Zuhai Cheng

Transient thermal effect of high power laser mirrors and its propagating characters of far-field

Summary The analytical solutions of the two-dimensional transient thermal effect of high power laser mirrors are developed by using integrate transform technique. Zernike polynomials are introduced to describe aberrations of laser-induced thermal deformations. And, the corresponding expressions of the Strehl ratio and the intensity distribution are gained. Some far-field optical patterns from which we can directly see the divergence of intensity are given. The predicted results are in agreement with that of experiment.

Experimental study of thermal deformation of honeycombed PCM cooled mirror in resonator

A new kind of phase change material (PCM) cooled mirror for use in the resonator of high power lasers is proposed. The PCM cooled mirror can minimize the thermal deformation on the reflecting surface effectively. A test setup was designed and the laser-induced thermal deformation of honeycombed PCM cooled mirror was measured at real time. Compared with a solid silicon mirror, it was only 0.45 µm at the tenth second of laser irradiation after absorbing 140 W net heat. In addition, the influence of the method of support on the thermal deformation was studied by experiments and showed that a simple support is compatible with a PCM cooled mirror.

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

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.

Numerical simulation on mirror's thermal distortion cooled by the organic phase-change material in high-power laser

A new mirror cooled by organic phase change material is presented in this paper. In virtue of the finite element analysis software—ANSYS, the transient numerical simulation on deformation of mirror surface in 10 seconds during laser working time is presented in the paper, which is based on heat transfer and thermal-elastic laws. The curves of temperature and thermal deformation along the radium are provided in the case of uniform beam and Gassian beam respectively. The result is showing that the passive cooling method with organic phase change materials to reduce thermal distortion on mirror surface is a g*ood way and small deformation can be achieved to a satisfied extent.

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.

Studies on high energy pulsed CO2 laser

The recent results on pulsed CO2 laser, including the effects of preionization structure, electrode profile, and discharge circuit are reported. Two sizes of discharge gap, 70 mm and 100 mm were selected. For the 70 mm gap, lateral preionization was used; for the 100 mm gap, preionization beneath the meshed electrode, and mixed preionization beneath and beside the electrodes were all tested. Strong lateral discharge was found in the case of near-Rogowski profile or Stappaerts profile, and strong central discharge was found in the case of Ernst profile. When the near- Rogowski profiled electrode was paired with an Ernst profiled electrode, better performance was obtained. Discharge distribution corresponding to the spark pins under the mesh grid was found obviously, which means that the uniformity of preionization should be further improved. Small signal gain coefficient was measured to evaluate the discharge. For the experimental setup with 70 mm discharge gap and 4 liters discharge volume, the highest small signal gain coefficient was 2.94% cm-1. When a laser cavity was installed, the highest pulsed energy obtained from this setup was 169 J. When the discharge volume was enlarged to 16 liters, pulsed laser energy up to 580 J was obtained.

Studies on large aperture volume discharge of TEA CO2 laser

The studies on large aperture volume discharge of TEA (transversely excited atmospheric) CO2 laser are reported. To obtain stable and uniform large aperture volume discharge, the effects of electrode profiles, preionization methods and parameters of discharge circuit have been studied experimentally. For a discharge construction with 70 mm gap and pumped by a 3-stage Marx high voltage pulse generator circuit isolated by resistance with 0.5 μF capacitance each stage, 150 J pulse energy was obtained from a discharge volume about 4 l.

The Characteristics of Wavefront and Far-Field Optical Pattern from High Power Laser Mirror

This paper presents the characteristics of wavefront and far-field optical pattern from high power laser mirror which suffered from thermal deformations. Zernike polynomials are introduced to describe aberrations of laser-induced thermal deformations. And, the far-field optical patterns of radiation are calculated by using Fourier transformation and measured by interference experiments. The result indicates that main aberrations are defocusing and primary coma, and that the tilt cannot be ignored. Some optical patterns and contour maps which depend on the laser beam power applied to the mirror and the exposure time are given. Finally a new clamping method is suggested, which can reduce the tilt of mirror and increase the pointing stability of high power laser beam.

Investigation of coaxial unstable resonator cooled by solid-liquid phase transition material

The innovative structure and working properties of a high- power co-axial phase change cooled unstable resonator which is cooled by solid-liquid phase transition material are reported in this paper. The experimental result shows it is a useful resonator to be used in high-power lasers.

Good processing method of interference patterns

A new pretreatment method and a modified Deutsch thinning algorithm for interference patterns are presented. Based on different characteristics of fringe patterns, different pretreatments are described. The problems of Deutsch algorithm are analyzed and then the modified algorithm is proposed in this paper. Several experimental results are also presented to support the validity of the method.