Mid-infrared spectroscopic studies and lasing in GaAs–AlGaAs quantum cascade devices

Complementary intersubband and interband optical measurements have been employed in order to study GaAs–AlGaAs quantum cascade light-emitting diode and laser structures. Using these techniques, we have measured the redistribution of electrons throughout the bridging regions and upper states in the active regions of the diode device with increasing bias. The high quality of the sample gives very narrow line widths in the optical spectra, permitting the resolution of transitions involving closely spaced energy levels. This has allowed the direct observation of level alignment at the onset of current flow through the device. In addition, stimulated emission at λ=9.7 μm has been observed from a GaAs–AlGaAs laser structure under pulsed operation. A threshold current density of 6.5 kA/cm² and peak power 300 mW are measured at 10 K and lasing operation is observed up to 200 K.     

基于波长调制光谱技术的线宽测量理论及其应用研究

气体吸收谱线的线宽主要包括碰撞作用引起的洛伦兹线宽和分子热运动引起的高斯线宽, 是可调谐二极管激光吸收光谱技术(TDLAS)的重要参数. 本项研究在弱吸收条件下, 通过对波长调制法中二次与四次谐波峰值比进行理论分析和仿真计算, 发现无论洛伦兹和高斯线宽如何变化, 二次和四次谐波峰值比具有恒过不动点的特征. 本文基于该不动点提出了一种线宽在线测量的方法, 并以CO₂分子6982.0678 cm^-1 吸收谱线为例进行实验验证. 实验结果表明, 该方法可以精确测量线宽, 进而根据测量得到的线宽确定气体分压和总压, 可有效地提高TDLAS技术在工业现场中的测量精度.     

Tunable anticrossing of gated and ungated plasma resonances and enhancement of interlayer terahertz electric field in an asymmetric bilayer of density-modulated two-dimensional electron gases

We study theoretically the terahertz (THz) response of a bilayer of density-modulated two-dimensional electron gases, which we employ to model the actual double-quantum-well electron channel of a grid-gated field-effect transistor in which strong THz photoresponse was recently observed. We have shown that such a system can be driven into the anticrossing regime between gated and ungated plasma resonances by tuning the gate voltage. The amplitude of the interlayer THz electric field in the ungated (double-layered) portions of the channel increases dramatically in the anticrossing regime. This strong interlayer THz electric field may strongly affect interlayer electron tunneling which, in turn, may contribute to the physical mechanism underlying the strong THz photoresponse observed in recent experiments.     

Material parameters of InGaAsP and InAlGaAs systems for use in quantum well structures at low and room temperatures

The set of material parameters for quantum well structures is of immense importance because of its usage in the development of theories, extraction of experimental data, and the proper design of devices. In particular, the (Al,In)GaAs/GaAs, InGaAs/InP and (In,Ga)AlAs/InGaAs quantum well systems have drawn a lot of attention. They form the center core of materials used for fundamental basic research and device applications. Despite the presence of some review articles and reference books, there is a lack of clear reference on the accurate determination of the material parameters for quantum wells. This review aims to provide a comprehensive and systematic set of material parameters for the above quantum well systems grown on (1 0 0) substrates at two different temperatures, below 10 K and at around 300 K. The parameters are compared against experimental data from various fabrication sources, measurement techniques, and quantum well structures. The values presented here serve as an accurate and up to date source of reference.     

Realization of optoelectronic integrated transmitter and receiver for POF Applications

The optoelectronic integrated transmitter and receiver for 650 nm plastic optical fiber (POF) communication applications realized in 0.5 μm BCD (Biplor, CMOS and DMOS) process is first described in this paper. The 650 nm resonant cavity light emitting diode (RCLED) is used as light source. It is first proposed for optoelectronic integration of the transmitter by bonding RCLED to the driver chip. Temperature compensation technology is employed in the driver circuit to compensate for the modulation current. In the monolithic optoelectronic integrated receiver, large area multi-finger PIN photodetector (PD) that is compatible with standard IC process, transimpedance amplifier and post amplifier are presented. Measurement results show that the responsivity and capacitance of PD is 0.25 A/W and 5 pF, respectively. The sensitivity of receiver is −14.6 dBm at 180 Mb/s and BER is less than 10⁻⁹ for 650 nm input light by POF. A clear eye diagram is demonstrated for 180 Mb/s PRBS. These indicate that optoelectronic integrated chips can be employed in high-speed POF-based Fast Ethernet systems for broadband access network applications.     

