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     

High speed quantum-well lasers and carrier transport effects

Carrier transport can significantly affect the high-speed properties of quantum-well lasers. The authors have developed a model and derived analytical expressions for the modulation response, resonance frequency, damping rate, and K factor to include these effects. They show theoretically and experimentally that carrier transport can lead to significant low-frequency parasitic-like rolloff that reduces the modulation response by as much as a factor of six in quantum-well lasers. They also show that, in addition, it leads to a reduction in the effective differential gain and thus the resonance frequency, while the nonlinear gain compression factor remains largely unaffected by it. The authors present the temperature dependence data for the K factor as further evidence for the effects of carrier transport     

Linewidth rebroadening due to nonlinear gain and index induced by carrier heating in strained quantum-well lasers

The carrier heating has been recently recognized as one of the main origins of nonlinear gain, in particular in strained quantum-well lasers. The asymmetry of this effect introduces a nonlinear refractive index. The joint effects of nonlinear gain and nonlinear refractive index that are both due to carrier heating together with the spatial-hole-burning give rise to an increase in the carrier density and in the linewidth enhancement factor. These effects can explain the linewidth rebroadening at high power in phase-shifted single-mode DFB lasers.     

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     

Gain, refractive index change, and linewidth enhancement factor inbroad-area GaAs and InGaAs quantum-well lasers

We report experimental and theoretical results for the injection-level dependence of the gain, refractive index variation, and linewidth enhancement factor (α) for four different quantum-well (QW) laser structures. Two of the lasers have GaAs QW layers that vary in width while the other two have InGaAs active layers that vary in QW depth. Experimental Hakki-Paoli data are used to compare gain, index change, and α-parameter between these pairs of devices. The results of two simulations are compared to the experimental data. The first is based on the approximation of parabolic bands for both the conduction and valence bands while the second employs the k·p method to refine the calculation of the valence bands. Our findings include: (1) narrower and deeper QWs yield lower α values; (2) modeling results from the k·p method are only slightly improved over those from the parabolic band model; (3) at high injection levels, stimulated emission below threshold is a prominent effect in these devices; and (4) at high injection levels, carriers in the barrier energy states above the well are shown to be responsible for increasing α values     

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.     

The effect of background plasma in the undulator on free electron lasers

A new unconventional FEL scheme (named PFEL), which has injected rarefied ionized background plasma in the undulator, is studied theoretically. It is shown that there exists a critical density n_p; for the background plasma. When the background plasma density n_p is higher than n^p _c, the FEL can no longer be excited. When n_p ? n^p _c, the laser gain for Raman FEL may be increased noticeably.     

Influence of electrostatic confinement on optical gain in GaInNAs quantum-well lasers

There remains controversy surrounding the cause of the magnitude and temperature sensitivity of the threshold current density of 1.3-/spl mu/m GaInNAs quantum-well (QW) lasers, with several authors attributing the strong temperature sensitivity to hole leakage, due to the relatively low valence band offset in GaInNAs/ GaAs QW structures. We use a Poisson solver along with a ten-band k.p Hamiltonian to calculate self-consistently the influence of electrostatic confinement on the optical gain in such lasers. We find that the inclusion of such effects significantly reduces the hole leakage effect, with the electrostatic attraction of the electrons significantly increasing the binding of heavy holes in the QW region. We conclude by comparison with previous theoretical and experimental studies that the room temperature threshold current is generally dominated by monomolecular recombination, while the temperature sensitivity can be explained as predominantly due to Auger recombination.     

Modulation response of quantum-well lasers with carrier transport effects under weak optical feedback

We theoretically investigate the influence of optical feedback on the modulation response of quantum-well lasers. Controlled weak optical feedback is shown to be useful to improve the high frequency performance, compensating the limitations imposed by carrier-transport effects.     

Linewidth enhancement factor for quantum-well lasers

The linewidth enhancement factor ? is calculated for an idealised GaInAs quantum-well laser. Values between 1 and 2 are found for a range of possible lasing energies. Somewhat larger values are expected for GaInAs quantum wells with InP barriers. It is concluded that dramatic reductions in ? are not to be expected for quantum wells.     

Experimental and theoretical analysis of the carrier distributionin asymmetric multiple quantum-well InGaAsP lasers

The authors present an experimental and theoretical analysis of the carrier distribution in multiple quantum-well (MQW) lasers and the effect of this carrier distribution on the gain of wells at different locations in the active region. An experimental technique using mirror image asymmetric multiple quantum-well (AMQW) lasers is described which provides quantitative information on the degree to which the carrier distribution affects the gain of quantum wells (QWs) in the active region. A gain model for AMQW lasers is developed and used to explain some important characteristics of AMQW devices. A rate equation model is presented which incorporates the effects of fields across the p-i-n junction active region. The model is able to predict experimental results measured from thirteen AMQW laser structures to within experimental uncertainty     

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.     

