Physical Layer Cabling: Fiber Optics

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Physical Layer Cabling: Fiber

Optics

Introduction

A fiber-optic

network is surprisingly simple. It is comprised of the following

elements:

A fiber-optic

transmission strand can carry the signal (in the form of a modulated

light beam) a few feet or even hundreds or thousands of miles. A cable

may contain three or four hair-like fibers or a bundle of hundreds of

such fibers.

A source of invisible

infrared radiation -- usually a light-emitting diode (LED) or a

solid-state laser -- that can be modulated to impress digital data or

an analog signal on the light beam.

A photosensitive detector

to convert the optical signal back into an electrical sign at the

receiver.

Efficient optical

connectors at the light source-to-cable interface and at the

cable-to-photo detector interface. These connectors are also critical

when splicing the optical cable due to excessive loss that can occur at

connections.

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The advantages of optical

communication links compared to copper conductors are enormous and include

the following:

Extremely

Wide System Bandwidth: The intelligence is impressed on the light by varying

the light's amplitude. LEDs have a 5 ns response time which results in a

maximum bandwidth of about 100MHz. Laser light offer data rates offer

10Gbps with a single-mode fiber. The amount of information multiplexed on

such a system, in the hundreds of Gbps, is staggering.

Immunity to

electrostatic interference: External electrical noise and lightning do not affect

energy in a fiber-optic strand. However, this is true only for the

optical strands, not the metallic cable components or connecting

electronics.

Elimination

of Crosstalk:

The light in one glass fiber does not interfere with, nor is it

susceptible to, the light in an adjacent fiber. Recall that crosstalk

results from the electromagnetic coupling between two adjacent copper

wires.

Lower

signal attenuation than other propagation systems: Typical attenuation of a 1

GHz bandwidth signal for optical fibers is 0.03 dB per 100 ft., compared

to 4.0 dB for RG-58U coaxial.

Lower Costs: Optical fiber costs are

continuing to decline. The costs of many systems are declining with the

use of fiber, and that trend is accelerating.

Safety: In many wired systems, the

potential hazard of short circuits requires precautionary designs.

Additionally, the dielectric nature of optic fibers eliminates the spark

hazard.

Corrosion: Given that glass is

basically inert, the corrosive effects of certain environments are not a

problem.

Security: Due to its immunity to and

from electromagnetic coupling and radiation, optical fiber can be used in

most secure environments. Although it can be intercepted or tapped, it is

very difficult to do so.

The Nature of Light

The speed of

light in free space is 3 * 108 m/s but is

reduced in other media, including fiber-optic cables. The reduction as

light passes into denser material results in refraction of light.

Refraction causes the light wave to be bent. The amount of bend provided

by refraction depends on the refractive index of the two materials

involved. The refractive index, n, is the ratio of the speed of light in

free space to the speed in a given material. For most purposes a single

value is accurate enough.

In the fiber-optic industry,

spectrum notation is stated in nanometers (nm) rather than in frequency

(Hz) simply because it is easier to use.

3 * 1014 Hz, or 300THz, is

equivalent to 1000 nm.

The one exception to this

naming convention is when discussing dense wavelength division

multiplexing (DWDM), which is the transmission of several optical

channels, or wavelengths, in the 1550-nm range, all on the same fiber.

For DWDM systems, notations, and particularly channel separations, are

stated in terahertz (THz).

Optical Fiber Cable

Intro

The commonly

used wavelengths in today's fiber-optic systems are:

Multimode

Fiber: 850 and 1310 nm

Single-mode

Fiber: 1310

and 1550 nm

Fiber to

the Home/Business: 1600-1625 nm

The Typical construction of

an optical fiber:

Core:

is the

portion of the fiber strand that carries the transmitted light.

Cladding:

is the

material surrounding the core. It is almost always glass, although

plastic cladding of a glass fiber is available but rarely used. The

cladding must have a lower index of refraction to keep the light in the

core.

