Pulse generation by distributed feedback dye lasers

Equations describing the pulse generation by distributed feedback dye lasers are derived and solved by analytical methods. The dependence of the laser output on macroscopic parameters is discussed.On leave from the Department of Physics, Friedrich-Schiller-University, DDR-6900, Jena, German Democratic Republic     

Optimization of dye-laser output coupling by consideration of the spatial gain distribution

The problem of output-coupling optimization of a transversely or longitudinally pumped dye laser is treated in terms of a rate equation approach under steady state considerations. The theoretical study considers the spatial variation of the gain distribution and such typical features of a dye laser as reabsorption, triplet effects and excited state absorption. Numerical results obtained with realistic values of typical laser parameters (cross-sections for stimulated emission and reabsorption, number density of active molecules etc.) indicate that a higher degree of output coupling and consequently higher output powers are feasible upon a complete consideration of the spatial gain distribution     

多波长染料激光系统

本文报道由四台染料激光器组成的四波长染料激光系统、测量了四个波长激光合束后的激光参量和时间特性,简要地讨论了影响有效染料激光功率的诸因素.本系统输出总功率8.9W.     

Picosecond and subpicosecon pulse generation in synchronously pumped mode-locked cw dye lasers

Generation of tunable near-infrared picosecond and subpicosecond pulses in a synchronously pumped cw mode-locked Oxazine-1-perchlorate dye laser has been investigated for different values of the important system parameters. The experimental results confirm the predictions of a simple theoretical model which describes the steady-state pulse duration in terms of gain modulation strength, pump power, intracavity bandwidth, pump pulse length and cavity detuning. For a pump-pulse length of 100 ps and a bandwidth of more than 500 Å for the intracavity tuning element dye-laser pulses as short as 0.35 ps have been obtained. Under these conditions up to 100 mW of average output power were provided. In addition, mode-locking of an Oxazine 750 dye laser by synchronous pumping with the cw train of pulses obtained from the Oxazine-1-Perchlorate laser is reported.     

Numerical calculations of intracavity dye Q-switched ruby laser

A mathematical model describing the dynamic emission of the Q-switched ruby laser has been adapted. The suggested model allows the investigation of the effects of a dye cell on the mode characteristics of the ruby laser and, moreover, the study of the effect of the laser input parameters on the output laser pulse. This model simulates the nonlinear effects of dye pulse modulation on the laser emission.In addition, a numerical solution of a nonlinear rate equation system of the adapted model is discussed. The solution estimates the density of the emitted radiation, population inversion and energy transfer processes of the ruby laser rod and dye cell for different emission regimes (one pulse regime, free running pulses, repetition periodic pulses). The estimated results of the laser output pulse characteristics are in a good agreement with the other calculated and experimental results.     

Pressure Dependence of the Small-Signal Gain and Saturation Intensity of a Copper Bromide Laser

A pair of copper bromide lasers in an oscillator-amplifier configuration is used to investigate the small signal gain and saturation intensity as amplifying parameters and output power of lasers, versus pressure of buffer gas. It is shown that the amplifying parameters and laser output power have a maximum value at optimum buffer gas pressure of 11 Torr. The challenge between microscopic parameters such as stimulated emission cross section, laser upper level lifetime, and population inversion, which determine the values of laser characteristics respective to the operational pressure of buffer gas, are investigated. Thus an optimum delay time of about 10 ns is determined, and a maximum output power equivalent to about 12 W is extracted. The amplifying parameters and measured output power of laser versus delay times show some local maxima and minima at the delay time interval of 6-43 ns.     

Laser dye stability part 1. Flash frequency and fluid filtration effects

A commercial coaxial xenon flashlamp was used to measure laser dye stability. The output energy of each lasing pulse was recorded and periodic measurements were made of the untuned lasing wavelength, the temporal flashlamp output, and the temporal lasing output. The present study demonstrates that it is possible to have a number of different types of apparent dye laser system “half-lifes” through the use of filtration and frequency of flash parameters. The use of near-zero flash frequency and moderate pore size filters allows a determination of the lifetime that is more representative of the dye as opposed to the lifetime of the dye laser system.     

