A brief history of lasers

During the nineteenth and twentieth centuries, scientists made many improvements to our life. One of the most important was a more accurate understanding of light, currently defined as traveling electromagnetic waves. Like the ocean’s waves, light also has an amplitude (which determines its power) and a frequency (which determines its color and energy). The better we understand light, the more uses we find for it in our life. For example, today we are faced with many technological devices based on light, such as printers, CD writers and readers, and fiber optic devices for telecommunications.

Many scientists are very interested in photons, for they can be used in communication, computation, and many other fields. Also, many researchers think that the technology of the future will be built on optoelectronics”photons and electrons.

The invention of lasers is a very important step in the science of optics. While lasers started out as a major component of science fiction stories, science fiction is rapidly becoming scientific reality due to continual improvements and discoveries.

Lasers defined

By definition, a laser is amplified light. However, its amplification is very different from a normal amplification, for this amplification makes the photons coherent by causing them to have the same energy and same direction. Such coherence enables a laser light to travel over long distances without diverging. If the laser beam is kept in a dispersionless media, theoretically it can keep the same waist size forever. However, the only media that currently can serve as a dispersionless media is a vacuum.

In a laser system, many atoms have to have electrons in the same high energy levels. If this is the case, any effect that stimulates the atoms’ system will emit coherent light. For the emission to continue, the system should be constructed so that there are always some electrons changing their energy level.

Observing several laser systems will give us a clearer understanding of lasers.

Ruby lasers

The first lasing structure was the ruby crystal (see Figure 1), devised by Dr. T. H. Maiman in 1960. This was a surprising development, for researchers thought that gases would be the first lasers. The ruby crystal is Al2O3 (called sapphire), and has an impurity level of 0.05% Cr+3 ions.

The ruby laser consists of a ruby crystal surrounded by a flash tube enclosed within an aluminum cylindrical cavity that is cooled by forced air. The laser cavity is pumped by a flash light. When the light’s power exceeds a certain limit, it begins to re-excite some ions inside the ruby crystal to higher state. The cavity ends are coated with evaporated silver. However, one side has a lower reflection ability so that some light can pass through it.

Gas lasers

Most elements and many molecules can be made to lase in a gaseous state. The first example of a gas laser is the HeNe (helium neon) laser, as depicted in Figure 2. In a high voltage tube, colliding helium and neon atoms transfer energy to neon atoms, which then assume a meta-stable state. After this, spontaneous emission occurs when neon atoms transit from a higher energy level to lower energy level. Like other lasers, the HeNe laser also needs to have a population inversion. The high population for neon’s meta-stable state is achieved by applying a high voltage to the tube. Although the stimulated emission decreases the number of atoms in the meta-stable state, the high voltage pumps the system back into the population inversion condition.

Having many different wavelengths (colors), HeNe laser are useful for all sorts of applications, from semiconductor technology to construction leveling.

Improvements in semiconductor technology have made many contributions to laser technology. Data storage on CDs, computer, printers, and telecommunication tools are just a few examples of the places where semiconductor lasers are used.

Semiconductor lasers

Three different materials have the properties necessary to serve as electron (carrier) conductors: metals, insulators, and semiconductors. Metals are good conductors for carriers, whereas insulators do not conduct electricity. In a solid state material, electrons stay in the bands determined by the attraction between positive and negative charges (electrons and nucleus). The further band for an electron is called the conduction band. In metals, the conduction band is partially filled, while in insulators the conduction band is totally empty. There is also a very large energy difference between the conduction band the valence band (the band just before conduction band). Thus, a large amount of energy has to be supplied in order to produce some carriers in the conduction band.

Conduction occurs when electrons are present in a conduction band, for they are somehow free in that band. They are not so free that they can escape it, but they are free enough to walk around in it. Research is revealing many other surprises or gifts that the All-Wise Creator has put in front of us.Our discovery of certain materials’ ability to serve as insulators and/or conductors has made our life much easier.

A very important step in the field of semiconductors is the use of optics during experiments. The electron in the conduction band can loose energy by radiating light, and one can use this energy to build lasers. The laser’s wavelength mainly depends on the energy gap between the conduction and the valence bands. If this energy gap is known, researchers can grow appropriate semiconductor structures to lase.

As growth techniques for semiconductors improve, the quality and variety of semiconductor lasers increase. Early semiconductor lasers were built from bulk structures. But after the 1980s, scientists discovered that layering different semiconductors could increase optical efficiency. The commercial state-of-art now is semiconductor quantum well lasers. In these structures, the electron’s mobility is restricted on a plane, giving carriers a two-dimensional freedom. Lasers using quantum dots (quasi-zero dimensional structures with superior optical properties) also have appeared during the last 5 years.

Conclusion

To see how our life will change via improvements in optics, just look at how fast communication has become, thanks to telecommunication lasers. Old thick and slow copper wires are being replaced by fast thin fibers. A computer and a camera gives one access to visual telecommunication via the Internet. All of this used to belong to science fiction. Not any more!

