Fiber Optic Comm System

8/4/2019 Fiber Optic Comm System

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Fiber Optic Communication System

Analysis and Design


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-Input which could either be analog or digital-Light Source (Laser or LED)-Modulator

-Fiber optic line-Amplifier-Detector

Applications:Cable TV Networks, Telephone Networks, Database Networks, LAN Networks

etc.

Fiber Optic Communication System


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Flowchart of the of the Analysis and Design


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The system designers must proceed through the

following five steps in order to develop a fiberoptic communication system:

1.Specify the system's operational

requirements.2.Describe the physical and environmental

requirements.

3.Compute the signal optical power

budget.

4.Perform a signal bandwidth analysis.

5.Review the system design.


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Define the application-Cable TV Broadcast, Telephone Network, Computer Network etc.

Determination of the signal-to-noise ratio-Depends on the bandwidth or data rate for an applicationThis implies a choice of signal types, either analog or digital.-The goal is to establish what optical power level will be required at theoptical detector inside the receiver unit.

STEP 1 (System Operational Requirements)


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Analog Signals-Video and audio can directly modulate optical output by causing the optical emitter tobrighten and dim.

Digital Signals-A digital pulse can be formed by turning the source "on" for a brief instant.

Bit error rate (BER)

-a parameter for system performance

-The majority of digital systems achieve a BER of 1 X 109.(1 error in 109 bits = 1 error in 1,000,000,000 bits).

-BER is a function of length because, the farther a pulse has to travel downa fiber the more distortion occurs.

-Amplifiers are used to build up weak signals.


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The resulting optical power level required at thedetector is a function of the data rate or bandwidth.

These levels for digital and analog signals are indicatedfor silicon detectors at 850 nm below.


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-To determine the components necessary to complete a fiberoptic system requires detailing run lengths and determining

system operating environments and components.

-Schematic drawing of the orientation and structure of the system

-The system designer should develop a layout schematic.

STEP 2 (System Layout)


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A fiber optic layout should detail distances between each fiber segment.


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STEP 3 (Signal Optical Power Budget)

-With the system layout and components known, it's now possible for the designer tocompute expected losses at each point in the system.


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-Every component including fiber has a range of optical loss due to variations inmanufacturer.

-An LED device, for example, will be specified with a minimum, average, and maximumoptical output power.

-Detectors also have sensitivity ranges. It is up to the system designer to determine theoptical power necessary at the detector surface from information supplied by themanufacturer.

-Once the receiver and transmitter power levels have been established it is possible toconsider the power transmitted by various cable lengths. This can be seen by plottingthe power on a diagram.


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-Taps, Splices and fiber length contributes to loss the system will have.- Use either peak or average optical power values for determining attenuationthroughout the system. Be consistent in your choice throughout the system

analysis.

Figure 28 - Typical optical power level in system with a tap and a splice.


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Figure 29 - Optical source-to-fiber power coupling chart for various emitters.

-Allow approximately 4 to 6 dB to account for thermal variations in the optical fiber,repair of damaged cables, and source degradation over time.

-Power coupled to various fiber types by a few typical source emitters is detailed inFigure 29.


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Fiber Selection

-Several Fiber Optics are available to suit the needs of different

communication system.


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-Choices for most LAN or data systems, for example, currently centers on the all-silicafibers.-Currently 3 sizes are most often considered:

-Video and CATV systems often employ 50/125 and single mode fibers because of theirhigh bandwidth and low loss performance characteristics.

-Modern intercity telephone trunks also employ single mode fibers.

-Fibers may be selected in a variety of bandwidths and attenuation

- Attenuation of optical fibers will vary depending on the source wavelength of thetransmitter.


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* Values for 850nm wavelength.


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STEP 4 (Bandwidth Analysis)

-The System engineer must assure that a sufficient bandwidth is provided to

ensure the transmission of the signal.- Long-haul telephone systems employ large distances between repeaters andrequire the 100,000 MHz-km fiber bandwidths associated with single modefiber.

-The transmitter and optical fiber should then have bandwidths about 1.5 to 2 timesgreater than the receiver.

-For digital systems the system bandwidth will depend on the data rate and thecoding format according to:

BW system = R/Kwhere:

K equals 1.4 for (NRZ) coding format and 1.0 (RZ) format.R in bits per second

-The system bandwidth is limited by the lowest bandwidth component in the link.


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The approximate relationship between the total cable bandwidth (BWCo) and onekilometer section fiber bandwidth (BWf)

BWf= BWCo (L) x

where:

BWCo total cable bandwidthL is the fiber length in kilometers.x equals 1.0 for cable run lengths (L) of one kilometer or less and x equals 0.75 for fiber incable run lengths greater than 1 kilometer.


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-System designer to review all of the pieces to determine that all work together to deliverthe right signal to the right place at the right time.

-The number of fibers or a cable depends on the number of channels or signal carryingcapacity desired. Cables employing fibers with special high bandwidths are available ascustom products.

-Specific materials and multi-fiber construction have resulted in numerous cable designs

which incorporate a variety of fibers to meet specific applications.

STEP 5 (System Review)


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The complete cable structure can be established using the following criteria:

Cable Construction:Hybrid All Dielectric Metal Strength Members

Jacket Materials:PVC Polyurethane Polyethylene Other

Environmental Protection Flame RetardancySunlight Resistance

Water Resistance

Water Blocking (gel fill)

Rodent Protection (armor)Nuclear Radiation Resistance

Other

Chemical Resistance:To Oil Acid Alkali Solvents

Fiber Features:Number ofFibers

Fiber TypeCore Size

Wavelength

Attenuation

Bandwidth

NA (numerical aperture)

Double Window

Number and type of electrical conductors


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Design and Analysis of a Fiber Optic

Local Area Network1.Specify the system's operational requirements.

A typical LAN network consists of:-nodes, or computers

-a medium for connection, either wired ,wireless or fiber-special network equipment, such as routers or hubs-

Determine the use/primary function of the systemSingle mode or Multi Mode Fiber optic

LAN network usually shares-Data, applications, and resources, such as printers and computing poweretc.-Digital Modulation require 20 to 600 MHz-km


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Determine the topology suited

2.Describe the physical and environmental requirements

(System Layout)

Linear bus topology

A star topology A tree topology

Mesh topology


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4.Perform a signal bandwidth analysis

3.Compute the signal optical power budget.

-Local area networks (LAN) typically require 20 to 600 MHz-km fiber bandwidth-Consideration of the encoding to be used because, it affects the bandwidth ofthe system

Example of a Optical Link Budget

Typical: 20 dB loss. It may vary according to the length of the fiber optic cable used.


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5.Review the system design

- Final reassessment of the system design component, specifications chosen.

Typical: CAT5e and RJ45

Fiber optic mostly used: Multi-mode optical fiber is usually chosen for

LAN

1.25Gbit/sec multimode fiber

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Fiber Optic Comm System