Effects of well number on temperature characteristics in 1.3-μmAlGaInAs-InP quantum-well lasers

Effects of well number on temperature characteristics have been investigated in 1.3-μm AlGaInAs-InP compressively strained multiple-quantum-well lasers. Well-number dependence of threshold currents (I_th), external quantum efficiencies (η_d ), characteristic temperatures of I_th and η_d arid maximum operation temperatures have been experimentally determined and analyzed. The characteristic temperature of the threshold current (T₀) and the maximum operation temperature (T_max ) were found to increase with increasing the number of quantum wells and a record high pulsed T_max of 220°C has been achieved in lasers with ten wells. In contrast, the characteristic temperature of the external efficiency (T_η) was found to decrease with increasing the number of wells. Because of this opposite well-number dependence of the T₀ and T_η, each of them alone is not necessarily a good measure to optimize the number of wells. Therefore, in this work, me also evaluated a power reduction at a constant current with increasing temperature, which depends on both T₀ and T_η and thus should be a more practical measure of the temperature characteristics, and discuss the optimum number of the quantum wells     

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     

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     

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     

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     

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     

Experimental analysis of temperature dependence in 1.3-μmAlGaInAs-InP strained MQW lasers

We have analyzed experimentally the temperature and pressure dependences of the lasing characteristics of 1.3-μm AlGaInAs-InP strained multiple-quantum-well lasers, by focusing on the ratio of the nonradiative recombination current to the total current. The temperature dependence of the radiative current was studied by observing the spontaneous emission through a window in the substrate. It was found to increase linearly with temperature, exactly as expected for an ideal quantum well over the entire temperature range from 100 to 360 K. Further, it was shown that pure radiative recombination dominated the total current below a breakpoint temperature T_b of 220 K. Above this temperature, the onset of loss processes including Auger recombination caused a superlinear increase in the threshold current. Analysis of the linear and nonlinear components allowed us to determine the ratio of the nonradiative to radiative currents at threshold. We find that, relative to similar GaInAsP/InP lasers, there is a decrease in the nonradiative component of the current, resulting in a higher characteristic temperature T₀ in the AlGaInAs-InP lasers. At 300 K, the radiative recombination current is more than 70% of the total threshold current. This result is consistent with the observation that the threshold current increases by about 8% in 12-kbar hydrostatic pressure, while in GaInAsP lasers, a decrease of 10% or more is always observed over this pressure range     

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     

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.     

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     

High-pressure studies of recombination mechanisms in 1.3-/spl mu/m GaInNAs quantum-well lasers

The pressure dependence of the components of the recombination current at threshold in 1.3-/spl mu/m GaInNAs single quantum-well lasers is presented using for the first time high-pressure spontaneous emission measurements up to 13 kbar. It is shown that, above 6 kbar, the rapid increase of the threshold current with increasing pressure is associated with the unusual increase of the Auger-related nonradiative recombination current, while the defect-related monomolecular nonradiative recombination current is almost constant. Theoretical calculations show that the increase of the Auger current can be attributed to a large increase in the threshold carrier density with pressure, which is mainly due to the increase in the electron effective mass arising from the enhanced level-anticrossing between the GaInNAs conduction band and the nitrogen level.     

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.     

1.3-μm spot-size-converter integrated laser diodes fabricated bynarrow-stripe selective MOVPE

High-performance 1.3-μm spot-size-converter integrated laser diodes (SSC-LDs) have been developed by using narrow-stripe (<2.0 μm) selective MOVPE. In order to decrease leak current at high temperature, a p-n-p-n current blocking structure was added using a self-alignment process. These LD's no longer require a semiconductor etching process. Superior lasing characteristics, such as a low driving current of 56 mA for output power of 10 mW, and high-slope efficiency at 85°C, were achieved by using a high-quality multiple-quantum well (MQW) active layer of narrow-stripe selective MOVPE and a p-n-p-n current blocking structure. A narrow radiation angle of 12° was obtained by optimizing the tapered-waveguide profile. A high-coupling efficiency of -2.8 dB was achieved between a LD chip and a single-mode fiber (SMF). This SSC-LD is very appropriate as a light source for access network systems, which require a low-cost LD module. It has excellent coupling efficiency, using a SMF, and a simple fabrication process, using selective MOVPE     

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     

High-power 1.55-μm mass-transport-grating DFB lasers forexternally modulated systems

