Optical Technology

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Optical Technology

Light, like radio, consists of electromagnetic waves. The major difference between the two is that light waves are much shorter than radio waves. The use of electromagnetic waves for long-distance communications was the beginning of an industry known first as wireless and later as radio. This industry was the foundation for electronics, which brought the world so many fascinating technologies.

When electronic circuits replaced the mechanical components in computers, the electronics were so fast compared to the older mechanical methods that no one ever thought the speed of the calculations would be limited by the speed of electrical signals. When engineers demanded even faster electronic circuits, the very short pulses and the tiny dimensions of their integrated circuits begged for the use of light energy rather than electrical energy. The use of light signals rather than electrical signals allowed for the design of very small and fast systems.

One of the first examples of replacing electrical signals with light was in communications. Special wires called transmission lines distribute telegraph, telephone, and television signals. In the mid-nineteenth century Samuel Morse's telegraph was extended from the East to the West Coast of the United States. The fledgling industry painfully discovered how signals transmitted long distances through wires could be degraded. Signals had to be regenerated every 10 to 20 kilometers (6 to 12 miles) depending on the quality of the transmission line. When the telephone industry came along, the situation was even worse. When cable television distributed television signals through transmission lines in the 1960s, amplification was required every few blocks. The problem of signal degradation increases with bandwidth . Television signals are nearly 2,000 times wider than voice signals.

Light energy can be constrained to a thin glass fiber much as electrical energy can be constrained to a wire. However, unlike wires, glass fibers can generate very little distortion even for signals of extremely wide bandwidth. Therefore, with the use of glass fiber, or fiber optics , broadband signals can be transmitted for much longer distances without amplifying or repeating. With fiber optics, communications systems with literally hundreds of television channels can be distributed with minimal distortion.

The glass fiber is very thin, extending from about 10 to 125 micrometers in diameter. In order for the fiber to contain light energy without loss, the index of refraction of the glass must be less on the surface of the fiber than at its center. The index of refraction is a measure of the speed of light in a medium—in this case, glass. The higher the index of refraction, the slower the light waves propagate.

The slowing of the light energy causes the light path to bend. In the glass fiber, the light energy is bent back toward the center of the fiber. This phenomenon is called total internal reflection. It insures that very little energy is lost by the fiber.

The light energy for the fiber is monochromatic—it has only one color or wavelength. Although the concepts of the glass fiber and total internal reflection have been known for many years, it was not until the invention of the solid-state laser diode and the light emitting diode (LED) that glass fibers could be used for communications.

The optimum wavelength for use with a glass fiber depends on the composition of the glass. The most common wavelengths are about 1,300 nanometers , which is longer than visible light in the infrared region. Although both the laser and the LED are commonly used, the highest performance systems use laser diodes.

The state of the art in fiber optics is the use of wavelength division multiplexing (WDM), which uses a number of different wavelengths to increase the capacity of the system. The signals of different wavelengths can coexist in the fiber without mutual interference. This is similar to how a number of radio stations can operate on the same FM broadcast band without interference. The radio receiver is capable of selecting one of the stations while ignoring others.

The advantages of transmitting signals with light energy can be realized not only for long distances but for short distances as well. Even though the dimensions of a computer and the integrated circuits used to make them are quite small, a certain time is required for the signals to travel around the computer. In the process of computation, signals move from one part of the computer to another before a final answer is reached. If the calculation is complex, the signals may spend considerable time traveling. Because light signals can travel faster than electrical signals, the use of light signals will enhance the computer.

Modern conventional computer circuits can switch electrical signals on and off in less than one nanosecond . Some specialized logic circuits can switch in a fraction of a nanosecond, but these circuits are not suited for large integrated circuits because they consume large amounts of power and it would be impossible to remove the heat from the chip.

Light energy can be switched in times measured in picoseconds . If light signals could be switched on and off by other light signals, computer logic elements could be constructed. Then, by connecting the logic elements together with light signals, an entire computer could be constructed using only light signals.

To take advantage of the inherent speed of optical computing, even the computer architecture can be adapted for speed. Most calculations require sequential operations. For example, to find the hypotenuse of a right triangle, the first side is squared and then the second. The two are then added together and the square root is taken of the result. Taking the square root is a sequence of operations. In many cases the calculations are taken eight bits at a time or even a single bit at a time. A faster way of achieving this would be to square the two sides at the same time or to do the operations in parallel using two computers. It sounds wasteful to employ two computers just to solve a simple triangle. But when thousands of numbers are to be squared and added, two computers would make the process go twice as fast; three computers, three times as fast; and so on.

An architecture called "massively parallel" involves a large number of processors where each processor performs a part of the required calculation. This can involve as many as 1,000 processors. This technique is only useful if the processors can perform fast calculations and can communicate with high speed. Optical computation using massively parallel architecture would result in the fastest computers on Earth.

Switching light signals involves esoteric materials that exhibit what are called non-linear optical properties. One light signal can effect the propagation of another light signal in these materials and can be used to switch another light signal off and on. Switching is the main ingredient for making logic elements.

One important application of optical technology is mass storage. Early computers used crude storage such as paper tape and punched cards . One of the first computer storage media for random access memory (RAM) used the properties of ferromagnetic materials. Ferromagnetics are materials that are primarily made of iron. Everyone is familiar with magnetized iron and steel and how the north and south poles effect other magnets or pieces of iron or steel. Imagine a simple iron bar that has been magnetized. There are two ways the bar can be magnetized. One orientation could represent a logic zero, and the other—with the north and south poles reversed— a logic one. The first random access memories used small donut-shaped, magnetized cores. These cores would retain their magnetized state when the computer power was removed.

For removable storage, ferromagnetic films are used on tapes, strips, disks (both hard and floppy), and for credit cards, employee badges, and similar items. Magnetic storage is vulnerable because the stored film could be erased if it is subjected to a magnetic field.

The density (bits per unit area) of the ferromagnetic film was good, but the demand for high-density storage was growing stronger. The first use of optics for high-density data storage was not for computers but for recording television signals. However, the optical videodisc failed to gain acceptance in the marketplace. The videocassette recorder (VCR) uses magnetic storage. The main advantage of the VCR was its ability to record. An off-shoot of the videodisc was a smaller version called the compact disc (CD). This device was first used for digital recording of music and later for computer data; the latest application is motion pictures.

Optical disk storage technology relies on recessed pits on a reflecting surface. Light energy reflected from a laser onto the smooth polished surface of the disk would be reflected directly back to the laser. If the surface is not smooth, the energy is scattered, with only a small amount reflected directly back to the laser. As the CD is rotated, a photo detector senses the change as the pits pass by. The disk has a very long track on which the pits are positioned, and the laser light traces out the track. The ones and zeros are encoded by the change of the reflected light intensity. The length of the pit is used to encode a number of digital bits.

Because the wavelength of the light is short, the storage density of CDs is very high. The spiral track of a CD, if it were unwound, would be more than 5,000 meters (16,400 feet) long. The space between the tracks is 1.6 micrometers and the pits are between 0.833 and 3.05 micrometers long. CDs are very inexpensive to make, are resilient to damage, and are completely immune to magnetic fields. As of early 2002, a CD can store between 500 and 800 megabytes (MB) of data.

see also Music; Robotics.

Albert D. Helfrick