Lasers

Visible lasers emit light with a wavelength in the visible spectrum, which may be red, green, violet or blue. Ultraviolet excimer lasers are very powerful and only used for cutting and burning operations. Most lasers are defined by their lasing medium; some are gases like neon or CO2, some liquid and others are considered solid state lasers and include fiber lasers and use materials like crystal or glass. CO2 lasers, YAG lasers and helium neon lasers use gas as the lasing medium. Dye lasers use a dye solution, which is liquid, and solid state lasers like ND:YAG lasers and diode lasers, which are semiconductor lasers, use a solid material as the lasing medium. Depending on the strength of the laser, there are many different applications across the medical, manufacturing, construction and electronics industries. The weakest lasers, such as laser modules, are used as pointers, leveling tools and surveyors. Hospitals and doctors' offices use medical lasers for surgeries and optical procedures. The most powerful laser systems are used in material fabrication. These industrial lasers are strong enough to cut, weld, etch, engrave and heat treat metals and plastics.

Lasers have a high stability output and will operate reliably over long term periods, requiring minimal service. Laser manufacturers provide dependable solutions for high-duty cycle/continuous-use applications. Depending on the repetition rate, lasers can operate continuously for hundreds or even thousands of hours. All lasers consist of an optical cavity, a gain medium and a pumping system. The optical cavity contains the media, which is the source of the laser light, and the mirrors that excite the media and direct the produced photons back along the same general path. The laser medium can be a solid, a gas (e.g. argon), liquid dye or semiconductors as is the case in diode lasers. Pumping systems transfer energy to the media in three basic ways. In optical pumping, the system utilizes photons from another source, such as a xenon gas flash tube. Collision pumping transfers energy using an electrical discharge within the pure gas or gas mixture media. Pumping systems may also rely on the binding energy released in chemical reactions in order to raise the media to the lasing state. While all laser manufacturers combine these three components, their products can differ greatly in their size, output, beam quality, power consumption and operating life.

Lasers are demarcated according to the lasing media that they employ. Solid state lasers, such as neodymium-yttrium aluminum garnet lasers, also denoted as ND:YAG lasers or simply YAG lasers, have lasing material distributed in a solid state. The most common gas lasers use helium, helium-neon and CO2 lasers. CO2 and YAG lasers are used for deep cutting and welding applications. Excimer lasers use a mixture of reactive gases, such as chlorine and fluorine, and inert gases (e.g. argon, krypton or xenon). Dye lasers have the ability to be tuned over a wide range of wavelengths. They make use of complex organic dyes in liquid solution or suspension as a lasing medium. Semiconductor lasers, also called diode lasers, are typically small, electronic devices that use low power. They may be built into larger arrays for use in applications, such as the writing source in laser printers or CD players.

Exposure to lasers can cause severe damage. Lasers are divided into 5 classes, according to their power output: Class I, Class II, Class IIIa, Class IIIb and Class IV. Laser manufacturers are required to build in engineering controls in laser systems to provide added safety measures. Class I lasers are the weakest kind and pose zero biological hazard. Their power is less than 1mw, and they are mostly used as pointers. Class II lasers are also safe to use, though they can damage eye tissue if focused there. They are powered up to 1mw. Class IIIa lasers are a little more dangerous. At 1 to 5mw, eye injury is possible, but they will not burn any materials. IIIb, 5 to 500mw, will cause eye damage and may burn materials. Finally, class IV lasers are the most dangerous at 500mw or more. They burn any material on contact, including human skin and hard metals. All lasers, no matter what their power, must have an enclosure around them that limits access to the laser beam. Class IV systems contain the most dangerous lasers and require a master switch that, when disabled, prohibits all but authorized personnel from operating the laser. A beam stop or attenuator, which significantly reduces beam emission when the laser is on standby, is permanently attached to all Class IV lasers and is recommended for Classes IIIa and IIIb as well. Other safety precautions that may be instituted include access restriction to the lasing area, eye protection, area controls, barriers, shrouds, education and training.

