Optical computer
An optical computer is a computer that performs its computation with photons as opposed to the more traditional electron-based computation.
Electronics computations sometimes involve communications via photonic pathways. Popular devices of this class include FDDI interfaces. In order to send the information via photons, electronic signals are converted via lasers and the light guided down the optical fiber. The conversion process between electronic signal and photonic one takes time and adds complexity to the device.
No true optical computers are declassified or otherwise known to exist. Some devices that are best classified as switches have been tested in the laboratory. However, transistors that are composed entirely of optical components, if they exist, are currently only in the development stage.
A fully functional computer is composed of many transistors. The number of them required to constitute a computer is arguable, but probably at least 10 and more often 106 transistors are required to do general computing tasks.
In light of this (so to speak), no true optical computers yet exist. The problems of design seem to stem from eliminating the conversion from photons to electrons and back. This conversion is necessary now because we don't have all-optical versions of all the myriad switching devices required by a computer.
Interestingly, modern (normal) electronic computers are getting closer to being optical in any case. The frequency of the system clocks on fast systems is currently in the single gigahertz range. As part of circuit design, any electronic signal varying that fast is giving off radio waves at that frequency. This means that a wire in a computer has a dual function as a conductor of electricity and as a waveguide for a gigahertz frequency radio wave.