Nonlinear gain suppression in quantum-well lasers due to carrier heating: the roles of carrier energy relaxation, electron-hole interaction, and Auger effect

The influences of carrier energy relaxation and electron-hole interaction on the nonlinear gain coefficient are detailed inspected by a numerical comparison of carrier heating model with classical rate-equation model in a large-signal analysis. It is shown that the nonlinear gain coefficient due to carrier heating is in proportion to an effective carrier energy relaxation time and has a nonlinear relation to the electron-hole energy exchange time. Accordingly, an empirical formula is deduced. In addition, the influence of Auger heating on the modulation dynamics is also considered, which can not be described by a single phenomenological nonlinear gain coefficient. Furthermore, the dependence of the nonlinear gain coefficient on the laser emission wavelength of distributed feedback laser is also demonstrated quantitatively.     

Unambiguous determination of quantum capture, carrier diffusion,and intrinsic effects in quantum-well laser dynamics usingwavelength-selective optical modulation

A novel wavelength-dependent optical modulation technique capable of explicitly delineating the effects of quantum capture, carrier diffusion, and other intrinsic effects in quantum-well laser dynamics is described. Results for a compressively strained multiple-quantum-well laser are presented     

Rabi oscillations for subpicosecond pulses in quantum-well optical amplifiers: interplay of carrier heating, nonlinear, and spectral effects

Using the Foreman effective mass Hamiltonian for strained In/sub x/Ga/sub 1-x/As--In/sub y/Ga/sub 1-y/As/sub z/P/sub 1-z/ quantum wells, the propagation of subpicosecond pulses in a 1.55-/spl mu/m optical amplifier was calculated. The multisubband carrier dynamics as well as the polarization dynamics were taken into account. Carrier heating and coherent light-carrier interactions as well as the interplay of these nonlinear processes and the amplifier dispersion are studied. Strong Rabi oscillations occur in the optical field of a propagated pulse, its frequency chirp, as well as in the carrier density and temperature. While the Rabi oscillation imposes negative frequency chirp and hence red-shifts the pulse spectrum, positive frequency chirp can occur due to the local gain dispersion, where the higher frequency components of the pulse have larger gain. Due to the Rabi oscillation, the spectrum of the amplified pulse is considerably distorted and sidebands emerge. For a linearly chirped input pulse, the spectrum of the output pulse can be either red-shifted or blue-shifted with respect to its center frequency, depending on its initial chirp. For strong pulse propagation, a pronounced pulse break up occurs when a 175-fs pulse propagates in the gain regime, while a significant pulse compression occurs when the pulse propagates at the transparency point.     

Effects of electronic current overflow and inhomogeneous carrier distribution on InGaN quantum-well laser performance

Laser performance of several InGaN quantum-well (QW) lasers with an emission wavelength of 392-461 nm are numerically studied with a LASTIP simulation program. Specifically, the effects of electronic current overflow and inhomogeneous carrier distribution on the laser performance of InGaN QW lasers operating at different wavelengths are investigated. Simulation results indicate that the use of an AlGaN blocking layer can help reduce the electronic current overflow and, in addition to the dissociation of the InGaN well layer at a high growth temperature during crystal growth, the inhomogeneous carrier distribution in the QWs also plays an important role in the laser performance. From the simulation results, we conclude that the lowest threshold current density is obtained when the number of InGaN well layers is two if the emission wavelength is shorter than 427 nm and one if the emission wavelength is longer than 427 nm, which are in good agreement with the results observed by Nakamura et al. in their experiments.     

Influence of carrier diffusion on the dynamics of a two-mode laser

The two-mode semiconductor laser rate equations are generalized to account for the diffusion of free carriers. Using the diffusion coefficient as a control parameter we analyze the competition between small-scale and large-scale gratings in the laser dynamics. Many solutions become unstable as the diffusion coefficient decreases     

A small-signal analysis of the modulation response of high-speedquantum-well lasers: effects of spectral hole burning, carrier heating,and carrier diffusion-capture-escape

We present a small-signal analysis of the modulation response by simultaneously considering the effects of spectral hole burning, carrier heating, and carrier diffusion capture-escape. An explicit form of the small-signal modulation response is obtained and the nonlinear gain coefficients associated with each physical process are defined. Further simplifications in our results will give analytical forms for calculating the resonant frequency and damping rate of the modulation response. One of the simplified versions of our results is shown to agree with previous investigations. The effects of the carrier dephasing time, energy relaxation time, and diffusion-capture-escape times on the high-speed performance of QW lasers are theoretically investigated     

