In this blog post, we’ll explore the principles behind why lasers produce light that is far more intense than ordinary light sources, using the concepts of stimulated emission, optical pumping, population inversion, and the directionality of light.
“Fire the laser beam!” This is a line often heard in robot animated movies. The green laser beams fired from robotic arms are usually powerful, and enemies are invariably defeated. Defeating enemies with lasers isn’t just a story from cartoons. In the past, Rheinmetall, a renowned German artillery manufacturer, unveiled a laser weapon called “HEL” as a new weapon to replace conventional artillery. It is said to be a formidable weapon capable of shooting down unmanned aerial vehicles. In response, the U.S. Naval Research Laboratory introduced a naval laser weapon called “LaWS.” It is a powerful weapon capable of easily sinking a small vessel. At this point, the power of lasers seems to surpass that of most bullets—or even bombs. How exactly can a laser produce such an intense beam of light? Let’s examine why lasers are so powerful by looking at how they work.
Before addressing “why” they are powerful, let’s first determine exactly “how” powerful they are. The intensity of laser light ranges from a minimum of 10³W to a maximum of 10¹²W. Does that not quite sink in? At around 10³W, we could put on a laser show. We wouldn’t need fluorescent lights or streetlights—the night sky would be filled with nothing but lasers. If we increase the intensity slightly to around 10⁶W, we could cut through bone—or even cut through steel—with a laser. The maximum intensity of 10¹²W is equivalent to the power of several missiles.
When we compare this intensity to other light sources, the power of lasers becomes even more apparent. Imagine focusing all the light from an incandescent bulb onto a single, tiny point. The temperature at that point would range from a few hundred K to, at most, a few thousand K. In contrast, when a laser is focused on that spot, the temperature rises to tens of millions of K. In other words, even if you gathered a thousand incandescent bulbs, their combined intensity would still fall short of that of a single laser. As such, lasers are far more powerful than any other man-made light source.
So why are they so superior? To put it simply, it’s because of “stimulated emission.” Imagine a tube of toothpaste with the cap off. If left alone, the toothpaste barely leaks out, but if you squeeze it hard, it squirts out. This is precisely because we have “induced” the emission of the toothpaste. The same applies to light. While conventional light sources emit light “spontaneously,” lasers “induce” the emission of light. This stimulated emission is the core operating principle of a laser and is the source of its various characteristics. In fact, the term “laser” is derived from the English expression meaning “light amplification by stimulated emission of radiation” (laser: light amplification by stimulated emission of radiation). Therefore, by understanding stimulated emission, we can grasp why lasers are particularly powerful. Now, let’s take a closer look at stimulated emission.
Stimulated emission refers to the process by which light is emitted in response to stimulation from external light. To understand the process of stimulated emission, let’s break down the laser material into very small units. If we break it down and observe it at a scale of about 10⁻¹⁵ to 10⁻¹⁸ m in diameter, we find particles called protons, neutrons, and electrons. Among these, electrons play a crucial role in emitting light. Each electron has its own unique energy, and the energy levels that electrons can occupy in a laser material are limited to a finite number of values.
Here, for simplicity, we will consider a case where only two energy states, E₁ and E₂, are possible. A proton is a particle that makes up the atomic nucleus—the center of the atom—and carries a positive charge. A neutron is a particle that makes up the atomic nucleus; it has a mass similar to that of a proton but no charge. An electron is one of the subatomic particles that orbits the atomic nucleus and carries a negative charge.
Now, the electrons have two homes to choose from. The first home has low energy. After using that energy for basic heating and cooling, there’s nothing left, so the home is simply quiet and peaceful. In contrast, the second home has an enormous amount of energy. There’s so much that it gets unbearably hot, and the energy is so uncontrollable that it shakes wildly. If you were an electron, which home would you choose? Naturally, it would be the first one. Just as water flows to lower ground and apples fall to the ground, electrons also tend to seek out lower energy states. In other words, they prefer the E₁ state over the E₂ state.
The day of the housing lottery has arrived for the electrons. Naturally, the more popular house is the E₁ house. If left alone, the E₁ house would surely fill up with electrons, while the E₂ house would obviously remain completely empty. However, a laser will never allow this situation to occur. In a laser, energy is forcibly applied to the electrons in the E₁ state, forcing most of them into the higher-energy E₂ state. As a result, the E₂ state ends up containing more electrons. This process of applying energy to electrons in a lower energy state is called optical pumping. Furthermore, the resulting state—where there are actually more electrons in the higher-energy state—is called population inversion. Creating a state of population inversion can be considered a preparatory step for stimulated emission.
