Transcript CHAPTER 5

CHAPTER 5
FIBER OPTIC SYSTEM DESIGN
System Design Considerations
 Design is based on
 Application

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
Type of signal
Distance from transmitter to detector
Performance standards
Resource constraints (time, money, etc.)
Implementation

Components

Format, power, bandwidth, dynamic range
Amplification, amplitude, and spacing
 Multiplexing
 Security requirements
 Acceptable noise levels

System development schematic
Partition functions
into implementable
pieces
User Service
Requirements
Generate and
organize
appropriate
functions
Subsystem
Requirements
Partition functions
into discrete
platforms
Logical
Architecture
Physical
Architecture
Subsystem
Requirements
Subsystem
Requirements
Focus on
User Needs
Focus on
Functions
to satisfy User
Needs
Focus on
Partition
of Functions to
Systems and
Locations
Focus on
Implementation
of subsystems
Design of optical networks
 Design proceeds at several levels (rough
classification)
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Physical: fiber, amplifiers, ADMs (hardware)
Data link: Ethernet, SONET (encoding, access control)
Network: ATM, IP (addressing, routing)
 There is interaction among these layers
 SONET may require particular physical layer configuration,
e.g., rings
 Ethernet, especially GigE or 10GigE will require switches
Steps for physical layer design
 Determine topology needed
 Point-point
 Star
 Ring
 Determine key functional requirements
 Data rates
 Error rates
 Make initial design
 Use manufacturer data to complete/modify design
 Satisfy budgets
 Meet performance goals
System factors for designing from scratch
Factor
Available choices
Type of fiber
Single mode, multimode, plastic
Dispersion
Repeaters, compensation
Fiber nonlinearities
Fiber characteristics, wavelengths
used, transmitter power
Operating wavelength
(band)
780, 850, 1310, 1550, 1625 nm typical
Transmitter power
~0.1 to 20 mw typical; usually
expressed in dBm
Light source
LED, laser
Receiver characteristics
Sensitivity, overload
Multiplexing scheme
None, CWDM, DWDM
System factors (continued)
Factor
Available choices
Detector type
PIN diode, APD, IDP
Modulation scheme
OOK, multilevel, coherent
End-end bit error rate
<10-9 typical; may be much lower
Signal-to-noise ratio
Specified in dB for major stages
Max number of
connectors
Loss increases with number of
connectors
Max number of splices
Loss increases with number of
splices
Environmental
Humidity, temperature, sunlight
exposure
Mechanical
Flammability, strength,
indoor/outdoor/submarine
System factors (continued)
Factor
Available choices
Amplifiers
Type, spacing
Switches
OEO, all optical
Add/drop multiplexers
Number, location
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
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Amount of power lost or gained in each component
1. Optical link loss (attenuation)
2. Dispersion
3. Signal-to-noise ratio
System power margin
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Allows for component tolerances, system degradation, repairs and splices
System Design Considerations
 Power at the Source
 Transmitter must be appropriate for the application
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Number of signals
Wavelength of signal
Type of transmitter device (LED, Laser)
Modulation
Mode structure
Tunability
WDM and amplification capability
Coupling efficiency
System Design Considerations
 Power in the Fiber
 Matching
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Source output pattern, core-size, and NA of fiber
Coupling is critical
 Power at the Detector
 Sensitivity is the primary purpose of the detector
 Must support the dynamic range of the power levels
System Design Considerations
 Fiber Amplification
 For those fibers that require amplification
 Two types:
Repeaters are rarely used.
 Optical amplifiers are the preferred amplification.
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Use manufacturers specifications to ensure optimization of the
input signal.
