Nd-glass belt lasers with improved beam quality

Nd-doped phosphate glass belt lasers pumped by laser diodes are demonstrated. The Nd-glass belt with a large cross-section and a small Fresnel number is air-cooled to provide around 18-W continuous wave (CW) output power with a beam quality factor of My2 × Mx2 = 16 × 3.2. With 6 pairs of V-grooves scribed on the edges of the glass belt, the beam quality is improved to My2 × Mx2 = 3.5×2.8 with output power of 13.5 W.     

Microscopic modelling of bulk and quantum-well GaAs-based semiconductor lasers

The dynamical behaviour of Fabry-Perot type semiconductor lasers is modelled, including the relevant many-body Coulomb effects of the excited carriers. Conditions are given under which a parametrization of the full model is possible, allowing simple analytic relations for local gain, refractive index and linewidth enhancement factor. The parameters of the simplified model are uniquely determined by the microscopic theory and have to be optimized for the respective operating conditions. The theory is evaluated for bulk and quantum-well GaAs active material and a variety of laser structures, including strongly and weakly index-guided structures, as well as purely guided single-and twin-stripe lasers.     

Complete rate equation modelling of quantum cascade lasers for the analysis of temperature effects

The effect of temperature on the dynamics of a GaAs-based quantum cascade laser (QCL) is analysed using a complete rate equation model. The analytical expressions for the threshold current density and the output power are derived using the model and the thermal behaviour of these parameters is examined. A better conformity of the threshold current density with experiment at higher temperatures is achieved. The effect of temperature on the 3 dB optical bandwidth is further investigated using the same model. A comparative analysis of the model is performed with the recently reported rate equation models. It is observed that the 3 dB optical bandwidth falls more rapidly at higher operating temperatures that highlight the effects of leakage and backscattering processes present in the device.     

Hole capture rate of GaInAs/InP strained quantum-well lasers

The property of hole capture of quantum wells is important in the static properties of lasers above threshold, such as the differential efficiency and light output power. We investigate experimentally the hole capture rate and its influence on the carrier overflow in the optical confinement layers for compressive-strained, tensile-strained and unstrained GaInAs/GaInAsP/InP quantum-well lasers emitting at 1.5 m by measuring the spontaneous emission from the optical confinement layers above threshold. The carrier density in the optical confinement layers increases with current owing to finite hole capture rates. This increase is dependent on well thickness and barrier height determined by the strain. This increase is comparable in the tensile-strained and unstrained lasers with relatively low threshold, while in the compressive-strained laser it is about double that in the other two types. The dependence of this increase on threshold carrier density is also observed, that is the carrier density in the optical confinement layers increases rapidly in high-threshold samples, in particular, in the tensile-strained laser with large hole barrier height. From these results, laser operation with high output power and high efficiency is expected by reducing threshold carrier density in the tensile-strained laser and by increasing well numbers in the compressive-strained laser as long as the inhomogeneous injection between wells is not severe. By fitting measurements with theory, the hole capture time is estimated as 0.1 to 0.25 ps in these strained and unstrained lasers.     

Valley density profile for highly efficient x-ray lasers with good beam quality

A plasma with a valley density profile at the required electron density for high-gain operation of an x-ray laser could be obtained using a normally incident 300 ps laser pulse followed by a grazing-incident 300 ps laser pulse. Sudden heating of the valley plasma region by another grazing-incident 300 fs laser pulse would then yield a highly efficient x-ray laser beam with a deflecting angle of 2 mrad and a divergence angle of 4 mrad. Saturation operation of the x-ray laser beam at 32.6 nm could be achieved with a total pump energy of less than 100 mJ.     

High beam quality broad-area diode lasers by spectral beam combining with double filters

A modified spectral beam combining(SBC) approach based on double asymmetrical filters was proposed. By using this scheme, the high-order lateral modes at the edge of the far-field pattern can be suppressed in the external cavity, and the beam quality in the slow-axis direction was improved from 16.1 to 13.4 compared to the conventional SBC. In the meanwhile, the electrical-to-optical efficiency from the modified SBC was more than 40% with an output power of 34.1 W, which is similar to that of the conventional SBC.     

