Carrier dynamics and microwave characteristics of GaAs-basedquantum-well lasers

We investigate the effects of carrier capture and re-emission on the electrical impedance, equivalent circuit, and modulation response of quantum-well (QW) laser diodes. The electrical impedance is shown to be a sensitive function of the time constants associated with carrier capture/transport and carrier re-emission. We compare the theoretical results with measured values of the electrical impedance of high-speed InGaAs-GaAs multiple-quantum-well lasers fabricated using different epilayer structures with a common lateral structure. The experimental results agree well with the theoretical model, allowing us to extract the effective carrier escape time and the effective carrier lifetime in the QWs, and to estimate the effective carrier capture/transport time     

Quantum capture and escape in quantum-well lasers-implications ondirect modulation bandwidth limitations

Analytic expressions for carrier capture and escape currents into quantum wells are derived. The authors find that the escape rate can be as large as the capture rate under typical operating conditions in quantum-well lasers so that the damping and inertia of the relaxation oscillation are significantly increased in these lasers. Implications for the limitation of the modulation bandwidth of quantum-well lasers and its dependence on the quantum-well structure are discussed     

Carrier DC and AC capture and escape times in quantum-well lasers

A theoretical model is proposed to study the carrier DC and AC capture and escape times in the small signal modulation response of quantum-well lasers. We derive the DC and AC capture and escape times by calculating the carrier net capture current. Our numerical results indicate that the AC capture and escape times are smaller than the DC capture and escape times. In some cases, they may be even smaller by one order of magnitude. We also find that the AC capture/escape time ratio is larger than the DC capture/escape time ratio by a factor of two. Therefore, conventional theoretical models that do not distinguish the differences between the DC and AC capture and escape times may overestimate the resonant frequency and underestimate the damping rate in the modulation response of quantum-well lasers, i.e., the AC capture and escape times limit the modulation bandwidth of quantum-well lasers more severely than that predicted by the DC capture and escape times     

Temperature dependence of dynamic and DC characteristics ofquantum-well and quantum-dot lasers: a comparative study

Dynamic and static characteristics of high-speed 1.55- and 1-μm wavelength tunneling injection quantum-well lasers and 1-μm wavelength self-organized quantum-dot lasers, have been measured as a function of temperature. While differential gain of the quantum-well lasers greatly increased with lowering of temperature (by a factor of 50), gain compression increased along with it, resulting in about the same intrinsic damping limit (K-factor) over a wide range of temperatures and only moderate increases in bandwidth (20-35 GHz). This suggests that increase in differential gain alone is not sufficient to improve modulation characteristics directly. Because of the mechanism of gain compression, lasers which are damping limited may not see a large improvement in modulation bandwidth simply by operating at lower temperature. In contrast, the modulation bandwidth of the quantum-dot lasers increased from 5-6 GHz at room temperature to larger than 20 GHz at 90 K. This behavior is explained by considering electron-hole scattering as the dominant mechanism-for electron capture in quantum-dots. The measured temperature dependence of the K-factor is analyzed with consideration of electron-hole scattering, and the value extracted for the electron intersubband spacing from this analysis, 60 meV, agrees with the theoretically calculated value of 56 meV     

Impedance characteristics of quantum-well lasers

We derive theoretical expressions for the impedance of quantum-well lasers below and above threshold based on a simple rate equation model. These electrical laser characteristics are shown to be dominated by purely electrical parameters related to carrier capture/transport and carrier re-emission. The results of on-wafer measurements of the impedance of high-speed In_0.35Ga_0.65 As/GaAs multiple-quantum-well lasers are shown to be in good agreement with this simple model, allowing us to extract the effective carrier escape time and the effective carrier lifetime, and to estimate the effective carrier capture/transport time     

Carrier capture and escape times in In/sub 0.35/Ga/sub 0.65/As-GaAs multiquantum-well lasers determined from high-frequency electrical impedance measurements

We present experimental results on the high-frequency electrical impedance of In/sub 0.35/Ga/sub 0.65/As-GaAs multiquantum-well lasers with varied p-doping levels in the active region. The analysis of the data, using a simple three rate equation model, provides information about the dynamical time constants (the carrier lifetime, the effective carrier capture and escape times) under the laser operation conditions. The addition of p-doping increases the carrier escape time at threshold from 0.7 ns, extracted for the undoped devices, up to a value higher than 2 ns for the p-doped lasers. The effective capture time is estimated to be between 2 and 5 ps.     

