KAHIBARO
Table of Contents
Light Amplification by Stimulated Emission
A laser is a device that produces a very special kind of light. The word laser stands for Light Amplification by Stimulated Emission of Radiation. Ordinary light bulbs emit light in many directions, with many wavelengths, and with waves that are not well organized. Laser light is different. It is usually very narrow, very directional, often nearly a single color, and highly coherent, meaning its waves keep a stable phase relationship.
Lasers are important in science, medicine, communications, industry, and everyday technology. Barcode scanners, fiber optic communication, eye surgery, distance measurement, and laboratory experiments all make use of lasers.
The Basic Idea
A laser works by making atoms or molecules emit light in a controlled way. The key process is stimulated emission, which is closely related to atomic energy levels and atomic transitions. In a laser, many particles are prepared in excited states. When the conditions are right, one photon can trigger an excited atom to emit another photon that is identical to it in energy, direction, phase, and polarization.
This causes light to be amplified. If this amplification happens inside a carefully designed optical system, the result is a strong beam of laser light.
The essential idea of a laser is this: excited atoms undergo stimulated emission, producing photons that match the incoming photon. This creates amplification of light.
Three Essential Ingredients
Every laser needs three main parts. First, it needs an active medium, also called a gain medium. This is the material whose atoms, ions, or molecules produce the light. Second, it needs a pumping mechanism, which supplies energy to the medium. Third, it needs an optical cavity, which helps the light bounce back and forth and grow stronger.
The active medium can be a gas, a liquid dye, a crystal, a semiconductor, or an optical fiber. The pumping mechanism may be an electric discharge, another light source, a chemical reaction, or an electric current. The optical cavity usually contains two mirrors facing each other.
Population Inversion
Under normal conditions, most atoms are in lower energy states. For laser action, that is not enough. A laser requires population inversion, which means that more particles are in an excited state than in a lower state relevant to the laser transition.
This is unusual because systems naturally prefer lower energy. The pumping system must continuously supply energy to create and maintain this inverted population.
If there is no population inversion, absorption dominates over stimulated emission, and amplification does not occur.
Laser action requires population inversion.
Without population inversion, the medium does not amplify light.
The Optical Cavity
The optical cavity, or resonator, is usually made of two mirrors. One mirror reflects almost all the light. The other reflects most of the light but lets a small portion escape. That escaping part becomes the laser beam.
As light travels back and forth between the mirrors, it passes repeatedly through the gain medium. Each pass can produce more stimulated emission, so the light intensity grows. Only certain wave patterns fit well inside the cavity, so the cavity also helps select the frequencies that can survive and build up.
How Laser Oscillation Begins
At first, the medium may emit some spontaneous photons in random directions. A few of these happen to travel along the cavity axis. Those photons are reflected back and forth by the mirrors. If the gain is large enough, they stimulate more emission and the light becomes stronger on each round trip.
When the amplification overcomes the losses in the cavity, sustained laser action begins. This condition is called threshold.
A laser starts operating only when gain exceeds losses. This condition is called the laser threshold.
Why Laser Light Is Special
Laser light has several characteristic properties. It is useful to compare it with ordinary light.
| Property | Ordinary light | Laser light |
|---|---|---|
| Direction | Spreads widely | Highly directional |
| Color | Many wavelengths | Often nearly monochromatic |
| Phase relation | Random | Coherent |
| Intensity | Usually moderate | Can be very high |
Monochromatic does not mean perfectly one wavelength, but usually a very narrow range of wavelengths. Coherence means that the wave maintains a predictable phase pattern over time and space. Directionality means the beam spreads very little.
Energy Balance and Gain
The growth of light in a laser medium depends on gain and loss. If a beam of intensity $I$ passes through a gain medium over distance $x$, a simple model writes
$$
I(x) = I_0 e^{gx}
$$
where $g$ is the gain coefficient. In real lasers, losses in the cavity reduce this growth. Laser action requires the net amplification over one cavity round trip to be greater than or equal to the total losses.
