Unique constraints encountered in the III-N semiconductor system, such as a lack of cleaving planes and resistance to wet etchants, make conventional approaches to the fabrication of laser diodes difficult to implement, and result in the need for novel cavity designs. Improvements in in-plane cavities include etching facets using a focused ion beam and the incorporation of gratings to decrease losses associated with poor mirrors. Towards the fabrication of an electrically pumped VCSEL, the issue of the bottom mirror can be addressed by the use of dielectric mirrors, and either the removal of the sapphire substrate or its incorporation into the cavity by using a curved backside mirror. This paper will review recent developments in these areas at UC Santa Barbara.

III-nitride VCSEL structures designed for electron-beam pumping have been grown by molecular beam epitaxy (MBE). The structures consist of a sapphire substrate on which an AlN/GaN distributed Bragg reflector (DBR) with peak reflectance >99% at 402 nm is deposited. The active region consists of a 2-(lambda) cavity with 25 In_0.1Ga_0.9N/GaN multiquantum wells (MQWs) whose emission coincides with the high reflectance region of the DBR. The thicknesses of the InGaN wells and the GaN barriers are 35 angstrom and 75 angstrom respectively. The top reflector consists of a silver metallic mirror which prevents charging effects during electron-beam pumping. The structure was pumped from the top- side with a cw electron-beam using a modified cathodoluminescence (CL) system mounted on a scanning electron microscope chamber. Light output was collected from the polished sapphire substrate side. Measurements performed at 100 K showed intense emission at 407 nm with narrowing of the linewidth with increasing beam current. A narrow emission linewidth of 0.7 nm was observed indicating the onset of stimulated emission.

Using self-consistent laser simulation, we analyze the performance of nitride Fabry-Perot laser diodes grown on sapphire. The active region contains three 4 nm InGaN quantum wells. It is sandwiched between GaN separate confinement layers and superlattice AlGaN/GaN cladding layers. AlGaN is used as an electron barrier layer. Pulsed lasing is measured near 420 nm wavelength and at temperatures up to 120 degrees Celsius. Advanced laser simulation is applied to link microscopic device physics to measurable device performance. Our two-dimensional laser model considers carrier drift and diffusion including thermionic emission at hetero-boundaries. The local optical gain is calculated from the wurtzite band structure employing a non-Lorentzian line broadening model. All material parameters used in the model are evaluated based on recent literature values as well as our own experimental data. Simulation results are in good agreement with measurements. Multi-lateral mode lasing is calculated with a high order vertical mode. The carrier distribution among quantum wells is found to be strongly non-uniform leading to a parasitic (absorbing) quantum well. The influence of defect recombination, vertical carrier leakage and lateral current spreading is investigated. The reduction of such carrier losses is important to achieve lower threshold currents and less self-heating. Several device optimization options are proposed. Elimination of the parasitic quantum well is shown to substantially enhance the device performance.

Three-dimensional electric-thermal-optical numerical simulator is developed and applied to model group-III-nitride-based intracavity-contacted vertical-cavity surface-emitting lasers (VCSELs) with InGaN multi-quantum-well active region. The optical model based on the effective frequency method is combined with electrical-thermal simulator using the control volume method. Isothermal (pulsed regime imitation) and CW modes of operation are calculated over a range of voltages, covering sub-threshold spontaneous emission and lasing emission. Effects of current crowding at the active-region periphery are examined, and in particular an impact on mode profiles of spatial hole burning superimposed on nonuniform gain distribution is studied. In order to reduce the current crowding and provide more uniform gain distribution within the active region, a semitransparent p-side contact design is proposed.

We demonstrate that the optical properties of GaInN/GaN/AlGaN quantum wells are governed to a large extent by the spontaneous and piezoelectric polarization fields arising from the strongly polar nature of wurtzite nitrides. The spontaneous emission from single quantum wells is strongly red-shifted and exhibits a very small oscillator strength, particularly for wide wells. In multiple quantum wells the electric fields lead to spatially highly indirect transitions between nearest and next nearest neighbor quantum wells. Screening of the fields due to injected carriers leads to a strong blue-shift of the emission. Internal fields due to spontaneous polarization are much more elusive than piezoelectric ones. By manipulating the surface coverage using an electron beam we demonstrate that such spontaneous fields in fact exist and that their direction is opposite to the piezoelectric fields. In this way, the emission of a nitride quantum well can be shifted by as much as 500 meV in a metastable manner. We discuss the impact of internal polarization fields on LED efficiency. We show that nonradiative recombination is subject to reduction by field effects in a similar manner as the radiative one.

Incorporation of a few percent of nitrogen into conventional III-V compounds to form III-N-V alloys such as GaNAs and GaNP leads to a large reduction of the fundamental band gap. We show experimentally and theoretically that the effect originates from an anti-crossing interaction between the extended conduction-band states and a narrow resonant band formed by localized N states. The interaction significantly alters the electronic band structure by splitting the conduction band into two nonparabolic subbands. The downward shift of the lower conduction subband edge is responsible for the N-induced reduction of the fundamental band-gap energy.

The variation of the value of the linewidth of an excitonic transition in InGaAsN alloys (1% and 2% nitrogen) as a function of hydrostatic pressure using photoluminescence spectroscopy is studied at 4K. The excitonic linewidth increases as a function of pressure until about 100 kbar after which it tends to saturate. This pressure dependent excitonic linewidth is used to derive the pressure variation of the exciton reduced mass using a theoretical formalism based on the premise that the broadening of the excitonic transition is caused primarily by compositional fluctuations in a completely disordered alloy. The linewidth derived ambient pressure masses are compared and found to be in agreement with other mass measurements. The variation of this derived mass is compared with the results from a nearly first-principles approach in which calculations based on the local density approximation to the Kohn-Sham density functional theory are corrected using a small amount of experimental input.

The nature of our work in the area of research and development of nitride-based LEDs and laser diodes requires a fast modular tool for numerical simulation and analysis. It is required that the modeling tool reflects the primary physical processes of current injection, quantum well bound state dynamics, quantum well capture, radiative and non-radiative transitions. The model must also have the flexibility to incorporate secondary physical effects, such as induced piezoelectric strain fields due to lattice mismatch. A one-dimensional model with a phenomenological well capture process, similar to that developed by Tessler and Eisenstein, has been implemented. The radiative processes are calculated from first principles, and the material band structures are computed using k (DOT) p theory. The model also features the incorporation of such effects as thermionic emission at heterojunctions, Shockley- Read-Hall recombination, piezoelectric strain fields, and self-consistent calculation of the quantum well bound states with dynamic device operation. The set of equations underlying the model is presented, with particular emphasis on the approximations used to achieve the previously stated goals. A sample structure is analyzed, and representative physical parameters are plotted. Finally, as an example of the modeled secondary physical processes, results showing the effects of incorporating strain-induced piezoelectric fields due to lattice mismatch are given.

The optical gain spectra for GaInNAs/GaAs quantum wells are computed using a microscopic laser theory. From these spectra, the peak gain and carrier radiative decay rate as functions of carrier density are determined. These dependences allow the study of the lasing threshold current density of GaInNAs/GaAs quantum well structures.

We have developed a current controlled numerical model by solving the semiconductor device drift-diffusion equations in an isothermal condition to study the current-voltage characteristics of heterostructure GaN/6H-SiC pnpn thyristors. The temperature sensitive parameters such as the density of states, lifetime, mobility, and bandgap energy are included for the electrical characterization. The modeling work is aimed at designing four-layer pnpn thyristors using GaN, 6H- SiC, or combination of thereof, for high power high temperature switching applications. Preliminary simulated results show high power switching at high temperature.

Photonics IC is an attractive information processing means to fully utilize the enormous bandwidth carried by the optical signals. The full integration of photonics devices, such as tunable lasers, modulators and photodetectors have to be developed and which can be obtain by using the Quantum Well Intermixing technology. This paper will explore on the wavelength tunability using different Quantum Well Intermixing techniques, such as impurity induced diffusion and impurity- free vacancy diffusion. Emphasis will be put on the development of the very recent innovations and applications.

