Fiber-Optic Communications James N. Downing. Chapter 3 Characteristics of Optical Fibers

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.


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


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


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.


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.


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.


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


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.


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.



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.



Total Dispersion– Total dispersion is due to all types of dispersion

polchromtot tttt 22mod2


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.


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


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


3.4 Types of Fiber

Step-Index Fiber– Most common– Two distinct refractive indices– Core refractive index constant


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


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



Dispersion-Shifted Fiber– Adjusts for pulse spreading caused by material

and waveguide dispersion


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


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



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 L-bands

– Reduced Cladding Fibers—cladding has been reduced from 125µm to 80µm


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.

Documents

Fiber-Optic Communications James N. Downing. Chapter 3 Characteristics of Optical Fibers