Video Transcript
In this video, we’re talking about
the components and operation of lasers. As complicated as they may seem,
all lasers essentially consist of three parts. In this lesson, we’ll talk about
those parts and how each one works, and we’ll also see how they all fit together to
create laser light.
Now, we mentioned that every laser
has three basic parts. Here are their names. There’s something called an active
medium, which provides the material — the atoms — whose electronic transitions are a
necessary part of the lasing process. And second, all lasers have an
energy source, often called a pump. This is what provides energy to the
active medium. And lastly, the third vital
component to a laser is called the resonant cavity. As we’ll see in a bit, it’s this
cavity that enables amplification of laser light. Let’s now look at these three
components in a bit more depth.
We’ll start with the active
medium. The active medium of a laser is
some material. And it could be a solid, like we’ve
sketched here, or a liquid or a gas whose atoms have energy levels that are well
suited to the production of laser light. Here’s what we mean by that. Say that the atoms that make up our
active medium possess two energy levels — we’ve called them 𝐸 one and 𝐸 two — that
electrons in those atoms can occupy. Now, the natural state we could
call it of electrons in any atom is to occupy the lowest energy level available to
them.
Unless we do something to this
system, unless we add energy to it, the electrons will remain where they are. But if they do, then we can never
make a laser. That’s because laser radiation
requires an electron to be in an excited state, like this one is here. And then for a photon with just the
right amount of energy to come along and stimulate this electron to return to its
lower energy level. In that process, the electron emits
a photon essentially identical to the one incident on it in the first place. This process is known as stimulated
emission of radiation, and it’s an essential part of creating laser light.
For this all to work then, to
create a laser, we require that the majority of electrons in our system occupy an
excited energy state as compared to the ground energy state. There’s an obstacle, though, to
making this happen. Say that we add energy to our
system and, by doing that, are able to get two electrons in the excited energy
level. It turns out that once excited,
electrons will naturally decay — it’s called — back down to their lower original
energy state. In fact, this is such a well-known
phenomenon that we even know the approximate amount of time that an electron will
spend in an excited state before decaying back down. It’s only about 10 billionths of a
second.
This means that to create laser
light from these two electrons, we only have a time window of about 10 nanoseconds
for a photon with the right energy level to come along and stimulate emission. Generally speaking, that’s just not
enough time. So rather than interacting with an
incoming photon, it’s much more likely that these electrons will just spontaneously
decay back to their original level. In the process, they do emit
photons, but not photons that can contribute to a laser beam. In order for a given material to
make a good active medium for a laser, the atoms in the material need to have more
than two possible energy levels for electrons.
Now, if there are three levels,
like we see here, with 𝐸 three being less than 𝐸 two but greater than 𝐸 one, then
it is possible using this atomic structure to create a laser. In fact, the very first laser
developed in 1960 had a three-level atomic structure, like this one. If the atoms in our active medium
have this kind of structure, then here’s how the system works. Just like before, we start out with
the electrons in this system, all at the lowest energy state. But then, using our energy source,
also called the pump, we elevate a number of these electrons so that they occupy the
level 𝐸 two, what we can call the excited state.
As we’ve seen, though, electrons
don’t stay in this state for long. When they spontaneously decay,
though, now, instead of going back down to this lowest energy level, called the
ground state, they instead move to this intermediate energy level. And now, there’s a very important
quality that this energy level, 𝐸 three, has. In contrast to the ground state and
the excited state, this level is known as a metastable state. This name comes from the fact that
when an electron occupies this level, it tends to stay there for quite some
time. The lifetime, we could say, of an
electron in this state is significantly longer than 10 nanoseconds. In fact, it can be a thousand or
even a million times longer than this. So, when electrons are in a
metastable state then, they’re much more available to interact with incoming
radiation. And when this happens, stimulated
emission can take place.
Before we go further, though, let’s
notice that as our system currently stands, there are more electrons in an excited
energy state, our metastable state, than there are at the ground level. This is not something that could
happen if we weren’t putting energy into our active medium via our pump because as
we mentioned earlier the default level for electrons in an atom to occupy is the
ground state. There’s a specific name for any
atomic system where more electrons are energized above the ground state than are in
that state. When this happens, it’s called a
population inversion. This word “population” refers to
all the electrons in the system, and “inversion” tells us that the way things
normally are has been flipped upside down.
Ground-state electrons typically
outnumber electrons in an excited state. But by putting energy into our
active medium, we’re able to invert this typical scenario. Achieving a population inversion is
a necessary part of creating a working laser. Recall that we weren’t able to
create this inversion when we just had a two-level system with a ground state in an
excited state. Because after being excited,
electrons so quickly decayed back down to the ground state. But now that we have a three-level
system with a metastable state, a population inversion is possible.