激光等离子体相互作用中量子电动力学效应对激光能量吸收的影响

为了研究强激光与等离子体相互作用中的量子电动力学(QED)效应对激光能量吸收的影响,用理论方法对强度为I>10^22wcm^-2、反方向传播的两束圆极化强激光与等离子体固体靶相互作用过程进行研究。探讨经典和QED情况下的辐射阻尼效应对激光能量吸收的影响。结果表明,在QED情况下,在相互作用过程中能够产生高能电子和高能辐射。这些高能电子集体振荡过程中的辐射阻尼导致激光能量的强吸收。QED情况比经典情况具有更好的实际应用价值。     

长脉冲激光能量时空分布对金属热作用的影响

为研究激光能量的时空分布对金属基体材料热作用效果的影响,建立了空间轴对称有限元计算模型.在平均功率相等和考虑吸收率随温度变化的前提下,分别模拟了平均功率相等的连续激光和长脉冲激光(脉宽为1 ms,脉冲频率分别为25、50、100和250 Hz)与Al板作用过程中气化前的热作用,并利用热焓法处理了固-液相变过程,对比了能量的时空分布不同的激光热作用效果的影响.结果表明:由于基体材料的吸收率同温度之间相互促进的关系,在上述前提下,热作用效果随重复频率的减小显著增加,并且金属吸收率的温度相关性对温升过程庞大影响.     

Monte Carlo analysis of the carrier relaxation processes in linear and parabolic-GRINSCH quantum well laser structures

The carrier relaxation process is widely acknowledged to have a strong bearing on the modulation limit of quantum well lasers. As a first and crucial step toward achieving a better understanding of this phenomenon, we have developed a numerical technique to study such processes in graded-index separate confinement heterostructure quantum well laser structures having an arbitrary grading profile. We base our approach on ensemble Monte Carlo simulation of the carrier transport in the 3-D graded-index region and in the 2-D quantum well. We also introduce a technique to handle the carrier capture and re-emission processes within the Monte Carlo method. The results obtained from our calculations for a number of structures with quantum well sizes 50-100 Å indicate that the overall carrier capture time is about 5-7.5 ps under low injection condition for the linearly graded structures, and significantly longer for the parabolically graded structures. On the other hand, the carrier capture efficiency is found to be higher for the parabolic graded-index structures. We also compare our calculations to published experiments and find good agreement     

Measurements of heating and energy storage in flashlamp-pumped Nd:YAG and Nd-doped phosphate laser glasses

We have experimentally measured the energy stored and the heat generated in flashlamp-pumped Nd:YAG and six different commercially available Nd-doped phosphate glasses. We find that the normalized heating parameterX, defined as the heat deposited per unit stored energy, is 1.5-2 times the value expected from the known spectroscopy of the Nd ions in these hosts and the emission spectrum of Xenon flashlamps. We discuss the heat sources in Nd-doped glasses. Using a theoretical model, we calculate the influence of Nd concentration, flashlamp pulse length, and current density onX, and we point out the effects ofXon practical laser designs.     

A quantum-well model and the optical absorption edge in structurally nonuniform a-Si:H-based alloys

This paper describes studies of the microstructure and optical absorption edge of films of a-Si: H and a-SiN_x: H alloys (x=0.0–0.72), obtained by decomposing gas mixtures in an rf glow discharge. For the a-Si: H films, the gas mixture was monosilane diluted by hydrogen, for the a-SiN_x: H alloys it was SiH₄+NH₃. Structurally nonuniform films with “device-quality” optoelectronic characteristics were obtained when the rates of growth were increased (to 8 Å/s). Atomic force microscopy and infrared absorption spectroscopy are used to identify a characteristic feature of the microstructures of these films: the presence of islets ～500 Å in diameter, whose boundaries are formed by clusters of hydrogen atoms (in the case of a-Si: H) or of hydrogen and nitrogen (in the case of a-SiN_x: H). In this case the optical band gap of a-Si: H (a-SiN_x: H) is determined by the concentration of SiH (SiN) bonds in the interior of the islets and is not sensitive to changes in the content of hydrogen (nitrogen) at their boundaries. This result is explained by a quantum-well model which takes into account the characteristic sizes of the microstructures formed by hydrogen or nitrogen atoms.     

A comparison of classical, state-space and neural network control to avoid slip

A neural network controller has been implemented on a microcomputer in Pascal to control the slip in the drive of a film-copying machine. Two conventional controllers were also used; these were a simple integral controller and a state-space controller with an estimator. Control with varying loads showed a step response could be followed with no overshoot and with a setting time of less than 0.1 s by the neural controller. The neural network was 50% faster than the other controllers in responding to step velocity input preventing slip from occurring at very small axle load. The response to a sine wave velocity input was shown to be non-linear but of the same amplitude for the neural network. Experiments with four times faster sampling gave a 30% improvement in response times.     

