OPTICAL COMPUTER



OPTICALCOMPUTER

Authors

B.Mallikarjuna Reddy K.Gnanendra Chekravarthy

II B.Tech (CSE) II B.Tech (CSE)

Roll: 07L21A0502 Roll: 07L21A0538

Email: mallikarjuna.502@ Email:gnanipuri@

Cell: 9441496893 Cells: 9966131963

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VAAGDEVI INSTITUTE OF TECHNOLOGY AND SCIENCES PEDDASETTIPALLI (VILL), PRODDATUR,

KADAPA DT.ANDHRA PRADESH

OPTICAL COMPUTER

CONTENTS

1. Introduction

2. Optical Computer

3. Optical Components for binary digital computer

4. Basic components for digital computer

5. The elements of a binary digital computer

6. Advantages

7. Conclusion

OPTICAL COMPUTER

Introduction:

Now a days we are using the electronic computer, just think for a while if all the electronic components are replaced with optical one. Optical computer is the computer which performs operations ten or more times faster than the conventional electronic computers. This optical computer uses the infrared beams to manipulate store, transmit data.

Optical Computer:

Optical computer is a computer that uses light instead of electricity (photons instead of electrons) to manipulate, store and transmit data. It uses the IR beams to perform digital computations, which results an optical computer which perform operations ten or more faster than an electronic computer.

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Visible light and IR beams and unlike electric currents, pass through each other with out interacting. Several laser beams can be shone so their paths intersect, but there is not interference among the beams, even when they are confined essential to two dimensions. Electric currents must be guided around each other, and this make three dimensional wiring necessary. Thus an optical computer besides being much faster than an electronic one, might also be smaller.

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Electrical crossovers (top) require three dimensions, but optical crossovers (bottom) require only two dimensions because light beams don’t intersect.

Optical components for binary digital computer:

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The fundamental building block for modern electronic computers is transistor. To replace electronic components with optical ones, an equivalent optical transistor is required. This is achieved using materials with a non-linear refractive index. In particular, materials exist where the intensity of incoming light affects the intensity of the light, transmitted through the material in a similar manner to the voltage response of an electronic transistor. This “optical transistor” effect is used to create logic gates, which in turn are assembled into the higher level components of the computer CPU.

Basic components for digital computer:

In optical computer which utilizes photons as information carriers instead of electrons. An important step in building this optical computer has been construction of an optical alternative for the electronic transistor.

Basic components:

➢ A switch

o An electronic switch.

o Photos carrying information

o Optical bistability

o Optical switch

A switch:

A binary digital computer needs to be able to represent two states, “1” and “0”, the true and the false. So some mechanical, electronics, optical, whatever component is needed that controlled by this both states and, what is more important, it has to be possible to go from one state go other state in a conventional way, i.e. according to some pre determined non-linear function. In other words we have to be able to switch between these two stable states.

An electronic switch:

In electronics, switching is done by the transistor.

The transistor consists of three layers: the emitter, collector and base. The base is the middle layer and is made of semi conducting material. This means that it can acts either as an insulator between emitter and collector, or as a conductor. If a small current flows from base to collector, some electrons traverse the base. This changes the base from an insulator to a conductor. If there is not current from base to collector, the base acts as an insulator again. Now we have an electronic switch, because if the base acts as a conductor and we let some (large) current flow from emitter to collector, we can stop this current by stopping the (small) current base to collector. However, this switch is subject to some limitations. There is a limit to the speed by which electrons can traverse the base, and in modern VSLI design, this limit is reached with approximately a nanosecond. But there might be other media to transport the information in a computer, thus attaining a higher speed. Photons carrying information.

Photons carrying information:

The highest speed ever attainable is the speed of light. So it seems logical to see light, or electromagnetic radiation in general, as the perfect way of pushing computing to its limits.

First of all, the electrons affect each other at distance, while photos do not. In particular, electrons repel each other because of their negative magnetic load. This property is an advantage in switching a transistor, in changing the base from an insulator to a conductor and vice versa.

And this where another advantage of photos come in: two beams of light can cross without affecting each other, provided their angle is not less than 10. This increase the number of possible interconnections, something we come back to later.

Optical bistability:

To build gates and storage (the real hardware components of the conventional digital computer), taking the transphasor as a staring point, we will explain something more about a property of some materials, called optical bistability.

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Figure: Nonlinear refraction with hysteresis loop.

The way we depicted the relation between the incident and the transmitted intensity of a beam of light going through some material as described in the previous section is only partly true. It is actually so, that the diagram representing an increasing incident beam slightly differs from the diagram representing a decreasing incident beam. That is, the “switching-intensity” (the incident intensity needed for the steep line in the diagram) of the incident beam differs. Representing this phenomenon in one diagram results in the one depicted in figure. It has a loop called the hysteresis loop.

By this hysteresis loop we again have two stable states. If we keep the incident beam at an intensity in the domain of the hysteresis loop, the transmitted intensity remains at the same level (high or low).

