F-NUMBER The focal length of lens divided by its usable diameter. In the case
of a laser the usable diameter is the diameter of the laser beam or a smaller
aperture which restricts a laser beam.
FABRY-PEROT INTERFEROMETER Two plane, parallel partially reflective optically
flat mirrors placed with a small air gap separation (1-20 mm) so as to produce
interference between the light waves (interference fringes) transmitted with multiple
reflections through the plate.
FAILSAFE INTERLOCK An interlock where the failure of a single mechanical or
electrical component of the interlock will cause the system to go into, or remain
in, a safe mode.
FEMTOSECONDS 10(-15) seconds.
FIBEROPTICS A system of flexible quartz or glass fibers with internal reflective
surfaces that pass light through thousands of glancing (total internal) reflections.
FLASHLAMP A tube typically filled with Krypton or Xenon. Produces a high intensity
white light in short duration pulses.
FLUORESCENCE The emission of light of a particular wavelength resulting from
absorption of energy typically from light of shorter wavelengths.
FLUX The radiant, or luminous, power of a light beam; the time rate of the
flow of radiant energy across a given surface.
FOCAL LENGTH Distance between the center of a lens and the point on the optical
axis to which parallel rays of light are converged by the laser.
FOCAL POINT That distance from the focusing lens where the laser beam has the
FOCUS As a noun, the point where rays of light meet which have been reflected
by a mirror or refracted by a lens, giving rise to an image of the source. As
a verb, to adjust focal length for the clearest image and smallest spot size.
FOLDED RESONATOR Construction in which the interior optical path is bent by
mirrors; permit compact packaging of a long laser cavity.
FREQUENCY The number of light waves passing a fixed point in a given unit of
time, or the number of complete vibrations in that period.