Protective

Jacket: A

plastic coating surrounding the cladding to provide protection.

Another measure of a fiber's

light acceptance is numerical aperture which is a basic specification

provided by the manufacturer that indicates the fiber's ability to accept

light and shows how much light can be off-axis and still propagate.

The first communication-grade

fibers (early 1970s) had light-carrying core diameters about equal to the

wavelength of light. They could carry just a single waveguide mode.

The difficulty of coupling

significant light into such a small fiber led to development of fibers

with cores of about 20 to 100 um. These fibers support many waveguide

modes and are called multimode fibers. The first commercial fiber-optic

systems used multimode fibers with light at 800-900 nm wavelengths. A

variation of multimode fiber was developed, termed graded-index fiber.

Provides greater bandwidth capability.

Single-mode fibers were found

to provide lower losses and even higher bandwidth. Their use was at 1300

nm, 1550 nm, and 1625 nm in telecommunication and fiber-to-home

applications.

The application now

determines the type used. The following major criteria affect the choice

of fiber type:

Signal Losses

Ease of light coupling and

interconnection

Bandwidth

The Three modes(that is,

multimode) of propagation:

The lowest-order

mode is seen traveling along the axis of the fiber.

The middle-order mode is

reflected twice at the interface.

The highest-order mode is

reflected many times and makes many trips across the fiber.

As a result of these variable

path lengths, the light entering the fiber takes a variable length of time

to reach the detector. This results in a pulse-broadening or dispersion

characteristic. This effect is termed pulse dispersion and limits the

maximum distance and rate at which data (pulse of light) can be

practically transmitted. The output pulse has reduced amplitude as well as

increased width. The greater the fiber length, the worse this effect will

be. Fiber is rated in bandwidth per length, such as 400MHz/km. Current

network standards limit multimode fiber distances to 2 km. Longer

transmission paths are attained by locating regenerators at appropriate

locations. Step-index multimode fibers are rarely used in networking.

Graded-Index Fiber

In an effort to

overcome the pulse-dispersion problem, the graded-index fiber was

developed. The index of refraction is tailored to follow a parabolic

profile, this results in low-order modes traveling through the

constant-density material in the center. High-order modes see a lower

index of refraction material farther from the core, and thus the velocity

of propagation increases away from the center. All modes tend to traverse

the fiber length in about the same amount of time. These fibers can

therefore handle higher bandwidths and/or provide longer lengths of

transmission before pulse dispersion effects destroy intelligibility and

introduce bit errors.

Graded-index multimode fiber

with 50 um-diameter cores and 125 um cladding are used in many telecom

systems at up to 100Mbps over 50-km ranges without repeaters. Graded-index

fiber with up to a 100 um core is used in short-distance applications that

require easy coupling from the source and high data rates, such as for

video and high-speed local area networks (LANs). The larger core affords

better light coupling than the 50 um core and doesn’t degrade the

bandwidth capabilities. Fibers are specified by the diameters of their

core and cladding, 50/125 fiber and 62.5 /125 fiber.

Single-Mode Fibers

A technique used

to minimize pulse dispersion effects is to make the core extremely small

-- a few micrometers. This type accepts only a low-order mode, thereby

allowing operation in high-data-rate, long-distance systems. This fiber

is typically used with high-power, highly directional modulated light

sources such as a laser. These are called single-mode fibers. Core

diameters of only 7-10 um are typical.

Single-mode are widely used

in long-haul and wide area networks (WAN) applications. They permit

transmissions of about 10Gbps and a repeater spacing of up to 80km. These

are constantly being upgraded by new developments.

Mode field diameter is the

term commonly sued when describing the core size of single-mode fibers.

Mode field diameter is the actual guided optical power distribution

diameter, which is typically 1 um or so larger than the core diameter.

The actual value depends on the wavelength being transmitted.

Fiber Attenuation and

Dispersion

Attenuation

Attenuation is

the loss of power introduced by the fiber. This loss accumulates as the

light is propagated through the fiber strand. It is expressed in dB/km

(decibels per kilometer) of length.

