Novel Closed-Form Model for Multiple-State Quantum-Dot Semiconductor Optical Amplifiers

Novel closed-form model for multiple-state quantum-dot semiconductor optical amplifiers (QD-SOAs) is derived. The model takes into account the effect of the ground state, excited state and the wetting layer. The model is simple, accurate and exhibits negligible computational time compared with numerical simulation. In addition, the derived model is valid for arbitrary applied current and input photon density and is interesting for device design and optimization. Analytical expressions for the optical gain, effective saturation density, maximum output density and the transparency current are also derived. Our model revealed that the effective saturation density of QD-SOAs strongly depends on the photon density and the applied current.     

Numerical Analysis of Gain Saturation, Noise Figure, and Carrier Distribution for Quantum-Dot Semiconductor-Optical Amplifiers

The gain saturation behaviors and noise figure are numerically analyzed for quantum-dot semiconductor optical amplifiers (QD-SOAs). The carrier and photon distributions in the longitudinal direction as well as the photon energy dependent facet reflectivity are accounted in the rate equations, which are solved with output amplified spontaneous emission spectrum as iterative variables. The longitudinal distributions of the occupation probabilities and spectral-hole burning are presented for electrons in the excited and ground states of quantum dots. The saturation output power 19.7 dBm and device gain 20.6 dB are obtained for a QD-SOA with the cavity length of 6 mm at the bias current of 500 mA. The influences of the electron intradot relaxation time and the QD capture time on the gain spectrum are simulated with the relaxation time of 1, 30, and 60 ps and capture time of 1, 5, and 10 ps. The noise figure as low as 3.5 dB is expected due to the strong polarization sensitive spontaneous emission. The characteristics of gain saturation and noise figure versus input signal power for QD-SOAs are similar to that of semiconductor linear optical amplifiers with gain clamping by vertical laser fields.     

Effect of Doping on the Optical Characteristics of Quantum-Dot Semiconductor Optical Amplifiers

The influence of p-type and n-type doping on the optical characteristics of a quantum-dot semiconductor optical amplifier (SOA) is studied using a rate equation model that takes into account the effect of the multidiscrete energy levels and the charge neutrality relation. Our calculations show that the amplifier optical gain can be greatly enhanced through p-type doping where the doping concentration should not exceed the certain level. We find that increasing the acceptor concentration increases the unsaturated optical gain but at the same time decreases the saturation density and the effective relaxation lifetime. Also our calculation reveals that the use of p-type doping will be associated with an increase in the transparency current where the increase in the transparency current depends on the incident photon energy. On the other hand, we find that it is possible to increase the saturation density and enhance the linearity of the SOA by using n-type doping.      

双抽运光参量放大的增益特性

在相同条件下(如抽运光功率、信号光功率等),与单抽运相比,双抽运光参量放大(OPA)的增益大,增益带宽宽.由于信号光的增益特性与参量增益系数直接相关,因此,在建立高非线性光纤(HNLF)光参量放大的理论模型的基础上,对相位匹配条件、参量增益以及信号光饱和增益的特性进行了分析讨论,采用有限差分法,对增益饱和的特性进行了模拟.结果表明,参量增益和光纤参数、输入抽运光以及信号光的波长、功率有关,而信号光的饱和增益和输入抽运功率有关.     

Correlation of theory with experimental SOAs

Predictions for the near-traveling wave amplifier (NTWA) with multiple-quantum-well structures have been developed. The continuity equation for quantum wells (QWs) with high carrier densities is combined with the amplifier TW gain equation expressed in terms of stimulated lifetime. This formulation allows the signal gain to be related to the bias current and the optical input signal through Fermi energies. The charge neutrality condition also plays an important role for high carrier density QW amplifiers. Auger recombination and heating effects are incorporated as essential components of the model. Experimental measurements of gain versus bias current and output power for both /spl lambda/= 850- and 1550-nm devices are found to be very well matched by the calculated results.     

Theoretical study of device sensitivity and gain saturation ofseparate absorption, grading, charge, and multiplication InP/InGaAsavalanche photodiodes

In this paper, we present a theoretical investigation into the performance of a separate absorption, grading, charge, and multiplication InP/InGaAs photodiode (SAGCM) using a two-dimensional drift-diffusion model. The analysis examines the sensitivity of the device performance to variations within the overall geometry of the photodiode. Specifically, we explore modifications to the p⁺ diffusion at the surface of the device, the thickness of the multiplication region, and the relative doping within the mesa charge sheet. It is found that variations within the design specifications may lead to considerable perturbations in the current-voltage response. In addition to the sensitivity analysis, the extent of gain saturation due to space charge effects within the diode is investigated. The effect of gain saturation is observed to agree with analytical based predictions. To the authors' knowledge, this is the first demonstration of gain saturation due to space charge effects in a SAGCM APD     

Effects of high electric fields and temperature on conductivity of a-Si

At high electric fields the Poole-Frenkel effect and thermally-assisted tunneling give rise to an enhanced electric conductivity of amorphous silicon (a-Si). The occupancy of states in the gap, free, and trapped charge carrier concentrations and electric conductivity of a homogenous a-Si in steady-state conditions are calculated on the basis of space-charge neutrality and with time-unchangeable concentrations of charge carriers. Simplifying approximations are introduced, thus enabling easier calculations of carrier concentrations and conductivity. The theory of capture-emission dynamics in a-Si at high fields is further extended by expressing occupancy functions and nonequilibrium quasi-Fermi levels. Effects of the density of states distribution in a-Si and added impurities upon carrier concentrations and conductivity are revealed.     