Nonlinear gain suppression in quantum-well lasers due to carrier heating: the roles of carrier energy relaxation, electron-hole interaction, and Auger effect

The influences of carrier energy relaxation and electron-hole interaction on the nonlinear gain coefficient are detailed inspected by a numerical comparison of carrier heating model with classical rate-equation model in a large-signal analysis. It is shown that the nonlinear gain coefficient due to carrier heating is in proportion to an effective carrier energy relaxation time and has a nonlinear relation to the electron-hole energy exchange time. Accordingly, an empirical formula is deduced. In addition, the influence of Auger heating on the modulation dynamics is also considered, which can not be described by a single phenomenological nonlinear gain coefficient. Furthermore, the dependence of the nonlinear gain coefficient on the laser emission wavelength of distributed feedback laser is also demonstrated quantitatively.     

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     

A versatile SPICE model for quantum-well lasers

A SPICE equivalent-circuit model for the design and analysis of quantum-well lasers is described. The model is based on the three-level rate equations which include, in their characterization of charge dynamics, the role of gateway states at the quantum well. The model is versatile in that it permits both small- and large-signal simulations to be performed. Emphasis here is placed on validating the model via a comparison of simulated results with measured data of the small-signal modulation response, obtained over a wide range of optical output powers from two lasers with different lengths of the separate-confinement heterostructure (SCH). Using a set of tightly specified model parameters, all the important trends in the experimental data are reproduced. The consideration of gateway states is found to be important, with regard to predicting the small-signal response, only for the laser with the longer SCH. This highlights the significance of the interplay between the roles of transport through the SCH and capture/release via the gateway states at the quantum well     

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.     

Free-carrier effect on index change in 1.3 /spl mu/m quantum-dot lasers

Below threshold measurements of the carrier induced refractive index change in 1.3 /spl mu/m quantum-dot lasers at resonance and also below bandgap were carried out. It is demonstrated that free-carrier absorption contributes approximately half the total index change, and that its relative contribution is a function of device length. This result gives a lower bound to phase-amplitude coupling in quantum-dot lasers.     

MBE growth and characteristics of periodic index separate confinement heterostructure InGaAs quantum-well lasers

We have used solid-source molecular beam epitaxy (MBE) to grow InGaAs quantum-well lasers emitting at 980nm in a novel configuration of periodic index separate confinement heterostructure (PINSCH). Periodic multilayers (GaAs/AlGaAs) are utilized as optical confinement layers to reduce the transverse beam divergence as well as to increase the maximum output power. The multilayers are grown by temperature modulation MBE without any shutter operation. The heterointerfaces in the multilayers are linearly graded such that the energy barrier heights are greatly decreased. This has led to a drastic reduction in the series resistance which is essential in the performance of high output power. The 5μm × 750μm device has far-field angles of 10° by 20°, a threshold current of 45 mA, an external differential quantum efficiency of 1.15 mW/mA (90%), and an output power of 620 mW, all measured at room temperature under CW operation. A record high fiber coupling efficiency of 51% has been achieved and more than 130 mW of power is coupled into a 5μm-core single mode fiber.     

Carrier-induced index change in AlGaAs double-heterostructure lasers

A large carrier-induced index change is reported for conventional 8 ?m-stripe oxide-isolated AlGaAs double-heterostructure lasers. At threshold, the index change of the active layer is ?0.05 to ?0.07, which is a factor of 5 to 10 larger than previously reported. It is accompanied by an even greater change in dispersion. These effects cannot be explained by a free-carrier effect, and are most likely caused by a carrier-induced shift of the absorption edge.     

Influence of facet coating on the dual wavelength operation of asymmetric InGaAs-GaAs quantum-well lasers

A low-reflectivity SiN/sub y/O/sub z/ film was deposited on one facet of asymmetric In /sub x/Ga/sub 1-x/As-GaAs quantum-well (QW) laser diodes containing three different QW compositions with the indium fractions x=0.25,0.21, and 0.15, located, respectively, from the n-doped to p-doped sides of the device. Lasing is only observed on the two higher In-content QWs. The reduction of the reflectivity of one facet, with the other as-cleaved, leads to an increase in the transition cavity length, below which the diode initially lases on the shorter wavelength QW (x=0.21) at threshold and above which the diode lases on the longer wavelength QW (x=0.25). No lasing occurs on the shortest wavelength QW (x=0.15). With sufficient current injection simultaneous lasing on the two QWs is observed. When dual-wavelength emission occurs the facet reflectivity determines whether the short or long wavelength emitting QW lases first as the pump current is increased. A separation of up to 31 nm between the two wavelengths is observed under the dual wavelength emission conditions.