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Glossary of Laser Terms

ABLATION: The removal of material or tissue by melting, evaporation, or vaporization.

ABSORB: To transform radiant energy into a different form, usually with a resultant rise in temperature.

ABSORBANCE: The ability of a medium to absorb radiation depending on temperature and wavelength. Expressed as the negative common logarithm of the transmittance.

ABSORPTION COEFFICIENT: The amount of radiant energy absorbed per unit or path-length.

ACTIVE MEDIUM: A medium in which lasing will take place, rather than absorption, at a given wavelength.

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

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 amp stimulated emission on each pass through the active medium.

AMPLITUDE: The maximum value of the electromagnetic wave, measured from the mean to the extreme; put simply, the height of the wave. (See drawing under "POLARIZATION.")


ANGSTROM UNIT: A unit of measurement for a wavelength of light (written Å), equal to one ten billionth of a meter (10-10 meter). Occasionally still used.

ANODE: An electrical element in laser excitation which attracts electrons from a cathode. An anode can be cooled directly by water or by radiation.

AR COATINGS: Anti-reflection coatings, used on the backs of laser output mirrors to suppress unwanted multiple reflections which reduce power.

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.

AXIAL-FLOW LASER: The simplest and most efficient of the gas lasers. 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 state. (See "GAS DISCHARGE LASER.")

AXIS, OPTICAL AXIS: The optical center-line for a lens system; the line passing through the centers of curvature of the optical surfaces of a lens.

BEAM BENDER: Hardware assembly or optical device, such as a mirror, capable of changing laser beam direction; used to re-point the beam and in "folded," compact delivery systems.

BEAM DIAMETER: The diameter of that portion of the beam which contains 86% of the output power.

BEAM EXPANDER: Optical device increasing beam diameter and reducing divergence. Result: A smaller focused spot for more distance between lens and part.

BEAM SPLITTING: Optically splitting a laser beam into two or more beams, allowing work on more than one side of a part at the same time—but at somewhat less power than with a multiple-output beam system.

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 of vertically polarized light. Non-standard on industrial lasers, but a must if polarization is desired.

BRIGHTNESS: The visual sensation of the luminous power of a light beam, as opposed to scientifically measured power of the beam.

CALORIMETER: An instrument which measures the heat generated by absorption of the laser beam—another way to measure laser power.

CATHODE: The element providing the electrons for the electrical discharge used to excite the lasing medium.

CO2 LASER: A laser largely used in industry in which the primary lasing medium is carbon dioxide.

COAXIAL GAS: Most laser welding is done with a shield of inert gas flowing over the work surface to prevent plasma oxidation and absorption, to blow away debris, and to control heat reaction. The gas jet has the same axis as the beam so the two can be aimed together.

COHERENT LIGHT, COHERENT RADIATION: Radiation composed of wave trains vibrating in phase with each other. Simply expressed: parallel rays of light.

COLLIMATED LIGHT: Divergent light rays rendered parallel by means of a lens or other device, allowing a sharp image of the object to be focused at the focal plane of the lens.

COLLIMATION: The process by which divergent rays (white, or natural, light) are converted into parallel rays (coherent light).

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

CURRENT SATURATION: Maximum flow of electric force in a conductor; in a laser, the point at which further electrical charge will not increase lasing action.

CW: The continuous-emission mode of a laser, as opposed to pulsed operation.

DEPTH OF FIELD: The working range of the beam, a function of wavelength, diameter of the unfocused beam, and focal length of the lens. To achieve a small diameter spot size, and thus a high power density, a short depth of field must be accepted.

DIVERGENCE: The angle at which the laser beam spreads in the far field; the bending of rays away from each other, as by a concave lens or convex mirror.

DRIFT, ANGULAR: All undesirable variations in output (either amplitude or frequency); angular drift of the beam, measured in milliradians before, during, and after warm-up.

DUTY CYCLE: The length of time the laser beam is actually cutting, drilling, welding, or heat-treating, as compared to the entire work cycle time.

ELECTRIC VECTOR: The electric field associated with a light wave and having both direction and amplitude. Commonly represented by a line with an arrowhead.

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

EMISSIVITY, EMITTANCE: Rate at which emission takes place; the ratio of the radiant energy emitted by a source or surface to that emitted by a blackbody at the same temperature.

ENHANCED 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.

EXPOSURE: A measure of the total radiant energy incident on a surface per unit area; radiant exposure.

FAR-FIELD IMAGING: An imaging technique with solid-state lasers that has several limitations: non-uniform energy distribution, very short working distances, and poor control of hole geometry.