Optical link loss budget
 Key calculations in designing a simple fiber optic link
 Objective is to determine launch power and receiver
sensitivity
 Variables
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Environmental and aging
Connector losses
Cable losses
Splices
Amplifier
Other components
Optical link loss budget
0
-5
-10
-15
+2
db
+2
db
+2
db
0-3
db
0-2
db
-4 db
+1
db
0-2
db
0-3 db
-20
-30
-35
-40
-45
Source variables
Fiber
Connector
Connector
Receiver
Optical link loss budget (continued)
Connector
-0.5 dB
Connector
-0.5 dB
Transmitter
-10 dBm
Splice
-0.1 dB
Receiver
-10 to –25 dBm
Splice
-0.1 dB
15.5 km @ 0.35 dB/km
Optical link loss budget (continued)
Item
(a)
Description
Optical fiber loss at 1310 nm: 15.5 km
length at 0.35 db/km
(c)
(d)
(e)
Splice loss: 2 splices at 0.1 db/splice
Connection loss: 2 connections at 0.5
db/connection
Other component losses
Design margin
(f)
Total link loss (a)-(e)
(g)
(h)
(I)
(j)
Transmitter avg. output power
Receiver input power (g-f)
Receiver dynamic range
Receiver sensitivity at BER 10-9
(h)
Remaining margin (h-j)
(b)
Amount
5.4 db
0.2 db
1.0 db
0.0 db
2.0 db
8.6 db
-10.0 dBm
-18.6 dBm
-10 to -25 dBm
-25 dBm
6.4 dB
Optical link loss budget—example
 Point-to-point fiber optic link between 2 computers
 Path length measured as 1.2 km
 Multimode fiber to be used
 Patch panel at each end to facilitate connections
 3 fusion splices required
 Transmitter power: -10 dBm
 Receiver sensitivity: -20 dBm
 Problem: choose type of fiber to be used
Example
Patch panel
-1.0 dB
Patch panel
-1.0 dB
Transmitter
-10 dBm
Splice
-0.1 dB
Splice Splice
-0.1 dB -0.1 dB
1.2 km
Receiver
-10 to –25 dBm
Available fiber
Fiber size
50/125
50/125
62.5/125
62.5/125
100/140
100/140
Attenuation
(db/km)
3.0
2.7
3.5
3.0
5.0
4.0
Maximum
allowable loss
(dB) at 850 nm
2.0
2.0
5.0
5.0
9.5
9.5
Maximum
length (km)
0.6
0.7
1.4
1.6
1.5
1.8
Example (continued)
 Using 62.5/125 with 3.0 db/km loss:
Item
(a)
(b)
(c)
(d)
(e)
(f)
Description
Optical fiber loss at 850 nm: 1.2 km
length at 3.0 db/km
Splice loss: 3 splices at 0.1 db/splice
Connection loss: 2 connections at 1.0
db/connection
Other component losses
Design margin
Total link loss (a)-(e)
Amount
3.6 dB
0.3 dB
2.0 dB
0.0 dB
2.0 dB
7.9 db
> 5 dB allowable
Example (continued)
 Using 100/140 with 4.0 db/km loss:
Item
(a)
(b)
(c)
(d)
(e)
(f)
Description
Optical fiber loss at 850 nm: 1.2 km
length at 4.0 db/km
Splice loss: 3 splices at 0.1 db/splice
Connection loss: 2 connections at 1.0
db/connection
Other component losses
Design margin
Total link loss (a)-(e)
Amount
4.8 dB
0.3 dB
2.0 dB
0.0 dB
2.0 dB
9.1 db
< 9.5 dB allowable
Example (continued)
 Power at receiver: -10 dBm – 9.1 dBm = -19.1 dBm
 OK, since receiver sensitivity –25 dBm
 Amplifier Placement
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Depends on
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Type of amplifier
Transmitter
Receiver
Rise time
Noise and error analysis
Can be inserted
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Before regeneration
Between regenerators
 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.
 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.
 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.
