We discuss the methods and results of the temporal, spatial and spectral modeling of doped fiber amplifiers. Particular emphasis is placed upon energy extraction with single pulse propagation.

The effective index method has been used to calculate eigenmodes of A1GaAs optical waveguides to determine optimal process parameters for their fabrication and to compare the advantages and disadvantages of several different fabrication technologies for the integration of waveguides and lasers.

For planar multilayer waveguides with multiple quantum wells (MQW), integral formulas for the optimal refractive index when the MQW are replaced by a single uniform layer are derived by considering linear perturbations of the fields and propagation constants for solutions of the wave equation. These integral formulas simplify to familiar discrete averages if the spatial variations of the fields over a period of the MQW are small. The discrete average result for TE modes is applied to a waveguide with a single MQW guiding region and the relative magnitude of the errors in the power distribution and propagation constant for symmetric and antisymmetric modes are explained.

A wavepacket propagation method is presented for analyzing perpendicular-field electroabsorption in quantum wells. The method evolves the pair envelope wavefunction for electrons and holes under Coulomb interaction and an applied electric field by a split-step algorithm. The power of the method is demonstrated by calculating ground-state properties, excitonic spectra, and certain features of electron-hole quantum dynamics in coupled double quantum wells (CDQW). First, an initial wavefunction tailored to the ground state is evolved in imaginary time and used to compute pair energies, exciton binding energies, and pair probability distributions. Second, an initial wavepacket tailored to the absorption spectrum is evolved in real time and used to record autocorrelation functions, mean positions of electrons and holes, and the excitonic spectra from the Fourier transform of the autocorrelation function. It is shown that the experimentally observed characteristics of the quantum-confined Stark effect are reproduced, and that significant migration of carriers occurs during the formation of the field-induced dipole moment.

We demonstrate that quantum-mechanical (or optical) path integrals for both time-invariant and time-varying potentials corresponding to longitudinally invariant and longitudinally varying refractive index distributions can be evaluated by a new set of accurate propagation procedures based on expressions for the exponential of any number of noncommuting operators as the product of exponentials of the individual operators.

We review our photoluminescence results concerning the structural disorder of the interfaces in GaAs/AlAs quantum well structures. In the highest quality samples structural disorder exists as monolayer—high islands. We show the types of possible luminescence spectra which can occur in quantum wells in which the bottom and top interfaces have differently-sized islands. It is shown how luminescence spectra are sensitive to both large and small island structures —whether they occur on the same interface or occur separately on the top and bottom interfaces.

We review recent theoretical studies of the symmetry properties of charge injection transistors. These studies, based on continuation modeling and transient device simulation, incorporate self-consistently the electron energy and real-space transfer currents over heterojunction interfaces. Inspection of the full device phase—space reveals a variety of instabilities and a striking novelty of multiply-connected current-voltage characteristics. We have found anomalous steady states in which hot-electron injection occurs in the absence of any voltage between the emitter electrodes. In these states, some of which are not only stationary but also stable with respect to small perturbations, the electron heating is due to the fringing field from the collector electrode. Some of the anomalous states break the reflection symmetry in the plane normal to the channel at midpoint. The study elucidates the formation of hot-electron domains in real-space transfer.

In our previous work, we reported the optimum design for 10 km and 50 km length fibers for a distributed erbium doped fiber amplifier(DEDFA), based on a comprehensive computer model. This design contained an erbium doping concentration of 20 - 22 parts per billion, V of 1.775 with a signal wavelength of 1555 nm using low signal powers, -30 dBm, and bidirectionally pumping at 1480 nm. This paper investigates the effects of operating a DEDFA at the optimum doping concentration with large input signal power in which the ASE has a greater effect on the overall performance. The final issue addressed is the noise figure under similar circumstances.

We theoretically investigate phase modulators based on the quantum-confined Stark effect. Our analysis incorporates electronic wavefunctions derived from the transfer matrix method in the effective mass approximation and is therefore more accurate than variational procedures. To describe exciton broadening we include phonon scattering and the electric-field induced tunneling out of the quantum well. Our results model correctly most characteristics of the measured refractive index change.