The AlGaAs/GaAs multi-quantum well waveguide defined by impurity free vacancy diffusion is analyzed using an advanced model. The waveguiding properties such as the modal propagation constant, full width half maximum of the guided field profile are found by the semi-vectorial wave equation. By choosing various parameters like the cap width, the cap thickness, the quantum well thickness and the operating wavelength properly, single-mode operation with strong optical confinement can be achieved. This method of defining guided channels is attractive in the view of integrating photonic devices with a tuneable wavelength operation.

A simple and accurate model is presented for the analysis of ion-implanted AlGaAs-GaAs multiple-quantum-well (MQW) single channel waveguide. Our model proposed that the interdiffusion is vacancy enhanced. So we simulate the interdiffusion mechanism by solving the coupled diffusion equation of vacancy and interdiffusion numerically. The modal propagation constants full width half-maximum (FWHM) and field profiles of the guided modes of the waveguide are solved numerically using a semi-vectorial wave equation. MQW optical waveguides defined by ion-implantation-induced intermixing are shown to have similar optical properties as conventional dielectric rib optical waveguide. They also provide a more flexible control over the waveguiding characteristics by changing parameters such as periods of MQW layers, mask width, ion implant energy, and diffusion time.

The linear light propagation in semiconductors is analyzed using a microscopic theory. Numerical results are compared with various approximation schemes based on additional boundary conditions (ABCs).

The purpose of this paper is to examine the dimensional aspects of the optical properties of quantum well and dot systems, without assuming that the carriers are localized to the geometrical extent of the confining potential. We show that optical absorption normal to the plane of a well cannot be expressed as an absorption coefficient but should be specified as a fraction of light transmitted or absorbed per well. The modal gain for light propagating along the plane of a well does not scale with well width and the variation of the material gain inversely proportional to the well width is a consequence of the definition of the confinement factor and has no independent physical significance. Optical absorption by quantum dots should be expressed as a cross section per dot. The radiative recombination rate is correctly expressed in terms of a 2D recombination coefficient and use of an equivalent 3D coefficient introduces an artificial dependence on well width which can lead to errors in the comparison of quantum well systems.

Polarization sensitivity of quantum-well structures with delta-strained layers is investigated by numerical analysis. The effects of delta-strained layers on optical gains are very sensitive to the location and the number of the layers. Among the various structures investigated, the quantum well with two delta-strained layers has the least polarization sensitivity for wide wavelength and injected carrier density ranges.

A fully quantum mechanical theory for a system of photons and Coulomb interacting electron-hole pairs in semiconductors is investigated. The resulting semiconductor luminescence equations are discussed and evaluated for a variety of examples. For a quantum-well system, it is shown how luminescence at the exciton resonance can result from an incoherent electron-hole plasma. Also changes in carrier lifetimes due to radiative recombination are studied.

We describe a single-pass technique for the independent measurement of optical modal gain and internal mode loss in semiconductor lasers structures using a single, multi-section device which gives the loss and the gain spectrum in absolute units and over a wide current range. Comparison of the transverse electric and transverse magnetic polarized gain spectra also identifies the transparency point, provides the quasi-Fermi level energy separation and a second means for determination of the mode loss. Measurements are described for AlGaInP quantum well laser structures with emission wavelengths close to 670 nm, yielding an internal loss of 10 cm^-1 and peak gain values up to 4000 cm^-1 for current densities up to 4 kAcm^-2. We have also made an independent measurement of the spontaneous emission spectrum through a top-contact window on the same device structure and have converted this to local gain using the usual thermodynamical relationship. By this means we have been able to confirm the validity of this relation between gain and emission for excited semiconductor structures of this type.

In this paper, the static and dynamic performance of multi quantum-well (MQW) 1.3 micrometer InGaAsP Fabry Perot lasers is assessed experimentally and theoretically to identify the mechanisms responsible for impaired high speed performance at elevated temperature. Initially, threshold currents and spontaneous emission spectra are characterized for a range of temperatures from room temperature to 85 degrees Celsius to indicate a significant increase in non-radiative current contributions. Preliminary estimates are made for the contributions of leakage and Auger recombination rates, found from the dependence of integrated spontaneous emission with carrier density. Drift-diffusion modeling is found to accurately predict the trend of threshold currents over temperature. Using gain modeling good agreement is found between the measured and predicted integrated spontaneous emission intensity. Gain measurements at 85 degrees Celsius indicate a reduction in RIN frequency to 63% of the 25 degree Celsius value which matches well with experimental small signal performance.

In this paper we demonstrate a prototype pump-probe system comprising of a pair of gain-switched semiconductor lasers and present results of a study of the influence of the dc and rf components on pulse shape and position. We demonstrate the system with and without a pulse compression stage consisting of a length of dispersion compensating fiber (DCF).

Cavity solitons appear as bright spots in the transverse intensity profile. They are similar to spatial solitons, but arise in dissipative systems. Here we consider a broad area vertical cavity resonator, driven by an external coherent field, at room temperature. The active material is constituted either by bulk GaAs, or by a Multiple Quantum Well GaAs/AlGaAs structure (MQW). A general model valid for both configurations is presented and a set of nonlinear dynamical equations is derived. The linear stability analysis of the homogeneous steady states is performed in a general form, holding for the two cases. Then, the nonlinear susceptibilities are specified: in the bulk case, we basically work in the free-carrier approximation, with some phenomenological corrections, such as the Urbach tail and the band-gap renormalization. For the bulk case, some numerical results concerning spatial pattern formation and cavity solitons are given. In the MQW case, on the contrary, we derive a full many-body theory, with the Coulomb enhancement treated in the Pade approximation.

We discuss mechanisms of polarization switching (PS) in Vertical Cavity Surface Emitting Lasers (VCSELs) within a mesoscopic approach based on an explicit form of a frequency- dependent complex susceptibility of the QW semi-conductor material. Cavity anisotropies, spin carrier dynamics and thermal shift of the gain curve are also taken into account in this framework. For large birefringence we find a PS due to thermal shift. For small birefringence we find a different PS, from the high-gain to the low-gain polarization state, that occurs at constant temperature. We characterize polarization partition noise in terms of power spectra. Transverse effects for PS in gain guided VCSELs are also considered.

We present a theoretical analysis of the optical physics of tapered oxide apertures in long- and short-cavity VCSELs. We apply our quasi-exact vector finite element model to a USC (long cavity) and U. Texas (short cavity) VCSEL to compute the electric field distribution, transverse confinement factor, diffraction rate, and threshold gain of the fundamental lasing mode. Making qualitative reference to the Hegblom, et al model, we analyze our results to deduce the fundamental physical effects of the tapered oxide aperture. We find that tapered oxides reduce diffraction loss through two separate physical phenomena: (1) a reduction in transverse confinement yielding a flatter phase front, and (2) an effective lens which acts to refocus the naturally diffracting wave front. We further find that in most VCSELs an inherent trade-off exists between minimizing the diffraction loss and maximizing the optical mode-to-gain interaction. To achieve the ultimate goal of (near) thresholdless lasing, this trade-off must be overcome: diffraction loss must be eliminated while simultaneously minimizing the mode volume. We conclude with a suggestion for a novel cavity design, which in theory achieves this goal.

It has been established both theoretically and experimentally that semiconductor microcavities are capable of modifying the spontaneous emission rate of electron-hole pairs within the microcavity. In particular, the microcavity may be used to re- shape the spectral distribution of radiation from a dipole source in order to enhance emission at certain frequencies and suppress emission other frequencies. It is desirable to exploit this effect to increase the efficiency of LEDs and SLDs. Numerical calculation may be used to evaluate the magnitude of the modification of spontaneous emission in practical microcavity structures. The Green's function based VCSEL mode solver we have developed is uniquely well-suited for such calculations. The method we have employed for calculating spontaneous emission rates in microcavities is presented, along with calculated results and comparisons with experimental data.

