Компьютерная техника. Еремина Н.В. - 29 стр.

demonstration will come 22 years after the theory behind optical computers was first
predicted by researchers from the computer company IBM. "However, there is still a
large gap between what theoretical physicists believe can be done, and what
electronic engineers know is possible.
       In theory, optical switches leave their electronical counterparts standing. It is
like comparing the speed of light with the speed of electricity. •"Optical switches are
so fast and yet so small that an optical device of one square centimetre can resolve
107 separate spots of light and each can be switched on and off at a speed of 30
nanoseconds. This means that an optical device one square centimetre in area could,
in theory at least, handle 3 x 1014 bits per second. This rate is equivalent to everybody
in the world having a telephone conversation at the same time.
      The optical switch works on the principle of optical "bistability". Usually,
when a beam of light is passed through a transparent material, the relationship
between the intensities of light entering and the light leaving is linear. However,
under certain circumstances a non-linear relationship occurs. A small increase in the
intensity of light entering the material leads to a much greater increase in the intensity
of light leaving the material.
       In optical switches, the material is placed inside a resonant cavity. In practice,
this means that the edges of the material are highly polished and parallel to each
other. With such materials some of the light entering becomes "trapped" inside as it
bounces back and forth against each polished surface. In other words, it resonates.
This changes the refractive index of the material, with the result that for a given
intensity of light entering the switch are two possible intensities of light leaving it.
       In other words, there is the equivalent to an "off" position and an "on" position
because there are two stable states and the material shows optical bistability. Up to
now a switching speed of 10'13 seconds has been achieved, although the power
needed to generate this is in the kilowatt range. A speed of one nanosecond (10"9) is
possible in the milliwatt power range.
5.3 Exercises
      5.3.1 Translate the sentences marked with an asterisk

      5.3.2 Think and say about

      a) theoretical suppositions behind optical computers;
      b) practical achievements in the field.

      5.4 Home exercises (to be done in writing)