Physics and history

The range of sizes in which lasers exist is immense, extending from microscopic diode lasers (top) to football field sized neodymium glass lasers (bottom) used for inertial confinement fusion.The first working laser was made by Theodore H. Maiman in 1960 at Hughes Research Laboratories in Malibu, California, beating several research teams including those of Charles Townes at Columbia University, and Arthur L. Schawlow at Bell laboratories. Maiman used a solid-state flashlamp-pumped synthetic ruby crystal to produce red laser light at 694-nanometres wavelength. In the same year the Iranian physicist Ali Javan invented the gas laser. He later received the Albert Einstein Award.

The basic physics of lasers centres around the idea of producing a population inversion in a laser medium by "pumping" the medium; i.e., by supplying energy in the form of light or electricity, for example. The medium may then amplify light by the process of stimulated emission. If the light is circulating through the medium by means of a cavity resonator, and the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise exponentially. Eventually it will get so strong that the gain is saturated (reduced). In continuous operation, the intracavity laser power finds an equilibrium value which is saturating the gain exactly to the level of the cavity losses. If the pump power is chosen too small (below the "laser threshold"), the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers.

Population inversion is also the concept behind the maser. The first maser was built by Charles H. Townes and graduate students J. P. Gordon, and H. J. Zeiger in 1953. Townes later worked with Arthur L. Schawlow to describe the theory of the laser, or optical maser as it was known. The word laser was coined in 1957 by Gordon Gould. Gordon also coined the words iraser, intending "aser" as the suffix and the spectra of light emitted at as the prefix (examples: X-ray laser = xaser, UltraViolet laser = uvaser) but these terms never became popular. Gordon was also credited with lucrative patent rights for a gas-discharge laser in 1987, following a protracted 30 year legal battle.

The first maser, developed by Townes, was incapable of continuous output. Nikolai Basov and Alexander Prokhorov of the USSR worked independently on the quantum oscillator and solved the problem of continuous output systems by using more than two energy levels. These systems could release stimulated emission without falling to the ground state, thus maintaining a population inversion. In 1964, Charles Townes, Nikolai Basov and Alexandr Prokhorov shared a Nobel Prize in Physics "for fundamental work in the field of quantum electronics, which has led to the construction of oscillators and amplifiers based on the maser-laser principle."

A further note on the terminology is necessary. As laser stands for light amplification by stimulated emission of radiation, it should be understood that the word light is here meant in the expansive sense, as photons of any energy; not as simply photons in the visible spectrum. Hence there are X-ray lasers, IR-lasers, UV-lasers, microwave lasers, radio lasers, etc. However, microwave lasers and radio lasers are usually called masers in the modern terminology. In addition, some argue that all lasers can be thought of as masers (see the article on maser for details).

Additionally, laser light can be highly intense—able to cut steel and other metals. While the beam emitted by a laser often has a very small divergence (highly collimated), a perfectly collimated beam cannot be created, due to the effect of diffraction. Nonetheless, a laser beam will spread much less than a beam of light generated by other means. A beam generated by a small laboratory laser such as a helium-neon (HeNe) laser spreads to approximately 1 mile (1.6 kilometres) in diameter if shone from the Earth's surface to the Moon. Some lasers, especially semiconductor lasers due to their small size, produce very divergent beams. However, such a divergent beam can be transformed into a collimated beam by means of a lens. In contrast, the light from non-laser light sources cannot be collimated by optics as well or much. Using a waveguide such as an optical fibre though, diffraction laws governing divergence no longer apply. Other interesting effects happen in nonlinear optics.

Graph showing the history of maximum laser pulse intensity throughout the past 40 years.An unforeseen discovery in 1992, lasing without maintaining the medium excited into a population inversion, was discovered in sodium gas in and again in 1995 each in sodium and rubidium gas by various international teams. Normally, electrons in the ground state absorb the pumping and emitted radiation, thwarting the laser gain by heating up the medium. So media with electron levels and transitions amenable to the driving current are desired, and generally those which involve three or four energy levels rather than two make better lasers because the electrons are kept above the ground state, excited, and optically-transparent so as not to heat up, but such media are prone to noisy beams. By using an external maser to induce "optical transparency" in the media by introducing and destructively interfering the ground electron transitions between two paths, the likelihood for the ground electrons to absorb any energy has been cancelled. Though there were initial hopes that this discovery would allow an increase in efficiency (higher than the .01 to .3 for typical media and wavelengths), the idea never panned out commercially or otherwise and remains little more than a backwater in laser research.

In 1985 at the University of Rochester's Laboratory for Laser Energetics a breakthrough in creating ultrashort-pulse, very high-intensity (terawatts) laser pulses became available using a technique called chirped pulse amplification, or CPA, discovered by Gérard Mourou. Later, in 1994, it was discovered by Mourou and his team at University of Michigan that the balance between the self-focusing refraction (see Kerr effect) and self-attenuating diffraction by ionization and rarefaction of a laser beam of terawatt intensities in the atmosphere creates "filaments" which act as waveguides for the beam thus preventing divergence. If a light filament drops below the intensity needed for this dynamic balance, called modulation instability, it can merge with another filament and continue propagating without broadening as with all earlier means of sending light. The filaments, having made a plasma, though turn the narrowband laser pulse into a broadband pulse having a wholly new set of applications

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