1. FIBER OPTIC SYSTEM DESIGN
CHAPTER 5
FIBER OPTIC SYSTEM DESIGN

2. 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, amplitude, and spacing
Multiplexing
Security requirements
Acceptable noise levels

3. System development schematic

4. Design of optical networks
Design proceeds at several levels (rough classification)
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

5. 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

6. System factors for designing from scratch
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

7. System factors (continued)
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

8. System factors (continued)
Available choices
Amplifiers
Type, spacing
Switches
OEO, all optical
Add/drop multiplexers
Number, location

9. 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
1. Optical link loss (attenuation)
2. Dispersion
3. Signal-to-noise ratio
System power margin
Allows for component tolerances, system degradation, repairs and splices

10. 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

11. 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
Must support the dynamic range of the power levels

12. 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.

13. Optical link loss budget
Key calculations in designing a simple fiber optic link
Objective is to determine launch power and receiver sensitivity
Variables
Environmental and aging
Connector losses
Cable losses
Splices
Amplifier
Other components

14. Optical link loss budget
Other allowances (repair, splice, safety margin
Temperature
Insertion loss
Fiber coupling
Insertion loss
Attenuation
Aging
Temperature -5
Launch
-10
+2 db
+2 db
+2 db
0-3 db
0-2 db
-4 db
+1 db
0-2 db
0-3 db
-15
Receiver Sensitivity Range
-20
-30
-35
-40
Source variables
Fiber
Receiver
-45
Connector
Connector

15. Optical link loss budget (continued)
Connector
-0.5 dB
Connector
-0.5 dB
Transmitter
-10 dBm
Receiver
-10 to –25 dBm
Splice
-0.1 dB
Splice
-0.1 dB
dB/km

16. Optical link loss budget (continued)

17. 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

18. Example Transmitter Receiver -10 dBm -10 to –25 dBm 1.2 km Patch panel
Splice
-0.1 dB
Splice
-0.1 dB
Splice
-0.1 dB
1.2 km

19. Available fiber

20. Example (continued)
Using 62.5/125 with 3.0 db/km loss:

21. Example (continued)
Using 100/140 with 4.0 db/km loss:

22. Example (continued) Power at receiver: -10 dBm – 9.1 dBm = -19.1 dBm
OK, since receiver sensitivity –25 dBm

23. Amplifier Placement Depends on Can be inserted Type of amplifier
Transmitter
Receiver
Rise time
Noise and error analysis
Can be inserted
Before regeneration
Between regenerators

24. 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.

25. 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.

26. 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.

27. Fiber Rise Time Total 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

28. 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

29. 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).

30. 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

31. Optical Power Measurement

32. 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).

33. 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.

34. 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

35. 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

36. 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

37. 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 , and singlemode cables using TIA/EIA (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.

38. 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.

39. 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.

40. 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.

41. 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

42. 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

43. 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 , and singlemode cables using TIA/EIA (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.

44. 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

45. Insertion Loss
There are two methods that are used to measure loss, a "patchcord test" which we call "single-ended loss" (TIA FOTP-171) 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.

46. 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.

47. OTDR Testing

48. 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