Connecting the World
May 6th 2002
Coherent, Monochromatic and Collimated
At some point in creation, it is said; the Supreme Being declared, “Let there be light”. From then we have been illuminated by one of the most ubiquitous substances in the universe, light. For many a century light had confounded the wise men and no one could really answer the question, “What is light?”
The classical view of light is that it is an electromagnetic wave, with the properties of frequency, wavelength and speed. The quantum mechanic viewpoint of light is that is it a stream of particles called photons. Photons have no mass and travel at the speed of light (do you not love the circular definition?). The compromise is that particles and waves are duals and both show properties of each other. Which of course, should be impossible, but isn’t.
Light as we know it is a rather complex animal. While it is relatively easy to create (use a light bulb), it is also simple to reflect, refract, focus, filter and absorb. Yet its composition is rather complicated. Ordinary light is composed of rays (or photonic streams) with many different frequencies, phases, polarization and directions. Note the hotchpotch of terminologies—while direction is a property of a traveling particle, the rest are properties of a wave.
Whatever light is, it has all the above properties—it travels rather fast, has direction and also has frequency, phase and polarization. Ever since scientists have admitted to all of this, they went on a quest to find the purest form of light, something that is quite easy to define, but very hard to manufacture.
The purest form of light (it had no name at this point) would be light that would be coherent, monochromatic and collimated. In simpler terms, coherent means that every ray of light is accurately phase-synchronized with each other. Monochromatic means that the light must be composed rays of exactly the same frequency. Finally collimated means that every ray would have exactly the same direction of travel.
The quest for such light stayed mainly in the dark, because generating light with such properties was found to be quite impossible. Every light source gives out light with many different wavelengths and phases and directions. Trying to process ordinary light into a coherent, monochromatic and collimated beam, is just impossible.
The first breakthrough was obtaining the intended result, not with light, but with microwaves. Using an innovative mix of science and black magic (or we should say, quantum physics), Charles Townes in 1953 demonstrated that he could inject energy into a substance, causing the electrons spinning around the atoms to get rather excited. Then due to a domino effect, these excited electrons jump back into the normal orbits; emit a brief flash of microwave energy. Since they all do it at the same time, and produce exactly the same amount of energy, the result is a highly coherent, single frequency, collimated burst of microwave energy.
Working with Townes, Gordon Gould wanted to do the same but make the end result light. In about 1957 they succeeded and discovered what later became known as the LASER—an acronym standing for Light Amplification by Simulated Emission of Radiation.
The first lasers were of course crude, bulky, very difficult to construct and expensive. A rod of high quality synthetic ruby was surrounded by a very powerful strobe light source. Each end of the rod had to be coated with a mirror of extreme accurate planar and angular parameters. Given a blinding dose of light the atoms in ruby did the dancing trick and produced a brief burst of coherent light. Ironically the light was not visible, as it was in the infrared spectrum.
The first use of lasers was in holography, the taking of three-dimensional pictures. Holography, though exciting in concept, proved to be quite a dud. The initial holograms were pieces of film that looked gray in normal light, but when held up to a laser source it produced a picture of an object that seems to hang in space—a kind of an optical illusion. In reality, these holograms were unbelievably difficult to produce, could be seen only with laser light and even then, very hard to see. Such holograms are called “Transmission Holograms”.
Later advances have made holograms much better. “Reflection Holograms” can be seen in ordinary light. Techniques for mass-producing holograms on plastic films have given rise to its use in prevention of counterfeiting of banknotes, or imprinting emblems on products. Holograms are interesting, but their utility is rather limited.
Yet, the laser has emerged as one of the most important things invented, ever. From the original ruby laser to the subsequent gas lasers, they have always been something big, expensive and delicate. Until rather recently they have been housed in large laboratories with “keep out” signs pasted all over. Then someone invented the semiconductor laser and the world of light suddenly changed.
Today a laser beam can be produced by a little wand about half the size of a pencil and costs about $10. These lasers, often called laser pointers, are often used in academic presentations to point at projections of slides. The laser pointers are great toys, and are a lot of fun to point at buildings and cars at night. They also make great irritants for unsuspecting victims.
The use of lasers today is so far reaching and widespread that a comprehensive enumeration is impossible. A laser in the CD player reads the CD used in reproducing music. The transcontinental telephone calls are carried by glass fiber, illuminated with laser beams. Lasers drive the printers we use to print computer output, both in color and black and white. Use of lasers in medical and industrial applications is amazingly widespread.
Consider the CD player. The CD is a plastic disk with contains music encoded as bits (1’s and 0’s). Each bit is a tiny pit on the surface of the CD. To read the music the player first spins the disk. Then it shines a laser on the CD. Since the laser is highly collimated, the beam can be passed though a lens causing it to focus into a very tiny spot—a spot so small, no beam of light from an ordinary source could be focused so perfectly. This focused light bounces of the pits as the CD spins and just the recorded music is lifted off of the disk without any mechanical contact.
The eye surgery called LASIK (short for laser-assisted in situ keratomileusis) is becoming very popular as a method of eliminating the need for corrective lenses. In LASIK a laser beam is focused onto the cornea of the eye. The laser essentially evaporates a few atoms from the surface of any tissue it touches. To correct near-sightedness, this beam is rapidly moved (in precise pre-calculated moves) over center of the cornea, slowly flattening it and reducing the power of the eye (something that is conventionally done by negative-power lenses). The technique can also be used to correct farsightedness and astigmatism.
The fundamental properties utilized in the applications of lasers are that the laser is a beam of light that is collimated and monochromatic. Thus a laser beam can travel over large distances without dispersing (beams of ordinary light spread out and hence lose intensity). A laser beam can also be focused down into a spot of light smaller than an atom. This allows enormous amounts of heat to be generated at a very precise spot.
The above properties are quite useful. A mirror left on the moon, in one of the NASA missions, allows scientists to shine lasers at it to measure the distance of the moon to an accuracy of a few centimeters (hence track its orbit very accurately). Passing the monochromatic beam through materials and measuring its reflections allow us to study the structure of materials and its chemical compositions. The possibilities seem to be limited only by our imaginations.
While peacetime uses of lasers abound, they have been used quite effectively to wage wars. Laser guided bombs are heat seeking missiles which find and destroy targets, that have been a-priory warmed, up by a aircraft mounted high-power laser gun.
The bar code reader is probably the most gimmicky use of lasers. At the supermarket checkout an item is passed over a glass panel and a laser beam scans all over the object to locate and read the bar code imprinted on it. After the bar code is read, the price of the item is found and inventory control systems are informed of the sale. A beam of light could have achieved the same, but lasers are cheap, easier to focus and make the scanner optically easier to manufacture.
A laser beam passing though a lens focuses to a tiny point, and hence looking into a laser is almost always harmful to the eye. An old urban legend says that a MIT laboratory containing one early laser had a sign saying, “Do not look into laser, with remaining eye”.
Partha Dasgupta is on the faculty of the Computer Science and Engineering Department at Arizona State University in Tempe. His specializations are in the areas of Operating Systems, Cryptography and Networking. His homepage is at http://cactus.eas.asu.edu/partha
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