Effects of carrier transport on injection efficiency and wavelengthchirping in quantum-well lasers

Using the steady-state solution to the carrier transport rate equation model for the quantum-well laser that they had previously proposed, the authors derive analytic expressions for the laser internal efficiency, carrier injection efficiency, and wavelength chirping under current modulation and show that the various carrier transport times can have a significant effect on these quantities. They present experimental data and theoretical calculations that clearly demonstrate that, as in the case of device optimization for high-speed operation, one has to minimize the transport time across the optical and current confinement regions and maximize the escape time out of the quantum-well active region in order to maximize the internal and the injection efficiencies and minimize the wavelength chirping     

On the internal quantum efficiency and carrier ejection in InGaAsP/InP-based quantum-well lasers

The optimization of InGaAsP/InP quantum-well laser heterostructures that had various configurations and emitted in the wavelength range from 1.26 up to 1.55 µm was carried out with the aim of maximizing the internal quantum efficiency and output optical power. It was experimentally shown that the heterolasers based on the laser structure with a broadened three-step waveguide have the highest quantum efficiency of stimulated radiation. In heterolasers of the suggested configuration, a decrease in the electron ejection out of the active region into the waveguide was observed. The power of the optical radiation of 4.2 W in a continuous-wave lasing mode was obtained in laser diodes with a mesa-stripe width of 100 µm. The quantum efficiency was 85% for the internal optical losses of 3.6 cm^?1.     

Effects of hot phonons on carrier heating in quantum-well lasers

The hot phonon effects on carrier heating in quantum-well lasers are theoretically investigated. We show that the neglect of the finite lifetime of LO phonons will significantly underestimate the carrier energy relaxation time and thus underestimate the effect of carrier heating in quantum-wed lasers. We investigate the effects of carrier heating with hot phonons on the saturation and degradation of the resonant frequency in high-speed quantum-well lasers. The implications of the hot phonon effects on the design of high-speed quantum-well lasers are also discussed     

Ultralow internal optical loss in separate-confinement quantum-well laser heterostructures

Internal optical loss in separate-confinement laser heterostructures with an ultrawide (>1 smm) waveguide has been studied theoretically and experimentally. It is found that an asymmetric position of the active region in an ultrawide waveguide reduces the optical confinement factor for higher-order modes and raises the threshold electron density for these modes by 10–20%. It is shown that broadening the waveguide to above 1 μm results in a reduction of the internal optical loss only in asymmetric separate-confinement laser heterostructures. The calculated internal optical loss reaches ∼0.2 cm⁻¹ (for λ≈1.08 μm) in an asymmetric waveguide 4 μm thick. The minimum internal optical loss has a fundamental limitation, which is determined by the loss from scattering on free carriers at the transparency carrier density in the active region. An internal optical loss of 0.34 cm⁻¹ was attained in asymmetric separate-confinement laser heterostructures with an ultrawide (1.7 μm) waveguide, produced by MOCVD. Lasing in the fundamental transverse mode has been obtained owing to the significant difference in the threshold densities for the fundamental mode and higher-order modes. The record-breaking CW output optical power of 16 W and wallplug efficiency of 72% is obtained in 100-μm aperture lasers with a Fabry-Perot cavity length of ∼3 mm on the basis of the heterostructures produced. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 38, No. 12, 2004, pp. 1477–1486. Original Russian Text Copyright ? 2004 by Slipchenko, Vinokurov, Pikhtin, Sokolova, Stankevich, Tarasov, Alferov.     

The influence of gain saturation on the output power of quantum-well semiconductor lasers

The light-power characteristic of a quantum-well semiconductor laser is theoretically studied taking into account the gain saturation effect. It is shown that, at high drive current densities, this light-current characteristic becomes nonlinear. The results obtained are in a good agreement with the experimental data.     