Lasers and Laser Manufacturers Image Provided by Schmidt Marking Systems

Laser Types

• CO2 lasers use contained CO2 gas to produce their energy. CO2 lasers, which can run for thousands of hours before a new CO2 supply is needed, work very well with most metals, wood, plastics, ceramics, glass and quartz.
• Diode lasers, also referred to as "semiconductor lasers," utilize microscopic chips made of gallium-arsenide or another semiconductor to generate their source of coherent light. Diode lasers are usually smaller and less powerful than other lasers and can be found in CD-ROM drives, CD players, barcode scanners and laser printers.
• Embedded lasers have a higher inherent capability than the laser system into which they are incorporated, where the lower assigned class number of the system is appropriate to the engineering features that limit accessible emission.
• Excimer lasers produce short, intense ultraviolet (UV) light pulses and have the greatest power and versatility of any light source in the UV range of the electromagnetic spectrum. Excimer lasers are widely used in the scientific, industrial and medical industries.
• Fiber lasers are powerful laser systems used for cutting industrial settings. 
• Helium neon lasers are lasers that are generated through the electrification of helium and neon in an enclosure. 
• Industrial lasers are laser systems used in industrial settings for cutting, welding, singeing, etching and other processes.
• Internal mirror lasers are lasing devices constructed with the reflecting cavity mirror attached to the containment envelope that houses the gas.
• Laser modules are small laser systems that are often portable, low power and used for more consumer applications than industrial or commercial applications; a few examples are laser pointers and laser levels.
• Laser systems are used for many applications, including metal, dieboard and plastic cutting, metal and non-metal marking and micro-machining.
• Marking lasers are used to scribe letters, words, designs, etc., on different materials, such as ceramics, wood and glass. Different lenses provide variations in laser diameter, which in turn increases or decreases the thickness of the marking.
• Medical lasers are used in lieu of scalpels and other medical instruments because of their precision.
• ND YAG lasers use neodymium-doped YAG crystals.
• Solid state lasers are laser systems in which a laser beam is generated by the electrification of a solid gain medium. Gain media are the sources of light and the mechanism of light amplification in laser systems.
• Welding lasers present a quick exposure time with little material outside the focus area being heated. Welding lasers, which operate with a very dense energy, are extremely precise and can perform welds that a human welder cannot.
• Yttrium aluminum garnet lasers, also referred to as "Nd:YAG lasers" or "YAG lasers," are solid state lasers that use neodymium-doped YAG crystals as the lasing mediums. YAG lasers are available in constant and pulsing configurations and are generally used for such applications as laser marking, cutting and welding.

Laser Terms

Absorb - To transform radiant energy into a different form, with a resultant rise in temperature.

Absorption - Transformation of radiant energy to a different form of energy by the interaction of matter, depending on temperature and wavelength.

Absorption Coeficient - Factor describing light's ability to be absorbed per unit of path length.

Accessible Emission Level - The magnitude of accessible laser (or collateral) radiation of a specific wavelength or emission duration at a particular point as measured by appropriate methods and devices. Also means radiation to which human access is possible in accordance with the definitions of the laser's hazard classification.

Accessible Emission - The maximum accessible emission level.

Limit (AEL) - permitted within a particularly class. In ANSI Z- 136.1, AEL is determined as the product of Accessible Emission Maximum Permissible Exposure limit (MPE) and the area of the limiting aperture (7mm for visible and near infrared lasers).

Active Medium - Collection of atoms or molecules capable of undergoing stimulated emission at a given wavelength.

Afocal - Literally, "without a focal length"; an optical system with its object and image point at infinity.

Aiming Beam - A laser (or other light source) used as a guide light. Used coaxially with infrared or other invisible light may also be a reduced level of the actual laser used for surgery or for other applications.

Amplification - The growth of the radiation field in the laser resonator cavity. As the light wave bounces back and forth between the cavity mirrors, it is amplified by stimulated emission on each pass through the active medium.

Amplitude - The maximum value of the electro-magnetic wave, measured from the mean to the extreme; simply stated: the height of the wave.

Angle of Incidence - See Incident Ray

Angstrom Unit - A unit of measure of wavelength dual to 10(-10) meter, 0.1 nanometer, or 10(-4) micrometer, no longer widely used nor recognized in the SI system of units.

Anode - An electrical element in laser excitation which attracts electrons from a cathode.

Aperture - An opening through which radiation can pass.

Apparent Visual Angle - The angular subtense of the source as calculated from the source size and distance from the eye. It is not the beam divergence of the source.

AR Coatings - Antireflection coatings used on optical components to suppress unwanted reflections.

Argon - A gas used as a laser medium. It emits blue/green light primarily at 448 and 515 nm.

Articulated Arm - CO(2) laser beam delivery device consisting of a series of hollow tubes and mirrors interconnected in such a manner as to maintain alignment of the laser beam along the path of the arm.

Attenuation - The decrease in energy (or power) as a beam passes through an absorbing or scattering medium.

Autocollimator - A single instrument combining the functions of a telescope and a collimator to detect small angular displacements of a mirror by means of its own collimated light.

Average Power - The total energy imparted during exposure divided by the exposure duration.

Aversion Response - Movement of the eyelid or the head to avoid an exposure to a noxious stimulant, bright light. It can occur within 0.25 seconds, and it includes the blink reflex time.

Axial-Flow Laser - A laser in which an axial flow of gas is maintained through the tube to replace those gas molecules depleted by the electrical discharge used to excite the gas molecules to the lasing. See gas discharge laser.

Axicon Lens - A conical lens which, when followed by a conventional lens, can focus laser light to a ring shape.

Axis, Optical Lens - The optical centerline for a lens system; the line passing through the centers of curvature of the optical surfaces of a lens.

Beam - A collection of rays that may be parallel, convergent, or divergent.

