2011 Vol. 4, No. 1
The working parameters of the discharge pulse-periodic XeCl lasers with pulse duration of 25-40 ns are researched. Results demonstrate that the lasers generate the pulses of radiation with energies of 0.2-0.7 J and operate with a repetition rate up to 100 Hz. It is shown that the total laser efficiency of 2.6% and the intrinsic efficiency of 3.8% are realized with the pumping power of 2.8-3.3 MW/cm.
Basic characteristics of the laser-based engine are compared with theoretical predictions and important stages of further technology implementation are investigated under low frequency resonance. Relying on a wide cooperation of different branches of science and industry organizations, it is very possible to use the accumulated potential for launching nano-vehicles during the upcoming years.
Generation of an electric signal under interaction of pulsed HF laser radiation with water column surface was investigated. It was found that the electric signal demonstrates pronounced a two-peak structure with time interval between peaks linearly depending on the laser energy. The second pike was determined to appear upon the collapse of the vapor cavity produced at the bottom of water column. It was also found that the amplitude of electric signal strongly depends on the presence of a thin water layer which is dilated and pressed in the process of the water column motion initiated by laser pulse. Tenfold increase of electric signal is achieved if the thin water layer is presented between the upper border of the cell and quartz plate which closes the cell and touches the upper surface of water column.
The high power nonchain repetitively pulsed HF laser is developed and the possibility of realizing the self-sustained volume discharge in SF6-based mixtures in the discharge gap with a high edge enhancement of the electric field without any additional stabilization measures in a pulsed discharge as well as in pulse-periodic modes is investigated. The obtained laser energy is 67 J at the efficiency in 3% and pulse repetition rate in 20 Hz.
This paper reports on investigating the ionization instability in active media of electric discharge non-chain HF(DF) lasers due to electron impact detachment of electrons from negative ions. This instability has been triggered in large volumes of SF6-based mixtures, spatially separated from electrodes, through the gas heating by a pulsed CO2 laser radiation. A self-organization phenomenon in a Self-sustained Volume Discharge(SSVD) on a laser-induced gas heating and resulting in formation of quasi-periodic plasma structures in the bulk of discharge gap is experimentally studied. Special attention is given to the evolution of these structures on changing the gas temperature and specific electric energy depositions. A plausible relation of the found self-organization effect to the electron detachment instability treated is discussed. Also suggested is the mechanism of moving a single plasma channel in working media of HF(DF) lasers owing to destruction of negative ions by electron impact.
The ignition of stoichiometric CH4∶ O2 mixture in a closed cylindrical quartz chamber was investigated. Two methods of combustion initiation were used:mixture ignition by freely localized pulse HF laser produced laser spark and mixture ignition by direct gas heating with the help of a pulse CO2 laser. In the latter case, the small admixture of SF6 strongly absorbing CO2 laser radiation was used for pulse heating of flammable mixture. Dynamics of combustion was investigated and the spectral characteristics of combustion glow were determined. The experiment demonstrates that rapid laser heating of a certain amount of reactive gas can significantly accelerate the process of inflammation and can realize the detonation mode of combustion gas mixture, even in a small reactor volume. Initial gas temperature distributions along chamber axis for threshold conditions are obtained.
Formations of stable micro-protrusions and microstructures on the surfaces of liquid metals during multipulse action of a UV-IR laser in different ambient gases was discovered for the first time. The rate of the structure formation was measured, which can reach 5-20 m/pulse depending on the metal and ambient gas sort. Single micro-protrusions of 1-2 mm in length were formed with the diameter approximately two times greater than the focal spot size. The possibility to control the microstructure shape was shown and their potential applications were also discussed.
The radiation torque induced by a focused laser beam has been widely utilized for the optical manipulation of small particles in various fields such as physics, biology, and optofluidics. To isolate the contribution of each scattering process to the radiation torque and to clarify the physical mechanism of radiation torque, the Debye Series Expand(DSE) is introduced to analysis of the radiation torque exerted on a homogeneous sphere by Gaussian beam. When Debye mode p ranges from 1 to a value pmax large enough, the result obtained by DSE is identical to that by Generalized Lorenz-Mie Theory(GLMT). Furthermore, the radiation torque corresponding to each Debye mode p is mainly analyzed. Results show that when incident beam is linearly polarized, all scattering processes for p in 1-5 can produce a transverse torque while the directions of torque are different. When the beam is circularly polarized, the torques for p in 1 and 0 are much larger than that for p in 1-4, and the direction of torque for p in 0 is opposite with that for other mode p.