Simulation of carrier transport and nonlinearities in quantum-welllaser diodes

The two-dimensional (2-D) quantum-well (QW) laser diode simulator Minilase-II is presented in detail. This simulator contains a complete treatment of carrier dynamics including bulk transport, quantum carrier capture, spectral hole burning, and quantum carrier heating. The models used in the simulator and their connectivity are first presented. Then the simulator is used to demonstrate the effects of various nonlinear processes occurring in QW lasers. Finally, modulation responses produced by Minilase-II are compared directly with experimental data, showing good quantitative agreement     

Anomalously high damping in strained InGaAs-GaAs single quantum well lasers

Measurements of the relative intensity noise spectra of strained, single-quantum-well, separate-confinement-heterostructure (SCH) InGaAs-GaAs lasers indicate that their frequency response is strongly damped. The ratio of the damping rate to the square of the resonance frequency is k=2.4 ns. This intrinsically limits the 3-dB modulation bandwidths of these lasers to about 4 GHz, negating the predicted increase in modulation bandwidth due to the large differential gain often associated with quantum-well devices. The damping behavior of these lasers is inconsistent with previous predictions of damping in bulk lasers due to spectral hole burning. A structure-dependent damping mechanism is proposed for quantum-well lasers.<>     

High-Performance Quantum Dot Lasers and Integrated Optoelectronics on Si

This paper provides a review of the recent developments of self-organized In(Ga)As/Ga(Al)As quantum dot lasers grown directly on Si, as well as their on-chip integration with Si waveguides and quantum-well electroabsorption modulators. A novel dislocation reduction technique, with the incorporation of self-organized In(Ga,Al)As quantum dots as highly effective three-dimensional dislocation filters, has been developed to overcome issues associated with the material incompatibility between III-V materials and Si. With the use of this technique, quantum dot lasers grown directly on Si exhibit relatively low threshold current (J _th=900 A/cm²) and very high temperature stability (T ₀=278 K). Integrated quantum dot lasers and quantum-well electroabsorption modulators on Si have been achieved, with a coupling coefficient of more than 20% and a modulation depth of ~100% at a reverse bias of 5 V. The monolithic integration of quantum dot lasers with both amorphous and crystalline Si waveguides, fabricated using plasma-enhanced chemical-vapor deposition and membrane transfer, respectively, has also been demonstrated.     

Effective differential gain and modulation bandwidth reduction inquantum well lasers due to confinement factor modulation

A mechanism that may reduce the effective differential gain due to the modulation of the confinement factor with carrier density in quantum-well lasers is described. This mechanism may limit modulation bandwidth for quantum-well lasers with high threshold carrier density and narrow confining layer     

Analysis of the performance of 1.55-μm InGaAs-InP tensilestrained quantum-well lasers

We fabricated 1.55-μm tensile strained InGaAs quantum-well (QW) lasers into broad-area and ridge waveguide lasers, and their performance was analyzed and compared with compressive strained and lattice-matched QW lasers. It is seen that the limitation on the tensile strain to a value less than 0.7%, which is required to prevent the emission wavelength being shorter than 1.55 μm, imposes restrictions on the performance enhancement in several aspects. Broad-area InGaAs QW lasers with a tensile strain of 0.7% show a larger gain coefficient and smaller transparency current density per well than those with InGaAsP QW lasers with a compressive strain of 1.0%. However, the internal quantum efficiency is much smaller than that for compressive ones and the internal optical loss increases rapidly as the number of QW's increases. These are thought to be caused by a smaller conduction band offset and the onset of dislocation generation at the well-barrier interfaces with the number of QW's, respectively. Ridge waveguide lasers with two QW's with tensile strain of 0.7%, which is designed not to exceed the critical thickness for dislocation generation, show smaller modal gain coefficients and inferior temperature characteristics as compared to those with six 0.7% compressive strained QW's and those with three lattice matched InGaAs QW's. However, the modulation bandwidth is measured to be larger than that for one that is compressively strained. It is believed to originate from the small effective capture time of the carriers due to thicker wells     