Beam Bender - A hardware assembly containing an optical device, such as a mirror, capable of changing the direction of a laser beam; used to repoint the beam, and in "folded," compact laser systems.

Beam Diameter - The distance between diametrically opposed points in the cross section of a circular beam where the intensity is reduced by a factor of e(-1) (0.368) of the level (for safety standards). The value is normally chosen at e(-2) (0.135) of the peak level for manufacturing specifications.

Beam Divergence - Angle of beam spread measured in radians more milliradians (1 milliradian = 3.4 minutes-of-arc or approximately 1 mil). For small angles where the cord is approximately equal to the arc, the beam divergence can be closely approximated by the ratio of the cord length (beam diameter) divided by the distance (range) from the laser aperture.

Beam Expander - An optical device that increases beam diameter while decreasing beam divergence (spread). In its simplest form consists of two lenses, the first to diverge the beam and the second to re-collimate it. Also called an upcollimator.

Beam Splitter - An optical device using controlled reflection to produce two beams from a single incident beam.

Blink Reflex - See aversion response.

Brewster Windows - The transmissive end (or both ends) of the laser tube, made of transparent optical material and set at Brewster's angle in gas lasers to achieve zero reflective loss for one axis of plane polarized light. They are non-standard on industrial lasers, but a must if polarization is desired.

Brightness - The visual sensation of the luminous intensity of a light source. The brightness of a laser beam is most closely associated with the radio-metric concept of radiance.

C.I.E. - Abbreviation for Commission International de l'Eclairage, the French translation for: International Commission on Illumination.

Calirometer - An instrument which measures the energy, usually as heat generated by absorption of the laser beam.

Carbon Dioxide - Molecule used as a laser medium. Emits far energy at 10,600 nm (10.6 ?m).

Cathode - A negatively charged electrical element providing electrons for an electrical discharge.

Closed Installation - Any location where lasers are used which will be closed to unprotected personnel during laser operation.

CO(2) Laser - A widely used laser in which the primary lasing medium is carbon dioxide gas. The output wavelength is 10.6 ?m (10600 nm) in the far infrared spectrum. It can be operated in either CW or pulsed.

Coaxial Gas - A shield of inert gas flowing over the target material to prevent plasma oxidation and absorption, blow away debris, and control heat reaction. The gas jet has the same axis as the beam,so the two can be aimed together.

Coherence - A term describing light as waves which are in phase in both time and space. Monochromaticity and low divergence are two properties of coherent light.

Collimated Light - Light rays that are parallel. Collimated light is emitted by many lasers. Diverging light may be collimated by a lens or other device.

Collimation - Ability of the laser beam to not spread significantly (low divergence) with distance.

Combiner Mirror - The mirror in a laser which combines two or more wavelengths into a coaxial beam.

Continuous Mode - The duration of laser exposure is controlled by the user (by foot or hand switch).

Continuous Wave (CW) - Constant, steady-state delivery of laser power.

Controlled Area - An locale where the activity of those within are subject to control and supervision for the purpose of laser radiation hazard protection.

Convergence - The bending of light rays toward each other, as by a positive (convex) lens.

Corrected Lens - A compound lens that is made measurably free of aberrations through the careful selection of its dimensions and materials.

Crystal - A solid with a regular array of atoms. Sapphire (Ruby Laser) and YAG (Nd:YAG laser) are two crystalline materials used as laser sources.

Current Regulation - Laser system regulation in which discharge current is kept constant.

Current Saturation - The maximum flow of electric current in a conductor; in a laser, the point at which further electrical input will not increase laser output.

CW - Abbreviation for continuous wave; the continuous-emission mode of a laser as opposed to pulsed operation.

Depth of Field - The working range of the beam in or near the focal plane of a lens; a function of wavelength, diameter of the unfocused beam, and focal length of the lens.

Depth of Focus - The distance over which the focused laser spot has a constant diameter and thus constant irradiance.

Dichroic Filter - Filter that allows selective transmission of colors desired wavelengths.

Diffraction - Deviation of part of a beam, determined by the wave nature of radiation and occurring when the radiation passes the edge of an opaque obstacle.

Diffuse Reflection - Takes place when different parts of a beam incident on a surface are reflected over a wide range of angles in accordance with Lambert's Law. The intensity will fall-off as the inverse of the square of the distance away from the surface and also obey a Cosine Law of reflection.

Diffuser - An optical device or material that homogenizes the output of light causing a very smooth, scattered, even distribution over the area affected. The intensity will obey Lambert's law (see Diffuse Reflection).

Divergence - The increase in the diameter of the laser beam with distance from the exit aperture. The value gives the full angle at the point where the laser radiant exposure or irradiance is e(-1) or e(-2) of the maximum value, depending upon which criteria is used.

Dosimetry - Measurement of the power, energy, irradiance or radiant exposure of light delivered are two crystalline materials used as laser to tissue.

DPSS - Laser Diode Pumped Solid State Laser; a laser where the lasing medium is a solid state material, e.g. Nd:YVO4 and the energy for the laser is supplied by a diode laser.