A comparative study between microscopic and macroscopic models describing polarization switching in optically anisotropic VCSELs is presented. The microscopic model includes: (1) steady-state, many-body multi-band microscopic theory for the optical response of semiconductor quantum wells; (2) the polarization properties of vectorial eigenmodes of VCSEL structures including mode-dependent losses and frequencies; (3) realistic model for optical anisotropies resulting from intentional or unintentional strain in an active quantum-well layer. A macroscopic model is derived from this microscopic model. It provides a rigorous generalization of the phenomenological approaches to the description of polarization properties of VCSELs used commonly in the literature. The optical anisotropy of the VCSEL structure is assumed to result from anisotropic strain of the active quantum well materials. The valence band anisotropy and heavy-hole light-hole mixing effects determine the system characteristics like anisotropic gain and refractive index. The results of microscopic and macroscopic models agree very well for input/output characteristics of anisotropic VCSELs. Also, the stability properties of polarization eigenmodes are qualitatively the same, although the ranges of stability are quantitatively different for both approaches. Incorporation of many-body effects into the analysis usually diminishes the agreement between microscopic and macroscopic theories.

Transverse mode dynamics of a 20-micrometer-diameter vertical- cavity surface-emitting laser (VCSEL) undergoing gain switching by deep current modulation is studied numerically. The direct current (dc) level is set slightly below threshold and is modulated by a large alternating current (ac). The resulting optical pulse train and transverse-mode patterns are obtained numerically. The ac frequency is varied from 2.5 GHz to 10 GHz, and the ac amplitude is varied from one-half to four times that of the dc level. At high modulation frequencies, a regular pulse train is not generated unless the ac amplitude is large enough. At all modulation frequencies, the transverse spatial profile switches from single-mode to multiple-mode pattern as the ac pumping level is increased. Optical pulse widths vary in the range 5 - 30 ps, with the pulse width decreasing when either the frequency is increased or the ac amplitude is decreased. The numerical modeling uses an approximation form of the semiconductor Maxwell-Bloch equations. Temporal evolution of the spatial profiles of the laser (and of carrier density) is determined without any assumptions about the type or number of modes.

We are currently developing 2 semiconductor laser simulators built on a first-principles microscopic physics basis. The first is a PC-based, plane-wave simulator for both component and system-level design of low-power optoelectronic devices. The second is a supercomputer-based simulator that models the fully time-dependent and spatially-resolved optical, carrier, and temperature fields for arbitrary geometry, high-power semiconductor lasers. Both simulators are based on a comprehensive gain model that includes the relevant bandstructure of the quantum wells and confining barrier regions together with a fully quantum mechanical many-body calculation that takes all occupied bands into account.

Our simulations on InP-based edge-emitting RW-lasers have been essentially based on the drift-diffusion approach. The model comprises a self-consistent description of the electronic properties and the optical field under modifications caused by heating processes. Calculations on the device performance have been done by means of the semiconductor device simulation package TESCA with energy transport equation. The quantum confined carriers are described by a (8X8) kp-Hamiltonian, self-consistently coupled to the Poisson equation and exchange-correlation potentials to give a guess for Coulomb effects. Using the package KPLIB for band structure calculations, modifications of the optical gain, caused by the presence of the strained multi quantum wells, have been simulated and will be discussed.

Many-body Coulomb effects influence the operation of quantum- well (QW) laser diode (LD) strongly. In the present work, we study a two-band electron-hole plasma (EHP) within the Hatree- Fock approximation and the single plasmon pole approximation for static screening. Full inclusion of momentum dependence in the many-body effects is considered. An empirical expression for carrier density dependence of the bandgap renormalization (BGR) in an 8 nm GaAs/Al_0.3Ga_0.7As single QW will be given, which demonstrates a non-universal scaling behavior for quasi-two-dimension structures, due to size-dependent efficiency of screening. In addition, effective mass renormalization (EMR) due to momentum-dependent self-energy many-body correction, for both electrons and holes, is studied and serves as another manifestation of the many-body effects. Finally, the effects on carrier density dependence of the alpha factor is evaluated to assess the sensitivity of the full inclusion of momentum dependence.

This paper presents our initial work on high speed laterally coupled semiconductor diode lasers, where the structures to be studied will be two laterally coupled semiconductor diode lasers are expected to increase the modulation bandwidth by using the principles of coupling. We will show a study of the modeling used to find the static and spatial behavior of theses devices, obtaining the near and far field profiles and light power-current characteristic. Our primary goal will be to present the static and spatial behavior of the two InGaAsP laterally coupled semiconductor diode lasers using the effective-index method and Beam Propagation Method (BPM). Our results provide the static evolution of the current density profile, carrier density in the active layer and effective index shape at different injection current levels of a specific InGaAsP structure operating at 1.3 micrometer.

Recent developments in the research in quantum cascade laser technology in university of Neuchatel are reviewed. We report operation of quantum cascade lasers at high temperature (up to 10 mW peak power at 90 degrees Celsius) and of distributed feedback quantum cascade lasers with single-mode surface emission. New results in the investigation of mid-IR emission from InGaN/GaN LED's are also discussed.

Interband cascade (IC) lasers are a new class of mid-IR light sources, which take advantage of the broken-gap alignment in type-II quantum wells to reuse electrons for sequential photon emissions from serially connected active regions. Here, we describe recent advances in InAs/GaInSb type-II IC lasers at emission wavelengths of 3.6 - 4 micrometer; these semiconductor lasers have exhibited significantly higher differential quantum efficiencies and peak powers than previously reported. Low threshold current densities (e.g., approximately 56 A/cm² at 80 K) and power efficiency exceeding 9% were observed from a mesa-stripe laser in cw operation. Also, these lasers were able to operate at temperatures up to 250 K in pulsed mode and to 120 K in cw mode. We observed from several devices at temperatures above 80 K a slope efficiency of approximately 800 mW/A per facet, corresponding to a differential external quantum efficiency of approximately 500%. Peak optical output powers exceeding 4 W per facet were observed from several type-II IC laser at 80 K.

The active region of a type-II mid-IR quantum cascade laser with improved high-temperature performance is investigated by multiband k(DOT)p methods. The calculated results show that internal losses due to intervalence absorption have been suppressed successfully in this active region. A fourteen-band k(DOT)p approach is used and results for short period type-II InAs/GaSb superlattices are compared with an eight-band k(DOT)p method and a plane wave pseudopotential method.

Optical intersubband response of a multiple quantum well (MQW)-embedded microcavity driven by a coherent pump field is studied theoretically. The n-type doped MQW structure with three subbands in the conduction band is sandwiched between a semi-infinite medium and a distributed Bragg reflector (DBR). A strong pump field couples the two upper subbands and a weak field probes the two lower subbands. To describe the optical response of the MQW-embedded microcavity, we adopt a semi- classical nonlocal response theory. Taking into account the pump-probe interaction, we derive the probe-induced current density associated with intersubband transitions from the single-particle density-matrix formalism. By incorporating the current density into the Maxwell equation, we solve the probe local field exactly by means of Green's function technique and the transfer-matrix method. We obtain an exact expression for the probe absorption coefficient of the microcavity. For a GaAs/Al_xGa_1-xAs MQW structure sandwiched between a GaAs/AlAs DBR and vacuum, we performed numerical calculations of the probe absorption spectra for different parameters such as pump intensity, pump detuning and cavity length. We find that the probe spectrum is strongly dependent on these parameters. In particular, we find that the combination of the cavity effect and the Autler-Townes effect results in a triplet in the optical spectrum of the MQW system. The optical absorption peak value and its location can be feasibly controlled by varying the pump intensity and detuning.

Design issues relating to high-power broad-area semiconductor lasers are reviewed. Device optimization is performed using a phenomenological model. This model provides a simple means of predicting the laser performance and assessing a high-power laser design. Emphases are placed on parameters that impact the power conversion efficiency,junction temperature, optical intensity, and near field uniformity.

