WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?

Lasers by Hal Hellman, is part of the HackerNoon Books Series. You can jump to any chapter in this book here. WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?

WHAT’S SO SPECIAL ABOUT COHERENT LIGHT?

So desirable are the qualities of coherent light that the complicated filtering process described above has actually been used. For example, one British experimenter, Dennis Gabor, used it in the 1940s in an attempt to make a better microscope. But so great was the effort, and so meager the resulting light, that this project was abandoned.

In the course of Dr. Gabor’s experiments, however, he did manage to make a special kind of picture, using coherent light, which he called a hologram. He derived the name from two Greek words meaning a whole picture. We shall see why in a moment.

Ordinary black and white photographs merely record darks and lights, or the intensity of the illumination, thereby providing a scale of grays, nothing more. But because waves of coherent light consistently maintain their relative spacing, they can be used to record additional information, namely the distance from objects.

For example, if we shine a beam of coherent (laser) light between two objects we can, knowing the light wavelength, determine the distance between them to a high degree of accuracy. The basic idea is diagramed in Figure 12. It can be seen that the number of waves times the wavelength gives the precise distance (to within 1 wavelength of light) from the laser source to each object. But this would be a difficult process to implement.

A better way, and one that is already in operation, is to use conventional methods to measure the approximate distance and use the laser beam for precise or fine measurement. In the device shown in Figure 2, the beam is split into two parts. One part is kept in the instrument itself to act as a reference. The other is aimed at a reflector, which sends it back to a detector in the main device, where it is automatically compared with the reference beam. If the two beams are in phase (that is, if their crests are superimposed), the waves combine and produce a high intensity beam at the detector. As the reflector moves closer to or farther away from the laser source the beam intensity decreases and then increases 20again as the wave crests move in and out of phase. The instrument counts the changes and displays the distance the reflector moves, as a function of the wavelengths, on the control cabinet meters.

Figure 12 Principle of distance measurement using coherent light. Wavelength times number of waves gives precise distance between laser and object.

Distance to be measured

Laser

Object No. 21 Wavelength

Object No. 1

Since the word for the interaction of the waves in a system like this is “interference”, the measurement process is called interferometry (pronounced in ter fer OM e try). Although not new, it can now be applied for the first time in machine tool applications, providing the accuracy needed in this age of space technology and microminiaturization. Measurements with a laser interferometer can be made with an accuracy of 0.5 part per million at distances up to 200 inches. Such precision was previously unheard of in a machine shop environment, having been limited to laboratory measurements, and only at a range of a few inches. Under similar laboratory conditions, measurements by laser interferometry now detect movements of 10⁻¹¹ centimeter, a distance approaching the dimensions of an atomic nucleus.

Now let us suppose we expand the laser beam as shown on page 22, and, with the aid of a mirror, direct part of it (the reference beam) at a photographic plate. The remaining portion of the diverging beam is used to illuminate the object to be photographed. Some of this light (the object beam) is reflected toward the plate and carries with it information about the object, as indicated by the wavy line. In the region where these two beams intersect, interference occurs, and a sample of this interference is recorded within the photographic emulsion. Where two crests meet a dark spot is recorded; where the waves are out of phase the processed plate is clear. The result is a hologram, a complex pattern of “fringes”, characteristic of the contour and light and dark areas of the object, as well as its distance from the plate. These fringes have the ability to diffract light rays. When light from a laser, or a point source of white light, is directed at the hologram from the same direction as the reference beam, part of the light is “bent” so that it appears to come from the place once occupied by the object. The result is a remarkably realistic 3-dimensional image.

There, in a nutshell, is the incredible new technique of holography. The extreme order of laser light is illustrated by the regularity of the dots on the cover of this booklet.

This strange kind of light provides us with yet other advantages. Indeed, one of the most important is the fact that the energy of the laser is not being sprayed out in all directions. All of it is concentrated in the narrow beam that emerges from the device. And it stays narrow. Laser light has already been shone on the moon, the beam spreading out to only a few miles in traveling there from earth. The best optical searchlight beam would spread wider than the moon itself, thus dissipating its energy.

It is for this reason, as well as its temporal coherence, that laser light is being considered for communications. A narrow beam is particularly important for space communications because of the long distances involved.

But it is also possible to focus laser light as no light has ever been focused before. At close range a laser beam can be focused down to a circle just a few wavelengths across, concentrating its energy and making it possible to drill holes only 0.0002 inch in diameter. The photo on page 52 shows the exquisite control that can be exercised.

Let us see what this focusability means in terms of power. Consider, by way of analogy, a dainty 100-pound lady in a pair of spike-heeled shoes. As she takes a step, her weight will be concentrated on one of those heels. If the area of the heel is, say, one quarter of a square inch 22(½ × ½ inch), the pressure exerted on the poor tile or carpet rises to 400 pounds per square inch (4 × 100) and if the heel is only ¼ inch on a side, the pressure will be 1600 pounds per square inch!

Making and Viewing a Hologram

MAKING A HOLOGRAM

Object

Object beam

Holographic plate

Mirror

Reference beam

Laser

VIEWING A HOLOGRAM

Hologram

Image

Eye

Coherent light source

What we are getting at, of course, is the fact that the coherence of the laser beam permits it to be concentrated into a tiny area. Thus whatever total energy is being sent out by the laser can be concentrated to the point where its effective energy is tremendous. The sun emits some 6500 watts per square centimeter. Laser beams have already reached 500 million watts per square centimeter.

But the power of the laser does not derive solely from its ability to be focused. Even an unfocused beam is several times more powerful than the sun’s output (per square centimeter).

Figure 13 The typical hologram, looks like a geometric design, but it contains more information than would an ordinary photograph. The images below, made from a hologram, show the detail, apparent solidity, and parallax effect of the reconstructed light waves. The parallax effect is the ability to see around the objects just as one could if they were really there. (See frontispiece.)

Tank, from another angle

The crucial difference between the sun’s light or any ordinary kind of light and laser light lies in the extent to which the emission of energy can be controlled. In the production of ordinary light the atoms, as we know, emit spontaneously, or in an uncontrolled fashion. But if the atoms could be forced to take in the proper amount of energy, store it, and release it when we wanted them to, we would have stimulated, rather than spontaneous, emission.

This, however, is practically the same as the amplification principle we discussed earlier. In that case, a small radio signal is jacked up into a large one by stimulating an available power source to release its energy at the same wavelength and in step with the smaller signal.

The question is, how can we do this with light?

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This book is part of the public domain. Hal Hellman (2021). Laser. Urbana, Illinois: Project Gutenberg. Retrieved October 2022 https://www.gutenberg.org/cache/epub/65512/pg65512-images.html

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