Drift - All undesirable variations in output either amplitude or frequency).

Angular Drift - Any unintended change in direction of the beam before, during, and after warmup; measured in mrad.

Duty Cycle - Ratio of total "on" duration to total exposure duration for a repetitively pulsed laser.

Electric Vector - The electric field associated with a light wave which has both direction and amplitude.

Electromagnetic Radiation - The propagation of varying electric and magnetic fields through space at the velocity of light.

Electromagnetic Spectrum - The range of frequencies and wavelengths emitted by atomic systems. The total spectrum includes radiowaves as well as short cosmic rays. Wavelengths cover a range from 1 hz to perhaps as high as 1020 hz.

Electromagnetic Wave - A disturbance which propagates outward from an electric charge that oscillates or is accelerated. Includes radio waves; X-rays; gamma rays; and infrared, ultraviolet, and visible light.

Electron - Negatively charged particle of an atom.

Embedded Laser - A laser with an assigned class number higher than the inherent capability of the laser system in which it is incorporated, where the systems lower classification is appropriate to the engineering features limiting accessible emission.

Emergent Beam Diameter - Diameter of the laser beam at the exit aperture of the system in centimeters (cm) defined at e(-1) or e(-2) irradiance points.

Emission - Act of giving off radiant energy by an atom or molecule.

Emissivity - The ratio of the radiant energy emitted by a any source to that emitted by a blackbody at the same temperature.

Emittance - The rate at which emission occurs.

Enclosed Laser Device - Any laser or laser system located within an enclosure which does not permit hazardous optical radiation emission from the enclosure. The laser inside is termed an "embedded laser."

Energy - The product of power (watts) and duration (seconds). One watt second = one Joule.

Energy (Q) - The capacity for doing work. Energy is commonly used to express the output from pulsed lasers and it is generally measured in Joules (J). The product of power (watts) and duration (seconds). One watt second = one Joule.

Energy Source - High voltage electricity, radiowaves, flashes of light, or another laser used to excite the laser medium.

Enhance Pulsing - Electronic modulation of a laser beam to produce high peak power at the initial stage of the pulse. This allows rapid vaporization of the material without heating the surrounding area. Such pulses are many times the peak power of the CW mode (also called "Superpulse").

Etalon - A Fabry-Perot interferometer with a fixed air gap separation. Such a device also serves as a basic laser resonant cavity.

Excimer - "Excited Dimer," a gas mixture used as the active medium in a family of lasers emitting ultraviolet light.

Excitation - Energizing a material into a state of population inversion.

Excited State - Atom with an electron in a higher energy level than it normally occupies.

Exempted Laser Product - In the U.S., a laser device exempted by the U.S. Food and Drug Administration from all or some of the requirements of 21 CFR 1040.

Extended Source - An extended source of radiation can be resolved into a geometrical image in contrast with a point source of radiation, which cannot be resolved into a geometrical image. A light source whose diameter subtends a relatively large angle from an observer.

F-Number - The focal length of lens divided by its usable diameter. In the case of a laser the usable diameter is the diameter of the laser beam or a smaller aperture which restricts a laser beam.

Fabry-Perot Interferometer - Two plane, parallel partially reflective optically flat mirrors placed with a small air gap separation (1-20 mm) so as to produce interference between the light waves (interference fringes) transmitted with multiple reflections through the plate.

Failsafe Interlock - An interlock where the failure of a single mechanical or electrical component of the interlock will cause the system to go into, or remain in, a safe mode.

Femtoseconds - 10(-15) seconds.

Fiberoptics - A system of flexible quartz or glass fibers with internal reflective surfaces that pass light through thousands of glancing (total internal) reflections.

Flashlamp - A tube typically filled with Krypton or Xenon. Produces a high intensity white light in short duration pulses.

Fluoroscence - The emission of light of a particular wavelength resulting from absorption of energy typically from light of shorter wavelengths.

Flux - The radiant, or luminous, power of a light beam; the time rate of the flow of radiant energy across a given surface.

Focal Lenght - Distance between the center of a lens and the point on the optical axis to which parallel rays of light are converged by the laser.

Focal Point - That distance from the focusing lens where the laser beam has the smallest diameter.

Focus - As a noun, the point where rays of light meet which have been reflected by a mirror or refracted by a lens, giving rise to an image of the source. As a verb, to adjust focal length for the clearest image and smallest spot size.

Folded Resonator - Construction in which the interior optical path is bent by mirrors; permit compact packaging of a long laser cavity.

Frequency - The number of light waves passing a fixed point in a given unit of time, or the number of complete vibrations in that period.

Gain - Another term for amplification.

Gas Discharge Laser - A laser containing a gaseous lasing medium in a glass tube in which a constant flow of gas replenishes the molecules depleted by the electricity or chemicals used for excitation.

Gas Laser - A type of laser in which the laser action takes place in a gas medium.