Analysis of differential gain in InGaAs-InGaAsP compressive andtensile strained quantum-well lasers and its application for estimationof high-speed modulation limit

A simplified model that furnishes an intuitive insight for the change in quantum-well (QW) laser gain due to QW strain and quantum confinement is presented. Differential gain for InGaAs-InGaAsP compressive and tensile strained multi-quantum-well (MQW) lasers is studied using the model. The comparison between the calculated and experimental results for lattice-matched and compressive strained MQW lasers shows that this model also gives quantitatively reasonable results. It is found that the variance-band barrier height strongly affects the differential gain, especially for compressively strained MQW lasers. The tensile strained MQW lasers are found to have quite high differential gain, due to the large dipole matrix element for the electron-light-hole transition, in spite of the large valence-band state density. Furthermore, a great improvement in the differential gain is expected by modulation p doping in the tensile strained MQW lasers. The ultimate modulation bandwidth for such lasers is studied using the above results     

Mode-switching in semiconductor lasers

In this paper, we experimentally analyze the modal dynamics of quantum-well semiconductor lasers. Modal switching is the dominant feature for semiconductor lasers that exhibit two or several active longitudinal modes in their time-averaged optical spectrum. In quantum-well lasers, these dynamics involve a periodic switching among several longitudinal modes, which follows a well-determined sequence from the bluest to the reddest mode in the optical spectrum. This feature is radically different from the well-known noise-driven mode-hopping occurring in bulk lasers which involves only two main modes. We analyze the differences in modal dynamics for these two kinds of laser by comparing the modal switching statistics and by studying the effects of noise and modulation in the pumping current.     

Generation and detection of high-speed pulses of mid-infraredradiation with intersubband semiconductor lasers and detectors

Semiconductor lasers and detectors based on intersubband electron transitions are used to generate and measure high-speed pulses of mid-infrared radiation. In particular, we use a commercial comb generator to gain-switch a state-of-the-art 8-μm quantum cascade laser mounted in a high-speed package. The output pulses of this device are then detected with a small-area quantum-well infrared photodetector, also packaged for high-speed operation. Pulse widths shorter than 90 ps are directly measured with this system. Accounting for the finite response time of the detection electronics, a deconvolved duration of approximately 45 ps is extrapolated     

Tunable monolithic colliding pulse mode-locked quantum-well lasers

The tunabilities of both the wavelength and the pulse-width of monolithic mode-locked semiconductor lasers are demonstrated. Pulses shorter than 1.6 ps, tunable over 8.8 mu m, have been generated by a temperature-tuned monolithic colliding pulse mode-locked (CPM) quantum-well laser. For a fixed wavelength, the pulse-width is independently controlled from 1.2 ps to longer than 3 ps by external bandpass filters. Near transform-limited time-bandwidth products of 0.34 were maintained throughout the tuning processes.<>     

In(Ga)As/GaAs self-organized quantum dot lasers: DC andsmall-signal modulation properties

