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Lasers

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

German: Laser

Category: lasers

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“Laser” (rarely written as l.a.s.e.r.) 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 principle of operation (exploiting stimulated emission from excited atoms or ions), the term is now mostly used for devices generating light based on the laser principle. More specifically, one usually means laser oscillators, but sometimes also includes devices with laser amplifiers, also called master oscillator power amplifier (MOPA).

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 Townes' group 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:

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

In laser technology, a wide range of optical components such as laser crystals, laser mirrors, polarizers, Faraday isolators and tunable optical filters are used; see the article on laser optics.

How a Laser Works

Basic Principle

A laser oscillator usually comprises an optical resonator (laser resonator, laser 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.

setup of an optically pumped laser
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 is then below the so-called laser threshold and only emits some luminescence light. Significant power output is achieved only for pump powers above the laser threshold, where the gain can reach the level of the resonator losses.

If the gain is larger than the losses, the power of the light in the laser resonator rises very rapidly, starting e.g. with low levels of light from fluorescence. Note that the resonator round-trip time is usually very small (e.g. a few nanoseconds, sometimes even much less), so that even a small net round-trip gain implies rapid exponential growth of the intracavity power. 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 (→ gain clamping). 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).

Coherence of Laser Radiation

A high degree of spatial coherence of the laser radiation can be achieved, essentially because the light emission is triggered (stimulated) by the intracavity radiation (i.e., the light is circulating in the laser resonator), rather than occurring spontaneously. In the stimulated emission process, the laser-active ions are made to emit light in the direction of already existing light, and also with the same optical phase. The resulting amplitude and phase profile of the laser beam is largely determined by the properties of the laser resonator, not by the laser gain medium.

Temporal coherence is a different issue, and it has completely different origins. Some laser gain media can emit light only in a narrow spectral range. Even if that is not the case, a laser often emits light only at a precisely defined wavelength or frequency, because the conditions are such that a net zero round-trip gain is possible only for that wavelength. A laser may be tuned to the exact wanted wavelength e.g. by using a tunable intracavity bandpass filter, such as a Lyot filter. Again, the mechanism of stimulated emission is of crucial importance: the laser-active ions can be made to emit at exactly the optical frequency of already existing light. The smaller the emission linewidth (i.e., the narrower the optical spectrum of emitted light), the higher the degree of temporal coherence.

Interestingly, even ultrashort pulses can exhibit a very high degree of temporal coherence, in that case involving coherence between subsequent pulses in a regular pulse train. This is related to the formation of a frequency comb as the optical spectrum.

Generation of Light Pulses

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). Often, a laser medium can accumulate some amount of energy over some “pumping” time in order to then release it within a much shorter time.

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 and Cr:ZnSe allow wavelength tuning over hundreds of nanometers.

Laser Noise

Due to various influences, the output of lasers always contains some noise in properties such as the output power or phase. For pulsed lasers, additional quantities can come into play, for example the timing jitter. For more details, see the article on laser noise.

Types of Lasers

Laser technology is a rather diverse field, utilizing a wide range of very different kinds of laser gain media, optical elements and techniques. Common types of lasers are:

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 not strictly lasers, but are nevertheless often called laser sources:

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 optical intensities achievable, but there are also other hazards related to laser sources. See the article on laser safety for details.

Laser Applications

There is an enormously wide range of applications for a great variety of different laser devices. They are largely based on various special properties of laser light, many of which cannot be achieved with any other kind of light sources. See the article on laser applications for an overview.

On the other hand, many potential laser applications cannot be practically realized so far because lasers are relatively expensive to make – or more precisely, because they are so far mostly made with relatively expensive methods. Most lasers are fabricated in relatively small volumes and with a limited degree of automation. Another aspect is that lasers are relatively sensitive in various respects, for example concerning the precise alignment of optical components, mechanical vibrations and dust particles. Therefore, there is ongoing research and development for finding more cost-effective and robust solutions.

For business success, it is often vital not just to develop lasers with high performance and low cost, but also to identify the best suited applications, or develop lasers which are best suited for particular applications. Also, the knowledge of the application details can be very important. For example, in laser material processing it is vital to know the exact requirements in terms of laser wavelength, beam quality, pulse energy, pulse duration etc. for optimum processing results.

Suppliers

The RP Photonics Buyer's Guide contains 267 suppliers for lasers. Among them:

Bibliography

[1]A. L. Schawlow and C. H. Townes, “Infrared and optical masers”, Phys. Rev. 112 (6), 1940 (1958) (ground-breaking work; also contains the famous Schawlow–Townes equation)
[2]T. H. Maiman, “Stimulated optical radiation in ruby”, Nature 187, 493 (1960) (first experimental demonstration of a laser)
[3]T. H. Maiman, “Optical maser action in ruby”, Br. Commun. Electron. 7, 674 (1960)
[4]P. P. Sorokin and M. J. Stevenson, “Stimulated infrared emission from trivalent uranium”, Phys. Rev. Lett. 5 (12), 557 (1960) (the first four-level laser)
[5]A. Javan, W. R. Bennett Jr., and D. R. Herriott, “Population inversion and continuous optical maser oscillation in a gas discharge containing a He–Ne mixture”, Phys. Rev. Lett. 6 (3), 106 (1961)
[6]G. Smith, “The early laser years at Hughes Aircraft Company”, IEEE J. Quantum Electron. 20 (6), 577 (1984)
[7]R. E. Slusher, “Laser technology”, Rev. Mod. Phys. 71, S471 (1999)
[8]J. M. Gill, “Lasers: a 40-year perspective”, IEEE J. Quantum Electron. 6 (6), 1111 (2000)
[9]"Bright idea: the first lasers", American Institute of Physics (2010)
[10]J. Hecht, “Short history of laser development”, Opt. Eng. 49, 091002 (2010)
[11]A. E. Siegman, Lasers, University Science Books, Mill Valley, CA (1986)
[12]O. Svelto, Principles of Lasers, Plenum Press, New York (1998)
[13]F. Träger (ed.), Handbook of Lasers and Optics, Springer, Berlin (2007)
[14]R. Paschotta, Field Guide to Lasers, SPIE Press, Bellingham, WA (2007)

(Suggest additional literature!)

See also: laser light, laser optics, laser applications, laser physics, laser design, laser resonators, laser crystals, gain media, diode-pumped lasers, lamp-pumped lasers, solid-state lasers, fiber lasers, waveguide lasers, upconversion lasers, semiconductor lasers, gas lasers, molecular lasers, X-ray lasers, mode-locked lasers, Q-switched lasers, visible lasers, laser threshold, slope efficiency, laser noise, linewidth, coherence, wavelength tuning, laser safety, laser specifications
and other articles in the category lasers


Dr. R. Paschotta

This encyclopedia is authored by Dr. Rüdiger Paschotta, the founder and executive of RP Photonics Consulting GmbH. How about a tailored training course from this distinguished expert at your location? Contact RP Photonics to find out how his technical consulting services (e.g. product designs, problem solving, independent evaluations, training) and software could become very valuable for your business!

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