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# Glossary

Absorb
To transform radiant energy into a different form, with a resultant change in temperature.
Absorption:
Transformation of radiant energy to a different form of energy by the action of matter, depending on temperature and wavelength.
Absorption coefficient
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. Also means radiation to which human access is possible in accordance with the definitions of the laser's hazard classification.
Accessible emission limit (AEL)
The maximum accessible emission level permitted within a particular class of lasers. In ANSI Z-136.1, AEL is determined as the product of Accessible Emission Level Maximum Permissible Exposure (MPE) limit and the area of the limiting aperture (7mm for visible and near infrared lasers).
Achromatic lens
A usually two-element lens that is corrected for chromatic aberration to bring two specified or distinct wavelengths to a common focal point.
Active medium
Collection of atoms 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, an aiming beam may also be a reduced level of the actual laser used for surgery or for other applications.
Ambient noise
The noise at a given location (such as in a room, a compartment, or a place outdoors), usually a composite of a number of sources.
Ambient temperature
The prevailing temperature of air or other media in a designated area, particularly the area surrounding an object.
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 electromagnetic wave, measured from the mean to the extreme. Stated simply, the height of the wave.
Analog modulation
Analog modulation is a type of modulation that requires an analog input signal to vary the output power of a laser. Our analog lasers are capable of modulation from CW to 20MHz. A benefit of analog modulation is that it allows users to adjust a laser’s output to a desired level from
Analog signal
A continuously varying signal (voltage or current), not pulsed or discrete in nature.
Anamorphic correction
An optical technique using one or two prisms to change an elliptical laser beam into a round beam.
Angstrom unit
A unit of measurement of wavelength equal to 10-10 meter, 0.1 nanometer, or 10-4 micrometer. No longer widely used or recognized in the SI system of units.
Angular drift
Any unintended change in direction of the beam before, during, and after warm-up, measured in mrad.
Antireflection coating
A thin layer of material applied to the surface of a lens to reduce reflected energy.
Anode
An electrical element in laser excitation that attracts electrons from a cathode.
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.
Aperture
A passageway through which radiation or matter may pass.
AR coatings
Antireflection coatings used on optical components to suppress unwanted reflections off the surfaces.
Argon
A gas used as a laser medium. It emits blue/green light primarily at 448 and 515 nm.
Articulated arm
CO2 laser beam delivery device consisting of a series of hollow tubes and mirrors interconnected so as to maintain alignment of the laser beam along the path of the arm.
Astigmatic correction
An optical technique using a cylindrical lens to correct for the astigmatism inherent in laser diodes so that emissions from both the parallel and perpendicular planes come to focus at the same image plane.
Astigmatism
A lens aberration in which light rays from a single point fail to converge in a single focal point.
Attenuation
The decrease in energy (or power) as a beam passes through an absorbing or scattering medium.
Attenuator
An electronic transducer that reduces the amplitude of a signal without distorting its waveform.
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.
Automatic power control mode
Laser diode drivers operating in this mode have a photodiode mounted inside the laser diode package. The photodiode generates a feedback signal that allows gauging and adjustment of the current supply, keeping the output power of the laser constant over temperature changes and time.
Average power
The total energy imparted during exposure divided by the exposure duration.
Aversion response
Movement of the eyelid or the head to avoid exposure to a noxious stimulant (e.g., a bright light). It can occur within 0.25 seconds, and it includes the blink reflex time.
Axicon lens
A conical lens that, when followed by a conventional lens, can focus laser light to a ring shape.
Axis, optical axis
The optical centerline for a lens system. The line passing through the centers of curvature of the optical surfaces of a lens.
Ballast resistor
This mechanism is placed in series with a laser to promote a stable electrical discharge and to prevent the laser from acting as an oscillator.
Bandpass
The range of frequencies that will pass through a filter.
Bandpass filter
A filter that ideally passes all frequencies between two non-zero finite limits and that bars all frequencies not within the limits.
Beam
A collection of rays that may be parallel, convergent, or divergent.
Beam diameter
Along any specified line that (a) intersects the beam axis and (b) lies in any specified plane normal to the beam axis, the distance between two exactly opposite points at which the irradiance is a specified fraction (e.g., 1/e2) of the beam’s peak power.
Beam divergence
The tendency of a laser beam to expand in diameter as it moves away from the source, measured in milliradians (mrad) at specified points.
Beam expander
An optical system designed to increase the diameter of a laser beam. Term generally refers to a telescope that transforms a collimated input beam with a small diameter into a collimated output beam with a larger diameter, thereby reducing the divergence of the beam.
