كيف يعمل الليزر Laser
Lasers are amazing light beams powerful enough to zoom miles into the sky or cut through lumps of metal. Once the stuff of science fiction, they have proved themselves to be among the most versatile inventions of modern times. The miniaturized laser beam that reads music in a CD player can also guide missiles, send emails down fiber-optic telephone lines, and barcode scan goods at the supermarket checkout.
The basic idea of a laser is simple. It's a tube that concentrates light over and over again until it emerges in a really powerful beam. But how does this happen, exactly? What's going on inside a laser? Let's take a closer look!
Photo: A scientific experiment to check the alignment of optical equipment using laser beams, carried out at the US Navy's Naval Surface Warfare Center (NSWC). Photo by Greg Vojtko courtesy ofUS Navy.
How is laser light different from ordinary light?
Lasers are more than just powerful flashlights. The difference between ordinary light and laser light is like the difference between ripples in your bathtub and huge waves on the sea. You've probably noticed that if you move your hands back and forth in the bathtub you can make quite strong waves. If you keep moving your hands in step with the waves you make, the waves get bigger and bigger. Imagine doing this a few million times in the open ocean. Before long, you'd have mountainous waves towering over your head!
A laser does something similar with light waves. It starts off with weak light and keeps adding more and moreenergy so the light waves become ever more concentrated. The "white" light produced by an ordinary flashlight contains many different light rays of different wavelengths that are out of step with one another (scientifically, that's known as "incoherent"). But in a laser, all the light rays have the same wavelength and they are coherent(absolutely in step). This is what makes laser light such a powerful concentration of energy.
Before you can understand how a laser works, you need to know how an atom can give off light. If you're not sure how this happens, take a look at the box how atoms make light in our introductory article about light.
Photo: It's much easier to make laser beams follow precise paths than ordinary light beams, as in this experiment to develop better solar cells. Picture by Warren Gretz courtesy of US DOE/NREL(Department of Energy/National Renewable Energy Laboratory).
How lasers work
A laser is effectively a machine that makes billions of atoms pump out trillions of photons (light particles) all at once so they line up to form a really concentrated light beam.
A red laser contains a long crystal made of ruby (shown here as a red bar) with a flash tube (yellow zig-zag lines) wrapped around it. The flash tube looks a bit like a fluorescent strip light, only it's coiled around the ruby crystal and it flashes every so often like a camera's flash gun.
How do the flash tube and the crystal make laser light?
- A high-voltage electric supply makes the tube flash on and off.
- Every time the tube flashes, it "pumps" energy into the ruby crystal. The flashes it makes inject energy into the crystal in the form of photons.
- Atoms in the ruby crystal (large green blobs) soak up this energy in a process called absorption. When an atom absorbs a photon of energy, one of its electrons jumps from a low energy level to a higher one. This puts the atom into an excited state, but makes it unstable. Because the excited atom is unstable, the electron can stay in the higher energy level only for a few milliseconds. It falls back to its original level, giving off the energy it absorbed as a new photon of light radiation (small blue blob). This process is called spontaneous emission.
- The photons that atoms give off zoom up and down inside the ruby crystal, traveling at the speed of light.
- Every so often, one of these photons hits an already excited atom. When this happens, the excited atom gives off two photons of light instead of one. This is called stimulated emission. Now one photon of light has produced two, so the light has been amplified (increased in strength). In other words, "light amplification" (an increase in the amount of light) has been caused by "stimulated emission of radiation" (hence the name "laser", because that's exactly how a laser works!)
- A mirror at one end of the laser tube keeps the photons bouncing back and forth inside the crystal.
- A partial mirror at the other end of the tube bounces some photons back into the crystal but lets some escape.
- The escaping photons form a very concentrated beam of
- powerful laser light.
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