Tangential Plane

When an off-axis object is focused by a spherical lens, the natural asymmetry leads to astigmatism (i.e., the system appears to have two different focal lengths). As shown in the figure below, the plane containing both the optical axis and the object point is called the tangential plane. Rays that lie in this plane are called tangential rays.


Tangential Ray

A light ray lying in the tangential plane of an astigmatic optic.


Abbreviation for Transverse Electromagnetic Modes. Used to designate the cross-sectional shape of the beam. The radial distribution of intensity across a beam as it exits the optical cavity.


The fundamental (lowest order) transverse electromagnetic (TEM) 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.


The input level at which lasing begins during excitation of the laser medium.

Total Internal Reflection (TIR)

Rays internally incident upon an air/glass boundary at angles greater than the critical are reflected with 100% efficiency regardless of their initial polarization state. The critical angle is given by:

qc(l) = arcsin(1/nl)

where nl is the index of refraction of the glass at wavelength l.


Passage of electromagnetic radiation through a medium.

Transverse Modes

Transverse modes have a field vector normal to the direction of propogation and are determined by the geometry of the laser or waveguide cavity and any limiting apertures. Waveguide modes are characterized as TE (transverse electric) with the electric vector normal, and TE (transverse magnetic) with the magnetic vector normal. In general, laser modes that do not have wall boundary conditions are designated TEM (transvers electric magnetic) with both vectors normal to the direction of propogation.

Tunable Laser

A laser system that can be "tuned" to emit laser light over a continuous range of wavelengths or frequencies.

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.