两路120 W固体激光高光束质量相干合成

通过主动相位控制实现了两路固体激光器的高光束质量相干合成输出,总输出功率达到240 W。建立了两路120 W板条激光放大器,通过光束整形获得了高光束质量方形光斑,并实现了高占空比光束拼接,填充因子高于92.4%。研制了光轴一致性探测与控制系统,采用基于压电陶瓷的快反镜实现了光轴的高精度闭环控制,两束激光光轴一致性优于2 rad（RMS）。设计了基于现场可编程门阵列（FPGA）的相位探测与控制系统,用随机并行梯度下降算法（SPGD）锁定了两束激光的活塞相位。相位闭环后,远场光斑峰值提高到开环状态的1.7倍,为理想值的84%。合成光束主瓣包含67%的激光总能量,光束质量（BQ）为1.1。     

两路120 W固体激光高光束质量相干合成

通过主动相位控制实现了两路固体激光器的高光束质量相干合成输出,总输出功率达到240W。建立了两路120W板条激光放大器,通过光束整形获得了高光束质量方形光斑,并实现了高占空比光束拼接,填充因子高于92.4%。研制了光轴一致性探测与控制系统,采用基于压电陶瓷的快反镜实现了光轴的高精度闭环控制,两束激光光轴一致性优于2μrad(RMS)。设计了基于现场可编程门阵列(FPGA)的相位探测与控制系统,用随机并行梯度下降算法(SPGD)锁定了两束激光的活塞相位。相位闭环后,远场光斑峰值提高到开环状态的1.7倍,为理想值的84%。合成光束主瓣包含67%的激光总能量,光束质量(BQ)为1.1。     

Powerful solid-state transversely diode-pumped YAG: Nd lasers with improved radiation quality

YAG: Nd lasers with transverse diode pumping of a cylindrical active (lasing) element are developed and studied. Pumping is carried out with continuous-wave (cw) laser diode array modules. At pump powers of 500, 820, and 1400 W, the output powers of the lasers operating in the multimodal regime are, respectively, 150, 350, and 500 W. The optical efficiency exceeds 30%. In the case of an unstable resonator operating in the negative branch of the stability diagram, the laser power equals 320 W with a beam divergence only twice as large as the diffraction limit. The design of a YAG: Nd laser pumped with pulsed-periodic laser diode array modules is presented. The total pump energy is 5 J, and the width and repetition rate of pulses are, respectively, 250 μs and 100 Hz.     

A simple method to determine the thermal focal length of solid-state lasers with rate equation

A simple method to measure the thermal focal length of solid-state laser with rate equation was presented. By measuring the output beam power of lasers, the beam waist of oscillating wave can be deduced from rate equation under the stable oscillation condition. Based on the ABCD matrix theory, the thermal focus of the gain medium was easily obtained. The thermal focus of a LD end-pumped Nd:GdVO₄ laser was measured and the experimental results agreed with the theoretical predication.     

Numerical analysis of Ytterbium-doped double-clad fiber lasers based on the temperature-dependent rate equation

The effects of temperature on the output power and optimal conditions of Ytterbium-doped double-cladding fiber lasers (YDDFLs) are discussed. Temperature-dependent rate equations for YDDFLs based on Boltzmann distribution are built in two-end pump model. The results show that the output power and the slope efficiency decrease with the increase of temperature. The increase of pump power results in that the effect of the temperature on the output performance increases, and the fiber length is a factor that the effect of the temperature depends on really. Moreover we combine the heat distribution formula with the rate equations and compare the numerical results considering with that not considering the temperature-dependence inside laser cavity, the effect of temperature can be ignored only when the pump power is lower, and the optimal fiber length becomes longer when considering the temperature-dependence.     

Demonstration of improved beam quality in an image-rotating optical parametric oscillator

We performed laboratory and numerical modeling studies of an optical parametric oscillator with 90 degrees intracavity image rotation. We found that the signal beam was more symmetric than that from comparable cavities without image rotation, and it had low values of the beam quality factor, M(2) . Oscillator performance agreed well with our numerical model.     

