Differential gain and linewidth-enhancement factor in dilute-nitride GaAs-based 1.3-/spl mu/m diode lasers

The effect of the quantum-well nitride content on the differential gain and linewidth enhancement factor of dilute-nitride GaAs-based near 1.3-/spl mu/m lasers was studied. Gain-guided and ridge waveguide lasers with 0%, 0.5%, and 0.8% nitrogen content InGaAsN quantum wells were characterized. Experiment shows that the linewidth enhancement factor is independent on the nitride content, and is in the range 1.7-2.5 for /spl lambda/=1.22--1.34 /spl mu/m dilute-nitride GaAs-based lasers. Differential gain and index with respect to either current or carrier concentration are reduced in dilute-nitride devices.     

Metalorganic Vapor Phase Epitaxial Growth of InAs/InGaAs Multiple Quantum Well Structures on InP Substrates

Device quality InAs/InGaAs multiple quantum well (MQW) structures were grown on InP substrates by metalorganic vapor phase epitaxy (MOVPE) and applied to lasers emitting at wavelengths longer than 2 mum. InAs/InGaAs MQWs with flat interfaces were obtained by adjusting the growth temperature between 460 degC and 510 degC. The photoluminescence peak wavelength of the MQWs increases from 1.93 to 2.47 mum as the thickness of InAs quantum wells increases from 2 to 7 nm. The structural and optical properties remained almost unchanged even after annealing at 620 degC. For 40-mu m-wide stripe broad-area lasers with 5-nm-thick InAs quantum wells, a lasing wavelength longer than 2.3 mum and an output power higher than 10 mW were achieved under continuous-wave operation at a temperature of 25 degC. These results indicate that InAs/InGaAs MQW structures grown by MOVPE are very useful for the active region of 2 mum wavelength lasers.     

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.     

1.3-/spl mu/m-range GaInNAsSb-GaAs VCSELs

1.3-/spl mu/m-range GaInNAsSb vertical-cavity surface-emitting lasers (VCSELs) with the doped mirror were investigated. GaInNASb active layers that include a small amount of Sb can be easily grown in a two-dimensional manner as compared with GaInNAs due to the suppression of the formation of three-dimensional growth in MBE growth. The authors obtained the lowest J/sub th/ per well (150 A/cm/sup 2//well) for the edge-emission type lasers due to the high quality of GaInNAsSb quantum wells. Using this material for the active media, the authors accomplished the first continuous wave operation of 1.3-/spl mu/m-range GaInNAsSb VCSELs. For the reduction of the threshold voltage and the differential resistance, they used the doped mirror grown by metal-organic chemical vapor deposition (MOCVD). By three-step growth, they obtained 1.3-/spl mu/m GaInNAs-based VCSELs with the low threshold current density (3.6 kA/cm/sup 2/), the low threshold voltage (1.2 V), and the low differential resistance (60 /spl Omega/) simultaneously for the first time. The back-to-back transmission was carried out up to 5 Gb/s. Further, the uniform operation of 10-ch VCSEL array was demonstrated. The maximum output power of 1 mW was obtained at 20/spl deg/C by changing the reflectivity of the front distributed Bragg reflector mirror. GaInNAsSb VCSELs were demonstrated to be very promising material for realizing the 1.3-/spl mu/m signal light sources, and the usage of the doped mirror grown by MOCVD is the best way for 1.3-/spl mu/m VCSELs.     

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     

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     

GaInNAs: a novel material for long-wavelength semiconductor lasers

GaInNAs was proposed and created in 1995 by the authors. It can be grown pseudomorphically on a GaAs substrate and is a light-emitting material having a bandgap energy suitable for long-wavelength laser diodes (1.3-1.55 μm and longer wavelengths). By combining GaInNAs with GaAs or other wide-gap materials that can be grown on a GaAs substrate, a type-I band lineup is achieved and, thus, very deep quantum wells can be fabricated, especially in the conduction band. Since the electron overflow from the wells to the barrier layers at high temperatures can he suppressed, the novel material of GaInNAs is very attractive to overcome the poor temperature characteristics of conventional long-wavelength laser diodes used for optical fiber communication systems. GaInNAs with excellent crystallinity was grown by gas-source molecular beam epitaxy in which a nitrogen radical was used as the nitrogen source. GaInNAs was applied in both edge-emitting and vertical-cavity surface-emitting lasers (VCSELs) in the long-wavelength range. In edge-emitting laser diodes, operation under room temperature continuous-wave (CW) conditions with record high temperature performance (T₀=126 K) was achieved. The optical and physical parameters, such as quantum efficiency and gain constant, are also systematically investigated to confirm the applicability of GaInNAs to laser diodes for optical fiber communications. In a VCSEL, successful lasing action was obtained under room-temperature (RT) CW conditions by photopumping with a low threshold pump intensity and a lasing wavelength of 1.22 μm     

A theoretical investigation of the characteristic temperature T/sub 0/ for semiconductor lasers

The temperature dependence of the characteristic temperature T/sub 0/ of semiconductor quantum-well lasers is investigated using detailed simulations. The critical-temperature-dependent processes are the optical gain and the nonradiative recombination. The gain model is based on k /spl middot/ p theory with the multiple quantum wells in the active layer represented by a superlattice. The Auger process is assumed to be thermally activated. It is shown that, with inclusion of the continuum state filling and interband mixing, the most important features experimentally observed in the temperature dependence of the T/sub 0/ value can be explained. The continuum state filling and band nonparabolicity cause a significant deviation from the ideal linear carrier density versus temperature relation for quantum wells. The results are compared to experiment for broad area devices lasing at 980 nm and 1.3, and 1.55 /spl mu/m, and show good agreement over a broad range of temperature.     

