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CURRENT TOPICS New Gallium-Arsenide Laser.--A junction laser which operates continuously at a power output of more than one watt was recently announced by the General Electric Research Laboratory, Schenectady, New York. The power output of the new gallium-arsenide laser demonstrates a tenfold improvement over what has previously been achieved with ordinary ruby and gas lasers. The new device also demonstrates remarkable efficiencies in the range of 20-300-/0, requiring only 5-6 watts of power to produce the one-plus watts of output. The new laser has achieved maximum performance when operated at about 20 degrees Kelvin. (liquid hydrogen temperature) but also has demonstrated excellent performance at somewhat higher temperatures. A unique heat-transfer design makes it possible for a current equivalent to 4,000 amperes per square centimeter to be passed through the gallium-arsenide junction. The junction in the Garfinkel-Engeler laser described today is about 1/1000th of a square centimeter in cross section. The coherent light emitted is in the near infrared (8400 Angstroms). Although "power approaching a watt has been reported for some sophisticated descendants of the ruby laser," General Electric scientists believed the more-than-one-watt output of their device represented the highest
power yet achieved by a CW (continuous-wave) laser. Advantages of the new device over other lasers include not only higher power but also the ability to modulate the coherent laser beam by simply varying the electric current applied. High input-output efficiency is also important since the only previous lasers exceeding an output of a few milliwatts have been driven by more than a kilowatt of power. In the new laser, the gallium-arsenide crystal diode containing the light-emittlng junction is made extremely thin and is surrounded by another piece of gallium-arsenide which serves simultaneously as heat conductor and electric insulator. Becuase the junction crystal and the surrounding conductor-insulator are essentially the same material, previous problems related to thermal expansion have been eliminated and much more effective methods and materials can now be employed to remove heat from the diode.
Drinking Water from the S e a . British scientists are perfecting a new and less expensive method of preparing drinking water from the sea. The technique has important applications in the field of nuclear power. Known as the Thornton system, the new method involves a system called the "mist heat transfer" which requires