Information AboutLaser |
| CATEGORIES ABOUT LASER | |
| lasers | |
| optical devices | |
| quantum optics | |
| acronyms | |
| directed-energy weapons | |
| photonics | |
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)]] A laser is a device that emits light through a specific mechanism for which the term laser is an and Thermodynamical process discussed in more detail below. As a light source, a laser can have various properties, depending on the purpose for which it is designed and calibrated. A typical laser emits light in a narrow, low- Divergence beam and with a well-defined Wavelength (corresponding to a particular Color if the laser is operating in the Visible Spectrum ). This is in contrast to a light source such as the Incandescent Light Bulb , which emits into a large solid angle and over a wide Spectrum of wavelength. These properties can be summarized in the term Coherence . A laser consists of a ''gain medium'' inside an ''optical cavity'', with a means to supply energy to the gain medium. The Gain Medium is a material (gas, liquid, solid or free electrons) with appropriate optical properties. In its simplest form, a Cavity consists of two mirrors arranged such that light bounces back and forth, each time passing through the gain medium. Typically, one of the two mirrors, the Output Coupler , is partially transparent. The output laser beam is emitted through this mirror. Light of a specific wavelength that passes through the gain medium is Amplified (increases in power); the surrounding mirrors ensure that most of the light makes many passes through the gain medium. Part of the light that is between the mirrors (i.e., is in the cavity) passes through the partially transparent mirror and appears as a beam of light. The process of supplying the energy required for the amplification is called Pumping and the energy is typically supplied as an electrical current or as light at a different wavelength. In the latter case, the light source can be a Flash Lamp or another laser. Most practical lasers contain additional elements that affect properties such as the wavelength of the emitted light and the shape of the beam. The first working laser was demonstrated in May 1960 by Theodore Maiman at Hughes Research Laboratories . Recently, lasers have become a multi-billion dollar industry. The most widespread use of lasers is in Optical Storage devices such as Compact Disc and DVD players, in which the laser (a few millimeters in size) scans the surface of the disc. Other common applications of lasers are Bar Code readers and Laser Pointer s. In industry, Lasers Are Used For Cutting steel and other metals and for inscribing patterns (such as the letters on computer keyboards). Lasers are also commonly used in various fields in Science , especially Spectroscopy , typically because of their well-defined wavelength or short pulse duration in the case of pulsed lasers. Lasers are also used for military and medical applications. PHYSICS 5. Laser beam]] demonstration at the Kastler-Brossel Laboratory at Univ. Paris 6 . The glowing ray in the middle is an electric discharge producing light in much the same way as a neon light. It is the Gain Medium through which the laser passes, ''not'' the laser beam itself, which is visible there. The laser beam crosses the air and marks a red point on the screen to the right.]] .]] See Also: Laser science Laser construction A laser is composed of an '' Active Laser Medium '', or ''gain medium'', and a resonant Optical Cavity . The gain medium transfers external energy into the laser beam. It is a material of controlled purity, size, concentration, and shape, which amplifies the beam by the Quantum Mechanical Process of Stimulated Emission , predicted by Albert Einstein , while he studied the Photoelectric Effect . The gain medium is energized, or '' Pumped '', by an external energy source. Examples of pump sources include electricity and light, for example from a Flash Lamp or from another laser. The pump energy is absorbed by the laser medium, placing some of its particles into high-energy (" Excited ") Quantum State s. Particles can interact with light both by absorbing photons or by emitting photons. Emission can be spontaneous or stimulated. In the latter case, the photon is emitted in the same direction as the light that is passing by. When the number of particles in one excited state exceeds the number of particles in some lower-energy state, Population Inversion is achieved and the amount of spontaneous emission due to light that passes through is larger than the amount of absorption. Hence, the light is amplified. Strictly speaking, these are the essential ingredients of a laser. However, usually the term ''laser'' is used for devices where the light that is amplified is produced as spontaneous emission from the same gain medium as where the amplification takes place. Devices where light from an external source is amplified are normally called Optical Amplifier s. The light generated by stimulated emission is very similar to the input signal in terms of wavelength, Phase , and polarization. This gives laser light its characteristic coherence, and allows it to maintain the uniform polarization and often monochromaticity established by the optical cavity design. The Optical Cavity , a type of Cavity Resonator , contains a coherent beam of Light between reflective surfaces so that the light passes through the gain medium more than once before it is emitted from the output aperture or lost to diffraction or absorption. As light circulates through the cavity, passing through the gain medium, if the gain (amplification) in the medium is stronger than the resonator losses, the power of the circulating light can rise Exponentially . But each stimulated emission event returns a particle from its excited state to the ground state, reducing the capacity of the gain medium for further amplification. When this effect becomes strong, the gain is said to be ''saturated''. The balance of pump power against gain saturation and cavity losses produces an equilibrium value of the laser power inside the cavity; this equilibrium determines the operating point of the laser. If the chosen pump power is too small, the gain is not sufficient to overcome the resonator losses, and the laser will emit only very small light powers. The minimum pump power needed to begin laser action is called the '' Lasing Threshold ''. The gain medium will amplify any photons passing through it, regardless of direction; but only the photons aligned with the cavity manage to pass more than once through the medium and so have significant amplification. The beam in the cavity and the output beam of the laser, if they occur in free space rather than waveguides (as in an '', that is being parallel without Diverging . However, a perfectly collimated beam cannot be created, due to Diffraction . The beam remains collimated over a distance which varies with the square of the beam diameter, and eventually diverges at an angle which varies inversely with the beam diameter. Thus, a beam generated by a small laboratory laser such as a Helium-neon Laser spreads to about 1.6 kilometers (1 mile) diameter if shone from the Earth to the Moon . By comparison, the output of a typical semiconductor laser, due to its small diameter, diverges almost as soon as it leaves the aperture, at an angle of anything up to 50°. However, such a divergent beam can be transformed into a collimated beam by means of a Lens . In contrast, the light from non-laser light sources cannot be collimated by optics as well or much. The output of a laser may be a continuous constant-amplitude output (known as ''CW'' or '' Continuous Wave ''); or pulsed, by using the techniques of Q-switching , Modelocking , or Gain-switching . In pulsed operation, much higher peak powers can be achieved. Some types of lasers, such as ''dye lasers'' and ''vibronic solid-state lasers'' can produce light over a broad range of wavelengths; this property makes them suitable for generating extremely short pulses of light, on the order of a few Femtosecond s (10-15 s). Although the laser phenomenon was discovered with the help of Quantum Physics , it is not essentially more quantum mechanical than other light sources. The operation of a Free Electron Laser can be explained without reference to Quantum Mechanics . It is understood that the word ''light'' in the acronym ''Light Amplification by Stimulated Emission of Radiation'' is typically used in the expansive sense, as photons of ''any'' energy; it is not limited to photons in the Visible Spectrum . Hence there are '' Infrared lasers'', '' Ultraviolet lasers'', '' X-ray lasers'', etc. For example, a source of atoms in a coherent state can be called an Atom Laser . |
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