Numerical model of wavelength conversion through cross-gain modulation in semiconductor optical amplifiers

A steady-state numerical model of wavelength conversion through cross-gain modulation in semiconductor optical amplifiers is described, which includes the spatial variations of the carrier density, gain coefficient, differential gain, and internal loss. Of particular interest is the analytic gain coefficient model, which is applied to the semiconductor optical amplifier converter problem for the first time to our knowledge. The model is used to compare performances of upconverters and downconverters for cases of long and short device lengths, and in large and small signal regimes. Comparisons with results of other studies are presented.     

Thermal instabilities in semiconductor amplifiers

We introduce here a model which includes the thermal dynamics in the time evolution of a semiconductor multiple quantum well microresonator, driven by a coherent holding beam. The active layer is electrically pumped, in order to obtain population inversion, but it is maintained below the lasing threshold. We show that the inclusion of thermal effects introduces a Hopf instability which may dominate the dynamical behaviour of the system in some operational regimes. In those cases our numerical simulations show that both spatial patterns and cavity solitons perform a drift motion in the transverse direction. This motion develops over the slow time scale which characterizes thermal effects.     

Optical bistability in vertical-cavity semiconductor optical amplifiers

We present an overview of the properties and applications of optical bistability in vertical-cavity semiconductor optical amplifiers (VCSOAs). The basic physics and analytical models of this optical nonlinearity are discussed. Experimental results obtained from a VCSOA operated in the 850 nm wavelength region are presented. Counterclockwise hysteresis loops are obtained over a range of initial phase detuning and bias currents. One hysteresis loop is observed experimentally with an input power as low as 2 muW when the device is biased at 98% of its lasing threshold. Numerical simulations based on the Fabry-Perot resonator model and rate equations we developed show good agreement with our experimental observations. In addition, a low input intensity high contrast (10:1) optical and gate and 2R regeneration are demonstrated. We believe that bistable VCSOAs can significantly advance the prospect of a dense two-dimensional array of low-switching-intensity all-optical logic and memory elements.     

Ultrafast dynamics of quantum-dot semiconductor optical amplifiers

We report on a series of experiments on the dynamical properties of quantum-dot semiconductor optical amplifiers. We show how the amplifier responds to one or several ultrafast (170 fs) pulses in rapid succession and our results demonstrate applicability and ultimate limitations to application of quantum-dot amplifiers in e.g. amplification of signals in a telecommunications system. We also review experiments on pulse propagation control and show the possibility to slow down or speed up 170 fs pulses in a quantum-dot based device.     

Operation of quantum-dot semiconductor optical amplifiers under nonuniform current injection

The influence of nonuniform current injection along the active region, on the linear operation of a quantum-dot semiconductor optical amplifier (QD-SOA) is investigated. For this purpose, we have utilized some functions to generate various nonuniform current injection profiles. These profiles have been considered in our numerical calculations, where the rate equation model is employed to construct different characteristics of the QD-SOA. We have found that the gain, as well as the crosstalk, of a QD-SOA is closely associated to the variance of the carrier density along the cavity. Simulation results show that nonuniform current injection can be used as a technique for gain enhancement as well as crosstalk suppression.     

理论研究垂直腔半导体光放大器脉冲放大特性

从速率方程和薄膜光学理论出发,对垂直腔半导体光放大器(Vertical Cavity Semiconductor Optical Amplifiers,简称VCSOAs)在脉冲工作情况下的动态增益进行了数值模拟.在计算中考虑了载流子和光强沿光轴方向的不均匀性,以及腔内介质折射率的不连续性对光波传输的影响.详细分析了反射模式下VCSOA在脉冲通过时载流子密度和瞬时增益的变化、输出光脉冲的畸变以及抽运光功率、分布布拉格反射镜(Distributed Bragger Reflector,简称DBR)周期数、输入脉冲能量以及脉冲宽度等参量对脉冲放大中的能量增益的影响.结果表明,能量增益随抽运光功率增大而增加;在峰值功率一定时输入脉冲宽度的增加将减小能量增益;输入脉冲能量的增加也会引起能量增益的下降,而适当减少顶部DBR的周期数可以改善这种脉冲放大的能量增益饱和特性.     