FLASHLAMP: Source of powerful light; often in the form of a helical coil and used to excite photon emission in a solid-state laser.

FLEXOGRAPHY: A printing process involving laser-engraved, seamless rubber plates.

FLUORESCENCE: The glow induced in a material when bombarded by light. Brewster windows of fused silica fluoresce in UV light, increasing absorption of laser radiation and degrading laser mode and output.

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

FOCUS: Noun: The point where rays of light meet which have been converged by a lens, giving rise to an image of the source. Verb: To adjust focal length for the clearest image.

FOCAL POINT: Same as first definition under "FOCUS;" in laser work, the focal point of the beam relative to the work surface has a critical effect, such as the depth and shape of drilled holes. When the focal point is at the surface, holes are of uniform diameter. When the focus is below the surface, conical holes are drilled.

FOLDED RESONATOR: Construction in which the interior optical path is bent by mirrors mounted on corner blocks bolted into pre-aligned position, permitting compact packaging of a long laser cavity.

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

GAIN: Another term for amplification, usually referring to the efficiency of a lasing medium in attaining a population inversion. High gain is typically more than 50% per pass of the light wave between cavity mirrors.

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. The discharged gas can be filtered and 90% recycled for economy.

GAS JET ASSIST: An assisting coaxial gas, such as oxygen, argon, or nitrogen, which may be used to achieve very high power levels for cutting certain metals.

GAS TRANSPORT: A laser design which generates very high beam power within a fairly small resonator structure. Long electrodes parallel the axis and gas is circulated across the resonator cavity.

GAUSSIAN: The "normal curve," or normal distribution, an example of which is the symmetrical bell shape of the holes created by the uncorrected, unfocused laser beam in its optimum mode. A Gaussian laser beam has most of its energy in the center.

HAZ: Heat-Affected Zone, or the area where laser beam and metal (or other) surface are in contact.

HELIUM-NEON LASER: ("HeNe"), Laser in which the active medium is a mixture of helium and neon, which is in the visible range. Used widely in industry for alignment, recording, printing, and measuring, it is also valuable as a pointer or aligner of invisible CO2 laser light.

HEAT SINK: A substance or device used to dissipate or absorb unwanted heat, as from a manufacturing process (or, with lasers, from reflected rays).

HERTZ: The approved international term, abbreviated Hz, which replaces CPS for cycles per second.

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.

INTENSITY: The magnitude of radiant energy (light) per unit, such as time or reflecting surface.

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. The ionization process creates a population inversion for lasing to occur. A research laser useful for some industrial applications.

IONIZATION: The process by which ions are formed.

IRRADIATION: Exposure to radiant energy, such as heat, X-rays, or light; the product of irradiance and time.

JOULE: One watt per second; a measurement frequently given for laser output in pulsed operation.

KEYHOLING: In welding, the deep-penetration holes, which fill quickly with molten metal, that can be made in a few milliseconds by laser.

LASER: An acronym of Light Amplification by Stimulated Emission of Radiation. A laser is a cavity that has mirrors at the ends and is filled with lasable material such as crystal, glass, liquid, gas, or dye. These materials must have atoms, ions, or molecules capable of being excited to a metastable state by light, electric discharge, or other stimulus. The transition from this metastable state back to the normal ground state is accompanied by the emission of photons which form a coherent beam.

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

LASER HARDENING: Laser-beam traversal of metal to harden quenching process producing the maximum hardness for most metals.

LASER OSCILLATION: The buildup of the coherent wave between laser cavity end mirrors. In CW mode, the wave bounding back and forth between mirrors transmits a fraction of its energy on each trip; in pulsed operation, emission happens instantaneously.

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.

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.

LIGHT: The range of electromagnetic radiation frequencies detected by the eye, or the wavelength range from about 400 to 750 nanometers. It is sometimes extended to include photovoltaic effects and radiation beyond visible limits.

A form of power regulation in which output power is maintained at a constant level by controlling discharge current.

LUMINANCE: Commonly called illumination; the luminous or visible flux per unit area on a receiving surface at any given point.

MENISCUS LENS: The lens used primarily in CO2 lasers by Coherent, Inc. It has one side convex, the other concave.

METASTABLE, METASTABLE STATE: Unstable condition in which the energy of a molecule is at some discrete level above the lowest, or ground state. It is this condition which is necessary for emission of photons in a laser. (From quantum theory.)

MICROMETER: A "MY-kro' mee-ter" is a unit of length equal to one millionth of a meter, or one micron. (Not to be confused with my-KROM'-a-ter, a measuring instrument used with a telescope, microscope, or laser for fine focusing).

