Optical fibers guide light through internal reflection using a core and cladding material where the core has a higher refractive index. There are two main types of optical fibers: single-mode fibers which support only one propagation mode and multimode fibers which support multiple propagation modes. A key difference is that multimode fibers have larger cores but also suffer from intermodal dispersion where different modes arrive at different times.

1. Optical Fiber Modes and
Configurations: Fiber Types
• Optical Fiber: dielectric (normally cylindrical)
waveguide that operates at optical
frequencies.
• Transmission properties are dictated by fiber
structural characteristics
• The propagation of light along a waveguide
can be described in terms of a set guided
electromagnetic waves called modes of the
waveguide

2. Fiber Types
• These modes are referred to
as bound or trapped modes of
waveguides
• Optical Fiber structure
– Core
– Cladding
• Reduces scattering loss that
results from discontinuities at core
• Mechanical support
• Protect core from contaminants
– Coating

3. Fiber Types
• Single mode
sustains on mode
of propagation
• Multimode
supports many
modes

4. Fiber Types
• Advantages of Multimode
– Larger core radii makes it easier to launch optical power
into the fiber and facilitate connecting of similar fiber
– LEDs can be used
• Disadvantage
– Intermodal dispersion (when optical pulse is launched into
fiber, optical power is distributed over all of the modes.
Each mode travels at slightly different velocity. This means
modes arrive at the fiber end at slightly different times,
causing pulse to spread out in time. This is known as
intermodal dispersion or intermodal dispersion
• Intermodal dispersion can be reduced using graded
index profile. Thus, graded index fiber have much
larger bandwidth than step index fiber

5. Fiber Optics Propagation
• Electromagnetic light guided along a fiber can be represented
by a superposition of bound modes. Each mode consists of
simple EM configurations. For light field with radian frequency
w, a mode traveling in z direction has time and z dependence
» ej(wt-βz)
– β (z component of wave propagation constant).
– For guided modes, β can assume discrete value
• Two methods
– Ray tracing
• Good approximation to light acceptance and guiding properties of fiber
when fiber radius to wave length is large
• More direct physical interpretation of light propagation characteristics
– Modal Analysis uses electromagnetic analysis
• Single mode fiber
• Coherence, interference phenomena
• Fiber bent loss

6. Step Index Fiber
• For step index fiber,
» n2 = n1 (1-Δ),
» Δ is core-cladding index difference or index difference,
value nominally 1-3% for multimode and 0.2 to 1 % for
single mode.
• Since n1>n2, EM energy is made to propagate
along fiber through internal reflection

7. Ray Optics Representation
• From snell’s law, for total internal reflection
» Sin(Φmin)=n2/n1
» n sin θ0,max = n1 sin θc = (n1
2
-n2
2
)1/2
» θc=Π/2-Φc
• Numerical aperture
» NA =n sin θ0,max = (n1
2
-n2
2
)1/2
≈ n1 (2Δ)1/2

8. Example
• Compute the numerical aperture and
acceptance angle for the symmetrical AlGaAs
slab waveguide where n1=3.6, n2=3.55
• Solution
NA = (3.62
-3.552
)1/2
=0.598
θo=36.7o
Thus, all light incident within +/- 36.7o
is accepted
NA =n sin θ0,max = (n1
2
-n2
2
)1/2
≈ n1 (2Δ)1/2

9. EM spectrum

10. Particle theory
• Ibn al-Haytham (Alhazen, 965-1040) proposed a particle theory of light in his Book of Optics
(1021). He held light rays to be streams of minute energy particles[4]
that travel in straight lines at
a finite speed.[5][6][7]
He states in his optics that "the smallest parts of light," as he calls them, "retain
only properties that can be treated by geometry and verified by experiment; they lack all
sensible qualities except energy."[4]
Avicenna (980-1037) also proposed that "the perception of
light is due to the emission of some sort of particles by a luminous source".[9]
• Pierre Gassendi (1592-1655), an atomist, proposed a particle theory of light which was published
posthumously in the 1660s. Isaac Newton studied Gassendi's work at an early age, and preferred
his view to Descartes' theory of the plenum. He stated in his Hypothesis of Light of 1675 that
light was composed of corpuscles (particles of matter) which were emitted in all directions from
a source. One of Newton's arguments against the wave nature of light was that waves were
known to bend around obstacles, while light travelled only in straight lines. He did, however,
explain the phenomenon of the diffraction of light (which had been observed by
Francesco Grimaldi) by allowing that a light particle could create a localised wave in the aether.
• Newton's theory could be used to predict the reflection of light, but could only explain refraction
by incorrectly assuming that light accelerated upon entering a denser medium because the
gravitational pull was greater. Newton published the final version of his theory in his Opticks of
1704. His reputation helped the particle theory of light to hold sway during the 18th century.
The particle theory of light led Laplace to argue that a body could be so massive that light could
not escape from it. In other words it would become what is now called a black hole. Laplace
withdrew his suggestion when the wave theory of light was firmly established. A translation of
his essay appears in The large scale structure of space-time, by Stephen Hawking and George F.
R. Ellis.