite GaN-AlxGa1-xN quantum-well lasers

The electronic band structures, density-of-states, and optical gain spectra for wurtzite GaN-Al_xGa_1-xN quantum wells are studied theoretically based on the Hamiltonian derived using the k.p method. We investigate the dependence of the optical gain and transparent current density on the well width, barrier height, and strain using a numerical approach with high accuracy. The mole fraction of Al in the barrier material is progressively increased to study the effects of quantum confinement and compressive strain. A higher Al mole fraction in the barrier leads to improvement of the TE optical gain and suppression of the TM optical gain. Furthermore, we demonstrate that a reduction of the well width offers improved modal gain over all radiative current densities. We also predict a transparent current density of 250 A/cm² for the GaN-Al_xGa_1-x N single quantum-well (QW) structure. Our results suggest that a suitable combination of thin well width and large barrier height should be selected in improving the TE optical gain in wurtzite GaN-Al_x Ga_1-xN single QW     

Analysis of differential gain in InGaAs-InGaAsP compressive andtensile strained quantum-well lasers and its application for estimationof high-speed modulation limit

A simplified model that furnishes an intuitive insight for the change in quantum-well (QW) laser gain due to QW strain and quantum confinement is presented. Differential gain for InGaAs-InGaAsP compressive and tensile strained multi-quantum-well (MQW) lasers is studied using the model. The comparison between the calculated and experimental results for lattice-matched and compressive strained MQW lasers shows that this model also gives quantitatively reasonable results. It is found that the variance-band barrier height strongly affects the differential gain, especially for compressively strained MQW lasers. The tensile strained MQW lasers are found to have quite high differential gain, due to the large dipole matrix element for the electron-light-hole transition, in spite of the large valence-band state density. Furthermore, a great improvement in the differential gain is expected by modulation p doping in the tensile strained MQW lasers. The ultimate modulation bandwidth for such lasers is studied using the above results     

Theoretical study on enhanced differential gain and extremelyreduced linewidth enhancement factor in quantum-well lasers

Low-chirp lasing operation in semiconductor lasers is addressed in a theoretical investigation of the possibility of reducing the linewidth enhancement factor (α factor) in quantum-well (QW) lasers to zero. It is shown that in reducing the α factor it is essential that lasing oscillation be around the peak of the differential gain spectrum, not in the vicinity of the gain peak. The condition for such lasing oscillation is analytically derived. The wavelength dependence of the material gain, the differential gain, and the α factor are calculated in detail taking into account the effects of compressive strain and band mixing on the valence subband structure. The effect of p-type modulation doping in compressively strained QWs is discussed. It is shown that the α factor, the anomalous dispersion part in the spectrum, crosses zero in the region of positive material gain, which makes is possible to attain virtual chirpless operation by detuning     

Resonance frequency, damping, and differential gain in 1.5 μmmultiple quantum-well lasers

A systematic investigation is presented into the intrinsic frequency response of quantum-well lasers, using parasitic-free relative intensity noise (RIN) measurements. There is shown to be a strong dependence of the resonance frequency on the number of quantum wells in the active region, originating from variations both in internal losses and in differential gain. The differential gain is found to have values higher than in corresponding bulk lasers, but only in devices with a large number of wells. The damping is also found to vary in a manner consonant with the changes in differential gain; however, comparison with bulk lasers indicates substantially stronger gain suppression in the quantum-well lasers studied     

Effect of active-region modulation doping on simultaneous ground-state and excited-state lasing in quantum-dot lasers

The lasing spectra and light-power characteristics of lasers based on InAs/InGaAs quantum dots with p-type modulation doping are studied over a broad range of pump currents. It is shown that p-type doping leads to a significant increase in the threshold current for the onset of lasing at the excited-state transition and makes it possible to attain higher output powers for lasing at the ground-state transition as compared to lasers with an undoped active region. An explanation for the observed features of two-level oscillation in quantum-dot lasers is suggested.     

Effective differential gain and modulation bandwidth reduction inquantum well lasers due to confinement factor modulation

A mechanism that may reduce the effective differential gain due to the modulation of the confinement factor with carrier density in quantum-well lasers is described. This mechanism may limit modulation bandwidth for quantum-well lasers with high threshold carrier density and narrow confining layer     

Theoretical investigation of gain and linewidth enhancement factorfor 1.55-μm tensile strained quantum-well lasers

The performance of quantum-well laser diodes with tensile strained wells was theoretically calculated. Using 4×4 Luttinger-Kohn Hamiltonian, valence band dispersion was calculated and used for the calculation of material gain. Linewidth enhancement factor was obtained by calculating the change of refractive index due to interband transition and free carrier plasma motion. The tensile well shows smaller material and differential gain compared to the compressive strained one. But smaller linewidth enhancement factor is obtained due to the absence of free carrier plasma effect. Linewidth enhancement factor is further reduced by p-type modulation doping in the barrier     

Linewidth rebroadening due to nonlinear gain and index induced by carrier heating in strained quantum-well lasers

The carrier heating has been recently recognized as one of the main origins of nonlinear gain, in particular in strained quantum-well lasers. The asymmetry of this effect introduces a nonlinear refractive index. The joint effects of nonlinear gain and nonlinear refractive index that are both due to carrier heating together with the spatial-hole-burning give rise to an increase in the carrier density and in the linewidth enhancement factor. These effects can explain the linewidth rebroadening at high power in phase-shifted single-mode DFB lasers.     

