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 again 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.
[Illustration: 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. 2
1 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 (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!
[Illustration: 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).
[Illustration: 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.)_]
[Illustration: Model tank]
[Illustration: 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?