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SHAOUL EZEKIEL: I'm sure you'll agree
with me when I say that lasers are fantastic devices.
In fact, every day we hear of more and more applications.
I don't think there is a field that has not
been touched by the laser.
Of course, most of us are familiar with supermarket
checkout counters with the laser barcode readers,
as well as laser disks and laser printers,
and now, we're having laser fiber optic communication
systems, but there are all sorts of other applications
that some of us don't know too much about.
For example, laser surgery is now
becoming very important, as well as retinal welding,
and so on, materials processing.
When it comes to sensors or laser sensors,
lasers can measure almost anything,
and sometimes, they do it very well,
for example, for navigation systems, laser gyroscopes that
are used right now in launch some airplanes.
Also, we have things like 3D imaging using holography,
which we've never had before.
Even optical computers are now being taken a little bit
more seriously, as well as the numerous applications
in fundamental physics, chemistry, and so on.
So you can see there are lots of applications for lasers.
Now, to learn about lasers and the associated optics,
one normally takes a course or gets the information
from books and papers, but there's always
something that's missing in this kind of learning,
and that is the actual seeing of the optical phenomena
associated with lasers, and also the related physical optics.
So we have tried to fill this need
by creating these videotaped demonstrations.
In fact, we have about eight hours of them,
five hours in optics, and about three hours
in laser fundamentals.
These taped demonstrations are intended
for the individual learner, studies by himself or herself
or in small groups, and also, it's
intended for the instructor, people
who teach lasers and optics and related areas.
So we have both of these people in mind
when developing these tape demonstrations.
Now, what I'd like to do is tell you a little bit more
about these demonstrations.
First, let me start with those demonstrations relating
to physical optics.
First set is related to polarization.
The first one in the area of polarization
is associated with a linear polarizer.
And here, all we do is show the use of a linear polarizer
in determining the state of polarization of a laser.
The next one, we look at the rotation
of the plane of polarization using a pair of polarizers,
and then we go on and illustrate the uses
of a quarter-wave plate and a half-wave plate.
And then in number five, we actually
take advantage of what we learned about polarization
components to demonstrate an optical isolator
or an optical diode where light can only go in one direction
and not in the other.
And then finally, under the area of polarization,
we look at the scattered light in a dielectric
and we look at it's polarization as a function of the input
polarization of the light.
Then we go on and look at the behavior
of light at dielectric boundaries,
for example, a piece of glass.
Here, we look at air-glass boundary
first, where we demonstrate the Brewster angle, where
for a sudden polarization, there's
no reflection at the boundary.
And the next one, which is even more fascinating,
which is the glass-air boundary, where we can also
look at the total internal reflection,
as well as the Brewster condition.
And finally, here, we look at the phase shift
for different polarizations in total internal reflection.
Then we look at two-beam interference
where the phenomena are much more dynamic to see.
In two-beam interference, we start first
with a simple Michelson interferometer
using collimated beams and show what fringes look like.
And then we switch to diverging beams or spherical beams,
and we see rings instead of line fringes.
And we look at both the light that's
transmitted through the interferometer
as well as the light reflected through an interferometer.
Then we get to a fun one here, and that
is what happens to the light or where
does the light go when we have destructive interference?
When we have actually no light coming out
of the Michelson interferometer, where does the light go?
And in fact, I would like to give you a little preview
of this demonstration.
So let's go to the monitor and see this demonstration.
Now, as we see on the monitor, we
see that Michelson interferometer on the right,
and the output of the Michelson's interferometer
in the inset above, and as I lean on the table,
we see that the light goes all the way
from very bright to completely dark or no light at all.
Now, just to show that indeed, there
is light inside interferometer, what I'm going to do
is block one arm with a card, and then
if we look in the close up, we see indeed,
there is light in interferometer when that arm is blocked.
Now, when I block the other arm, again, we
see that indeed, there's light being reflected
from each on the front of it.
When I take the card off, I can make the light disappear
completely, which means I have complete destructive
interference.
So the whole question is, where does the light
go when I have complete destructive interference?
The next demonstration in the area of two-beam interferometry
relates to fringe contrast.
As we see, we have a number of them.
One, we look at the effect of vibrations on fringe contrast,
then we look at the effect of different intensities
in the two arms of the interferometer
and how it relates to fringe contrast,
then we look at an interesting one,
the effect of different polarizations in the two
arms of the interferometer.
And finally, here, we look at the fringe contrast
as a function of path length difference between the two
arms of the interferometer.
And as we know, if, indeed, the laser is not single frequency,
then the fringe contrast will be very poor for certain path
length differences.
Now, in demonstration number 17, we
do quite a comprehensive study of fringe contrast or coherence
length and its relationship to the spectrum of the laser.
Now, I'd like to show you a little bit
from this demonstration.