 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
Optical Power
 Most basic fiber optic measurement
 The basis for loss measurements as well as the power
from a source or presented at a receiver
 Typically both transmitters and receivers have
receptacles for fiber optic connectors, so measuring
the power of a transmitter is done by attaching a test
cable to the source and measuring the power at the
other end
 For receivers, one disconnects the cable attached to
the receiver receptacle and measures the output with
the meter
Optical Power
 Optical power is based on the heating power of the
light, and some optical lab instruments actually
measure the heat when light is absorbed in a detector
 Optical power meters typically use semiconductor
detectors since they are sensitive to light in the
wavelengths and power levels common to fiber optics
 Most fiber optic power meters are available with a
choice of 3 different detectors, silicon (Si),
Germanium (Ge), or Indium-Gallium-Arsenide
(InGaAs).
Optical Power
 Table 1. Optical power levels typical of fiber optic
communication systems
Network Type
Wavelength, nm
Power Range, dBm
Power Range, W
Telecom
1310, 1550
+3 to -45 dBm
50 nW to 2mW
Datacom
650, 850, 1300
0 to -30 dBm
1 to 100uW
CATV, DWDM
1310,1550
+20 to -6 dBm
250 uW to 10mW
Optical Power Measurement
Optical Power Measurement
 Measuring power requires only a power meter (most
come with a screw-on adapter that matches the
connector being tested), a known good fiber optic cable
(of the right fiber size, as coupled power is a function of
the size of the core of the fiber) and a little help from the
network electronics to turn on the transmitter
 when measure power, the meter must be set to the
proper range (usually dBm, sometimes microwatts, but
never "dB" - that's a relative power range used only for
testing loss and the proper wavelength , matching the
source being used in the system (850, 1300, 1550 nm for
glass fiber, 650 or 850 nm for POF).
Optical Power Measurement
 To measure power, attach the meter to the cable attached
to the source that has the output you want to measure
(see diagram to the right). That can be at the receiver to
measure receiver power, or using a reference test cable
(tested and known to be good) that is attached to the
transmitter to measure output power
 Turn on the transmitter/source and give it a few minutes
to stabilize. Set the power meter for the matching
wavelength and note the power the meter measures.
Compare it to the specified power for the system and
make sure it's enough power but not too much.
Optical wavelength
 The wavelengths we use for transmission must be the
wavelengths we test for losses in our cable plants.
Our power meters are calibrated at those
wavelengths so we can test the networking
equipment we install
 The three prime wavelengths for fiber optics, 850,
1300 and 1550 nm drive everything we design or
test. NIST (the US National Institute of Standards
and Technology) provides power meter calibration at
these three wavelengths for fiber optics
Optical wavelength
 Multimode fiber is designed to operate at 850 and
1300 nm, while singlemode fiber is optimized for
1310 and 1550 nm.
 The difference between 1300 nm and 1310 nm is
simply a matter of convention, harking back to the
days when AT&T dictated most fiber optic jargon
 Lasers at 1310 nm and LEDs at 1300 nm were used
in singlemode and multimode fiber respectively
Fiber Optic Testing
 Testing is used to evaluate the performance of fiber
optic components, cable plants and systems.
 As the components like fiber, connectors, splices,
LED or laser sources, detectors and receivers are
being developed, testing confirms their performance
specifications and helps understand how they will
work together.
 Designers of fiber optic cable plants and networks
depend on these specifications to determine if
networks will work for the planned applications
The following test:
 Continuity testing to determine that the fiber routing
and/or polarization is correct and documentation is
proper.
 End-to-end insertion loss using an OLTS power meter
and source. Test multimode cables using TIA/EIA 52614, and singlemode cables using TIA/EIA 526-7
(singlemode). Total loss shall be less than the calculated
maximum loss for the cable based on Loss Budget
calculations using appropriate standards or customer
specifications.
 Optional OTDR testing may be used to verify cable
installation and splice performance. However, OTDR
testing should not be used to determine cable loss,
especially on longer cables. Use of an OTDR in premises
applications may be inappropriate if cables are too short.
The following test:
 If the design documentation does not include cable
plant length, and this is not recorded during
installation, read the length from the distance
marking on the cable jacket or test the length of the
fiber using the length feature available on an OTDR,
or some OLTSs.
