Theoretical analysis of high-temperature characteristics of1.3-μm InP-based quantum-well lasers

By taking into account the electrostatic deformation in the band profiles and the temperature dependence of the optical dephasing time, we study the temperature sensitivity of the differential gain, threshold carrier density, and radiative current density in 1.3-μm InP-based strained-layer quantum-well (QW) lasers. Electrostatic deformation is analyzed by the self-consistent numerical calculation of Poisson's equation, the scalar effective-mass equation for the conduction band, and the multiband effective-mass equation for the valence band. The optical dephasing time is then obtained from the intrasubband scattering rates for electrons and holes within the fully dynamic random phase approximation including carrier-carrier and carrier-phonon interactions on an equal basis. It is clarified that the electrostatic band-profile deformation is one of the dominant mechanisms For determining the temperature sensitivity Of the differential gain, while the optical dephasing time has a pronounced influence on the transparent condition at elevated temperatures. We demonstrate that the electrostatic band-profile deformation and the temperature-dependent optical dephasing play essential roles in determining the high-temperature characteristics of InP-based QW lasers     

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     

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     

Orientation dependence of optical gain in zincblende-GaNstrained-quantum-well lasers

Optical gains in zincblende-GaN strained-quantum-well (QW) lasers are analyzed theoretically for various crystal orientations. Valence subbands are calculated based on the 6×6 Luttinger-Kohn model. It is found that the gains in zincblende-GaN strained-QW lasers with (110) orientation are much higher than those in wurtzite-GaN lasers with (0001) orientation     

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     

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     

Performance and physics of quantum-dot lasers with self-assembledcolumnar-shaped and 1.3-μm emitting InGaAs quantum dots

This paper reports recent developments of our self-assembled InGaAs quantum-dot (QD) lasers and their unique physical properties. We achieved a low-threshold current of 5.4 mA at room temperature with our originally designed columnar-shaped QD's, and also, room-temperature 1.3-μm continuous-wave (CW) lasing with self-assembled dots grown at a decreased growth rate and covered by a strained InGaAs layer. We discuss influence of homogeneous broadening of single-dot optical gain on lasing spectra, influence of nonradiative carrier recombination on temperature characteristics of threshold currents, a model for the origin of the homogeneous broadening, a finding of random telegraph signals, and suppression of temperature sensitivity of interband emission energy by covering dots with a strained InGaAs layer     

Self-Organized (100)-/(001)-Preferred Orientation in Pb(Zr,Ti)O3 Films Grown on Polycrystalline Substrates by Metalorganic Chemical Vapor Deposition

Lead zirconate titanate [Pb(Zr _x Ti_{1 ? x} )O₃, PZT] films were grown on (100), (110) and (111)SrRuO₃//SrTiO₃ substrates at 600°C by metalorganic chemical vapor deposition (MOCVD). The crystal orientation dependence of the growth rate was investigated for these films. The growth rate of (100)-/(001)-oriented epitaxial films was approximately 1.7 and 2.0 times higher than that of (110)-/(101)- and (111)-/(11&1macr;)-oriented epitaxial films, respectively. On the other hand, the growth rate of (100)-/(001)-preferred oriented PZT films grown on (111)Pt/TiO₂/SiO₂/(100)Si substrates was almost the same with that of (100)-/(001)-oriented epitaxial films. The deposition rate of these films was approximately 1.5 μm/h. High growth rate of (100)-/(001)-oriented PZT grains makes (100)-/(001)-preferred orientation on (111)Pt/TiO₂/SiO₂/(100)Si substrate. From transmission electron microscopy observation, (100)-/(001)-oriented grains were found to be directly grown on (111)-oriented Pt grains without obvious another oriented grains. As a result, orientation-controlled PZT films were successfully grown on (100)Si substrates having (111)-oriented Pt bottom electrodes.     

