FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

core. The central primary light-conducting region of a
material medium, such as an optical fiber, the refractive
index of which must be higher than that of
its cladding in order for the lightwaves to be
totally reflected or refracted. Most of the optical
power is in the core.
coupler. In optical transmission aystems, a component
used to interconnect two or more optical fibera.
Also see connector; bulk coupler; electronic&llycontrollable
coupler; reflective star-coupler; 3-dB
coupler.
coupling. The connection, attachment, or binding of
optical elements, electric circuit elementa, electric
and magnetic fields, propagation modes, or electromagnetic
wave component, such as surface waves
and evanescent waves, to internal waves in waveguides,
dielectric slabs, or other interdependent
associations and interactions of events and materials
in a system. For example, two optical fibers or certain
elements in an integrated optical circuit may
be coupled together in some manner to preserve signal
continuity. See evanescent field coupling.
coupling coefficient. Synonym for coupling ratio.
coupling efficiency. In fiberoptic transmission, the
ratio of the optical power on one side of an interface
to the optical power on the other side. For
example, the ratio of the power developed by a light
aource to the power accepted by a bundle of fibers,
or the power received at the end of a bundle of
fibers to the power that impinges on a photodetector.
For light sources with emitting areas larger than
fiber core diameters, the product of fiber numerical
aperture (N.A.) and core diameter is a good indicator
of maximum coupling efficiency. For other
sources, such as small laser diodes with emitting
areas small than the fiber core diameter, the N.A.
alone is a relevant indicator of coupling efficiency,
usually expresaed as a percentage.
critical angle. The angle, with the normal, at which
an electromagnetic wave incident upon an interface
surface between two dielectric media, at which total
reflection of the incident ray first occurs as the
incident angle with the normal to the incident aurface
is increased from zero, and beyond which total
internal reflection continues to occur although with
increased attenuation at a rate determined not only
by the electromagnetic parameters of the transmission
medium, but also by the frequency and the incidence
angle. The wave is guided along the reflecting
surface with no average transport of energy into
the second medium, and the intensity of the reflected
wave is exactly equal to the intensity of the
incident wave. The wave in an optical fiber will
be confined to the fiber for all incidence angles
greater than the critical angle. The critical angle
is given by sin ec = (~2/~1) 1/2 where 9C is the
critical angle and Cz and c1 are the permittivities
of the transmitted (outside) and incident medium
(inside), respectively, and where El is always greater
than =2; e.g. , the case for an optical fiber (conducting
a wave), and air. In terms of refractive
indices, the critical angle is the incidence angle
from a denser medium, at an interface between the
denser and less dense medium, at which all of the
light is refracted along the interface, i.e., the
angle of refraction is 90°. When the critical
angle is exceeded, the light is totally reflected
back into the denaer medium. The critical angle
varies with the refractive indices of the two media
with the relationship, sin Oc = n2/nl, where n2 is
the index of refraction of the less dense medium, nl
is the refractive index of the denser medium, and 9C
is the critical angle, as above. In terms of total
internal reflection in an optical fiber, the critical
angle is the smallest angle made by a meridional
ray in an optical fiber that can be totally reflected
from the innermost interface and thus determines
the maximum acceptance angle at which a meridional
ray can be accepted for transmission along a fiber.
Also see total internal reflection.
critical radius. The largest radius of curvature of an
optical fiber, containing an axially propagated
electromagnetic wave, at which the field outside the
fiber still detaches itself from the fiber and radiates
into space because the phase-front velocity
must increase to maintain a proper relationship with
the guided wave inside the fiber. Thia velocity cannot
exceed the velocity of light, aa the wavefront
sweeps around the outside of the curved fiber. This
causes attenuation due to a radiation loss. The
field outside the fiber decays exponentially in a
direction transverse to the direction of propagation.
It is the radius of curvature of an optical
fiber at which there is an appreciable propagation
mode conversion loss, due to the abruptness of the
transition from straight to curved. For a radius
of curvature greater than the critical value, the
fields behave essentially as in a straight guide.
For radii smaller than the critical value, considerable
mode conversion takea place.
coupling loss. In a fiberoptic coupling, the optical
power loss caused by the coupling itself, a loss
that would not occur if the optical fiber were continuous
without the coupling.
coupling ratio. The ratio of power on the output side
of a coupling to the power on the input side. The
coupling ratio is always less than unity. Synonymous
with coupling coefficient. Also see 3-dB coupler.
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D
dark current. ~’e current that flows in a photodetector
when there is no radiant energy or luminous flux
incident upon its aensitive surface, i.e., when there
is total darkness. Dark current generally increaaea
with increaaed temperature for most photodetectors.
For example, in a photoemissive photodetector, the
dark current is given by:
Id = AT2eq’$lkT
where A is the surface area constant, T is the absolute
temperature, q is the electron charge, $ ia
the work function of the photoemisaive surface material,
and k is Boltzmann’s constant.
darkfield sensor. In fiberoptic, a sensor in which
the optical power tapped and modulated by the sensor
is a small fraction of the total optical power fed
to or available to the sensor. Contrast with brightfield
sensor.
data. Representation of facts, concepts, or instructions
in a manner suitable for communication, interpretation,
or processing by human, manual, semiautomatic,
or fully-automatic means. The characters
used as data may assume any form or pattern to which
meaning may be assigned in order to represent infor-