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CURRENT TOPICS 89 a coded message in the form of varying loudness, or amplitude. This is called amplitude modulation. The new laser technique makes possible a similar procedure with the light emitted by the laser. Furthermore the process also permits frequency modulation, in which the frequency rather than the amplitude of the laser beam is varied. The new technique also permits the laser to be turned on and off 100,000 times a second. This capability means that a practical laser radar system is now a real possibility, especially for use in outer space.
The experimental system employs a liquid-nitrogen-cooled calcium-fluoride laser crystal containing traces of dysprosium, a rare earth. The crystal is placed at one focus of an elliptical chamber and activated by a powerful tungsten lamp placed at the other focus. At either end of the crystal is a coil of wire with which an alternating magnetic field is generated and applied to the crystal. Around the whole chamber, in turn, is another coil which generates a steady-state magnetic field.
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