Laser Optics.

TM 9-258

b. In laser radiation, the ends of the ruby rod

photons, and these photons continue interacting to

produce still more photons. When a threshold energy of

are highly polished so that light can pass through almost

the total photonic energy within the resonant cavity is

without absorption. A mirror is placed at each end and

attained, a pulse, consisting of a very intense laser beam

aligned perpendicularly to the principal axis of the rod.

formed by photon waves, bursts out of the partially

When the rod is illuminated with an intense photoflash

reflective end of the ruby rod.

light, it emits a fluorescent light which reflects back and

e. In practical applications the highly polished ends of

forth between the two mirrors. This increase in intensity

the ruby rod can be mirrored or coated with a dielectric

of the light is produced by the oscillation of the ruby light

material, such as magnesium fluoride or cerium dioxide

within an optically resonant cavity formed by the rod and

(B, fig 6-2). When this is done one end is fully coated

the two reflecting surfaces of the mirrors.

c. The removal of the laser energy, or

and the other is partially coated so that the emitted laser

light can pass through it. This laser beam, as it emerges

radiation, from the resonant cavity is accomplished by

from the ruby rod, has a slight divergence.

making the rear reflecting mirror 100% reflective and the

front reflecting mirror partially reflective. This allows

6-5.

Laser Optics.

some of the laser light generated within the resonant

a. The extremely tight beam produced by the

cavity to pass through as a laser beam of the same

laser, with final tightening obtained by external. optical

diameter as the ruby rod

systems, gives it the ability to travel extreme distances

d. During the optical pumping of the ruby rod

with little divergence. The laser output can be focused to

by the flashlamp, some of the chromium in the ruby rod

a parallel beam where spreading is expected to be less

become excited. This causes their electrons to move

than one foot per mile of travel. A beam of light from a

away from the atoms and position themselves at higher

ruby laser would suffer so little divergence that, for

energy levels from which they spontaneously fall back to

example, it would be concentrated in an area ten miles

their normal energy states. During this transition, each

across when it reached the moon; an ordinary search

of these electrons produces a photon of light. These

light with the same intensity would place a beam over

photons now oscillate by reflecting from one mirror

25,000 miles wide on the moon.

surface to the other within the resonant cavity. On their

b. Brightness may approach millions of times

way to the mirror, some of the photons collide with one or

that of the sun, on a relative bandwidth basis. Spectral

more atoms which are in an excited state due to the

narrowness allows good signal to background ratios to

flashlamp pumping, and interact with them to produce a

be realized. Compared to microwave systems, laser

photon or photons identical in energy and frequency with

devices will allow construction of equipment with an

the initial photon. The newly formed photons continue to

antenna only inches across.

interact with other excited atoms, producing more

6-3