We outline physical models and simulations for suppression of self-focusing and filamentation in large aperture semiconductor lasers. The principal technical objective is to generate multi-watt CW or quasi-CW outputs with nearly diffraction limited beams, suitable for long distance free space transmission, focusing to small spots or coupling to single-mode optical fibers. The principal strategies are (1) optimization of facet damage thresholds, (2) reduction of the linewidth enhancement factor which acts as the principal nonlinear optical coefficient, and (3) design of laterally profiled propagation structures in lasers and amplifiers which suppress lateral reflections.

We describe a new approach to achieving high brightness emission from laser diodes using a compound waveguide containing an unusual high order mode. This mode is predominantly single lobed and unusually localized in the low index region of the waveguide. To examine the utility of this mode we calculated the single-mode fiber butt coupling efficiency for an AlGaAs/GaAs compound waveguide of 61 micrometer aperture width, emitting at a wavelength of (lambda) equals 980 nm. Our results show a 2.3 times enhancement of the fiber coupling efficiency when compared with a simple broad area waveguide. Furthermore the coupling efficiency is still almost a factor of two greater than the coupling efficiency of the emission from a single mode ridge waveguide.

A comprehensive model has been developed to study the operating characteristics of high-power high-brightness lasers consisting of a ridge-waveguide section coupled to a tapered region. The model, based on the Beam Propagation Method (BPM), includes a non-linear gain coefficient, current spreading due to junction voltage, and thermal effects taking into account for the first time to our knowledge a longitudinal gradient in the device temperature. We first demonstrate that during operation unwanted radiation that does not couple into the lateral mode of the waveguide, systematically propagates into the tapered region, and leads to the deterioration of the beam quality. To deflect and scatter this radiation, the use of specific cavity-spoiling elements, consisting of grooves etched down through the active region, appears necessary. We also study the role of the ridge section length in the operation of the device. A long ridge-waveguide region, providing both a well defined fundamental mode in the ridge- waveguide, and a gain saturation in the tapered region, improves the beam stability, but can lead, on the other hand, to optical self-focusing. Thermal effects are also investigated. We show how thermal lensing induces a lateral quadratic phase curvature and therefore alters the astigmatism of the device.

This paper will review the use of the contactless methods of photoreflectance (PR), contactless electroreflectance (CER), and surface photovoltage spectroscopy (SPS) for the nondestructive, room temperature characterization of a wide variety of wafer-scale semiconductor device structures. Some systems that will be discussed include heterojunction bipolar transistors such as graded emitter GaAlAs/GaAs and AlInAs/InGaAs as well as GaInP/GaAs (including the determination of the built-in fields/doping levels in the emitter and the collector regions, doping level and minority carrier lifetime in the base, alloy composition, and the degree of ordering in the GaInP), pseudomorphic GaAlAs/InGaAs/GaAs high electron mobility transistors (including the determination of the composition, width, and two-dimensional electron gas density in the channel), quantum well edge emitting lasers [InGaAsP/InP (including the detection of p-dopant interdiffusion), graded index of refraction separate confinement heterostructure GaAlAs/GaAs/InGaAs], vertical-cavity surface-emitting lasers (determination of fundamental conduction to heavy-hole excitonic transition and cavity mode), and InAs/GaAs quantum dot lasers. These methods are already being used by more than a dozen industries world-wide for the production-line qualification of these device structures.

Over-heating of semitransparent fused silica (quartz) pieces within MetalOrganic Chemical Vapor Deposition (MOCVD) reactors may result in parasitic deposition on reactor walls, leading to loss of precursors. Although growth on the substrate (epitaxial growth) is diffusion-limited, parasitic deposition occurs at colder temperatures and is therefore, rate-limited. The modeling of low-temperature deposition requires complex chemical mechanisms, which account not only for the kinetics of decomposition, but also the kinetics of adsorption and desorption at the surfaces. In this article, the role of complex chemistry has been demonstrated for growth of Gallium- Arsenide in a commercial horizontal reactor (Crystal Specialties 425). Numerical computations were performed for a wide range of operating conditions. Comparison of numerical predictions with experimental data clearly indicates the need for the development and use of detailed chemistry in modeling parasitic deposition in commercial reactors.

Process control based on first-principles models is possible if the information from the reactor simulations can be represented in a compact model. This is necessary since on- demand reactor simulation in real time for control purposes is not currently realistic. We have developed a neural network based model for applications in process control and process design.

We show experimentally that semiconductor lasers with a double cavity or with an injected signal behave dynamically as excitable media. We perform experimental tests in order to characterize excitable pulses. We also present experimental evidence of coherent resonance as the amount of noise is increased in the system.

A passively Q-switched laser produces regular pulses above threshold. Here we show that this type of laser, described by the Yamada model, exhibits excitability below threshold. There is a stable equilibrium, the off-solution, from which the system can be triggered by a sufficiently large, but small perturbation, to produce a single pulse after which it settles back to the off-solution. In order to study possible applications, such as pulse reshaping and clock recovery, we consider the reaction of the laser to a triggering input pulse. Furthermore, we demonstrate that when the laser is subjected to injected optical noise below threshold it reacts by producing a train of pulses, whose coherence is maximal and, equivalently, the normalized jitter is minimal for a particular noise level. This shows that the Yamada model with optically injected noise constitutes an example of all-optical coherence resonance.

In this paper we report the observation of low-frequency phenomena in the behavior of a stochastic system governed by a pair of coupled ordinary differential equations, which models a directly modulated laser diode. The appearance of these fluctuations, which resembles an intermittent-like behavior, is very similar to those that have been observed in an external cavity laser diode where extensive work has been carried out recently to explain their origin. Here we show that these bursts are observed in both, the stochastic and the deterministic model of the solitary laser dynamics.

In this work, we studied the effect of structural chirping on optical bistability in (lambda) /4-shifted semiconductor distributed-feedback, (DFB) devices, such as an etalon with nonlinear mirrors, a (lambda) /4-shifted DFB waveguide, and a (lambda) /4-shifted DFB laser amplifier. We found that chirped DFB devices exhibit bistable switching at a lower input power.

This work describes a new technique to visualize the electron flow in semiconductor quantum structures using carrier drag effect. The visualization method is based on ultra-short laser pulse excitation and time-resolved micro-photoluminescence measurements at low temperatures. Here, time-integrated and time-resolved images corresponding to the electron flow in modulation-doped n-type GaAs/AlGaAs quantum wells and edge- wires are demonstrated. It is found that the relaxation time of exciton-electron scattering and exciton-lattice scattering are very long and that the relaxation time in the edge-wires are longer than those in the wells, resulting in high exciton mobility in the edge wires.

We study the impact of carrier dynamics on the characteristics of InGaAsP/InP multi-quantum well lasers through detailed simulations and experiments. The device characteristics were simulated including carrier transport, capture of carriers into the quantum wells, quantum mechanical calculation of the levels and optical gain in the wells and solution for the optical mode. The simulations were self consistent for each value of device bias. The device characteristics studied include static light-current-voltage curves, dynamic small signal impedance and the small signal modulation of the light output. The comparison between simulation and experiment constrains the capture rate for these devices. The simulations suggest that the modulation response of these devices is not fundamentally limited by the carrier transport for the frequency range studied. The trends are understood in terms of sequential transport through the multi-quantum well active region.

An electrical equivalent circuit for quantum well absorptive bistable laser diode has been developed from rate equations. The carrier transport effects are studied by simulating the circuit for dc sweep and transient conditions using circuit simulator Pspice. The hysterisis width is found to vary nonlinearly for lower values of transport time and linearly for higher values. The turn on delay increases with transport time in transient operation.

We have studied the potential profile of gateless MESFET devices using electro-optic probing. We have used smooth samples as previous work has shown how rough devices produce excessive noise generated from the Fabry-Perot effect. The profiles measured show non-linear behavior at low fields but high duty cycles. These non-linearities were more noticeable at the edges of the devices and we believe they are associated with device heating which would be prominent at the edges due to 'edge effect.' To remove this effect we have used very low duty cycles and the resulting potential profiles are as expected. Using low duty cycles and applying high electric fields allows us to study non-linear transport behavior in these devices. The samples were designed to exhibit non-linear behavior due to the Gunn Effect. At high applied electric fields the current saturates and becomes noisy, indicative of non-linear behavior. We show the first reported device field profiles under these conditions measured using electro-optic probing. The observed non-linear behavior can be explained in terms of the Gunn Effect.

