How is a Laser Beam Produced?
By definition the laser is a device that amplifies light by means of stimulated emission of radiation. In other words, the laser is a generator and amplifier of radiation in the form of light. To help understand how the laser beam is produced, let’s begin by looking at how light is created at the atomic level.
Atoms are composed of a nucleus with the electrons orbiting it. When a certain amount of energy is supplied to the atom, the electron’s path can change and move to a higher orbit. The atom is said to go from the ground state energy level to the excited level. The atom will then calm when the energy is no longer supplied and the electrons will return to the ground state in their original lower orbit. When they return, they release some of the energy that was applied to them earlier. This energy release is in the form of photons which are particles of light. You can see this happen when you turn on an electric stove and apply electricity to the heating coil. The heating coil gets very hot as well as gives off light. The same happens with an incandescent light bulb or a fluorescent tube. When light is released from these sources, a rainbow of color is given off. Each color of light has its own wavelength; therefore, most light sources emit many wavelengths of light.
A laser light has three properties that make it unlike any other light source: First, lasers are monochromatic which means they produce light of one wavelength or of one color. Second, the light released from a laser is coherent, which means that it is ordered and controlled. Each photon moves in unison with the other photons and they move in waves matching the pattern of the other light waves. Third, laser light has a specific direction unlike other forms of light that move out randomly in many directions. These three properties are created by a process called stimulated emission.
Let’s look at the first laser invented, the ruby laser, to see how the laser beam is generated. The ruby laser uses a flash lamp in the shape of a coil wound around a ruby crystal rod. The ruby rod is composed of the same material as the ruby gemstone, corundum, whose molecules are made of two atoms of aluminum and two of oxygen. The ruby rod is known as the lasing media. The color of a ruby is due to a small chromium content. The two ends of the ruby rod are covered with mirrors with one end fully reflective and the other end only partially reflective.
The ruby rod is irradiated on the sides by light from a flash lamp operated usually for a few milliseconds at a time. The atoms and their electrons of the ruby rod begin to absorb the energy and move to an excited level. The electrons quickly return to the ground states between each operation of the flash lamp thus releasing red photons particles horizontally along the length of the ruby rod. These particles begin to bounce back and forth within the rod between the two mirrors on the ends, building in intensity until they exit out the end with the mirror that is only partially reflective. The light that discharges from the rod is of one wavelength (color), is coherent (ordered), and moves in a single direction (beam).
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