So then, these electrons in our
metastable state are good candidates for interacting with incoming photons,
stimulating an electron to return to the ground state, and, in so doing, emitting a
photon that’s identical in phase, direction, and frequency with the one that
stimulated its transition. These photons are then capable of
interacting with other metastable-state electrons, leading to even more stimulated
emission events. So, we’re starting to see on a very
small scale how laser light can be produced.
If we think about the active medium
of our laser on a macroscopic scale though, we can start to see that the probability
of a photon with just the right frequency being incident on one of these excited
atoms may be fairly small. That is, it may be possible for
such a photon to pass all the way through the active medium without interacting with
any electrons. This is where the third component
of our laser, the resonant cavity, comes in.
The purpose of the resonant cavity
is to contain the active medium with basically two big mirrors on either end. Now, in reality, one of these ends
is actually a full mirror, 100 percent reflective of light incident on it, while the
other end is a partial mirror, reflecting upwards of 90 percent of the light
reaching it but letting some through. So then, for this photon that we’ve
said has just the right frequency to stimulate an electronic transition from 𝐸
three to 𝐸 one — even though on its first pass through, it didn’t run into any of
these excited electrons — the partial mirror is likely to send it back the way it
came for another pass. And if on this pass it doesn’t
reach any excited electrons, then it will just reflect off the mirror at the other
end of the resonant cavity.
If we send a photon with the right
frequency back and forth through our active medium enough times, eventually it will
interact with an electron in the metastable state of an atom, stimulate an emission
of an identical photon, and then these two photons will continue on in this
direction. As this process continues, more
photons are added to what is becoming a beam of laser light. We can start to see that in this
central portion of our active medium, more and more laser light will be produced and
will bounce back and forth along this path. As we mentioned, not all of this
light is reflected when it reaches the mirror on the right end of our cavity, the
partial mirror. And it’s only that light that is
not reflected, some small fraction of the overall amount, that makes it through and
is the actual laser beam produced by our laser.
So to recount this process, if we
start out with an active medium whose electron energy levels allow for an excited
state, a metastable state, along with the ground state, then by applying energy to
this medium using our pump, we’re able to elevate a majority of electrons out of the
ground state. And once they occupy a metastable
state in between the excited and ground states, we can say then that we’ve achieved
a population inversion. At this point, these electrons are
ripe for stimulated emission by incoming photons. These emission events take place
within a cavity called a resonant cavity, which is basically two mirrors at either
end of the active medium, which is designed to compound or cascade all of the
stimulated emission of radiation.
Lastly, a fraction of that
radiation is transmitted through the partial mirror, and this is the beam of light
we may detect coming from our laser. Knowing all this, let’s get some
practice now through an example exercise.
Which of the following most
correctly describes the quality of the active medium of a laser that is relevant to
its ability to produce lasing? (A) An active medium contains
completely ionized atoms. (B) An active medium contains atoms
with unstable nuclei. (C) An active medium contains atoms
in which electrons tend to transition to excited states at the same rate that they
tend to transition to relaxed states. (D) An active medium contains atoms
in which electrons tend to transition to excited states at a greater rate than they
tend to transition to relaxed states. (E) An active medium contains atoms
in which electrons tend to transition to relaxed states at a greater rate than they
tend to transition to excited states.
Okay, so this question is all about
what’s called the active medium of a laser. A laser’s active medium could be a
solid, like this here, or a liquid or even a gas. But in any case, the material of
the active medium is carefully chosen to enable lasing. In our various answer options, we
see that some of them refer to what are called excited states as well as relaxed
states. These states have to do with the
particular energy level structure of the atoms that make up the active medium.
Let’s say that we represent those
energy states this way. We can let this line here represent
the ground-state energy level of the atoms in our active medium. We’ll call this level 𝐸 sub g for
the energy of that ground state. And then, let’s say that this
thicker line right here represents all of the excited states of our atom
together. So, we’re grouping our atoms’
excited states all together, and we’re saying that they have an energy called 𝐸 sub
e. Now, when it comes to electrons in
this system, we know that an electron can either occupy the ground state — this, by
the way, is where electrons naturally tend — or if some energy was added into the
system, then an electron could be bumped up to an excited energy level.
Now, another name for a ground
energy level is a relaxed energy level or a relaxed state. So if an electron moves up like
this, we say that it’s excited. And if an electron moves from an
excited state to the ground state, we say it has relaxed. So that’s the meaning of these
terms “excited states” and “relaxed states” that appear in some of our answer
options. So that we can see all five of our
answer options on the same screen, let’s paraphrase the ones we saw on the previous
screen. Those were answer options (C), (B),
and (A).