I believe that one we will develop a technology to transport material instantly, as stated in Qur’an: One who had knowledge of the Book said: I will bring it to you within the twinkling of an eye! When (Solomon) saw it placed firmly before him, he said: This is by the Grace of my Lord! – to test me whether I am grateful or ungrateful! If anyone is grateful, truly his gratitude is (a gain) for his own soul. But if any is ungrateful, truly my Lord is free of all needs, supreme in honor! (27:40).

In conclusion, we have to learn how to read the Book of the Universe and to understand it so that we can make even more beneficial discoveries.

References

• Arakawa, Y. and H. Sakaki. Multidimensional Quantum Well Laser and Temperature Dependence of Its Threshold Current. Appl. Phys. Lett. 40, no. 11 (June 1982): 939-41.
• Davis, Christopher C. Lasers and Electro-Optics: Fundamentals and Engineering. Cambridge Univ. Press: 1996.
• Hecht, Eugene. Optics. 4th ed. Addison-Wesley: 2001.
• Hitz, Breck et al. Introduction to Laser Technology. 3d ed. IEEE: 2001.
• http://home.achilles.net/~jtalbot/
• Kirstdter, N. et al. Low Threshold, Large T Injection Laser Emission from (InGa) as Quantum Dots. Electron. Lett. 30, no. 17 (Aug. 1994): 1416-17.
• Ledentsov, N. N. et al. Quantum-dot Heterostructure Lasers. IEEE J. Select. Topics Quantum Electron. 6 (May-June 2000): 439-51.
• Maiman, Theodore. The Laser Odyssey. Laser Press: 2000.
• Svelto, Orazio (ed.). Principles of Lasers. Translated by David C. Hanna. 4th ed. Plenum Publishing Corp.: 1998.

WHY OPTICAL?

Despite the many benefits that classical computers (‘classical’ here means computers in which the signals are carried electrically) have brought to our lives, they have some limitations which prevent any improvement in the speed or volume of signals carried. These limitations are inherent to the way these computers work.

For example, classic electric circuits carry information units serially, one by one, and there are some lower limits beyond which such circuits cannot be built-below that limit they simply cannot process the information reliably. Another handicap is that electrons floating in circuits can interfere with each other-and this interference, incidentally, is one reason why engineers cannot produce smaller circuits. By contrast, photons, light particles, which are the main signal or information carrying agent simply do not interact with each other because they do not carry a charge.

An optical computer could be run faster than one running electrons, theoretically at the speed of light, along optical fibres which are specifically designed guide-wires to transfer light-photons in and out between chips in an optical computer without distortion.

One of the main advantages of optical computers is their capability of processing more than one piece of information at the same moment. That means multi-beams can be processed in one chip. This would allow engineers to use parallel processing which greatly enhances the speed of the computer.

THE DIFFICULTIES

Lasers would, naturally, be the source of light in this new generation of computers. Scientists and engineers all over the world are trying to build appropriately tiny lasers emitting precise frequencies of infrared light. But they face a number of practical hurdles. One has to do with making lasers of appropriate size and efficiency. Current technology does not have the means to build optical chips comparable in size to ‘classical’ ones. The efficiency of the lasers is not high enough for the specifications required. Most of the energy to run these lasers escapes as heat and is not used. Since one or at most two percent of this energy can be transformed into the useful form of light, the rest can generate a lot of heat which is dangerous to the condition of the chips.

Making the right lasers is not the only problem on the way to fully optical computers. Switches are at the heart of optical computers, but as photons do not interact with each other, there are substaintial difficulties in building switches.

SOME PROPOSED SOLUTIONS

One solution to this problem is to build computers which are part electrical, part optical. Many scientists now believe that the most viable use for optical technology is in this type of hybrid system combining optics and electronics. Researchers are now focusing their work on optical interconnections between chips, which could be a reality in as little as one or two years. This type of connection can vastly increase the amount of data moving in and out of chips.

Such a machine would have to contain prisms, mirrors, and lasers to channel the light, as well as gallium arsenide chips that convert pulses of laser light into electrons so as to function as switches. If all this does happen, there will be a need for new computer architectures, that is, new computer structures.

However, there are some scientists following a different route. They are trying to find ways to use current transistor technology so as to detect laser beams in the information processing. NPN type transistors without a metal cover would be appropriate because they are faster. This approach also allows for adaptation of existing designs, with all the advantages in time and savings that brings.

FUTURE

The first optical processor developed at AT&T Bell Labs measured about two feet by two feet. Scientists hope some day to fit it all into three square inches. A fully optical computer is more than five years away.

Scientists have set themselves a target for the year 2000: 1,000 I/O (input and output) channels running at 1 giga-bit/sec. That is a thousand times faster than current modern computers.

It is a pity that we must wait for a decade, while scientists and engineers try to accomplish this difficult task. But what an exciting wait!

•  FURTHER READING
• ‘Bright future’, Scientific American, May 1990.
• ‘Now easier optical’, Electronics, May 1990.
• ‘Slacken lights up’, Scientific American, July 1991.
• ‘Optical computer no longer lighters away’, Byte, April 1992.
• ‘Optical computing sheds ‘blue sky’ image’ Electronics, April 1990.