Performance of Widely Tunable Multi-Quantum-Well and Bulk Laser Diodes and the Main Limiting Factors

The output power and tuning performance of multi-quantum-well (MQW) and bulk InGaAsP/InP-distributed Bragg reflector (DBR) tunable laser diodes (TLDs) are investigated over a wide wavelength tuning range using physics-based PICS3D and VPI laser simulation tools within the travelling-wave formalism. The key result of our simulations is the discovery of a new effect in TLDs due to intervalence band absorption (IVBA) in passive phase and DBR sections, which limits the wavelength tuning range. The physical mechanism responsible for such a behavior is a collapse of the spectral-mode selectivity by the DBR due to large IVBA losses in the phase or/and DBR sections. We fundamentally demonstrate different roles played by the IVBA in the active and passive sections of a TLD. It is shown that the IVBA in passive sections and the carrier relaxation broadening (CRB) of the Lorentzian lineshape function in the lasers' active and passive sections play a crucial role in TLD tuning operation. The IVBA coefficient k IVBA and the intraband relaxation time τ in are the major limiting factors that define the output power variation and the achievable tuning range of the lasers. Both bulk and MQW lasers with small k IVBA demonstrate a wide wavelength tuning range above 30 nm, while for large k IVBA , the tuning range drops below 10 nm. We show that the output power variation with tuning due to the CRB parameter τ in is qualitatively different in bulk and MQW TLDs. The TLD tuning and power performance is also strongly affected by the shapes of the net gain and the cavity mirror loss spectra and their mutual positioning with respect to the lasing cavity mode during the tuning. The limiting parameters k IVBA and τin as well as gain and mirror loss spectra must be thoroughly evaluated in each TLD structure prior to the device design and optimization in order to achieve the best performance in terms of the wavelength tuning and the output power stability

Carrier-induced refractive index change and optical absorption in wurtzite InN and GaN: Fullband approach

Based on the full band electronic structure calculations, first we consider the effect of n-type doping on the optical absorption and the refractive index in wurtzite InN and GaN. We identify quite different dielectric response in either case; while InN shows a significant shift in the absorption edge due to n-type doping, this is masked for GaN due to efficient cancellation of the Burstein-Moss effect by the band gap renormalization. For high doping levels the intraband absorption becomes significant in InN. Furthermore, we observe that the free-carrier plasma contribution to refractive index change becomes more important than both band filling and the band gap renormalization for electron densities above 10$^{19}$~cm$^{-3}$ in GaN, and 10$^{20}$~cm$^{-3}$ in InN. As a result of the two different characteristics mentioned above, the overall change in the refractive index due to n-type doping is much higher in InN compared to GaN, which in the former exceeds 4\% for a doping of 10$^{19}$~cm$^{-3}$ at 1.55~$\mu$m wavelength. Finally, we consider intrinsic InN under strong photoexcitation which introduces equal density of electron and holes thermalized to their respective band edges. The change in the refractive index at 1.55~$\mu$m is observed to be similar to the n-doped case up to a carrier density of 10$^{20}$~cm$^{-3}$. However, in the photoexcited case this is now accompanied by a strong absorption in this wavelength region due to $\Gamma^v_5 \to \Gamma^v_6$ intravalence band transition. Our findings suggest that the alloy composition of In$_x$Ga$_{1-x}$N can be optimized in the indium-rich region so as to benefit from high carrier-induced refractive index change while operating in the transparency region to minimize the losses. These can have direct implications for InN-containing optical phase modulators and lasers.Comment: Revised with an appendix, two additional figures, and more discussions; 10 pages, 14 figures; published versio

Polar optical phonon scattering and negative Kromer-Esaki-Tsu differential conductivity in bulk GaN

Cataloged from PDF version of article.GaN is being considered as a viable alternative semiconductor for high-power solid-state electronics. This creates a demand for the characterization of the main scattering channel at high electric fields. The dominant scattering mechanism for carriers reaching high energies under the influence of very high electric fields is the polar optical phonon (POP) emission. To highlight the directional variations, we compute POP emission rates along high-symmetry directions for the zinc-blende and wurtzite crystal phases of GaN. Our treatment relies on the empirical pseudopotential energies and wave functions. The scattering rates are efficiently computed using the Lehmann-Taut Brillouin zone integration technique. For both crystal phases, we also consider the negative differential conductivity possibilities associated with the negative effective mass part of the band structure. (C) 2001 Elsevier Science B.V. All rights reserved

Comparative analysis of zinc-blende and wurtzite GaN for full-band polar optical phonon scattering and negative differential conductivity

