Influences of unconfined states on the optical properties ofquantum-well structures

The population of the unconfined states, with energies above the band edge of the barrier layers, can be significant in some regions of the active volume in high power lasers and amplifiers. This paper analyzes the influences of these states on optical properties, such as gain, refractive index, differential gain, and linewidth enhancement factor, for different quantum-well (QW) structures. Our results show that at high excitation levels, the unconfined band contributions to the real part of the optical susceptibility can be significant, especially in structures with weak quantum confinement potentials. This is in agreement with recent measurements of peak gain and carrier-induced refractive index change versus carrier density, for InGaAs-GaAs QW laser structures     

Orientation dependence of optical properties in long wavelengthstrained quantum-well lasers

The dependence of optical properties on crystal orientation is analyzed for long wavelength strained quantum-well (QW) GaAsP-InGaAsP lasers. The calculation is based on the multiband effective mass theory which enables us to consider the anisotropy and the nonparabolicity of the valence-band dispersions. It is found that the optical gain increases as the crystal orientation is inclined from (001) toward (110). This is due to the reduced valence-band density of states. The differential gain is about 1.6 times larger for the (110)-oriented 1.55-μm strained QW's than for equivalent (001)-oriented QW's. It is also shown that the threshold current density in 1.3-μm strained QW lasers decreases to two-thirds of that in the (001)-oriented laser as the orientation is inclined away from (001) by 40°-90     

Analysis of gain in determining T0 in 1.3 μmsemiconductor lasers

Rapid decrease of differential gain has been determined to dominate the temperature dependence of threshold current in 1.3-μm multiquantum well and bulk active lasers giving rise to low values of T ₀. Extensive experimental characterization of each type of device is described. Results are presented for the dependence of gain on chemical potential and carrier density as a function of temperature. The data indicate the important role of the temperature-insensitive, carrier density dependent chemical potential in determining differential gain. Modeling of the temperature dependence of threshold carrier density in MQW and bulk active lasers based on a detailed band theory calculation is described. The calculated value of T₀ depends on the structure of the active layer, e.g., multiquantum well versus bulk. However, the calculated values are substantially higher than measured     

Non-Markovian gain of strained-layer wurtzite GaN quantum-welllasers with many-body effects

A theoretical model for the optical gain of strained-layer wurtzite GaN quantum-well (QW) lasers is developed taking into account valence-band mixing, many-body effects and non-Markovian relaxation. The valence-band structure is calculated from a 6×6 multiband effective mass Hamiltonian for the wurtzite structure taking into account built-in strain due to lattice mismatch. The theoretical foundation for the optical processes is based on the time-convolutionless reduced-density operator formalism given in previous papers for an arbitrary driven system coupled to a stochastic reservoir. Many-body effects are taken into account within the time-dependent Hartree-Fock approximation and the optical gain with Coulomb (or excitonic) enhancement is derived by integrating the equation of motion for the interband polarization. It is predicted that the Coulomb enhancement of gain is pronounced with increasing magnitude of compressive strain in the QW     

Large- and small-signal dynamic behavior of high-speeddual-polarization quantum-well semiconductor lasers

The transmission-line laser model is modified to model both transverse-electric (TE) and transverse-magnetic (TM) modes so that it is applicable to quantum-well (QW) dual-polarization lasers and polarization-insensitive semiconductor optical amplifiers (SOAs). The effects of carrier transport are also included in the model. The resulting dual-polarization transmission-line laser model is used to study large- and small-signal dynamic behavior of dual-polarization lasers. We find from large-signal simulations that the polarization asymmetry (ratio of the transverse-modal powers) varies on a nanosecond time scale in dual-polarization single-quantum-well (SQW) devices. We show that unequal transverse-modal differential gains and gain nonlinearities are responsible for this temporal polarization asymmetry. In addition, our numerical simulations show that the steady-state polarization asymmetry is a strong function of the gain nonlinearity. Small-signal dynamic simulations show that the modulation response of the polarization-unresolved output of dual-polarization SQW lasers follows that of the transverse mode with the lowest gain nonlinearity coefficient, regardless of the transverse-modal differential gains     