MILLIJOULE: One thousandth of a Joule.

MODE: A particular functioning arrangement, setup, or condition for laser operation, such as continuous emission, pulses, or grouped pulses. "Mode" also describes the cross-sectional shape of the beam. (See "TEM.")

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. Since no light is completely monochromatic, it usually consists of a very narrow band of wavelengths. Lasers provide the narrowest bands.

NANOMETER: A unit of length in the International System of Units (SI) equal to one billionth of a meter (10-9 meter). Once called a millimicron, it is used to represent wavelength. Abbreviated "nm."

NEAR FIELD IMAGING: A solid-state laser imaging technique offering control of spot size and hole geometry, adjustable working distance, uniform energy distribution, and easily produced range of spot sizes.

Nd:GLASS LASER: A solid-state laser of Neodymium:glass offering high power or short pulses, or both, for specific industrial applications.

Nd:YAG LASER: A solid-state laser of Neodymium:Yttrium-Aluminum Garnet, similar to the Nd:glass laser. Both are pumped by flashlamp.

NEMA: National Electrical Manufacturers' Association, a group which defines and recommends safety standards for electrical equipment.

NOISE: Unwanted, minor currents or voltages in an electrical system.

OBJECT: The subject matter or figure imaged by, or seen through, an optical system.

OPTICAL DENSITY: Protection factor provided by a filter (such as used in eyewear, viewing windows, etc.) at a specific wavelength. Each unit of OD represents a 10x increase in protection.

OPTICAL FIBER: Filament of quartz or other optical material capable of transmitting light along its conformation and emitting it at the end.

OPTICAL PUMPING: Exciting the lasing medium by the application of light rather than electrical discharge from anode and cathode.

OUTPUT COUPLER: The resonator mirror which transmits light; the one at the opposite end is totally reflective.

OUTPUT POWER: The energy per second emitted from the laser in the form of coherent light, usually measured in watts for continuous-wave operation and joules for pulsed operation.

OXYGEN ASSIST: In certain cutting operations, coaxial oxygen initiates an exothermic reaction to enhance the cutting rate for thick metals; in other words, oxygen actually does the cutting, with the reaction being maintained by the laser beam.

PHOTOACOUSTIC EFFECTS: Arises with the use of very short-duration high-energy laser pulses, at pulse durations typically below 10 microseconds. Significant amounts of energy are absorbed and a rapid expansion occurs in the tissue, generating an acoustic shock wave that causes mechanical disruption to cellular structures.

PHOTOCHEMICAL EFFECTS: Effects that occur from long exposure durations at incident power levels insufficient to cause damaging photothermal effects. It is an energy dependent process (a function of the total quantity of radiation absorbed rather than its rate of absorption).

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.

PHOTOTHERMAL EFFECTS: The damage mechanism for acute laser injury (i.e. for injury immediately following exposure). The radiation incident at the surface is absorbed in the underlying tissue, increasing the temperature of the tissue to the level at which damage can occur, and laser burns result. It is a power dependent process (a function of the RATE at which energy is absorbed rather than the total quantity of energy involved).

PLASMA: In laser welding, a metal vapor that forms above the spot where the beam reacts with the metal surface. Also used to describe the laser tube (plasma tube, discharge tube) which contains the completely ionized gas in certain lasers.

POINTING ERRORS: Beam movement and divergence, often preventable by using short path-lengths.

POLARIZATION: Restriction of the vibrations of the electromagnetic field to a single plane, rather than the innumerable planes rotating about the vector axis. This prevents optical losses at interfaces between the lasing medium and optical elements. Various forms of polarization include random, linear (plane), vertical, horizontal, elliptical, and circular. Of two polarization components (so-called), S and P, the P component has zero losses at Brewster's angle. (See "BREWSTER WINDOWS.") l = wavelength E = electric vector H = magnetic vector.

POPULATION INVERSION: When more molecules (atoms, ions) in a laser are in a metastable state than in the ground state (a situation needed for sustaining a high rate of stimulated emissions), a "population inversion" is said to exist. Without a population inversion, there can be no lasing action.

POWER DENSITY: The amount of radiant energy concentrated on a surface.

POWER METER: An accessory used to monitor laser beam power at the rear reflector, tune the beam for optimum power, or monitor power delivered to the work station.

POWER RAMPING: A controlled change in the power level of a laser beam, either linearly, as up a ramp, across, and down again—or in several discrete steps. Useful for smooth completion of circular welds and for preventing fractures from rapid cooling.

