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Computers work with binary, on or off, states. A completely optical computer requires that one light beam can turn another on and off. This was first achieved with the photonic transistor, invented in 1989 at the Rocky Mountain Research Center. This demonstration eventually created a growing interest in making photonic logic componentry utilizing light interference.
Light interference is very frequency sensitive. This means that a narrow band of photon frequencies can be used to represent one bit in a binary number. Many of today's electronic computers use 64 or 128 bit-position logic. The visible light spectrum alone could enable 123 billion bit positions.
Recent research shows promise in temporarily trapping light in crystals. Trapping light is seen as a necessary element in replacing electron storage for computer logic.
While photonic computing is still seen as impractical by many[who?], research is being pushed along by strong market forces already implementing networking and, thus, creating opportunities. Recent years have seen the development of new conducting polymers which create transistor-like switches that are smaller, and 1,000 times faster, than silicon transistors.
Optical switches switch optical wavelengths. Optical switching, while not all-optical, has already become important in networking environments. 100 terabit-per-second data-handling is expected within the decade. Existing technologies include:
micro-electro-mechanical systems, or MEMS, which use tiny mechanical parts such as mirrors.
Thermo-optics technology, derived from ink-jet technology, creates bubbles to deflect light.
liquid crystal display switching changes (e.g., by filtering and rotating) the polarization states of the light.
acousto-optic modulator uses the acousto-optic effect to diffract and shift the frequency of light using sound waves (usually at radio-frequency).
photonic integrated circuits.
Optical computers
An optical computer (also called a photonic computer) is a device that uses the photons in visible light or infrared (IR) beams, rather than electric current, to perform digital computations. An electric current creates heat in computer systems, the more processing speed is needed, the more electricity is required; all this extra heat is extremely damaging to the hardware. Light however doesn't create significant amounts of heat no matter how much is used and therefore more powerful processing systems can be produced. By applying some of the advantages of visible and/or IR networks at the device and component scale, a computer might someday be developed that can perform operations 10 or more times faster than a conventional electronic computer.
Visible-light and IR beams, unlike electric currents, pass through each other without interacting. Several (or many) laser beams can be shone so their paths intersect, but there is no interference among the beams, even when they are confined essentially to two dimensions. Electric currents must be guided around each other, and this makes three-dimensional wiring necessary. Thus, an optical computer, besides being much faster than an electronic one, might also be smaller.
Some engineers think optical computing will someday be common, but most agree that transitions will occur in specialized areas one at a time. Some optical integrated circuits have been designed and manufactured. (At least one complete, although rather large, computer has been built using optical circuits.) Three-dimensional, full-motion video can be transmitted along a bundle of fibers by breaking the image into voxels. Some optical devices can be controlled by electronic currents, even though the impulses carrying the data are visible light or IR.
Optical technology has made its most significant inroads in digital communications, where fiber optic data transmission has become commonplace. The ultimate goal is the so-called photonic network , which uses visible and IR energy exclusively between each source and destination. Optical technology is employed in CD-ROM drives and their relatives, laser printers, and most photocopiers and scanners. However, none of these devices are fully optical; all rely to some extent on conventional electronic circuits and components.
Most research projects focus on replacing current computer components with optical equivalents, resulting in an optical digital computer system processing binary data. This approach appears to offer the best short-term prospects for commercial optical computing, since optical components could be integrated into traditional computers to produce an optical/electronic hybrid. Other research projects take a non-traditional approach, attempting to develop entirely new methods of computing that are not physically possible with electronics.
Optical components for binary digital computer
The fundamental building block of modern electronic computers is the 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's CPU.
Misconceptions, challenges and prospects
Another claimed advantage of optics is that it can reduce power consumption, but an optical communication system will typically use more power over short distances than an electronic one. This is because the shot noise of an optical communication channel is greater than the thermal noise of an electrical channel which, from information theory, means that we require more signal power to achieve the same data capacity. However, over longer distances and at greater data rates the loss in electrical lines is sufficiently large that optical communications will comparatively use a lower amount of power. As communication data rates rise, this distance becomes shorter and so the prospect of using optics in computing systems becomes more practical.
A significant challenge to optical computing is that computation is a nonlinear process in which multiple signals must interact to compute the answer. Light, which is an electromagnetic wave, can only interact with another electromagnetic wave in the presence of electrons in a material and the strength of this interaction is much weaker for electromagnetic wave light than for the electronic signals in a conventional computer. This results in the processing elements for an optical computer requiring high powers and larger dimensions than for a conventional electronic computer using transistors.
Photonic logic
Photonic logic is the use of photons (light) in logic gates (AND, NAND, OR, NOR, XOR, XNOR). Photonic logic refers to the usage of light (photons) to form logic gates. Switching is obtained using nonlinear optical effects when two or more signals are combined.
Resonators are especially useful in photonic logic, since they allow a build-up of energy from constructive interference, thus enhancing optical nonlinear effects.
Other approaches currently being investigated include photonic logic at a molecular level, using photoluminescent chemicals.
Photonics
The science of photonics includes the generation, emission, transmission, modulation, signal processing, switching, amplification, detection and sensing of light. The term photonics thereby emphasizes that photons are neither particles nor waves - they are different in that they have both particle and wave nature. It basically covers all technical applications of light over the whole spectrum from ultraviolet over the visible to the near, mid and far infrared. Most applications, however, are in the range of the visible and near infrared light. The term Photonics developed as an outgrowth of the first practical semiconductor light emitters invented in the early 1960s and optical fibers developed in the 1970s.
Refraction of waves of photons (light) by a prism
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