Beam noise
Undesirable, unpredictable fluctuations in the output of a laser module. Potential sources for noise include the laser diode drive circuit or any number of external interferences. Laser modules that operate in automatic power control mode are designed to compensate for and reduce low-frequency power fluctuations.
Beamsplitter
An optical device (e.g., a partially reflecting mirror) for splitting a single beam into two or more individual beams.
Beam waist
With Gaussian laser output, the point at which the curvature of the beam is zero. This corresponds to the point at which the beam diameter is at a minimum. For lasers with elliptical beam shapes, the beam diameter is given for both the major and the minor axes.
Bias current
Bias current is the steady current applied to a laser diode to overcome the threshold current of the laser diode.
See aversion response.
Brewster's 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 zeroreflective loss for one axis of plane polarized light. They are non-standard on industrial lasers but essential 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 radiometric concept of radiance.
Calorimeter
An instrument that measures energy, usually as heat generated by absorption of the laser beam.
Carbon dioxide
Molecule used as a laser medium. Emits far infrared energy at 10,600 nm (10.6 micrometers).
Cathode
A negatively-charged electrical element providing electrons for an electrical discharge.
Charge-coupled device (CCD)
An image-sensing detector that consists of a matrix of microscopic sensing elements, each of which corresponds to an image pixel. The CCD converts an optical image into electrical signals that represent the information contained within each pixel. These signals may be extracted sequentially and stored, or they may otherwise be processed in digital format for the purpose of transmitting or storing a digital representation of the optical image.
CIE
Abbreviation for Commission International de L'Eclairage, the French translation for International Commission on Illumination.
One or more layers of material of lower refractive index that surround the core of an optical fiber. Works to contain core light and protect against surface contaminant scattering.
Clear aperture (CA)
Also known as free aperture or objective aperture. The opening in the mount of an optical system or its components that restricts the extent of the bundle of rays incident on the given surface. It is usually circular and specified by its diameter.
Closed installation
Any location where lasers are used that will be closed to unprotected personnel during laser operation.
CO2 laser
A widely used laser in which the primary lasing medium is carbon dioxide gas. The output wavelength is 10.6 micrometers in the far infrared spectrum. It can be operated in either CW or pulsed mode.
Coaxial gas
A shield of inert gas flowing over the target material 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.
Coherence
Pertains to two or more waves that have a fixed phase relationship over time. When wave crests simultaneously meet other crests, while troughs meet other troughs, the waves are deemed to be in phase. When the crests of one wave simultaneously meet the troughs of another, they are out of phase. When crests and troughs meet randomly, they are deemed incoherent.
Coherence length
The distance over which energy in two separate waves remain in phase.
Collimation
The process by which divergent beams are converted into parallel beams (coherent light).
Collimated light
Light in which all of the rays are considered to be parallel to one another.
Combiner mirror
The mirror in a laser that combines two or more wavelengths into a coaxial beam.
Convergence
In optics, the bending of light rays toward one another.
Constant current mode
Laser diode drivers operating in this mode do not utilize a photodiode. Instead, they provide a set level of current to the diode. The diode output power will drift when the temperature changes due to the inherent properties of the semiconductor material. A thermoelectric cooler can help prevent power and wavelength drift by stabilizing the operating temperature of the laser diode.
Continuous wave (CW) laser modules
These modules emit energy continuously rather than in short pulses. Continuous wave applications require the laser to be on 100% of the time.
Continuous mode
The duration of laser exposure is controlled by the user (by foot or hand switch).
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.
Depth of field
The distance between the nearest and furthest parts of an object that are acceptably sharp while the lens is in focus for a specific distance.
Depth of focus
The range of image distances over which the system can deliver a sharp image (as defined by the user), e.g., the distance that photographic film or printing paper can be moved and still maintain an acceptably sharp focus.
Dichroic filter
Filter that allows selective transmission of desired wavelengths while reflecting others.
Diffraction
The deviation of a wavefront of light from the path predicted by geometric optics when the wavefront interacts with (or is restricted by) a physical object such as an aperture or an edge. Secondary wavefronts develop from this interference, and these wavefronts not only interact with the primary wavefront, but they will interfere with one another to form various diffractive patterns.
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 1/e or 1/e2 of the maximum value, depending on which criteria is used.
Dosimetry
Measurement of the power, energy, irradiance, or radiant exposure of light delivered to tissue.
Drift
All undesirable variations in output (either amplitude or frequency).
Duty factor
Also known as duty cycle, duty factor (Df) is the ratio of pulse duration to pulse period.
Effective focal length (EFL)
Distance from the principal point to the focal point of a lens.
Electric vector
The electric field associated with a light wave that has both direction and amplitude.