Theoretical analysis of diffused quantum-well lasers and optical amplifiers

Diffused quantum-well (QW) distributed feedback (DFB) lasers and optical amplifiers will be theoretically analyzed in this paper. For DFB lasers, a design rule will be proposed and the validity of the design rule will be discussed with respect to changes in the injected carrier density. The range of grating period, which can be used in the design, is discussed. As a consequence, the maximum tuning range of the emission wavelength can be estimated without involving the time-consuming self-consistent simulation. The features of polarization independence of optical amplifiers achieved by using diffused QWs are also discussed. Our theoretical results successfully explain why polarization independence can achieve in the long-wavelength tail of the modal gain and absorption coefficient but not at photon energies above the transition edge. This explanation applies to other tensile-strained QWs for polarization-independent applications. The understanding is crucial for optimizing polarization-independent devices. To conclude, our analysis of the diffused QW optical devices demonstrates that QW intermixing technology is a practical candidate for not only realizing monolithic photonic integrated circuit, but also enhancing optical device performance.     

Comprehensive modeling of diffused quantum-well vertical-cavitysurface-emitting lasers

A numerical model for investigating the thermal, electrical, and optical characteristics of vertical-cavity surface-emitting: lasers (VCSELs) with a diffused quantum-well (QW) structure is presented. In the model, the quasi-three-dimensional (quasi-3-D) distribution of temperature, voltage and optical fields as well as the quasi-two-dimensional (quasi-2-D) diffusion and recombination of carrier concentration inside the QW active layer are calculated in a self-consistent manner. In addition, the quasi-3-D distribution of implanted ions before and after thermal annealing are computed. The variation of electrical conductivity and absorption loss as well as the influence of impurity induced compositional disordering on the optical gain and refractive index of the QW active layer are also taken into consideration. Using this model, the steady-state characteristics of diffused QW VCSELs are studied theoretically. It is shown that significant improvement of stable single-mode operation can be obtained using diffused QW structure     

InGaAs-GaAs quantum-dot lasers

Quantum-dot (QD) lasers provide superior lasing characteristics compared to quantum-well (QW) and QW wire lasers due to their delta like density of states. Record threshold current densities of 40 A·cm ^-2 at 77 K and of 62 A·cm^-2 at 300 K are obtained while a characteristic temperature of 385 K is maintained up to 300 K. The internal quantum efficiency approaches values of ~80 %. Currently, operating QD lasers show broad-gain spectra with full-width at half-maximum (FWHM) up to ~50 meV, ultrahigh material gain of ~10⁵ cm^-1, differential gain of ~10^-13 cm² and strong nonlinear gain effects with a gain compression coefficient of ~10^-16 cm³. The modulation bandwidth is limited by nonlinear gain effects but can be increased by careful choice of the energy difference between QD and barrier states. The linewidth enhancement factor is ~0.5. The InGaAs-GaAs QD emission can be tuned between 0.95 μm and 1.37 μm at 300 K     

Effect of nitrogen on the band structure and material gain of In/sub y/Ga/sub 1-y/As/sub 1-x/N/sub x/-GaAs quantum wells

The conduction subband structure of InGaAsN-GaAs quantum wells (QWs) is calculated using the band anticrossing model, and its influence on the design of long-wavelength InGaAsN-GaAs QW lasers is analyzed. A good agreement with experimental values is found for the QW zone center transition energies. In particular, a different dependence of the effective bandgap with temperature when compared to the equivalent N-free structure is predicted by the model and experimentally observed. A detailed analysis of the conduction subband structure shows that nitrogen strongly decreases the electron energies and increases the effective masses. A very small N incorporation is also found to increase the nonparabolicity, but this effect saturates for higher nitrogen contents. Both the In content and well width decrease the effective masses and nonparabolicity of the conduction subbands. Material gain as a function of the injection level is calculated for InGaAsN-GaAs QWs for moderate carrier densities. The peak gain at a fixed carrier density is found to be reduced, compared to InGaAs, for a small N content, but this reduction tends to saturate when the N content is further increased. For the gain peak energy, a monotonous strong shift to lower energies is obtained for increasing N content, supporting the feasibility of 1.55-/spl mu/m emission from InGaAsN-GaAs QW laser diodes.     