Gated Pulse - A discontinuous burst of laser light, made by timing (gating) a continuous wave output - usually in fractions of a second.

Gaussian Curve Normal - Statistical curve showing a peak with even distribution on either side. May either be a sharp peak with steep sides, or a blunt peak with shallower sides. Used to show power distribution in a beam. The concept is important in controlling the geometry of the laser impact.

Ground State - Lowest energy level of an atom.

Half-Power Point - The value on either the leading or trailing edge of a laser pulse at which the power is one-half of its maximum value.

Heat Sink - A substance or device used to dissipate or absorb unwanted heat energy.

Helium-Neon (HeNe) Laser - A laser in which the active medium is a mixture of helium and neon. Its wavelength is usually in the visible range. Used widely for alignment, recording, printing, and measuring.

Hertz (Hz) - Unit of frequency in the International System of Units (SI), abbreviated Hz; replaces cps for cycles per second.

Hologram - A photographic film or plate containing interference patterns created by the coherence of laser light. A three dimensional image may be reconstructed from a hologram. Here are transmission, reflection or integral holograms.

Image - The optical reproduction of an object, produced by a lens or mirror. A typical positive lens converges rays to form a "real" image which can be photographed. A negative lens spreads rays to form a "virtual" image which can't be projected.
Incident Light - A ray of light that falls on the surface of a lens or any other object. The "angle of incidence" is the angle made by the ray with a perpendicular to the surface.

Infrared Radiation (IR) - Invisible Electromagnetic radiation with wavelengths which lie within the range of 0.70 to 1000 ?m. These wavelengths are often broken up into regions: IR-A (0.7-1.4 ?m), IR-B (1.4-3.0 ?m) and IR-C (3.0-1000 ?m).

Integrated Radiance - Product of the exposure duration times the radiance. Also known as pulsed radiance.

Intensity - The magnitude of radiant energy.

Intrabeam Viewing - The viewing condition whereby the eye is exposed to all or part of a direct laser beam or a specular reflection.

Ion Laser - A type of laser employing a very high discharge current, passing down a small bore to ionize a noble gas such as argon or krypton.

Ionizing Radiation - Radiation commonly associated with X-Ray or other high energy electro-magnetic radiation which will cause DNA damage with no direct, immediate thermal effect. Contrasts with non-ionizing radiation of lasers.

Irradiance (E) - Radiant flux (radiant power) per unit area incident upon a given surface. Units: Watts per square centimeter. (Sometimes referred to as power density, although not exactly correct).

Irradiation - Exposure to radiant energy, such as heat, X-rays, or light.

Joule (J) - A unit of energy (1 watt-second) used to describe the rate of energy delivery. It is equal to one watt-second or 0.239 calorie.

Joule/cm(2) - A unit of radiant exposure used in measuring the amount of energy incident upon a unit area.

KTP - Potassium Titanyl Phosphate. A crystal used to change the wavelength of a Nd:YAG laser from 1060 nm (infrared) to nm (green).

Lambertian Surface - An ideal diffuse surface whose emitted or reflected radiance (brightness) is dependent on the viewing angle.

Laser - An acronym for light amplification by stimulated emission of radiation. A laser is a cavity, with mirrors at the ends, filled with material such as crystal, glass, liquid, gas or dye. A device which produces an intense beam of light with the unique properties of coherency, collimation and monochromaticity.

Laser Accessories - The hardware and options available for lasers, such as secondary gases, Brewster windows, Q-switches and electronic shutters.

Laser Controlled Area - See CONTROLLED AREA.

Laser Device - Either a laser or a laser system.

Laser Medium - (Active Medium) material used to emit the laser light and for which the laser is named.

Laser Oscillation - The buildup of the coherent wave between laser cavity end mirrors producing standing waves.

Laser Product - A legal term in the U.S. See 21 CFR 1040.10, a laser or laser system or any other product that incorporates or is intended to incorporate a laser or a laser system.

Laser Rod - A solid-state, rod-shaped lasing medium in which ion excitation is caused by a source of intense light, such as a flashlamp. Various materials are used for the rod, the earliest of which was synthetic ruby crystal.

Laser Safety Officer (LSO) - One who has authority to monitor and enforce measure to the control of laser hazards and effect the knowledgeable evaluation and control of laser hazards.

Laser System - An assembly of electrical, mechanical and optical components which includes a laser. Under the Federal Standard, a laser in combination with its power supply (energy source).

Leading Edge Spike - The initial pulse in a series of pulsed laser emissions, often useful in starting a reaction at the target surface. The trailing edge of the laser power is used to maintain the reaction after the initial burst of energy.

Lens - A curved piece of optically transparent material which depending on its shape is used to either converge or diverge light.

Light - The range of electromagnetic radiation frequencies detected by the eye, or the wavelength range from about 400 to 760 nanometers. The term is sometimes used loosely to include radiation beyond visible limits.

