Glossary

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Appendix C – Glossary

Active medium. The atomic or molecular species which can provide gain for laser oscillation.

Also called laser medium, lasing medium or active material.

Attenuation. Reduction in intensity that results when optical radiation travels through an

absorbing or scattering medium. In optical fibre, attenuation (in decibels) equals

10 log (Po / Pin), where Po is the power at the output end of the fibre and Pin is the power

launched into the fibre.

Average power. In a repetitively pulsed laser, the energy per pulse times the repetition rate.

When the energy per pulse is expressed in joules and the repetition rate in hertz, the

average power is expressed in watts.

Beam diameter. The distance between the two opposing points at which the irradiance or

radiant exposure is a specified fraction (typically 1/e or 1/e2) of the irradiance or radiant

exposure of the emitted radiation.

Beam divergence. The increase in beam diameter with distance from the laser's exit aperture.

Measured in milliradians at specified points, usually where irradiance or radiant exposure

is 1/e or 1/e2 the maximum value, and expressed as the "full-angle" divergence.

Calorimeter. A type of detector that measures heat produced by absorption of radiation.

Coherence. A fixed phase relationship among various points of an electromagnetic wave in

space (spatial coherence) or in time (temporal coherence).

Continuous wave (CW) laser operation. Laser operation in which radiation is emitted

continuously.

Crystal laser. A type of laser in which the active medium is an atomic species in a crystal such

as ruby, YAG (yttrium aluminium garnet), or YALO (yttrium aluminate).

Detector. Any device which detects light, generally producting an electronic signal with

intensity proportional to that of the incident light.

Diode laser. See Semiconductor laser.

Divergence. See Beam divergence.

Dye laser. A type of laser in which the active medium is an organic dye, generally in solution

with the liquid either flowing or encapsulated within a cell. Experimental solid and gas

dye lasers also have been built. Also called organic-dye, tunable-dye or liquid laser.

Excimer laser. A laser in which the active medium is an excimer, a molecule which is

chemically unstable except in its excited state. The term often is applied to lasers in

which the active medium is a rare-gas halide (or monohalide) excimer such as KrF* or

XeF*.

Gas laser. A type of laser in which the active medium is a gas. The category is subclassified

according to the active medium into atomic (such as helium-neon), molecular (carbon

dioxide, hydrogen cyanide and water vapour), ionic (argon, krypton, xenon, and the

metal-vapour types such as helium-cadmium and helium-selenium), and excimer

(typically rare-gas halides). Loosely applied, "ion" means argon and krypton.

Hertz (Hz). The SI unit of frequency of periodic phenomena. It replaces the non-SI unit "cycles

per second." The number of pulses per second that a laser can produce may be expressed

in hertz.

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