Did you know that
lasers are all around us? We depend on lasers in many everyday activities:
to transmit data, to listen to music and watch DVDs, to perform surgery, to
drill and cut materials… What makes lasers such a versatile and useful
tool? Come and see for yourself!
In our traveling
laser show we are designing a number of hand-on demonstrations of basic
laser properties and some applications of laser technology.
Experiment 1: How
a laser works
demonstrates the basic principles of laser operation and the critical
difference between incoherent light (like a gas discharge lamp) and
coherent laser radiation.
With a high enough
electric potential, one can produce a glowing discharge in a neon gas –
we have neon signs everywhere! In such a discharge, atoms are excited
into high energy states and then decay, emitting photons. It is not hard
to check that such emission consists of many distinct colors, called
an emission spectrum. The exact combination of emission lines is
individual for every atom, and, for example, can be used to identify gas
The photo above shows
the discharge of a Ne tube. There are several important things
illustrated there. First, even though the discharge color is soft pink to
the eye, it in fact has many colors that you can separate using a
diffraction grating. Second, the emitted light goes in all different
directions. It is good for illuminating a room, but would make a very
lousy laser pointer.
One more critical
component is required to turn the incoherent light from a
discharge to a coherent laser beam – a feedback to amplify only a
specific fraction of the emitted light. In most lasers this is done by
placing two mirrors (called the resonator) at the end of a light-emitting
element (called the optical gain medium). The mirrors are aligned to let,
for example, only red light propagate between the two mirrors, bouncing
back and forth many times. As a result only this particular light gets
amplified. In this case a bright collimated beam appears.
By fine-tuning the
position of one of the mirrors, we can change the spatial distribution of
light inside the cavity, producing a variety of shapes in the output
Our huge thanks to Sam Goldwasser
for lending us his He-Ne laser system for the demonstrations. Do you want
to build a laser? Check out Sam’s page for He-Ne laser kits.
Liquid fiber optics
Did you know that most of the data
you see on-line or on television, and sometimes even phone calls, are transmitted
using light pulses traveling in optical fiber? This experiment is
designed to explain how optical fiber works.
Optical fibers are made of a very thin glass wire
where light pulses can propagate for very long distances. This is much more
efficient than sending an electrical signal through metal wires, since
people have learned to make fibers with extremely low losses. But what
makes light stay inside a fiber core?
Everyone knows that if a light beam hits an interface
between two materials, some of the light is transmitted, and some of the
light is reflected. How much of the original beam goes which way depends
on the properties of the media, in particular on their refractive
But it is not always possible to transmit the light
through the boundary even if two media are transparent! When a light beam
gees from a medium with ahigher refractive index to a medium with a lower
refractive index (for example from water or glass to the air) at a large
angle, all the light is reflected back, and nothing is transmitted. This
effect is called total internal reflection.
Optical fibers take full advantage of this effect. Commercial fibers are
made such that their core has a slightly higher refractive index than the
surrounding part of the wire (called cladding), so that the light is
always completely reflected off the interface.
But you don’t need a fancy optical fiber to see this
light guiding effect. In our experiment we can “trap” light inside a
water stream, using the same idea: the laser beam continuously reflects
off the water-air interface and thus follows the bending water.
Mechanical action of a laser
You can probably rather easily distinguish a laser
from a lamp, because light emitted by a lamp shines in every
direction, while the laser beam is collimated (i.e., forms a single
streight beam). Another important difference is that the spectrum of the
lamp consists of many colors, and a laser operates on a very specific
wavelength. In fact, there is a deeper difference: light emitted by the
lamp is incoherent, which means that any portion of the light us
independent from others. On the other hand, laser radiation is coherent,
and all the emitted light oscillates in unison. As a result, its action
on the object is much more pronounced.
We can see that difference in our
“Pop the balloon” experiment. First, a regular 20W light bulb is focused
on a balloon and nothing happens. Then a 200x weaker (100mW) laser beam
is directed to the balloon and pops it in a few seconds.
can put several balloons of different colors in a row to see how easily
our laser pops all of them with two exceptions. A baby pink balloon is
very light - it scatters more light than it absorbs. A green balloon is
also very hard to pop - even though you cannot see it in the clip, the
laser was shining on it for several minutes without any effect. Can you
guess why the green balloon is tough for the green laser to pop? The
balloon appears green because this is the color that penetrates it the best,
and thus it is more transparent for the green laser compare to all
can even use this effect to pop a balloon inside another balloon!
While popping balloons is just fun, it illustrates
much more serious applications of lasers in industry, medicine, and other
areas. A focused laser beam can cut out metal parts or shape a cornea in
a human eye with great precision.
At the same time think of these popping balloons next
time you are playing with a laser pointer. Human eyes are very delicate
objects, and can be damaged with as little as a few milliWatt of coherent
laser radiation. Never aim a pointer or any other laser at your eyes, or
at any people around you.
Please send all
questions, requests and suggestions to