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Encyclopedia of Laser Physics and Technology

© Dr. Rüdiger Paschotta

Last update: 2008-05-08

Definition: devices generating visible or invisible light, based on stimulated emission of light

"Laser" is an acronym for "Light Amplification by Stimulated Emission of Radiation", coined in 1957 by the laser pioneer Gordon Gould. Although this original meaning denotes an operation principle, the term is now mostly used for devices generating light based on the laser principle. The first laser device was a pulsed ruby laser, demonstrated by Theodore Maiman in 1960 [2,3]. In the same year, the first gas laser (a helium neon laser, [5]) and the first laser diode were made. Before this experimental work, Arthur Schawlow, Charles Hard Townes, Nikolay Basov and Alexander Prokhorov had published ground-breaking theoretical work on the operation principles of lasers, and a microwave amplifier and oscillator (maser) had been developed by the group of Townes in 1953. The term "optical maser" (MASER = microwave amplification by stimulated amplification of radiation) was initially used, but later replaced with "laser".

Laser technology is at the core of the wider area of photonics, essentially because laser light has a number of very special properties:

• It is usually emitted as a laser beam which can propagate over long lengths without much divergence and can be focused to very small spots.
• It can have a very narrow bandwidth, whereas e.g. most lamps emit light with a very broad spectrum.
• It may be emitted continuously, or alternatively in the form of short or ultrashort pulses, with durations from microseconds down to a few femtoseconds.

These properties, which make laser light very interesting for a range of laser applications, are to a large extent the consequence of the very high coherence of laser radiation. The articles on laser light and laser applications give more details.

How a Laser Works

The operation principles and basic components of a laser are explained in the following. A laser usually comprises an optical resonator (laser resonator, cavity) in which light can circulate (e.g. between two mirrors), and within this resonator a gain medium (e.g. a laser crystal), which serves to amplify the light. Without the gain medium, the circulating light would become weaker and weaker in each resonator round trip, because it experiences some losses e.g. upon reflection at mirrors. However, the gain medium can amplify the circulating light, thus compensating the losses if the gain is high enough. The gain medium requires some external supply of energy – it needs to be "pumped" e.g. by injecting light (optical pumping) or an electric current (electrical pumping → semiconductor lasers). The principle of laser amplification is stimulated emission.

Figure 1: Setup of a simple optically pumped laser. The laser resonator is made of a highly reflecting curved mirror and a partially transmissive flat mirror, the output coupler, which extracts some of the circulating laser light as the useful output. The gain medium is a laser crystal, which is side-pumped e.g. with light from a flash lamp.

A laser can not operate if the gain is smaller than the resonator losses; the device then only emits some fluorescence light. A higher gain can result from stronger pumping. Significant power output is achieved only for pump powers above the so-called laser threshold, where the gain can exceed the resonator losses.

As long as the gain is larger than the losses, the power of the light in the laser resonator quickly rises, starting e.g. with low levels of light from fluorescence. As high laser powers saturate the gain, the laser power will in the steady state reach a level so that the saturated gain just equals the resonator losses. Before reaching this steady state, a laser usually undergoes some relaxation oscillations. The threshold pump power is the pump power where the small-signal gain is just sufficient for lasing.

Some fraction of the light power circulating in the resonator is usually transmitted by a partially transparent mirror, the so-called output coupler mirror. The resulting beam constitutes the useful output of the laser. The transmission of the output coupler mirror can be optimized for maximum output power (see also: slope efficiency).

Some lasers are operated in a continuous fashion, whereas others generate pulses, which can be particularly intense. There are various methods for pulse generation with lasers, allowing the generation of pulses with durations of microseconds, nanoseconds, picoseconds, or even down a few femtoseconds (→ ultrashort pulses from mode-locked lasers).

The optical bandwidth (or linewidth) of a continuously operating laser may be very small when only a single resonator mode can oscillate (→ single-frequency operation). In other cases, particularly for mode-locked lasers, the bandwidth can be very large – in extreme cases, it can span about a full octave. The center frequency of the laser radiation is typically near the frequency of maximum gain, but if the resonator losses are made frequency-dependent, the laser wavelength can be tuned within the range where sufficient gain is available. Some broadband gain media such as Ti:sapphire or Cr:ZnSe allow wavelength tuning over hundreds of nanometers.

Due to various influences, the output of lasers always contains some noise in properties like the output power or optical phase.

Types of Lasers

Common types of lasers are:

• Semiconductor lasers (mostly laser diodes), electrically (or sometimes optically) pumped, efficiently generating very high output powers (but typically with poor beam quality), or small powers with good spatial properties (e.g. for application in CD and DVD players), or pulses (e.g. for telecom applications) with very high pulse repetition rates. Special types: quantum cascade lasers (for mid-infrared light), surface-emitting semiconductor lasers (VCSELs and VECSELs), the latter also being suitable for pulse generation with high powers.
• Solid-state lasers based on ion-doped crystals or glasses, pumped with discharge lamps or laser diodes, generating high output powers, or lower powers with very high beam quality, spectral purity, and/or stability (e.g. for measurement purposes), or ultrashort pulses with picosecond or femtosecond durations. Common gain media are Nd:YAG, Nd:YVO₄, Nd:YLF, Nd:glass, Yb:YAG, Yb:glass, Er:Yb:glass, Ti:sapphire, Cr:YAG, Cr:LiSAF, and Cr:LiCaF. A special type of ion-doped glass lasers are:
• Fiber lasers, based on optical glass fibers which are doped with some laser-active ions in the fiber core. Fiber lasers can achieve extremely high output powers (up to kilowatts) with high beam quality, allow for widely wavelength-tunable operation, narrow linewidth operation, etc.
• Gas lasers (e.g. helium-neon lasers, CO₂ lasers, and argon ion lasers) and excimer lasers, based on gases which are typically excited with electrical discharges. Frequently used gases include CO₂, argon, krypton, and gas mixtures such as helium/neon. Common excimers are ArF, KrF, XeF, and F₂.

Less common are chemical and nuclear pumped lasers, free electron lasers, and X-ray lasers.

Laser Sources in a Wider Sense

There are some light sources which are strictly speaking not lasers, but are nevertheless often called laser sources:

• In some cases, the term is used for amplifying devices emitting light without an input (excluding seeded amplifiers). One example are X-ray lasers, which are usually superluminescent sources, based on spontaneous emission followed by single-pass amplification. There is then no laser resonator. A similar situation occurs for optical parametric generators, where however the amplification is not based on stimulated emission. Light from such devices can have laser-like properties, such as strongly directional emission and a small optical bandwidth.
• In other cases, the term laser sources is justified by the fact that the source contains a laser, among other components. This is the case for sources based on nonlinear frequency conversion of laser radiation, e.g. in frequency doublers or optical parametric oscillators.

Safety Aspects

The work with lasers can raise significant safety issues. Some of those are directly related to the laser light, in particular to the high achievable optical intensities, but there are also other hazards related to laser sources. See the article on laser safety for details.

Bibliography

See also: laser light, laser applications, laser physics, laser design, photonics, cavities, laser crystals, gain media, diode-pumped lasers, lamp-pumped lasers, laser threshold, slope efficiency, laser noise, linewidth, coherence, wavelength tuning, solid-state lasers, fiber lasers, waveguide lasers, upconversion lasers, semiconductor lasers, gas lasers, X-ray lasers, mode-locked lasers, Q-switched lasers, visible lasers, laser safety

Category: lasers