This exponential form is similar in mathematics to many growth and decay processes in physics.
Types of Lasers
Lasers come in many forms depending on the active medium.
| Type of laser | Active medium | Common example |
|---|---|---|
| Gas laser | Gas atoms or molecules | Helium-neon laser |
| Solid-state laser | Crystal doped with ions | Ruby laser, Nd:YAG laser |
| Semiconductor laser | Semiconductor junction | Laser diode |
| Dye laser | Organic dye solution | Tunable dye laser |
| Fiber laser | Doped optical fiber | Erbium fiber laser |
A helium-neon laser produces a familiar red beam. A ruby laser was one of the earliest lasers. Semiconductor lasers are used in many small electronic devices because they are compact and efficient.
Three-Level and Four-Level Lasers
Not all laser systems use energy levels in the same way. In a three-level laser, particles are pumped from the ground state to a high level, then quickly fall to a metastable excited state. The laser transition occurs from that metastable state back to the ground state. This can be difficult because the lower laser level is the ground state, which is naturally heavily occupied.
In a four-level laser, the laser transition ends in a level above the ground state, and that lower laser level quickly empties. This makes population inversion easier to achieve. For this reason, four-level lasers are often more efficient.
Continuous and Pulsed Operation
Some lasers emit light continuously. These are called continuous-wave lasers. Others emit short bursts of light, called pulses. Pulsed lasers can produce extremely high peak power, even if the average power is moderate.
A pulse of energy $E$ lasting time $\Delta t$ has average pulse power
$$
P = \frac{E}{\Delta t}
$$
If $\Delta t$ is very small, the power can become very large. This is one reason pulsed lasers are useful for cutting materials, studying fast processes, and performing delicate medical procedures.
Tunability
Some lasers emit only one main wavelength fixed by their medium and cavity. Others can be tuned over a range of wavelengths. Tunable lasers are very useful in spectroscopy, where scientists study how matter absorbs and emits light.
The laser wavelength is related to the energy difference between two levels:
$$
E = hf = \frac{hc}{\lambda}
$$
So different energy level spacings correspond to different laser wavelengths.
The photon energy of laser light is determined by the transition energy:
$$
E = hf = \frac{hc}{\lambda}
$$
Common Examples
The helium-neon laser is a gas laser that commonly emits red light at about $632.8 \, \text{nm}$. It is stable and often used in teaching laboratories.
The ruby laser is a solid-state laser based on chromium ions in aluminum oxide. It was historically important as one of the first lasers.
Semiconductor laser diodes are extremely common. They are found in optical communication systems, laser pointers, disc readers, and many sensing devices.
The Nd:YAG laser is a powerful solid-state laser used in industry and medicine. It can also be frequency converted to produce other colors.
Applications
Lasers are useful because they can deliver energy and information very precisely. In communications, they carry signals through optical fibers. In medicine, they can cut tissue, reshape the cornea, or destroy abnormal cells. In industry, they are used for cutting, welding, drilling, and measuring. In science, lasers are used for spectroscopy, cooling atoms, interferometry, and precision metrology.
Their narrow beam and coherence make them valuable for accurate distance measurements and imaging systems.
Safety
Laser light can be dangerous, especially to the eyes. Because the beam is so concentrated, even a low power laser can damage the retina if viewed directly. Higher power lasers can also burn skin or ignite materials.
Reflections can also be dangerous, especially from shiny surfaces. Safe laser use requires proper eye protection, controlled beam paths, and awareness of the laser power and wavelength.
Never look directly into a laser beam.
Even reflected laser light can be hazardous.
Summary
A laser is a device that amplifies light by stimulated emission. It needs an active medium, a pumping mechanism, and an optical cavity. The crucial physical condition is population inversion. When gain exceeds losses, laser action begins. The resulting light is typically coherent, highly directional, intense, and nearly monochromatic.
Lasers are one of the most important practical applications of atomic physics, because they directly use quantized energy levels and controlled atomic transitions to produce light with remarkable properties.