Navigation: This is a basic explanation of interference-based photonic (optical digital) computing. Using the animated graphic above, the basics of how light speed photonic computing is accomplished are explained in detail. Enjoy.
The organization of photonic logic stages imitates the organization of logic stages in an electronic computer because we want the photonic computer to do the same things that regular computers do...only faster. The basic switching functions of digital computers use Boolean logic. Invented by George Boole in the middle of the 19th century, Boolean logic functions are easily generated by machines. From a hand full of logic operators, entire computing systems are built by interconnecting millions of them in information-flow and control architectures. The result can be an IBM PC, a Pentium Pro or a hand calculator, but they all work the same.
Operating logically
In binary data coding, when a pulse is on (at a specific time) the data bit is a "1", and when is absent, or off, the bit is a "0". In the illustration, ON bit light pulses are represented by the moving squares. They enter from the two input beam positions on the left and are combined by a special optical hologram (the big black thing shown in cross section.) Angles depicted are greatly exaggerated in order to clarify the explanation.
George Bool's logic requires at least two different types of "operators" or logic functions in order to produce all of his other logic functions, and from that, all of computing can be constructed just as is done in electronics. Some of his operators include the AND, NAND, OR, NOR, NOT, and the XOR (exclusive OR.) While NANDs and NORs are popular in electronic circuits, the OR and the XOR accomplish the same tasks in light computers.
Animation 1: Single Beam Animation, diffraction makes two pulses out of one.
Energy is spread over the mask area when it arrives at the mask.
(NOTE: during the trip, diffraction spreads out all of the energy.
It is depicted as separate pulses only because that is the portion
of the energy that is used to convey information.
Traveling at the speed of light, "on" pulses interact with the photonic transistor's hologram in different ways depending on the exact hologram, and the relationship of simultaneously-arriving pulses to each other. In this first animated illustration, (Animation 1) an OR, and an XOR are shown. Each of these functions can be produced with the same hologram, the difference being the location from which the output energy is extracted. Thus, information about both functions is actually generated simultaneously, with the output energy from each one being directed as needed to subsequent logic stages.
As with any two-input binary device, there are four possible combinations of the input beams being instantaneously on or off: both off, the upper one on, the lower one on, and both beams on. These various input combinations are held steady during the entire pulse length so as to produce equivalently long outputs. The input pulse combinations produce four different images, or energy distributions that are projected onto the separator. The moot, or null image is produced when both beams are simultaneously off. Since there is no energy input, there is no energy output either through the hole or by reflection. This is the state that occurs in the animation above in between on pulses.
To the right of the animation above, (Animation 1) the mask mirror is shown as seen from the hologram. in between pulses, no energy is available, and thus nothing goes through the exit hole. However, when the diffracted pulse reaches the mirror/mask the image component having the OR information exits the hole. The portion having the XOR information (the pulse heading downward away from the hologram,) is reflected by the mirror and on to the next logic stage, (not shown.)
A single pulse entering at either of the inputs will be spread out by diffraction to cover the surface of the separating mirror to the right.
Thus the mirror with its hole provides a separation of energy from different locations within the image produced during the various states of this photonic transistor. It is, therefore, both a mirror and a mask...an image component separator.
Looking at the projected image on the logic output separator, the right portion of the figure shows the distribution of energy in relation to the output hole, at the moment when the spreading pulse strikes the mirror. The view is from the hologram looking toward the mirror.
This second animation (Animation 2)
depicts the case where the lower input beam
contains an "on" pulse. Compare the inputs between
the single beam input above, and the one on the left.
The result is exactly the same as shown above (Animation 1) with the exception of a 180 degree phase change that occurs in the carrier wave (the electromagnetic energy itself) at the XOR output position, which will affect the way those pulses are handled in subsequent logic stages. This phase change occurs between the two input states depicted above, as to whether the energy comes from the upper input or the lower input, and is not the same as the usual phase change that occurs upon reflection.
The carrier phase remains the same at the OR output.
When both inputs are on simultaneously during the pulse time, the two beams interfere with each other at the hologram producing an image which is considerably different, called an "interference fringe." This redistribution of energy within the fringe is produced by constructive interference between the two pulse waves, which localizes the combined energy into areas we call CI areas. Since the energy from both beam pulses is redistributed into the CI areas at the separator, (the mirror/mask,) there is a dearth of energy in the DI areas. DI stands for "destructive interference."
Now compare the single pulse animation above (Animations 1 and 2) with this double pulse animation (Animation 3) an notice the important differences.
Destructive interference, and the initial constructive interference take place at the position of the hologram, when energy from one input is made to superposition on top of energy from the other beam. The tiny pattern produced at that time/location, is then projected onto to the mirror/mask where energy having different characteristics is separated to become the photonic transistor's various outputs.
As a result, all of the input possibilities are depicted in the animations. To help illustrate the point, the OR function is like the dome light in an automobile. When either or both of the front doors are open, the dome light is on. Both doors must be shut before the light goes out.
The XOR is like the light above a stair well. When both switches are up (on,) or off (down) together, the stairwell light is off. But if they are opposite, the light goes on. That is unless your electrician installed one of them up-side-down.
So, Why do some of the pulses turn "hot" pink in the animations?
In real photonic transistors, the laser light which supplies the input light does not actually change color (frequency or wavelength.) The localization of energy caused by interference increases the intensity of the output pulse by 4 times over what exits through either output when only one pulse traverses the transistor. The increased intensity is depicted by "hot pink" as opposed to plain red for the single beam intensity.
It might be asked, "Where does this extra energy come from?" When the single pulse arrives, and is spread into the CI area to become part of the OR output, and also into the DI area to become part of the XOR output, thus, there are 2 portions available. If there were no interference, then turning on the second beam would produce 2 more portions at each output, for a total of 4 energy portions. However, because of interference, all 4 portions arrive at the OR output. Since intensity of an electromagnetic pulse is the amount of energy per unit square area, which is proportional to the square of the amplitude, the OR output intensity is 4 times the single pulse intensity.
Two important things are occurring here. 1) the logic information has been extracted, and is captive within the modulation patterns of the two outputs. Since it is information that computers manipulate, the generation of this logic information from the modulated information represented by the blinking input beams is the vital step in producing computing. However, subsequent circuits must be presented this information in a useable form.
As with the 180 degree phase shifting that occurs at the XOR output between one single beam state and the other single beam state, there is an amplitude modulated component that exists along with the OR information in the OR output. CyberDyne has a number of photonic devices for dealing with this, and putting it to useful work.
2) The real advantage to the photonic transistor comes from its great speed. The transient propagation, or switching time is easily calculated since it is directly related to the distance the pulses must travel from the hologram to the output. While an electronic transistors having a propagation time in the order of a nanosecond (billionth of a second,) traverse a distance smaller than a point of a pin, light in air or vacuum covers 30 cm, or about 11 3/4 inches. A considerable number of photonic transistor logic stages can thus be built into a "nanosecond" far out-pacing its electronic counterpart.
In the animation to the right, (Animation 4) many pulses are shown producing the various logic outputs from the various combinations of inputs as the pulses pass one after the other through a photonic transistor. Unlike electronic transistors, logic calculations are able to be produced using pipelined pulses that exist simultaneously within a transistor without the pulse in front cross-talking with the pulse behind it. So, as long as the results of one logic operation are not needed while the pulses are still in the pipeline, a considerable amount of information can be processed by a single photonic transistor during the time it takes for an electronic transistor to just turn itself on.
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