Fiber-Optic Communications

James N. Downing
 Chapter 9

Fiber-Optic Communications Systems

9.1 System Design Considerations
Design is based on
Application
Type of signal
Distance from transmitter to detector
Performance standards
Resource constraints (time, money, etc.)
Implementation
Components
Format, power, bandwidth, dynamic range
Amplification
 9.1 System Design Considerations
Design is based on
Implementation
Components
Format, power, bandwidth, dynamic range
Amplification, amplitude, and spacing
Multiplexing
Security requirements
Acceptable noise levels
 9.1 System Design Considerations
System Power Budget
Most important parameter is throughput or transfer function.
Output power must be greater than the input sensitivity of
the receiver.
System budget
Amount of power lost or gained in each component
System power margin
Allows for component tolerances, system degradation, repairs
and splices
 9.1 System Design Considerations
Power at the Source
Transmitter must be appropriate for the application
Number of signals
Wavelength of signal
Type of transmitter device (LED, Laser)
Modulation
Mode structure
Tunability
WDM and amplification capability
Coupling efficiency
 9.1 System Design Considerations
Power in the Fiber
Matching
Source output pattern, core-size, and NA of fiber
Coupling is critical
Power at the Detector
Sensitivity is the primary purpose of the detector
Minimum sensitivity yet still meets standards
Must support the dynamic range of the power levels
 9.1 System Design Considerations
Fiber Amplification
For those fibers that require amplification
Two types:
Repeaters are rarely used.
Optical amplifiers are the preferred amplification.
Use manufacturers specifications to ensure
optimization of the input signal.
 9.1 System Design Considerations
Amplifier Placement
Depends on
Type of amplifier
Transmitter
Receiver
Rise time
Noise and error analysis
Can be inserted
Before regeneration
Between regenerators
 9.1 System Design Considerations
System Rise Time Budget
Determines the bandwidth carrying capability
Total rises time is the sum of the individual
component rise times.
Bandwidth is limited by the component with the
slowest rise time.
 9.1 System Design Considerations
Rise Time and Bit Time
Rise time is defined as the time it takes for the
response to rise from the 10% to 90% of maximum
amplitude.
Fall time is the time the response needs to fall
from 90% to 10% of the maximum.
Pulse width is the time between the 50% marks on
the rising and falling edges.
 9.1 System Design Considerations
Transmitters, Receivers, and Rise Time
Rise time of transmitter is based on the response
time of the LED or laser diode.
Rise time of the receiver is primarily based on the
semiconductor device used as the detector.
 9.1 System Design Considerations
Fiber Rise Time
Comes directly from the total dispersion of the
fiber as a result of modal, material, wave guide,
and polarization mode dispersion
Total Rise Time
Sum of all the rise times in the system
 9.1 System Design Considerations
Round Trip Delay
Time needed for the signal to reach the furthest point of the
network and return
Dispersion Compensation
Allows for lowering the fiber dispersion characteristics
add fiber with dispersion of the opposite magnitude
Only available type: chromatic dispersion
 9.1 System Design Considerations
Single Channel System Compensation
Implementation
Long length of small amplitude dispersion fiber
Short length of large amplitude dispersion fiber
(distributed compensation)
Multi-Channel System Compensation
Large effective area fibers
Reduced dispersion fibers
 9.1 System Design Considerations
Single Channel System Compensation
Noise and Error Analysis
Determines the type of amplification required
Minimizing System Noise
Additional Noise Sources
Extended pulse width
Modal properties of fibers
Chirp
Fresnel reflection
Feedback noise
 9.1 System Design Considerations
Multiple Channel System
Channel Density and Spacing
Standards have been defined by ITU-T
WDM, TDM, and Noise
Interchannel crosstalk: Data from adjacent channels gets
mixed
Dispersion in adjacent channels
Non-linearities at high powers causes interference
Narrow bandpass filtering at the receiver
 9.1 System Design Considerations
WDM Power Management
Methods must ensure that all power levels fall with
acceptable range.
Gain flattening is the process of adjusting the
amplitudes of wavelengths to be the same.
9.2 From the Global Network to the
Business and Home
Long-Haul Communications
Terrestrial cables
Telegraph cable across the English Channel in 1850
First transatlantic cable in 1866
Transatlantic telephone cable in 1957
Transatlantic fiber-optic cable in 1988
Optical amplifiers replaced repeaters in 1990s
9.2 From the Global Network to the
Business and Home
Undersea Cables
Must be capable of low loss and dispersion
Must limit optical noise
Must have a pressure resistant covering
Amplifier gain below 10 dB
Precise dispersion
Repeatered systems has pump laser and amplifier
Unrepeatered system has optical amplifiers spaced out over
the length of the fiber
 9.2 From the Global Network to the
Business and Home
Terrestrial Cables
Long-haul lengths
Easy repair
Amplification needed less often
When is terrestrial, satellite or undersea
cabling used?
Depends on politics and economy rather than
technology or geography
 9.2 From the Global Network to the
Business and Home
Metro and Regional Networks
PSTN: Public switched telephone networks for
regions (little population)
MANs: Metropolitan area networks (more densely
populated areas such as towns and universities)
LANs: Local area networks
WANs: Wide area networks
 9.3 Special Fiber-Optic
Communications Systems
Soliton Communications
Form of dispersion compensation
Combination of chromatic and self-phase
modulation
Coherent Communications Systems
Uses WDM bandwidth more efficiently
Possible improvement in receiver sensitivity
 9.3 Special Fiber-Optic
Communications Systems
Optical CDMA
Maximizes the bandwidth in LANs without special
filtering devices
Spreads the signal energy over a wider frequency
band than necessary
 9.3 Special Fiber-Optic
Communications Systems
Free Space Optics
Signal travels through space rather than a fiber
Relies on line of sight
Free of FCC regulations
Bandwidth is not held to that of the fiber used
Fiber Optics and the Future
Where you go, then so shall I.