In a single quantum well with no applied electric field, the typical exciton lifetime is subnanosecond. Excitons do not have time to reach thermodynamic quasi-equilibrium before recombining. Quasi-equilibrium requires both an average exciton kinetic energy of kT and thermalized occupation of the inhomogeneously broadened exciton density of states. In a coupled double quantum well under an electric field, lifetimes of the `indirect' excitons (i.e., electrons and holes mostly confined in different wells) are substantially longer, typically in the range 10 to 100 nsec. We have measured cw and time-resolved photoluminescence and photoluminescence excitation spectra as a function of temperature from a symmetric coupled double quantum well. The double wells consisted of 50 angstrom GaAs wells separated by a 40 angstrom Al_0.3Ga_0.7As barrier, and an electric field of 30 kV/cm was applied across the wells. The indirect exciton lifetime was measured to be 40 nsec for temperatures between 2 K and 30 K. Direct analysis of the data (discussed in detail in Refs. 3 and 4 shows that: (1) Below 6 K, the excitons are metastably trapped in spatial domains which are local energy minima; thermodynamic quasi-equilibrium is not achieved within the exciton lifetime. (2) Raising the temperature above 6 K increases the exciton kinetic energy and allows the excitons to be thermally activated out of these local minima; thermodynamic quasi-equilibrium is achieved. (3) The quasi-equilibrium occupation of the exciton density of states is very well approximated by a Fermi-Dirac distribution. This last observation is particularly noteworthy, in that excitons are bosons and there have been predictions of Bose-Einstein condensation in this system. Because of the electric dipole of the indirect excitons in coupled wells, there is a repulsion between the excitons. This repulsion is strong enough to cause the excitons to avoid each other, but sufficiently short ranged to be neglected for spatially separated excitons. Since the different energies in the inhomogeneously broadened density of states correspond to spatially different positions in the well, this exciton-exciton interaction can be modelled using non-interacting thermodynamic statistics but using a Fermi-Dirac distribution to approximately account for the repulsion.

The mobility of electrons and holes is calculated in silicon as a function of temperature and the concentration of impurities. Calculations are done for both majority and minority carriers. Special care has been taken in the calculation of the contribution from impurity scattering. Both the dielectric function, and the local field corrections, have been calculated as a function of temperature and impurity concentration. The results agree with the data at low temperature, and at high doping at room temperature.

For many years it has been assumed that nonradiative recombination plays a dominant role in determining the high temperature performance of long wavelength laser diodes. We show that this view is inconsistent with the measured temperature dependence of spontaneous emission from light emitting diodes. We conclude that net gain primarily determines the temperature sensitivity of threshold in long wavelength semiconductor lasers.

Two independent low-frequency noise sources originating from inhomogeneous current flow in thin dielectric films were revealed in thin film structures. One of them is connected with the existence of local leakage channels while the other is due to the modulation of the injection current by processes of charge capture and release by traps inhomogeneously distributed in the contact region. Physical models were developed which accounted for the high level of noise power and its frequency dependence.

Ten years ago Normandin and Stegeman demonstrated a novel surface harmonic generation geometry but the cross sections were too low for applications. Using periodic multilayers in the waveguide, cross sections were increased by over a million. Theory refinements and new results are presented for large angle light deflectors, integrated intracavity doubling visible lasers, and high resolution (< 10 GHz) optical spectrometers. Applications in display, telecommunication, interconnect, and coding are also discussed.

Interactions of sidemodes in a semiconductor gain medium in the presence of an oscillating mode are studied both for beat frequencies comparable to interband relaxation rates as well as those comparable to intraband relaxation rates. The former applies to the complete many-body theory, provided the strong mode Rabi frequency is small compared to the intraband relaxation rates. Population pulsations play an important role in both cases, with spectral hole burning playing an equally important role for the large beat frequency case.

A new model for the small signal intensity modulation response is described which utilizes a new analytic description of the quantum well laser based on the concept of a stimulated lifetime. The new analysis is able to qualitatively predict the modulation response of quantum well lasers without the concept of non-linear gain. The analysis uses the quasi-Fermi level separation as a parametric variable to couple the electron and photon rate equations. In particular, the Fermi level allows the effects of the diode losses to be an integral part of the small signal transfer function. It is shown that by including all the current components in the electron rate equation, the modulation response of the laser is fundamentally limited by the diode carrier lifetime. This is a consequence of the less than unity value of the electrical confinement factor within the quantum well. The recombination and diffusion current components outside the well are shown to be the cause of the increased damping of the laser resonance as the bias applied to the laser is increased.

