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A novel semi-analytical model for cross-gain modulation (XGM) in quantum dot vertical-cavity semiconductor optical amplifier has been derived. The derived model, which is simple, requires small computational time compared with numerical simulation and can efficiently be used to investigate the optical bistable/stable characteristics of the device. Large XGM bandwidth can also be obtained when the device operates in the bistable region and in the stable region close to the bistable edge. A figure of merit is proposed to optimize the complicated tradeoff between the XGM bandwidth, efficiency, applied current and the input pump to probe ratio. We find that optimum characteristics can be obtained in our investigated device when the probe wavelength is detuned by 9 Å above the resonant wavelength and when a specific CW probe density is applied to the input of the device. [ABSTRACT FROM AUTHOR]

The characteristics of optical bistability in quantum dot vertical cavity semiconductor optical amplifier integrated with MEMS membrane have been investigated, and a closed-form model is derived taking into account the effect of the quantum dot discrete states and the membrane deflection and reflectivity. We show that small membrane deflection significantly adjusts the resonant wavelength, the contrast ratios and the hysteresis width. The shape of the input-output characteristics of the device can be adjusted to display clockwise, butterfly and counterclockwise hysteresis loops depending on the membrane deflection and the initial wavelength detuning. Our analysis reveals that the contrast ratios of the upper and lower bistable levels are totally different. Also, it has been shown that the characteristics of the bistability are strong function of the active layer linewidth enhancement factor and the membrane reflectivity. [ABSTRACT FROM AUTHOR]

We have studied the characteristics of optical bistability of different types of optical modes amplified by small-size quantum dot vertical cavity semiconductor optical amplifiers operated in reflection. Our analysis reveals that TE 01 mode exhibits stronger intensity-dependent non-linearity in small radius devices, which results in stronger optical phase modulation and therefore larger hysteresis width compared with the other modes. The effect of the wavelength detuning of the input signal on the shape of the hysteresis loop is studied. We find that butterfly hysteresis loop exhibits the largest hysteresis width compared with clockwise and counterclockwise loops. Our analysis reveals that doping the quantum dots with p-type doping slightly reduces the hysteresis width while doping the dots with n-type doping clearly increases the hysteresis width for any wavelength detuning. We estimate that the hysteresis width of quantum dot active layer will exhibit higher hysteresis width compared with quantum well active layer having the same threshold gain. [ABSTRACT FROM AUTHOR]

A novel analytical model for spin-polarization in quantum dot VCSEL has been derived. The derived model includes the effect of spin relaxation of electron and hole in all discrete energy states. The model also takes into account the gain dispersion of the quantum dots, the birefringence, the linewidth enhancement factor and the doping concentration in the active layer. We find that spin relaxation of electrons has stronger effect on the polarization characteristics of quantum dot VCSEL than spin relaxation of holes (especially in p-type doped spin-injected quantum dot VCSEL). The derived model is accurate and exhibits negligible computational time compared with numerical models. In addition, the derived model is suitable for device design and optimization. [ABSTRACT FROM AUTHOR]

The spectral characteristics of the linear gain and the small-signal crosstalk of positively detuned and negatively detuned quantum dot gain-clamped semiconductor optical amplifiers (QDGC-SOA) have been studied. Our analysis reveals that the linear gain of the ground-state in positively detuned QDGC-SOA can be enhanced by doping the dots by p-type concentration, while the linear gain of the excited-state in negatively detuned QDGC-SOA can be enhanced by doping the dots by n-type concentration. The small-signal crosstalk is found to be proportional to the amplification where we find that positively detuned QDGC-SOA exhibits less crosstalk (by at least 3 dB) than negatively detuned QDGC-SOA. In addition, we find that positively detuned QDGC-SOA is capable of providing larger optical gain. [ABSTRACT FROM AUTHOR]

The polarization characteristics of doped spin-injected quantum dot vertical cavity surface emitting laser (QD-VCSEL) have been studied using multi-population spin-flip coupled rate equation model. The stability conditions of spin-injected VCSEL are also investigated for different device parameters. Our analysis reveals that QD-VCSEL emitting at high lasing energies, close to the first excited state energy, demonstrates high output ellipticity and high ellipticity gain compared to QD-VCSEL emitting at the ground state energy. Also, we find that increasing the p-type doping concentration of the quantum dots increases the output ellipticity gain of the device and significantly alters the stability conditions of the VCSEL. Large instability is observed when the VCSEL is emitting near $$\sim \!\!70\hbox {meV}$$ above the ground state energy and when the quantum dots are doped with $$\sim \!\!4\times 10^{10}\,\hbox {cm}^{-2}$$ of p-type concentration. Moreover, we find that QD-VCSEL exhibits good stability when its emission energy is approximately 10-20 meV higher than the ground state energy and when the dots are doped with more than $$4\times 10^{10}\,\hbox {cm}^{-2}$$ of p-type concentration. [ABSTRACT FROM AUTHOR]

Abstract: The spectral characteristics of up and down wavelength/energy conversion via cross-gain modulation in doped and undoped quantum dot (QD) semiconductor optical amplifier (SOA) have been studied using the multi-population coupled rate equation model. Our analysis reveals that flexible broadband conversion from low to high energy (∼90meV) and from high to low energy (∼30meV) can be obtained in P-type doped QD-SOA with <1dB output power variation. We find that the tuning range of undoped QD-SOA converter is limited by the contrast ratio for low to high energy conversion and by the conversion efficiency uniformity for high to low energy conversion. Our analysis reveals that doping the dots by P-type concentration significantly increases the contrast ratio and consequently enhances the tuning range of low to high energy conversion. However we find that at high P-type concentration, the contrast ratio and the modulation bandwidth will be improved but the conversion efficiency will be reduced. [Copyright &y& Elsevier]