CCD摄像机的输出调制度阈值分析

应用数理统计学的有关内容对CCD输出信号中的随机噪声进行了相关分析。引入了“噪声等效调制度”这一概念 ,利用计算机模拟技术对摄像机成像时所产生的随机噪声进行了模拟与计算。通过计算机模拟 ,推导出了噪声等效调制度与CCD摄像机系统信噪比之间的关系。将信噪比对CCD摄像机成像质量的影响予以量化 ,并对摄像机的输出调制度阈值进行了分析与计算     

面向光学器件透反射率测量的声光调制型激光功率稳定系统设计

在各类光学系统设计过程中,由于实验系统对于光学器件的透反射率要求不同,需要确定光学器件的透反射率。本文设计了一种对光学器件的透反射率进行精准测量及标定的系统,该系统通过对激光光源进行声光调制,使得激光输出功率的稳定度显著提高,避免了测量时光源不稳定带来的较大误差。实验结果表明,该系统能够稳定光源输出功率,稳定度维持在0.05%/h,甚至更高的水平,满足了光学器件透反射率测量的误差小,精度高等要求。     

Photocharacterization of Novel Ruthenium Dyes and Their Utilities as Oxygen Sensing Materials in Presence of Perfluorochemicals

Photophysical constants of three novel ruthenium dyes derived from tridentate pyridinediimine (pydim) ligands has been declared and their photoluminescent properties were investigated in solvents of dichloromethane (DCM), tetrahydrofuran (THF) and ethanol (EtOH) by UV-Visible absorption, emission and excitation spectra. The quantum yield, fluorescence decay time, molar extinction coefficient and Stoke’s shift values of the novel ruthenium complexes were determined. The perfluoro compound (PFC) nonadecafluorodecanoic acid which is also known as medical gas carrier has been used for the first time together with newly synthesized Ruthenium complexes in ethyl alcohol. The utilities of oxygen sensing materials were investigated in EtOH in presence of chemically and biochemically inert PFC.     

Effect of low-energy electron irradiation on the cathodoluminescence of multiple quantum well (MQW) InGaN/GaN structures

The effect of low-energy electron-beam (e-beam) irradiation on the InGaN-related cathodoluminescence in multiple quantum well (MQW) InGaN/GaN light-emitting diode (LED) structures has been studied. It is shown that the e-beam exposure leads to an increase of emission intensity and to a formation of new blue-shifted emission bands. The changes observed were explained by the enhancement of In diffusion stimulated by excess carrier recombination.     

GaAs光生载流子动力学机制的超快光谱学分析

利用飞秒激光泵浦探测技术,通过改变光学参数,如中心波长、功率,分别对未故意掺杂高纯n型砷化镓的差分反射谱进行研究,进而分析室温下砷化镓光生载流子动力学过程.首先,当泵浦光功率恒为100mW,探测光功率恒为10mW时,随着中心波长的增大,差分反射率峰值随之增大,信噪比也随之增加.其次,通过拟合不同延迟时间下泵浦光功率和差分反射率的实验曲线,并和理论模型比较后发现,在一定范围内的泵浦功率和差分反射率呈线性相关,未故意掺杂高纯n型砷化镓的饱和载流子浓度为(3.590 1±0.310 3)×1017 cm~(-3).在此基础上,把光生载流子动力学过程分为3个过程:804±67fs的光激发过程、134~268fs的初始散射过程、1ps和3~6ps的复合过程.研究表明,差分反射率与探测功率不存在显著的依赖性,但差分反射谱的信噪比与探测功率存在相关性.     

