Education and simulations with LaserCADIII

Esta Tesis Doctoral describe la física y las aplicaciones de dispositivos biestables de estructuras bulk y pozos cuánticos basados en simulaciones numéricas y computacionales; además se analiza los regeneradores ópticos basados en medidas experimentales. Se desarrolla un modelo numérico para el cálculo de la ganancia óptica en láseres de semiconductor de estructuras tipo bulk y de pozo cuántico para diferentes materiales, posteriormente los datos obtenidos se utilizan en el estudio de la Biestabilidad Óptica Dispersiva en este tipo de estructuras dentro de una cavidad Fabry-Pérot. Se realiza un modelo matemático para el estudio teórico de la Biestabilidad Óptica Dispersiva en Láseres de Pozo cuántico de ganancia logarítmica en una cavidad resonante Fabry-Pérot. Con los resultados obtenidos del modelo matemático realizamos una comparativa con la herramienta de simulación de dispositivos fotónicos llamada VPItransmissionMaker™. Las aplicaciones de la biestabilidad óptica también han sido estudiadas en esta Tesis, tales como, el estudio de la variación del ancho de línea de la señal de entrada a los láseres de semiconductor y cómo afecta dicha variación en la histéresis de la curva biestable del láser semiconductor. Simulación de la Célula Lógica Programable Óptica (Optically Programmable Logic Cell, OPLC) basadas en Láseres de Pozo Cuántico, capaces de proporcionar diferentes funciones lógicas booleanas y se propone teóricamente un circuito de recuperación de la señal de reloj utilizando OPLCs. Finalmente, se realiza un estudio experimental de un regenerador 2R, formado por un amplificador óptico de semiconductor y un electroabsorbedor implementados en la misma guía de onda. Realización de pruebas experimentales en estática y dinámica, comprobación de la capacidad de recuperar la forma de la señal óptica y determinar las mejoras de la relación de extinción y relación señal-ruido. -- Abstract This thesis describes the physics and applications of bistable devices made of bulk and quantum-well structures based on numerical and computational simulations; and analyzes optical regenerators devices based on experimental measurements. A theoretical model of the optical gain in bulk and quantum-well semiconductor structures has been developed to further study the optical bistability in both semiconductor structures within a Fabry-Pérot cavity. An analytical model has been developed for studying dispersive optical bistability in Fabry-Pérot quantum-well semiconductor lasers that exhibit a logarithmic dependence of the optical gain on carrier concentration. The laser is operated in transmission and reflection. Model results are shown for a Fabry-Pérot quantum-well laser and compared with the predictions of a commercial computer-aided design (CAD) software tool named VPItransmissionMaker™. Optical bistability for optical computing applications has also been studied in this thesis. Some of the applications proposed are the linewidth influence in photonics logic devices, where the variation of the linewidth in the optical input signal affects the hysteresis curve of the semiconductor laser. Also, it has been simulated an Optically Programmable Logic Cell based on quantum-well lasers capable to provide boolean logic functions in the optical domain. Additionally, a theoretical study for optical clock recovery using the Optical Programmable Logic Cell (OPLC) has been proposed. Finally, all optical 2R-regeneration based on the integration of semiconductor optical amplifiers and electroabsorbers in a single waveguide is experimentally demonstrated. Static transfer functions of concatenated structures show strong improvements of the nonlinearity. An extinction ratio improvement has been obtained under dynamics operation.

Monte Carlo markovian models of a dual-mode semiconductor laser with quantum well (QW) or quantum dot (QD) active regions are proposed. Accounting for carriers and photons as particles that may exchange energy in the course of time allows an ab initio description of laser dynamics such as the mode competition and intrinsic laser noise. We used these models to evaluate the stability of the dual-mode regime when laser characteristics are varied: mode gains and losses, non-radiative recombination rates, intraband relaxation time, capture time in QD, transfer of excitation between QD via the wetting layer... As a major result, a possible steady-state dual-mode regime is predicted for specially designed QD semiconductor lasers thereby acting as a CW microwave or terahertz-beating source whereas it does not occur for QW lasers.

Threshold characteristics of a semiconductor quantum-well (QW) laser are calculated using the global electroneutrality condition, which includes charge carriers both in the active and waveguide regions and thus presents an equality of the total charge of electrons to the total charge of holes in these two regions. It is shown that at the lasing threshold, the densities of electrons in the QW and the waveguide region are not equal to the densities of holes in these regions, i.e., the local electroneutrality condition is violated in each of the regions. Depending on the velocities of the carrier capture from the waveguide region into the QW, the electron density can be either higher or lower than the hole density (both in the QW and in the waveguide region). The charge of the carriers of each sign in the waveguide region is shown to be greater than that in the QW.

