13-08-2012, 03:24 PM
Ray theory transmission in optical fibers
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A Brief Historical Note
Beyond the middle ages:
–Newton (1642-1726) and Huygens (1629-1695) fight over nature of
light
• 18th–19th centuries
–Fresnel, Young experimentally observe diffraction, defeat Newton’s
particle theory
–Maxwell formulates electro-magnetic equations, Hertz verifies antenna
emission principle (1899)
• 20th century
–Quantum theory explains wave-particle duality
–Invention of holography (1948)
–Invention of laser principle (1954)
–1st demonstration of laser (1960)
–Proposal of fiber optic communications (1966)
–1st demonstration of low-loss optical fibers (1970)
–Optical applications proliferate into the 21st century:
nonlinear optics, fiber optics, laser-based spectroscopy, computing,
communications, fundamental science, medicine, biology,
manufacturing, entertainment, … (Let all flowers blossom!)
Light as waves, rays and photons
• Light is an electromagnetic wave.
• While light is a wave, it nevertheless travels along straight
lines or rays, enabling us to analyze simple optical
components (e.g. lenses and mirrors) and instruments in terms
of geometrical optics.
• Light is also a stream of photons, discrete particles carrying
packets of energy and momentum. (We will cover this after
mid-term.)
Plane waves and their associated rays
• Spherical waves – wavefronts are spherical (rays are
perpendicular to the wavefronts and point radially outwards)
• Plane waves – wavefronts are planes (rays are perpendicular
to plane wavefronts in the direction of propagation)
• Spherical waves from a point source at a far distance from an
observer appears as a plane wave
Silica optical fibers
• Both the core and the cladding are made from a type of glass known
as silica (SiO2) which is almost transparent in the visible and near-IR.
• In the case that the refractive index changes in a “step” between the
core and the cladding. This fiber structure is known as step-index fiber.
• The higher core refractive index (~ 0.3% higher) is typically obtained
by doping the silica core with germanium dioxide (GeO2).
*In Lab 1, we should be able to see the step boundary between the
core and the cladding, by end-illuminating the fiber and imaging the
output-end cross-section using a microscope.
Large-NA fibers?
Developing ways for fiber to collect light efficiently was an
important early step in developing practical fiber optic
communications (particularly in the 1970s)
• It seems logical to have optical fibers with NA as large as possible …
with as large Δ as possible … in order to couple maximum amount of
light into the fiber.
• Soon, we will find out that such large-NA fibers tend to be
“multimode” and are unsuitable for high-speed communications
because of a limitation known as modal dispersion.
• Relatively small-NA fibers are therefore used for high-speed optical
communication systems.
Typical fiber NA
Silica fibers for long-haul transmission are designed to
have numerical apertures from about 0.1 to 0.3.
The low NA makes coupling efficiency tend to be poor, but
turns out to improve the fiber’s bandwidth! (details later)
• Plastic, rather than glass, fibers are available for short-haul
communications (e.g. within an automobile). These fibers are restricted
to short lengths because of the relatively high attenuation in plastic
materials.
Plastic optical fibers (POFs) are designed to have high numerical
apertures (typically, 0.4 – 0.5) to improve coupling efficiency, and so
partially offset the high propagation losses and also enable alignment
tolerance.
Silica fibers for long-haul transmission are designed to
have numerical apertures from about 0.1 to 0.3.
The low NA makes coupling efficiency tend to be poor, but
turns out to improve the fiber’s bandwidth! (details later)
• Plastic, rather than glass, fibers are available for short-haul
communications (e.g. within an automobile). These fibers are restricted
to short lengths because of the relatively high attenuation in plastic
materials.
Plastic optical fibers (POFs) are designed to have high numerical
apertures (typically, 0.4 – 0.5) to improve coupling efficiency, and so
partially offset the high propagation losses and also enable alignment
tolerance.