In order to improve the output power of a laser pulse in the Inertia Confinement Fusion(ICF) system, an Arbitrary Generator(AWG) is designed to shape the laser pulse with an ideal condition. The ultra-narrow pulse is used to trigger the GaAs Field Effect Transistor(FET) to generate multiple negative pulses. The different time delays for each pulse are introduced by transmitting it through an analog time delay wire. Finally, a micro-strip line is obtained to couple the pulses and to get the overlapped pulse shape. The gate bias on FET is used to determine the magnitude of pulse, so the arbitrary pulse is gotten by overlapping the multiple pulses with various magnitudes and time delays. To satisfy the high amplitude requirement of electrical pulse for the ICF system, an ultra-broad bandwidth and voltage tunable amplifier is applied to the output signals and the pulse amplitude is adjustable. The designed AWG generates a pulse waveform with amplitude between 0 and 5 V, pulse width less than 10 ns and a square waveform rising edge in 520 ps, falling edge in 790 ps. The experiment shows the shaped laser pulse is obtained based on the shaping electrical pulse.
In order to analyze the impacts of stability of optical elements on the target positioning in a high power excimer laser Master Oscillator Power Amplification(MOPA) system, an analyzing model for the system was established and a mirror mount was also designed according to the analyzing results to meet the need for the system experiment. Firstly, in light of the unique feature of high power excimer laser MOPA system, the beam path was effectively simplified, and a three dimensional layout model to analyze the system was established. Then, in terms of the model and the requirements of target positioning precision of the system, the stability request of every optical element in this system was obtained by using the three dimensional coordinate conversion and ray-tracing. Finally, the stability parameters of the mirror mount designed by ourselves were measured. The simulating results indicate that the rotations of mirrors and the drift of lens vertical to the optical axis are the chief factors affecting the target positioning precision. In addition, the rotations of mirrors in the main amplifiers round X and Y axes should be controlled less than 0.8 and 1.6 rad, respectively. The experimental results for the mirror mount designed point out that the rotations round X and Y axes are not more than 0.6 and 0.81 rad, respectively, which means that the tolerance of coordinate parameters of each optical element corresponding to the target positioning precision satisfies the requirement of the system.
The polished nickel surface was scanned by a femtosecond laser beam with a pulse width of 35~65 fs and a central wavelength at 800 nm, and the multi-color pattern was obtained. The same processing was done for the stainless steel at different scanning speeds and laser fluences. The experimental results were analyzed by Electron Scanning Microscopy(ESM). The results show that the surface of the colorful metals are full of laser-induced periodic surface structures covered with nanostructures(NC-LIPSS), and the NC-LIPSS period on the nickel surface is about from 480 to 510 nm and that of on the stainless steel is about from 480 to 540 nm.
The effects of some metal targets subjected to the laser irradiation and tangential airflow are overviewed. It points out that the airflow usually affects the metal targets in such ways:cooling the targets, removing the melt layer, supporting the combustion reaction and breaking the target prior to melt-through. After analyzing on the airflow effects in details, it suggests that extensive experimental research should be performed for the combustion reaction and thermal-mechanical effect during laser irradiation. By this way the clearer laws can be understood and the unified physical model can be developed for engineering applications.
In order to measure the Two Photon Absorption(TPA) cross section of organic material films, a new approach to measure the TPA cross section of a azo dye film based on the nonlinear transmission methods by a low power picosecond Nd∶ YVO4 laser was presented. Firstly, a multi-layered equivalent film sample(5 layers, 20 mm20 mm) with the same thickness for each layer was prepared. The film from an azo dye material was spun and coated on a large PC substrate and then it was cut into 20 mm20 mm clips. Furthermore, a mode-locked Nd∶ YVO4 picosecond laser(1 064 nm, 56.8 MHz, 20 ps) was chosen as the pump source to excite the sample. Finally, the nonlinear transmittance of the five-equivalent layer film was measured and the curve of the experimental data was fitted. Obtained result shows the value of the TPA cross section of this dye molecule is 634.2 GM. Compared with other methods, this experimental scheme is very simple and valid.
The micro-structures of single crystal silicon surfaces produced by the cumulative radiation from 532 nm Nd∶ YAG nanosecond laser pulses were investigated under different ambient atmospheres. The results show that the ambient atmospheres have an important role on the silicon surface morphology. The microstructures produced in vacuum, N2 or SF6 were analyzed in detail, it was indicated that the density of the spikes formed in SF6 is larger than those formed in N2 or vacuum, and the spikes have higher aspect ratios than those in other ambient atmospheres. Furthermore, the dimensions of the microstructures formed in N2, vacuum or SF6 have decreased in turn. The experimental results suggest that the efficiency of laser-induced chemical etching in SF6 atmosphere is higher than those in vacuum or N2 atmospheres. Moreover, there are ripple microstructures formed on the edge of irradiation area, which is caused by the frozing of capillary waves.