0.98-μm multiple-quantum-well tunneling injection laser with98-GHz intrinsic modulation bandwidth

We demonstrate GaAs-based 0.98-μm multiple-quantum-well (MQW) tunneling injection lasers with ultrahigh-modulation bandwidths. Electrons are injected into the active region via tunneling, leading to a “cold” carrier distribution in the quantum wells (QWs). The tunneling time (2 pS) measured by time resolved differential transmission spectroscopy agrees with the capture time extracted form the electrical impedance measurement. The tunneling barrier prevents electrons from going over the active region into the opposite cladding layer. The carrier escape time in tunneling injection lasers is larger than that in conventional QW lasers. Enhanced differential gain, minimized gain compression and improved high frequency performance have been achieved. The -3-dB modulation bandwidth is 48 GHz and the maximum intrinsic modulation bandwidth is as high as 98 GHz     

Theoretical and experimental analysis of 1.3-/spl mu/m InGaAsN/GaAs lasers

We present a comprehensive theoretical and experimental analysis of 1.3-/spl mu/m InGaAsN/GaAs lasers. After introducing the 10-band k /spl middot/ p Hamiltonian which predicts transition energies observed experimentally, we employ it to investigate laser properties of ideal and real InGaAsN/GaAs laser devices. Our calculations show that the addition of N reduces the peak gain and differential gain at fixed carrier density, although the gain saturation value and the peak gain as a function of radiative current density are largely unchanged due to the incorporation of N. The gain characteristics are optimized by including the minimum amount of nitrogen necessary to prevent strain relaxation at the given well thickness. The measured spontaneous emission and gain characteristics of real devices are well described by the theoretical model. Our analysis shows that the threshold current is dominated by nonradiative, defect-related recombination. Elimination of these losses would enable laser characteristics comparable with the best InGaAsP/InP-based lasers with the added advantages provided by the GaAs system that are important for vertical integration.     

Recent Advances on InAs/InP Quantum Dash Based Semiconductor Lasers and Optical Amplifiers Operating at 1.55 μm

This paper summarizes recent advances on InAs/InP quantum dash (QD) materials for lasers and amplifiers, and QD device performance with particular interest in optical communication. We investigate both InAs/InP dashes in a barrier and dashes in a well (DWELL) heterostructures operating at 1.5 mum. These two types of QDs can provide high gain and low losses. Continuous-wave (CW) room-temperature lasing operation on ground state of cavity length as short as 200 mum has been achieved, demonstrating the high modal gain of the active core. A threshold current density as low as 110 A/cm² per QD layer has been obtained for infinite-length DWELL laser. An optimized DWELL structure allows achieving of a T₀ larger than 100 K for broad-area (BA) lasers, and of 80 K for single-transverse-mode lasers in the temperature range between 25degC and 85degC. Buried ridge stripe (BRS)-type single-mode distributed feedback (DFB) lasers are also demonstrated for the first time, exhibiting a side-mode suppression ratio (SMSR) as high as 45 dB. Such DFB lasers allow the first floor-free 10-Gb/s direct modulation for back-to-back and transmission over 16-km standard optical fiber. In addition, novel results are given on gain, noise, and four-wave mixing of QD-based semiconductor optical amplifiers. Furthermore, we demonstrate that QD Fabry-Perot (FP) lasers, owing to the small confinement factor and the three-dimensional (3-D) quantification of electronic energy levels, exhibit a beating linewidth as narrow as 15 kHz. Such an extremely narrow linewidth, compared to their QW or bulk counterparts, leads to the excellent phase noise and time-jitter characteristics when QD lasers are actively mode-locked. These advances constitute a new step toward the application of QD lasers and amplifiers to the field of optical fiber communications     

