1. Lecture 11 – Application of Photonics
-Optical Communication

2. Telecommunications Communications over 'long distances'
Information is encoded as signals
Signals are transmitted through a medium
Signals are directed to recipients
Many technologies
Electronic, radio, microwave
Fiber-optic, atmospheric optical

3. What is communicated? Data Voice: telephone, radio
Telegrams
Computer data, telemetry, etc.
Internet , files, web pages
Voice: telephone, radio
Video: sound and images
All are encoded as signals
Systems are converging

4. Signal Structure Signal modulates a carrier wave
Carrier wave is at much higher frequency
Amplitude modulation changes carrier intensity
Standard in fiber optics, AM radio
Frequency modulation changes carrier frequency
Standard for FM radio, television

5. Modulation formats Amplitude modulation of carrier signal Analog
Pulse Code Modulation (digital)

6. Signal Format and Structure-1
Analog signal is an analog of original source (e.g., electrical analog of voice)
Continuous signal levels
Cable television, home phones

7. Signal Format and Structure-2
Digital signal is digital coding
Bit patterns sample signal level at one time
Discrete signal levels (binary off-on)
Many digital codings possible
Long-distance phone, data
Sampling
interval
Digital signal
Wave

8. Bandwidth Information per unit time
Frequency in Hertz (cycles per second)
Bits per second
Depends on signal source and format
HDTV is highest video, analog NTSC lower
Stereo music more than telephone audio
Capacity depends on transmission medium
May vary with length of medium

9. Multiplexing Combines two or more signals
Multiple signals sent over one path
Dates back to telegraph
Reduces costs
Type
Frequency-division
Time-division
Wavelength-division

10. Frequency-division multiplexing
Example is radio broadcast
Each signal has its own carrier frequency
Combined into one signal
Signals modulated on carriers
Individual signal
Frequency

11. Time-division multiplexing
Starts with slow digital signals
Slow signals combined to make faster signal
Hierarchy of data rates
Bits or bytes interleaved
Slow inputs
Interleaved output
Multiplexer

12. Wavelength-division multiplexing
TDM input
Individual optical channels
Channel 1
Optical l 1
transmitter 1
Channel 2 l
Optical 2
l1, l2, l3, l4
transmitter 2
Optical
multiplexer
WDM Output
In one fiber
Channel 3 l 3
Optical
transmitter 3
Whole unit can be put in
One box as WDM
transmitter
Channel 4 l 4
Optical
transmitter 4

13. Transmission distance
Key figure of merit
Depends on
Transmitter power
Receiver sensitivity
Attenuation
May vary with signal bandwidth
Copper attenuation increases with frequency

14. Networks & Connectivity
Network distributes signals
Types of connectivity
Point to point
Point to multipoint (broadcast)
Switched
Networked

15. DEMAND FOR MORE BANDWIDTH OPTICAL COMMUNICATION
Trends
Internet: A Deriving force
SOME ACTUAL FACTS
12 Million messages in next minute
0.5 Million voice mail messages in next minute
3.7 Million people log on the net today
Next 100 days, Internet traffic doubles
100 Million additional internet users every year
Data based on the survey at Bell Laboratories, USA in Nov., 2000.
DEMAND FOR MORE BANDWIDTH
ONLY SOLUTION IS
OPTICAL COMMUNICATION

16. History of Fiber Optics
John Tyndall demonstration in 1870
In 1870, John Tyndall, using a jet of water that flowed from one container to another and a beam of light, demonstrated that light used internal reflection to follow a specific path. As water poured out through the spout of the first container, Tyndall directed a beam of sunlight at the path of the water. The light followed a zigzag path inside the curved path of the water. This idea know as total internal reflection. This idea is the basic of fiber optic.
Total Internal reflection is the basic idea of fiber optic

17. Fiber-optic communication
is a method of transmitting information from one place to another by sending light through an optical fiber.
The light forms an electromagnetic carrier wave that is modulated to carry information.

18. Fiber-optic communication
The process of communicating using fiber-optics involves the following basic steps:
Creating the optical signal using a transmitter,
relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak,
and receiving the optical signal and converting it into an electrical signal.

19. How Does fiber optic transmit light
When light enters the area between 2 difference materials it will produce two different indexes of refraction. The light will either entirely reflected or a portion of it will be refracted depending on the angle. If the light can be kept at an angle where it is entirely reflected, it will become trapped inside and transmitted along the fiber.