光脉冲在钕玻璃激光放大介质中的传播特性研究

建立了用于研究光脉冲在钕玻璃激光放大介质中传播特性的模拟软件.它综合考虑了激光放大介质中基质的克尔非线性效应、群速度色散、活性粒子的增益谱以及增益饱和对光脉冲传播特性的影响.所得结果对高功率钕玻璃激光放大器的设计和优化有参考价值.     

行波半导体光放大器的增益饱和及其在解复用技术中的应用

本文通过对行波半导体光放大器增益饱和及其瞬态增益特性的分析，指出了利用半导体光放大器增益饱和快的特点，可使半导体光放大器环路镜实现超高速的解复用功能。     

Optical gain and gain suppression of quantum-well lasers withvalence band mixing

The effects of valence band mixing on the nonlinear gains of quantum-well lasers are studied theoretically for the first time. The analysis is based on the multiband effective-mass theory and the density matrix formalism with intraband relaxation taken into account. The gain and the gain-suppression coefficient of a quantum-well laser are calculated from the complex optical susceptibility obtained by the density matrix formulation with the theoretical dipole moments obtained from the multiband effective-mass theory. The calculated gain spectrum shows that there are differences (both in peak amplitude and spectral shape) between this model with valence band mixing and the conventional parabolic band model. The shape of the gain spectrum calculated by the new model becomes more symmetric due to intraband relaxation together with nonparabolic energy dispersions. Optical intensity in the GaAs active region is estimated by solving rate equations for the stationary states with nonlinear gain suppression     

Propagation of short pulses in an active nonlinear two-core optical fiber

The switching dynamics of short pulses in an active nonlinear two-core fiber coupler is investigated theoretically. The analysis takes into account the effects of coupling-coefficient dispersion, gain bandwidth, and gain saturation. In particular, we demonstrate that the pulse breakup effect due to coupling-coefficient dispersion can be suppressed by limiting the bandwidth of the linear gain, whether gain saturation is considered or not.     

Carrier Dynamics of Quantum-Dot, Quantum-Dash, and Quantum-Well Semiconductor Optical Amplifiers Operating at 1.55 μm

We assess the influence of the degree of quantum confinement on the carrier recovery times in semiconductor optical amplifiers (SOAs) through an experimental comparative study of three amplifiers, one InAs-InGaAsP-InP quantum dot (0-D), one InAs-InAlGaAs-InP quantum dash (1-D), and one InGaAsP-In-GaAsP-InP quantum well (2-D), all of which operate near 1.55-mum wavelengths. The short-lived (around 1 ps) and long-lived (up to 2 ns) amplitude and phase dynamics of the three devices are characterized via heterodyne pump-probe measurements. The quantum-dot device is found to have the shortest long-lived gain recovery (~80 ps) as well as gain and phase changes indicative of a smaller linewidth enhancement factor, making it the most promising for high-bit-rate applications. The quantum-dot amplifier is also found to have reduced ultrafast transients, due to a lower carrier density in the dots. The quantum-dot gain saturation characteristics and temporal dynamics also provide insight into the nature of the dot energy-level occupancy and the interactions of the dot states with the wetting layer.     

Effect of the nonlinear saturation of the gain on the peak modulation frequency in lasers based on self-assembled quantum dots

Peak modulation frequency of lasers based on self-organized quantum dots is calculated taking into account the effect of nonlinear gain saturation. Because of a large nonlinear gain coefficient and a reduction in the differential gain with increasing optical losses, the peak modulation frequency is attained for an optimum loss level that is significantly lower than the saturated optical gain in the active region. For lasers based on multiply stacked arrays of quantum dots, the peak modulation frequency first increases with increasing number of quantum-dot layers before leveling off, with the limiting value being inversely proportional to the nonlinear gain coefficient.     

Nonlinear gain suppression in semiconductor lasers due to carrier heating

A simple model is presented for carrier heating in semiconductor lasers from which the temperature dynamics of the electron and hole distributions can be calculated. Analytical expressions for two new contributions to the nonlinear gain coefficient, in are derived, which reflect carrier heating due to stimulated emission and free carrier absorption. In typical cases, carrier heating and spectral holeburning are found to give comparable contributions to nonlinear gain suppression. The results are in good agreement with recent measurements on InGaAsP laser diodes.<>     

