Hello and welcome to this course. This is
Introduction to Photonics. I will start the
lecture today by just giving you a little
bit of background as to how to this course
was conceptualized in the first place.
You go through a certain level of optics in,
as part of your high school education where
you learn about the basic laws of reflection,
refraction and based on that you, you cover
a few topics.
And then when you come to college you are
typically experiencing more advance courses
such as optical communications, photonic integrated
circuits, optical sensors and biophotonics
and so on.
And what we thought is that we actually need
a bridge between the two because certain level
of fundamentals that are taught in the high
school level is not enough to clearly appreciate
the kind of concepts that you are encountering
at the advance level courses.
So we decided to float this course as sort
of a bridge between the two and, and of course
this course has been now offered for several
years to several sets of students where we
have clearly had an opportunity to look at
the finer aspects of the course and, you know
try to patch them up.
Now before we move on, let me ask you this
question. Why are you in this course? Why,
you know photonics? What are the typical things
that you use light for? At that point if some
of the students present here can participate,
I would appreciate it. What do we use light
for?
Sure, that is a very, very good example. Optical
communications is clearly one reason why,
in certain ways we can say it is one reason
why we have all these wireless communication
at this level because what you do when you
have your mobile phone?
You take a mobile phone and do a phone call
is you are trying to use electromagnetic waves
in the R F region to communicate to an antenna,
nearby antenna, right. And chances are, from
that antenna on to the exchange or from that
exchange to other exchanges different parts
of the country or different parts of the world,
the chances are that it is going to be carried
through optical communication.
So in certain ways, optical communications
has revolutionized this whole, you know field
of communication. So that is a very good example
as to why, why do we need to study about light.
What else?
So simple example is you know how you are
able to see me? The whole process of human
vision is, is based on light, right. So you
have a light source here. You have all these
lights, LED lamps and fluorescent lamps that
are illuminating. Light is falling on me,
getting scattered and you know you are actually
able to see this image in your eye. So your
eye is your detector in this case, right?
So the whole process of human vision is based
on the fact that we are able to use light
and we are able to detect light. And of course
an extension of that you can say is all the
imaging that is happening including, you know
what you do with your mobile phones is based
on that sort of a principle, this vision principle
which uses light. So what else?
Biomedical imaging clearly, you know lot of
things you are able to see, you know parts
of the human body like internal parts of human
body through optical probes, through endoscopy
you are able to do imaging of what is happening
inside a body through the endoscopes.
And of course even in terms of what is happening
in our eye, what is happening, you know at
the surface of our body we are able to image
using biomedical imaging. So we use, we use
light for that. What else?
Let us get a little more modern. Let us get
more up-to-date. Where else do we use light
these days? You have heard of Augmented Reality?
So what is Augmented Reality? It is basically,
you have a display that comes in, that mixes
2 different scenarios and it is able to give
you extra information than what you normally
have with your regular vision, right?
So that whole thing about Augmented Reality
which is actually one of the disruptive technologies
that is going around. There are lot of Fortune
500 companies, technology companies, the Microsofts,
the Apples, the Googles they are all out there,
you know trying to come up with Augmented
Reality solutions and certainly there is a
lot of development happening in that domain.
And then of course you can extend that to
things like autonomous driving. So you have
a driverless car. And, and you know a simpler
version of that could be a robot, right. So
robotics, the whole thing about robotics,
it is able to see the situa/situation, you
know things around it and able to take actions.
So all of those require light-based technologies
so, so light certainly plays fairly big role
in our everyday lives and it continues to
change the way we see things around us and
we get things done around us. So it is so
much so that by 2050 there is some prediction
that most of the Fortune 500 companies around
the world will be having something or the
other to do with photonics, Ok.
So that is why are here. We are here to understand
the properties of light, how to generate light,
how to detect light and how to manipulate
light, how to make light work for us, right?
So that is what this course is about.
