How do lasers work?

Fundamentals of lasers explained

Dec 19, 2021

Focus like a laser, not a flashlight - Michael Jordan

Greetings, fellow Bohron

A laser is a device that emits a narrow and focused beam of radiation. The term LASER is short for Light Amplification by Stimulated Emission of Radiation. Laser light differs greatly in its properties from ordinary light-emitting sources like incandescent light bulbs.

Comparison of light emitted by ordinary sources and lasers. *Source*

This cutting-edge technology is present all around us inside laser printers, barcode scanners, fibre optic cables and CD players. Because of the enormous amount of energy that can be plugged into a focused beam of radiation, lasers are employed for a wide variety of applications like cutting, welding, blasting, eye surgery, tattoo removal and missile defence systems.

Lasers and cats are a pure source of entertainment. *Source*

Development

Starting from Albert Einstein’s idea of stimulated emission in 1917, the dream of developing a device that could emit a monochromatic, coherent beam of radiation became a reality after 4 decades of detailed research by numerous physicists around the world.

Einstein(top-left), Townes(top-second from right), Maiman(bottom-right) and several others contributed to the invention of LASER.

The first-ever coherent beam of electromagnetic radiation was produced using an ammonia maser(microwave counterpart of laser) developed by Charles Townes, James Gordon and Herbert Zeiger in 1953 at Columbia University.

On 16 May 1960, American physicist Theodore Maiman operated the first-ever laser at Hughes Research Laboratories in California. It consisted of a ruby cylinder with its ends silver-coated to make them reflected. Photographic flashlamps were used as the pumping source.

Theodore Maiman with the first working laser. *Source*

Physics of Lasers

Before you can understand its functioning, it is better to know about two terms used frequently in optics:

Monochromatic: A beam of visible light is said to be monochromatic if it consists of a single colour or frequency.

Different colours are present in sunlight and hence it isn’t monochromatic. *Source*

Coherent: Two waves are said to be coherent if they are in phase with each other and have the same frequency.

A representation of coherent laser light waves. *Source*

Lasers require two things to function: a lasing medium and a pumping source. The laser beam is transmitted using a laser medium such as gas, crystal or diode. Then energy is pumped into the medium from outside using a pumping source like electricity or chemical reaction. This excites the atoms in the lasing medium and electrons absorb energy to jump to outer electron shells.

If more photons are incident on these unstable excited atoms, they cause the atoms to decay down to lower energy states with the release of more photons.

If two mirrors are placed at the ends of the laser tube, these photons do not escape quickly. They bounce back and forth and cause further stimulated emission in other atoms of the lasing medium, hence producing an avalanche of monochromatic and coherent photons in the process. This creates a very narrow beam of laser light that diverges hardly at all.

Classification

Depending upon the lasing medium and ways of pumping energy into the medium, lasers are classified as follows:

Gas Laser

Lasing Medium: Gases like helium and neon or carbon dioxide

Pumping Source: Radio waves or electricity

They are weak lasers and produce red beams. CO₂ lasers, however, are powerful and can be used for welding and cutting purposes. He-Ne lasers are the most common gas lasers.

Chemical Laser

A Chemical Oxygen Iodine Laser(COIL) system mounted on a Boeing 747. *Source*

Lasing Medium: Gas

Pumping Source: Chemical Reaction

They are powerful lasers capable of can generating megawatts of power. They have been utilized in military applications like shooting down short-range missiles. Examples include deuterium fluoride and hydrogen fluoride lasers.

Solid-State Laser(SSL)

Nd:YAG laser producing a 532 nm frequency green light. *Source*

Lasing Medium: Glass or crystals with added impurity atoms

Pumping Source: Flashlamps or diode laser

The first laser consisted of a ruby crystal. SSLs produce high energy ultrashort pulses of laser light. They are used in eye surgery, metal processing, laser printers and as optical weapons in F-35 jets. In Nd:YAG laser, the host material yttrium aluminium garnet (YAG) is doped with a tiny fraction of neodymium.

Semiconductor lasers, excimer lasers and dye lasers are other prominently used lasers.

When laser first came out in 1960, Maiman dubbed his invention as “a solution seeking a problem" because, at that time, very few of its applications were known. Science fiction popularized these devices when they were featured as death stars and ray guns in Star Wars and everyone wondered if such a destructive technology could become a reality in future. We will find this out next time.