Overview of Demonstrations in Lasers and Optics - Demonstrations in Lasers

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I'm sure you will 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's a
field that has not been touched by the
laser because most of us are familiar
with supermarket checkout counters with
the laser barcode readers as well as
laser discs 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 in for navigation systems
laser gyroscopes
that are used right now in lots of
airplanes also we have things like 3d
imaging using holography which we've
never had before the 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 they relate to physical
optics so we have tried to fill this
need by creating these videotaped
demonstrations in fact we have about
eight
all of them five hours in optics and
about three hours in laser fundamentals
the these tape 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 and related 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 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 polarisers 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 learnt 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 the 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
the here we look at an air glass
boundary first where we
demonstrate the Brewster angle where for
certain polarization there's no
reflection at the boundary in the next
one which is even more fascinating which
is a glass air boundary where we can
also look at the total internal
reflection and as well as the Brewster
condition and finally here we look at
the phase shift for different
polarizations in total internal
reflection then we we look at two beam
interference where the phenomena are
much more dynamic to to see in two beam
interference we have 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
into in the 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 Michaels inter
interferometer where does the light go
and in fact I would like to to give you
a little preview of this of this
demonstration so let's go to the monitor
and see this demonstration now as we see
in the on the monitor we see the
Michelson interferometer on the right
and the output of the Michelson
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 the 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 that indeed there is light in the
interferometer when that arm is blocked
now when I block the other arm
again we see that indeed is light being
reflected from each other in the frontal
but 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 is
that where does the light go when I have
complete destruction destructive
interference the next demonstration in
the 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 contrasts as a function of
path length difference between the two
arms of the interferometer and as we
know the if indeed the laser is not
single frequency then the fringe
contrast will be very poor in for for
certain path length differences now in a
demonstration number 17 we do a quite a
comprehensive study of Fringe contrast
or coherence length and its relationship
to the spectrum of the of the laser now
I'd like to show you a little bit from
this demonstration as you see on the
scope we show 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 at the free 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 around five centimeter path
length difference in the fringe contrast
looks looks pretty bad here ten close to
ten centimeter path length difference
you can hardly see the the fringes again
is around twelve is still pretty bad
then as I go further this is about 15
centimeters or so I see that the fringe
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 it would before now I'd like to to
show you other demonstrations that
relate to something even more exciting
than to beam into from tree which is
effective multiple beam interference and
under multiple beam interference
we have plane mirror cavity we start
with a plane mirror cavity using
collimated beams and again I would like
to show you a little bit from from this
fascinating demonstration on your screen
you see the plane parallel fabry-perot
cavity below with a spacing a 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 is no practically no light coming
out of the interferometer and as I
leaned on one of the mirrors I can bring
the transmitted light into maximum and
then into into a minimum it's a very
very dramatic demonstration of multiple
beam interference now the 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 the not just that transmitted
beam but we also look at 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 and 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 is being tuned here we are
we have a much larger mode and 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 under 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 the and this sort of
summarizes the interference
demonstrations now we want to turn to
diffraction and we'll start with front
Hoffer diffraction now when I
demonstrate diffraction I'd like to be
able to change things while while I
demonstrate I don't like these statics
static demonstrations so we have here we
have an adjustable state 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 multiple slits instead of just
showing you just the diffraction pattern
of lots of slits what we do here we
introduce each slit edit one slit at a
time so we start with one two three four
five six seven and
go on and you see how the patent 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 to those of narrow slits then we
go into two-dimensional front half of
diffraction patterns and we have a
variable rectangular aperture then here
is very pretty you look at the
diffraction pattern from that and then
we look at the circular apertures and
finally we look at the the diffraction
pattern associated with across multiple
slits essentially we have here is two
wrong key rulings that we cross and then
we vary the angle between them and look
at the look at the pattern and so after
four and half a diffraction then we show
you a little bit of Fornell diffraction
now on the Finnell diffraction again we
go back to the adjustable slit but here
what's interesting about this
demonstration that will show you the
transition from the front Hoffer
condition to the fernell using just that
adjustable pair of slits and it's a
fascinating experiment to see how the
Maxwell's equations are solved by by
this by this slit and then we end
Fornell diffraction by showing you the
pattern that relates to the Finnell
pattern that that relates to a number of
circular apertures and again as a
function of the curvature of the of the
field incident on the 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 that's what we do here we
show you the behavior of a single mode
fiber first and what happens to the two
the light coming out of the fiber as a
function of bending and so on and then
we'll switch to a multimode 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 in a single mode fiber and show how
how touchy it is and how you can change
the state of polarization coming out of
the fiber by simply just 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 in laser fundamentals is
just a simple laser so we start with a
simple laser we'll show you what a
simple days it looks like and then we
look at the very important obviously
important thing in lasers and that is
the the amplitude the amplifier the