So let me go down here and start making a
few notes. So clearly we are at course which
is Introduction to Photonics, right and we
just started discussing why photonics and
we started discussing some examples of what
is the use of light and we said the whole
process of human vision is based on light,
right?
And then you extend to imaging concepts, using
cameras, using endoscopes you are able to
see things around us and then somebody gave
this example of optical communications where
we use light to carry information which has
completely revolutionized this whole field
of communications.
And then we can list out things like material
processing. So you take once again your mobile
phone as an example. You have different parts
of the mobile phone, you know manufactured
with such high precision and some of these
may involve actually laser based marking;
laser based cutting and so on. So that is
lot of material processing applications for
which light is used.
And going forward, things like Augmented Reality
there is something called Gesture Recognition.
So Gesture Recognition is the next big thing
as far as man-machine interface is concerned.
We are always trying to work on concepts which
can break down this interface between the
man, machine and this is the next level wherein
you just, based on some gestures you are able
to communicate with the machine and you are
able to, you know, you are able to make the
computer understand that you are giving certain
commands, certain instructions for the computer
to do things for us.
So, so we can keep going on in that and some
of the more recent, you know developments
or towards going to the next generation of
computers based on quantum computing and so
on. So all of these are essentially, you know
based on things where we manipulate light,
Ok. So from that perspective you can go back
and say, when we what mean by photonics is
photonics is the science of light.
Through understanding this science of light
we can, we can do certain things like what
are the properties of light, right, we can
try to understand what are the properties
of light, we can try to understand the generation
and detection of light and then probably more
importantly, you can look at the manipulation
of light.
So what do we mean by manipulation of light?
Light has certain properties. So if you want
to describe light, what are the terms that
you use to describe light? Very good, so let
us list this out. Wavelength is one characteristic
or in other words, in colloquial terms the
color of light, right?
So I am using different colors here obviously
to, you know denote different things and that
essentially means that these different colors
represent different wavelengths of light.
What else? Intensity. So you have amplitude
of light and of course through that you can
look at the intensity of light. What else?
Phase, certainly is another property and another
property which is not...
(Professor – student conversation starts)
Polarization
Polarization, very good.
(Professor – student conversation ends)
So we have an opportunity to change certain
properties of the light and through that realize
certain functionalities, right? So that is
what we are going to be, that is one of the
things we are going to be looking at as far
as this course is concerned. So at this point,
maybe I should step back and show you the
outline of the course, what exactly we are
going to be seeing as far as this course is
concerned.
So you have one module. There was a first
module which corresponds to understanding
the properties of light and when we talk about
understanding the properties of light, the
first thing that we look at is this important,
very important principle called wave-particle
duality.
You know, you in colloquial terms you would
see that certain people refer to the science
of light as optics, and certain other section
of people look at it as photonics. Are the
different? Not really. Possibly when somebody
is talking about optics what they are looking
at is things that are, properties that are
based on the wave nature of light.
And when somebody is talking about photonics
they are talking about physical processes
which involve the particle nature of light,
Ok. So we are going to go into some of those
details and try to appreciate where we can
use the wave nature of light and where we
need to use the particle nature of light and
all that.
So we will look at some of those to start
with. And then we will go into this important
discussion which is probably a fulcrum as
far as this course is concerned because when
we look at the properties of light you start
understanding that light is in, in general,
random in nature, right.
In terms of, you know the emission of photons,
in terms of detection of photons you start
understanding that there is a certain statistics
associated with, with those processes.
And in fact it is only to explain those statistics
which we try to do in terms of coherence property
of light, you start appreciating that, may
be it does not help just to treat light as
waves. May be you need to look at the particle
nature of light as well. Ok.
So this really, this topic is really the motivation
to look closely at the particle nature of
light and so, you know beyond that we are
looking at the properties of a photon, the
properties, the statistics of the photon and
so on.
And then beyond that we are at a position
to understand interaction of these photons
with atomic systems, with matter, Ok. So we
will start looking into how this absorption
and emission processes happen and through
that process you come up with this realization
that there is something called stimulated
light emission, Ok
And if you look at, you know stimulated light
emission you start understand that may be
things like light amplification is possible,
right and then once we have realized that
we are at a point where we can start making
light sources.