In a state of population inversion, the electrons in E₂ are like a massive sandcastle. Just as sand becomes heavy and unstable as it piles up, the accumulated energy puts them in a highly precarious state. Like a sandcastle that would collapse at the slightest touch, even the tiniest stimulus causes them to immediately release energy and attempt to return to the E₁ state. Now, let’s turn on the laser switch. When we do, light from outside the laser is transferred to one of the electrons in the E₂ state, initiating the first stage of stimulated emission. The excited electron releases energy and drops to the E₁ state. The amount of energy released by the electron is, of course, (E₂ – E₁). This energy is emitted outside the laser in the form of light.
What’s interesting here is that as this light travels outward, it excites another electron in the E₂ state. The second electron is excited and drops to the E₁ state. Let’s not confuse this with optical pumping. Here, the light does not transfer energy to the electron, as it does in optical pumping. It simply gives the electron a nudge so it can drop to the E₁ state. Now, the second electron also emits light corresponding to the energy difference (E₂–E₁). Through the same process, the third electron, the fourth electron, and eventually countless electrons in the E₂ state drop to the E₁ state and emit light. A stream of light corresponding to the energy difference (E₂–E₁) is generated for each electron that drops to the E₁ state. Once all electrons in the E₂ state have dropped to the E₁ state, stimulated emission ends.
Now that we understand stimulated emission, why does this process make a laser so powerful? There are three reasons. The first reason lies in the preparation process for stimulated emission: optical pumping. Conventional light sources do not undergo optical pumping. In other words, population inversion does not occur, so only a very small number of electrons are in the high-energy E₂ state. Consequently, the number of electrons that will decay to the E₁ state is naturally small, and the number of emitted light beams is also small.
Second, with stimulated emission, there is no energy loss when the light emitted by each electron combines with the others. Light is, by its very nature, a wave that vibrates up and down as it travels forward. Imagine waves crashing on a beach. You’ve probably noticed that waves that looked large from a distance appear infinitely smaller when you’re on the beach. This is because the waves approaching the shore meet the waves receding from the shore and lose their energy. Conversely, let’s imagine a situation where two waves moving in the same direction merge. If the wave coming from behind can overtake the wave in front, the combined wave will reach an even greater height. Stimulated emission falls into the latter category. This is because when light emitted by one electron excites another electron, that electron is also caused to emit light in its own direction. As a result, the energy emitted by each electron can be precisely superimposed in the same direction, and the total energy of this superimposed light becomes {(E₂ – E₁) × (number of excited electrons)} without any loss. This phenomenon is impossible in spontaneous emission, where electrons fall out of the system one by one and emit light in different directions.
Third, stimulated emission allows a laser to focus light into a single point. You may have seen in a presentation how a laser beam fired by a speaker appears as a small dot on a large screen. Even when fired from Earth to the Moon, the diameter of this laser beam is less than 3 km. It hardly spreads at all. To concentrate light in this way, the light beams emitted by the laser must be parallel to one another. Let’s examine the structure of a laser. A laser makes the light travel back and forth tens of thousands of times within a very narrow tube, reflecting it to ensure the beams remain parallel. If the light emitted by each electron were directed in a different direction, most of these light beams would cancel each other out as they crossed paths—much like waves colliding. However, as mentioned earlier, the light produced by stimulated emission all travels in the same direction, so there is no cancellation. This is why, among countless light-emitting devices, only lasers can focus light into a single point. What if light that would otherwise spread in all directions were concentrated into a single small point? Naturally, its intensity would increase by tens of times.
By understanding stimulated emission, we’ve now seen why lasers are particularly powerful. Lasers have a vastly greater number of electrons emitting light. Furthermore, stimulated emission allows all those light beams to combine without energy loss and enables the emitted light to be focused into a single point. In this way, laser light is the most powerful and superior light ever created by humans, with a source entirely different from ordinary light. Consequently, lasers are currently used as valuable tools in various industrial fields, such as laser printers, laser surgery, laser lighting, and laser welding. Since lasers were developed relatively recently, they represent a technology with boundless potential for advancement. No one knows whether lasers will one day bring about a new revolution for humanity. But we must be cautious. Just as lasers have been used to create weapons, such powerful technology can always be dangerous.