Effect of the confinement-layer composition on the internal quantumefficiency and modulation response of quantum-well lasers

The authors show that the transport factor X is related to the internal quantum efficiency η_i of the quantum-well laser, and the enhancement in X contributes significantly to a reduction of η_i. It is also shown that suppression of the thermionic emission of carriers out of the quantum well is essential to prevent the degradation of the effective differential gain in high-speed quantum-well lasers     

Phase dampings of optical dipole moments and gain spectra in semiconductor lasers

Homogeneous broadenings and gain spectra in semiconductor lasers have been theoretically estimated, involving non-Markovian relaxation processes. The estimated spontaneous emission spectrum is in fair agreement with the observed one for a GaAlAs laser. The advantages of the present model, compared to conventional Lorentzian and delta-function models, are discussed.     

Carrier distribution, gain, and lasing in 1.3-/spl mu/m InAs-InGaAs quantum-dot lasers

In this paper, we present the results of a theoretical model built to describe the temperature-dependent lasing characteristics of InAs-InGaAs quantum-dot (QD) lasers operating at 1.3 /spl mu/m. From the model, we find that traditional carrier distribution theories are inadequate to describe the performance of these lasers. We therefore introduce an improved model that allows for both free carriers and excitons in the dots. The new model provides threshold current and characteristic temperature T/sub 0/ values that are in good agreement with experimental data. The results of our modeling reveal that, while it is the excitons that mainly contribute to the gain, the ratio of excitons to free carriers significantly affect the T/sub 0/ of QD lasers. Our model results also indicate that the wetting layer current plays little role in QD laser performance. In addition, the model correctly predicts other experimental observations such as; increased T/sub 0/ for increased number of QD layers and p-doped structures, and the oscillatory behavior of T/sub 0/, lending further credibility to the model.     

Fourier-transform-limited pulses from gain-switched distributed-Bragg-reflector lasers using simultaneous modulation of gain and phase sections

Fourier-transform-limited pulses ( Delta tau . Delta nu approximately=0.35) have been obtained from a microwave-modulated DBR laser at 1.56 mu m. The chirp of the gain-switched pulse is compensated for by applying a fraction of the microwave signal to the phase section, thereby creating an instantaneous frequency shift of opposite sign. The generation of short coherent pulses from such a monolithic source can be of great interest for long haul soliton transmission.<>     

Theory of gain, modulation response, and spectral linewidth in AlGaAs quantum well lasers

We investigate theoretically a number of important issues related to the performance of AlGaAs quantum well (QW) semiconductor lasers. These include a basic derivation of the laser gain, the linewidth enhancement factor α, and the differential gain constant in single and multiple QW structures. The results reveal the existence of gain saturation with current in structures with a small number of wells. They also point to a possible two-fold increase in modulation bandwidth and a ten-fold decrease in the spectral laser linewidth in a thin QW laser compared to a conventional double heterostructure laser.     

Absorption, emission, and gain spectra of 1.3 µm InGaAsP quaternary lasers

The spontaneous emission spectrum of mesa lasers was analyzed to determine the absorption and gain spectra at threshold. The radiative current density at threshold was found to be 4.4 kA cm^-2μm^-1, which is 60 percent of the total current density for the lowest threshold mesa laser. The increase in radiative lifetime due to reabsorption of emitted radiation was calculated to be 1.5, using file measured absorption and emission spectra. Contrary to other studies, our investigation of an LED and 3 lasers of different types yielded no evidence of carrier heating.     

单掺铥固体激光器热效应研究进展与理论分析北大核心CSCD

随着中红外2μm波段固体激光器在工业、医学、军事和科学研究等领域中的不断扩展,其研究的重要性越来越明显。单掺铥固体激光器的输出波长恰好处于2μm附近,因此成为人们研究的热点方向,但单掺铥晶体由于发射截面小,上转换、重吸收效应严重等原因导致其热效应明显,严重影响了激光器的输出性能,使激光器的发展受到严重限制。因此研究激光晶体中的热效应对激光器性能的提升具有重要意义。本文综述了自研究热效应以来国内外基于各种基质的单掺铥固体激光器的热效应的研究成果,同时对固体激光器热透镜效应的热传导理论以及对热透镜效应的发生环境和形成条件有影响的因素进行阐述和分析。最后针对分析过程中所涉及的热功率密度,光强等参数进行讨论,为热透镜效应的热焦距计算和测量奠定了良好的理论基础。