So we will jump into understanding the fundamentals
of lasers which is by itself, is a complete
course. But what we are trying to do is to
just look at some of the basic concepts as
far as lasers are concerned.
And then the most commonly used light sources
are the semiconductor light emitting diodes
and some of these light panels are based on
semiconductor light emitting diodes, and semiconductor
lasers like laser pointer, if I am using a
laser pointer I am using a semiconductor laser
typically, Ok.
And then we go on to understand light detection
and that once again, once we have understood
the basics of semiconductor p n junctions
we can also figure out how light detection
can be possible using your semiconductor light
detectors. There are photo diodes, what goes
into your mobile camera for example, you know
all those principles.
And beyond that we will go on to looking at
the manipulation of light, so that is our
last module, right. So we will look at how
light can interact with R F waves that is
electromagnetic waves, radio frequency electromagnetic
waves and acoustic waves.
Can you believe that? You can manipulate light
using acoustic waves, you know. How are we
able to do that? Those are some of the principles
we are going to look at in week 10.
And, and beyond that we can also look at how
to manipulate photons through non-linear properties
of material, non-linear response of material.
So the most part in the course we are looking
at interaction of light with material as if
it is a linear response that we are getting
from the material but towards the last portion
we will look at what if the material responds
non-linearly to the light that is incident
on it.
So what can we do with, with that sort of
a property, right? So this is essentially,
I have charted it out as for 11 weeks. It
may actually spill over to 12 weeks but one
important aspect of this particular course,
the way it starts for you students here is
that it is going to be a theory-cum-practical
course.
So each week we will actually be doing a laboratory
session which is enhancing your understanding
on that particular concept that is taught
that week. It is to the point that your laboratory
sessions are essentially driving what we are
discussing in the theoretical aspects of this
course, Ok.
And for those of you that are doing this course
online, what we will be able to do on a weekly
basis is provide demonstration of those concepts
so that you can follow what is going on, essentially
what the students do, the students here do
in the laboratory you will be able to do that,
you will be able to watch that at least as
a in-class demonstration, Ok.
So, so and so clearly the lab sessions that
are defined over here are, you know, are enhancing
those, those practical aspects that we are
going to study as far as this course is concerned.
As far as the textbook for this course is
concerned I am going to be closely following
this excellent textbook which is written by
Saleh and Teich, Fundamentals of Photonics.
It is just that, you know Saleh and Teich
puts this material in such a way that you
can start from appreciating some of the wave
properties of light and then go on to appreciating
the photon, you know properties of light.
So it is nicely structured which is in tune
with what I want to teach as far as this course
is concerned. So we are going to adopt that
as the textbook and of course there are certain
other reference books that are provided which
can be helpful in understanding these concepts
at a deeper level, Ok.
So we looked at why photonics, why we are,
why we are offering this course and then closer
to why it makes sense to follow this course.
So, and then we also said essentially what
we are doing in this course is dealing with
photonics where we are looking at the properties
of photons, light in general then this generation
and detection of photons and manipulation
of photons, Ok.
So those are the three primary modules that
we are going to be studying as far as the
course is concerned. And let us first start
with understanding the properties of light.
And to do that, we will have to step back
and take an historical perspective of the
science of light, how it came about and it
all starts with a simple concept called ray
optics.
And ray optics is primarily based on this
observation by a scientist by name Fermat
in the early 1600s, right. Fermat essentially
hypothesized at that time that light travels
in path of least time, ok. So what does that
mean? Light travels in the path of least time.
In a homogenous medium it actually corresponds
to saying to light travels in straight lines.
If I use the light source over here I can
essentially model this light source as rays
of light that are, you know coming and hitting
me and from me, bouncing off to you, Ok.
So once you are able to say that light travels
in straight paths, you can use rays to represent
the propagation of light. And that is the
simplest way of explaining, you know how light
travels through different media, Ok.