The loss, or attenuation,

of the signal is due to the combination of four factors:

Scattering:

This is the

primary loss factor over the three wavelength ranges. It accounts for

96 percent of the loss. It is known as Rayleigh

scattering and is caused by refractive index fluctuations. It decreases

as wavelength increases.

Absorption:

This is the

second loss factor, a composite or light interaction with the atomic

structure of the glass. It involves the conversion of optical power to

heat. One portion of the absorption loss is due to the presence of OH

hydroxol ions dissolved in the glass during manufacture. These cause the

water attenuation or OH peaks shown above and other attenuation curves.

Macrobending:

This is the loss

caused by the light mode breaking up and escaping into the cladding when

the fiber bend becomes too tight. As the wavelength increases, the loss

in a bend increases.

Microbending:

This is a type of

loss caused by mechanical stress placed on the fiber strand, usually in

terms of deformation resulting from too much pressure being applied to

the cable.

Dispersion

Dispersion, or

pulse broadening, is the second of the two key distance-limiting

parameters in a fiber-optic transmission system. It is a phenomenon in

which the light pulse spreads out in time as it propagates along the

fiber strand. This results in a broadening of the pulse. If the pulse

broadens excessively, it can blend into the adjacent digital time slots

and cause bit errors.

There are three types of

dispersion:

Modal

dispersion:

The broadening of a pulse due to different path lengths taken through the

fiber by different modes.

This occurs

predominantly in multimode fiber. Light rays can take many paths when

traveling in a fiber, as a result, they arrive at the detector at

different times, and result in pulse broadening. The use of graded-index

fiber greatly reduces the effects of dispersion, given that only a

single mode is transmitted.

Chromatic

dispersion: The

broadening of a pulse due to different propagation velocities of the

spectral components of the light pulse.

This is present

in both single-mode and multimode fibers. The light source, both lasers

and LEDs, produce several different wavelength light rays when

generating the light pulse. Each light ray travels at a different

velocity, and as a result, these rays arrive at the receiver detector at

different times, causing the broadening of the pulse.

Polarization

mode dispersion:

The broadening of a pulse due to the different propagation velocities of

the X and Y polarization components of the light pulse.

This is the type

of dispersion found in single-mode systems and becomes of concern in

long-haul and WAN high-data-rate digital and high-bandwidth analog video

systems. In a single-mode fiber, the single propagating mode has two

polarizations, horizontal and vertical, or X axis and Y axis. The index

of refraction can be different for two components; this affects their

relative velocity.

Zero

dispersion wavelength: This happens near 1310 nm. By altering the refractive

index profile, this wavelength can be shifted to the 1550 nm region. These

fibers are called dispersion-shifted. The 1550 nm region exhibits a lower

attenuation that at 1310 nm. This becomes an advantage, particularly in

long-haul carriers because with minimum attenuation and minimum dispersion

in the same wavelength region, repeater and regenerator spacing can be

maximized.

Dispersion Compensation

Most fiber is use

today is called class Iva variety. They were optimized to operate in the

1310 nm region. It is often desired to add transmission capacity to the

older fiber cables by using the 1550 nm region, particularly because the

attenuation at 1550 nm is less than at 1310 nm. One major problem arises

at this point. The dispersion value is higher at 1550 nm, which severely

limits its distance capability.

To overcome this problem, a

fiber called dispersion compensation fiber was developed. The fiber acts

like an equalizer, negative dispersion canceling positive dispersion. The

fiber consists of a small coil normally placed in the equipment rack just

prior to the optical receiver input. This does introduce some insertion

loss (3-10 dB) and may require the addition of an optical-line amplifier.

A new device is a fiber

Bragg grating. This technology involves etching irregularities onto a

short strand of fiber, which changes the index of refraction and, in

turn, reflects slower wavelengths to the output before the faster ones.