We present detailed experimental studies of the physical properties of the stable high-gain modes of semiconductor lasers subject to delayed optical feedback. We demonstrate that a sufficient reduction of the linewidth enhancement factor (alpha) is an efficient concept to achieve permanent stable emission on one of these modes. In addition, we confirm major predictions of the Lang-Kobayashi rate equation model concerning the parameter dependence of this stable emission state. Furthermore, we investigate the stability of the stable modes against external perturbations. In particular, we demonstrate that the noise induced escape from the basin of attraction of the stable mode can be well understood if a potential is assumed. Finally, by applying both, external noise, and small signal modulation, we study the interplay of noise, periodic modulation, and determinism in the vicinity of the stable mode. Thereby, we obtain further information about the phase space structure around the stable mode.

We demonstrate numerically and experimentally that low- frequency fluctuations (LFF) in a laser diode subject to delayed optical feedback can be suppressed or stabilized by a second optical feedback with a short delay. The second feedback suppresses LFF by shifting antimodes far away from the external cavity modes in phase space, or by making them disappear, with the consequence that the crises that induce the power dropouts are no longer possible. Moreover, as the second feedback strength increases, the laser undergoes a bifurcation cascade with successive regions where it exhibits chaos or LFF and regions where it locks to a newly-born stable maximum gain mode. This all-optical stabilization technique is easier to implement from an experimental point of view than many existing methods since it does not require modification of any laser parameters or of the first optical feedback.

We present experimental data on the dynamic response to direct current modulation of off-the-shelf laser diode devices. We have been able to observe strong effects due to the light reflected at the fiber end, thus providing optical feedback. These effects are shown to have a major impact when the laser is close to a dynamical instability (period doubling bifurcation). We observe that around the frequency of resonance, the spectrum becomes periodic having several peaks spaced by the inverse of the external cavity round trip delay. A strong current modulation causes the laser diode to narrow this spectrum.

Possibilities are investigated for influencing the dynamical behavior of diode lasers by means of integrated passive dispersive reflectors (PDR). The specific configurations comprise DFB lasers complemented with different PDR, which consist of a phase tuning section and a passive grating section. Among others, the potential of these configurations will be investigated for generation and tuning the properties of self pulsations (SP), as e.g. the frequency and the modulation depth. Our considerations are based on the Traveling Wave Equations (TWE) coupled to carrier rate Equations. Together with numerical time domain computations of this system, a single mode approximation is applied and checked as possible tool for tailoring the dynamic effects.

Accurate modeling of photonic devices is essential for the development of new, higher performance optical components required by current and future high-bandwidth communication systems. This paper reviews one of the predominant techniques for such modeling, the beam propagation method (BPM), and describes several applications along with experimental results. BPM can be used for both mode solving and the simulation of propagation in nearly arbitrary structures, and as a result, is commonly used in commercial design tools.

Semiconductor optical amplifiers (SOA's) have been used as a key optical component for the high capacity communication systems. The monolithic integration is necessary for the stable operation of these devices and the wider applications. In this paper, the coupling technique between different waveguides and the integration of SSC's are discussed and the research results of optical devices integrated with SOA's are presented.

Noise in 1550 nm gain-clamped and conventional SOAs is studied using a detailed device model. By dividing the amplifier in M longitudinal sections were are able to account for longitudinal non-uniformity of the carrier density. We use a rate equation for locally averaged values of carriers and photons density for each section, obtaining a highly non- uniform spatial profile of carrier density for a conventional SOA, due to local saturation caused by ASE or signal photons. At least M equals 8 sections are required to accurately model the noise figure. The model is then applied to a DBR-type gain-clamped SOA, whose noise figure is studied as a function of input power and lasing wavelength. We show that changes in the spatial carrier profile caused by the input signal significantly affect the noise figure, even when the gain is constant. A new method for Gain-Clamped SOA noise figure reduction is also proposed, based on unbalanced Bragg reflectors. An improvement of noise figure as large as 2 dB is devised.

A novel structure of low-index diamond-like microprism on a single-mode symmetric Y-junction waveguide is proposed. Not only the phase-front mismatch can be compensated, but also the field mismatch can be ameliorated with the help of power pre- splitting effect. The optimal prism parameters are determined, and the results simulated by BPM also demonstrate that the transmitted loss is only 0.6 dB even though the branching angle is up to 20 degrees. Moreover, this structure takes many advantages of the simple structure, easy design procedure, being fairly insensitive to fabrication errors, and flexible selection of prism-index.

Semiconductor lasers subject to a weak external perturbation (optical injection, optical feedback) are unstable devices that generate pulsating intensities. Most of our understanding of these instabilities comes from intensive numerical simulations of simple model equations. These computations are long and delicate because the solution of the laser equations exhibits several time scales. Asymptotic methods may either simplify the laser original equations or lead to useful approximations of the solution of these equations. We illustrate these techniques by reviewing the Hopf bifurcation problem of the well known Lang and Kobayashi equations modeling a laser subject to optical feedback. Basic approximations are reviewed, a low pump problem is examined in detail, and analytical approximations of the (multiple) Hopf bifurcation line in the case of a short cavity are derived for the first time.

We use advanced techniques from bifurcation theory to examine the dynamics of single-mode semiconductor lasers with optical injection, modeled by three-dimensional rate equations. Key bifurcations, namely saddle-node, Hopf, period-doubling, saddle-node of limit cycle and torus bifurcations, are followed over a wide range of injection strengths and detunings for different fixed values of the linewidth enhancement factor (alpha) . Combining the stability diagram in parameter space with phase portraits provides a global and detailed view of complex dynamics of injected semi-conductor lasers. In particular, we concentrate here on different routes to phase locking, which can be surprisingly complicated. Our analysis reveals many regions of chaotic behavior and multistability in good agreement with experimental studies.

Chaotic dynamics have been found in a single mode semiconductor laser subject to optical injection experimentally or by numerical simulation. In this paper we study this laser system by means of rate equations, which mathematically are a three-dimensional vector field. To study different routes to chaos we start from the knowledge of bifurcation curves in the plane of injection strength and detuning in Ref. [1] of this issue. Our main tool is combining the continuation of bifurcation curves with computing the respective phase space objects. In this way, we obtain detailed knowledge of regions in parameter space of different types of chaos, and what transitions can be found at the boundaries of such regions. This gives new insight into chaotic output found in experiments. Furthermore, it allows relatively easy access to chaotic dynamics for applications such as chaotic data encryption schemes.

We propose different schemes for secure data transmission based on synchronization of chaotic semiconductor lasers. The sources are driven to chaos by light injection either from another laser or from a mirror. Synchronization is obtained by implementing a master/slave configuration, i.e., by controlled injection of the emission of one chaotic system into the other. In a first basic scheme (masking) encryption consists in hiding a message by superposition of a chaotic waveform. An alternative and better scheme (Chaotic Shift Keying) consists in the digital modulation of a suitable parameter (such as the supply current) of the master laser, which is located at the transmission end. Because of the very complex chaotic pattern of the transmitted waveform, an eavesdropper cannot detect the incoming bit stream by conventional time or frequency domain methods (such as by using filters or correlators). On the other side, decryption can be performed by the authorized listener, which owns tuned copies of the chaotic system, by implementing a suitable synchronization scheme.

Recent experiments using lasers subject to strong external injection [Simpson, Opt. Commun. 170, 93 (1999)] have demonstrated that adding a small reference current modulation to the dc-bias current can easily lock the oscillation frequency of the laser to the reference frequency. Tunable, locked outputs from 9.5 to 13.1 GHz have been obtained. We explain why synchronization is readily achieved at high injection rates. We describe the locking phenomenon in detail and derive useful analytical expressions for the frequencies and locking range in terms of the laser parameters.

