On the Dynamics of the Adenylate Energy System: Homeorhesis vs Homeostasis

Biochemical energy is the fundamental element that maintains both the adequate turnover of the biomolecular structures and the functional metabolic viability of unicellular organisms. The levels of ATP, ADP and AMP reflect roughly the energetic status of the cell, and a precise ratio relating them was proposed by Atkinson as the adenylate energy charge (AEC). Under growth-phase conditions, cells maintain the AEC within narrow physiological values, despite extremely large fluctuations in the adenine nucleotides concentration. Intensive experimental studies have shown that these AEC values are preserved in a wide variety of organisms, both eukaryotes and prokaryotes. Here, to understand some of the functional elements involved in the cellular energy status, we present a computational model conformed by some key essential parts of the adenylate energy system. Specifically, we have considered (I) the main synthesis process of ATP from ADP, (II) the main catalyzed phosphotransfer reaction for interconversion of ATP, ADP and AMP, (III) the enzymatic hydrolysis of ATP yielding ADP, and (IV) the enzymatic hydrolysis of ATP providing AMP. This leads to a dynamic metabolic model (with the form of a delayed differential system) in which the enzymatic rate equations and all the physiological kinetic parameters have been explicitly considered and experimentally tested in vitro. Our central hypothesis is that cells are characterized by changing energy dynamics (homeorhesis). The results show that the AEC presents stable transitions between steady states and periodic oscillations and, in agreement with experimental data these oscillations range within the narrow AEC window. Furthermore, the model shows sustained oscillations in the Gibbs free energy and in the total nucleotide pool. The present study provides a step forward towards the understanding of the fundamental principles and quantitative laws governing the adenylate energy system, which is a fundamental element for unveiling the dynamics of cellular life.

不同腔结构下的声光调Q双包层光纤激光器特性研究

报道了半导体激光器端面抽运不同结构的声光调Q的双包层光纤激光器的脉冲输出特性.对前向、后向不同抽运方式的掺镱调Q双包层光纤激光器在输出平均功率,调Q脉冲宽度及脉冲稳定性进行了对比及讨论;其中后向抽运的光纤激光器,在10kHz重复频率调制下,获得了斜效率为60%的平均功率输出,其脉冲宽度为52ns,单脉冲能量为0·3mJ.最后利用不同抽运方式下的速率方程,理论分析调Q脉冲的特性,分析结果与实验相符.

双调Q开关热退偏补偿全固态激光器

为了有效地补偿激光二极管(LD)侧向抽运1000 Hz重复率电光调Q Nd:YAG激光器棒状增益介质内存在的热致双折射损耗，设计了一种新颖的双调Q晶体开关复合谐振腔结构。实验结果表明，设计的双调Q晶体开关结构Nd:YAG激光器输出激光脉冲能量比单调Q晶体开关结构的非补偿腔输出能量提高了56%，当侧面抽运半导体激光器输出功率达到450 W时，激光输出达到30 mJ/pulse，输出光束偏振度优于10:1，激光脉冲宽度约14 ns。并获得6.7%的光-光转换效率。通过对双调Q开光激光谐振腔进行建模，并用求解速

Comparison of Yb-doped fiber laser with one-end and double-end pumping configuration

A high-power Ytterbium-doped fiber laser (YDFL) with homemade double clad fiber (DCF) is introduced in this paper. The output power characteristics of a linear cavity fiber laser have been studied theoretically by solving the rate equations and experimentally tested with single- and double-end-pumping configurations. When both ends of the fiber are pumped by two high-power laser diodes with a launched power of similar to 300 W each, a maximum CW output of 444 W is obtained with a slope efficiency of similar to 75%. (c) 2006 Elsevier Ltd. All rights reserved.

一种改进型金属-半导体-金属光电探测器数学模型

以E. Sano的金属-半导体-金属光电探测器(MSM-PD)模型为基础,提出了一种改进型的模型。该模型以多个电流源和电容并联的形式构造,以吸收区过剩电子和空穴总数为研究对象,求解速率方程。另外计算了电容,给出了暗电流与端电压的非线性计算式,改进了传统模型中暗电流的线性计算方法。通过线性叠加给出了该模型光电流的数学解析解。通过在Matlab中的模拟计算,表明该模型具有计算量小、准确度高的特点,它不仅能反映一定偏压和光照下光电流的变化,而且能展示光电子在器件中的转化过程。这种模型也能较好地应用于微弱信号的检

上转换效应对Er3+-Yb3+共掺磷酸盐玻璃被动调Q 激光输出的影响

To study the effects of upconversion in Erbium, a set of rate equations that simulates the performance of the passively Q-switched Er:Yb:glass laser with a Co²⁺:MgAl2O4 saturable absorber was set up. The dynamics of the Er³⁺ excited state and the effect of upconversion on the passively Q-switched laser are obtained through numerical simulation of the model. It is found that the impact ratios of upconversion effect on the peak power of the passively Q-switched laser pluse and the repetition rate are both decreased with the increase of pump power.

Correlations between the performance characteristics of a liquid crystal laser and the macroscopic material properties.

