MaximumPC 2007 H

r & d BREAKING DOWN TECH —PRESENT AND FUTURE

76 MAXIMUMPC HOLIDAY 2007

HOW THEY WORK

W

hen science-fi ction authors got wind

of the concept of lasers, they immedi-

ately weaved the technology into their story

lines as heinous instruments of interstel-

lar destruction—not surprising, when you

consider that the word “laser” is actually an

acronym for “light amplifi cation by stimu-

lated emission of radiation.”

But as you sit in front of your PC,

you’re likely to be in close proximity to

several lasers, none of which is capable

of setting paper on fi re, much less blow-

ing apart a spaceship. The same goes

for those traveling shows that use such

focused beams of light to create halluci-

nogenic displays to a soundtrack of Pink

Floyd and Led Zeppelin.

Lasers today are the key technology

behind CD, DVD, HD DVD, and Blu-ray play-

ers and burners. They create the images

produced by laser printers, and they precisely

track the movement of laser mice. How did

a concentrated beam of light become so

important to so many aspects of modern

computer technology?

UP AND ATOM

To understand lasers, we must start with the

atom, which—as anyone with the slightest

exposure to science education knows—is the

basic component of just about everything in

our known universe. The atom, however, can

be broken down into even smaller elements;

namely, neutrons and protons, which form the

atom’s nucleus. Neutrons and protons exert

a positive electrical charge, while a cloud of

negatively charged electrons circulate around

the outside of the nucleus.

Light—any type of light—is created when

electrons are energized by an external source,

such as electricity. Once that is accomplished,

the electrons move into a higher orbit around

the atom, and the atom becomes unstable.

This state is only temporary, however; the

electrons soon return to their normal orbit, and

this is when the good stuff happens. As the

electrons return to a state of equilibrium, they

release their excess energy in the form of par-

ticles called photons: light.

When the electrons inside the atoms of

conventional light sources—such as incandes-

cent light bulbs, fl uorescent tubes, fl ashlights,

and even the sun itself—are excited, they emit

photons randomly. The “white” light gener-

ated by these sources contains a wide variety

of incoherent rays of different wavelengths

(wavelength being determined by the energy

difference between the atom’s excited and

relaxed states). The light is described as being

white because it’s the sum of many different

wavelengths. A laser device, on the other

hand, is capable of compelling atoms to emit

photons in a highly organized fashion.

THE LIGHT FANTASTIC

The key concept behind laser light is stimu-

lated emission. If the photon emitted by an

atom encounters another atom with an elec-

tron in the same excited state, it can provoke

that second atom to throw off a photon that

exhibits the same wavelength and moves in

the same direction.

A laser consists of a gain medium, which

is a material with specifi c optical properties

that render it capable of amplifying light of

a specifi c wavelength. The gain medium is

housed in a cavity capped by a mirror at one

end and a partially transparent mirror at the

other. As energy (in the form of light, or in the

case of the semiconductor lasers, electricity) is

pumped into the gain medium (which can be a

gas, liquid, or solid), it excites these electrons.

The electrons then emit energy in the form of

photons as they return to their relaxed state.

The photons then bounce back and forth

between the two mirrors, repeatedly pass-

ing through the gain medium, exciting other

electrons and stimulating the emission of

even more photons. This cascading effect

continues as long as energy is applied to the

gain medium. Some of these photons escape

through the partially transparent mirror, also

known as an output coupler. Since all the

escaped photons are of the same wavelength

and are all traveling in the same direction, they

form an intense, monochromatic, highly direc-

tional column of light: a laser beam.

STORAGE APPLICATIONS

Semiconductor lasers are the most common

type of laser; low-powered semiconductor

lasers are used in the construction of every-

thing from laser printers and optical drives to

laser pointers and measuring devices. The

semiconductor laser in a common CD-ROM

drive emits light with a wavelength of 780

nanometers (very near infrared, which ranges

from 750nm to 1mm) and is projected through

a lens with a numerical aperture of 0.45. As a

lens’ numerical aperture increases, so does its

ability to create a focused spot of light.

White Paper: How Lasers Function

Energy injected into a laser’s gain medium excites the atoms within it, causing the electrons circling those atoms to

throw off particles known as photons. These photons exhibit the same wavelength and move in the same direction,

resulting in a powerful, monochromatic beam of light.

The computer industry has tapped Stimulated emission is the key concept

this exotic technology for a host of

everyday applications. Sadly, no one

has yet devised a means of equip-

ping sharks with lasers beams.

BY GORD GOBLE

Mirrored Atoms

Surface

Partially

Mirrored

Surface

Energy Source

Laser Light

Energy Source

Photons

Atoms