In this work, the injection-detection in a laser diode is studied as a special case of coherent detection scheme. Signal waveform, amplitude and signal-to-noise ratio are calculated as functions of target distance and total optical attenuation. Several semiconductor laser sources are considered (i.e. Fabry-Perot, DFB and external cavity), and signal dependence on the physical parameters of the sources is analyzed.

Optical second harmonic generation is very sensitive to surfaces or interfaces if the bulk material is inversion symmetric. We present the results of second harmonic studies of the interface between silicon and silicon dioxide. Previous results have shown that the technique is sensitive to roughness at this interface. To develop a further understanding of why roughness influences second harmonic generation, we investigate the dependence of the second harmonic on the photon energy of the incident pulses. Measurements are made on a series of samples, with varying vicinal angles, in order to determine what role miscut and step edges play.

The adiabatic elimination of the polarization dynamics often invoked in treatments of pulse propagational and pulse interactions in semiconductor optical waveguides is critically investigated. By employing a systematic expansion of the polarization equation and staying within a third-order [(chi) ⁽³)] regime we are able to derive explicit corrections to the response function due to non-adiabatic effects. Several such effects are identified and analyzed. This provides a unified view of different results reported previously in the literature, but also identifies a new effect which apparently hasn't been analyzed so far. The effect relates to virtual excitation of the total carrier density, as opposed to the well-known adiabatic following effect of individual two-level systems, and we derive simple analytical expressions that relate this effect to the gain and index dispersion. We find it to give a sizable contribution to the well-known instantaneous effects of two-photon absorption and Kerr nonlinearity.

An InGaAsP/InP variable power splitter utilizing the multimode interference (MMI) and the linear electro-optic (LEO) effect is designed and analyzed. The splitter consists of two MMI waveguides and phase-shifting waveguide section between them. The input MMI waveguide acts as a 3-dB splitter for the TE input signal. In the phase-shifting waveguides, the relative phase of split input signals is changed by using the LEO effect. The output MMI waveguide combines the phase-modulated signals at output ports. Depending on the amount of phase change induced by reverse bias voltages of 0 approximately 2.5 V, the splitting ratio varies from 100(DOT)0 to 50:50 continuously.

Nonlinear quadratic interactions near the band edge of a finite length photonic band gap structure are studied. A strong enhancement of the nonlinear response is found due to the essential role played by the electromagnetic density of modes and phase matching conditions. This open the door to a new generation of very compact nonlinear devices. The blue and green light emission conditions are discussed.

In this paper, the experimental realization and promises of three-dimensional (3D) photonic crystals in the infrared and optical wavelengths will be described. Emphasis will be placed on the development of new 3D photonic crystals, the micro- and nano-fabrication techniques, the construction of high-Q micro- cavities and the creation of 3D wave-guides.

Further investigations on a hot electron barrier light emitter (HEBLE), which has a potential for use in the area of wavelength domain multiplexing (WDM), have been undertaken. These investigations follow the works, which were presented over the last two years in Photonics West 98 and 99. The structure of the device is based on Al_xGa_1-xAs-GaAs system with a single quantum well of GaAs. The novelty of the device is how it is operated. Unlike a normal light emitter, HEBLE has a barrier between the n-doped region and the quantum well, which prevents the electrons flooding into the quantum well, hence no light output, when it is forward biased. To obtain the light output, not only the device must be forward biased, but also the n-doped region must be electrically heated, so the electrons will have enough energy to overcome the barrier and move into the quantum well. The devices have been fabricated following the results of the computer simulation, which was presented last year in Photonics West 99. The results of these devices, which have been studied in many aspects including spectral analysis of the light output at different temperatures, will be presented.

We consider a p⁺nn⁺-heterodiode with an additional nn-heterojunction in the n-base that plays a role of the base/collector junction. Such a device can be a very effective light-emitting phototransistor or transistor. A depletion layer of the added intermediate heterojunction splits the diode n-base into two new bases: wider-bandgap phototransistor (transistor) base 1 and narrower-bandgap light emitting base 2 that serves simultaneously as the collector region of the phototransistor (transistor). A coefficient of light-to-light conversion (of an input light absorbed in base 1 into an output light emitted from base 2) can be very high due to very photo transistor gain. Effects of avalanche multiplication on the depletion layer of the intermediate heterojunction, edge multiplication in an accumulation layer of the same junction, and thermionic emission across a conduction band discontinuity barrier are taken into account. This discontinuity barrier can be high enough (> 0.5 - 0.7 eV) for effective implementation of the suggested transistor, phototransistor and light-to-light converter. We discuss various material systems appropriate for implementation of these devices (namely InAs/GaSb/AlSb-system and InGaAs/InAlAs/InP-system). Criteria of stability of the described transistor regime are considered, and transition to a thyristor (light emitting photothyristor) regime is discussed.

A few major schemes for all-optical switching/modulation is considered and the comparison of the values of nonlinear index change required for the device operation is presented. Each design is assumed to be optimized to meet the requirement for insertion losses of 1 dB (about 80% transmission or reflectance depending on the geometry of the device) and the switching contrast of 10 dB. A number of numerical results of transmission and reflectance in those devices are proposed in this paper. The advantage of the design of optimized integrated optical switchers/modulators with losses include extremely low switching refractive index change (delta) n. In addition, we suggest one new optimized design based on Bragg grating.

We present numerical analysis of traveling-wave (TW) multiple quantum well (MQW) electro-absorption modulator which can be used for wide-band applications, covering DC to 30 GHz or higher frequencies. Considerations in design of TW modulators are microwave characteristics such as, waveguide attenuation and phase velocity matching between guided lightwave and microwave. In this study, we simulate a 1.3 micrometer InGaAs/InGaAsP TW MQW EAM using the 3D Finite Difference-Time Domain method, and investigate frequency dependent parameters by using the Fourier transform for analysis of microwave characteristics in detail. We identify that as the distance between signal and ground electrode increase, the characteristic ridge-type TW EAM change from planar CPW to that of microstrip structure. It is believed that our calculated data provide useful information to optimize and fabricate ridge-type TW CPW EAM.

The use of microwave semiconductor devices as photodetectors or optically controlled circuit elements have attracted growing interest. We have systematically characterized the optical response of p-channel pseudomorphic MODFET as a function of the drain voltage, gate voltage, and optical power of the illumination. Physical mechanisms responsible for the variation of the device characteristics due to the optical illumination are discussed and analytic models are developed for strong non-linear behavior of the threshold voltage and the photoresponsivity with the optical power of the illumination.

Strained layer superlattices based on Sb semiconductor material are proposed for long wavelength photodetector applications. Theoretical studies on the band structure of AlGaSb/GaSb, AlInSb/InSb, and GaInSb/InAs superlattices show that the wavelength coverage can be extended from 0.5 to 30 micrometer. Optical absorptions of the superlattices are calculated taking into account the intermixing effect at different diffusion lengths. Responsivity and detectivity of the GaInSb/InAs superlattice detector are also analyzed. Blue shift of responsivity is observed for increased intermixing and the detectivity D^* at 77 K is increased by more than one fold in magnitude as R₀A increases linearly with intermixing.

Stable mode-locked pulses with a repetition rate of 10 GHz were generated from an Er-doped fiber laser with a semiconductor optical amplifier in the cavity. By tuning the current of the semiconductor optical amplifier, complete harmonic mode-locking was obtained and the supermode noise in the RF spectrum of mode locked laser was removed for certain current range of the semiconductor optical amplifier.

We investigate, both analytically and numerically, the effectiveness of the nonlinear gain to suppress the background instability in bandwidth-limited soliton transmission. Different types of analytical solutions of the complex Ginzburg-Landau equation (CGLE), namely solutions with fixed amplitude and solutions with arbitrary amplitude, are discussed. The conditions for the stable pulse propagation are defined within the domain of validity of the soliton perturbation theory. The CGLE is solved numerically assuming various input waveforms with different phase profiles, amplitudes and durations. Relatively stable pulse propagation can be achieved over long distances by the use of suitable combination of linear and nonlinear gains. For the cubic CGLE, truly stable propagation of arbitrary amplitude solitons can be achieved in a system with purely nonlinear gain. A new soliton compression effect is demonstrated both for fixed- amplitude and for arbitrary-amplitude solitons. This compression can be particularly significant when the system parameters are chosen near the singularity of the fixed- amplitude solution.

