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Light Amplification by Stimulated Emission of Radiation -
(LASER)
A Laser is a device that emits coherent light of essentially one wavelength in a narrow beam. Lasers can be made using gaseous, liquid or solid state. In optical networks, solid state or semiconductor lasers are used as high performance light sources. Photons are generated in the semiconductor by application of a voltage. Photons with the right wavelength, phase, and direction of travel are selected by an optical cavity in the laser. See also Optical Cavity, LED. Laser light is monochromatic (it has only one frequency or "color," although infrared or ultraviolet lasers produce optical frequencies that are not visible to the human eye), in contrast to white light that has, at the other extreme, many frequency components, and the phase of the sine wave electromagnetic waveform maintains a fixed (coherent) timing relationship over a long interval of time. The peak amplitudes of each cycle of a coherent wave form occurs at absolutely uniform time intervals. Lasers create monochromatic coherent light by combining the light radiation due to the oscillation of individual electrons in individual atoms as these electrons change their electric charge configuration from a higher energy level to a lower energy level. First these electrons are "pumped" up to a higher energy level by a primary source of power. We call these the "excited" electrons. One way to excite electrons is to accelerate the atoms in a gas by applying a high voltage to electrodes at two ends of a container of the gas, so that some atoms collide with each other and transfer their kinetic (motion related) energy to some of the electrons, or alternatively by shining a non-coherent light source of higher frequency (shorter wavelength) than the desired output light frequency on a solid material. Some excited electrons in an atom subsequently "fall" to a lower energy level, and when doing this they emit light at a frequency proportional to the difference in energy between the high and low energy levels. The energy difference (E2-E1) is related to the frequency of the light, f, by the formula E2-E1= hf, where h is Planck's constant. Excited electrons "fall" naturally, but at unpredictable times, from the higher to the lower energy level for no apparent reason. One way to make even more electrons fall from the high energy level to the low energy level is to shine a light on these atoms, that light having the same frequency f as the expected output light frequency. This latter process is called stimulated emission. Because the gas or solid is made up of atoms having the same energy level structure, all the atoms then emit light at the same frequency. By placing two parallel reflecting mirrors at two ends of the material, and precisely locating the distance between these mirrors so that their distance is an integral number of wavelengths of the light in question, a standing wave of multiply reflected light is set up in the gas or solid. This is what makes the light emission from all the different electrons coherent. By making one of the two mirrors either partially reflecting and partly transparent, or by having a small transparent spot on one otherwise fully reflective mirror, some of the light is able to escape in a straight, monochromatic, coherent light beam. This is the laser beam. It can be guided into the core of an optical fiber for communication purposes. The light output of the laser can be turned on and off electrically to produce light pulses to convey digital information.
This figure shows the basic operation of a semiconductor light amplification stimulated emission of radiation (LASER) device. This example shows that a semiconductor LASER is constructed of a specific type of p-type and n-type semiconductor material. When a forward current is applied to the device, photons are produced within the optical cavity. As photons travel down the cavity, the produce other photons along their same path. This diagram shows that the optical cavity has a fully reflective mirror on one end that reflects all photons back into the cavity. At the other end, the mirror is partially reflective allowing some of the photons to exist from the LASER. This diagram shows that photons exit from the LASER in the same direction (coherent light).
LASER Diagram
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