Yanzhao Lu

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 5.3 μm.

Simulation of Microhollow Cathode Discharge With Planar and Hollow Anodes

The 3-D fluid models of microhollow cathode discharge (MHCD) with both planar- and hollow-anode structures are presented. In the planar-anode structure, the highest densities are ~1018 and ~1019 m-3 for electron and ion, respectively, and the electron mean energy is in tens of electronvolts. In the hollow-anode structure, densities and electron mean energy are higher than in planar one, which indicates that this structure is more attractive than the planar one. The influences of pressure on the MHCD have also been discussed.

Pulse behavior of a short pulse discharged TEA CO2 laser

The studies on a short pulsed TEA (Transversely Excited Atmospheric pressure) CO2 laser pumped by a magnetic pulse exciter are reported. The exciter includes a magnetic pulse compression circuit, which compresses the pulse duration below 100 ns. According to our experiments, the maximum output energy of a single pulse can be as high as 2.84 J and the Full Width at Half Maximum (FWHM) of the laser pulse is 70ns approximately. The pulse shapes and characteristic times are measured. The results indicate that the single pulse energy keeps a linear relation with the gas pressure and charge voltage. And there is an optimized composition that the maximum output pulse energy or shortest pulse width can be obtained. Based on this free oscillation laser, a tunable TEA CO2 laser is developed. Over 62 laser lines from 9.16 to 10.79μm have been obtained and the output pulse energy of 10P(20) line is 0.5 J.

Numerical analysis of cutting glass by dual CO2- laser beams

Cutting brittle materials such as ceramics and glass by lasers or traditional saw method, costly fractures and associated damage such as chips and cracks can result. In most cases, these problems were caused as a result of the stress was not controlled properly and exceeded the critical value of the fracture. In this study, a dual-laser-beam method was proposed to avoid fractures in glass laser cutting, where an off-focused CO2-laser beam was scanning on the top surface of glass periodically and repeatedly and a preheated-band which has a proper temperature was built. This preheated-band will reduce the temperature gradients when the glass is cut by the focused CO2-laser beam. Under these conditions, glass can be cut with melting method without any fractures. The process of cutting glass by dual CO2-laser beams was simulated numerically with FEA method and the distribution of temperature and thermal stress was investigated. The relationships between the cutting parameters, such as laser beam diameter, laser power, cutting speed, and the profile of the cutting groove were also discussed. The result showed that thermal stress decreased with the increasing width of preheated-banding, and the smaller the diameter of laser beam, the better the quality of the cutting groove was in the same laser power situation.

Theoretical research of α-RF discharge in slab oxygen iodine lasers

A theoretical model is established to describe the α-RF discharge in slab Oxygen Iodine lasers, according to the continuity equation of electron density, the electron energy equilibrium equation and the continuity equation of current density. Assuming a Maxwellian energy distribution, the spatial distributions of electron density and electric field in RF plasma are obtained by numerical method. The effects of parameters on discharge characteristics have been analyzed. The results show that the current density has a big effect on the electron density in discharge area. The influences of excitation frequency on the maximum value of electric field and the thickness of boundary layer are also discussed. And the spatial distributions of electron energy and excitation efficiency of singlet delta oxygen have been calculated. The influences of gas mixture on the excitation efficiency of singlet delta oxygen are discussed. It provides references of parameters for slab discharge in singlet delta oxygen generating.

An efficient optically pumped terahertz laser without metal-mesh mirrors

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.

Evolution of Shock Wave in TEA gas laser

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.

Experimental research of pulsed discharge in oxygen mixtures

The pulsed discharge in oxygen and its mixtures to produce singlet delta oxygen has been studied experimentally at gas pressure of 30~ 936mBar. As singlet sigma state of oxygen O2(b1Σg+) and atomic oxygen O have great influence on the yield of singlet delta oxygen O2(a1▵g), the visible spectrum of O2(b1Σg+) at 762nm and O at 778nm have been measured, and the effects of argon, helium and carbon monoxide have been discussed. The electric stability was better in O2-He than in O2-Ar mixtures, while it could be increased quickly with a small amount of CO. In O2-He mixtures more oxygen were decomposed than in O2-Ar mixtures, while adding CO the decomposition was reduced, the amount of atomic oxygen was decreased, and the O2(b1Σg+) was increased comparatively. The time-resolved spectrum of the plasma has also been measured. The life-time of atomic oxygen was longer than O2(b1Σg+)s, and it could be shorten with argon.