Fiber-Optic Communications
James N. Downing
Chapter 3
Characteristics of Optical Fibers
Chapter 3 Characteristics of Optical Fibers
3.1 Light Propagation in Optical FibersAcceptance 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 FibersFiber 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 FibersV-numberwhere – 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 2
2
2VN
aNAV
Chapter 3 Characteristics of Optical Fibers
3.1 Light Propagation in Optical FibersModal 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 FibersModal 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 DispersionModal 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 DispersionMaterial 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 DispersionWaveguide 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 DispersionPolarization 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 DispersionTotal Dispersion– Total dispersion is due to all types of dispersion
polchromtot tttt 22mod2
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 FiberMultimode 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 FiberSingle-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 FiberStep-Index Fiber– Most common– Two distinct refractive indices– Core refractive index constant
Chapter 3 Characteristics of Optical Fibers
3.4 Types of FiberGraded-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 TypesPlastic 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 TypesDispersion-Shifted Fiber– Adjusts for pulse spreading caused by material
and waveguide dispersion
Chapter 3 Characteristics of Optical Fibers
3.5 Special Fiber TypesPolarization 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 TypesPhotonic 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 TypesOther 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 L-bands
– Reduced Cladding Fibers—cladding has been reduced from 125µm to 80µm
Chapter 3 Characteristics of Optical Fibers
3.5 Special Fiber TypesOther 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.