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 radio waves as well as short cosmic rays. Wavelengths cover a range from 1 Hz to perhaps as high as 1020 Hz.
Electromagnetic wave
A disturbance that 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.
Electrostatic discharge (ESD)
The sudden transfer of accumulated static charge (typically of high voltage at low current) from one object to another. ESD can be extremely harmful to semiconductor diodes, causing failure or a significant decrease in lifetime. Users of semiconductor lasers should always wear antistatic devices when working with these devices.
Embedded laser
A laser with an assigned class number higher than the inherent capability of the laser system in which it is incorporated, whereby the system's 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 1/e or 1/e2 irradiance points.
Emission
Act of giving off radiant energy by an atom or molecule.
Emissivity
The ratio of the radiant energy emitted by any source to that emitted by a blackbody at the same temperature.
Emittance
The rate at which emission occurs.
Emitter size
The size of the emitter of a laser diode expressed in microns. Typical sizes are 1um by 3µm for low power index-guided diodes. High power gain-guided laser diodes have emitter sizes from 1µm by 50µm to 1µm by 500µm. Bars and arrays are typically made up of several 1µm by 500µm gain-guided "chips" spaced evenly across distances of up to 10mm (10,000µm).
Enclosed laser device
Any laser or laser system located within an enclosure that 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 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.
Enhanced pulsing
Electronic modulation of a laser beam to produce high peak power at the initial stage of the pulse. 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").
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 that 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.
Failsafe interlock
An interlock whereby 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
Ten to the negative 15th seconds.
Fiber coupling
A method of directing a laser’s output into a single mode or multi-mode fiber optic.
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.
Fluorescence
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.
F-number
The focal length of lens divided by its useable diameter. In the case of a laser, the useable diameter is the diameter of the laser beam or a smaller aperture that restricts a laser beam.
Focal length
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. The effective focal length (EFL) is the distance from the principal point to the focal point. The back focal length (BFL) is the distance from the vertex of the last lens to the second focal point. The front focal length (FFL) is the distance from the first lens surface to the first focal point.
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 that 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. Permits 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 of time.
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 beam
A beam of light that has most of its energy in the center. The irradiance of the beam is symmetric about the beam axis and varies with radial distance r from the axis as
$I(r)=I_0*exp(\frac{-2r^2}{r_1^2})$
where r1 is the radial extent of the beam where the irradiance has dropped to 1/e2 of its value on the beam axis, I0.
Gaussian curve
Statistical curve showing a peak with normal, 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
The most widely-used gas laser. Available in a variety of wavelengths from the green to the red to the infrared, the most common wavelength being 633 nm (red). Emits highly monochromatic radiation.
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.
Image
The optical reproduction of an object, produced by a lens or mirror. A typical positive lens converges rays to form a "real" image that can be photographed. A negative lens spreads rays to form a "virtual" image that cannot 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.
Incoherence
Pertains to two or more waves that do not have a fixed phase relationship with respect to one another.
Invisible electromagnetic radiation with wavelengths that lie within the range of 0.70 to 1000 micrometers. This region is often broken up into IR-A, IR-B, and IR-C.
Product of the exposure duration and radiance. Also known as pulsed radiance.
Intensity
Interferometer
A device that uses the principle of interference of electromagnetic waves for measurement purposes (e.g., to measure wavelength or temperature).
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.
Radiation commonly associated with X-ray or other high energy electro-magnetic radiation that will cause DNA damage with no direct, immediate thermal effect. Contrasts with non-ionizing radiation of lasers.
Iris diaphragm
Device that uses thin overlapping plates to change the diameter of a central opening and, therefore, to control the amount of light allowed through it. Enables users to adjust the diameter of a beam.
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.)
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/cm2
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 an Nd:YAG laser from 1060nm (infrared) to 532nm (green).
Lambertian surface
An ideal diffuse surface whose emitted or reflected radiance (brightness) is dependent on the viewing angle.
Laser
An acronymn for light amplification by stimulated emission of radiation. A cavity with mirrors at the ends, filled with material such as crystal, glass, liquid, gas, or dye. A device that produces an intense beam of light with the unique properties of coherency, collimation, and mono-chromaticity.
Laser medium
Also known as active medium. Material used to emit 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 measures to the control of laser hazards and affect the knowledgeable evaluation and control of laser hazards.
Laser system
An assembly of electrical, mechanical, and optical components that includes a laser. Under the Federal Standard, a laser in combination with its power supply (energy source).
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 that, depending on its shape, is used to either converge or diverge light.
LIDAR
An acronymn for light detection and ranging that describes a remote sensing technique that uses a laser beam to probe the characteristics of a surface target.