Cataloged from PDF version of article.For high-power electronics applications, GaN is a promising semiconductor. Under high electric fields, electrons can reach very high energies where polar optical phonon (POP) emission is the dominant scattering mechanism. So, we undertake a full-band analysis of POP scattering of conduction-band electrons based on an empirical pseudopotential band structure. To uncover the directional variations, we compute POP emission rates along high-symmetry directions for the zinc-blende (ZB) crystal phase of GaN. We also compare the results with those of the wurtzite phase. In general, the POP scattering rates in the zinc-blende phase are lower than the wurtzite phase. Our analysis also reveals appreciable directional dependence, with the Gamma-L direction of ZB GaN being least vulnerable to POP scattering, characterized by a scattering time of 11 fs. For both crystal phases, we consider the negative differential conductivity possibilities driven by the negative effective mass part of the band structure. According to our estimation, for the ZB phase the onset of this effect requires fields above similar to 1 MV/cm. (C) 2000 American Institute of Physics. [S0003-6951(00)02743-1]

On the Distribution of Traffic Volumes in the Internet and its Implication

Getting good statistical models of traffic on network links is a well-known, often-studied problem. A lot of attention has been given to correlation patterns and flow duration. The distribution of the amount of traffic per unit time is an equally important but less studied problem. We study a large number of traffic traces from many different networks including academic, commercial and residential networks using state-of-the-art statistical techniques. We show that the log-normal distribution is a better fit than the Gaussian distribution commonly claimed in the literature. We also investigate a second heavy-tailed distribution (the Weibull) and show that its performance is better than Gaussian but worse than log-normal. We examine anomalous traces which are a poor fit for all distributions tried and show that this is often due to traffic outages or links that hit maximum capacity. We demonstrate the utility of the log-normal distribution in two contexts: predicting the proportion of time traffic will exceed a given level (for service level agreement or link capacity estimation) and predicting 95th percentile pricing. We also show the log-normal distribution is a better predictor than Gaussian or Weibull distributions

Surface scattering velocities in III-nitride quantum well laser structures via the emission of hybrid phonons

We have theoretically and numerically studied nitride-based quantum well (QW) laser structures. More specifically, we have used a QW made with III-nitride where the width of the barrier region is large relative to the electron mean free path, and we have calculated the electron surface capture velocities by considering an electron flux which is captured into the well region. The process is assisted by the emission of the longitudinal optical phonons as predicted by the hybrid (HB) model. The results of surface capture velocities via the emission of HB phonons are compared to the emission of the dielectric continuum phonons (Zakhleniuk et al 1999 Phys. Status Solidi a 176 79). Our investigation shows that the two different phonon models predict almost the same results for the non-retarded limit. Furthermore, the surface capture velocities strongly depend on the size of the structure and the heterostructure materials. Lastly, a comparison to the recent experimental values shows that our model could accurately describe the experimentally measured parameters of the quantum capture processes

On the Distribution of Traffic Volumes in the Internet and its Implications

In this edition of the Voice, the College’s Career Planning Placement Service offers a variety or workshops include one on life planning. Wooster Chief of Security and Dr. Startzman of the campus wellness center, speak to students on the topic of rape and safety at the College. The Wooster Board of Trustees begins the process to select a new president of the College of Wooster. The Art Center offers classes on quilting, plants, printmaking, drawing, and other artistic mediums, to students for eight weeks. Additionally, an article discusses the, then up and coming, Bicentennial of the United States.https://openworks.wooster.edu/voice1971-1980/1131/thumbnail.jp

Solution of the 1D Schrödinger equation in semiconductor heterostructures using the immersed interface method

Due to the enormous progress in computer technology and numerical methods that has been achieved over the past several decades, the use of numerical simulation methods in all scientific disciplines gain more and more importance. In the physics field, these methods have provided remarkable numerical solutions to problems considered analytically intractable. The solution of the Schrödinger equation in semiconductor heterostructures is a good example. However, many of these numerical schemes are cumbersome to implement for nonexperts in numerical computing. With this reason as motivation, a novel method simple enough to implement yet powerful enough to solve Schrödinger equation in semiconductor devices with high accuracy is presented herein. © 2012 Elsevier Inc.

Travelling-wave modelling of the modulation dynamic performance of wavelength-tunable laser diodes using the integrated VPI and PICS3D software

© The Institution of Engineering and Technology. The modulation dynamic performance of a three-section bulk InGaAsP/InP tunable laser diode (TLD) operating at 1550-nm under direct intensity modulation during discontinuous tuning is investigated with the travelling-wave approach, using commercial software tools VPI and PICS3D. The authors demonstrate a strong effect of the gain spectra shape on the modulation response of the TLDs. Two models have been developed for simulation of the modulation response which incorporate real gain spectra of TLDs obtained either from experiment or ab-initio calculations: (i) the VPI + PICS3D integrated model, and (ii) the fitted parabolic shape gain model. The results obtained for both models are in good agreement. A significant ~3 times increase of the relaxation oscillation frequency and the corresponding modulation bandwidth was observed under blue wavelength tuning of the TLD over a 21-nm range from the initial 1550-nm lasing wavelength. The authors show that the main physical reason for this increase is a dispersion of the differential gain which increases about 4 to 5 times when the lasing wavelength decreases over the above tuning range. The reported enhancement of the modulation response of TLDs is important for their practical applications