Editorial Feature

How Does a Laser Work?

The LASER (Light Amplification by Stimulated Emission of Radiation) is one of the most ubiquitous and versatile pieces of technology in the modern world. They are crucial in fields as diverse as data storage, vision-correcting surgery, displays, high-precision measurements and surveying, and even nuclear fusion research.

Lasers take advantage of a quantum effect called "stimulated emission" to produce a beam of coherent, monochromatic light - the light waves which make up the beam oscillate perfectly in sync.

This is in contrast to most other light sources, which produce unfocused light over a broad range of wavelengths - beams of laser light are extremely straight and bright as a result. The special properties of laser light open up many applications which ordinary light is unsuitable for.

Figure 1. Minute Physics outline the basic principles of lasers, and explain how stimulated emission works.

Discovery of Lasers

The concept of using stimulated emission to generate a beam of tightly focused, coherent electromagnetic radiation was first brought to realisation in 1954, although our understanding of the physical principles involved had been developing for some decades before that point, beginning with Einstein in 1917.

Charles Townes at Columbia University in New York, and, independently, Dr. Basov and Dr. Prochorov at the Lebedev Institute in Moscow, developed the first "MASER" - effectively a laser which emits microwave radiation instead of visible light. All three scientists were rewarded with a Nobel Prize.

By 1960, the available technology had progressed far enough to allow the first visible-light laser to be developed - the energy required to excite a light-emitting material is much higher than the amount required for a maser. Theodore Maiman built the first ruby laser at the Hughes Research Laboratories, which emitted a pulsed beam of laser light at 694.3 nm.

Structure and Operation of a Laser

The most basic laser consists of a gain medium within an optically resonant cavity with a mirror at both ends.

When the particles in the gain medium are energized by an electrical current, an ordinary light source, or even another laser, the electrons are pumped into a temporary excited state. When this state decays, the particle emits a photon of a specific wavelength.

Across the whole laser, this produces a very large number of photons of the same wavelength. the net effect of billions of photon-photon collisions inside the material is that the photons become like clones of each other - they all travel in the same direction, oscillating in perfect synchronization at the same frequency.

The frequency of the laser light is dependent on the gain medium, which can be a crystal, a semiconductor, a gas, or even a coloured liquid. The material used for a particular laser depends on the wavelength of light required for the application.

The photons emitted by particles in the gain medium will collide with more particles, stimulating more emission - once a certain threshold of energy input has been passed, more emission than absorption occurs, and the light is amplified.

The system of mirrors reflects the emitted light backwards and forward through the medium, increasing its intensity with each pass. After some time, the energy of the light levels off, and the light becomes true laser light.

One of the mirrors at the end of the laser cavity is semi-silvered - this reflects most of the light back into the cavity (typically 1 - 5%), whilst letting some of it exit. Most laser designs include an optical switch, which only allows the light to leave the cavity once the gain has become saturated.

Figure 3.Energy level diagram for a ruby laser. Blue and green light from the broad-spectrum pumping light excites electrons in the ruby's chromium ions to broad 4F states. The electrons then decay via a metastable 2E doublet state to produce photons with a wavelength of 694.3 nm.

Table 1. Some common laser materials and their output wavelengths

Gain Medium Type Wavelength Applications
CO2 Gas 10.6 µm Surgery, laser cutting/marking, welding
InGaAsP / InP Semiconductor 1000-1650 nm Fibre optic communications
Titanium-doped sapphire Crystal 650-1100 nm Spectroscopy, photonics, research
Ruby Crystal 694.3 nm Rangefinding, holographic materials testing
AlGaInP/GaAs Semiconductor 635-670 nm DVD players, laser pointers
He-Ne Gas 632.8 nm Alignment, interferometers
Rhodamine 6G Dye 566 nm Tuneable lasers, spectroscopy
InGaN/GaN on SiC Semiconductor 380- 470 nm Data storage

Pulse vs. Continuous Wave Lasers

Pulse Lasers

In a pulse laser, such as a ruby laser, once the light is allowed to escape from the laser cavity, the energy which had built up in the laser is drained in a short pulse of light which only lasts for a fraction of a second.

The laser then immediately begins to recharge, emitting regular, short pulses, which can be extremely intense. The most powerful pulse lasers, used in research facilities attempting to ignite a fusion reaction, can output trillions of watts of peak power.

Continuous Wave Lasers

Some lasers can operate in "continuous wave" mode, emitting a steady stream of light rather than pulses. The most common continuous wave lasers are Helium-Neon lasers and dye lasers. Not all gain media are amenable to continuous operation, however. The laser must be constantly recharged, which can damage the material if the power required to keep the laser charged is too high.

References and Further Reading

Will Soutter

Written by

Will Soutter

Will has a B.Sc. in Chemistry from the University of Durham, and a M.Sc. in Green Chemistry from the University of York. Naturally, Will is our resident Chemistry expert but, a love of science and the internet makes Will the all-rounder of the team. In his spare time Will likes to play the drums, cook and brew cider.

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