Modeling of Q-switched lasers with top-hat pump beam distribution

For laser diode end-pumped solid-state lasers, the pump beam can be considered to be a Gaussian distribution or a top-hat distribution or between them. The rate equations for Q-switched lasers have been treated before, by considering the pump beam as a Gaussian distribution. This paper deals with the rate equations and the solutions for Q-switched lasers under a top-hat pump beam distribution. The normalized rate equation is obtained and solved numerically. A group of general curves are given and the comparison with the results under the Gaussian pump beam distribution and the plane-wave approximation is made. The results show that the solutions of the rate equations under a top-hat pump beam distribution are different from those under a Gaussian pump beam distribution, but the difference is very small. The solutions of the rate equations under both situations can give much more precise theoretical results than those under the plane-wave approximation. PACS 42.60.Gd; 42.55.Xi; 42.55.Ah     

Generalised expressions for the association and dissociation rate constants of molecules with multiple binding sites

ABSTRACT

Many proteins exhibit multiple binding patches. A patch may harbour a key chemical modification site, but may also simply act as a trap for the binding to another site. Here we consider the scenario in which one molecule (enzyme) binds another molecule (substrate) which contains two sites. We present microscopic expressions for the rate at which the enzyme binds to a particular site on the substrate, both for the scenario in which the enzyme directly binds the site without first visiting the other site, and for the case in which it may visit the other site an arbitrary number of times before binding to the site of interest. We also present the expressions for the corresponding dissociation reactions. These expressions can be used to compute in a single rare-event simulation of the dissociation pathway not only both the intrinsic and effective dissociation rate constants but also both association rate constants.     

高功率激光器腔镜热变形对输出光束质量的影响

利用有限元分析法结合Fox-Li迭代法,考虑腔内本征模式与腔镜热形变的相互耦合作用,计算模拟了正支共焦非稳腔的本征模式分布,定量分析了高功率激光器腔镜热变形对输出光束质量的影响,重点讨论了腔镜热变形所引起的腔内本征模式相位特性的变化,并从波前功率谱密度、Zernike像差系数及光束质量值等角度对腔镜发生热形变前后的激光器输出光束的光束特性进行比较分析。研究结果表明:高功率激光器腔镜热形变对输出光束的光束质量会产生一定的影响,且随着激光输出功率的增大,镜面热形变引起的输出光束波前相位高频比例及Zernike高阶像差均会有所增大,波前畸变程度也明显变大,光束质量逐渐变差。     

Rate equations for modelling of short-pulse rare-gas halide excimer lasers

In this work, on the basis of the analysis of data relating to relaxation processes and emission spectra of excimer molecules, the rate equations for modelling short-pulse rare-gas halide excimer lasers are formulated and discussed. The equations take into account relaxation processes important for short-pulse generation, such as decay of B-, C- and X-states, vibrational relaxation in these states, and B-C mixing. As a special case for these equations, simplified equations for KrF and XeCl lasers are introduced. The gain recovery curves and the dependences of saturation energy on pulse duration, computed both for KrF and XeCl, coincide well with the experimental data. Discussion of the factors influencing the gain dynamics and the saturation energy of these media, is also presented.     

高功率激光器腔镜热变形对输出光束质量的影响

利用有限元分析法结合Fox-Li迭代法,考虑腔内本征模式与腔镜热形变的相互耦合作用,计算模拟了正支共焦非稳腔的本征模式分布,定量分析了高功率激光器腔镜热变形对输出光束质量的影响,重点讨论了腔镜热变形所引起的腔内本征模式相位特性的变化,并从波前功率谱密度、Zernike像差系数及光束质量值等角度对腔镜发生热形变前后的激光器输出光束的光束特性进行比较分析。研究结果表明：高功率激光器腔镜热形变对输出光束的光束质量会产生一定的影响,且随着激光输出功率的增大,镜面热形变引起的输出光束波前相位高频比例及Zernike高阶像差均会有所增大,波前畸变程度也明显变大,光束质量逐渐变差。     

锥形太赫兹量子级联激光器输出功率与光束特性研究

针对锥形太赫兹量子级联激光器, 利用有限差分波束传播法和速率方程法, 建立了准三维的太赫兹有源器件仿真模型, 能够对具有轴向非线性波导结构的激光器进行模拟. 利用此模型, 研究了锥角大小对激光器输出光功率及光束质量的影响. 仿真结果表明, 考虑到器件之间的光耦合效率, 为了达到最大的有效输出光功率, 锥形太赫兹量子级联激光器的锥角存在一个最优值.