A comprehensive VCSEL device simulator

This paper deals with the design and implementation of a self-consistent electrothermooptical device simulator for vertical-cavity surface-emitting lasers (VCSELs). The model is based on the photon rate equation approach. For the bulk electrothermal transport, a thermodynamic model is employed in a rotationally symmetric body. Heterojunctions are modeled using a thermionic emission model and quantum wells are treated as scattering centers for carriers. The optical field is expanded into modes that are eigensolutions of the vectorial electromagnetic wave equation with an arbitrary, complex dielectric function. The open nature of the VCSEL cavity is treated by employing perfectly matched layers. The optical gain and absorption model in the quantum-well active region is based on Fermi's Golden Rule. The subbands in the quantum well are determined by solving the stationary Schrodinger equation and using a parabolic band approximation for the electrons, light and heavy holes. The photon rate equation is fully integrated into the Newton-Raphson scheme used to solve the system of nonlinear device equations. An efficient numerical optical mode solver is used, that is based on a Jacobi-Davidson type iterative eigensolver. The latter combines a continuation scheme with preconditioner recycling. The practical relevance of the implementation is demonstrated with the simulation of a realistic etched-mesa VCSEL device.     

Improved luminescence from quantum-dot nanostructures embedded in structurally engineered (In,Ga)As confining layers

Efficient luminescence of quantum-dot nanostructures embedded in active regions of lasers is important for low-threshold current density devices. This paper discusses an approach for structurally engineering confining (In,Ga)As layers into which InAs quantum dots are inserted to enhance their emission efficiency. It is shown that by inserting the dots at the center of compositionally graded In/sub x/Ga/sub 1-x/As layers, the relative emission efficiency can be increased by nearly an order of magnitude over the emission of dots inside a constant composition (In,Ga)As structure. This enhancement is thought to be a result of the high structural and optical quality of the confining layers.     

Microscopic modeling of gain and luminescence in semiconductors

The capabilities of a fully microscopic approach for the calculation of optical material properties of semiconductor lasers are reviewed. Several comparisons between the results of these calculations and measured data are used to demonstrate that the approach yields excellent quantitative agreement with the experiment. It is outlined how this approach allows one to predict the optical properties of devices under high-power operating conditions based only on low-intensity photo luminescence (PL) spectra. Examples for the gain-, absorption-, PL- and linewidth enhancement factor-spectra in single and multiple quantum-well structures, superlattices, Type II quantum wells and quantum dots, and for various material systems are discussed.     

High-Power Fiber Lasers and Amplifiers Based on Multimode Interference

In this paper, high-power fiber lasers and amplifiers based on multimode interference (MMI) in active large-core multimode optical fibers are proposed and their properties are investigated. Experimental results and simulations indicate that such fiber lasers and amplifiers are promising candidates for high-power miniature solid-state lasers. Utilization of the MMI leads to remarkable spectral and spatial features of fiber lasers and amplifiers such as generation of high-power diffraction-free beams.      

Design of a SiGe-Si quantum-well optical modulator

A light modulator consisting of modulation-doped SiGe-Si multiple quantum wells integrated in a silicon-on-insulator waveguide is designed. The device is based on the electrorefractive effect due to the variation of holes density in the SiGe wells, induced by applying a reverse bias on a PIN diode. This mechanism is simulated by numerical calculations of the hole distribution coupled with the optical guided mode propagation characteristics. The mode effective index variation of TE-polarized light at the 1.31-/spl mu/m wavelength can then be obtained as a function of the applied bias. The influences of the structure parameters such as the thickness and the doping level of the doped barrier layers or the number of SiGe wells is analyzed thanks to a design of experiment method. The optimization gives an effective index variation of 2.10/sup -4/ for an applied bias voltage of 6 V. To obtain optical intensity modulation, this structure has to be included in a Fabry-Perot cavity. The modulation performances are analyzed.     