So ray optics is a fairly powerful concept
as far as understanding properties of light
is concerned and that is going to be a starting
point of most of the discussions that we do
in the early part of the course, Ok.
And then there is this other scientist, so
Fermat said light travels in straight lines,
which we are calling as rays. And then this
other person by name Huygens in the mid 1600s,
he came up with the hypothesis that light
travels as waves just like, you know sound
waves Huygens hypothesized that light also
has, demonstrates wave-like property, Ok.
So that happens to be superseding what we
are seeing in ray optics, so you get into
what is called wave optics and what is the
important aspect of waves? What are we introducing
when we talk about waves?
(Professor – student conversation starts)
0:26:02.3
So wavelength
(Professor – student conversation ends)
So we start introducing things like wavelength
and this whole concept of phase that, that
light carries you know. So that is actually,
it is, it is easily explained when you have
a wave. So when you are looking at them as
rays, the rays do not represent any particular
color nor does it represent any accumulation
of its phase as it propagates, Ok. It just
tells about the direction of light.
But now when we discuss this in terms of waves
you start saying Ok, there are, there are
these other characteristics that come into
picture.
And then came this, you know, this, this declaration
from Maxwell around the mid 1850s, mid 1800s
where he declared that light travels as E
M waves, electromagnetic waves, Ok. And that
actually brought about another study based
on modeling light waves as electromagnetic
waves.
And what could possibly come out of something
like this? What do you think you can explain
you know when you are talking about light
as electromagnetic waves?
The last property we were talking about, light
polarization comes about this. So all the
discussion on polarization is something that
is well explained when you consider light
as electromagnetic waves.
And it is not until Max Planck around 1885,
he hypothesized that light emission as well
as absorption is quantized, Ok. So you can
say in certain ways that the modern optics
evolved from, you know this, this hypothesis
by Max Planck which essentially gives a much
bigger picture and that is this topic of Quantum
Optics, right.
Or some people like to call it as photonics
where you start looking at light emission
and absorption as quantized and then of course
the final piece in the puzzle, I think is
around 1915 that Einstein declared that light
itself comprises of quanta of energy which
was later coined as photons, right?
So all this thing about quantum nature or
the particle nature of light, you know, there
is lot more discussion and lot more research
that was happening beyond, beyond that particular
point. Ok.
So this is sort of a brief history of how
this field has developed and we are going
to try to spend some time trying to understand
what if you treat light as ray. What can you,
what are the kind of problems that you can
solve by just treating light in terms of rays
of light, Ok, propagation of light in terms
of rays of light?
And you would be surprised to find that pretty
much, you know more than half, I would say
it is a very large proportion of optical systems
can be modeled with just simple concept of
ray optics, Ok, and we are going to try to
take some examples of that.
So one, and then we will go on to the subsequent
lectures, we will say, Ok what are we missing
in ray optics and what can we capture in wave
optics and then we go on to what are we missing
in wave optics that we capture in electromagnetic
optics and so on, right. So that is, that
is how we are going to progress going forward,
Ok.
So let us just take an example of endoscopy,
right? So that is something we were throwing
out a little earlier. What is endoscopy? It
is about putting an optical probe through
your body to see inside parts of your body,
things we cannot see from just outside. And
that essentially is clearly facilitated by
an optical probe.
So let us go ahead and design an endoscope,
Ok. Shall we? So what do we need to, you know
design this optical probe? What are the principles
that we need to understand? And so happens
that there are two basic principles, one is
called law of reflection and another is called
law of refraction, Ok.
So what are we dealing with in terms of law
of reflection? You basically say, Ok you have
reflecting surface over here and then if you
have a wave that is incident on this, or a
light ray that is incident on this surface
at an angle let us say, theta i it is going
to get reflected at an angle theta r, right.
Now it can be proved that, you know you can
say this is, if this is the path of least
time if you start from Fermat's principles,
it is just a couple of steps that you can
use, one of the key clues in that is that
what if the light had done straight down,
right? You, you look at that and then you
fold it back and then you can prove that theta
r equals to theta i.
