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Tangential
Plane
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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.
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Tangential
Ray
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A
light ray lying in the tangential plane of an astigmatic optic.
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TEM
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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.
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TEM00
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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.
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Threshold
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The
input level at which lasing begins during excitation of the
laser medium.
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Total
Internal Reflection (TIR)
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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.
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Transmission
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Passage
of electromagnetic radiation through a medium.
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Transverse
Modes
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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.
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Tunable
Laser
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A
laser system that can be "tuned" to emit laser light over
a continuous range of wavelengths or frequencies.
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Tunable
Dye Laser
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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.
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