White Paper: How Lasers Function

Nathan Edwards

Energy injected into a laser’s gain medium excites the atoms within it, causing the electrons circling those atoms to throw off particles known as photons. These photons exhibit the same wavelength and move in the same direction, resulting in a powerful, monochromatic beam of light.

When science-fiction authors got wind of the concept of lasers, they immediately weaved the technology into their story lines as heinous instruments of interstellar destruction—not surprising, when you consider that the word “laser” is actually an acronym for “light amplification by stimulated emission of radiation.”

But as you sit in front of your PC, you’re likely to be in close proximity to several lasers, none of which is capable of setting paper on fire, much less blowing apart a spaceship. The same goes for those traveling shows that use such focused beams of light to create hallucinogenic displays to a soundtrack of Pink Floyd and Led Zeppelin.

Lasers today are the key technology behind CD, DVD, HD DVD, and Blu-ray players and burners. They create the images produced by laser printers, and they precisely track the movement of laser mice. How did a concentrated beam of light become so important to so many aspects of modern computer technology?

Up and Atom

To understand lasers, we must start with the atom, which—as anyone with the slightest exposure to science education knows—is the basic component of just about everything in our known universe. The atom, however, can be broken down into even smaller elements; namely, neutrons and protons, which form the atom’s nucleus. Neutrons and protons exert a positive electrical charge, while a cloud of negatively charged electrons circulate around the outside of the nucleus.

Light—any type of light—is created when electrons are energized by an external source, such as electricity. Once that is accomplished, the electrons move into a higher orbit around the atom, and the atom becomes unstable. This state is only temporary, however; the electrons soon return to their normal orbit, and this is when the good stuff happens. As the electrons return to a state of equilibrium, they release their excess energy in the form of particles called photons: light.

When the electrons inside the atoms of conventional light sources—such as incandescent light bulbs, fluorescent tubes, flashlights, and even the sun itself—are excited, they emit photons randomly. The “white” light generated by these sources contains a wide variety of incoherent rays of different wavelengths (wavelength being determined by the energy difference between the atom’s excited and relaxed states). The light is described as being white because it’s the sum of many different wavelengths. A laser device, on the other hand, is capable of compelling atoms to emit photons in a highly organized fashion.

The Light Fantastic

The key concept behind laser light is stimulated emission. If the photon emitted by an atom encounters another atom with an electron in the same excited state, it can provoke that second atom to throw off a photon that exhibits the same wavelength and moves in the same direction.

A laser consists of a gain medium, which is a material with specific optical properties that render it capable of amplifying light of a specific wavelength. The gain medium is housed in a cavity capped by a mirror at one end and a partially transparent mirror at the other. As energy (in the form of light, or in the case of the semiconductor lasers, electricity) is pumped into the gain medium (which can be a gas, liquid, or solid), it excites these electrons. The electrons then emit energy in the form of photons as they return to their relaxed state.

The photons then bounce back and forth between the two mirrors, repeatedly passing through the gain medium, exciting other electrons and stimulating the emission of even more photons. This cascading effect continues as long as energy is applied to the gain medium. Some of these photons escape through the partially transparent mirror, also known as an output coupler. Since all the escaped photons are of the same wavelength and are all traveling in the same direction, they form an intense, monochromatic, highly directional column of light: a laser beam.

Storage Applications

Semiconductor lasers are the most common type of laser; low-powered semiconductor lasers are used in the construction of everything from laser printers and optical drives to laser pointers and measuring devices. The semiconductor laser in a common CD-ROM drive emits light with a wavelength of 780 nanometers (very near infrared, which ranges from 750nm to 1mm) and is projected through a lens with a numerical aperture of 0.45. As a lens’ numerical aperture increases, so does its ability to create a focused spot of light.

The laser beam is directed at a spinning disc, which has a polycarbonate layer stamped with pits (surface areas without pits are called lands). The polycarbonate layer is backed by a reflective metal (aluminum, typically). As the disc spins beneath the laser, the light passes through the polycarbonate layer and bounces off the aluminum layer. Inside the drive, an optical pickup measures the difference between the pits and lands to create the binary ones and zeroes used to encode music, video, and other types of data.

DVDs pack more data into the same area by rendering the pits and lands smaller and closer together; DVD drives use lasers that emit light with a shorter wavelength, 650nm, projected through a lens with a higher numerical aperture: 0.65. The pits and lands on Blu-ray and HD DVD discs are even smaller and more tightly packed than those on DVDs­—players that read these discs use blue lasers that emit light with a 405nm wavelength. One reason Blu-ray delivers more storage capacity than HD DVD, despite both using blue lasers, is that Blu-ray devices use a numerical aperture of 0.85, compared to HD DVD’s 0.65.

Printer Applications

Next to optical drives, lasers are most commonly found in printers. And like optical drives, laser printers utilize lasers that emit light in wavelengths ranging from 650nm to 780nm (with higher-powered models using lasers with shorter wavelengths).
The laser is focused on a rotating drum inside the printer, which is coated with photo-conductive material. The drum initially receives a positive electrical charge from either a charged roller or a corona wire. The laser then emits a pulse of light for each dot that is to be printed, which discharges that area of the drum. Once this pattern of dots is created for the entire image on the page, the printer coats the drum with positively charged toner. The toner “sticks” to the discharged areas of the drum and is repelled by the areas that remain positively charged.

A sheet of paper (which the corona wire has endowed with a negative charge) is then rolled over the drum. Since the negative charge on the paper is stronger than the one on the drum, the paper pulls the toner away from the drum. The paper then passes through a fuser, which melts the toner and bonds it with the fibers in the paper.

More powerful lasers may be capable of cutting through steel, and the Department of Defense has made no secret of its efforts to weaponize laser technology, but the vast majority of lasers are used in peaceful applications such as these.

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