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«Glossary Of Laser Terminology By: Saleh J. Jany BDS MSc. HLD ABLS BLS A Ablation: The removal of material in industrial laser cutting, or tissue in ...»

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Glossary

Of Laser

Terminology

By:

Saleh J. Jany

BDS MSc. HLD ABLS BLS

A

Ablation: The removal of material in industrial laser cutting, or tissue in medical laser cutting,

by melting, evaporation, or vaporization.

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 Coefficient Factor: Describes light's ability to be absorbed per unit of path length.

Absorption of Radiation:

- Receiving electromagnetic radiation by interaction with the material, and transforming it to different form, which is usually heat (rise in temperature). The absorption process is dependent on the wavelength of the electromagnetic radiation and on the absorbing material.

Accessible Emission Limit: The maximum accessible emission level limit (AEL) permitted within a particular class. AEL is determined as the product of Accessible Emission times the Maximum Permissible Exposure (MPE) using the area of the limiting aperture (7 mm for visible and near infrared lasers).

Accessible Emission: The magnitude of accessible laser (or other 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 Radiation: Laser radiation that can expose human eye or skin in normal usage.

Active Medium: Collection of atoms or molecules which can be stimulated to a population inversion, and emit electromagnetic radiation in a stimulated emission.

Adaptive Optics: Recent advances in deformable mirror technology and laser guide stars allows most of the distortion produced by the atmosphere to be removed. This results in near diffraction-limited performance of ground-based receiving telescopes, giving image quality similar to that produced by the Hubble Space Telescope.

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.

2 Alignment: Laser alignment is a measure of the deviation of the optical axis of the laser beam with respect to the mechanical axis of the laser housing. Some laser diode modules feature an adjustable alignment for precise positioning applications.

Amplification: The process in which the electromagnetic radiation inside the active medium within the laser optical cavity increases by the process of stimulated emission or amplification of 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; simply stated: the height of the wave.

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

Anode: The positive electrode of a gas laser, used for electrical excitation of the gas in the tube.

APD Avalanche Photodetector: The solid-state equivalent of a PMT, usually restricted to operation in the red, near-infrared and infrared part of the spectrum. For photon-counting applications, they are operated in the so-called "Geiger Mode". The device is noisier than a PMT but has greater quantum efficiency. Maximum quantum efficiencies are about 80%.

Aperture: A small opening through which the electromagnetic radiation passes.

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 are 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.

Argon Laser: A gas laser in which argon ions are the active medium. This laser emits in the blue

- green visible spectrum, primarily at 488 and 515 nm..

Articulated Arm: CO2 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 radiation energy (power) as a beam passes through an absorbing or scattering medium.

3 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, such as bright light. It can occur within 0.25 seconds and it includes the blink reflex time. Aversion response action, such as closing of the eye or movement of the head, to avoid exposure to laser light 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.





Axicon Lens: A conical lens which, 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 center of curvature of the optical surfaces of a lens.

B Beam Bender: A hardware assembly containing an optical device, such as a mirror, capable of changing the direction of a laser beam; used to re-point the beam and used 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 1/e (0.368) of the peak level (for safety standards). The value is normally chosen at 1/e2 (0.135) of the peak level for manufacturing specifications.

Beam Divergence: Angle of beam spread measured in radians or 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.

4 Beam Splitter: An optical device used to divide the light from a laser into two separate beams the reference beam and the object beam. It consists of a partially transparent mirror that reflects part of the laser beam and transmits the rest.

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

Bins: A bin has a bandpass filter response with certain frequency bandwidth. A spectral analysis requires the signal to be divided into a large number of bins. The signal power spectral density at each bin frequency is thereby measured.

Blink Reflex: (See Aversion Response).

Brewster Windows: Windows at the ends of a gas laser or windows in 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.

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.

C Calorimeter: An instrument which measures the energy, usually as heat, generated by absorption of the laser beam.

Carbon Dioxide (CO2) Laser: A gas laser in which CO2 molecules are the active medium. This laser emits in the infrared spectrum, with the strongest emission line at 10.6 µm. It can be operated in either CW or pulsed mode.

Cathode: The negative electrode of a gas laser used for electrical excitation of the gas in the tube.

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

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: A property of electromagnetic waves which are in phase in both time and space.

Coherent light has monochromaticity and low beam divergence, and can be concentrated to high power densities. Coherence is needed for interference processes like holography.

5 Coherence: The alignment between light wave wavelength and the position of that wave in its oscillation cycle. When the crests and troughs of several light waves are in alignment, they are coherent.

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.

Collimator: Optical device consisting of two lenses separated by the sum of their focal length. It is used to provide desired beam diameter to meet specific beam delivery requirements.

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: a laser which with a continuous output that is greater than or equal to 0.25 s.

Continuous Wave: Continuous Wave (CW) transmission is when there is an electromagnetic carrier wave present and the message signal, be it voice, video or data, is impressed upon the carrier by modulating its amplitude, phase or frequency. It is the system employed for terrestrial radio and TV transmissions. When the wave is unmodulated, the signal is highly monochromatic. The average power in the modulated wave is approximately that in the unmodulated carrier component.

Controlled Area: A locale where the activities 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 electrical current in a conductor; in a laser, the point at which further electrical input will not increase laser output.

6 CW: Abbreviation for Continuous Wave; the continuous-emission mode of a laser as opposed to pulsed operation.

D 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.

Diachronic Filter: Filter that allows selective transmission of colors desired wavelengths.

Diffraction: A wave property which creates deviation from a straight line when the beam passes near an edge of an opaque object. 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.

Diffraction Limited: Electromagnetic waves diffract around the edges of opaque objects, or on passing through or reflecting off a finite aperture, like a dish, lens or mirror. Even if such a wave is perfectly collimated, so that the beam emitted is parallel, it will eventually spread out. The larger the aperture in relation to the wavelength, the smaller the beam spread. An electromagnetic beam is said to be "diffraction limited" when it is so collimated that it cannot be made more narrow in the near-field. Receivers are also said to be "diffraction limited" when the energy is focused into the smallest possible spot size at the focal plane. An optical receiver that is not "diffraction limited" may be said to be a "light bucket" - it collects photons, but cannot concentrate them into a very small tightly focused spot.

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.

Diode Laser: A laser that emits coherent light through the injection of electric current into a semiconductor diode.

Diode: An electronic device that conducts a current in only one direction.

7 Divergence: The angular measurement of laser beam spread with distance. The projected dot of a laser will increase in size the farther it is projected. Laser divergence is measured in milliradians (mrad).

Dosimetry: Measurement of the power, energy, irradiance, or radiant exposure of light delivered to tissue.

Drift (Angular): Any unintended change in direction of the beam before, during, and after warm-up; measured in mrad.

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

Duty: Cycle ratio of total "on" duration to total exposure duration for a repetitively pulsed laser.

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



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