Duluo Zuo

Tunable middle infrared radiation generation in a GaSe crystal

Tunable second harmonic generation of pulsed CO2 laser radiation in a GaSe crystal was demonstrated. The nonlinear optical properties of GaSe and phase-matching conditions were investigated, and the best fitted dispersion relationships for this crystal were also determined. External conversion efficiency of 0.1% with unfocused pumped radiation had been achieved and maximum single pulse energy of 0.19 mJ was generated at 5.3u2002μm.

Investigation of ion characteristics in CO 2 laser irradiating preformed tin-droplet plasma

Comparative study of CO 2 laser-produced tin-droplet plasma with and without pre-pulse laser has been presented. A pre-pulse laser and the CO 2 laser was combined and focused to tin-droplet with a diameter of 180 µm. The emitted Sn ions were detected by several Faraday cups to obtain angular distribution of ions in the laser-produced tin-droplet plasma. The influence of pre-pulse laser energy and delay time between pumping laser and pre-pulse laser on the ion characteristics was investigated. It is illustrated that ion average kinetic energy from CO 2 laser-produced plasma (LPP) can be reduced when the tin-droplet target has been replaced by the preformed Sn plasma. The obtained optimal delay time with the lowest ion average kinetic energy is about hundreds of nanoseconds. The ion time-of-flight spectra show a twin peak structure in laser-irradiating preformed Sn plasma. And a superimposed Maxwell–Boltzmann (MB) distribution is proposed to describe this twin peak ion time-of-flight spectra. The fitting results quite agree with the raw ion time-of-flight spectra in current experiment. Then, the fitted plasma temperatures and mass-center velocities with various delay times in laser-irradiating preformed plasma are obtained, and the fitted plasma temperatures can be comparable with ion average kinetic energy in double-pulse LPP, which justified the rationality using this superimposed MB distribution.

Spectral Property Investigation of Air Plasma Generated by Pulsed

In this paper, the laser propulsion plasma has been diagnosed experimentally using the method of optical emission spectroscopy. The spectral evolution of the air plasma generated by pulsed CO2 laser radiation with a parabolic reflector was studied, with a pulse energy of 2 J and a pulsewidth of 70 ns (full-width at half-maximum). In addition, most of the plasma spectral lines in the visible light region were identified, and the spectra of air plasma were dominated by the emission of single ionization of nitrogen and oxygen, with a weak emission of nitrogen and oxygen atom. Based on the local-thermodynamic-equilibrium state, the basic spectral properties of laser-induced plasma evolution, including the evolution of the plasma spectrum, electron temperature, and electron number density, were analyzed. We utilized the Boltzmann plot to evaluate the electron temperature and the Stark broadening and Stark center frequency shift to evaluate the electron number density. Results proclaimed that the maximum values of the electron temperature and electron number density were about 4.5 × 10 K and 1019 cm-3, respectively.

\hbox{CO}_{2}

The spectral characterization of laser-induced air plasma by a pulsed TEA CO2 laser with a GaAs lens is reported. The data were collected using an ICCD spectrograph. The spectra of laser-induced plasma in the range of 300-900 nm and their spatial and temporal behaviors were investigated. Emission spectra of the N II lines occurring between 490 and 520nm were analyzed in detail. The intensity peaks during the laser pulse approximately 0.7 μs and the peak emission spectra decay over the first 1.5 μs. The spectra of plasma at different position were measured in the directions both perpendicular and parallel to the laser propagation respectively. And the spectra of laser-induced air plasma produced by different focal length lens were compared. The affection of different focal length lens for the plasma was described. The shapes of horizontal directions were smaller with the shorter focal length and the intensities of the plasma were higher. In addition, the temperature of plasma was evaluated from the emission intensities. The peak temperature was more than 10000K.

Laser

This paper presents some results on the air-breathing propulsion experiments with a parabolic light craft and a self-made UV-preionized TEA CO2 laser with 115 J maximum pulse energy. In wire-guided vertical flight experiment, impulse-coupling coefficient 390 N/MW is obtained at the pulse energy 60 J. The influence of the shape of the lightcraft and the laser pulse profile on the coupling coefficient was discussed. The spectrum of the plasma was detected using monochromator, and the time evolvement process of line spectrum of the plasma was analyzed.