A simple, accurate, and fast multilayer feedforward artificial neural network (ANN) model for quantum-dot semiconductor optical amplifiers (QD-SOAs) is developed. The developed ANN model has demonstrated excellent precision and accurately Models the physical characteristics of QD-SOAs such as pulse amplification and four-wave mixing characteristics. Furthermore, the developed ANN model requires a very small computational time compared with numerical models, which is very attractive for computer-aided-design applications. The model is used to create very interesting design curves for QD-SOAs. In addition, the model is suitable for physical-parameter extraction from available measured data. [ABSTRACT FROM AUTHOR]

The linewidth enhancement factor (α) of inhomogenously broadened InAs/GaAs quantum dot (QD) lasers is studied taking into account the effect of the multi-discrete QD energy levels for both electrons and holes. The effect of size-fluctuation and the separation between the energy states on α for different applied bias is studied. Very small value of α (∼0.3) is obtained at high-applied bias. This value is smaller than the linewidth enhancement factor in quantum well lasers. An empirical formula for the linewidth enhancement factor is also derived. [ABSTRACT FROM AUTHOR]

Detailed theoretical analysis of the size fluctuation in InAs-GaAs quantum dot (QD) lasers is presented. Analytical expressions for the inhomogeneous line broadening and the optical gain are derived for a Gaussian size fluctuation distribution. The effect of size fluctuations on the QD carrier density, modal gain, and differential gain is studied. Red shifts in the gain peak is observed when size fluctuations increases. The energy detuning between the gain peak and the differential gain peak for a pyramidal quantum dot system having an average base length of 130 Å and standard deviation of 7 Å is about 12 meV. [ABSTRACT FROM AUTHOR]

An analytical model for the optical gain of quantum dot-semiconductor optical amplifier (QD-SOA) is presented. The model is derived by analytically solving the rate equations of the laser amplifier taking into account inhomogeneous line broadening, dot-to-dot electron tunneling and gain saturation due to spontaneous emission. Because of the unique electronic structure of quantum dots (QDs), we observed that using QD in the active region of a semiconductor optical amplifier increases the unsaturated optical gain and the output saturation intensity of the amplifier. [Copyright &y& Elsevier]

Detailed theoretical analysis of the gain characteristics of quantum-dot semiconductor optical amplifiers (QD-SOA) is presented. An analytical expression for the optical gain is derived from the quantum dot and wetting layer rate equations. Due to the better confinement of carriers in the quantum dots, our calculation shows that large unsaturated optical gain can be obtained at low operating current. Also, we found that the output saturation intensity of QD-SOA is higher than the output saturation intensity of bulk-SOA. This fact lends itself to the design of efficient low-power SOAs. [ABSTRACT FROM AUTHOR]

Novel closed-form model for the threshold conditions and the optical gains of vertical dual-cavity semiconductor optical amplifier (VDC-SOA) is derived. Expressions for the transfer functions that relate the average photon density inside the two cavities to the incident photon density are derived. The model is simple, accurate and easy to use for designing dual-wavelength laser amplifiers. Good agreement is obtained between the analytical model and numerical calculations. The transmitted and reflective bistability characteristics of quantum dot VDC-SOA have been investigated. The threshold condition of each mode is expressed in terms of the top-cavity and the bottom-cavity currents. The dual modes λ o + and λ o - show different bistability behavior as a function of the top cavity current when the device operates at fixed operating point below the threshold curve. We find that the contrast ratio of mode λ o + is a weak function of the device currents, and the hysteresis width increases when the top cavity current is increased. For mode λ o - large contrast ratio and hysteresis width are obtained when the two cavities exhibit equal currents. Our analysis also reveals that the spectral characteristics of mode λ o + are different from that of mode λ o - . [ABSTRACT FROM AUTHOR]

Examines efficient wavelength switching in an In[sub 0.4]Ga[sub 0.6]As/GaAs self-organized quantum dot laser with an intracavity absorber section. Wavelength shift caused by a shift of lasing between the bound states of the quantum dots; Heterostructure of the quantum dot laser; Light-current characteristics.

A novel type of multichannel and multistate all-optical switch using a single sample-grating quantum-dot-distributed feedback semiconductor optical amplifier has been proposed and theoretically demonstrated. The multichannel device, which operates below threshold, utilizes cross-gain modulation and the sample-grating technique. The multichannel outputs are strongly coupled and are utilized to get multistability at several wavelength channels. Three logic states can be obtained when the inputs are properly detuned to the sample-grating comb modes. The three logic states, which exhibit reasonable gain, are separated by wide hysteresis width and can be tuned to a different wavelength channels. The device characteristics are very useful for building all-optical logic gates, flip-flops, and decision circuits. [ABSTRACT FROM AUTHOR]

We propose and theoretically demonstrate a novel type of all-optical multistable device using single quantum dot distributed feedback semiconductor optical amplifier. The device utilizes cross-gain modulation with proper wavelength detuning to obtain multistability in a conventional DFB semiconductor optical amplifier. We find that dual bistability, which can be adjusted vertically or horizontally by applying small probe input, can be obtained in both the pump and the probe characteristics. Adjustable switching with contrast ratio as high as 14 dB is obtained from the probe signal. [ABSTRACT FROM AUTHOR]

Far-infrared spontaneous emission at 300 K and lower temperatures, due to intersubband transitions in self-organized In[sub 0.4]Ga[sub 0.6]As/GaAs quantum dots, has been characterized. Measurements were made with a multidot layer near-infrared (∼1 μm) interband laser. The far-infrared signal, centered at 12 μm, was enhanced after the interband transition reached threshold at 300 K. The results are explained in terms of the carrier dynamics in the dots. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]