PULSE ENERGY: The power of a single, brief emission from a laser programmed for pulsed behavior rather than continuous operation. Pulse energy can be several times greater than CW emission.

PULSE TAIL: Pulse decay time, which can be shortened (by using a special mixture of gases) to allow for fast repetition of laser pulses within a given length of time.

PUMP: To excite the lasing medium (see "OPTICAL PUMPING").

Q: The energy-storing efficiency of a laser resonator. The higher the "Q," the less energy loss.

Q-SWITCH: A device that has the effect of a shutter moving rapidly in and out of the beam to "spoil" the resonator's normal Q, keeping it low to prevent lasing action until a high level of energy is stored. Result: a giant pulse of power when normal Q is restored.

RADIAN: An arc in a circle, equal in length to the radius; an angle (57.3°) at the center of a circle, formed by 2 radii cutting off such an arc. Thus one rad = 57.3°.

RADIANCE: Brightness; the radiant energy per unit solid angle and per unit projected area of a radiating surface.

RADIANT ENERGY: Energy traveling as wave motion; specifically, the energy of electromagnetic waves (light, X-rays, radio, gamma rays).

RADIANT FLUX: The rate of emission or transmission of radiant energy.

RADIANT INTENSITY: Radiant power, or flux, expressed as emission per unit solid angle about the direction of the light in a given length of time.

RADIANT POWER: The amount of radiant energy available per unit; the radiant flux.

RAMAN EFFECT: Part of the energy in a photon is transferred to (or from) the vibrational/rotational energy of a molecule.

RAMPER: Power controls for accomplishing the ramping steps; may be computer-controlled.

REFLECTANCE: The ratio of the reflected flux to the incident flux, or the ratio of reflected light to light falling on the object.

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. Simply put, the bending of incident rays as they pass from one medium to another, such as air to water.

RESOLUTION: Resolving power, or the quantitative measure of the ability of an optical instrument to produce separable images of different points on an object; the capability of making distinguishable the individual parts of an object, closely adjacent images, or sources of light.

RESONATOR: The mirrors (or reflectors) making up the laser cavity containing the laser rod or tube. The mirrors reflect light back and forth to build up amplification under an external stimulus. Emission is through one of them, called a coupler, which is partially transmissive.

RMS: Units of electronic noise; the letters stand for Root-Mean-Square.

ROCKWELL C: A scale or test used to define hardness in metals, particularly steel and titanium.

ROTATING LENS: Beam delivery in a circular movement for cutting large-diameter holes. (De-focusing the beam for this lowers power density and increases drilling time.)

SPECTRAL RESPONSE: The response of a device or material to monochromatic light as a function of wavelength.

STABILITY: Temperature, electrical, dimensional, and power stability are important attributes in a working laser. The Everlase™ series of industrial lasers achieve such stability through circulating coolant, cast iron end pieces, permanent alignment, electrical control, and kinematic mounting. Result: long-term variation of only +/- 2% of rated power.

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 which is also at some metastable energy level, the second atom will be stimulated to emit a photon. Both photons will be of the same wavelength, phase, and spatial coherence. Light amplified in this manner is intense, coherent (collimated or parallel), and monochromatic. In short, laser light.

TEM: Abbreviation for Transverse Electromagnetic Mode, the cross-sectional shape of the working laser beam. An infinite number of shapes can be produced, but only a relatively small number are needed for industrial applications. In general, "the higher the TEM, the coarser the focusing."

  • TEM00: A Gaussian-curve mode that is the best collimated and produces the smallest spot of high power density for drilling, welding and cutting.
  • TEM01: Divided into two equal beams for special applications.

THRESHOLD: During excitation of the laser medium, this is the point where lasing begins.

TRANSFORMATION HARDENING: A process well-suited to lasers which involves hardening metal by heating it to the critical temperature for transformation, quenching it, and solidifying it with uniform distribution of its carbon content.

TRANSMISSION: In optics, the passage of radiant energy (light) through a medium.

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

TREPANNING THE BEAM: Relative motion of the beam with respect to the part, usually in a circular fashion (see "ROTATING LENS").

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

VISIBLE LIGHT TRANSMISSION/TRANSMITTANCE: The amount of visible light usable to the eye that passes through a filter. As a rule of thumb, as optical density increases, visible light transmission decreases — but not always.

WAVE: An undulation or vibration, a form of movement by which all radiant energy of the electromagnetic spectrum is thought to travel.

WAVELENGTH: The fundamental property of light—the length of the light wave, which determines its color. Common units of measurement (which is usually from crest to crest) are the micron, the nanometer, and (earlier) the angstrom.

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