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The scientific principle underlying laser physics was first developed by Albert Einstein in 1916. The word LASER is actually an acronym that stands for Light Amplification by Stimulated Emission of Radiation.
To understand what this means, take an ordinary light bulb. The light this bulb produces is caused by the “spontaneous emission” of photons. (Photons are particles of light energy.) When electricity enters the filament of a light bulb, the electrons of the resting atoms that make up the filament are excited by the electrical energy. The electrons of the filament’s atoms are excited (energized) by the electrical current, but as some absorb energy others will lose that extra energy, or “decay”—and when the electrons spontaneously decay they emit photons, or light. Each electron that spontaneously decays emits one photon of light.
All laser devices produce what is called “stimulated emission” of light. In stimulated emission, each time a photon impacts an already excited atom, that photon energy causes the atom to emit two photons, not just one (because the first photon is not absorbed, and that energy is passed on to excite other atoms). The two photons from first atom will collide with (and add energy to) two other excited atoms of the medium in the laser, and the four electrons stimulated in these two atoms will then emit four photons of light energy, which will then stimulate photon emission from eight other excited atoms, and so on. This geometrical increase in photon emissions by energized atoms is called “light amplification by stimulated emission of radiation”, or LASER.
The schematic diagram above shows how the interior of a laser is constructed. The first working laser device was made in the United States in 1960 by Theodore Maiman of the Hughes Aircraft Research Laboratory in Malibu, California. The energy source (for instance, electricity in low-level lasers) pumps energy into the resonating chamber containing the amplifying medium. The resting atoms of the medium (in the form of gas, solid state, or semiconductor) become excited as they absorb the incoming energy and begin to spontaneously emit photons. As these photons strike other excited atoms, stimulated emission begins.
One end of the resonating chamber is a fully reflecting mirror while the other end is a partially reflecting mirror. The elongated chamber and the mirror arrangement can do three things: 1) they increase the stimulation process and thereby amplify the intensity of the light being produced; 2) they allow the laser light to emerge in a parallel manner as a highly focused beam, and; 3) they cause the emitted light to be coherent. (The significance of coherent light is explained below.)
As the medium in the resonance chamber becomes more and more energized the number of excited atoms exceed the number of atoms that are not energized (resting). At that point, stimulated emission of light from this reservoir of charged atoms becomes “sustained emission” (the technical term is “population inversion”) and the laser is now fully functional.
The Importance of Coherent Light
Lasers are different from regular light sources and sunlight because they produce coherent light as compared with the non-coherent light from other sources. The coherent light energy from lasers is highly organized energy. The photon packets of light energy are well-ordered and the light waves are highly synchronized. (According to the findings of quantum physics, light can be understood to travel as both photons (particles) and waves.) Because the atoms in the resonance chamber are all vibrating at the same rate, laser light is monochromatic, which means that the light vibrates within a very narrow bandwidth (a more specific wavelength or frequency). The narrow bandwidth is necessary to the production of coherent light.
Coherent light is able to travel relatively long distances without spreading out compared with non-coherent light. Coherent light is therefore able to penetrate the skin of the body and affect deeper tissues; a higher percentage of non-coherent light is either reflected or absorbed by the skin. Therefore, the therapeutic effects of light energy in the body (see How Soft Laser Light Works in the Body) are more effectively transmitted by coherent laser light.