As you see on the scope, we show the spectrum of the laser,
and this indicates that the laser is oscillating
at three frequencies.
Below is the Michelson interferometer setup.
And at first I'll start with equal path lengths
in the two arms of the interferometer,
then we can look at the fringe contrast.
Here is a fringe contrast for equal path length.
Looks very, very good.
Now what I do, I start to change the length of one of the arms.
Here I am round 5 centimeter path length difference,
and the fringe contrast looks pretty bad here.
10, close to 10 centimeter path length difference,
you can hardly see the fringes.
Again, it's around 12.
It's still pretty bad.
Then as I go further, this is about 15 centimeters or so,
I see the difference contrast comes back.
Here I am getting better and better.
Here, around about 20 centimeter path length difference or so,
the quality of the fringes look as good as they were before.
Now, I'd like to show you other demonstrations that
relate to something even more exciting
than two-beam interferometer, which
is the effect of multiple beam interference.
And under multiple beam interference,
we have plain mirror cavity.
We start with a plain mirror cavity using collimated beams.
And again, I would like to show you
a little bit from this fascinating demonstration.
On your screen, you see the plane parallel Fabry-Pérot
cavity below with a spacing of few millimeters,
and right above it, you see the individual beams when
the cavity is misaligned.
Now, as I start to align the cavity,
the beams start to superimpose until I
reach the condition when they're all overlapping
and we see the interference.
And here we are.
In fact, there's practically no light
coming out of the interferometer.
And as I lean on one of the mirrors,
I can bring the transmitted light into maximum
and then into a minimum.
It's a very, very dramatic demonstration
of multiple beam interference.
Now, the next one in the area of multiple beam interference,
we look at again a plane mirror cavity with diverging
beams this time instead of collimated beams.
And here, we see the rings and the bullseye and so on,
and also, we look at not just the transmitted beam,
but we also look at the reflected beam from this cavity
to see what that looks like.
And then we switch from plane mirror cavities
to curved mirror cavities, and here, we
excite lots of transverse modes or radial modes in the cavity.
So I'd like, again, to show you just briefly a few
of these beautiful transverse modes.
On the screen, you see one of the higher transverse modes
as the cavity is being tuned.
Here we are.
We have a much larger mode.
Here is a another one.
They're very, very pretty.
And in addition to just looking at the transverse modes,
we also in this demonstration, we're
going to look at their frequencies.
And again, there's a lot to be learned from there.
The final one on the multiple beam interference,
it relates to the use of a curved mirror cavity
as an optical spectrum analyzer, the so-called confocal
resonator.
So we show you what happens at confocal
and what happens when you are below or above
confocal condition.
And this sort of summarizes the interference demonstrations.
Now, we want to turn to diffraction.
And we'll start with Fraunhofer diffraction.
Now, when I demonstrate diffraction,
I like to be able to change things while I demonstrate.
I don't like these static demonstrations.
So what we have here, we have an adjustable slit.
We'll start with an adjustable slit diffraction,
and we'll show what happens as a function of slit width,
then we go and look at what happens when we have two slits,
and then when it comes to multiple slits,
instead of just showing you just the diffraction pattern of lots
of slits, what we do here, we introduce each slit one
slit at a time.
So we start with 1, 2, 3, 4, 5, 6, 7, and so on,
and you'll see how the pattern changes
as a function of the number of slits.
Then we quickly look at the diffraction patterns
of thin wires and try to relate them to those of narrow slits.
Then we go into two-dimensional Fraunhofer diffraction
patterns, and we have a variable rectangular aperture.
And here, it's very pretty.
You look at the diffraction pattern from that.
And then we look at the circular apertures,
and finally, we look at the diffraction pattern associated
across multiple slits.
Essentially, we have here is two Ronchi rulings that we cross,
and then we vary the angle between them
and look at the pattern.
And so after Fraunhofer diffraction,
then we show a little bit of Fresnel diffraction.
Now, on the Fresnel diffraction, again, we
go back to the adjustable slit, but here,
what's interesting about this demonstration,
we show you the transition from the Fraunhofer condition
to the Fresnel using just that adjustable pair of slits.
And it's a fascinating experiment
to see how the Maxwell's equations are
solved by this slit.
And then we end Fresnel diffraction
by showing you the pattern that relates to--
the Fresnel pattern that relates to a number
of circular apertures, and again,
as a function of the curvature of the field
incident on the aperture.
And we close our demonstrations in physical optics
by looking at the propagation of light in fibers.
And as we know today, fibers are very, very important.
And what we do here, we show you the behavior
of a single-mode fiber first and what
happens to the light coming out of the fiber as a function
of bending and so on, and then we
switch to a multi-mode fiber where the output is no longer
a single spot that is consist of different spot,
just like in the resonators.