Answer option (C) is very similar
to options (D) and (E), except (C) says that an active medium contains atoms in
which electrons tend to transition to relaxed states at the same rate of transition
at which they move into excited states. So, note that option (D) says that
transitions to excited states happen at a greater rate than those to relaxed states,
while (E) has the opposite. It says transitions to relaxed
states occur at a greater rate than those to excited states. And then, as we saw, (C) describes
a rate of transition to relaxed and excited states which is the same. So that was answer option (C).
Answer option (B) said that the
active medium of a laser contains atoms with unstable nuclei. And then the very first answer
option, (A), said that the active medium of a laser contains completely ionized
atoms. Now that we’ve got all of these on
the same screen, let’s return to our sketch of these energy levels in the atoms that
make up the active medium.
Recall that we’re looking to
identify the quality of the active medium that relates directly to its ability to
lase. In order for that to happen, we
know that stimulated emission must take place. This involves an electron in an
excited energy level interacting with an incoming photon. If the frequency, in other words,
the energy of this photon, is just right, this interaction can stimulate the
electron to return to a relaxed state and, in the process, to emit a photon with an
identical frequency, phase, and direction as the original one. This process, repeated many times,
produces a beam of coherent radiation characteristic of laser light.
We see then that for lasing to
happen, electrons in the active medium must be in an excited state. If they weren’t — if they were
instead in the ground state like here — then when a photon of just the right
frequency came along, instead of stimulating the emission of another identical
photon, this photon would simply be absorbed and then propel the electron to an
excited state. When electrons are in an excited
state then, they can be stimulated to emit a photon. And this emitted photon adds to the
original one, whereas, on the other hand, if electrons are in the ground state and
they absorb a photon, there’s a net loss of photons one photon in that process. All this to say, for lasing to
occur, there needs to be what is called a population inversion. This is where the number of
electrons in excited states outnumber the electrons in the ground state.
Knowing this, we can start to
eliminate a few of our answer options. First, considering options (A) and
(B), a completely ionized atom has no bound electrons, but it’s just those
transitions made by bound electrons that we need for lasing. Completely ionized atoms, then, are
incapable of supplying the active medium of a laser. So, we’ll eliminate option (A). Option (B) talks about unstable
nuclei in our active medium. This description would indicate
impending nuclear decay in radioactivity. But these processes do not
contribute to the production of laser light. There’s no need for the atoms in
our active medium to have unstable nuclei. And in fact, we’d rather they
not. We’ll cross option (B) off our list
too.
As we consider the remaining three
options, we can see that they’re very similar to one another. All of them describe a transition
rate to excited states and relaxed states and compares those rates. To figure out which of these three
is the best answer, here’s the question we can ask. Which of the three different types
of rates described in these answer options will lead to a majority of electrons in
our atom being in an excited state compared to the ground state? With that question in mind, let’s
look again at answer option (C).
This says that the rate at which
electrons in our atom transition to an excited state is the same as the rate with
which they transition to a relaxed state. If that happened, though, we would
expect there to be the same number of excited electrons as relaxed ones. And this would mean we haven’t
achieved a population inversion. A photon incident on this atom
would be just as likely to be absorbed as it would be to stimulate the emission of
another photon. So when these rates are the same,
as option (C) claims, we won’t be able to amplify the light being produced by
stimulated emission. So, we’ll cross off this
option.
Next, option (E) says that the rate
at which electrons in the atom transition to relaxed states is greater than the rate
at which they transition to excited ones. If that were so, we would expect
our atomic system to look like this, with no excited electrons. But no excited electrons means no
stimulated emission. So, laser light can’t be produced
that way. We’ll cross off this choice
too.
Finally, option (D) says that the
rate at which electrons transition to excited states is greater than the rate at
which they move to relaxed states. Of all our answer options, this is
the only one that would lead to a true population inversion. This would then enable the
production of laser light. So, an active medium contains
atoms, in which electrons tend to transition to excited states at a greater rate
than they tend to transition to relaxed states.
Let’s summarize now what we’ve
learned about the components and operation of lasers. In this lesson, we saw that lasers
consist of three parts. The active medium, which can be a
solid, a liquid, or a gas, which provides material with the right kind of electronic
energy structure for lasing. All lasers also have some kind of
energy source, sometimes called an energy pump, which provides energy to the atoms
in the active medium. By doing so, more electrons in this
medium are in excited states than in the ground state, creating what is called a
population inversion.
We also learned that a population
inversion is enabled by what’s called a metastable energy state. This stimulated emissions of light
from this state are then amplified by the third main part of the laser known as the
resonant cavity. Thanks to all these parts working
together, a beam of laser light is ultimately produced.