Light Regulation - A form of power regulation in which output power is monitored and maintained at a constant level by controlling discharge current.

Limiting Angular Subtense - The apparent visual angle which divides intrabeam viewing from extended-source viewing.

Limiting Aperture - The maximum circular area over which radiance and radiant exposure can be averaged when determining safety hazards.

Limiting Exposure Duration - An exposure duration which is specifically limited by the design or intended use(s).

Longitudinal or Axial Mode - Determines the wavelength bandwidth produced by a given laser system controlled by the distance between the two mirrors of the laser cavity. Individual longitudinal mode standing waves within a laser cavity.

Lossy Medium - A medium which absorbs or scatters radiation passing through it.

Maintenance - Performance of those adjustments or procedures specified in user information provided by the manufacturer with the laser or laser system, which are to be performed by the user to ensure the intended performance of the product. It does not include operation or service as defined in this glossary.

Maximum Permissible Exposure (MPE) - The level of laser radiation to which person may be exposed without hazardous effect or adverse biological changes in the eye or skin.

Meniscus Lens - A lens which has one side convex, the other concave.

Metastable State - The state of an atom, just below a higher excited state, which an electron occupies momentarily before destabilizing and emitting light. The upper of the two lasing levels.

Micrometer - A unit of length in the International System of Units (SI) equal to one-millionth of a meter. Often referred to as a "micron".

Micron - An abbreviated expression for micrometer which is the unit of length equal to 1 millionth of a meter. See MICROMETER.

Microprocessor - A digital chip (computer) that operates, controls and monitors some lasers.

Mode - A term used to describe how the power of a laser beam is geometrically distributed across the cross-section of the beam. Also used to describe the operating mode of a laser such as continuous or pulsed laser.

Mode Locked - A method of producing laser pulses in which short pulses (approximately 10-12 second) are produced and emitted in bursts or a continuous train.

Modulation - The ability to superimpose an external signal on the output beam of the laser as a control.

Monochromatic Light - Theoretically, light consisting of just one wavelength. No light is absolutely single frequency since it will have some bandwidth. Lasers provide the narrowest of bandwidths that can be achieved.

Multimode - Laser emission at several closely-spaced frequencies.

Nanometer (nm) - A unit of length in the International System of Units (SI) equal to one-billionth of a meter. Abbreviated nm - a measure of length. One nm equals 10(-9) meter, and is the usual measure of light wavelengths. Visible light ranges from about 400 nm in the purple to about 760 nm in the deep red.

Nanosecond - One billionth (10(-9)) of a second. Longer than a picosecond or femto-second, but shorter than a micro-second. Associated with Q-switched lasers.

Nd:Glass Laser - A solid-state laser of neodymium: glass offering high power in short pulses. A Nd doped glass rod used as a laser medium to produce 1064 nm light.

Nd:YAG Laser - Neodymium:Yttrium Aluminum Garnet. A synthetic crystal used as a laser medium to produce 1064 nm light.

Nd:YVO4 Laser - Neodymium:Yttrium Ortho Vanadate.  A synthetic crystal used as laser medum to produce 1064 nm light in high energy efficiency lasers.

Near Field Imaging - A solid-state laser imaging technique offering control of spot size and hole geometry, adjustable working distance, uniform energy distribution, and a wide range of spot sizes.

NEMA - Abbreviation for National Electrical Manufactures' Association, a group which defines and recommends safety standards for electrical equipment.

Neodymium (Nd) - The rare earth element that is the active element in Nd:YAG laser and Nd:Glass lasers.

Noise - Unwanted minor currents or voltages in an electrical system.

Nominal Hazard Zone (NHZ) - The nominal hazard zone describes the space within which the level of the direct, reflected or scattered radiation during normal operation exceeds the applicable MPE. Exposure levels beyond the boundary of the NHZ are below the appropriate MPE level.

Nominal Ocular Hazard Distance (NOHD) - The axial beam distance from the laser where the exposure or irradiance falls below the applicable exposure limit.

Object - The subject matter or figure imaged by, or seen through, an optical system.

Opacity - The condition of being non-transparent.

Open Installation - Any location where lasers are used which will be open to operating personnel during laser operation and may or may not specifically restrict entry to observers.

Operation - The performance of the laser or laser system over the full range of its intended functions (normal operation). It does not include maintenance or services as defined in this glossary.

Optic Disc - The portion of the optic nerve within the eye which is formed by the meeting of all the retinal nerve fibers at the level of the retina.

Optical Cavity - (Resonator) Space between the laser mirrors where lasing action occurs.

Optical Density - A logarithmic expression for the attenuation produced by an attenuating medium, such as an eye protection filter.

Optical Fiber - A filament of quartz or other optical material capable of transmitting light along its length by multiple internal reflection and emitting it at the end.

Optical Pumping - The excitation of the lasing medium by the application of light rather than electrical discharge.

Optical Radiation - Ultraviolet, visible and infrared radiation (0.35-1.4 ?m) that falls in the region of transmittance of the human eye.
Optical Resonator See Resonator.