That is the angle of reflection is equal to
the angle of incidence, Ok. This is once again
something that you would have studied in high
school physics. You are all very familiar
with it, right.
The other thing you may be very familiar with
is this law of refraction where we say; Ok
you have an interface between two material,
Ok. And in optics what do we use to characterize
different material? Yes; refractive index
right.
So let us say this is n 1 and this is n 2
and this is the normal over here and then
if I have a light ray coming in with an angle
theta 1 in this case part of the light may
be reflected but the other part of the light
is going into this second medium with angle
theta 2 and then we have this famous law known
as Snell's law which says n 1 sin theta 1
equal to n 2 sin theta 2, right?
So this is also something that you are very
familiar with and of course, it is not too
difficult to prove this, you know just from
geometric perspective but it is more easily
probably proven from the electromagnetic perspective
considering the boundary condition between
the two media and all that, right. So, but
let us just take this for granted and move
on.
Now if you say you have this Snell's law and
if you consider a specific condition where
n 1 is greater than n 2, Ok this is already
going to imply that if you have to plug it
in Snell's law, it says that theta 2 is greater
than theta 1, right. If n 1 is greater than
n 2, then if you plug that into Snell's law
it comes up with this simple implication that
theta 2 has to be greater than theta 1.
And it is only, you know as a matter of some
other value of theta 1 equal to theta c where
theta 2 becomes pi by 2, right? So as you
keep increasing theta 1 it gets to a certain
angle where, you know which you can label
as theta c at which theta 2 equal to pi by
2.
Now if you write the Snell's law at that particular
point it basically says n 1 sin theta c equal
to n 2 sin of pi by 2 which is equal to 1
so this you say is n 2. And in other words,
if you say, if you define this angle theta
c, this is sin inverse of n 2 over n 1, right.
So that is fairly simple proof of what that
critical angle is.
And what happens beyond that critical angle?
If theta 1 is greater than theta c, total
internal reflection, right and total internal
reflection says that in the same case that
we were drawing over here, if you have your
interface here, if n 1 is greater than n 2
and theta 1 greater than theta c then all
the light that is incident on this interface
is going to get reflected, right?
So you have total internal reflection and
if you project this forward and say what if
we have one more interface over here which
is bounded by n 2, Ok. Then if this is theta
1 and these two interfaces are parallel then
this angle of incidence is also going to be
the same angle theta 1.
So you would have reflection having over here
as well. So essentially if you manage to get
this angle right when you are launching light
into this, into this structure, then it can
be confined within that structure and it can
propagate over certain distance as long as
these two interfaces are parallel to each
other, right?
So that is essentially the underlying principle
in endoscopy. You are launching light into
this structure and through this process of
total internal reflection it is going to carry
all that information to the other end, Ok.
Now of course, there is a, you know limit
to what angles it can gather and to examine
that limit you need to understand what is
happening at this interface at the launch
side, Ok
But we do not have time to discuss that right
now. So let us stop here and let us continue
in the next session how we can define what
is the cone of light rays that we can capture
within this wave guide as far as an endoscope
is concerned. And I would also want you to
think about this. This is leading on to the
next topic that we are going to discuss, Ok.
You can do this experiment with me, right.
Take your thumb and index finger, any other
finger that you like, Ok and look through
this. And when you look through this with
them far apart you can clearly see what is
going on the other side, right. And then you
take it closer, closer, closer and to the
point where you are touching the other finger,
then obviously you are not seeing anything
through that.
But just retract a little bit. Just before
you touch that other finger you will actually
see that you are not, well, you won't be able
to see through just before you touch that
other finger, Ok. Now you can try that now
or at home or in your room wherever but I
want you to come back the next session, when
Thursday and I want you to tell me what is
happening.
Why you are not able to see the light even
though there is a gap between the two fingers?
Ok and that will be the motivation for what
we are going to do next, OK, thank you.