1.3-μm InGaAsP-InP n-type modulation-doped strainedmultiquantum-well lasers

The use of n-type modulation doping to reduce the threshold current, the carrier lifetime, and the internal loss in 1.3-μm InGaAsP-InP strained multiquantum-well (MQW) lasers is experimentally demonstrated. The threshold current density, the carrier lifetime, and the internal loss were reduced by about 33%, 36%, and 28%, respectively, as compared with an undoped MQW laser. Moreover, the turn-on delay time in the n-type modulation-doped MQW lasers with a low-leakage buried heterostructure was reduced by about 35%. These results confirm the suitability of this type of laser for use in the basic structure of a monolithic laser array used as a light source for high-density parallel optical interconnection     

1.3-μm InP-InGaAsP lasers fabricated on Si substrates by waferbonding

1.3-μm InP-InGaAsP lasers have been successfully fabricated on Si substrates by wafer bonding. InP-InGaAsP thin epitaxial films are prepared by selective etching of InP substrates and then bonded to Si wafers, after which the laser structures are fabricated on the bonded thin films. The bonding temperature has been optimized to be 400°C by considering bonding strength, quality of the bonded crystal, and compatibility with device processes. Room-temperature continuous-wave (RT CW) operation has been achieved for 6-μm-wide mesa lasers with a threshold current of 39 mA, which is identical to that of conventional lasers on InP substrates. Additionally, the lasers fabricated on Si have exhibited higher output powers than the lasers on InP, which is due to higher thermal conductivity of Si substrates. From these results, the wafer bonding is thought to be a promising technique to integrate optical devices on Si and implement optical interconnections between Si LSI chips     

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     

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.     

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     

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     

Reliable wide-temperature-range operation of 1.3-μmbeam-expander integrated laser diode for passively aligned opticalmodules

A novel structure for a 1.3-μm beam-expander integrated (BEX) laser diode is demonstrated. It combines a thickness-tapered InGaAsP-InP multiple quantum-well (QW) crystal grown by a novel silicon shadow masked metalorganic vapor phase epitaxy and a simple reverse-trapezoid-ridge waveguide laser structure that offers smooth mode field expansion and improved high-temperature lasing performance. We found this new BEX laser quite suitable for operation over a wide range of temperatures above 85°C and highly efficient lens-free coupling to a single-mode fiber (SMF) of less than 3 dB. These excellent lasing properties along with reliability under severe environmental conditions make this BEX-LD a promising candidate for practical use for low-cost long-wavelength light-source modules using optical passive alignment techniques     

Lateral carrier confinement in miniature lasers using quantum dots

Although quantum-dot (QD) lasers are yet to reach their promise of ultralow threshold and high characteristic temperature because of QD size nonuniformity, we have found that they can be used to effectively limit the lateral diffusion of carriers in the active region, enabling the scaling of lasers to small lateral dimensions. Although oxide apertures continue to enable improved performance in vertical-cavity surface-emitting lasers (VCSEL's) by reducing optical losses and current spreading, lateral carrier losses remain uncontrolled. We investigate QD active material in which lateral diffusion is intentionally reduced. Cathodoluminescence (CL) results demonstrate reduced lateral diffusion in the material with which we expect 50% reduction in the threshold current for 1-μm-wide edge-emitters or 5-μm-diameter VCSEL's. We have made QD stripe lasers with submicrometer widths that lase from the ground state and have quantified the lateral carrier reduction in the QD laser active region. We show empirically that the degree of lateral carrier confinement is dependent on the quantum state from which lasing occurs and demonstrate 63% reduction in lateral carrier leakage for the ground-state lasers. Finally, the scaling of threshold current in QD VCSEL's is compared with that of quantum-well (QW) VCSEL's by numerical modeling for future design considerations     

The metal-organic chemical vapor deposition growth and propertiesof InAsSb mid-infrared (3-6-μm) lasers and LEDs

