Laser

A laser is a device that creates a special kind of light through a process called optical amplification, based on something known as stimulated emission of electromagnetic radiation. The word laser comes from an acronym meaning "light amplification by stimulated emission of radiation." The very first laser was built in 1960 by Theodore Maiman at Hughes Research Laboratories. This invention was based on earlier theories by scientists like Charles H. Townes and Arthur Leonard Schawlow.

Unlike normal lights, a laser produces light that is coherent, meaning the light waves are organized and travel together. This special property allows a laser beam to be focused into a very tiny spot, which is useful for tasks like laser cutting and making tiny patterns for computer chips. Laser beams can also stay narrow over long distances, making them perfect for tools like laser pointers and devices that measure distances, such as lidar.

Lasers are used in many everyday technologies, including fiber-optic communication, optical disc drives, laser printers, and barcode scanners. They are also important in medicine for laser surgery and skin treatments, as well as in industries for cutting and welding materials. Because of their precision and power, lasers are considered one of the most important inventions of the 20th century.

Terminology

The first device that used amplification by stimulated emission worked with microwave frequencies and was called a maser, short for "microwave amplification by stimulated emission of radiation." When similar devices were developed to work with light, they were called optical masers. Later, the word "microwave" was changed to "light," and the term laser was born.

Today, devices that work with frequencies higher than microwaves, such as infrared lasers, ultraviolet lasers, X-ray lasers, and gamma-ray lasers, are all called lasers. Devices that work with microwave or lower frequencies are called masers. The verb "to lase" means to give off coherent light, and it describes what happens when a laser is operating.

Fundamentals

Photons, the tiny packets of light, are released from energy levels in atoms and molecules. In everyday light sources like lightbulbs or stars, this energy comes from many different levels, creating a broad mix of energies. This is called thermal radiation.

In a laser, the process is different. When a photon passes by, it can trigger another photon to be released in exactly the same way. This is called stimulated emission. If many atoms are in the right excited state, these photons can trigger even more photons, creating a chain reaction. Special materials used in lasers stay excited longer, allowing this chain reaction to happen. Lasers can focus their light into very thin beams or spread it out over long distances, making them different from ordinary light.

Design

A laser is made up of three main parts: a gain medium, a way to give it energy, and something to help the light stay focused. The gain medium is a special material that can make light stronger. When light passes through it, the light gets more powerful.

To make the gain medium work, it needs energy, which can come from electricity or another light source. Most lasers use two mirrors on either end of the gain medium. The light bounces between the mirrors, getting stronger each time. One mirror lets some of the light escape, creating the laser beam.

Laser physics

See also: Laser science

Electrons and how they interact with electromagnetic fields are important in understanding chemistry and physics.

Stimulated emission

Main article: Stimulated emission

In simple terms, electrons in atoms can move between different energy levels. They can absorb energy from light or heat, causing them to jump to a higher energy level. Later, the electron can drop back down, releasing a photon of light.

When a photon with the right wavelength hits an excited electron, it can cause the electron to drop to a lower energy level and emit a new photon. This new photon is exactly the same as the one that triggered the drop — same wavelength, phase, and direction. This process is called stimulated emission.

Gain medium and cavity

The gain medium is a special material that can amplify light. It is put into an excited state by an external energy source, like a light or electrical signal. This material can be a gas, liquid, solid, or plasma. When enough particles are in an excited state, more stimulated emissions happen than absorptions, leading to light amplification.

The gain medium is placed inside a resonator, which usually has two mirrors. Light bounces back and forth between these mirrors, passing through the gain medium many times. This creates a powerful, coherent beam of light. If the gain is strong enough, the light can become very intense.

The light emitted

Most lasers start with a bit of random light, which is then amplified by stimulated emission. This process creates light that is very pure and coherent. Laser light can be very narrow in width and can stay focused over long distances, though it will eventually spread out due to diffraction.

In 1963, Roy J. Glauber showed how laser light can be described using quantum physics, for which he won the Nobel Prize in Physics. Laser beams can have different shapes and properties depending on the design of the laser and its resonator.

Quantum vs. classical emission processes

The production of laser light relies on stimulated emission, a process predicted by Albert Einstein. This process involves electrons moving between energy levels in atoms or molecules. In some special lasers, like free-electron lasers, atomic energy levels are not used, and the process can be explained without quantum mechanics.