Assessment of linear semiconductor amplifiers for upgrading WDM-PONs with wavelength reuse

In this work we have assessed the capacity of a linear semiconductor optical amplifier to compensate the fiber and component losses present in a wavelength division multiplexing passive optical network (WDM-PON) evolution from fiber-to-the-building (FTTB) to fiber-to-the-home access. The evaluation measurements confirm that the presence of a semiconductor optical amplifier placed at the entry of a group of optical network units that share the same wavelength channel can raise the loss budget that the link can tolerate in the fiber, compensating for the losses of a passive splitter up to a 1:16 division rate, allowing the upgrade of existing WDM-PON FTTB structures to make the fiber reach the final user's home.     

Design and implementation of cryogenic semiconductor amplifiers as interface between RSFQ circuits

Rapid single flux quantum (RSFQ) circuits create high interest in cryogenic amplifiers as interface to commercial room temperature electronics. The requirements for the amplifiers are an extremely high bandwidth (SFQ pulses with ), high voltage gain of about 10⁴ (common RSFQ output voltage level of about ), low power consumption (cryogenic environment at 4 K) and low noise. Hybrid amplifiers, based on commercial available p-HEMT transistors, can solve the problematic high-speed interface. In this paper, we present measurement results of a hybrid four stage coplanar amplifier in combination with a RSFQ Toggle Flip-Flop (RSFQ T-FF) and a Josephson array quantizer (JA-Q).     

Analytic modeling of erbium-doped fiber amplifiers on the basis of intensity-dependent overlapping factors

Rate equations based on intensity-dependent overlapping factors are integrated to obtain analytic solutions for pump, signal, and amplified spontaneous emission (ASE), even when the coupled signal varies with time. The equations can be applied without the imposition of any restraints on the values of the pump, signal, and ASE powers, the excited-state-absorption cross section, the erbium-density distribution, or other parameters that characterize the fiber. The methods used to calculate pump, signal, and ASE powers are discussed. Experimental techniques to characterize the doped fiber that were based on these analytic expressions are introduced.     

Theory of digital modulation of semiconductor laser diode remotely

The authors present a new theory that can be applied to modulate the semiconductor laser diode (LD) remotely. The proposed scheme utilizes the effect of incoherent external optical feedback (EOF) on the LD output optical signal. In particular, owing to the high increase in threshold carrier density, the LD will be impelled to turn off when the value of external reflectivity is equal to the laser back reflectivity. Thus, by exposing the LD, which is injected with a dc-current higher than threshold, to incoherent EOF digital signal (which conveys the transmitted information), the output optical signal of LD can be modulated accordingly.     

Analytic bond-order potentials for modelling the growth of semiconductor thin films

Interatomic potentials for modelling the vapour phase growth of semiconductor thin films must be able to describe the breaking and making of covalent bonds in an efficient format so that molecular dynamics simulations of thousands or millions of atoms may be performed. We review the derivation of such potentials, focusing upon the emerging role of the bond-based analytic bond-order potential (BOP). The BOP is derived through systematic coarse graining from the electronic to the atomistic modelling hierarchies. In a first step, the density functional theory (DFT) electronic structure is simplified by introducing the tight-binding (TB) bond model whose parameters are determined directly from DFT results. In a second step, the electronic structure of the TB model is coarse grained through atom-centered moments and bond-centered interference paths, thereby deriving the analytic form of the interatomic BOP. The resultant σ and π bond orders quantify the concept of single, double, triple and conjugate bonds in hydrocarbon systems and lead to a good treatment of radical formation. We show that the analytic BOP is able to predict accurately structural energy differences in quantitative agreement with TB calculations. The current development of these potentials for simulating the growth of Si and GaAs thin films is discussed.     