We describe the metal-organic chemical vapor deposition (MOCVD) growth of AlAs_1-xSb_x cladding layers and InAsSb-InAs multiple-quantum well (MQW) and InAsSb-InAsP strained-layer superlattice (SLS) active regions for use in mid-infrared emitters. The AlAs_1-xSb_x cladding layers were successfully doped p- or n-type using diethylzinc or tetraethyltin, respectively. By changing the layer thickness and composition of SLSs and MQWs, we have prepared structures with low temperature (<20 K) photoluminescence wavelengths ranging from 3.2 to 6.0 μm. We have made gain-guided injection lasers using undoped p-type AlAs_0.16Sb_0.84 for optical confinement and both strained InAsSb-InAs MQW and InAsSb-InAsP SLS active regions. The lasers and light emitting diodes (LEDs) utilize the semi-metal properties of a GaAsSb(p)-InAs(n) heterojunction as a source for electrons injected into active regions. A multiple-stage LED utilizing this semi-metal injection scheme is reported. Gain-guided, injected lasers with a strained InAsSb-InAs MQW active region operated up to 210 K in pulsed mode with an emission wavelength of 3.8-3.9 μm and a characteristic temperature of 29-40 K. We also present results for both optically pumped and injection lasers with InAsSb-InAsP SLS active regions. The maximum operating temperature of an optically pumped 3.7-μm strained-layer superlattice (SLS) laser was 240 K. An SLS LED emitted at 4.0 μm with 80 μW of power at 300 K     

The temperature dependence of 1.3- and 1.5-μm compressivelystrained InGaAs(P) MQW semiconductor lasers

We have studied experimentally and theoretically the spontaneous emission from 1.3- and 1.5-μm compressively strained InGaAsP multiple-quantum-well lasers in the temperature range 90-400 K to determine the variation of carrier density n with current I up to threshold. We find that the current contributing to spontaneous emission at threshold I_Rad is always well behaved and has a characteristic temperature T₀ (I_Rad)≈T, as predicted by simple theory. This implies that the carrier density at threshold is also proportional to temperature. Below a breakpoint temperature T_B, we find I α n^Z, where Z=2. And the total current at threshold I_th also has a characteristic temperature T₀ (I_th)≈T, showing that the current is dominated by radiative transitions right up to threshold. Above T_B, Z increases steadily to Z≈3 and T₀ (I_th) decreases to a value less than T/3. This behavior is explained in terms of the onset of Auger recombination above T_B; a conclusion supported by measurements of the pressure dependence of I_th. From our results, we estimate that, at 300 K, Auger recombination accounts for 50% of I_th in the 1.3-μm laser and 80% of I_th in the 1.5-μm laser. Measurements of the spontaneous emission and differential efficiency indicate that a combination of increased optical losses and carrier overflow into the barrier and separate confinement heterostructure regions may further degrade T₀ (I_th) above room temperature     

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     

Monolithic integration of spot-size converters with 1.3-μmlasers and 1.55-μm polarization insensitive semiconductor opticalamplifiers

To reduce packaging costs, it is necessary to use passive alignment between the laser diodes and optical fiber. Such an alignment requires low-coupling loss and large positional alignment tolerances. This is achievable with integrated spot-size converters, which permit to match the near field of a laser to that of a flat-end single-mode fiber (SMF). In this paper, we first review briefly the different technological approaches to realize spot-size converters. Then, we focus on the double-core structure developed both for 1.3-μm Fabry-Perot lasers and 1.55-μm semiconductor optical amplifiers (SOAs). The spot-size expansion is simulated using a two-dimensional (2-D) beam propagation method analysis. Short spot-size converters (100 μm) integrated with 1.3-μm lasers and 1.55-μm SOAs exhibit beam divergences as low as 12°×12° and 12°×15°, respectively. The performances of devices with integrated spot-size converters are reported and discussed. A 2-in wafer process is used thanks to the versatility of the double-core structure and its compatibility with buried ridge stripe technology