Modes of operation

A laser can work in two main ways: continuous or pulsed. In continuous mode, the laser beam’s power stays steady over time, which is useful for tasks needing a constant beam. In pulsed mode, the laser sends out bursts of light, which can be helpful for special jobs like cutting or researching very fast processes.

Some lasers can switch between these modes. For example, a continuous laser can be turned on and off to make pulses, though it’s still called a continuous-wave laser if the pulses are slow. Lasers that need to send out very short, powerful bursts use techniques like Q-switching or mode locking. These methods help create extremely short light pulses, useful for studying super-fast events in science.

History

In 1917, Albert Einstein described the basic idea for lasers and similar devices in a scientific paper. He explained how light could be amplified through a process called stimulated emission. Over the next few decades, scientists around the world built on this idea.

The first actual laser was created in 1960 by Theodore Maiman. He used a small piece of ruby crystal to produce the first beam of laser light. Since then, scientists have made many different types of lasers using various materials and methods, each with new and useful abilities.

Types and operating principles

Further information: List of laser types

Gas lasers

Main article: Gas laser

After the invention of the helium-neon gas laser, many other gases have been used to create lasers. The helium-neon laser can operate at many wavelengths, but most are designed to work at 633 nanometers. These lasers are common in schools and labs. Carbon dioxide lasers can emit powerful beams used in industries for cutting and welding. Argon-ion lasers can produce light at several colors, and nitrogen lasers are often used by hobbyists. Some gas lasers use metals and can create deep ultraviolet light.

Solid-state lasers

Main article: Solid-state laser

Solid-state lasers use materials like crystals or glass that are specially treated with atoms that give them energy. The first laser ever made was a ruby laser. Neodymium is a common material used in solid-state lasers and can produce strong infrared light. These lasers are used for cutting, welding, and medical procedures. Other materials like ytterbium and holmium are also used in solid-state lasers for different purposes.

Fiber lasers

Main article: Fiber laser

Fiber lasers use thin glass fibers to guide the laser light. These lasers can produce very powerful beams and are used in industries and medicine. The fibers help keep the laser cool and reduce distortions in the light beam.

Semiconductor lasers

Main article: Semiconductor lasers

Semiconductor lasers are small devices that use electricity to produce light. They are used in laser pointers, CD players, and many other devices. Some semiconductor lasers can produce very high power and are used in industries for cutting and welding. Scientists are also working on making lasers from silicon, which could be used in computers.

Uses

Main article: List of applications for lasers

When lasers were first invented, people wondered what they could be used for. Since then, lasers have become very common tools in many areas of life. They are used in consumer electronics, information technology, science, medicine, industry, law enforcement, entertainment, and the military. For example, lasers are important for fiber-optic communication, which helps send large amounts of data over long distances.

One of the first noticeable uses of lasers was in supermarket barcode scanners, introduced in 1974. Lasers are also found in many everyday devices, such as laser printers and CD players. They are used in many ways, including cutting and shaping materials, helping doctors in surgeries, guiding soldiers, and creating fun light shows.

Lasers have many important uses in medicine. They can help fix problems with the eyes, treat kidney stones, and improve the appearance of skin. Lasers are also used to treat some cancers by shrinking or destroying growths. This can be done with less pain and scarring than traditional surgery. However, doctors need special training to use these laser treatments.

Lasers can also be used as weapons. Some lasers are designed to damage or disable enemies, such as by causing temporary or permanent vision loss. International rules, like the Protocol on Blinding Laser Weapons, ban the use of weapons that cause permanent blindness. Some military groups use lasers that only temporarily blind or confuse enemies.

Safety

Even the very first laser was known to be dangerous. Even low-power lasers can be risky for your eyes if the light beam hits them directly or bounces off a shiny surface. The light from a laser can focus to a tiny spot on the retina, causing burns and permanent damage in just seconds.

Lasers are grouped into safety classes to show how dangerous they are:

• Class 1 lasers are safe because they are usually enclosed, like in CD players.
• Class 2 lasers are safe for normal use; your quick blink helps protect your eyes. These are often found in laser pointers.
• Class 3R lasers have a small risk of hurting your eyes if you stare at them too long.
• Class 3B lasers can harm your eyes right away.
• Class 4 lasers are very powerful and can burn skin and damage eyes, even from scattered light. Many industrial lasers fall into this category.

People who work with powerful lasers should wear special safety goggles to protect their eyes.