31/01/2009

1

Understanding

What is an Optical Network?

The laser or LED light flasheson (a digital 1) and off(a digital0) - you can think of this asbeing similar to Morse code.

The 1's and 0's carrying data is then sent on to a Laser system. This changes the data into pulses of laser light. For each 1, the laser switches on, and for a 0 it switches off.

These on/off pulses of infrared light can be: video, computer, or voice data

31/01/2009

2

What is "ask-ee" ?

ASCII (AMERICAN STANDARD CODE FOR INFORMATION INTER CHANGE)A standard for digital representation of letters, numbers, and control codes;understood by most computers

BIT (BINARY DIGIT) An acronym for Binary digit. A bit is a digit in binary form andcarries one of two values, 0 or 1.

BYTE A single computer character, generally 8 bits.

My name in binary code is:

J = 11010001 o = 10010110 e = 10000101

Direct or External Modulation• We now know that an optical network is digital, meaning that

data is represented by 1s and 0s

• Direct modulation is simply switching the laser on and off

• In external modulation , the laser’s output is constant - it is sent through a shutter that can open or close to switch the light output on and off

31/01/2009

3

Bandwidth Units of Measurement

� In digital systems, bandwidth is expressed as bits (of data) per second (bps)

� Bit = smallest unit of digital information

� byte = a set of 8 bits

� bps = bits per second

� Kbps = 1000 bps

� Mbps = 1,000,000 bps

� Gbps = 1,000,000,000 (one billion) bps

� Tbps = 1,000,000,000,000 (one trillion) bps

Fiber Advantages – a brief summary• It has exceptional bandwidth

• It has the ability to carry many signals concurrently

• It is immune to electromagnetic interference

• It has no electromagnetic emissions

• It does not corrode like copper based cabling does

• It is resistant to eavesdropping

• It is virtually "future proof“

• It has the capability to operate in conjunction with any current, or proposed, LAN/WAN standard

• It is light weight and easy to handle

• It’s pulling strength is much higher than that of copper

• …and on it goes

31/01/2009

4

Fiber versus Copper

• A single copper pair is capable of carrying 6 phone calls

• A single fiber pair is capable of carrying over 2.5 million simultaneous phone calls (64 channels at 2.5 Gb/s)

• A single copper pair is capable of carrying 6 phone calls

• A single fiber pair is capable of carrying over 2.5 million simultaneous phone calls (64 channels at 2.5 Gb/s)

Fiber Design

Core Cladding Coating Strength Jacket

Members

31/01/2009

5

Fiber Design

Plastic FibreStep Index Multimode

CORE AND CLADDING DIAMETER

Single-mode

MultimodeGlass Fibre

50125 125 62.5

9125 140 100

All measurements in µµµµm

A micron (short for micrometer) is one-millionth of a meter

How Fiber Works

CORE - (Silica)(Higher refractive index)

Cladding - (Silica)(Lower refractive index)

Coating - (Acrylic or Silicon)

Light

31/01/2009

6

Total Internal ReflectionTotal Internal Reflection: Light injected into the core and striking the core-to-cladding interface at an angle greater than the critical angle will bereflected back into the core

Cladding

Less Dense

Core

More Dense

This can only occur where light travels from a medium with a higher refractive index (slower) to one with a lower refractive index (faster)

The critical angle is the angle of incidence for which the angle of refraction is 90°. At this angle the refracted ray glances parallel to the boundary.

For angles greater in absolute value than the critical angle, all the light is reflected.

Total Internal Reflection

�When a wave hits a boundary with a medium that it can travel faster in(e.g. light going from the core into cladding or air) it will be refracted through a larger angle than its angle of incidence.

�The speed of light in a vacuum is equal to 300,000,000 m/secIR = 1

�The speed of light in glass is 200,000,000 m/sec IR = 1.5

31/01/2009

7

HOW OPTICAL FIBER IS MADE

1. Making the preform

2. Drawing the perform

See video!

OPTICAL FIBER PARAMETERS� The lowest visible frequency is red; the highest is violet

� Commonly known as the visible light spectrum

� They lie in the wavelength range between 400 and 700 nanometers

31/01/2009

8

ATTENUATION• Attenuation is the reduction of signal strength over the length of

the fiber• Rayleigh scattering accounts for the majority (about 96%) of

attenuation

• The second type of attenuation in fiber is absorption and accounts for 3-5 % of fiber attenuation

• This phenomenon causes a light signal to be absorbed by natural impurities in the glass

FIBER TYPES

• Multi-Mode: supports hundreds of modes• Single-Mode: supports a single mode of light

31/01/2009

9

Step-Index MM

�This has the effect of scattering the signal because the rays arrive at the far end at different times

�This is known as Inter-modal Dispersion (sometimes referred to as Differential Mode Delay, DMD)

�To ease the problem, graded index fibers were developed

Graded Index Fiber

� Has a high refractive index at the centre which gradually reduces to a low refractive index at the circumference

� Slows down the lower order modes allowing the higher order modes to catch-up

31/01/2009

10

Graded vs. Stepped Index MM Fiber

�Eliminates modal dispersion

�Enables very long distance transmission

�Small NA requires precise alignment of transmitter & connectors

�Requires more expensive transmitters, connectors, installation

�A single light beam is transmitted down the fiber anddoes not interact with the cladding/core boundary.