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 that 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 that is specifically limited by the design or intended use(s).
Linear polarization
With respect to radiation, a light beam in which the polarization direction is fixed.
Longitudinal mode
Also referred to as 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 modes are produced by standing waves within a laser cavity.
Lossy medium
A medium that absorbs or scatters radiation passing through it.
M2 value
Describes the deviation of the laser beam from a theoretical, ideal Gaussian beam profile. For a theoretical Gaussian beam, the M2 value is equal to 1.0. For a typical laser beam, the M2 value is greater than 1.0. Collimated TEM00 diode laser beams typically have an M2 value ranging from 1.1 to 1.7.
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 permissable exposure (MPE)
The level of laser radiation to which a person may be exposed without hazardous effect or adverse biological changes in the eye or skin.
Meniscus lens
A lens that has one side convex and the other concave.
Metastable state
The state of an atom, just below a higher excited state, that 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 one-millionth of a meter. See micrometer.
Microprocessor
A digital chip (computer) that operates, controls, and monitors lasers.
1/1000 of a radian (1x10<sup-3radians). Usually used to specify the divergence of a laser beam.
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 (e.g., continuous or pulsed).
Mode locked
A method of producing laser pulses in which short pulses (approximately 10- to 12-second) are produced and emitted in bursts or a continuous train.
Modulated lasers
Allow users to vary the output power of a laser by varying a control voltage. The laser is activated only when needed. Beam modulation may be used to synchronize a laser with an analytical instrument or camera. Several of our laser diode modules employ one of two types of modulation
analog or TTL.
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 (10-9) of a meter. The usual measure of light wavelengths. Visible light ranges from about 400nm in the violet to about 760nm in the deep red.
Nanosecond
One-billionth (10-9) of a second. Longer than a picosecond or femto- second, but shorter than a microsecond. Associated with Q-switched lasers.
Nd:Glass laser
A solid-state laser of neodymium glass offering high power in short pulses. An Nd-doped glass rod used as a laser medium to produce 1064nm light.
Nd:YAG laser
A solid-state laser that uses a rod of yttrium aluminum garnet (YAG) doped with neodymium to serve as the lasing medium.
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 that 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.
Neutral density filter
A gray filter that decreases the intensity of light without altering the wavelength.
Noise
Unwanted minor currents or voltages in an electrical system.
Nominal hazard zone (NHZ)
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.
Numerical aperture (NA)
Expresses the ability of a lens to resolve fine detail in an object being observed. Equal to the refractive index of the lens multiplied by the sine of the cone angle of the lens. Generally measured with respect to an object or image point, and will vary as the point is moved.
$NA=n_0*sin(q)$
where n0 is the index of refraction of the medium surrounding the image (normally 1.0 for air), and q is the angle of light as it narrows to the focal point.
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 in which lasers are used that 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 that is formed by the meeting of all the retinal nerve fibers at the level of the retina.
Optical cavity
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.
Ultraviolet, visible, and infrared radiation (0.35 to 1.4 æm) 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 a laser cavity that 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 that 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 10-34 Joule-sec).
Photosensitizers
Chemical substances or medications that 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.
Planoconcave lens
Lens with one cancave surface (curves in) and one flat surface.
Planoconvex lens
Lens with one convex surface (curves out) and one flat surface.
Plasma shield
The ability of plasma to stop transmission of laser light.
Pockel's cell
An electro-optical crystal used as a Q-switch.
Pointing 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 vector to a single plane. Prevents optical losses at interfaces between the lasing medium and optical elements.
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 a necessary prerequisite for laser action.
Power
The rate of energy delivery expressed in Watts (Jjoules per second). Thus, 1 Watt = 1 Joule x 1 second.>
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
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. 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.
Pulsed lasers
Lasers that emit energy in a series of short bursts, or pulses, and are inactive between each pulse. Pulsed lasers—not to be confused with modulated lasers—typically deliver several Watts of peak power per pulse.
Pulse mode
Operation of a laser when the beam is on intermittently in fractions of a second.
Pump
To excite the lasing medium. See pumping or optical 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 that stores energy in the laser media to produce extremely short, high-intensity bursts of energy.
Quantum efficiency (QE)
In an optical source or detector, the ratio of the number of generated photons per second to the number of electrons flowing per second.
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 (180/pi, numerically). Corresponds to an angle of 57.29578 degrees.
Brightness. The radiant power per unit solid angle and per unit area of a radiating surface.
Energy in the form of electromagnetic radiation, usually expressed in Joules.
The total energy per unit area incident upon a given surface. Used to express exposure to pulsed laser radiation in units of J/cm2.