The four wave mixing (FWM) process in nonlinear media can be used for all optical demultiplexing of high bit-rate optical time division multiplexed channels. Interaction of the optical time division multiplexed signal and a pulsed pump with a repetition frequency equal to the desired output bit-rate, results in generation of a new wave at a new wavelength which carries the information of one of the de-multiplexed channels. Dispersion shifted fiber (DSF) is an adequate nonlinear medium in which the four wave mixing process takes place. In this work a detailed theoretical study of an all-optical demultiplexer based on four wave mixing in dispersion shifted fibers is presented for different demultiplexing input/output bit-rates. The four wave mixing process in dispersion shifted fibers is studied through numerical simulation of the non- linear Schroedinger equation, taking into account all fiber nonlinearities. The performance of the demultiplexer is characterized in terms of efficiency, Q-factor, suppression of adjacent channels and eye pattern for each de-multiplexed channel. These characteristics are studied for different fiber lengths, pulsewidths, powers, etc. This detailed characterization of the operational conditions of the demultiplexer will reveal its limitations and hints for its optimal design will be proposed.

We propose a new modeling structure for long period fiber gratings. The proposed model has the multiport orthogonal FIR (finite impulse response) lattice filter configuration. Our multiport lattice filter model can be used to analyze (or to synthesize) phase-shifted long period fiber gratings, cascaded long period fiber gratings, and piecewise-uniform long period fiber gratings. The validity of the proposed model has been confirmed by comparing the calculated spectrum curves with the measured ones.

Nanoscale coherent insertions of narrow gap material in a single-crystalline matrix, or Quantum Dot (QD) provide a possibility to extend the basic principles of heterostructure lasers. The idea to use heterostructures with dimensionality lower than two in semiconductor lasers appeared a quarter of a century ago, simultaneously with the proposal of a quantum well laser. However, fabrication of quantum wire- and, particularly, QD heterostructure (QDHS) lasers appeared to be much more difficult. The breakthrough occurred when techniques for self-organized growth of QDs allowed fabrication of dense arrays of uniform in shape and size coherent islands free from undesirable defects. Recently, some key parameters of QD lasers were significantly improved as compared to those for QW devices. High-power operation, record low threshold current densities, strongly reduced chirp and extension of the wavelength range on GaAs substrates up to 1.3 micrometer range were demonstrated. It also became clear that unique properties of QDs may give rise to a new generation of semiconductor lasers, such as far and middle infrared light emitters based on interlevel electron transitions in QDs or single quantum dot vertical-cavity surface-emitting lasers.

Using molecular beam epitaxy we have developed GaInAs/GaAs quantum dot heterostructures by self organized growth for applications in distributed feedback (DFB) lasers with optimized high temperature performance and for single dot spectroscopy. By using a single active layer, lasers with low- threshold current densities (J_th equals 144 A/cm² for a 2 mm long device) and high internal quantum efficiencies (greater than 90%) were obtained. Ground-state lasing of the quantum dots was observed up to device temperatures of above 200 degrees Celsius. By the combination of ridge waveguide structures with lateral metal gratings complex coupled DFB lasers have been obtained. Threshold currents of 14 mA, differential efficiencies of 0.33 W/A and sidemode suppression ratios of over 50 dB have been achieved. Monomode operation was observed for temperatures from 20 to 213 degrees Celsius. This is the largest temperature range over which the operation a DFB laser has been reported up to now. By etching small mesa structures (typical dimension 100 nm X nm) single dots have been isolated from the dot layers. Single dot spectroscopy provides information on exciton and biexciton properties including e.g. the biexciton binding energy, the Zeemann splittings, polarization anisotropies etc. From optically active transitions of both, bright and dark excitons we determine values for the electron (e) and hole (h) g factors. Furthermore we determine the X singlet-triplet splitting which is found to be enhanced over bulk values by about an order of magnitude due to the increase of the e-h overlap in the QD's.

Quantum dots laser diodes using the dots-in-a-well (DWELL) structure (InAs dots in an InGaAs quantum wells) have exhibited significant recent progress. With a single InAs dot layer in In_0.15Ga_0.85As quantum well, threshold current densities are as low as 26 A cm^-2 at 1.25 micrometer. Quantum dot laser threshold current densities are now lower than any other reported semiconductor laser. In this work, the threshold current density is reduced to 16 A cm^-2 by HR coatings on the same device. Further investigation of performance reveals that use of multiple DWELL stacks improves the modal gain and internal quantum efficiency. It is suggested that carrier heating out of the quantum dots limits the T_O value of these DWELL lasers.

Theoretical study of threshold characteristics of a quantum dot (QD) laser in the presence of excited-state transitions is given. The effect of microscopic parameters (degeneracy factor and overlap integral for a transition) on the gain is discussed. An analytical equation for the gain spectrum is derived in an explicit form. Transformation of the gain spectrum with the injection current is analyzed. The threshold current density is calculated as a function of the total losses. The conditions for a smooth or step-like change in the lasing wavelength with the losses are formulated. Threshold characteristics of a laser based on self-assembled pyramidal InAs QDs in GaAs matrix are simulated. A small overlap integral for transitions in such QDs (and hence large spontaneous radiative lifetime) is shown to be a main possible reason for a low value of the maximum single-layer modal gain of the respective structure which is deficient to attain lasing at moderately short (several hundreds of micrometers) cavity lengths.

Electron beam pumped surface emitting lasers are of great interest for a variety of applications, such as Laser Cathode Ray Tubes (LCRT) in projection display technology and high power UV light sources for photolithography as an application of nitride emitters. Two distributed Bragg reflector (DBR) samples were grown by plasma assisted molecular beam epitaxy (PAMBE). The active regions of the samples are a GaN bulk layer and multihetero (MH) structure, respectively. Also, a separately grown single DBR stack was studied to find optical transmission and reflection properties which were compared to transfer matrix simulations. Scanning electron beam pumping at 80 K was performed on the two vertical cavity structures with an excitation energy of 40 keV in order to characterize the influence of the distributed Bragg reflectors on the resonator properties. Surface emission spectra measured for various electron beam currents revealed luminescence emission maxima located at about 3.45 eV at 80 K for the sample with the MH structure active region. Optical modes appeared for excitation powers greater than 0.85 MW/cm². Further increasing the excitation power density the number of modes increased and a broadening and redshift of the luminescence spectrum could be observed. Based on our experimental results, we discuss the dependence of optical parameters of the nitride vertical cavity and sample surface reactions on primary electron beam power.

Radiation coupled mode theory with first order perturbation is used to investigate the optimum shape and depth in trapezoidal gratings (which includes rectangular and triangular cases) for radiative and nonradiative applications. Optimum trapezoidal grating shape for nonradiative purposes is obtained for both DFB and DBR lasers. For radiative purposes Optimum triangular grating are obtained for DBR lasers.

Performance parameters of tapered distributed feedback (DFB) and distributed Bragg reflector (DBR) lasers are compared with earlier configurations, such as uniform grating DFB, (lambda) /4 phase-shifted DFB and uniform grating DBR lasers. Parameters such as oscillation conditions (above and below threshold), mode pattern along the laser length, output power, effects of nonzero end reflections on oscillation conditions, yield of the gain, linewidth an differential quantum efficiency are obtained for tapered DFB and DBR lasers. To find oscillation conditions a nonlinear model is used, which considers gain saturation due to longitudinal field intensity variation along the laser. Grating shape is of rectangular type. For tapering profile, a class of polynomial curves of hyperbolic form with a degree of freedom is used. For the analysis F-matrix approach is applied. For purpose of comparison, a constant effective coupling coefficient for all structures is defined. Some new important results have been obtained which show that, tapered DFB lasers are superior to phase shifted DFB lasers. Improvement of tapered DBR over uniform DBR depends on the type of performance parameter.