Optically Pumped Lasers - A type of laser that derives energy from another light source such as a xenon or krypton flashlamp or other laser source.

Output Coupler - Partially reflective mirror in laser cavity which allows emission of laser light.

Output Power - The energy per second measured in watts emitted from the laser in the form of coherent light.

Phase - Waves are in phase with each other when all the troughs and peaks coincide and are "locked" together. The result is a reinforced wave in increased amplitude (brightness).

Photocoagulation - Use of the laser beam to heat tissue below vaporization temperatures with the principal objective being to stop bleeding and coagulate tissue.

Photometer - An instrument which measures luminous intensity.

Photon - In quantum theory, the elemental unit of light, having both wave and particle behavior. It has motion, but no mass or charge. The photon energy (E) is proportional to the EM wave frequency (v) by the relationship: E=hv; where h is Planck's constant (6.63 x l0(-34) Joule-sec).

Photosensitizers - Chemical substances or medications which increase the sensitivity of the skin or eye to irradiation by optical radiation, usually to UV.

Picosecond - A period of time equal to 10-12 seconds.

Pigment Epithelium - A layer of cells at the back of the retina containing pigment granules.

Plasma Shield - The ability of plasma to shop transmission of laser light.

Pockel's Cell - An electro-optical crystal used as a Q-switch.

Point Source - Ideally, a source with infinitesimal dimensions. Practically, a source of radiation whose dimensions are small compared with the viewing distance.

Pointing Errors - Beam movement and divergence, due to instability within the laser or other optical distortion.

Polarization - Restriction of the vibrations of the electromagnetic field to a single plane, rather that the innumerable planes rotating about the vector axis. Various forms of polarization include random, linear, vertical, horizontal, elliptical and circular.

Population Inversion - A state in which a substance has been energized, or excited, so that more atoms or molecules are in a higher excited state than in a lower resting state. This is necessary prerequisite for laser action.

Power - The rate of energy delivery expressed in watts (joules per second). Thus: 1 Watt = 1 Joule x 1 Sec.

Power Meter - An accessory used to measure laser beam power.

PRF - Pulse Repetition Frequency. The number of pulses produced per second by a laser.

Protective Housing - A protective housing is a device designed to prevent access to radiant power or energy.

Pulse - A discontinuous burst of laser, light or energy, as opposed to a continuous beam. A true pulse achieves higher peak powers than that attainable in a CW output.

Pulse Duration - The "on" time of a pulsed laser, it may be measured in terms of milliseconds, microsecond, or nanosecond as defined by half-peak-power points on the leading and trailing edges of the pulse.

Pulse Mode - Operation of a laser when the beam is intermittently on in fractions of a second.

Pulsed Laser - Laser which delivers energy in the form of a single or train of pulses.

Pump - To excite the lasing medium. See Optical Pumping or Pumping.

Pumped Medium - Energized laser medium.

Pumping - Addition of energy (thermal, electrical, or optical) into the atomic population of the laser medium, necessary to produce a state of population inversion.

Q-Switch A device that has the effect of a shutter to control the laser resonator's ability to oscillate. Control allows one to spoil the resonator's "Q-factor", keeping it low to prevent lasing action. When a high level of energy is stored, the laser can emit a very high-peak-power pulse.

Q-Switched Laser - A laser which stores energy in the laser media to produce extremely short, extremely high intensity bursts of energy.

Radian - A unit of angular measure equal to the angle subtended at the center of a circle by a chord whose length is equal to the radius of the circle.

Radiance - Brightness; the radiant power per unit solid angle and per unit area of a radiating surface.

Radiant Energy (Q) - Energy in the form of electromagnetic waves usually expressed in units of Joules (watt-seconds).

Radiant Exposure (H) - The total energy per unit area incident upon a given surface. It is used to express exposure to pulsed laser radiation in units of J/cm(2).

Radiant Flux/Radiant Power - The time rate of flow of radiant energy. Units-watts. (One [1] watt = 1 Joule-per-second). The rate of emission of transmission of radiant energy.

Radiant Intensity - The radiant power expressed per unit solid angle about the direction of the light.

Radiant Power - See Radiant flux.

Radiation - In the context of optics, electromagnetic energy is released; the process of releasing electromagnetic energy.

Radiometry - A branch of science which deals with the measurement of radiation.

Rayleigh Scattering - Scattering of radiation in the course of its passage through a medium containing particles, the sizes of which are small compared with the wavelength of the radiation.

Reflectance/Reflectivity - The ratio of the reflected radiant power to the incident radiant power.

Reflection - The return of radiant energy (incident light) by a surface, with no change in wavelength.

Refraction - The change of direction of propagation of any wave, such as an electromagnetic wave, when it passes from one medium to another in which the wave velocity is different. The bending of incident rays as they pass from one medium to another (eg.: air to glass).

