High optical quality undoped and sulfur-doped gallium selenide crystals were grown from melts by the modified vertical Bridgman method. Detailed study of the damage produced under femtosecond pulse exposure has shown that evaluation of the damage threshold by visual control is unfounded. Black matter spots produced on crystal surfaces do not noticeably decrease either its transparency or its frequency conversion efficiency as opposed to real damage identified as caked well-cohesive gallium structures. For the first time it was demonstrated that optimally sulfur-doped gallium selenide crystal possesses the highest resistivity to optical emission (about four times higher in comparison with undoped gallium selenide).
A high power 4.65 µm single wavelength laser by second harmonic generation (SHG) of TEA CO 2 laser pulses in silver gallium selenide (AgGaSe 2 ) and zinc germanium phosphide (ZnGeP 2 ) crystals is reported. Experimental results show that the average output power of SHG laser is not only restricted by the damage threshold of the nonlinear crystals, but also limited by the irradiated power of fundamental wave laser depending on the operating repetition rate. It is found that ZnGeP 2 can withstand higher 9.3 µm laser irra diation intensity than AgGaSe 2 . As a result, using a parallel array stacked by seven ZnGeP 2 crystals, an aver age power of 20.3 W 4.65 µm laser is obtained at 250 Hz. To the best of our knowledge, it is the highest output power for SHG of CO 2 laser by far.
A new welding method has been developed for the tungsten inert gas process which can both improve the weld depth/width (D/W) ratio and protect the electrode from oxidation during the welding process. The effects of the welding parameters and the Marangoni convection patterns on the weld pool shapes are discussed. Results showed that the change of welding parameters directly change the temperature distribution on the pool surface and the concentration addition of the surface active element, hence, eventually affect the pattern and strength of the Marangoni convection. For the liquid pool on stainless steel, the weld shapes depends to a large extent on the pattern and strength of the Marangoni convection. It is possible to generate a heavy oxide layer on pool surface under a relatively high oxygen content to affect the surface tension stress. The heavy oxide layer may inhibit the heat flow on the weld pool surface, and reduce the D/W ratio.
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