﻿How a Laser Works EngineerGuy Series #4
For years lasers have been a hallmark of science fiction
yet much of our technology today depends on them
Range finding devices, optical communications, and of course,
bar code scanners.
The unique characteristics of laser light
make all these things possible.
However, its use by eye surgeons to
reattach retinas highlights them all.
Injury can cause the eye's retina to peel away
from the tissue supporting it.
Without rapid treatment the entire retina can become detached
causing blindness.
Surgeons use green laser light of nearly a single wavelength
because that color passes through the eye's lens
and vitreous humor
without being strongly absorbed, and thus without causing damage.
The laser beam then strikes the retina where the tissue
strongly absorbs that light, using the high intensity
light to weld the detached retina back into place.
The beam's narrowness allows the surgeon to affect
only the area of the retina that needs to be repaired
areas as small as 30 microns.
How a laser creates light with these three characteristics
is a tour de force of engineering.
Let me show you.
Any glow-in-the-dark toy can illustrate the basic principles
of creating laser light.
This glows because the zinc-copper based compound coating
the inside can absorb energy from a light source
and then later radiate it as light.
The light provides energy to electrons in the coating
promoting them to higher energy levels.
Once the light is off, these electrons slowly lose their
added energy and return to their lower energy ground states.
The energy lost is given off as light.
A closely related phenomena lies at the heart of a laser.
Let me tell you about the engineering
in the very first laser based on ruby.
Here I have a tiny piece of ruby
and some red colored glass beads.
When I shine the blue light on the glass beads nothing much happens
but shine it on the ruby and it glows red.
Unlike the glow-in-the-dark ball the light appears immediately
and when I shut off the blue light it disappears.
In 1960, Ted Maiman demonstrated the first laser
by taking a cylinder of ruby and surrounding it
with a xenon arc flash lamp used in aerial photography.
An intense burst of light from the lamp initiates lasing.
To see how it works let's look at what
happens with a weaker lamp.
A flash would promote a few electrons
from the ground state to an excited state.
They'd lose a bit of energy
fall to a lower energy state without emitting light
and then drop from there to the ground state
giving off a burst of light.
The light produced would be incoherent light
a spectrum of colors and intensities
just as my small laser made the ruby sphere glow.
To create a laser takes an extremely powerful lamp.
In the ruby laser repeated flashes
called pumping
make something amazing happens.
They supply so much energy that a population inversion occurs
Here more electrons in the energy level
just above the ground state than in the ground state.
Electrons from a population inversion returning
to the ground state release light that starts an avalanche
called stimulated emission.
The photon produced when an electron decays
induces other excited electrons to simultaneously decay
and release nearly identical photons.
That creates coherent light
meaning that the crests and troughs
of every light wave in the beam match up.
Now, at this point we have coherent light
but not yet the other two properties of laser light.
To get a narrow beam with all the light rays parallel
and of a nearly single wavelength requires
an addition to the ruby rod.
Maiman silvered the ends to reflect the light within the ruby cylinder.
He made the two ends of the rod astonishingly parallel to each other
From top to bottom the distance between these
two mirrors differs by no more than 200 nanometers.
Inside this resonant cavity, two things happen.
First, any light rays that don't line up with the axis
eventually just exit out the side of the cylinder.
And the light parallel to the axis becomes intensified
and narrowed in wavelength.
The mirrored ends create a standing wave
which means only light of particular wavelengths
can exist inside the cavity.
By choosing the rod length correctly
we can get the nearly single wavelength
of light characteristic of a laser.
A small hole in one of the mirrors or a partially silvered mirror
allows the light to escape creating the familiar beam.
Now, since the first Ruby laser was created
lasers have become easy and cheap to manufacture.
For instance this laser pointer uses a
semiconductor diode to produce light.
Although many innovations and improvements have occurred since
1960 the essential principles are the same.
I'm Bill Hammack, the engineer guy.
This video is based on a chapter in the book
Eight Amazing Engineering Stories.
The chapter features more information about this subject.
Learn more about the book at the address below.