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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 principle of operation, 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 ) 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: * 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 applications, are to a large extent the consequences of the very high degree of coherence of laser radiation. The articles on laser light and laser applications give more details. How a Laser Works
A laser 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.
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-calledlaser threshold and only emits some luminescence light. Significant power output is achieved only for pump powers above the laser threshold, where the gain can exceed the resonator losses. If 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 (→ 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...
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