Quaternary semiconductor alloy material systems for applications as highly efficient Distributed Bragg Reflectors were investigated for two operating wavelengths of 1.3 and 1.55 micrometer. Based on the calculations of the material's energy bandgaps and indices of refraction for the entire composition range and for the corresponding incident wavelength, four quaternary alloys have been selected from which the refractive index difference between two adjacent layers of 0.65 has been obtained. All quaternary alloys were lattice matched to InP substrate. The reflectivity calculations show that for the DBR with a large index of refraction contrast, 16 pairs of layers would result in a reflectivity of 99%. The resulting monolithic VCSEL structure would consist of fewer DBR layers, interface roughness would be reduced, and, therefore, reliability of the device would be improved.

The GaInAsP/InP device described in this work consists of an InP p-n junction with a GaInAsP quantum well placed on the n- side within the depletion region. This device is designed for 1.5 micrometer emission. Light emission is independent of the applied voltage polarity, and the device acts as an XOR optical logic gate. One potential application for this device is as a low cost VCSEL for optical access networks, since two diffused-in point contacts are used for longitudinal biasing. Hence, the current is injected directly into the active region without having to pass through the Distributed Bragg Reflectors (DBRs). Experimental results concerning the temperature dependence of photoluminescence and electroluminescence spectra, and light field characteristics are compared with model calculations. These include self- consistent numerical one-dimensional solutions of the Poisson and Schrodinger equations. We also studied the emission wavelength as a function of position of the GaInAsP quantum well within the built-in electric field of the InP p-n junction. The calculated overlap of the normalized electron and hole wavefunctions is in good agreement with the experimental results.

The generation mechanism of self-sustained pulsation in vertical cavity surface emitting lasers is analyzed theoretically. The influence of self-focusing, diffraction loss as well as optical feedback is taken into consideration. It is shown that the condition of self-sustained pulsation can be modified significantly by optical feedback. In addition, it is possible to obtain self-sustained pulsation in vertical cavity surface emitting lasers by using external mirror.

We present the results of our studies concerning temperature dependence of photoluminescence (PL) in Ga_xIn_1-xAs_{1- y}N_y/GaAs single quantum wells. Our results at low temperatures indicate the presence of a high density of compositional and/or structural disorder and hence poor PL efficiency, common to as-grown GaInAsN material. We show, however, that the optical quality of GaInAsN can be improved while achieving a red shift in the spectra. This is unlike the results obtained by rapid thermal annealing (RTA) or conventional annealing, which are widely employed as post- growth treatment techniques, where any increase in the PL intensity is almost always accompanied by an undesired blue- shift.

We study the effects of microwave loss and device length on the nonlinear characteristics of intensity modulation response in 1.55 micrometer traveling-wave coplanar waveguide InGaAsP bulk electro-absorption modulator. By using device length segmentation scheme optical transmission curve reflecting the change of the electro-absorption effects at each segmented position due to the microwave loss is obtained, and then the intermodulation distortion and spurious free dynamic range characteristics of RF signal are analyzed. Device length decreases V_b3, which is the bias voltage minimizing the third order and increases the third order intermodulation distortion (IMD3). On the other hand, microwave loss increases V_b3, and reduces IMD3.

The effects of carrier cooling on the saturation characteristics of quantum well absorbers are theoretically investigated. A comprehensive many-body model is employed, and both excitonic effects and bandgap renormalization are shown to play a significant role. The detailed interplay between band-filling and many-body effects at low carrier temperatures is shown to lead to saturation characteristics that are highly spectrally dependent, and that cannot in general be identified by a linear saturation characteristic. In addition, a spectral regime of enhanced fast saturation is identified at or slightly below the exciton peak energy, which may provide a mechanism for previously observed mode-locking behavior.

1.55 um PnpN optical thyristor as a smart optical switch has potential applications in advanced optical communication systems. It can be used as a header processor in optical asynchronous transfer mode (ATM) and as a hard limiter in optical code division multiple access (CDMA) system. For those applications, however, relatively slow switching speed of the optical thyristor is the major limiting factor. To enhance the switching characteristics, depleted optical thyristor (DOT) has been proposed, in which majority carriers in the center n- and p-layers can be fully depleted by applying a reverse-bias pulse. Recently, we proposed a novel waveguide type 1.55 micrometer InGaAsP/InP DOTs. In this presentation, using the finite difference method (FDM), we calculate the effects of such parameters as doping concentration, thickness of the outer and inner layers of the thyristor to find out the optimized structure in the view of fast and low power consuming operation, low reverse full-depletion voltage, high optical confinement factor. With these results, a waveguide type 1.55 micrometer DOT is fabricated with metal organic chemical vapor deposition (MOCVD) and measured on the switching voltage with the size. The results of the simulation are compared with those of the experiment.

Recent improvements of plastics optics performance and manufacturing technology of plastics lenses have resulte din rapid applicationfo plastics otical elements. On eof the factors indelyaing the applications of the plastics oticalelements is the existence of birefringence in palstics lenses. It gives us a challenge to resolve th proboem so that the plastics oticalelements can achieve much higher levels of performance. It is generally recongized that the mechanism of birefringence generation is relevant to the resin behaviors during the injection molding process. If this mechanism is fully understood by flow analysis, it may be a great contribution to the fabrication of plastics optical elements. However, convetnioanl two-dimensional flow anlaysis on onjection molding fails to graps th ephenomena of birfringence. in this paper, we have successfully indentified some phenomena on onjection molding that are closely related to the gneratioof birefringence.We anlayzed th ephenomean in detial sin filling pakcing, and colling processes durin th einjection modlin process using tthree-dimensional CAE system, called 3D TIMON. The anlayzed rsults were confirmed experimentally and theyenabled us to predict the generation of birefringence by CAE analysis.

In long distance high bit rate optical links, the combined effect of fiber dispersion, nonlinearities and polarization mode dispersion determines the system performance. Mid-span spectral inversion (MSSI) has been proposed for dispersion and nonlinearity compensation. Although the use of spectral inverters in more than one point along the optical link has been discussed in the past, their impact on the transmission performance has never been examined in a detailed and quantitative way. In this work a comparison of the transmission performance when using MSSI or multi-point spectral inversion (MPSI) is presented, for 10- and 40- Gb/s signal transmission and for various transmission conditions. The transmission is described by the non-linear Schroedinger equation and the performance is estimated through the Q-factor analysis of the received optical signal. The comparison starts assuming ideal inverters where the complex conjugate of their input signal is generated at the output. In this case, the MPSI method appears to be more effective in compensating the dispersion and the nonlinearities, allowing highly performing transmission. The results are different when a real spectral inverter based on four-wave mixing in highly nonlinear dispersion shifted fibers is considered. In this case no improvement in the transmission performance can be observed.

A Split-Step Reconstruction Technique (SSRT) is proposed to enhance the computational accuracy of FMAT. The main advantage of using SSRT is that the propagation error due to the split- step iteration approach can be eliminated. In addition, an adaptive mesh control algorithm, with which the allocation of mesh size depends on the gradient of soliton, is utilized to improve the computational efficiency. It is shown that the calcuation error can be reduced significantly when compared with the FMAT.

InP-based 1.6 micrometer microcavity light emitting diodes (MCLEDs) have been designed, fabricated and characterized. Oxide-confined MCLEDs with lateral aperture size down to 1 micrometer have been realized with enhanced output slope efficiency and excellent spectral and spatial properties. Both mirror-free cylindrical MCLEDs with oxide-aperture and 3D MCLEDs have been fabricated and a maximum output power of 0.03 mW at 30 mA for a 30 micrometer device has been realized. The effect of the aperture size on the slope efficiency and spatial properties have been systematically investigated, which shows an enhanced slope efficiency and narrower emission angle for small aperture size devices. The temperature dependent cavity properties have also been carefully examined for both the active region emission peak and cavity resonance peak.