High-output-power operation of 1.55-μm-wavelength distributed-feedback (DFB) lasers with a novel mass-transport grating (MTG) structure which is composed of InAsP buried with InP are reported. To improve high output power characteristics, we have investigated the influence of the width of the active layer on the light output power and the spectral linewidth at high injection current. It is confirmed that the increase of the active layer width is effective to realize high output power and to reduce the linewidth power product. The fabricated lasers show high single-longitudinal-mode output power of 180 mW, which is the highest value reported for 1.55-μm DFB lasers. They also exhibit narrow spectral linewidths less than 0.3 MHz and low noise characteristics of -159 dB/Hz. Moreover, we have obtained the mean time to failure of longer than 10⁵ h with a lifetime test over 200 h at 50°C     

Long-term wavelength stability of 1.55-μm tunable distributedBragg reflector lasers

To investigate physical mechanisms involved in long-term wavelength drift of tunable distributed Bragg reflector (DBR) laser, the evolution of the tuning characteristics as well as the Bragg section intensity modulation response of several DBR lasers have been simultaneously assessed by current injection in the Bragg section only. Current versus voltage I(V) characteristics under aging have also been recorded to compare the role of leakage current and nonradiative recombination defect evolution. The wavelength drift as well as the carrier lifetime of the tuning section varies following an exponential law A+Bexp(-t_A/τ) versus aging time t_A. The time constant τ is aging temperature and Bragg current dependant. The carrier lifetime decreases with time indicating a wavelength drift mainly due to nonradiative recombination defect increase. Modeling of the I_B(V) and λ_B(I_B) characteristics is presented, that fits nicely the experimental data. The exponential form of the wavelength drift is used to propose novel and adequate burning conditions of DBR lasers     

High-performance 1.55-μm wavelength GaInAsP-InPdistributed-feedback lasers with wirelike active regions

High-performance 1.55-μm wavelength GaInAsP-InP strongly index-coupled and gain-matched distributed-feedback (DFB) lasers with periodic wirelike active regions mere fabricated by electron beam lithography, CH₄/H₂-reactive ion etching, and organometallic vapor-phase epitaxial regrowth, whose index-coupling coefficient was more than 300 cm^-1. In order to design lasers for low threshold current operation, threshold current dependences on the number of quantum wells and the wire width mere investigated both theoretically and experimentally. A record low threshold current density of 94 A/cm² among 1.55-μm DFB lasers was successfully obtained for a stripe width of 19.5 μm and a cavity length of 600 μm. Moreover, a record low threshold current of 0.7 mA was also realized at room temperature under CW condition for a 2.3-μm-wide buried heterostructure with a 200-μm-long cavity. Finally, we confirmed stable single-mode operation due to a gain-matching effect between the standing-wave profile and the wirelike active region     

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     

Narrow-beam divergence 1.3-μm multiple-quantum-well laser diodeswith monolithically integrated tapered thickness waveguide

This paper describes the optimum design, fabrication, and performance of a 1.3-μm multiple-quantum-well (MQW) laser diode monolithically integrated with a tapered thickness spot-size transformer. The dependence of the lasing characteristics on the thickness distribution of the core layer and on the current injection profile of the device were analyzed. This integrated laser with its optimized structure performed at a low threshold current of 22.2 mA, even at 85°C. The integrated spot-size transformer effectively reduced the lateral and vertical far-field FWHM's to 8° and 9°, respectively. A very long lifetime of over 1×10⁵ h was estimated at 85°C and 8 mW under CW operation     

Ultralow threshold 1.3-μm InGaAsP-InP compressive-strainedmultiquantum-well monolithic laser array for parallel high-densityoptical interconnects

An ultralow-threshold 1.3-μm InGaAsP-InP 10-element monolithic laser array is achieved through careful optimization of a strained multiquantum-well active layer, especially the amount of strain, the well thickness, the barrier thickness, the number of wells, and the active laser width. This array has a record-low threshold current, highly uniform threshold current characteristics (1.3±0.09 mA and slope efficiency of 0.37±0.01 W/A), extremely low operating current of 14 mA under 5-mW output power, and long-term reliability. This array is suitable as light sources for a parallel high-density optical interconnection system. In addition, a record low CW threshold current of 0.58 mA at 20°C and 1.62 mA at 90°C, as a long-wavelength laser, is obtained by employing a short cavity (100 μm) uith high-reflection coatings