This results in a compressed, or narrower, light pulse, minimizing

inter-symbol interference (ISI).

Optical Components

Intro

Fiber-optic

communication systems use two kinds of light sources: the diode laser

(DL) and the high-radiance light-emitting diode (LED).

Diode lasers and LEDs bring

to systems different characteristics:

Power Levels

Temperature Sensitivities

Response Times

Lifetimes

Characteristics of Failure

The diode laser is a

preferred source for moderate-band to wideband systems. Fast response

time (typically less than 1 ns) and can couple high levels of useful

optical power (usually several mW) into an optical fiber with a small

core and a small numerical aperture. Used as the source for single-mode

fiber.

Systems that operate at

slower bit rate and require modest levels of fiber-coupled optical power

(50-250 uW). This allows for the use of high-radiance LEDs. LED is

cheaper, less complex driving circuitry, and no thermal or optical

stabilizations needed.

The narrow spectra of an DL

is advantages in systems with high bit rates since the dispersion

effects of the fiber on pulse width are reduced, and thus pulse

degradation over long distances is minimized.

A distributed feedback

(DFB) laser, uses techniques that provide optical feedback in the laser

cavity. This enhances output stability, which produces a narrow and more

stable spectral width (within the range of 0.01-0.1 nm). This allows the

use of more channels in dense wavelength division multiplex (DWDM)

systems. Vertical cavity surface emitting lasers (VCSELs) are lasers

that can support a much faster signal rate than LEDs, including gigabit

networks. VCSELs have the simplicity of LEDs with the performance of

lasers.

Most lasers emit a fixed

wavelength, but there is a class called tunable lasers in which the

fundamental wavelength can be shifted a few nanometers, but from a

modulation point of view as in frequency modulation. Primarily used in

network operations where DWDM is involved. They are used along with

either passive or tunable WDM filters.

Intermediate Components

Typical

fiber-optic telecommunication link is a light source or transmitter and

light detector or receiver interconnected by a strand of optical fiber,

light pipe, or glass.

Intermediate Components

include:

Isolators: An isolator

is an inline passive device that allows optical power to flow in one

direction only.

Attenuators: Attenuators are used to

reduce the received signal level (RSL). They are available in fixed and

variable configurations.

Branching

devices:

Branching devices are used in simplex systems where a single optical

signal is divided and sent to several receivers, such as

point-to-multipoint data or a CATV distribution system.

Splitters: Splitters are used to

split, or divide, the optical signal for distribution to any number of

places.

Wavelength

division multiplexers: Wavelength division multiplexers combine or divide

two or more optical signals, each having a different wavelength. They

are sometimes called optical beam splitters.

Optical-line

amplifiers:

are analog amplifiers. Placement can be at the optical transmitter

output, midspan, or near the optical receiver.

Detectors

The devices used

to convert the transmitted light back into an electrical signal are a

vital link in a fiber-optic system. This important link is often

overlooked in favor of the light source and fibers.

The important

characteristics of light detectors are as follows:

Responsivity:

This is a measure of output current for a given light power launched

into the diode.

Response Speed: This

determines the maximum data rate capability of the detector.

Spectral Response: This

determines the responsivity that is achieved relative to the wavelength

at which responsivity is specified.

Optical fibers are joined

either in a permanent fusion splice or with a mechanical splice. The

connector allows repeated mating and unmating. These connectors must

lose as little light as possible, and this relies on correct alignment

of the core of one fiber to another, or to a source or detector. Losses

for properly terminated fusion and mechanical splices is typically 0.2

dB or less. Axial misalignment typically causes the greatest loss.

There are two techniques to

consider for splicing:

Fusion Splicing is a long-term method, in which two

fibers are fused or welded together. The two ends are stripped of their

coating, cut or cleaved, and inserted into the splicer. The fiber ends

are aligned and an electric arc is fired across the ends, melting the

glass and fusing the two ends together.

Mechanical

Splicing

Fiber Connectorization

Optical Networking

Safety