 If testing shows variances from expected losses,
troubleshoot the problems and correct them.
Component Testing
 Fiber optic inspection microscopes are
used to inspect connectors to confirm
proper polishing and find faults like
scratches, polishing defects and dirt.
 They can be used both to check the
quality of the termination procedure and
diagnose problems.
 A well made connector will have a
smooth , polished, scratch free finish and
the fiber will not show any signs of
cracks, chips or areas where the fiber is
either protruding from the end of the
ferrule or pulling back into it.
Continuity Testing
 Perform continuity testing of optical fibers using a
visual fiber tracer, visual fault locator, or OLTS
power meter and source.
 Trace the fiber from end to end through any
interconnections to ensure that the path is properly
installed, and that polarization and routing are
correct and documented.
Visual Tracing
 Continuity checking with a visual fiber
tracer makes certain the fibers are not
broken and to trace a path of a fiber from
one end to another through many
connections, verifying duplex connector
polarity for example.
 It looks like a flashlight or a pen-like
instrument with a light bulb or LED source
that mates to a fiber optic connector
 Attach the fiber to test to the visual tracer
and look at the other end of the fiber to see
the light transmitted through the core of
the fiber. If there is no light at the end, go
back to intermediate connections to find
the bad section of the cable
Visual Fault Location
 A higher power version of the fiber tracer
called a visual fault locator (VFL) uses a
visible laser that can also find faults.
 The red laser light is powerful enough for
continuity checking or to trace fibers for
several kilometers, identify splices in
splice trays and show breaks in fibers or
high loss connectors.
 You can actually see the loss of light at a
fiber break by the bright red light from
the VFL through the jacket of many
yellow or orange simplex cables
(excepting black or gray jackets, of
course.)
 It's most important use is finding faults in
short cables or near the connector where
OTDRs cannot find them
Insertion Loss
 Insertion loss refers to the optical loss of the
installed fibers when measured with a test source
and power meter (OLTS). Test multimode cables
using TIA/EIA 526-14, and singlemode cables using
TIA/EIA 526-7 (single mode).
 The insertion loss measurement is made by mating
the cable being tested to known good reference
cables with a calibrated launch power that becomes
the "0 dB" loss reference.
Insertion Loss
 a) Test multimode fiber at 850 and 1300 nm, and
singlemode fiber at 1310 and 1550 nm, unless
otherwise required by other standards or customer
requirements.
 b) Test reference test cables to verify quality and
clean them often.
 c) Cabling intended for use with high speed systems
using laser sources may be tested with appropriate
laser sources to ensure that tests verify performance
with that type of source
Insertion Loss
There are two methods that are used to measure loss, a
"patchcord test" which we call "single-ended loss" (TIA FOTP171) and an "installed cable plant test" we call "double-ended
loss" (TIA OFSTP-14 (MM) and OFSTP-7 (SM).) Single-ended
loss uses only the launch cable, while double-ended loss uses a
receive cable attached to the meter also.
OTDR Testing
 OTDRs are powerful test instruments for fiber optic cable
plants. When used by a skillful operator, OTDRs can locate
faults, measure cable length and verify splice loss. Within
limits, they can also measure the loss of a cable plant
 OTDR) uses optical radar-like techniques to create a picture of
a fiber in an installed fiber optic cable. The picture, called a
signature or trace, contains data on the length of the fiber,
loss in fiber segments, connectors, splices and loss caused by
stress during installation
 OTDRs are used to verify the quality of the installation or for
troubleshooting. However, OTDR testing shall not be used to
determine cable loss. OTDRs have limited distance resolution
and may show confusing artifacts when testing short cables
typical of premises applications. If OTDR testing of premises
cables is desired, experienced personnel should evalute the
appropriateness of the tests.
OTDR Testing
Bit Error Rate (BER) Testing
 Bit error rate(BER) is a fundamental measure of
digital transmission quality. BER is essentially an
error probability of digital bits in the received signal;
it is also known as bit e.rror probability