How Things Work: Lasers
A laser is a device that can read data from a CD, provide a light spectacle, and even produce a concentrated, coherent beam of light capable of cutting through materials.
A laser (Light Amplification by Stimulated Emission of Radiation) is essentially an energy emitter.
Atoms have different energy levels, or orbits, containing electrons. When these electrons are in their lowest energy orbit, the atom is considered to be in its ‘ground state.’ When an atom absorbs energy, its electrons transition from lower-energy orbits to higher-energy orbits. When these electrons return to their lowest energy orbit, they produce energy that takes the form of light or, in this case, a laser beam.
Lasers ‘feed’ atoms energy, causing electrons in the atoms to transition to higher energy orbits. The source that feeds atoms, called the source of excitation, may be a light or another laser. The source emits energy in pulses or a continuous wave.
The material that absorbs energy from the source of excitation is called the lasing medium. The wavelength of light depends on this material, which varies from laser to laser. Alexandrite lasers, for instance, use gemstones as their lasing mediums, whereas dye lasers use dyes that are dissolved in a solvent.
This resulting wide range of light wavelengths allows lasers to be used in a variety of disciplines, including medicine, tattoo removal, and electronics. Carbon dioxide lasers, for example, emit infrared light that is hot enough to melt steel. A ruby red laser, on the other hand, emits visible light that is suitable for removing freckles.
Laser beams have unique characteristics when compared to light emitted from a light bulb. First, laser beams are monochromatic. The color depends on the amount of energy produced by the atom’s electrons. Second, lasers produce light waves that are coherent, or in phase with one another. In other words, the parallel light waves travel in sync. Last, a laser produces light with a concentrated beam. This beam is produced by a process known as ‘stimulated emission.’
In stimulated emission, an atom emits a photon of energy, which interacts with another atom of the same material, causing it to emit a second photon that has the same amount of energy as the first photon. This process propagates throughout the material, leading to a cascade of energy emissions.
Lasers also contain mirrors that reflect the photons back and forth. As the photons reflect between the two mirrors, more electrons return to their lowest energy level, thereby producing more light. Light passes through one of the mirrors to exit the laser.
In medicine, lasers operate based on the technique of selective photothermolysis. This technique involves the removal of target tissues and preservation of surrounding tissues by using the appropriate energy and wavelength of laser light.
In vision correction, for instance, excimer lasers (which produce ultraviolet light) are used to eliminate corneal tissue around the eye. This procedure improves the eye’s ability to focus light. To correct nearsightedness, excimer lasers flatten the cornea, moving the eye’s focal point closer to the retina. In farsighted vision correction, excimer lasers create a steeper slope about the cornea, moving the eye’s focal point closer to the retina.
Dye lasers are used to remove vascular lesions. One type of vascular lesion is a port-wine stain, which is a deformed part of a blood vessel that connects arteries to veins. These deformed blood vessels appear during birth, but they can be removed by dye lasers without harming skin tissue.
In addition to providing medical treatment, lasers are also used to remove tattoos. When laser light shines on a tattoo, the pigment absorbs the light, causing the ink to segment into tiny pieces, which are removed by the body’s white blood cells. Q-switched lasers are often used to remove tattoos without scarring skin. These lasers emit short pulses of energy at a high voltage. The laser’s effectiveness, however, depends on the pigment color. Green and blue laser pigments tend to absorb red light, whereas red and orange laser pigments tend to absorb green light best.
Many electronic devices, on the other hand, use lasers to read data off of a disk. In particular, CD players contain small lasers. These lasers read music off of the CD based on a pattern of bumps that appear on the CD’s aluminum surface.
As the CD spins, the laser beam reflects off of the CD’s aluminum surface and enters a light sensor. The sensor detects the presence of a bump on the CD when the laser changes its reflection angle. The optical sensor converts these readings into a digital signal, which is then turned into musical sound.
Lasers have many other uses, including the creation of holographs, measurement of distances, and the cutting of steel. Lasers are also used in physics to study the properties of plasma, a gas that contains free electrons and composes most stars.
From objects as large as stars to devices as small as CD players, lasers allow doctors, artists, and engineers to perform tasks that could not be performed with conventional technologies.