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

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

The role of carrier transport on the current injection efficiency of InGaAsN quantum-well lasers

A theoretical and experimental study demonstrates that the current injection efficiency of InGaAsN quantum-well (QW) lasers can be significantly affected by carrier transport in the separate confinement heterostructure (SCH) region. An offset QW design is utilized to show the impact of hole transport on the temperature dependence of the external differential quantum efficiency and above threshold injection efficiency. A reduction of the current injection efficiency is found for structures which have significant hole transport times in the SCH.     

A comparison of amplitude-phase coupling and linewidth enhancementin semiconductor quantum-well and bulk lasers

The amplitude-phase coupling factor α (linewidth enhancement factor) is compared for typical semiconductor quantum-well and bulk double heterostructure lasers. As a direct consequence of the reduction of the differential gain, there is no reduction of α in single-quantum-well lasers compared to bulk lasers. The number of quantum wells strongly affects the amplitude-phase coupling in quantum-well lasers. It is shown that the interband transition induced amplitude-phase coupling dominates that induced by the plasma effect of carriers in typical quantum-well lasers. By considering the spontaneous emission factor in the spectral linewidth, the authors show that there is an optimal number of quantum wells for achieving the narrowest spectral linewidth     

Increase in the internal optical loss with increasing pump current and the output power of quantum well lasers

Semiconductors - The operating characteristics of semiconductor quantum-well lasers, calculated with consideration for an increase in the internal optical loss in the waveguide region with...     

Optimum output efficiency of homogeneously broadened lasers with constant loss

The exact solution of a laser power extraction model is analyzed that relates the extraction efficiency with the laser parameters such as output coupling, small-signal gain, absorption loss, and laser cavity length. The model assumes a stable optical resonator and a homogeneously broadened gain medium. Optimum output coupling and resulting maximum extraction efficiency are determined for a range of values of the small-signal gain and absorption loss per pass. A relation is derived that allows determination of the intrinsic laser parameters from output power measurements when output mirrors are utilized with three different reflectances.     

Gain spectra of quantum-well lasers

The gain spectrum and its sensitivity to carrier density is calculated for a model quantum-well heterostructure semiconductor laser for a range of quantum-well widths. The gain spectra, especially for narrow wells, show better mode-to-mode gain discrimination than for the equivalent bulk laser. Good carrier confinement helps obtain this desirable feature.     

Internal optical loss measurements in InGaAs-InAlGaAs quantum-well lasers operating around 1550 nm

A multisection device technique is employed to carry out internal optical loss measurements in two types of InGaAs-InAlGaAs quantum-well structures. One structure consists of conventional identical-width quantum wells and the other, a broader spectral-width material, consists of multiple-width quantum wells in the active region. The temperature dependence of the internal optical losses is also investigated for both structures.     

The theory of strained-layer quantum-well lasers with bandgaprenormalization

A theoretical model for the strained-layer quantum-well laser is presented taking into account the valence-band mixing and the bandgap renormalization. Our theoretical approach for the electronic properties is based on the Luttinger-Kohn Hamiltonian, including the strains and the carrier-induced bandgap shifts using the Hartree-Fock approximation. The effects of the biaxial compressive and tensile strains on the gain, the output characteristics, the bandgap renormalization, and the modulation response of strained-layer quantum-well lasers are studied. We present new results incorporating the many-body effects in the form of the bandgap renormalization with the valence-band mixing and the multisubband effects. It is found that the bandgap renormalization depends strongly on the nature of strain applied to the quantum well. The differential gain that determines the upper frequency limit of the direct current modulation is calculated from the total derivative of the equigain surface with respect to the carrier- and the photon-densities near the threshold condition. Our approach to the differential gain yields reasonable agreement between theory and experiment for the 3-dB modulation bandwidth. Both InGaAs-AlGaAs and InGaAs-InP strained quantum-well systems are considered     

High single-mode power conversion efficiency vertical-cavitytop-surface-emitting lasers