1.3-μm InAsP n-type modulation-doped MQW lasers grown bygas-source molecular beam epitaxy

The effect of n-type modulation doping as well as growth temperature on the threshold current density of 1.3-μm InAsP strained multiple-quantum-well (MQW) lasers grown by gas-source molecular beam epitaxy (GSMBE) was investigated for the first time. We have obtained threshold current density as low as 250 A/cm² for 1200-μm long devices. The threshold current density per well for infinite cavity length J_th/N_w∞ of 57 A/cm² was obtained for the optimum n-doping density (N_D=1×10¹⁸ cm^-3) and the optimum growth temperature (515°C for InP and 455°C for the SCH-MQW region), which is about 30% reduction as compared with that of undoped MQW lasers. A very low continuous-wave threshold current of 0.9 mA have been obtained at room temperature, which is the lowest ever reported for long-wavelength lasers using n-type modulation doping, and the lowest results grown by all kinds of MBE in the long-wavelength region. The differential gain was estimated by the measurement of relative intensity noise. No significant reduction of differential gain was observed for n-type MD-MQW lasers as compared with undoped MQW lasers. The carrier lifetime was also reduced by about 33% by using n-type MD-MQW lasers. Both reduction of the threshold current and the carrier lifetime lead to the reduction of the turn-on delay time by about 30%. The 1.3-μm InAsP strained MQW lasers using n-type modulation doping with very low power consumption and small turn-on delay is very attractive for laser array application in high-density parallel optical interconnection systems     

Carrier capture times in quantum-well, -wire, and -boxdistributed-feedback lasers characterized by dynamic lasing emissionmeasurements

We report experimental and theoretical studies of carrier capture times in low-dimensional semiconductor distributed feedback lasers with three different typical active region structures: quantum well (QW), wire, and box. The total effective carrier capture times of the lasers are directly determined by means of dynamic lasing emission measurements. By systematical comparison of QW, wire and box lasers, we evidence a strong dependence of the total effective carrier capture time on the packing density of the active region, which indicates the significant contribution of the local quantum capture time to the total effective carrier capture time, as revealed firstly by Kan et al. (1993). The intrinsic local carrier quantum capture time can be deduced from this kind of study. The determined local quantum carrier capture time for the InGaAs-InGaAsP QW laser is about 3 ps at 2 K, which is well consistent with a detailed quantum mechanics calculation. Furthermore, by comparison of box lasers with an approximate equal box size (70 nm) but different box densities, we find that the determined local quantum capture time of the box lasers is only about 2.4 ps at low temperature. We believe that this is a direct experimental indication of the existence of an efficient channel for carrier capture and relaxation in the investigated quantum-box system. The systematic comparison of QW, wire and dot lasers reveals the dominant limitation of the geometry effect on the high speed modulation of quantum wire and dot lasers, except when the quantum wires and dots are packed with a quite high density     

Modification of modal gain in InGaAs-GaAs quantum-well lasers dueto barrier-state carriers

This paper describes the effects of barrier-state carriers on the modal gain of InGaAs-GaAs quantum-well (QW) lasers emitting at 980 nm. Experimental studies and numerical simulations are used to examine several drive configurations, each having a unique effect on the laser response. These include compound drive current shapes, optical excitations and fast electrical drives with rise times shorter than 100 ps. We demonstrate that a large barrier-state carrier density affects the index of refraction sufficiently so as to cause a reduction in the confinement factor and modal gain which is large enough to turn the laser off     

Exciton relaxation and dephasing in quantum-dot amplifiers from room to cryogenic temperature

We present an extensive experimental study of the exciton relaxation and dephasing in InGaAs quantum dots (QDs) in the temperature range from 10 K to 295 K. The QDs are embedded in the active region of an electrically pumped semiconductor optical amplifier. Ultrafast four-wave mixing and differential transmission spectroscopy on the dot ground-state transition are performed with a sensitive heterodyne detection technique. The importance of the population relaxation dynamics to the dephasing is determined as a function of injection current and temperature. Above 150 K dephasing processes much faster than the population relaxation are present, due to both carrier-phonon scattering and Coulomb interaction with the injected carriers. Only at low temperatures (<30 K) does population relaxation of multiexcitons in the gain regime fully determine the dephasing.     