We use a comprehensive computer model to evaluate the performance of erbium doped optical power amplifiers (EDFA) in the 800 nm band. The pump wavelength dependence of the amplifier gain and noise is considered for aluminophosphate (APS-EDFA) and fluorophosphate (FP-EDFA). The existence of band optimum length, where the output remains constant and close to the peak output for a pump band, has been shown for both APS-EDFA and FP-EDFA under saturated conditions. It has been shown that band optimum length exists for both APS-EDFA and FP-EDFA unlike in the case of small signal gain, where the BOL existed only for APS-EDFA. The influence of the signal excited state absorption (ESA) at various pump powers and wavelengths is studied. The performance of power EDFAs with a pump reflecting mirror has been compared with bidirectional pumping and co-directional pumping for both APS-EDFA and FP-EDFA. For small pump powers (P_p approximately equals 10 mW) the configuration with pump reflecting mirror is better than bi-directional pumping, both in terms of large signal gain and noise figure (NF) for APS-EDFA and FP-EDFA. At large pump powers (P_p approximately equals 200 mW) the output signal powers are comparable and the NF is better for the configuration with the reflecting mirror for APS-EDFA. The signal gain and noise performance is better in the case of FP-EDFA even at P_p equals 200 mW.

The average energy loss rate (AELR, in an ensemble sense) of hot electrons via interactions with quasi-equilibrium electron-only and electron-hole plasmas in bulk GaAs is calculated exactly, within the random phase approximation (RPA). Results are presented for plasma densities ranging from 4 X 10¹⁵ cm^-3 to 1 X 10¹⁹ cm^-3, and for temperatures ranging from 100 K to 300 K. Comparisons are made with simpler, static screening and plasmon pole approximations. Using the full RPA result, a critical plasma density is obtained beyond which the AELR decreases. A quantitative analysis of this behavior shows that it is due to both the Pauli exclusion principle, and to a blocking mechanism that occurs when plasmon emission processes become kinematically forbidden.

A quantum well resonant cavity structure is modelled using energy balance considerations by computing the Poynting vectors across the absorbing region. The quantum efficiency (eta) is shown to yield terms that depend on the Q of the cavity, the position of the quantum wells in the structure, and the relative reflectivities of the two stacks at either end of the cavity. The bandwidth B.W. of the detector on the other hand depends only on the cavity Q. Thus an optimum (eta) X B.W. product can be found. Experimental results corroborate the model.

The temperature dependence of photodiodes quantum efficiency for different processes (deep diffused and ion implanted) and resistivities (10 and 100 (Omega) .cm) were measured. To better predict their behavior, a comparison was made with a simple uni-dimensional p-n junction model. This includes band-gap, depletion region width, diffusion constants, mobilities, intrinsic carrier concentration, absorption coefficient, and refractive index temperature functions. The surface recombination length of the minority carriers and the concentration of recombination centers were fitted to the experimental data.

Waveguide parameters for Pr⁺-doped fluoride fiber amplifiers have been optimized for low and moderate values of relative refractive index difference. A cut-off wavelength of 750 nm gives maximum gain for NA equals 0.1 and 0.2. A pump reflecting mirror etched at the output end of the amplifier gives higher gain at smaller lengths (approximately equals 1/3) in both the small and large signal regimes.

A comprehensive analysis of noise in rare-earth doped fiber optical amplifiers has been carried out using both the minimum noise figure (NF) and the noise figure with amplified spontaneous emission (ASE) noise spectral density. According to these different analytical methods, we have analyzed the noise performance for different applications. We found that the method calculating the noise figure with ASE spectral density gave more accurate correlation with experimental results. Furthermore, we confirmed that the NF in erbium-doped fiber amplifiers (EDFA), with pumping at 980 nm wavelength, is 2 dB less than that at 1480 nm, and the 3 dB quantum limit can also be obtained. The noise figure in concatenated EDFAs is approximately 8 dB after transmission through 9000 km. the noise figures in cascaded amplifiers, with the signal-spontaneous beat noise dominating, are almost the same for several stages. The noise figure in a distributed amplifier is more sensitive to the doping concentration. The 3 dB quantum-limited noise figure can also be obtained in a praseodymium-doped fluoride fiber optical amplifier operating at a signal wavelength of 1.3 micrometers .

Recombination via deep level centers within a delta doped sheet in GaAs diodes is investigated as a function of deep level energy and local sheet position within the junction. Current-density versus voltage (jV) characteristics of pn-junctions are calculated varying the sheet position and energy of a single level, and a double level deep center (Ti). With homogeneous photogeneration, the changes in open circuit voltage V_oc and fill factor of photodiodes are determined as a function of such delta doping. Schottky barriers on MBE-grown GaAs with a planar thin Ti doped sheet are examined by capacitance-voltage (CV) measurements. The corresponding charge profiles N(W) are compared with modeling results.