In order to understand how the performance of a liquid-crystal laser depends on the physical properties of the low molar mass nematic host, we have studied the energy threshold and slope efficiency of ten optically pumped liquid-crystal lasers based on different hosts. Specifically, this leads to a variation in the birefringence, the orientational order parameter, and the order parameter of the transition dipole moment of the dye. It is found that low threshold energies and high slope efficiencies correlate with high order parameters and large birefringences. To a first approximation this can be understood by considering analytical expressions for the threshold and slope efficiency, which are derived from the space-independent rate equations for a two-level system, in terms of the macroscopic liquid crystal properties.

Low short term timing jitter in an integrated monolithic extended cavity semiconductor mode-locked laser

Jitter measurements were performed on a monolithically integrated active/passive cavity multiple quantum well laser, actively mode-locked at 10 GHz via modulation of an absorber section. Sub-10 ps pulses were produced upon optimization of the drive conditions to the gain, distributed Bragg reflector, and absorber sections. A model was also developed using travelling wave rate equations. Simulation results suggest that spontaneous emission is the dominant cause of jitter, with carrier dynamics having a time constant of the order of 1 ns.

Direct observation of coherent spin transfer processes in an InGaAs/GaAs quantum well via two-color time-resolved Kerr rotation measurements

A two-color time-resolved Kerr rotation spectroscopy system was built, with a femtosecond Ti:sapphire laser and a photonic crystal fiber, to study coherent spin transfer processes in an InGaAs/GaAs quantum well sample. The femtosecond Ti:sapphire laser plays two roles: besides providing a pump beam with a tunable wavelength, it also excites the photonic crystal fiber to generate supercontinuum light ranging from 500 nm to 1600 nm, from which a probe beam with a desirable wavelength is selected with a suitable interference filter. With such a system, we studied spin transfer processes between two semiconductors of different gaps in an InGaAs/GaAs quantum well sample. We found that electron spins generated in the GaAs barrier were transferred coherently into the InGaAs quantum well. A model based on rate equations and Bloch-Torrey equations is used to describe the coherent spin transfer processes quantitatively. With this model, we obtain an effective electron spin accumulation time of 21 ps in the InGaAs quantum well.

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 rum at the bias current of 500 mA. The influences of them 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.

Effects of coupling lens on optical refrigeration of semiconductors

Optical refrigeration of semiconductors is encountering efficiency difficulties caused by nonradiative recombination and luminescence trapping. A commonly used approach for enhancing luminescence efficiency of a semiconductor device is coupling a lens with the device. We quantitatively study the effects of a coupling lens on optical refrigeration based on rate equations and photon recycling, and calculated cooling efficiencies of different coupling mechanisms and of different lens materials. A GaAs/GaInP heterostructure coupled with a homo-epitaxial GaInP hemispherical lens is recommended.

Optical bistability in InP/GaInAsP equilateral-triangle-resonator microlasers

Optical bistability is reported in InP/GaInAsP equilateral-triangle-resonator (ETR) microlasers, which are fabricated by planar technology. For a 30 mu m side ETR microlaser with a 2-mu m-wide output waveguide connected to one of the vertices of the ETR, hysteresis loops are observed for the output power versus the injection current from 215 to 235 K. The laser output spectra are measured in the upper and lower states of the hysteresis loop, which show strong mode competition among transverse modes. The hysteresis loops are demonstrated by two-mode rate equations with asymmetric cross gain saturation and different output efficiencies. (C) 2009 Optical Society of America

Magnetic field switching in parallel quantum dots

We show that the Coulomb blockade in parallel dots pierced by magnetic flux Phi completely blocks the resonant current for any value of Phi except for integer multiples of the flux quantum Phi(0). This non-analytic (switching) dependence of the current on Phi arises only when the dot states that carry the current are of the same energy. The time needed to reach the steady state, however, diverges when Phi -> n Phi(0). Copyright (C) EPLA, 2009

Antiphase state in passively Q-switched Yb : YAG microchip multimode lasers with a saturable absorber GaAs

We report on recent experimental results of the spontaneous antiphase dynamics that occurs in a laser-diode-pumped multimode passively Q-switched microchip Yb:YAG (where YAG is yttrium aluminum garnet) lasers with a saturable absorber GaAs. We observe that the pulse sequence of the first mode characterized by one, two, and three pulses as a group and all the modes display an antiphase state as the pumping ratio rises. We modify the multimode rate equations to account for nonlinear absorption due to GaAs in the presence of spatial hole burning. We perform numerical simulations based on the proposed rate equations and reproduce the observed antiphase state of two and three active modes.

Photon iterative numerical technique for steady-state simulation of gain-clamped semiconductor optical amplifiers

The photon iterative numerical technique, which chooses the outputs of the amplified spontaneous emission spectrum and lasing mode as iteration variables to solve the rate equations, is proposed and applied to analyse the steady behaviour of conventional semiconductor optical amplifiers (SOAs) and gain-clamped semiconductor optical amplifiers (GCSOAs). Numerical results show that the photon iterative method is a much faster and more efficient algorithm than the conventional approach, which chooses the carrier density distribution of the SOAs as the iterative variable. It is also found that the photon iterative method has almost the same computing efficiency for conventional SOAs and GCSOAs.