Lasers are devices that produce intense beams of monochromatic, coherent radiation.  The original term "laser" (dating to about 1960) was an acronym for "light amplification by stimulated emission of radiation", although its original meaning has generally been overshadowed by regular use of the lowercase term. This original acronym, however, provides an excellent description of a laser.

Lasers are essentially specialized light sources which emit a narrow beam of coherent light. It is important to note that the term light does not only refer to visible light, but is more accurately used to represent electromagnetic radiation of any kind; lasers are capable of emitted non-visible (ultraviolet, infrared, etc.) radiation as well.

Coherence represents an ideal state for any wave and exists in two forms: temporal and spatial. While coherence is a complicated subject, it may be oversimplified by stating that a fully coherent wave is fully in phase with itself. When discussing lasers, coherence represents the ability of a wave function — which describes the action of photon particles — to interfere with other wave functions. (In this case, this type of interference is desirable.)

The atoms and molecules that make up all matter can be excited from a low to a high energy state (usually) by heat and emit light as they return to their low energy state. If excited by an ordinary light source, atoms radiate light of many different wavelengths and at different times. If, however, light of an exact wavelength is passed through the atom at the exact instance that the atom is excited, the atom is stimulated to emit and amplify this new radiation; this resulting radiation is temporally and spatially coherent. The latter concept effectively describes a laser's operating mechanism and is actually represented by the last three letters of the laser acronym: "stimulated emission of radiation."

A laser's production of this coherent beam is important for a number of reasons relating to their many applications:

• It allows light to be focused on a very tight spot.
• Spatial coherence keeps the beam collimated (in-line) over long distances.
• Temporal coherence allows a focus on a very narrow wavelength and permits very rapid pulsing.

Design / Construction

All lasers consist of three main components: an energy source (also referred to as pump or pump source), a gain medium (or laser medium), and an optical resonator formed by two or more optical lenses. The image of a deconstructed laser pointer below illustrates these three components; more information about each component can be found below the image.

In the case of the image:

• The laser diode functions as the energy source (pump).
• The gain medium is a neodymium / yttrium vanadium oxide crystal (represented by the label Nd: YVO).
• An expanding lens, collimating lens, and IR filter function collectively as the optical resonator.

Image credit: Sam's Laser FAQ

Energy Source / Pump

A laser's energy source supplies it with light. Common sources include electrical discharges, lamps, another laser, and chemical reactions. A laser's pump source depends upon its gain medium; for example, an Nd: YAG laser would be supplied by a laser diode or flash lamp, while a HeNe device is supplied by an electrical discharge.

Gain Medium

Most properties of a laser are determined by its gain medium, and for this reason the gain medium defines the laser's type and application. Hundreds of materials are suitable as laser media, including:

• Liquids (dyes) — A liquid solution's chemical configuration determines the laser's wavelength.
• Gases — carbon dioxide, argon, krypton, helium-neon, etc. Gases are typically pumped by electrical discharge.
• Solids — crystals and glasses such as yttrium aluminum garnet (YAG) and sapphires. The crystal/glass host is doped with one of various impurities to render it suitable as a gain medium.
• Semiconductors — movement of electrons across diode junctions causes laser action.

Optical Resonators

The optical resonator, also known as the optical cavity, refers to the area between the gain medium and the laser output. The cavity may contain several components, including:

Two or more optical mirrors close to the gain medium. The mirrors reflect transmitted light back into the gain medium — sometimes hundreds of times — in order to suitably amplify it for output. The mirror nearest the output is known as the output coupler, as it allows some light to leave the cavity, effectively creating the laser's output beam.

A variety of other optical components — such as filters, lenses, mirrors, and modulators — that determine the laser's effect. In the laser pointer image above, note that the expanding lens and collimating lens work to shape the laser's output, while the IR filter removes any infrared radiation from the beam.

Applications

For years after their development in the late-1950s, lasers were seen as novelty devices unsuited to serious technological application. Since then, they have become ubiquitous in nearly every industry for an enormous range of uses. Some industries and specific applications are listed below.

Commercial — laser printing, optical drives, thermometers, barcode scanning

Defense — as a blinding weapon, target marking, tracking, missile defense, weapons guiding

Industrial — cutting, welding, marking

Medical/healthcare — surgery, dentistry, eye treatment, skin treatment hair removal

Research — spectroscopy, microscopy, scattering, microdissection

Specifications

Important specifications for lasers include laser type, wavelength and output modes.

Types

Lasers are typically classified by their gain medium, although some are specified by their intended application. For example, an alignment laser is any laser used to create a precise reference point or line for aligning machines.

The table below characterizes many common laser types.

Wavelength

The visible spectrum, showing the relative positions of the infrared and ultraviolet ranges.  Image credit: Suffolk University

As discussed above, a laser's wavelength is determined by its gain medium. Its wavelength also determines the color of the laser's output beam. Common colors/wavelengths include:

• Ultraviolet (1 to 390 nm)
• Violet (390 to 455 nm)
• Blue (455 to 492 nm)
• Green (492 to 577 nm)
• Yellow (577 to 597 nm)
• Orange (597 to 622 nm)
• Red (622 to 780 nm)
• Infrared (.78 to 1000 μm)

Lasers which are specified as tunable can be adjusted — via an external cavity — to emit one of several different wavelengths, effectively rendering them several lasers within the same package.

Output Modes

A laser may be specified as having one of several output types.

A device with continuous output simply emits a beam continuously.

Pulsed output consists of a series of single emissions which occur in sequence.

Q-switched emission (sometimes called giant pulse formation) is a specialized type of pulsing in which each pulse has a much higher peak power than a continuous mode laser. Q-switching results from the addition of an attenuator in the optical resonator so that the output beam is not allowed to emit until it achieves a high level of optical gain and energy storage, resulting in a high power emission.

A pulsed laser in operation. Video credit: arijitc123

Standards

Lasers may be manufactured, tested, and used based on a number of standards. Because of the very wide application of lasers across numerous industries, related standards are particularly numerous and varied. Examples of laser standards include:

ANSI Z136.1 - Safe use of lasers

AWS C7.2M - Recommended practices for laser welding, cutting, etc.

BS EN 60825-1 - Safety of laser products

ISO 17526 - Optics and optical instruments Lasers and laser-related equipment - lifetime of lasers

References

Columbia University - Properties of Laser Beams

Robert Aldrich - Laser Fundamentals

Rockwell Laser - Laser Standards and Classifications

Image credits:

World StarTechnologies | OSI Optoelectronics