Fiber-Optic Communications
James N. Downing
Chapter 3
Characteristics of Optical Fibers
Chapter 3 Characteristics of
Optical Fibers
3.1 Light Propagation in Optical Fibers
Acceptance Angle and Numerical Aperture
– Acceptance angle is the angle cone of light
transmitted down the fiber.
– Numerical aperture is the sine of ½ of the
acceptance angle.
Chapter 3 Characteristics of
Optical Fibers
3.1 Light Propagation in Optical Fibers
Fiber Modes
– Fiber mode refers to the way waves propagate
down a fiber.
– The geometry of the fiber as well as the existence
of waves traveling forward and backward allows
only certain ray angles to propagate.
– Bessel functions describe which modes yield
numerical results: V-number
Chapter 3 Characteristics of
Optical Fibers
3.1 Light Propagation in Optical Fibers
V-number
2  aNA
V 
where

– N is the number of modes
– a is the radius of the fiber
– λ is the wavelength of light
For single-mode fiber, V < 2.405
N 
V
2
2
Chapter 3 Characteristics of
Optical Fibers
3.1 Light Propagation in Optical Fibers
Modal Properties
– Ideally all angles carry equal amounts of energy.
– Actual mode distribution differs due to launch conditions,
coupling, and leaky modes.
– Mode coupling describes how energy is transferred between
modes.
– Leaky modes are the highest order modes that transmit into
the cladding or transmitted back into the core.
Chapter 3 Characteristics of
Optical Fibers
3.1 Light Propagation in Optical Fibers
Modal Properties
– Mode distribution describes how evenly the energy is
distributed across all modes.
– Mode scrambler is used to achieve steady state for
measurement purposes on short fibers.
– Cutoff wavelength is the minimum propagation wavelength
that can be transmitted.
– Mode-field diameter (output spot size) is approximately the
core diameter for multimode fibers.
Chapter 3 Characteristics of
Optical Fibers
3.2 Fiber Dispersion
– Dispersion is the spreading of a light pulse
as it propagates down the fiber.
– Dispersion may be either modal or
chromatic.
Chapter 3 Characteristics of
Optical Fibers
3.2 Fiber Dispersion
Modal Dispersion
– The temporal spreading of a pulse in an optical
waveguide caused by modal effects
– Intermodal, or modal, dispersion occurs only in
multimode fibers.
– Contributes to pulse broadening
Chapter 3 Characteristics of
Optical Fibers
3.2 Fiber Dispersion
Material Dispersion
– Material dispersion occurs because the spreading of a light
pulse is dependent on the wavelengths' interaction with the
refractive index of the fiber core.
– Material dispersion is a function of the source spectral width,
which specifies the range of wavelengths that can propagate
in the fiber.
– Material dispersion is less at longer wavelengths.
Chapter 3 Characteristics of
Optical Fibers
3.2 Fiber Dispersion
Waveguide dispersion
– Waveguide dispersion occurs because the mode
propagation constant is a function of the size of
the fiber's core relative to the wavelength of
operation.
– Waveguide dispersion also occurs because light
propagates differently in the core than in the
cladding.
Chapter 3 Characteristics of
Optical Fibers
3.2 Fiber Dispersion
Polarization Mode Dispersion
– Polarization mode dispersion (PMD) occurs when
different planes of light inside a fiber travel at
slightly different speeds, making it impossible to
transmit data reliably at high speeds.
Chapter 3 Characteristics of
Optical Fibers
3.2 Fiber Dispersion
Total Dispersion
– Total dispersion is due to all types of dispersion
 t tot 
t
2
mod
 t
2
chrom
 t
2
pol
Chapter 3 Characteristics of
Optical Fibers
3.3 Fiber Losses
– Absorption loss occurs at wavelengths greater than
1.55µm due to infrared vibration.
– Scattering can be significant at shorter wavelengths.
– Attenuation describes the total loss of a optical fiber
system
– Bending loss occurs when total internal reflection
deteriorates because of installation procedures.
Chapter 3 Characteristics of
Optical Fibers
3.4 Types of Fiber
Multimode Fiber
–
–
–
–
–
Can transmit more than a single mode
Relatively inexpensive
Easy to couple with LEDs and detectors
Large bandwidth
NA ~ 0.20
Chapter 3 Characteristics of
Optical Fibers
3.4 Types of Fiber
Single-Mode Fiber
–
–
–
–
–
–
–
Allows only a single mode to propagate
Difficult to handle and couple
More expensive
Requires a laser source
Large bandwidth
High speed/large bandwidth systems
NA ~ 0.12
Chapter 3 Characteristics of
Optical Fibers
3.4 Types of Fiber
Step-Index Fiber
– Most common
– Two distinct refractive indices
– Core refractive index constant
Chapter 3 Characteristics of
Optical Fibers
3.4 Types of Fiber
Graded-Index Fiber
– Refractive index varies between the central core
and the cladding
– More expensive
– Dispersion and bandwidth improved
– Works best for multimode fiber
– Rays refract continuously
Chapter 3 Characteristics of
Optical Fibers
3.5 Special Fiber Types
Plastic Fiber
–
–
–
–
–
High attenuation
Less expensive than glass
Easy to work with
Step-index fibers
Used in automobiles, consumer products, industrial control,
and small LANs
Chapter 3 Characteristics of
Optical Fibers
3.5 Special Fiber Types
Dispersion-Shifted Fiber
– Adjusts for pulse spreading caused by material
and waveguide dispersion
Chapter 3 Characteristics of
Optical Fibers
3.5 Special Fiber Types
Polarization Maintaining Fiber
– Used in lithium niobate modulators and Raman
amplifiers
– Maintains polarization of the incoming light
– Minimizes cross-coupling between polarization
modes
Chapter 3 Characteristics of
Optical Fibers
3.5 Special Fiber Types
Photonic Crystal (Holey) Fibers
–
–
–
–
–
–
Dispersion can be controlled
Nonlinear properties
Single-mode
Wide wavelength
Cladding region consists of air holes
Two categories: High-index and low-index guiding fibers
Chapter 3 Characteristics of
Optical Fibers
3.5 Special Fiber Types
Other Fibers
– Low OH Fiber—low water content
– Rare-Earth Doped Fiber—gain media fro amplifiers and
lasers. Erbium doped fiber amps used for over C- and Lbands
– Reduced Cladding Fibers—cladding has been reduced
from 125µm to 80µm
Chapter 3 Characteristics of
Optical Fibers
3.5 Special Fiber Types
Other Fibers
– High-Index Fibers—used in couplers and DWDM
components
– Photosensitive Fibers—change their refractive index
permanently when illuminated with UV radiation
– Lensed Fibers—used to launch light from transmitters into
fibers. May add curvature to an end and be more cost
effective than wasting energy due to mismatches.