FIBEROPTIC SENSOR TECHNOLOGY HANDBOOK

slab dielectric optical waveguide. An optical waveguide
consisting of rectangular layers or ribbons
of materials of differing refractive indices that
support one or more lightwave transmission modes,
with the energy of the transmitted waves confined
primarily to the layer of highest refractive index,
the lower indexed medium serving aa cladding, jacketing,
or surrounding medium. Slab dielectric optical
waveguides are used in integrated optical circuits
for geometrical convenience, in contrast to
the optical fibers in cables for long-distance
transmission.
slab dielectric waveguide. An electromagnetic waveguide
consisting of a dielectric transmission medium
of rectangular cross section. The width and thickness
of the guide may be controlled to support
specific propagation modes; it may be cladded, protected,
distributed, and electronically controllable;
and it may be mounted on integrated electrooptical
circuit substrate.
Snell’s law. When electromagnetic wavea, such as light,
pass from a given transmission medium to a medium
of higher refractive index (denser) medium, its path
is deviated toward the normal. When paasing into
a less dense medium, its path is deviated away from
the normal. Often called the law of refraction,
Snell’s law defines this phenomenon
the relation between the incidence
by describing
angle and the
refraction angle as follows:
sin@l/sine2 = n2/nl
where 81 is the incidence angle, ’32 is the refraction
angle, n2 is the refractive index of the medium
containing the refracted ray, and nl is the refractive
index of the medium containing the incident
ray. Stated in another way, both laws, that of reflection
and of refraction, are attributed to Snell,
namely, when the incident ray, the normal to the
surface at the point of incidence of the ray on the
surface, the reflected ray, and the refracted ray
all lie in a single plane, the angle between the incident
ray and the normal is equal in magnitude to
the angle between the reflected ray and the normal.
The ratio of the sine of the angle between the normal
and the incident ray, to the sine of the angle
between the normal and the refracted ray, is a constant
for a given wavelength of incident light.
Also see refraction.
source. In fiberoptic sensors, as in communications,
that part of a syatem from which signals or messages
are considered to originate. A fiberoptic sensor is
the source of baseband signals in a fiberoptic system.
The source may be an optical source (unmodulated)
or a signal source used to modulate the optical
source. See light source.
source-free medium. In fiberoptic, a transmission
medium that does not contain a source of electromagnetic
radiation, such as electric charges or magnetic
poles, other than a propagating or standing
electromagnetic wave.
space-division multiplexing. The use of spatial separation
between light beams, conductors, optical
fibers or other transmission media in order to obtain
channel isolation. For example, the combining
of several independent and isolated fibers or wires
in a single bundle or cable in order to use each
fiber (or bundle) as a separate communication path,
channel, or set of channels. A typical arrangement
for multiplexing might be to use time-division multiplexing
on each space-division multiplexed fiber pair
in an optical cable.
splitter.
See beam splitter.
~. In a laser, emission that does
not bear an amplitude, phase, or time relationship
with an applied signal and is therefore a random
noiselike form of light radiation. Alao see stimulated
emission.
spreading.
star-coupler.
See geometric spreading.
See reflective star-coupler.
step-index fiber. A fiber in which there is an abrupt
change in refractive index between the core and
cladding along a fiber diameter, with the core refractive
index being higher than the cladding refractive
index. There may be more then one layer,
each layer with a different refractive index that
is uniform throughout the layer, but usually with
decreasing indices in the outside layers.
stimulated emission. In a laser, the emission of light
caused by a signal applied to the laser such that
the response is directly proportional to, and in
phase coherence with, the electromagnetic field of
the stimulating signal. This coherency between applied
signal and response contributes to the usefulness
of the laaer. Also see spontaneous emission.
stimulated emission of radiation. See light amplification
by stimulated emission of radiation (laser).
strength member. In fiberoptic cables, a component of
the cable that provides tensile strength and bending
resistance, therefore limiting stress (and strain)
on the optical fibers in the cable.
stripper.
See cladding mode stripper.
substrate. A material used to support or serve as a
foundation, vehicle, or carrier for other material
that has the required characteristics for specific
application but does not have the proper phyaical
strength to support itself, e.g., a block of material
upon which active materials may be deposited by
evaporative techniques or on which active materials
may be bonded by cementing or etching techniques.
The substrate is usually inert or passive relative
to the active material mounted upon it.
switch.
See waveguide switch.
solid state. Pertaining to the conduction of electric
currents or magnetic flux, or the propagation of
electromagnetic waves, within materials other than
gases or other than a vacuum.
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