Modeling of strained quantum-well lasers with spin-orbit coupling

A complete model with the spin-orbit coupling for strained quantum-well lasers is presented. Explicit formulas for the momentum-matrix elements are given. The improvement in the threshold current density of tensile strained quantum-well lasers, as compared with that of the unstrained quantum well, is shown to result from the enhanced momentum matrix. The differential gain and the linewidth enhancement factor are calculated. The theoretical results show a smaller linewidth enhancement factor for compressively and tensile strained quantum wells than that of the unstrained structure, as has been experimentally observed. The temperature behavior of both the radiative component and the Anger component of the threshold current density is shown. Due to a decrease of gain and differential gain with increasing temperature, the threshold carrier density in unstrained quantum wells is increased with a large increment of the Auger recombination current at high temperature. For strained quantum wells, this increment is moderate because of the smaller threshold carrier density     

Amplified spontaneous emission spectroscopy in strainedquantum-well lasers

Amplified spontaneous emission spectroscopy is used to extract the gain and refractive index spectra systematically. We obtain the gain and differential gain spectra using the Hakki-Paoli method. The refractive index profile, the induced change in refractive index by an incremental current, and the linewidth enhancement factor are measured from the Fabry-Perot peaks and the current-induced peak shifts in the amplified spontaneous emission spectra. The measured optical gain and refractive index are then compared with our theoretical model for strained quantum-well lasers. We show that a complete theoretical model for calculating the electronic band structure, the optical constant, and the linewidth enhancement factor agrees very well with the experiment. Our approach demonstrates that amplified spontaneous emission spectroscopy can be a good diagnostic tool to characterize laser diodes, extract the optical gain and index profiles, and confirm material parameters such as the strained quantum-well band structure parameters for a semiconductor structure under carrier injection     

Progress in GaN-based quantum dots for optoelectronics applications

Our recent progress in GaN-based quantum dots (QDs) for optoelectronics application is discussed. First, we discussed an impact of the use of GaN-based QDs on semiconductor lasers, showing theoretically that reduction of threshold current by using the QDs in GaN-based lasers is much more effective compared to those in GaAs-based or InP-based lasers. Then discussed are our growth technology including self-assembling growth of InGaN QDs on sapphire substrates by atmospheric-pressure metalorganic chemical vapor deposition. Using the self-assembling growth technique, we have succeeded in obtaining lasing action in an edge-emitting laser structure with the InGaN QDs embedded in the active layer under optical excitation with the emission wavelength of 410 nm. Toward UV light wavelength emission, we have recently established self-assembled GaN QDs of high quality and high density under very low V-III ratio. We clearly observed two photoluminescence peaks from both the QDs and the wetting layer at room temperature, which clearly shows the nanostructures are formed with the Stranski-Krastanow growth mode.     

Structural dependence of 1.3-μm narrow-beam lasers fabricated byselective MOCVD growth

The effect of structural parameters on the lasing characteristics of 1.3-μm narrow beam lasers has been investigated. Monolithically integrated vertically tapered multiquantum-well (MQW) waveguide, fabricated by use of selective metal-organic chemical vapor deposition (MOCVD), is used for the expansion of the optical spot size. It is experimentally shown that the energy separation between the gain and waveguide regions that is formed simultaneously by selective MOCVD is shown to be an important parameter in order to achieve low-threshold current density and good temperature characteristics. The lengths of gain and waveguide regions have been investigated in terms of temperature characteristics of threshold current and far-field angle. A lower threshold current density and a higher characteristic temperature were obtained for longer gain region, We also have estimated the waveguide loss of the mode-field converter lasers diodes (MFC-LD's). High performance of 1.3-μm integrated vertically tapered waveguide lasers were achieved in an optimized device