We report advances in the power conversion (wall-plug) efficiency of vertical-cavity top-surface-emitting lasers. The devices were fabricated from molecular beam epitaxial layers using deep proton implants to define gain-guided lasers. The epitaxial structure included low resistance, piecewise linearly graded n-type and p-type mirrors, a triple In_0.2Ga_0.8As quantum-well active region, and a delta-doped contact layer. Power conversion efficiencies as high as 12.7% for continuous-wave single-mode operation were measured after several hours of device operation     

Effects of strain on the high speed modulation of GaAs- andInP-based quantum-well lasers

A small-signal numerical analysis of pseudomorphic GaAs- and InP-based Fabry-Perot quantum-well lasers using calculated optical gain spectra with strain effects included is reported. Examination of the effect of lifetime broadening shows that the resonance frequency increases at a rate of ~250-MHz/meV reduction in the lifetime broadening for a GaAs-based strained layer laser. The modulation speed is limited by either device heating or facet damage. If the limitation is imposed by the optical power then the modulation speed increases as the laser cavity becomes shorter and the number of quantum wells increases. If the limitation is imposed by the injection current density, however, then the modulation speed decreases for the laser with shorter cavity length. The highest modulation speed is given by an optimum well number. A resonance frequency of ~16 GHz is predicted for a pseudomorphic GaAs-based laser with 30% excess In and average output power of ~5 mW     

Gain coefficient, quantum efficiency, transparency current density,and internal loss of the AlGaAs-GaAs-based lasers on Si substrate

The AlGaAs-GaAs based lasers on Si substrate with GaAs quantum-well and island-like active regions are fabricated by metal-organic chemical vapor deposition. The parameters of internal quantum efficiency, gain coefficient, transparency current density, and the internal loss that describe the operating characteristics of laser diodes are investigated. The optical confinement factor is calculated with the assumption that the light emission occurs from the island regions only. In addition, longer minority carrier lifetime obtained for the lasers with island-like active regions reveals their improved characteristics over conventional quantum-well lasers     

Dependence of differential quantum efficiency on the confinementstructure in InGaAs/InGaAsP strained-layer multiple quantum-well lasers

An internal efficiency of 91% was obtained with In_0.7Ga _0.3As/InGaAsP strained-layer multiple quantum well (MQW) lasers emitting at a wavelength of 1.5 μm. The dependence of the reciprocal differential quantum efficiency on the length of the laser cavity shows that the absorption loss in the InGaAsP (λ=1.3 μm) confinement layer caused by carrier overflowing into the confinement layer reduces the internal efficiency     

Intrinsic equivalent circuit of quantum-well lasers

An intrinsic equivalent circuit of quantum-well lasers that takes the capture of carriers into the quantum well into account is presented. Several qualitative features in the modulation response and the device impedance are revealed. The dependences on the carrier capture and escape rates are discussed     

Gain of TE-TM modes in quantum-well lasers

Semiclassical laser theory is rigorously applied to semiconductor lasers in order to obtain both the complete TE and TM linear gain. The resulting expressions for the modal gain in heterostructure lasers differ in form from those conventionally accepted. In particular, the conventional modal gain written as the product of a confinement factor and a bulk gain is only an approximation of the true modal gain derived. The conventional expression relies on an explicit definition of the active region of the laser, which can be ambiguous when certain heterostructures, such as parabolic quantum wells, are to be treated. This ambiguity is eliminated by the gain expressions as a more natural active region defined by the product of electron and hole wave functions emerges. The relevant approximations which allow the newly derived gain equations to be written in forms similar to the conventional expressions for single quantum well, multiquantum well (MQW), and in wide active region lasers are explicitly shown     

Nonlinear analysis of quantum-well lasers with the effects ofcarrier transport

An improved nonlinear quantum-well (QW) laser model, which takes into account the effects of quantum capture and escape processes, is presented, based on laser rate equations and Volterra theory. This model is further expanded by proper parameter transform to include the effect of carrier diffusion in the separate confinement heterostructure region. Various QW laser distortions have been evaluated using this model and compared with the results obtained from the previous model where the transport effects are absent. The results shows that the effect of transport processes on laser dynamic nonlinearity can be significant