Also known as radiant power. The time rate of flow of radiant energy. Units-Watts. (1 Watt = 1 Joule-per-second). The rate of emission of transmission of radiant energy.
The radiant power expressed per unit solid angle about the direction of the light.
In the context of optics, electromagnetic energy is released. The process of releasing electromagnetic energy.
A branch of science that 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 or 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 medium in which the wave velocity is different. The bending of incident rays as they pass from one medium to another (e.g., air to glass).
Repetitively pulsed laser
A laser with multiple pulses of radiant energy occurring in sequence with a prf > 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.
Scintillation
Term 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 or latch).
Self-contained lasers
Include the laser, optics, and power supply in a single case.
Semiconductor laser
A type of laser that produces its output from semiconductor materials such as GaAs.
Separated geometry lasers
Offer added flexibility by housing the diode and lens in their own laser head assembly, separate from the power supply. This configuration gives users a choice among laser head sizes and makes incorporation into an application easier.
Service
Performance of adjustments, repair, or procedures on a non-routine basis. Required to return the equipment to its intended state.
Single mode
Laser emission in a single transverse mode, or laser emission at a single primary wavelength.
Solid angle
The ratio of the area on the surface of a sphere to the square of the radius of that sphere. Expressed in steradians (sr).
Source
Either a laser or laser-illuminated reflecting surface, i.e., source of light. A physical source of radiation such as a laser diode.
Source size
The physical size of a source of radiation, such as from a laser diode. Also see emitter size.
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.
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 to 1000 times per second), with peak powers per pulse higher than the maximum attainable in continuous wave mode. Average powers of superpulse are always lower than the maximum in continuous wave. Process often used on CO2 surgical lasers.
TEM
Abbreviation for transverse electro-magnetic modes. Used to designate the cross-sectional shape of the beam.
TEM00 mode
Fundamental (lowest order standing wave) transverse electromagnetic mode in a laser. With TEM modes, both the electric and magnetic field vectors are normal to the propagation of the beam. The TEM00 mode has a Gaussian energy distribution across the beam.
Thermal relaxation time
The time to dissipate the heat absorbed during a laser pulse.
Thermoelectric (TE) cooler
A device that helps maintain a cool, stable temperature for a laser diode. The benefits of temperature control include prolonging the life of the laser diode, maintaining a stable output power, and promoting wavelength stability.
Threshold current (Ith)
The minimum driving current required to stimulate laser output.
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 mode
Mode that has a field vector normal to the direction of propagation. In TE (transverse electric) mode, the electric field vector is normal. That is, the electric field is perpendicular to the direction of propagation. In TM (transverse magnetic) mode, the magnetic field vector is normal, meaning that it is perpendicular to the direction of propagation. In TEM (transverse electomagnetic) mode, both field vectors are normal to the direction of propagation. The lowest order transverse mode of a laser is the Gaussian TEM00.
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.
Tunable laser
A laser system that can be "tuned" to emit laser light over a continuous range of wavelengths or frequencies.
TTL modulation
Type of modulation that requires a 0 or 5VDC, TTL-compatible input signal to control laser output. Unlike analog modulated lasers, TTL modulated lasers cannot produce any level of fractional power. They are either ON or OFF. A benefit of TTL modulation is that users can program it to enable, inhibit, or modulate a laser, making it an excellent resource for synchronized applications.
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 avapor.
Vignetting
The loss of light through an optical element when the entire bundle of light rays does not pass through. Also, an image or picture that shades off gradually into the background.
Electromagnetic radiation that can be detected by the human eye. Commonly used to describe wavelengths that lie in the range between 400nm and 700 to 780 nm.
Watt
A unit of power (equivalent to one Joule per second) used to express laser power.
Watt per cm2
A unit of irradiance used in measuring the amount of power per area of absorbing surface or per area of CW laser beam.
Wave
A sinusoidal undulation or vibration. A form of movement by which all radiant electromagnetic energy travels.
Wavefront
A surface connecting all field points of electromagnetic energy that are equidistant from the source.
Waveguide
A device (e.g., optical fiber) that confines or guides electromagnetic waves along a path defined by the construction of the guide.
Wavelength
The distance between successive peaks, troughs, or identical parts of a wave, measured in nanometers or microns. Equal to the speed of the wave divided by its frequency.
Wave number
The number of complete wave cycles of an electromagnetic field that exist in a given distance of linear space, expressed in reciprocal centimeters (cm-1).
Window
A piece of glass with plane parallel sides that admits light into or through an optical system and excludes dirt and moisture.
YAG laser
A solid-state laser that uses a rod of yttrium aluminum garnet (YAG) doped with neodymium to serve as the lasing medium.