Repetitively Pulsed Laser - A laser with multiple pulses of radiant energy occurring in sequence with a PRF greater than or equal to 1 Hz.

Resonator - The mirrors (or reflectors) making up the laser cavity including the laser rod or tube. The mirrors reflect light back and forth to build up amplification.

Rotating Lens - A beam delivery lens designed to move in a circle and thus rotate the laser beam around a circle.

Ruby - The first laser type; a crystal of sapphire (aluminum oxide) containing trace amounts of chromium oxide.

Scanning Laser - A laser having a time-varying direction, origin or pattern of propagation with respect to a stationary frame of reference.

Scintillation - This term is used to describe the rapid changes in irradiance levels in a cross section of a laser beam produced by atmospheric turbulence.

Secured Enclosure - An enclosure. to which casual access is impeded by an appropriate means (e.g., door secured by lock, magnetically or electrically operated, latch, or by screws).

Semiconductor Laser - A type of laser which produces its output from semiconductor materials such as GaAs.

Service - Performance of adjustments, repair or procedures on a non routine basis, required to return the equipment to its intended state.

Solid Angle - The ratio of the area on the surface of a sphere to the square of the radius of that sphere. It is expressed in steradians (sr).

Source - The term source means either laser or laser-illuminated reflecting surface, i.e., source of light.

Spectral Response - The response of a device or material to monochromatic light as a function of wavelength.

Specular Reflection - A mirror-like reflection.

Spontaneous Emission - Decay of an excited atom to a ground or resting state by the random emission of one photon. The decay is determined by the lifetime of the excited state.

Spot Size - The mathematical measurement of the diameter of the laser beam.

Stability - The ability of a laser system to resist changes in its operating characteristics. Temperature, electrical, dimensional and power stability are included.

Steradian - (sr) The unit of measure for a solid angle.

Stimulated Emission - When an atom, ion or molecule capable of lasing is excited to a higher energy level by an electric charge or other means, it will spontaneously emit a photon as it decays to the normal ground state. If that photon passes near another atom of the same frequency, the second atom will be stimulated to emit a photon.

Superpulse - Electronic pulsing of the laser driving circuit to produce a pulsed output (250-1000 times per second), with peak powers per pulse higher than the maximum attainable in the continuous wave mode. Average powers of superpulse are always lower than the maximum in continuous wave. Process often used on CO(2) surgical lasers. 

TEM - Abbreviation for: Transverse Electro-Magnetic modes. Used to designate the cross-sectional shape of the beam.

TEM(oo) - The lowest order mode possible with a bell-shaped (Gaussian) distribution of light across the laser beam.

Thermal Relaxation Time - The time to dissipate the heat absorbed during a laser pulse.

Threshold - The input level at which lasing begins during excitation of the laser medium.

Transmission - Passage of electromagnetic radiation through a medium.

Transmittance - The ratio of transmitted radiant energy to incident radiant energy, or the fraction of light that passes through a medium.

Transverse Electromagnetic Mode - The radial distribution of intensity across a beam as it exits the optical cavity. See TEM.

Tunable Laser - A laser system that can be "tuned" to emit laser light over a continuous range of wavelengths or frequencies.

Tunable Dye Laser - A laser whose active medium is a liquid dye, pumped by another laser or flashlamps, to produce various colors of light. The color of light may be tuned by adjusting optical tuning elements and-or changing the dye used.

Ultraviolet (UV) Radiation - Electromagnetic radiation with wavelengths between soft X-rays and visible violet light, often broken down into UV-A (315-400 nm), UV-B (280-315 nm), and UV-C (100-280 nm).

Vaporization - Conversion of a solid or liquid into a vapor.

Vignetting - The loss of light through an optical element when the entire bundle of light rays does not pass through; an image or picture that shades off gradually into the background.

Visible Radiation (Light) - Electromagnetic radiation which can be detected by the human eye. It is commonly used to describe wavelengths which lie in the range between 400 nm and 700-780 nm.

Watt - A unit of power (equivalent to one Joule per second) used to express laser power.

Watt/cm(2) - A unit of irradiance used in measuring the amount of power per area of absorbing surface, or per area of CW laser beam.

Wave - An sinusoidal undulation or vibration; a form of movement by which all radiant electromagnetic energy travels.

Wavelength The length of the light wave, usually measured from crest to crest, which determines its color. Common units of measurement are the micrometer (micron), the nanometer, and (earlier) the Angstrom unit.

Window - A piece of glass with plane parallel sides which admits light into or through an optical system and excludes dirt and moisture.

YAG - Yttrium Aluminum Garnet; a widely used solid-state crystal which is composed of yttrium and aluminum oxides which is often doped with a small amount of the rare-earth neodymium and then called Nd:YAG.

YVO4 - Yttrium Ortho Vanadate; a crystal used in energy efficient diode-pumped solid-state lasers, which is composed of yttrium and vanadium oxides.  The crystal is often doped with a small amount of the rare-earth neodymium and then called Nd:YVO4.