Self-organized growth of InGaAs/GaAs strained epitaxial layers gives rise to an ordered array of islands via the Stranski-Krastanow growth mode, for misfits >1.8%. These islands are pyramidal in shape with a base diagonal of ~20 nm and height of ~6-7 nm, depending of growth parameters. They therefore exhibit electronic properties of zero-dimensional systems, or quantum dots. One or more layers of such quantum dots can be stacked and vertically coupled to form the gain region of lasers. We have investigated the properties of such single-layer quantum dot (SLQD) and multilayer quantum dot (MLQD) lasers with a variety of measurements, including some at cryogenic temperatures. The experiments have been complemented with theoretical calculations of the electronic properties and carrier scattering phenomena in the dots. Our objective has been to elucidate the intrinsic behavior of these devices. The lasers exhibit temperature independent threshold currents up to 85 K, with T₀⩽670 K. Typical threshold currents of 200-μm long room temperature lasers vary from 6 to 20 mA. The small-signal modulation bandwidths of ridge waveguide lasers are 5-7.5 GHz at 300 K and increased to >20 GHz at 80 K. These bandwidths agree well with electron capture times of ~30 ps determined from high-frequency laser impedance measurements at 300 K and relaxation times of ~8 ps measured at 18 K by differential transmission pump-probe experiments. From the calculated results we believe that electron-hole scattering intrinsically limits the high-speed performance of these devices, in spite of differential gains as high as ~7×10^-14 cm² at room temperature     

Tunneling injection lasers: a new class of lasers with reduced hotcarrier effects

In conventional quantum-well lasers, carriers are injected into the quantum wells with quite high energies. We have investigated quantum-well lasers in which electrons are injected into the quantum-well ground state through tunneling. The tunneling injection lasers are shown to have negligible gain compression, superior high-temperature performance, lower Auger recombination and wavelength chirp, and better modulation characteristics when compared to conventional lasers. The underlying physical principles behind the superior performance are also explored, and calculations and measurements of relaxation times in quantum wells have been made. Experimental results are presented for lasers made with a variety of material systems, InGaAs-GaAs-AlGaAs, InGaAs-GaAs-InGaAsP-InGaP, and InGaAs-InGaAsP-InP, for different applications. Both single quantum-well and multiple quantum-well tunneling injection lasers are demonstrated     

Improvements in mode-locked semiconductor diode lasers using monolithically integrated passive waveguides made by quantum-well intermixing

By using the technique of quantum-well intermixing (QWI), monolithically integrated passive, and active waveguides can be fabricated. It is shown that mode-locked extended cavity semiconductor lasers with integrated low-loss passive waveguides display superior performance to devices in which the entire waveguide is active: the threshold current is a factor of 3-5 lower, the pulsewidth is reduced from 10.2 ps in the all active laser to 3.5 ps in the extended cavity device and there is a decrease in the free-running jitter level from 15 to 6 ps (10 kHz-10 MHz).     

Thermal frequency drift suppression in tunable DFB lasers by plasmainduced frequency shift enhancement

The authors propose enhanced-plasma-effect (EPE) lasers for coherent optical frequency division multiplexed networks. In the EPE lasers, the blue frequency shift due to the plasma effect is enhanced by incorporating a very thick p-side optical guide layer as a carrier reservoir for multiple quantum-well distributed feedback laser and it surpasses the red frequency shift due to the thermal effect. The results of demonstrating a frequency step response maintained in the blue shift region and an enhanced blue shift frequency modulation response over the whole modulation frequency range from DC are presented for the first time     

A quantitative comparison of the classical rate-equation model withthe carrier heating model on dynamics of the quantum-well laser: therole of carrier energy relaxation, electron-hole interaction, and Augereffect

In this paper, a quantitative theoretical comparison of the classical rate-equation model with the carrier heating model for large signal dynamic response of 1.5-μm InGaAs-InGaAsP single-mode quantum-well (QW) lasers Is performed. The contributions of carrier energy relaxation, electron-hole interaction, and Auger effect to the nonlinear gain are inspected in detail by a numerical comparison of the two models at room temperature (293 K) and low temperature (50 K). It can be shown that contribution of the carrier heating to the nonlinear gain coefficient is proportional to an effective carrier energy relaxation time, and the contribution of the electron-hole energy exchange time shows a nonlinear relation. Furthermore, the influence of Auger heating on the modulation dynamics is also considered and is found to be indescribable by a single phenomenological nonlinear gain coefficient. The dependence of the nonlinear gain coefficient on the laser emission wavelength of distributed feedback lasers is also demonstrated quantitatively for the first time