Technology -> Optical Computers

Optical Computers

Since the dawn of the silicon age, computer processors have worked in essentially the same way: bits of data travel through the circuits of your computer in the form of electrons. It works well; it's fast, and cheap. But chipmakers are beginning to run up against absolute physical limits to making microprocessors even faster. The next significant advance in computer technology will require new materials or new ways of transporting data.

Optical Computers: Why are they needed

The traditional computers process information by crunching 0's and 1's. Higher speeds are achieved by miniaturizing electronic components to a very small micron-size scale so that those electrons need to travel only very short distances within a very short time.

This is method to increase the spead is only a short term solution because there are physical limits to how small a circuit can be made. Using Very Large Scale Integration (VLSI) technology over 300 million transistors can be fitted on a single silicon chip. It is also estimated that the number of transistor switches that can be put onto a chip doubles every 18 months. Further miniaturization of lithography introduces several problems such as dielectric breakdown, hot carriers, and short channel effects etc. All of these factors combine to seriously degrade device reliability.

Optical interconnections and optical integrated circuits provide a way out of this by enabling us to use light to perform computations.

Optical Computers: Advantages

One of the major advantages of optical computing is the increase in the speed of computation. Light travels at 186,000 miles per second, 982,080,000 feet per second. In one nanosecond, photons of light travel just a bit less than a foot. Just right for doing things very quickly in microminiaturized computer chips.

Apart from the speed the optical interconnections have several other advantages over their magnetic counterparts. They are immune to electromagnetic interference, and free from electrical short circuits. They have low-loss transmission and provide large bandwidth, capable of communicating several channels in parallel without interference. They are capable of propagating signals within the same or adjacent fibers with essentially no interference or cross-talk. They are compact, lightweight, and inexpensive to manufacture, and more facile with stored information than magnetic materials.

Another advantage of optical methods over electronic ones for computing is that optical data processing can be done much easier and less expensive in parallel than can be done in electronics. Parallelism is the capability of the system to execute more than one operation simultaneously. Electronic computer architecture is, in general, sequential, where the instructions are implemented in sequence. This implies that parallelism with electronics is difficult to construct. On the other hand a photon-based processor using different wavelengths of light that represent color to human eyes could quickly generate many parallel processes, drastically increasing computing speed and complexity.

To appreciate the difference between both optical parallelism and electronic think of an imaging system of as many as 1000x1000 independent points per mm 2 in the object plane which are connected optically by a lens to a corresponding 1000x 1000 points per mm˛ in the image plane. For this to be accomplished electrically, a million nonintersecting and properly isolated conduction channels per mm 2 would be required.

Another advantage of light results because photons are uncharged and do not interact with one another as readily as electrons. Consequently, light beams may pass through one another in full-duplex operation, for example without distorting the information carried. In the case of electronics, loops usually generate noise voltage spikes whenever the electromagnetic fields through the loop changes. Further, high frequency or fast switching pulses will cause interference in neighboring wires. Signals in adjacent fibers or in optical integrated channels do not affect one another nor do they pick up noise due to loops. Finally, optical materials possess superior storage density and accessibility over magnetic materials.

Optical Computers: Current Work

New logic gates are being designed at NASA because the current logic gates don't function in the new medium. The diagram below is a schematic of the new nanosecond all-optical AND logic gate setup designed at NASA.

This optical logic gate controls one light beam with another. It is "on" when the device transmits light, and "off" when it blocks the light."Optical bistable devices and logic gates such as these are the equivalent of electronic transistors they operate as very high speed on-off switches and are also useful as optical cells for information storage."

Optron Systems is working with researchers at NASA's Marshall Space Flight Center to develop new ways of getting photons from point A to point B - in essence, creating a new kind of circuit board for light. They used thin polymer films, made from organic materials, which can be cut into specific patterns with a laser. The main problem they faced with these organic materials was that they develop polymer aggregates - clumps of other materials within the pathways - that could scatter the light impulses used to transmit data. The only way to eliminate them is to grow the films in outer space - on the U.S. space shuttle, for instance. Which obviously is not a very viable alternative.

Don Frazier, the director of the Marshall project at NASA, hopes to have a prototype of an optical imaging device by 2001. Sometime after that, Optron Systems hopes to bring an optical computer to market. When that happens, Silicon Valley may have to change its name.

Optical Computers: Article Followup

This article was written in early 2000, for 'The Vector' which is NJIT's student newspaper. Since it was written there have been some advances in this technology but unfortunately Optical Computers are still mostly theoretical. No one has built a complete working prototype that would work outside of lab conditions.

I am still interested in this field and will be writing a followup essay on this sometime in the near future. So check back regularly for updates. Incase you are really interested in my writing more about this, send me an email asking me to do so. That should motivate me to write the article sooner.

- Suramya
May 2003