A model to characterize the effect of cross phase modulation on polarization in an optical fiber transmission system

This paper presents a model that described the process of inter-influence between cross phase modulation (XPM) and polarization effects such as polarization mode dispersion (PMD) and polarization dependant loss (PDL) in an optical fiber communication system. Previous papers have discussed PMD-PDL and PMD-XPM effects in separate models, but in this work we developed a model that is capable of characterizing and evaluating PMD, PDL and XPM simultaneously. From the model, we found that PMD and PDL can be affected by XPM. The presence of XPM changed the vector directions of PMD and PDL in Stokes space.     

Numerical simulation of phase modulation method used as high sensitive self-mixing vibrometry in vertical-cavity surface-emitting laser

The self-mixing interference technique is a compact sensing technique that has widely used for measuring vibrations. For purpose of increasing the accuracy of the vibration measurement system, a new phase modulation technique based on the self-mixing interferometer is presented. The phase modulation is obtained by varying the length of external cavity periodically. The vibration information of the external target is determined by the Fourier analysis method. Theoretical analysis and numerical simulations of phase modulation method used as high sensitive self-mixing vibrometry based on vertical-cavity surface-emitting laser are given in this paper.     

A detailed analysis of cross-phase modulation effects on OOK and dpsk optical WDM transmission systems in the presence of GVD,SPM, and ASE noise

A performance analysis is carried out to evaluate the effect of cross-phase modulation (XPM) on a dispersion-managed 20 Gb/s optical wavelength-division multiplexing (WDM) transmission system using either the on-off keying (OOK) or the different-phase-shifting keying (DPSK) modulation, in the presence of the group-velocity dispersion (GVD), self-phase modulation (SPM), and amplified spontaneous emission (ASE). It is found that to achieve a bit error rate (BER) of 10⁻⁹ at a distance of 160 km, a 1.0 dB XPM power penalty is incurred for input channel power of 3 dBm in the OOK transmission and 7 dBm in the DPSK transmission. The power penalty increases with input channel powers and is inversely proportional and exhibits oscillations with respect to the channel separation. The oscillation is evenly spaced for the DPSK but not for the OOK and suggests the presence of optimum separation values. The XPM penalty decreases when a high dispersion fiber is used and increases linearly with increasing dispersion slope. Small residual dispersion can reduce the penalty of nonlinear effects.     

Influence of the modulation index of Mach-Zehnder modulator on RoF link with ASK millimeter-wave signal

In this paper, the influence of the modulation index of LiNbO₃ Mach-Zehnder modulator on the radio over fiber (RoF) link based on single sideband (SSB) optical millimeter (mm)-wave with ASK signal is theoretically and numerically investigated. Our investigation shows that there exists an optimal modulation index to generate the SSB optical mm-wave with a maximal RF photocurrent. Although the fiber dispersion distorts the code form and degrades the performance of the RF signal demodulated from the SSB optical mm-wave after fiber transmission, it does not cause the closure of the eye diagram. However, the influence of the fiber dispersion becomes more obvious as the modulation index increases. For the duplex RoF link with the optical carrier of the uplink recovered from the downlink, a larger modulation index of the downlink causes a worse crosstalk from the downlink to the uplink.     

Thermal modulation in Zeno and anti-Zeno effects on quantum measurement

We have proposed the mathematical formulations of quantum Zeno and anti-Zeno effects at non-zero temperature on vibrational population transfer and dissociation of a diatomic system. The proposed formulation has been tested for diatomic systems HBr⁺ and HI using time-dependent Fourier grid Hamiltonian method. The effect of temperature on the dynamics is incorporated through the population distribution at different vibrational states following Boltzmann distribution formula. It has been found that with increase in temperature Zeno effect increases and anti-Zeno effect decreases in case of survival probability on ground vibrational state. In case of dissociation, with increase in temperature Zeno effect increases while anti-Zeno effect remains almost same.     

Impacts of higher-order dispersions and saturable nonlinearities on modulation instability in negative-refractive metamaterials

On the basis of the standard linear stability analysis and Drude electromagnetic model,the impacts of higher-order dispersions and three kinds of typical saturable nonlinearities on modulation instability(MI) have been analyzed and calculated for negative-refractive metamaterials(MMs).Our results show that the MI gain spectra consist of only one spectral region instead of one or two regions in ordinary materials,which may be close to or far from the zero point.Particularly,the spectrum far from the zero point has a high cut-off frequency but a narrow spectral width,which is obviously beneficial to the generation of high-repetition-rate pulse trains.Moreover,MI characteristics here will vary with the normalized angular frequency which can be modified by adjusting the structures of negative-refractive MMs,signifying the controllability of bistable solitons and MI based applications.The effects of saturable nonlinearities are similar to those in ordinary materials.     