And finally, here, we look at the polarization,
the polarization issue in a single-mode fiber
and show how touchy it is and how
you can change the state of polarization
coming out of the fiber by simply just stressing it
or just even breathing on it.
And this concludes about five hours or so,
as I said before, of demonstrations
in physical optics.
Now, let's look at our demonstrations
in laser fundamentals.
The first one in laser fundamentals
is just a simple laser.
So we start with a simple laser.
We show you what a simple laser it looks like,
and then we look at the very important, obviously important
thing in lasers, and that is the amplifier, the light amplifier,
which is the heart of the laser.
So what I'd like to do is show you a little bit
from this demonstration.
On your screen, we now see the amplifier down below,
and we have a laser beam that enters the amplifier
from the right and exits on the left,
and then it goes through a detector,
and above, we have a scope that we monitor the laser intensity
on, and also a digital volt meter that, again,
monitors the laser intensity.
Now, when we turn on the amplifier-- and you
can see it nice and bright.
We turn on the amplifier, you see that, indeed, we
get a 5% increase in the light going through the amplifier.
And we do that a few times to convince you that indeed, we
are observing gain.
And we also check on initial conditions and so on,
but this is just, as I say, a glimpse of this experiment.
Now, in the next demonstration, we're
going to look at the polarization of laser light.
We have two kinds of lasers, one with internal mirrors,
and one with the external mirrors.
And then after that, we look at the spectrum of the laser light
and see whether it's indeed oscillating single frequency,
multiple frequency, and so on, and also,
the polarization of each of these frequencies
that comes out from the laser.
Then we have included an interesting little
demonstration here that shows that the light inside a laser
cavity is much brighter than the light outside.
Most of us who understand the theory of lasers believe this,
but there's nothing like demonstrating it and observing
it.
Then we show a little bit about the optics of laser beams.
We look at the beam coming out straight from the laser,
then we spatially filter it, clean it up,
and it looks fascinating, that Gaussian distribution, and then
we focus it and observe it by looking at the scattered light
in water.
We change the focal length of lenses
and show you a little bit about the shape of the Gaussian
spherical beam.
And then the next one, in the seventh experiment
on the demonstration on the lasers,
we look at the transverse modes of a laser.
Now, these are not normally observed in commercial lasers,
because commercial lasers are not
supposed to have transverse modes, but in this case,
we've got a laser that by adjusting their mirrors
and what have you, we can illustrate
these transverse modes, and then I'd
like to show you a little bit from this demonstration.
On your scope now, you see on the left, we see a donut mode.
And this is different, quite a bit
different from the normal single spot
that we get from commercial lasers.
And in the bottom right-hand corner,
I'm adjusting the alignment of this laser
to bring in a variety of these very, very pretty transverse
modes.
Now, in the demonstration, we also
look at the spectrum of these modes,
not just the shape of them.
We also look at the spectrum, and we
add a few little more interesting things
about such behavior.
Number eight on the lasers is associated
with the laser sprectal width.
Now, the laser speckle width, the intrinsic laser spectral
width is normally not observed because it's very, very narrow,
and it's usually corrupted by vibrations
or what have you and shaking of the cavity,
but here, we do some clever little tricks to give you
a feel for, indeed, the very extremely narrow line
width that comes out from a laser
without the effect of this cavity shaking.
The next one, the next demonstration on the reflection
back into the laser, this gives us
a feel of how lasers misbehave when we reflect the light back
into the laser cavity.
We show here that you get a lot of frequency instability,
as well as amplitude instability when the laser light is
reflected back into the cavity.
Then the next demonstrations, we then
change from helium neon laser to an argon laser,
and in this one, number 10, we show you
how powerful even an unfocused beam from an argon laser
is just by simply letting it fall on a piece of cardboard
and setting it on fire.
The next one, we show that an argon laser lays
in several wavelengths, and we can, using a grating,
we can show that indeed, a laser can oscillate simultaneously
at several wavelengths.
And here, we look at a method of achieving single frequency
behavior in a long argon laser by the use
of an intracavity etalon, and then
we show what happens if the etalon is adjusted right,
and also, what happens when it's not adjusted right.
Then we go to the very colorful laser,
which is the tunable dye laser where
we can tune the wavelength anywhere from green to red.
And I would like to just take a few moments to show you
the changing color coming out from this dye laser.
So on your screen now, we see that the output
from the dye laser now.
We just diffuse it and let it fall on a screen.
You can see it's going from green to sort of orange.
And if your screen is set right, now it's
going to red, and even deep red by simply tuning the dye laser.
And in the last two demonstrations,
we use the dye laser to excite atomic sodium.
We look at the strong fluorescence, resonant
fluorescence from atomic sodium when
the dye laser is tuned to it.
And in the final one, we look at fluorescence
from molecular iodine.
