Paraxial Ray

In optics, rays that are always very close and nearly parallel to the optical axis. In this region, lens surfaces are always very nearly normal to the optical axis, and hence all angles of incidence and refraction are small. As a result, the sines of the angles of incidence and refraction are small (as used in Snell's law) and can be approximated by the angles themselves (measured in radians).


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).


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 x10-34 Joule-sec).


Ceramic-based actuators with a non-symmetric crystal structure capable of moving and measuring at the nanometric level. The most common piezoelectric crystal is lead zirconate titanate (PZT).

PIN Photodiode

A silicon photodiode with a high-resistance intrinsic area between the P and N layers, thus the name positive-intrinsic-negative photodiode.

Plasma Shield

The ability of plasma to stop transmission of laser light.

Pockel's Cell

An electro-optical crystal used as a Q-switch.


With respect to light radiation, the restriction of the vibrations of the magnetic or electric field vector to a single plane. The polarization direction is the direction of the electric field vector and is normal to the direction of beam propagation. When the direction of polarization is fixed, it is termed linear polarization. If the electric field vector has two orthogonal components that cause the resultant polarization direction to rotate about the direction of propagation, it is termed elliptical polarization. A special case of elliptical polarization is circular polarization.

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.


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

Power Meter

An accessory used to measure laser beam power.

Protective Housing

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


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 millisecond, 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.

Pulse Repetition Frequency (PRF)

The number of pulses produced per second by a laser.

Pulsed Laser

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


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