Measurements of low-frequency electrical noise (LFN) in quantum-well (QW) semiconductor lasers have been conducted using index guided AlGaInP lasers. To investigate location and origin of the LFN in QW lasers, temperature dependence of the LFN is investigated over a wide range of injected current from 10-7 to 6 X 10-2 A, at temperatures between 0 degree(s)C and 65 degree(s)C. The effects of a short duration of burn-in process on the LFN have been investigated by measuring the LFN in the virginal device and the device after 20 hours stress (current I equals 45 mA, temperature T equals 40 degree(s)C). We find, (1) there are different noise mechanisms associated to the observed terminal current noise when laser diodes operate above and below threshold current; (2) it is much more clear to see the effects of the stress on the LFN versus injected current (SI - ID) than in current versus voltage (ID - V) and optical-power versus injected current (PO - ID); (3) over the wide range of injected current, we did not observe the temperature dependence of the 1/f, though different g-r components appear in the spectra of the LFN measured at different temperatures. We have qualitatively analyzed the noise mechanisms and their location. We will also demonstrate that the noise measurement can be used as a diagnostic tool for the reliability of QW laser diodes.

Experimental evidence of a physical phenomenon characteristic of semiconductor lasers, i.e., cooling of the electrons above the threshold for stimulated emission, is reported. We show that this effect is directly related with the internal quantum efficiency (ηint), which is one of the central physical quantities in the theory of semiconductor lasers. As a model system we selected the terahertz quantum-cascade laser that is particularly suitable for the investigation of nonequilibrium electronic ensembles. The reported procedure for the assessment of ηint can be easily extended to other model systems, enlightening the relevance of including hot-electron distributions in semiconductor laser modeling.

Compact monolithic passively mode-locked semiconductor lasers offer repetition frequencies in the Multi-GHz range and the generation of picosecond-short optical pulses. Their pulse train timing stability however, is considerable lower as compared to fiber-based pulsed lasers. All-optical self-feedback configurations have been studied that allow to substantially improve the timing stability. We present recent experimental results on the pulse train stabilization of a monolithic two-section quantum-well (QW) semiconductor laser emitting at 1070 nm by different fiber-based feedback configurations. Besides the timing jitter (TJ), we study the repetition frequency control and its dependence on the feedback-cavity parameters (e.g. feedback strength, coarse and fine delay lengths).

A two-dimensional model for quantum-well lasers that solves, self-consistently, the semiconductor equations together with the complex scalar wave equation is described. It incorporates a position- and wavelength-dependent gain function which is derived from a quantum mechanical calculation. Such a model enables one to predict the characteristics of a quantum-well laser with a minimal number of empirical parameters. The output of the model includes light-current characteristics, the current distribution, and the optical field intensity distribution, obtained simultaneously in the calculation. Examples for modeling GRIN-SCH SQW (graded-index separate confinement heterostructure single quantum well) ridge wave guide lasers are given, and good agreement with experimental results is obtained. The model is used to optimize the geometry of a GRIN-SCH SQW laser for minimum threshold current and maximum efficiency.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

To circumvent the numerous performance-limiting effects of hot-carrier phenomena in semiconductor quantum-well lasers, we have demonstrated the innovative approach of directly injecting carriers into the lasing subband by tunneling. These lasers, made with a variety of material systems, have shown evidence of reduced hot-carrier effects. Specifically, measured small-signal modulation bandwidth of approximately 50 GHz and maximum intrinsic bandwidth of 110 GHz have been achieved with 0.98 micrometers lasers. These are the highest measured modulation bandwidths in any laser. Auger recombination has been virtually eliminated in 1.55 micrometers lasers and reduced chirp and temperature dependence are also demonstrated. Significant reduction of hot-carrier and carrier leakage effects have also been recently demonstrated in small-area vertical-cavity surface-emitting lasers. These experimental results are supported by recent simulations that identify gain suppression in high speed lasers to be caused by a coupling between the electron temperature and the quasi Fermi level. Lasers with quantum dots as gain media promise high differential gain, very low threshold current, temperature-insensitive operation and high modulation bandwidth. We have investigated room-temperature single-mode ridge-waveguide quantum box lasers in which the quantum box gain regions are realized by self-organized growth and carrier injection is achieved by conventional means over hetero-barriers and by tunneling. The tunneling injection quantum dot lasers show improvements in both differential gain (6 X 10<SUP>-14</SUP> cm<SUP>2</SUP>) and modulation bandwidth. These results will be presented and discussed.

We experimentally investigate the complex dynamics of a multi-mode quantum-dot semiconductor laser with time-delayed optical feedback. We examine a two-dimensional bifurcation diagram of the quantum-dot laser as a comprehensive dynamical map by changing the injection current and feedback strength. We found that the bifurcation diagram contains two different parameter regions of low-frequency fluctuations. The power-dropout dynamics of the low-frequency fluctuations are observed in the sub-GHz region, which is considerably faster than the conventional low-frequency fluctuations in the MHz region. Comparing the dynamics of quantum-dot laser with those of single- and multi-mode quantum-well semiconductor lasers reveals that the fast low-frequency fluctuation dynamics are unique characteristics of quantum-dot lasers with time-delayed optical feedback.