Investigation of high-speed optical FSK generation scheme based on carrier suppression and phase modulation

We propose and numerically investigate a novel high-speed (40-Gb/s and above) optical frequency shift-keying (FSK) transmitter scheme. By optical carrier-suppressed modulation and differential phase-shift-keying (DPSK) to intensity modulation (IM) conversion, only one light source is needed in the proposed scheme to generate high-speed optical FSK signals. By using a Mach-Zehnder modulator (MZM) as phase modulator and an additional delay interferometer (DI) cascaded after the carrier-suppressed MZM to suppress the remaining carrier, the performance of our scheme can compete with the double-light-source counterparts. The transmission performances of the FSK signal generated with the proposed scheme as well as detuning and bandwidth tolerance of the demodulation filter are also carefully investigated. Simulation results show that the proposed FSK generation scheme is very suitable for the next-generation optical access network and optical label switching network. A potential application of our scheme in high-speed passive optical network is proposed.     

双势阱玻色-爱因斯坦凝聚体系的自俘获现象及其周期调制效应

研究了双势阱玻色-爱因斯坦凝聚体系(BEC)的自俘获现象(self-trapping). 在平均场近似下通过相平面(phase space)分析的方法研究了两种自俘获的机理：1)势阱中的粒子数在平衡位置附近振动，而相对相位随时间单调变化(running-phase); 2) 势阱中的粒子数和相对相位都在平衡点附近振动. 研究了周期调制场对自俘获现象的影响，发现发生自俘获现象的相变参数能够被周期场非常有效的调制，从而在弱相互作用BEC体系中也可以观察到自俘获现象. 还研究了多体量子涨落对自俘获现象的影响，讨论了在现有的实验条件下对凝聚体自俘获现象进行观察和周期调制. 关键词： 玻色-爱因斯坦凝聚 自俘获 双势阱 周期调制     

Analysis of an optical logic gate using a symmetric coupler operating with pulse position modulation (PPM)

In this paper we propose the operation of an all-optical logical gate based in a symmetric nonlinear directional coupler (NLDC) operating with a pulse position modulation (PPM). The performance of a symmetric NLDC realizing two-input AND/OR logical functions, which can be applied in transmission and processing of signals in all-optical form in TDM systems, is examined. This integrated symmetric NLDC logical gate operates with two ultrashort soliton light pulses (2 ps), which are modulated in agreement with the technique of pulse position modulation (PPM). Initially, we evaluate the effect resulting of an increment in the PPM coding parameter offset (ε), for the temporal position of the output pulse, considering the anomalous group-velocity dispersion (GVD), nonlinear self phase modulation (SPM) and without loss propagation regime of input pulses, in the cores 1 and 2 of the NLDC. In this situation, we analyze the four possible situations for the two-input logical gate, modulating the 1 and 2 input pulses through temporal displacement and allowing a variation in the coding parameter offset. We can conclude that is possible to get AND/OR logical operations for the cores 1 or 2, without to insert PPM error, since a phase control (Δ?) exists applied in agreement with the logical level of the input pulse in the core 1. Finally we define the truth table, considering the adequate phase difference and coding parameter offset for the stable operation of the AND/OR logical gate based in the symmetric NLDC.     

Nonequivalent spectra of unpaired electrons in field and frequency modulation

We report a difference in the spectral lineshapes of continuous-wave (CW) electron paramagnetic resonance (EPR) spectroscopy between field and frequency modulation. This finding addresses the long-standing question of the effect of modulation in EPR absorption. We compared the first-derivative EPR spectra at 1.1 GHz for lithium phthalocyanine crystals, which have a single narrow linewidth in the EPR absorption spectrum, using field and frequency modulation. The experimental findings suggest that unpaired electrons have different behaviors under perturbation due to field and frequency modulation.