FIBER OPTIC SYSTEM DESIGN

CHAPTER 5

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

System development schematic

User Service Requirements

Logical Architecture

Physical Architecture

Focus on

User Needs

Focus on Functions

to satisfy User Needs

Focus on Partition

of Functions to Systems and

Locations

Generate and organize

appropriate functions

Partition functions into discrete

platforms

Subsystem Requirements

Subsystem Requirements

Subsystem Requirements

Focus on Implementation of subsystems

Partition functions into implementable

pieces

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

Steps for physical layer design

Determine topology needed Point-point Star Ring

Determine key functional requirements Data rates Error rates

Make initial designUse 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

Amount of power lost or gained in each component1. Optical link loss (attenuation)2. Dispersion3. Signal-to-noise ratio

System power margin Allows for component tolerances, system degradation, repairs

and splices

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

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

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.

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

Optical link loss budget

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Optical link loss budget (continued)

Transmitter-10 dBm

Receiver-10 to –25 dBm

Splice-0.1 dB

Splice-0.1 dB

Connector-0.5 dB

Connector-0.5 dB

15.5 km @ 0.35 dB/km

Optical link loss budget (continued)

Item Description Amount

(a)Optical fiber loss at 1310 nm: 15.5 km length at 0.35 db/km 5.4 db

(b) Splice loss: 2 splices at 0.1 db/splice 0.2 db

(c)Connection loss: 2 connections at 0.5 db/connection 1.0 db

(d) Other component losses 0.0 db(e) Design margin 2.0 db

(f) Total link loss (a)-(e) 8.6 db

(g) Transmitter avg. output power -10.0 dBm(h) Receiver input power (g-f) -18.6 dBm(I) Receiver dynamic range -10 to -25 dBm(j) Receiver sensitivity at BER 10-9 -25 dBm

(h) Remaining margin (h-j) 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 usedPatch panel at each end to facilitate

connections3 fusion splices requiredTransmitter power: -10 dBmReceiver sensitivity: -20 dBm

Problem: choose type of fiber to be used

Example

Transmitter-10 dBm

Receiver-10 to –25 dBm

Splice-0.1 dB

Splice-0.1 dB

Patch panel-1.0 dB

1.2 km

Patch panel-1.0 dB

Splice-0.1 dB

Available fiber

Fiber sizeAttenuation

(db/km)

Maximum allowable loss (dB) at 850 nm

Maximum length (km)

50/125 3.0 2.0 0.650/125 2.7 2.0 0.7

62.5/125 3.5 5.0 1.462.5/125 3.0 5.0 1.6100/140 5.0 9.5 1.5100/140 4.0 9.5 1.8

Example (continued)

Using 62.5/125 with 3.0 db/km loss:

Item Description Amount

(a)Optical fiber loss at 850 nm: 1.2 km length at 3.0 db/km 3.6 dB

(b) Splice loss: 3 splices at 0.1 db/splice 0.3 dB

(c)Connection loss: 2 connections at 1.0 db/connection 2.0 dB

(d) Other component losses 0.0 dB(e) Design margin 2.0 dB

(f) Total link loss (a)-(e) 7.9 db> 5 dB allowable

Example (continued)

Using 100/140 with 4.0 db/km loss:

Item Description Amount

(a)Optical fiber loss at 850 nm: 1.2 km length at 4.0 db/km 4.8 dB

(b) Splice loss: 3 splices at 0.1 db/splice 0.3 dB

(c)Connection loss: 2 connections at 1.0 db/connection 2.0 dB

(d) Other component losses 0.0 dB(e) Design margin 2.0 dB

(f) Total link loss (a)-(e) 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 Depends on

Type of amplifier Transmitter Receiver Rise time Noise and error analysis

Can be inserted 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 measurementThe basis for loss measurements as well as the

power from a source or presented at a receiverTypically 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 526-14, 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 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.

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