Strain-dependence of the gain saturations in InGaAsP/InP quantum-well gain media

Numerical analyses of polarization-dependent optical gain saturations are given for quantum-well (QW) lasers in the presence of strain in the well regions in order to investigate the strain dependence of polarization-bistable operations. Gain saturation coefficients are obtained from nonlinear susceptibilities calculated in the perturbative analyses of density matrices. Band dispersions and dipole matrix elements, which are put into the density matrices, are calculated by diagonalizing Luttinger's Hamiltonian, including valence band mixing. The strain induces a change in band dispersions and wavefunctions, leading to strain-dependent saturation coefficients. The self-saturation coefficients and the cross-saturation coefficients (with orthogonal optical polarizations) pertinent to InGaAsP/InP QW vertical-cavity surface-emitting lasers are calculated. We find that the relative magnitudes of self- and cross-saturation coefficients are strongly dependent on the strain; in the presence of compressive strain, the cross-saturation coefficients are larger than the self-saturation in the wide range of the linear gain spectra, especially in the vicinity of the gain peak, indicating that the compressively strained structure is more favorable for the polarization-bistable operations.     

Nonlinear saturation behaviors of high-speed p-i-n photodetectors

We present numerical simulations of the ultrafast transport dynamics in an ultrahigh-speed double-heterostructure p-i-n photodetector. Nonlinear saturation behaviors under high field and high power illumination are investigated with the external circuit response considered. Damping constants and diffusion constants are both treated as electric-field- and carrier-concentration dependent in our model in order to take into account the effect of carrier scattering. We have also considered the carrier trapping at the heterostructure interfaces for the first time. Besides the drift-induced space charge screening effect, we find that saturation of external circuit and carrier-trapping-induced screening effect are also the dominant mechanisms contributed to the nonlinear bandwidth reduction under high power illumination. On the other hand, previously reported plasma oscillations are found to be greatly suppressed by including strong carrier diffusion effect in the model     

Nonlinear Optical Response of Conjugated Polymer to Electric Field

The organic π-conjugated polymers are of major interest materials for the use in electro-optical and nonlinear optical devices. In this work, for a selected polyacetylene chain, the optical absorption spectra in UV/Vis regime as well as the linear polarizabilitiy and nonlinear hyperpolarizability are calculated by using quantum chemical ab initio and semiempirical methods. The relationship of its optical property to electric field is obtained. Some physical mechanism of electric field effect on molecular optical property is discussed by means of electron distribution and intramolecular charge transfer.     

Dynamic Spatiotemporal Speed Control of Ultrashort Pulses in Quantum-Dot SOAs

We present theoretical and experimental results on the propagation of ultrashort pulses in quantum-dot (QD) laser amplifiers. The propagation time of the light pulses is controlled by the pulse itself (self-induced speed control) or by injection of a second pump pulse (external speed control). Our simulations on the basis of spatially and temporally resolved QD Maxwell–Bloch equations reveal that the excitation and relaxation dynamics induced by the propagating pulse or a pump pulse within the active charge carrier system leads to a complex gain and index dynamics that may either speed up or slow down the propagating light pulse. The physical effects allowing for the dynamic speed control could be ascribed to complex (coherent and incoherent) level dynamics leading to dynamic gain saturation and index dispersion. The dependence of the propagation time on injection current density and pulse energy is discussed. The numerical results of pulse reshaping and propagation times in the gain and absorptions regime are compared to experimental results.     

Sideband evolution from spontaneous into amplified emission in a free electron laser

A theoretical study of the evolution of sidebands, originating from large-scale (nonlinear) oscillations in the electron dynamics, is reported for a combined wiggler and guide field system operating near magnetoresonance. These sidebands, already existent in the spectrum of spontaneous emission, are followed up through small signal gain computations, and are finally recovered in the simulations of the nonlinear saturation process. A comparison with a radiation spectrum of an experiment done at the Naval Research Laboratory (NRL) shows good agreement. This indicates that the sidebands, observed in the experiment at saturation power level, are essentially due to the specific structure of the spectrum of spontaneous emission.     

Temperature dependence of the threshold current density of aquantum dot laser

Detailed theoretical analysis of the temperature dependence of threshold current density of a semiconductor quantum dot (QD) laser is given. Temperature dependences of the threshold current density components associated with the radiative recombination in QDs and in the optical confinement layer (OCL) are calculated. Violation of the charge neutrality in QDs is shown to give rise to the slight temperature dependence of the current density component associated with the recombination in QD's. The temperature is calculated (as a function of the parameters of the structure) at which the components of threshold current density become equal to each other. Temperature dependences of the optimum surface density of QD's and the optimum thickness of the OCL, minimizing the threshold current density, are obtained. The characteristic temperature of QD laser T_o is calculated for the first time considering carrier recombination in the OCL (barrier regions) and violation of the charge neutrality in QDs. The inclusion of violation of the charge neutrality is shown to be critical for the correct calculation of T_o. The characteristic temperature is shown to fall off profoundly with increasing temperature. A drastic decrease in T_o is shown to occur in passing from temperature conditions wherein the threshold current density is controlled by radiative recombination in QD's to temperature conditions wherein the threshold current density is controlled by radiative recombination in the OCL. The dependences of T_o on the root mean square of relative QD size fluctuations, total losses, and surface density of QDs are obtained