Cavity waveguide-output laser

We propose for the first time a laser configuration which couples the cavity standing wave field directly with an optical fibre or optical fibre taper using its polished face as an output coupling mirror. A method for direct, active and paraxial launching of laser emission into an optical fibre, adaptable for various laser active media, has been achieved on this basis. Some of the important parameters of this laser are expressed in analytical form. Experimental verification of the proposed idea has been conducted on the basis of a dye laser. Excellent intensity distribution has been obtained beyond the optical fibre output. Launching efficiency exceeding 80% has been predicted and measured experimentally.     

Mathematical modeling of ruby laser as a pumping source of a “self Q-switched” distributed feedback dye laser

A mathematical model describing the dynamic emission of the ruby laser as a pumping source of a distributed feedback dye laser (DFDL) has been adapted. The suggested model allows the temporal behavior investigation of the ruby laser and the DFDL on mode characteristics and, moreover, investigating the affect of laser input parameters on the output laser pulses in the ruby laser and in the DFDL.The numerical solutions of a coupled nonlinear rate equations system of the adapted model that predict the generation of picoseconds pulses, with neglecting the effect of refractive index variation, are discussed (feedback process is achieved only by optical gain). The model estimates the density of the emitted radiation, energy density of the first excited state, and the output power of the DFDL. The adapted mathematical model is in good agreement with the available experimental data.     

High power cw dye lasers

The dependance of the output power of cw dye lasers on the pump power and the system-and stream parameters is calculated and experimentally tested. It was found that the pump power for a given system is limited to a critical value due to the influence of thermal inhomogeneities. This critical pump power is predominantly dependant on the dye solvent, the flow velocity, the focussing of the pump laser and the stability range of the dye laser system. Possibilities for the reduction of the thermal inhomogeneities are discussed and attainable output powers are calculated.     

Polarization and power characteristics of pulsed dye lasers with transverse pumping

Starting from the classical theory on molecular luminescence in liquids, rate equation for two-polarization-mode laser oscillation under transverse pumping with intense excitation have been set forth neglecting the influence of orientational relaxation of the dye molecules. Hence, the laser output power and polarization under different configurations have been obtained. The results show that the dye laser is partially polarized even when the pump beam is unpolarized and no polarizing elements exist in the cavity. If we define a reference plane consisting of the axes of the pump beam and the dye laser, the laser system has to adopt the plane perpendicular to the reference plane as the polarization direction in order to yield a highly efficient, linearly polarized laser output. Experiments on a copper-vapor laser pumped dye laser verified these analytical results.     

The effect of varying the hydraulic parameters of a flow cell in a dye laser pumped by a copper vapor laser

A simple method for measuring hydrodynamic parameters that influence the copper vapor laser (CVL) pumped dye laser stability is reported. A specially designed converging diverging curved flow duct is employed. The flow gap at the laser pump region is varied from 0.4 to 0.6 mm and the Reynolds number is varied from 1000 to 20,000. As a result, the channel aspect ratio and effective curvature ratio are varied from 41.6 to 62.5, and 69 to 102, respectively, at the CVL pump region. The variation in the intensity of a He-Ne laser beam transmitted through the water flowing in the test dye cell at different flow rates correlated with the numerically estimated hydrodynamic parameters. The impacts of these hydrodynamic parameters on a 5.5 kHz CVL pumped dye laser output are discussed.     

Efficient amplification of sub-picosecond pulses of a non-CW dye laser

Efficient amplification in a dye laser amplifier is investigated theoretically and experimentally. A five-level rate equation approach is considered including rotational relaxation of the dye molecules. The effects of the pump pulse duration and of the parameters of the input pulse are discussed. The results are compared with experimental data for 0.5 ps pulses of a pulsed dye laser. Conversion efficiencies >10% are achieved for a single pass amplifier using Nd:YAG pump pulses of 2 ns while an effective fluorescence lifetime of 1.7±0.2 ns is determined for the gain medium rhodamine 6G. The triple pass amplifier stage of the laser system achieves an energy conversion of 4% with 40 J output pulses.     