light amplifier which is the heart of
the of the laser so I'd like to do is
show you a little bit from this
demonstration on your screen we now see
the amplified down below and we have a
laser beam that enters the amplifier
from the right and exits on the left and
then goes to a detector and above we
have a scope that we monitor the laser
intensity on and also a digital
voltmeter that again monitors the laser
intensity now when we turn on the the
amplifier you can see it nice and bright
when turn on the amplifier you see that
indeed we get a five percent 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 but this is just as I say
a glimpse of this experiment now in the
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 the spectrum of the
year of the laser light you see whether
it's indeed oscillating single frequency
multiple multiple frequency and so on
and also the polarization of each each
of these frequencies that comes out from
the laser then we have we've included a
interesting little demonstration here
that shows that the light inside laser
cavity is much brighter than the light
outside most of us who understand the
theory of lasers believe this but it'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 straight coming out straight
from the laser
then we spatially filter it clean it up
and looks looks fascinating that
Gaussian distribution and then we focus
it and observe it by looking at the
scattered light in in water we change
the focal length of lens's and show you
a little bit about the how the shape of
the Gaussian spherical beam and then
then the next one in the seventh
experiment in the demonstration under
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 got a laser that by adjusting the
mirrors and what-have-you we can
illustrate these transverse modes and
then I'd like to show you a little bit
from from this demonstration on your
scope now you see on the Left we see a
doughnut 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
the of this laser to bring in a variety
of these very very pretty
transfers transverse modes now in the in
the demonstration we also look at the
spectrum of these of these modes not
just the shape of them we also look at
the other spectrum and we add a few
little more interesting
things about about such behavior number
eight under lasers is associated with
the laser spectral width now the laser
spectral 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 of the in shaking of the of the
cavity but here we we do some clever
little tricks to to give you a feel for
the indeed the very extremely narrow
line width
that comes out from a 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
the light back into into the laser
cavity we show here that the get a lot
of frequency instability as well as
amplitude instability when the laser
light is reflected back into 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
being 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 can
lays in several wavelengths and we can
using using a grating we can 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 intra
cavity at LAN and then we show what
happens if the at LAN is is adjusted
right and also what happens when it's
not adjusted right then we go to to the
very colourful laser which is the
tunable dye laser where we
to the wavelength anywhere from green to
red and I would like to just take a few
moments to show you the change in color
from coming out from this dye laser so
on your on your screen now we see that
the output from from the dye laser now
we we just diffuse it and let it fall on
the screen you can see it's going from
green the 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
to excite atomic sodium we look at the
the strong fluorescence resonance or
essence 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 in this
demonstration we we actually show the
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 color is identical in
some cases they're not identical at all
and that makes a very very nice
demonstration now the in in viewing
these demonstrations you may want to use
black and white instead of color because
of the limitations in in video recording
especially VHS you may want to use black
and white in certain cases so what I'd
like to do now is illustrate in in one
of the diffraction experiments the
crossbeam diffract 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 front
half a diffraction pattern
of cross multiple slits or wrong key
rulings in the left bottom corner you
see me adjusting the orientation of one
of the wrong key rulings with respect to
the other one you can see the pattern
which can stitute many many dots you see
it rotates as I rotate one of the wrong
key 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 the red doesn't now
doesn't show the bleeding and 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 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 the output
in the output of the Michelson
interferometer now the most of the
demonstrations were actually recorded in
here in the studio on this table using a
laser like this and in fact this lays in
most cases and some of the optical
components few of the other
demonstrations for example one of the
argon laser and the dye laser were done
in my laboratory at MIT now let me
remind you one important thing that in
all these demonstrations I do not
present any theory nor equations nor a
diagram I think the the setup that I
will you
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 fouls
and another one optics by by Hecht and
of course other ones and you may have
your your favorite book on optics and
you can certainly use that to review
background material in the area of
lasers I have selected
introduction to optical electronics by
Yariv another one lasers by Sigmund and
lazy electronics by Verde n and finally
a very readable simplified treatment of
lasers with a title introduction to
lasers and their applications bio Shake
Allen and and Rhodes and as I mentioned
before I'll mention some background
reading if if you need to connect them
the background reading with the with the
demonstrations now I'd like to turn to
some hints but people who are going to
be using or viewing these tapes now for
the individual or people or maybe in
small groups I think what you want to do
first is to review the material so that
it's 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 that I
posed in the demonstrations and finally
I'd like you to go back and re-examine
the background material and hopefully
that you'll get more out of these
demonstrations 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 I would like you to do
before showing these demonstrations in
class is
is look at these demonstrations select
appropriate sections and then only show
these appropriate sections in class as
you as you discuss the 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 describe being
describing the effects because in some
of the cases the 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 turn off
the audio and 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
videotape demonstrations they certainly
wouldn't have been possible if it wasn't
for the able contributions of two of my
graduate students Steven Smith and
Farhad Zuri niche in addition I'd like
to give credit to the very able 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 asked 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
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