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     

High-Efficiency InGaN/GaN Light Emitters Based on Nanophotonics and Plasmonics

We report novel methods to enhance light emission efficiencies from InGaN/GaN quantum wells (QWs) based on nanophotonics and plasmonics. First, the nanoscopic optical properties were observed and characterized based on the carrier localization and the quantum confinement Stark effect depending on the In composition of InGaN. Based on the results, we proposed that the emission efficiencies should be improved by making nanostructures, and showed actual enhancement of photoluminescence (PL) intensities by using fabricated random nanodisk and arrayed nanopillar structures. Moreover, surface plasmon (SP) coupling technique was used to enhance blue and green light emissions from InGaN/GaN QWs. We obtained a 14-fold increase in the PL intensity along with a 7-fold increase in the internal quantum efficiency (IQE) of light emission from InGaN/GaN when nanostructured Ag layers were deposited 10 nm above the QWs. The possible enhancement mechanism was discussed and reproduced by using the 3-D finite-difference time-domain simulations. Electron–hole pairs in InGaN QWs couple to electron oscillations at the metal surface and produce SPs instead of photons or phonons. This new path increases the spontaneous emission rate and the IQEs. The SP-emitter coupling technique would lead to superbright and high-speed solid-state light-emitting devices that offer realistic alternatives to conventional fluorescent light sources.      

Submilliampere threshold current InGaAs-GaAs-AlGaAs lasers andlaser arrays grown on nonplanar substrates

High performance buried heterostructure InGaAs-GaAs-AlGaAs quantum-well lasers and laser arrays with tight spatial confinement of the electrical current and the optical fields have been fabricated by metalorganic chemical vapor deposition. The lasers ace fabricated in a single growth step, using nonplanar substrates as a template for the active region definition. CW room temperature threshold currents, as low as 0.5 mA and 0.6 mA, are obtained for as-cleaved double and single quantum-well lasers, respectively. External quantum efficiencies exceeding 80% are obtained in the same devices. High-reflectivity facet-coated lasers have room temperature CW threshold currents as low as 0.145 mA with 10% external quantum efficiency. Lasers made by this technique have high yield and uniformity, and are suitable for low threshold array applications     

Continuous-Wave (CW) operation of GaInP-AlGaInP visiblecompressively strained multiple quantum-wire (CS-WQWR) lasers

GaInP-AlGaInP compressively strained multiple quantum-wire layers were fabricated by the in situ strain induced lateral layer ordering process, during gas source molecular beam epitaxial (GS-MBE) growth. The effect of compositional modulation was described in terms of PL spectra, and TEM images for GaInP-AlGaInP MQWR lasers with 18 period (GaP)_1.5-(InP)_1.5 SPBS active layers. Based on transmission electron microscopy (TEM) images, the size of quantum-wire width was estimated, and the size fluctuation of quantum wires were discussed. Quantum-wire effect was discussed in terms of anisotropic lasing characteristics and EL polarization, which were reflected by an anisotropic oscillation strength in quantum wires and the comparison with GaInP-AlGaInP compressively strained quantum-film lasers was examined in terms of threshold current density. The condition under which quantum wires were formed by strained induced lateral layer ordering process was discussed in terms of anisotropic behaviors of lasing characteristics, such as threshold current density and lasing wavelength for GaInP-AlGaInP MQWR lasers with (GaP)_m/(InP) _mSPBS active layers. The lowest obtained J_th value was 278 A/cm² under the room temperature (r.t.) pulsed condition. The first CW operation of GaInP-AlGaInp quantum-wire laser was described. Threshold current was 294 A/cm² and CW operation up to 70°C was obtained     

Continuous-wave low-threshold performance of 1.3-μm InGaAs-GaAsquantum-dot lasers

The understanding of material quality and luminescence characteristics of InGaAs-GaAs quantum dots (QD's) is advancing rapidly. Intense work in this area has been stimulated by the recent demonstration of lasing from a QD active region at the technologically important 1.3-μm wavelength from a GaAs-based heterostructure laser. Already, several groups have achieved low-threshold currents and current densities at room temperature from In(Ga)As QD active regions that emit at or close to 1.3 μm. In this paper, we discuss crystal growth, QD emission efficiency, and low-threshold lasing characteristics for 1.3-μm InGaAs-GaAs QD active regions grown using submonolayer depositions of In, Ga, and As. Oxide-confinement is effective in obtaining a low-threshold current of 1.2 mA and threshold-current density of 19 A/cm² under continuous-wave (CW) room temperature (RT) operation. At 4 K, a remarkably low threshold-current density of 6 A/cm² is obtained     

High-temperature operation of 1.3-μm tapered-active-stripe laserfor direct coupling to single-mode fiber

High-temperature operation of 1.3-μm wavelength multiquantum-well (MQW) lasers with an active stripe horizontally tapered over whole cavity, for direct coupling to single-mode fibers (SMFs), are reported. The lasers have reduced the output-beam divergence in a simple structure which does not contain an additional spot-size transformer. To improve high-temperature characteristics, we have investigated the influence of the thickness of separate-confinement-heterostructure layers and the number of quantum wells (QWs) on the threshold current and the output-beam divergence at high temperature. As a result, the fabricated lasers show low-threshold current (~18 mA) and high-slope efficiency (~0.4 mW/mA) with narrow output-beam divergence (~12°) at 85°C. Moreover, we have obtained maximum coupling efficiency of -4.7 dB in a direct coupling to a SMF, and the reliability of longer than 10⁵ h (MTTF) by a lifetime test of over 2000 h at 85°C     

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