 Different from a neon light radiating in any directions, a laser is a beam of coherent light radiating in the same direction with high intensity and a particular wavelength. All lasers contain an amplifying medium, located in a cavity between two mirrors, that can increase the intensity of light beam passing through it. This increased intensity will give additional energy into the beam itself.

 The energized amplifying medium has more atoms or molecules of the medium in the excited state to create a population inversion. Then, an input photon with a particular wavelength strikes an excited atom of the medium and pushes it back to a lower energy state to emit a photon with the same wavelength and in phase and in the same direction as the input photon. The process is called stimulated emission.

 The beam is repeatedly reflected and amplified between two mirrors. One of the mirrors reflects almost the incident beam ; the reflection efficiency of the other mirror is between 20% and 80%. So, the unflected light passes through the mirror and becomes the laser.

 Lasers are applied in many fields such as consumer electronics, medicine, surveying, manufacturing, construction industry, scientific instrumentation, and military systems. Besides, it is also used for ignition of a fusion reaction, laser printers, compact disc and scan bar codes etc.

 In telecommunications, laser can replace electrical transmission over traditional copper wires, but laser beams are easily unfavorably interfered by external conditions, such as rain, fog, and clouds.

 The invention of optical fiber enables to transmit voice, data, pictures and video by light. For example, semiconductor lasers can transmit billions of bits of information per second by light pulses over glass fibers. By wavelength division multiplexing technology, several wavelengths of light can transmit trillions of bits simultaneously over a single optical fiber.