A comparison of classical and Bayes risks when the quality varies randomly

The present study analyses the performance of Bayes and classical acceptance sampling plans when consumer's and producer's quality specifications are available. Bayes and modified classical risks have been compared for a single sampling plan when the quality varies randomly.     

激光制导半实物仿真中弱激光能量测试和控制

介绍了激光制导半实物(HWIL)仿真中1 064 nm激光散射微弱能量的测量方法,设计并构建了激光散射弱能量测量系统.结合室内大气路径气溶胶Mie散射理论进行激光散射微弱能量预测模型分析,利用测量系统进行了实地验证,预测模型结果与实际测量结果符合较好,该系统可以满足实验室激光大气传输弱激光能量测量要求.分析了影响实际仿真结果的主要因素,指出在仿真测试过程中应根据测试需求和测试手段采取合理的空间布局和必要的干扰能量控制措施以减小额外干扰能量对仿真结果的影响.     

Thermodynamic and spectroscopic measurements of the electron-hole drop binding energy in Ge: An old problem revisited

One can expect to get the electron-hole (EHD) binding energy φ either from spectroscopic or from threshold measurements. While the value obtained by the former is in excellent agreement with theory, the results given by the latter did not seem to lead to φ in a simple way. We present here new optical threshold studies in which we think that the right conditions to obtain a direct thermodynamic measurement of φ have been mastered. Besides, our results present novel features like a sharp jump of the EHD luminescence at threshold above 2.4 K and observation of hysteresis in the whole temperature range of our work.     

A study of the static and multigigabit dynamic effects of gainspectra carrier dependence in semiconductor lasers using atransmission-line laser model

The addition of carrier diffusion and gain curve peak carrier dependence to the transmission-line laser model allows the study of both the temporal and the spectral effects of this dependence. The model, including a photodiode model, is described and results for a static 860-nm device and an 8-Gb/s bit-sequence-modulated 1550-nm constricted-mesa device compare favorably with the previously obtained numerical, analytical, and experimental data     

A computer aided system for research on the quantitative laser spectroscopy of ethylene

Spectral line strength, self- and N₂-broadened widths of 28_2,27–28_1,27 transition of ethylene v₇ band have been obtained at room temperature and were agree well with the results of theoritical calculation. With those spectral parameters, the temperature dependence of C₂H₄ absorption coefficients at CO₂laser transition 10P(14)were measured. The results showed that the temperature dependence of the linewidth was close to T^?0.4. A computer aided system was used so that the accuracy of the measurement was well improved than that we did before.     

高功率激光与材料相互作用机理研究进展

本文综述了高功率激光和材料相互作用物理机理研究现状和存在问题,着重阐述了笔者在这方面已进行的理论和实验工作,并指出了本方向可继续进行的研究工作。     

Intraocular Nd:YAG laser surgery: laser-tissue interaction, damagerange, and reduction of collateral effects

The damage mechanisms of intraocular Nd:YAG laser surgery and their respective damage ranges were investigated in vitro using bovine cornea specimens as a model tissue. The main damage mechanisms are plasma formation and expansion, emission of acoustic transients, and cavitation with jet formation. When a sequence of laser pulses is applied, the interaction of the acoustic transients with gas bubbles remaining from preceding laser exposures is also important. To distinguish the effects caused by the different physical mechanisms, laser pulses were aimed directly onto the corneal endothelium, through the cornea, and parallel to the cornea at various distances. Simultaneously, the cavitation bubble size was determined. The damage range of the acoustic transients produced by a 4 mJ laser pulse is several millimeters, when they can interact with small gas bubbles attached to the corneal endothelium     

Effects of carrier mobility, energy gap, and excitation size on the performance of single layer organic solar cells

A model of universal single layer organic solar cells in metal-insulator-metal （MIM） representation involving field-depen- dent carrier mobility is set up. The current-voltage characteristics as well as the distribution of electron density, hole density and recombination rate on a set of parameters are simulated. Subsequently, the dependences of the short-circuit current density （Jsc） and open-circuit voltage （Voc） on the electron and hole zero-field mobility, excitation generation rate, energy gap, as well as electron-hole pair distance in an excitation are investigated. It is demonstrated that the enhancement of either the electron mobility or the hole mobility can contribute to the increase of Jsc in the devices. The increase of the hole mobility can lead to the improvement of both Jsc and Voc, and the simultaneous increase of the electron mobility and hole mobility will greatly elevate Jsc but maintain a steady Voc. Additionally, all the increases of the excitation generation rate, energy gap and electron-hole pair distance are beneficial to both the remarkable increases of Jsc and Voc of the devices.