In most modern computers, electrons travel between transistor switches on metal wires or traces to gather, process and store information. The optical computers of the future will instead use photons traveling on optical fibers or thin films to perform these functions. But entirely optical computer systems are still far into the future. Right now scientists are focusing on developing hybrids by combining electronics with photonics. Electro-optic hybrids were first made possible around 1978, when researchers realised that photons could respond to electrons through certain media such as lithium niobate (LiNbO3). To make the thin polymer films for electro-optic applications, NASA scientists dissolve a monomer (the building block of a polymer) in an organic solvent. This solution is then put into a growth cell with a quartz window. An ultraviolet lamp shining through this window creates a chemical reaction, causing a thin polymer film to deposit on the quartz.
An ultraviolet lamp causes the entire quartz surface to become coated, but shining a laser through the quartz can cause the polymer to deposit in specific patterns. Because a laser is a thin beam of focused light, it can be used to draw exact lines. A laser beam's focus can be as small as a micron-sized spot (1 micron is 1-millionth of a meter, or 1/25,000 of an inch), so scientists can deposit the organic materials on the quartz in very sophisticated patterns. By painting with light, scientists can create optical circuits that may one day replace the electronics currently used in computers.
NASA scientists are making these organic thin films on the Space Shuttle to overcome problems caused by convection. Convection is a circular motion in air or in a liquid created from uneven heating. On Earth's surface, when a gas or liquid is heated it expands, becoming lighter and less dense. This lighter material rises, mixing with cooler and denser material from above. Such turbulence occurs in the world's weather patterns or even in a pot of water boiling on the stove.
Convection creates difficulties when trying to create a uniform film. A UV lamp or laser light will raise the temperature of the film solution, causing the hotter solution to rise. Aggregates of solid polymers often form in the solution, and convective flows that develop in the solution can carry these aggregates to the surface of the quartz. Because aggregates on optical films can cause light to scatter, the goal is to make the films as smooth and uniform as possible.
Convection is actually caused both by heating and the Earth's gravity. The microgravity conditions of space reduce the effects of convection because there is no up direction for the heated material to head towards. Any aggregates in space-produced films can only reach the quartz through the slower process of diffusion. Because microgravity reduces convection, films made in space have fewer polymer aggregates than those made on Earth.
Convection causes other problems for the production of optical films. Convection can affect the distribution of molecules in a fluid, so films created on Earth can have regions that are rich or poor in certain molecules rather than evenly dispersed throughout. Films made in microgravity often have more highly aligned and densely packed molecules than Earth formed films. Because there is little convection in a microgravity environment, scientists can produce smoother and more uniform films in space.
The thin films being developed by NASA are composed of organic molecules, which often are more sensitive than inorganics to changes in light intensity. Organics can perform a large number of functions such as switching, signal processing and frequency doubling, all while using less power than inorganic materials. While silicon and other inorganics are often used in electronic computer hardware, the all optical computers of the future will probably use mostly organic parts. There will be a gradual hybridisation in which computers using both organic and inorganic parts make use of photons and electrons. These hybrid devices will eventually lead to all optical computer systems.
In the optical computer of the future electronic circuits and wires will be replaced by a few optical fibers and films, making the systems more efficient with no interference, more cost effective, lighter and more compact.
Smaller, more compact computers are often faster because computation time depends on shorter connections between components. In the search for speed, computer chips have grown ever smaller, it is estimated that the number of transistor switches that can be put onto a chip doubles every 18 months. It is now possible to fit 300 million transistors on a single silicon chip, and some scientists have predicted that in the next few decades computer technology will have reached the atomic level. But more transistors mean the signals have to travel a greater distance on thinner wires. As the switches and connecting wires are squeezed closer together, the resulting crosstalk can inadvertently cause a digital signal to change from a 1 to a 0. Scientists are working on developing newer, better insulators to combat this problem. But optical computers wouldn't need better insulators because they don't experience crosstalk. The thin-films used in electro-optic computers would eliminate many such problems plaguing electronics today.
The thin films allow us to transmit information using light. And because we're working with light, we're working with the speed of light without generating as much heat as electrons. We can move information faster than electronic circuits, and without the need to remove damaging heat.
Multiple frequencies of light can travel through optical components without interference, allowing photonic devices to process multiple streams of data simultaneously. And the optical components permit a much higher data rate for any one of these streams than electrical conductors. Complex programs that take 100 to 1,000 hours to process on modern electronic computers could eventually take an hour or less on photonic computers.
The speed of computers becomes a pressing problem as electronic circuits reach their maximum limit in network communications. The growth of the Internet demands faster speeds and larger bandwidths than electronic circuits can provide. Electronic switching limits network speeds to about 50 gigabits per second (1 gigabit (Gb) is 1 billion bits).
Terabit speeds are already needed to accommodate the 10 to 15 percent per month growth rate of the Internet, and the increasing demand for bandwidth-intensive data such as digital video (1 Tb is 1 trillion bits). All optical switching using optical materials can relieve the escalating problem of bandwidth limitations imposed by electronics.
Last year Lucent Technologies' Bell Laboratory introduced technology with the capacity to carry the entire world's Internet traffic simultaneously over a single optical cable. Optical computers will someday eliminate the need for the enormous tangle of wires used in electronic computers today. Optical computers will be more compact and yet will have faster speeds, larger bandwidths and more capabilities than modern electronic computers.
Optical components like the thin-films developed by NASA are essential for the development of these advanced computers. By developing components for electro-optic hybrids in the present, NASA scientists are helping to make possible the amazing optical computers that will someday dominate the future.
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