20. The Race for Bandwidth 1995 2001 World Wide Web Users 6 Million
World Wide Web Servers
100K
17+ Million
Monthly Internet Traffic
31 Terabytes
350,000 Terabytes
Internet Backbone Demand
Doubles Every 6 Months

21. Exploding Demands for Bandwidth

22. Optical Fiber Applications

23. Fiber to the Home

24. Optical Fiber: Advantages
Capacity: much wider bandwidth (10 GHz)
Crosstalk immunity
Immunity to static interference
Lightening
Electric motor
Florescent light
Higher environment immunity
Weather, temperature, etc.

25. Optical Fiber: Advantages
Safety: Fiber is non-metalic
No explosion, no chock
Longer lasting
Security: tapping is difficult
Economics: Fewer repeaters
Low transmission loss (dB/km)
Fewer repeaters
Less cable
Remember: Fiber is non-conductive
Hence, change of magnetic field has
No impact!

26. Disadvantages Higher initial cost in installation Interfacing cost
Strength
Lower tensile strength
Remote electric power
More expensive to repair/maintain
Tools: Specialized and sophisticated

27. Copper vs. fiber bandwidth
Fiber loss does not change
until very high frequency
Copper loss
rises steadily
with frequency
Power
Frequency
Hecht: Understanding Fiber Optics. (C) 2006 Pearson Education, Upper Saddle River, NJ, All Rights Reserved.

28. Optical Fiber Architecture
TX, RX, and Fiber Link
Transmitter
Input
Signal
Coder or
Converter
Light
Source
Source-to-Fiber
Interface
Fiber-to-light
Detector
Amplifier/Shaper
Decoder
Output
Fiber-optic Cable
Receiver

29. Optical Fiber Architecture – Components
Input
Signal
Coder or
Converter
Light
Source
Source-to-Fiber
Interface
Fiber-to-light
Detector
Amplifier/Shaper
Decoder
Output
Fiber-optic Cable
Receiver
Light source:
Amount of light emitted is proportional to the drive current
Two common types:
LED (Light Emitting Diode)
ILD (Injection Laser Diode)
Source–to-fiber-coupler (similar to a lens):
A mechanical interface to couple the light emitted by the source into the optical fiber
Light detector:
PIN (p-type-intrinsic-n-type)
APD (avalanche photo diode)
Both convert light energy into current

30. Global network Local network Regional network National network
International
network
Switch
Switch
Switch
Switch
Switch
Switch
Switch

31. Components of global network
Submarine cables
High capacity intercontinental
Shorter, regional cables
National backbone networks
Regional networks
Local networks
Satellites play minor role

32. Network nodes Present in long-haul, regional and metro
Hubs or terminal points
Ends of cables where signals are switched and re-organization
Signals typically regenerated
Signals may be broken down to slower data rates or multiplexed to higher rates
Add/drops
Only a few optical channels are added/dropped

33. Optically controlled gate
Wavelength conversion
Input signal
Input modulates amplification of
CW laser source at different wavelength
CW laser
Semiconductor amplifier

34. Optical Communication Systems
First Generation, ~1975, 0.8 mm
MM-fibre, GaAs-laser or LED
Second Generation, ~1980, 1.3 mm, MM & SM-fibre
InGaAsP FP-laser or LED
Third Generation, ~1985, 1.55 mm, SM-fibre
InGaAsP DFB-laser, ~ 1990 Optical amplifiers
Fourth Generation, 1996, 1.55 mm
WDM-systems
1.8
0.8
1.0
1.2
1.4
1.6
0.9
1.1
1.3
1.5
1.7
Wavelength (mm)
Attenuation

35. Fiber Structure A Core Carries most of the light, surrounded by
A Cladding, Which bends the light and confines it to the core, covered by
A primary buffer coating which provides mechanical protection, covered by
A secondary buffer coating, which protects primary coating and the underlying fiber.

36. Types Of Optical Fibre Light ray n1 core n2 cladding
Single-mode step-index fibre
no air
n1 core
n2 cladding
Multimode step-index fibre
no air
Variable n
Multimode graded-index fibre
Index porfile

37. Dispersion

38. Absorption Losses In Optic Fiber
Windows of operation: nm nm nm 6
Rayleigh scattering
& ultraviolet
absorption 5 4
Loss (dB/km) 3
Peaks caused
by OH- ions
Infrared
absorption 2 1
0.7
0.8
0.9
1.0
1.1
1.2
1.3
1.4
1.5
1.6
1.7
Wavelength (mm)
Single-mode Fiber Wavelength Division Multiplexer
(980/1550nm, 1310/1550nm, 1480/1550nm, 1550, 1625nm)

39. Fiber Alignment Impairments
Axial displacement
Gap displacement
Angular displacement
Imperfect surface finish
Causes of power loss as the light travels through the fiber!

40. Optical Fiber System

41. Global Undersea Fiber systems