An optical switch:

At this point we take a closer look at the conventional transistor. In the electronic transistor, the two currents of electrons do not really interact. The semi conducting material acts as an intermedium. If we want to build a switch and follow the idea of the transistor, we have to find a material isomorphic to the semi conducting material i.e. a device of which we can change the properties just by sending a beam of light through it. Perhaps we can find a device that sometimes (dependent on another beam) is opaque, and sometimes transparent. In 1896 the French physicist Charles Fabry and Alfred Petor invented their interferometer. It is simply consists of two partially reflecting mirrors, placed parallel to each other. This might be the basis for an optical transistor. If a beam of light strikes the first mirror, some percentage of the light is reflected, and some goes through. The same happens at the other mirror. But if we take two mirrors that let only 10 percent of the light go through, only 1 percent of the light goes through both mirrors (the transmitted beam) and some of the light stays between the mirrors (in what is called the cavity) for a while.

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A property of light is that we may look at it in two different ways: as particles (photons) and as waves. Now we are going to use the latter. Waves interfere under certain circumstances. This means that if two waves are aligned, they may reinforce or destruct each other, dependent on whether they are in equal or opposite phase. If the mirrors are placed at a distance equal to halfway an integral number of half wavelengths, the beams of light that are in the cavity interfere constructively. This means that the transmitted beam is far more that 1 percent, it might even be a 100 percent. On the other hand, if the mirrors are placed at an integral number of half wavelengths, the waves interfere destructively. The transmitted beam is even less than 1 percent. Thus, we have some sort of a switch. But so far it only switches by placing the mirrors at another distance.

Now we come back to the major problem of how to switch without moving parts. It would be nice to be able to change the wavelength of a beam. The development of lasers, sources of very powerful coherent radiation, it was discovered that the refractive index of some materials changes if the intensity of incoming the beam of light exceeds a certain boundary. This is called nonlinear refraction, because we get a nonlinear diagram if the incident intensity is plotted against the transmitted intensity. This results in two levels: a low level and a high level.

The elements of a binary digital computer:

Optical bistability and switching provide us with the means of building a binary digital computer. Gates and storage elements are useful transform a transphasor in to a logic gate and how to build a storage elements using optical bistability.

Gates:

The logic performed by a conventional computer is done with sixteen Boolean functions, but two of them (AND, OR and NOT) are sufficient, because we can combine these to perform one of the other fourteen. We now show that it is very easy to transform a transphasor in either an AND or an OR gate. Because there is no need for optical bistability, a transphasor without hysteresis is needed.

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An AND gate is formed by taking two incident beams acting as the two inputs of the gate. The high level intensities of both beams must be lower than the switching intensity of the transphasor, but higher than half the switching-intensity. Both incident beams are aimed at the same spot on the first mirror. Only if both incident beams have an intensity. If both incident beams, or one of them, has an intensity below its high-level, the transmitted beam will be of low-level intensity. This is exactly like an AND gate in electronics.

To make an OR gate we only have to make sure that the high-level intensities of the incident beams are equal to the switching-intensity of the transphasor. If one or both incident beams have high-level intensities, the transmitted beam has a high-level intensity. Otherwise, both incident beams must have a low-level intensity. Again the working of the optical OR gate is very analogous to the working of the electronic one.

The optical NOT gate is constructed by taking the reflected beam as the output. As the reflected beam is the inverse of the transmitted beam, an increase of incident intensity produces low output while decreasing the incident beam provides high output.

Storage elements:

In a binary computer there is a need for storage elements able to represent two stable states. If the high-level intensity represents a “1” and the low-level a “0”, putting a “1” in the device can be done by just by adding some other beam for a short while, such that the added intensity is just enough to get a high-level transmitted intensity. Putting a “0” in the device can be done by just stopping the beam for a short while.

Assembling the elements:

In order to communicate between all the elements interconnection is needed. A reliable outcome signals need to be synchronized by some sort of clock signals. It uses function/interconnection module and a pipelined processor.

The function/interconnection module:

The base of the design are the function/interconnection modules that are programmable with 16 customizing inputs.

The idea is to combine signal-pairs (a signal pair consists of a signal and its inverse, say A and A or B and B) using four tri-input AND gates. This is called functional logic block.

Two functional logic blocks (having two signal-pairs as input and one signal-pair as output) can be combined yielding a functional logical cell. It is clear that the output of such a cell can be used as one of the input pairs of another cell.

Grouping two functional logic cells (having a total of two input pairs, two output pairs and sixteen customizing inputs) gives a function/interconnection module usable both to perform logic operations as to interconnect various logic functions.

A pipelined processor:

The function/interconnection module are cascadable to form a pipelined processor, programmable do every wanted computation. Synchronization is done using a clock signal. The clock signal can control the customizing inputs of the various function/interconnection modules. Latches (storage elements that preserve the signal during one clock cycle) between the modules, also controlled by clock signal, data can flow through the pipeline.

Advantages:

1. One of the major advantages of optical computing is to increase the speed of computation, light travels at 186,000 miles per second i.e. in one nano second photos of light travels just a bit less than a foot. It is enough to do things very quickly in micro miniaturized computer chips.

2. Optical computer is immune to electro magnetic interference and free from electrical short circuits.

3. They have low loss of transmission and provide large bandwidth capable of propagating signals within the same or adjacent fibers with essential no interference or cross talk.

4. Another major advantage of optical methods over electronic ones is computing is the optical data processing can be done much easier and less expensive in parallel than can be done in electronics.

Conclusion:

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Researches are working to replace all electronic components with optical ones. Soon we can see computers which have the capability of computing more than ten or more times faster than now present conventional electronic computers.

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