To make these three properties occur takes something called stimulated emission . This does not occur in ordinary flashlight -- in a flashlight, all of the atoms release their photons randomly. In stimulated emission, photon emission is organized.
Population Inversion For a laser you need a continuous number of available electrons for transition to a lower state emitting photons. In equilibrium most systems have a Boltzmann distribution of states and therefore the ground state always has the largest number of electrons. By electrical or optical pumping, carriers can be excited to higher states and the population of the states can be changed. For lasing, the upper radiative transition states must have a large population of electrons than the ground state i.e. population inversion. This also requires the lifetimes of each state to be correct to allow population inversion. (Most lasers, however, use four levels with a fast depopulation of the lower radiative transitions state but it is easier to illustrate population inversion using a three level laser system).
Key to a laser is a pair of mirrors , one at each end of the lasing medium.
Photons, with a very specific wavelength and phase, reflect off the mirrors to travel back and forth through the lasing medium.
In the process, they stimulate other electrons to make the downward energy jump and can cause the emission of more photons of the same wavelength and phase.
A cascade effect occurs, and soon we have propagated many, many photons of the same wavelength and phase. The mirror at one end of the laser is "half-silvered," meaning it reflects some light and lets some light through.
The light that makes it through is the laser light.
Laser mode is the possible standing em waves in laser cavity.
Longitudinal (Axial) Modes
Axial standing em waves within the laser cavity.
Laser Resonator or Laser Cavity
The optical mirrors, active medium and pumping system form the laser resonator , which is also called Laser Cavity. Laser cavities can be divided into Stable Cavities and Unstable Cavities according to whether they make the oscillating beam converge into the cavity or spread out from the cavity.
Lasers are designated by the type of lasing material employed:
Solid-state Lasers: Lasing Material Distributed in a Solid Matrix ruby or Nd:YAG (Neodymium: Yttrium- Aluminum Garnet ;1,064 nm).
Gas Lasers: He, HeNe, Most common gas lasers; visible red light. CO 2 lasers emit energy in the far-infrared, used for cutting.
EXCIMER lasers: (the name is derived from the terms excited and dimers ) use reactive gases, such as chlorine and fluorine, mixed with inert gases such as Ar, Kr or Xe. When electrically stimulated, a pseudo molecule (dimer) is produced. When lased, the dimer produces light in the ultraviolet range.
Dye lasers: use complex organic dyes, such as rhodamine 6G, in liquid solution or suspension as lasing media. They are tunable over a broad range of wavelengths.
Semiconductor lasers: diode lasers, are not solid-state lasers. These electronic devices are generally very small and use low power. They may be built into larger arrays, such as the writing source in some laser printers or CD players .
Class I - These lasers cannot emit laser radiation at known hazard levels.
Class I.A. - This is a special designation that applies only to lasers that are "not intended for viewing," such as a supermarket laser scanner. The upper power limit of Class I.A. is 4.0 mW.
Class II - These are low-power visible lasers that emit above Class I levels but at a radiant power not above 1 mW. The concept is that the human aversion reaction to bright light will protect a person.
Class IIIA - These are intermediate-power lasers (cw: 1-5 mW), which are hazardous only for intrabeam viewing. Most pen-like pointing lasers are in this class.
Class IIIB - These are moderate-power lasers.
Class IV - These are high-power lasers (cw: 500 mW, pulsed: 10 J/cm 2 or the diffuse reflection limit), which are hazardous to view under any condition (directly or diffusely scattered), and are a potential fire hazard and a skin hazard. Significant controls are required of Class IV laser facilities
such as Boeing’s Airborne Laser which can be mounted on a 747 jet is able to burn the skin off enemy missiles .
A long range laser rangefinder LRB20000 is capable of measuring distance up to 20 km; mounted on a tripod with an angular mount. The resulting system also provides azimuth and elevation measurements. A laser rangefinder is a device which uses a laser beam in order to determine the distance to a reflective object. it works on the principle of time of flight.
Revolver equipped with laser sight. Revolver equipped with laser sight. The laser has in most military applications been used as a tool to enhance the targeting of other weapon systems. For example, a laser sight is a small, usually visible-light laser placed on a handgun or rifle aligned to emit a beam parallel to the barrel. Most laser sights use a red laser diode Others use an infrared diode .
Operates autonomously, above the clouds, outside the range of threat weapons but sufficiently close to enemy territory
Engages early, destroying ballistic missiles in their boost phase of flight over launch area
Cues and tracks targets, communicating with other joint theater assets for layered defense system
This lidar scanner may be used to scan buildings, rock formations, etc., to produce a 3D model. The lidar can aim its laser beam in a wide range: its head rotates horizontally, a mirror flips vertically. The laser beam is used to measure the distance to the first object on its path.
Geology and Meteorology for detecting faults and measuring uplifts.
Physics and Astronomy to measures the distance to reflectors placed on moon.
Biology and conservation
Military and law enforcement for vehicle speed measurement
COMMERCIAL USE Lasers used for visual effects during a musical performance
Laser printer A laser printer is a common type of computer printer that rapidly produces high quality text and graphics on plain paper. As with digital photocopiers and MFPs , laser printers employ a xerographic printing process but differ from analog photocopiers in that the image is produced by the direct scanning of a laser beam across the printer's photoreceptor HP LaserJet 4200 series printer
Laser engraving is the practice of using lasers to engrave or mark an object. The technique can be very technical and complex, and often a computer system is used to drive the movements of the laser head. Despite this complexity, very precise and clean engravings can be achieved at a high rate Laser marked electronic part