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 resident 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 resident cavity, comes in.
The purpose of the resident 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 resident 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 resident 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 and 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 resident cavity. Thanks to all these parts working together, a beam of laser light is ultimately produced.