Electrothermal dynamics of semiconductor nanowires under local carrier modulation

Charge transfer, surface/interface, defect states, and internal fields strongly influence carrier statics and dynamics in semiconductor nanowires. These effects are usually probed using spatially resolved scanning current techniques, where charge carriers are driven to move by diffusion force due to a density gradient, drift force due to internal fields, and thermoelectric force due to a temperature gradient. However, in the analysis of experimental data, analytical formulas are usually used which are based on the assumption that a single component of these forces dominates the carrier dynamics. In this work we show that this simplification is generally not justified even in the simplest configurations, and the scanning microscopy data need to be analyzed with caution. We performed a comprehensive numerical modeling of the electrothermal dynamics of free charge carriers in the scanning photocurrent microscopy configuration. The simulation allows us to reveal and predict important, surprising effects that are previously not recognized, and assess the limitation as well as potential of these scanning current techniques in nanowire characterization.     

Analysis of carrier modulation in channel of ferroelectric-gate transistors having polar semiconductor

We have been fabricated and characterized a ferroelectric-gate thin-film transistors (TFTs) using ZnO as a channel polar semiconductor and YMnO₃ as a ferroelectric gate. A typical n-channel transistor property showing clear drain current saturation in I_D-V_D (drain current - drain voltage) characteristics was recognized. When the 3 V of the gate voltage is applied under the 4 V of drain voltage, the large drain current of about 1.1 mA is obtained. These controlled-polarization-type ferroelectric-gate TFTs using ZnO-channel TFTs operate in the accumulation-depletion mode and the ON/OFF state of the ferroelectric-gate TFTs strongly depends on the polarization switching of P_SFe. In this paper, therefore, the polarization switching of P_SFe in the TFT is carefully examined and the relationship between the polarization switching and the carrier accumulation (depletion) state is discussed using impedance spectroscopy and Capacitance-Voltage (C-V) measurements at applied the gate voltage.     

半导体非线性光学材料的第一性原理研究

通过第一性原理研究Ⅱ-Ⅳ-Ⅴ2族CKP半导体中的CdSiAs2，计算了其双折射性，量化了双折射性同应力的线性关系，它的负双折射性使之能够通过应力、温度调节以及同CdGeAs2混合来设计非临界位相匹配材料。计算显示，少量的参杂Ge（ 〈5％），能够实现非临界位相匹配I类二次谐波产生（SHG）在CO2激光谱线范围可调谐，它可能具有很高的有效χ^（2）。     

Effects of two-photon absorption on all optical logic operation based on quantum-dot semiconductor optical amplifiers

We investigate all-optical logic operation in quantum-dot semiconductor optical amplifier (QD-SOA) based Mach–Zehnder interferometer considering the effects of two-photon absorption (TPA). TPA occurs during the propagation of sub-picosecond pulses in QD-SOA, which leads to a change in carrier recovery dynamics in quantum-dots. We utilize a rate equation model to take into account carrier refill through TPA and nonlinear dynamics including carrier heating and spectral hole burning in the QD-SOA. The simulation results show the TPA-induced pumping in the QD-SOA can reduce the pattern effect and increase the output quality of the all-optical logic operation. With TPA, this scheme is suitable for high-speed Boolean logic operation at 320 Gb/s.     

Steady-state photoresponse theory for transition-metal-doped semiconductor photoanodes in aqueous electrolytes

The effect of the dopant ions on the steady-state photoresponse of transition-metal-doped semiconductor electrodes in aqueous electrolytes is examined with particular attention to minority carrier lifetime and transport characteristics. An approximate treatment is introduced which takes into account the variation in hole lifetime with position in the depletion region and the effect of the impurity centers on the effective hole mobility. The results obtained clearly resemble the known photocurrent-voltage characteristics of a number of such systems.