Single-mode Fiber

31/01/2009

11

DISPERSION� Note: Dispersion is not a loss of light; it's a distortion

of the signal.

� Thus, dispersion and attenuation are two very different and unrelated problems:

1. Modal dispersion - found only in MM fibers

2. Chromatic Dispersion - present in both MM and SM fibers

�Modal dispersion … the arrival of different modes of the light at different times is called Modal Dispersion .

• Chromatic dispersion … is a result of different wavelengths of light traveling at different velocities in the core - the pulses spread out as they travel through the fiber

31/01/2009

12

Differential Mode Delay (DMD)

OM3 OM1or 2

Mode conditioning principle

31/01/2009

13

1 and 10 Gb/s Optical Modules vs. Fiber Type vs. Re ach

¹Mode Conditioning Patch-cords required

31/01/2009

14

Let’s Recap

NUMERICAL APERTURE (NA)� NA defines the light-gathering ability of the fiber

� Imagine a cone coming from the core. Light entering the core from within this cone will be propagated by total internal reflection

� Light entering from outside the cone will not be propagated

� A large NA makes it easier to inject more light into a fiber, while a small NA tends to give the fiber a higher bandwidth

� You cannot mix and match fibers! Trying to connect SM to MM fiber can cause 20 dB loss - that's 99% of the power gone!

Transmit Receive

31/01/2009

15

Transmitters and Light Sources� LEDs cannot modulate faster than 622 Mb/sec

� Vertical cavity surface emitting lasers (VCSELs) are low-cost lasers that operate at 850 nm on multimode fiber to support gigabit speeds.

� Single-mode transmitters are typically made up of either a Distributed feedback (DFB) or a Fabry-Perot (FP) laser

Laser

VCSEL

LED

Laser: Light amplification by stimulated emission of radiation

Fiber Amplifiers

�To boost the signal for greater distances

Optical Fiber

OutputSignal

InputSignal

PumpLaser

(co-pumping)

PumpLaser

(counter-pumping)

Erbium Doped Fiber Amplifiers (EDFAs) Raman Amplification

Electronic Repeater

31/01/2009

16

Optical Fiber Attenuators� Optical fiber attenuators are used in optical systems where

the optical power from a source is too high for the equipment in use.

� A fiber optic attenuator reduces the power of an optical signal traveling through the attenuator without distorting the waveform.

MODE-FIELD DIAMETER (MFD)

• Not all light travels through the core of the fiber, but is distributed through both the core and the cladding. The "mode field" is the distribution of light through the core and cladding of a fiber. MFD defines the size of the power distribution.

• MFD measurements can be found on product spec sheets provided by the manufacturer.

CoreDiameter

MFD

31/01/2009

17

Wavelength Division Multiplexing (WDM)

Multiplexing

De-multiplexing

Detectors

Lasers

Dense Wavelength Division Multiplexing (DWDM) communication systems use multiple WDM’s to transmit multiple channels through fiber optic cables.

The act of combining several differentwavelengths on the same fiber is known asmultiplexing .

At the receiving end, these wavelengths needto be separated again, which is known, logicallyenough, as de-multiplexing .

Wavelength Division Multiplexing (WDM)

�The exact wavelengths of light being used are usually aroundthe 1550 nanometer region, the wavelength region in whichoptical fiber performs the best

�Each different wavelength will be separated by a 0.8nmspacing, which is the frequency separation

�So, if you have four wavelengths, you could have them at1549.2nm, 1550nm, 1550.8nm, and 1551.6nm

Systems being deployed at present will typically have 32 wavelengths

31/01/2009

18

FTTH: fiber to the home - bringing fiber all the way to the home of the subscriber

FTTP: fiber to the premises - same thing, different term, also encompasses businesses

FTTC: fiber to the curb (with wire into the home)

FTTN: Fiber to the node (with wire into the home)

FTTx: to refer to all types or for those who can’t decide what to call it!

What is FTTx?

The alternatives to FTTH are inadequate for future bandwidth needs:

DSL won’t have the bandwidth over most installed co pper

Wireless won’t have the bandwidth, and the spectrum may not be affordable

BPL (broadband over power lines) won’t have the ban dwidth

If the alternatives do not offer the bandwidth, the y cannot be cost effective.

So why put off the inevitable?

The architecture used by Verizon for triple play ser vices

EDFA = erbium doped fiber amplifier

WDM = wavelength division multiplexing

FDF = fiber distribution frame

FDH = fiber distribution hub

ONT = optical network terminal

OLT = optical line terminal

31/01/2009

19

PON relies on optical splitters and couplers to direct signals without regenerating them, which allows providers to build out fiber optic networks without active electronics.

Without those electronics, which require external power, PON is less expensive to deploy - ?

Split Nom. splitting loss (dB)

Avail. fiber loss

(dB)

Nom. Distance

(km)

4 7 11 44

8 10.5 7.5 30

16 14 4 16

32 17.5 2.5 10

64 21 -1 -4

FSO is a line-of-sight technology that uses beams of light to provide optical bandwidth connections.

Today, FSO technology can transmit voice, data, and video at bandwidths up to 1.5 Gbps.

The infrared wavelengths used are just visible to the human eye as red, and are most clearly seen at night.

What is Free Space Optics (FSO)?

31/01/2009

20

QUESTIONS