Spectral characterization of laser-induced air plasma by pulsed CO2 laser

The experiment collected some spinach leaves which were irradiated by pulsed CO2 laser with energy 5.6J, 8.0J and 9.5J respectively. Each of them was soaked in three kinds of solvents (water, ethanol, the mixture of ethanol and petroleum ether) respectively. The experiment shows that the ethanol solution which contains the irradiated leaves turn dark green than the ethanol solution which contains the intact leaves and the color of solution with the leaves irradiated by CO2 laser with 9.5J changes the most significantly. Further, selective excitation on the molecular level of the cell wall were used to explain the phenomenon.

Some experimental studies on the UV-preionized TEA CO2 laser propulsion

Experimental studies on a tunable efficient high-efficient pulsed NH3 terahertz (THz) lasers pumped by TEA CO2 lasers are presented. When NH3 is pumped by the different lines with the CO2 lasers, the generation of different terahertz radiations is discussed. The lines of the CO2 lasers are 9R(08), 9P(20), 10R(14), 10R(08), and 10R(06). To improve THz laser energy and photon conversion efficiency, different higher power of the CO2 laser can effectively improve THz laser energy and photon conversion efficiency. When the 9P(20) CO2 lasers with 9.68 J and 4.12 J pump NH3, the corresponding energy conversion efficiencies are 0.28% and 0.19%, increasing by a factor of about 1.5. The generation of terahertz radiations with energy as high as 27.29 mJ and 7.73 mJ are obtained, respectively, increasing by a factor of about 3.5. Meanwhile, for 10R(14) line, the energy conversion efficiencies increase to 8.5 times and the energy of THz lasers increase to 32 times.

Theoretical investigation on breaking plant cell wall by laser

The efficiency of a simply designed optically pumped terahertz laser is studied experimentally. The terahertz laser cavity only consists of a quartz glass tube, an antireflection-coated Ge window and a SiO2 window. The Ge crystal acts as the high-reflectivity mirror of terahertz radiation and the input coupler of pump laser instead of complicated metal-mesh mirrors. The Ge crystal is near 3 mm thick, whose exact thickness is designed according to etalon effects to maximize terahertz reflectivity. NH3 gas is filled in the cavity as the active medium and pumped by a TEA CO2 laser. As high as 25.9 mJ terahertz radiation at the wavelength of 151.5 μm is extracted from 1.76 J pump energy. The corresponding photon conversion efficiency of this terahertz laser reaches 41.5%. A 4.7-mm-thick GaAs crystal and a 6-mm-thick ZnSe crystal are also chosen to be the input coupler. The experimental results show that the efficiency of the Ge window is 54% and 66% higher than that of the GaAs and ZnSe windows, respectively. The reason of the higher efficiency of the Ge window is demonstrated experimentally and theoretically in this paper.

Tunable high-efficient pulsed NH3 terahertz lasers

The evolution of shock wave generated by discharge in laser chamber is one of the key factors which affect laser beam quality, discharge stability, and repetition rate of TEA gas laser. In this paper, Mach-Zehnder interferometer is applied to observe both the longitudinal and transversal shock waves between electrodes as well as the acoustic waves originated by preionization in the discharge pumping zone of TEA gas laser. By changing the discharge voltage, gas pressure and gas composition concentration, the developing processes in different conditions are compared and analyzed. It is observed that the shock waves originating from cathode is different from the anodes ones even in the symmetric electrode construction. And the carbon dioxide concentration in helium-buffered working gas can affect the speed of the wave obviously. However, the increasing trend of shock wave speed, when increasing discharge voltage or reducing discharge gas pressure, is inconspicuous.

An efficient optically pumped terahertz laser without metal-mesh mirrors

In this article we report on studies aimed at a theoretical description and an experiment design of generation of intense THz pulsed laser. Through solving the time-independent rate equations, the absorption coefficient and the gain coefficient of intense THz pulsed laser are obtained with the assumption of the pump rate and the induced transition rate much larger than collision rate. The analysis further shows that at low pressure the absorption coefficient is proportional to the second power of pressure and is in inverse proportion to pump intensity, and that the gain coefficient has the same feature expect for being in inverse proportion to THz radiation intensity. Meanwhile the expressions of the saturation pumping intensity and THz radiation intensity are given under the same assumption. On the other hand the expression of energy absorbed from pump beam is derived from solving the time-dependent rate equations, and we demonstrate that path length, gas pressure, and pulse width are the important factors impacting on the energy absorbed from pump beam. When transmission of pump laser is determined, it is found that gain molecule mass, gas pressure and pulse width exist an inner confine relation. Finally according to the theoretical analysis and some parameters assumed, we present an experiment design of the intense THz pulsed laser.