We consider research results on the effect of the active medium temperature, which varies with the operation of semiconductor and solid-state (DPSS) lasers of the visible range, on the output spectral characteristics. The paper presents the study results of the spectral-optical radiation parameters of semiconductor lasers, their coherence lengths, and the dependence of the spectral maximum position on temperature. This study is necessary for selection of the most optimal laser, which in the future is planned to be used in medical ophthalmologic diagnostics. The experiment was carried out by solid-state (DPSS) and semiconductor laser modules based on a laser diode. Spectral dependences on the active medium temperature of lasers were obtained, ranging from 300 to 370 K. The spectra were recorded with the use of an automated spectral complex based on the MDR-23 monochromator. We show determination possibility of the internal stabilization damage of laser modules without mechanical intervention but only by applying their spectral characteristics. The obtained data give the possibility to take into account the temperature characteristics and perform further optimization for parameters of such lasers

An experimental and theoretical study of relative intensity noise (RIN) spectra of side-mode injection-locked Fabry–Pérot semiconductor lasers is reported. It is shown that the injection-locking technique effectively increases the relaxation oscillation frequency from 4.5 GHz (free-running mode) to 12 GHz (injection-locked mode) and enhances relaxation peaks of the slave laser RIN spectra. Results from our theoretical model, which include the key parameters for semiconductor quantum-well lasers, such as the linewidth enhancement factor, the nonlinear gain saturation coefficients, and optical confinement factor, show good agreement with our experimental results.

This paper summarizes a simple single-mode theory of a semiconductor laser and two kinds of multimode extensions. The theories are based on an quasi-equilibrium Fermi-Dirac model of a two-band semiconductor laser gain medium. We include cavity boundary conditions and find the laser single-mode steady-state oscillation intensity. The question as to when sidemodes can build up leads to consideration of a theory of multiwave mixing in the semiconductor medium. This theory is also useful in saturation spectroscopy and phase conjugation using such media, but it does not predict the saturation behavior of the sidemodes. We mention a third-order multimode theory of the laser that allows for sidemode saturation and includes the many-body effects of band-gap renormalization and Coulomb enhancement. These multimode theories assume that the intermode beat frequencies are small compared to the carrier-carrier scattering rate, an assumption that should be valid for external-mirror semiconductor lasers. Using a simple model for the beat frequencies comparable to the carrier-carrier scattering rate, we find two-level inhomogeously broadened sidemode gain and coupling coefficients. Population pulsations and spectral hole burning play approximately equal roles in this theory.© (1990) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Strained layer quantum-well heterostructure semiconductor lasers and other optoelectronic devices are having an impact on nearly all applications of optoelectronics and especially fiber optic telecommunications. In this special issue on strained-layer optoelectronic materials and devices, the most recent advances in this fast-moving area of research are represented by 11 invited papers and 18 contributed papers. The research reported here covers an impressive range from detailed calculations and experimental data on the valence band structure and optical gain of strained layer heterostructures to the laser operating characteristics of devices made from these structures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

In this paper, we present a rigorous approach based on split-step time-domain dynamic modeling (SS-TDM) algorithm using to solve the time-dependent coupled wave equations of the time-domain traveling-wave (TDTW) model to investigate the effect of facets reflectivity on the Light-Current characteristics of (DFB) semiconductor laser for different values the coupling coefficient. The simulation results show that using high reflection coating on the front facet and low reflection coating on the back facet with a proper diffraction grating can significantly improve the power-conversion efficiency of the (DFB) semiconductor laser.

There are several equation sets that are used to characterize of the performance parameters of a quantum well (QW) lasers. One of simple and important equations groups which are used to study of laser characteristics in the almost powerful and expensive simulation tools such as Crosslight, ISE TCAD and SILVACO is of rate equations. A numerical analysis of rate equations is a powerful tool to study of carriers and photons behavior in the semiconductor lasers. The differential rate equations are used to estimate simulated and spontaneous emissions of laser in the all semiconductor simulation softwares [1-7]. A new visual and analytical model to study effects of structure parameters on laser performance of InGaN single quantum well lasers based on solving rate equations by using user friendly programming software, Delphi, is presented. This model has provided a graphical user interface (GUI) for researchers by which one can work directly with all laser parameters throughout the simulation work and analysis without the need to access source code[1]. We have investigated effects of different parameters such as quantum well thickness, separate confinement heterostructure (SCH) thickness and cavity length on the principle characteristics of the laser which include laser time response (P-t), and output power-current (P-I) characteristics. Meanwhile related features such as turn-on delay time of lasing, threshold current and slope efficiency have been investigated. Figure 2 shows the graphical user interface (GUI) of the main program, which uses to input parameters such as QW and SCH thickness and etc. Simulation results of P-I characteristics of InGaN SQW laser with our software has shown in figure 2. Also figure 3 shows the P-t characteristics of the simulated InGaN SQW laser. In conclusion, the presented work demonstrates a new visual, open source and analytical model based on solving rate equations by user friendly Delphi programming software. Our model in comparison with other analytical software and expensive simulators provides directly access to the parameters of lasers throughout the simulation and analysis.