脱氧核糖核酸柔性的分子动力学模拟:Amber bsc1和bsc0力场的对比研究

脱氧核糖核酸(DNA)的结构柔性对DNA生物功能的实现具有重要作用,全原子分子动力学模拟是一种研究DNA结构柔性的重要方法.DNA的分子动力学力场在Amber bsc0基础上有了进一步的发展,即Amber bsc1.本文采用基于最新bsc1力场和先前bsc0力场的分子动力学模拟对DNA的宏观柔性和微观柔性进行对比研究,发现力场的改进对DNA宏观柔性参量的预测有一定改善,即所预测的拉伸模量和扭转-伸缩耦合比与实验值更为接近,而弯曲持久长度和扭转持久长度两种力场结果皆与实验值一致.微观分析发现,除了滑移量稍变大,bsc1力场得到的微观结构参量如扭转角和倾斜角与实验值更为接近,且新力场下DNA宏观柔性的改善与DNA的微观结构参量及其涨落紧密相关.     

多纵模短腔染料激光放大前后光谱的比较

实验观测了短腔染料激光器输出的多纵模激光及其经一级染料放大的激光光谱，比较了光谱特性。在一定情况下，短腔染料激光器的多纵模激光经放大器放大可产生一个至几个新纵模，新纵模与短腔染料激光器输出的纵模有相似的频率间隔、线宽及频率牵引等特征。新纵模产生属于三阶四光子混频（即四波混频）为主导的非线性光学效应。存在频率牵引表明，增益介质中的多波混频存在频率失配，在这一四波混频中光子能量并不守恒     

Application of the maximum information entropy principle to selforganizing systems

Self-organizing systems are systems which can acquire macroscopic spatial, temporal, or spatio-temporal structures by means of internal processes. Hitherto the distribution functions of the order parameters governing the macroscopic structures could be calculated by microscopic theories only. In the present paper we derive them from macroscopic quantities, where we demonstrate the procedure explicitly by means of the single and multimode laser close to the lasing threshold.     

Characterizing laser dye performance

Several measures of laser performance are investigated for a generic dye laser operating under conditions where the input energy is limited by a maximum value and the output is required to be above some minimum value. Dimensionless expressions are developed that show the interrelationships between the efficiency, the number of laser pulses for a unit volume of dye solution, and the total laser output per volume of dye solution. For one of the performance measures, the total laser output per unit volume of dye solution, the initial slope efficiency divided by a parameter characteristic of the degradation process,k ₀/c, is identified as a reasonable figure of merit. It is readily evaluated from experimental degradation data and has reasonably simple properties relative to transfer between laser systems.     

Spectral optimisation of a synchronously mode locked femtosecond dye laser

Through optimisation of the spectrum and dispersion parameters of a synchronously mode locked Rhodamine B dye laser, pulses as short as 270 fs have been directly generated without using any saturable absorber or pulse compressor. In addition a system of tunable output coupling with well characterised dispersion has been demonstrated.     

A narrowband single-mode dye laser

The linewidth of a cw-single-mode tunable dye laser has been investigated. It was found that the linewidth is determined by microscopic velocity fluctuations of the turbulent flow within the dye cell of the laser and by fluctuations of the pump power. The linewidth is further influenced by mechanical disturbances and by temperature fluctuations of the dye solution. The results of the investigations made it possible to achieve a reduction of the velocity fluctuations. It is shown that under adequate operating conditions the free-running dye laser has a linewidth of 2 MHz.     

Photoinduced fission of the doubly even uranium isotopes 234U, 236U and 238U

A measurement of the angular distributions and yields of fission fragments in the photofission of ²³⁴U has been performed between 5.2 and 6.4 MeV. As γ-source, the bremsstrahlung from a microtron was used. For the detection of the fission fragments, solid-state track detectors were used. The present data for ²³⁴U have been analysed together with earlier obtained data for ²³⁶U and ²³⁸U. The values of the fission barrier parameters obtained are compared to results in theoretical macroscopic and microscopic fission potential energy calculations.