Now, in this demonstration, we actually
show the incident laser light, the color of the incident laser
light, and simultaneously, we show
the color of the iodine fluorescence induced
by the laser.
In some cases, the colors are identical, in some cases,
they're not identical at all, and that makes a very, very
nice demonstration.
Now, in viewing these demonstrations,
you may want to use black and white instead of color because
of the limitations in video recording, especially on VHS,
you may want to use black and white in certain cases.
So what I'd like to do now is illustrate
in one of the diffraction experiments,
the cross beam across multiple slit diffraction experiment.
I'd like to show you how things improve,
the viewing improves if we switch to black and white.
Now, on your screen, you see the Fraunhofer diffraction pattern
of cross multiple slits or Ronchi rulings.
In the left bottom corner, you see
me adjusting the orientation of one of the Ronchi rulings
with respect to the other one.
You can see the pattern, which consists of many, many dots.
You see it rotates as I rotate one of the Ronchi rulings.
So now, let's turn off the color and go to black and white.
And indeed, you can see the resolution now is higher
because the red now doesn't show the bleeding,
and then you can see some of the finer spots.
Again, let's go back to full color.
Here we are, back in full color, and then
the switch back to black and white
to show the higher resolution.
Now, you may want to do that wherever
you think you need more resolution to help
you observe the effect.
Now in other cases, for example, in the case
of where does the light go in destructive interference
in a two-beam interferometer, you
may wish to use freeze frame if you have that freeze frame
capability, and then you can admire the complete darkness
of the output, the output of the Michelson interferometer.
Now, most of the demonstrations were actually recorded here
in the studio on this table using a laser like this.
And in fact, this laser, in most cases,
and some of the optical components.
Few of the other demonstrations, for example,
of argon laser and the dye laser,
were done in my laboratory at MIT.
And let me remind you of one important thing,
that in all these demonstrations,
I do not present any theory, nor equations, nor a diagram.
I think the setup that I will use
in a lot of the demonstration is quite simple to follow,
and I describe it very, very slowly.
But I do give some references for background reading.
In fact, I have listed over here some of them.
In the area of optics, one recommended text
is Introduction to Modern Optics by Fowles,
and another one, Optics by Hecht,
and of course, there are other ones,
and you may have your favorite book on optics,
and you can certainly use that to review background material.
In the area of lasers, I've selected
Introduction to Optical Electronics by Yariv.
Another one, Lasers by Siegman, and Laser Electronics
by Verdeyen, and finally, a very readable simplified treatment
of lasers with the title Introduction to Lasers
& Their Applications by O'Shea, Callen, and Rhodes.
And as I mentioned before, I'll mention some background reading
if you need to connect the background reading
with the demonstrations.
Now, I'd like to turn to some hints
for people who are going to be using or viewing these tapes.
Now, for the individual, or maybe in small groups,
I think what you want to do first
is to review the material so that you appreciate
the demonstrations a little bit more,
and then I'd like you to answer some of the questions
that I leave unanswered or the question I
pose in the demonstrations.
And finally, I'd like you to go back and reexamine
the background material, and hopefully, you'll
get more out of these demonstration this way.
Now, for the instructor, of course,
we all know that instructors already know their background
material, so you don't need to review that,
but what I would like you to do before showing
these demonstrations in class is look at these demonstrations,
select appropriate sections, and then only show
these appropriate sections in class as you discuss the theory
or whatever the background that you give the students.
And I think in this way, it will come
at a very opportune moment.
Finally, I think in some cases, you
may want to turn off the audio and substitute your own voice,
describing the effects, because in some of the cases,
the demonstrations are elaborate and take a while,
and if you don't have that much time in class,
then there's no reason why you can't turn off the audio
and describe it yourself, and then
you can make the experiment, the demonstration
seem much shorter.
As I mentioned earlier, we have eight hours
of videotaped demonstrations.
It certainly wouldn't have been possible
if it wasn't for the able contributions of two
of my graduate students, Stephen Smith and Farhad Zarinetchi.
In addition, I'd like to give credit to the variable CES
video staff for developing these demonstrations with us.
As you can imagine, it's not like your normal video program
where you're only recording people talking.
And finally, I'd like to thank all my students over the years,
both in regular MIT classes, as well as in the summer
session, all those students who ask good questions,
because it's these questions that
really encouraged us to develop these video demonstrations.
And in closing, before I let you go and view
these demonstrations, I'd like to say
that I hope you'll enjoy looking at them and learn from them,
and more important, I hope that they
will increase your excitement and enthusiasm for the laser
and optics field.
Now, these demonstrations are by no means perfect.
I'm sure there are lots of improvements that can be made.
But I think that they illustrate what they're supposed
to illustrate quite clearly.
And finally, after viewing these demonstrations,
if you have any comments or some suggestions
for other demonstrations that we may have left out,
we certainly would be grateful if you would
contact us and let us know.