oc6thsem,power budget

8/9/2019 oc6thsem,Power Budget

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Design Considerations

Link Power Budget

There is enough power margin in the system to

meet the given BER

Rise Time Budget

Each element of the link is fast enough to meet

the given bit rate

These two budgets give necessary conditions

for satisfactory operation


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Simple point-to-point link

System Requirements

1. Transmission Distance2. Data Rate for a given BER

This p-p link forms the basis for examiningmorecomplex systems


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Selecting the Fiber

Other factors to consider:

Attenuation Bandwidth-Length product

Then decide

Multimode or single mode

Step or graded index fiber

Bit rate and distance are the major factors


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Selecting the

Optical Source Emission wavelength

Spectral line width

(FWHM) and numberof modes

Output power

Stability

Emission pattern

Effective radiating area

LED

LAS

ER


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Selecting the detector

Type of detector

APD: High sensitivity but complex, high bias voltage

(40V or more) and expensive

PIN: Simpler, thermally stable, low bias voltage (5V orless) and less expensive

Responsivity

Operating wavelength and spectral selectivity

Speed Sensitivity


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Typical bit rates at different

wavelengthsWavelength LEDSystems LASERSystems.

800-900 nm

(TypicallyMultimode Fiber)

150 Mb/s.km 2500 Mb/s.km

1300 nm (Lowest

dispersion)

1500 Mb/s.km 25 Gb/s.km

(InGaAsP Laser)

1550 nm (Lowest

Attenuation)

1200 Mb/s.km Up to 500

Gb/s.km

(Best demo)


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Optical power-loss model

ystem MarginT s R c sp f P P P ml nl L S E! !

: Total loss; : Source po er; : x sensitivity

connectors; splices

T s

n

ystem MarginT s c sp f l nl L S E! !

: Total loss; : Source po er; : x sensitivity

connectors; splices

T s

n


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ystem arginT s c sp f l nl S E! !

Optical power-loss model (Cont..)

PT :- Total Loss

PS

:- Source Power

PR : - Receiver Sensitivity

lsp :- Splice loss

n :- No. of splices

lc :- Connector lossm :- No. of Connectors


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Example 1A network provider choose thecomponents with thefollowing characteristics-

Receiver sensitivity= - 50dBm (at 1Gb/s)

Fiber loss 0.5 dB/km

Transmitter launch power into the fiber is 0 dBmDetector coupling loss is 1 dB.

It is anticipated that in addition, 20 splices each of loss 0.5

dB are required.

Calculate the power budget to construct an optical link of40km and bandwidth of 1Gb/s. Also determine whether

the system operates with sufficient power margin or not.


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TransmitterOutput = 0 dBm

ReceiverSensitivity = -50dBmAvailable Power = 50dB

System Loss

Fiber Loss of 0.5dB/Km, 40Km = 20dB

Detector Coupling Loss = 1dB

Total Splicing Loss (0.5dBX20) = 10dB

System margin = 6dB

Total Attenuation = 37dB

Hence theexcess power margin is 13dB. It is sufficient for

the operation of the optical link.

Solution


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Example 2

Consider a LED source ofemitted power 0dBm.

Available length of fiber is 2km. Assume that source

coupling loss is 3dB, detector coupling loss is 1dB,

system margin is 5dB, splice loss is 0.2dB per splice and

attenuation is 2dB/km for the operating wavelength850nm. Determine the maximum permissible link

length. Receiver sensitivity is 30dBm.


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Solution

TransmitterO

utput = 0 dBmReceiverSensitivity = -30dBm

Available Power = 30dB

Loss due to source and detector coupling and system margin =

3dB + 1 dB + 5 dB = 9dB

Thus Power left = 30dB 9dB = 21dB

Let length of the link = L km (Length ofeach fiber section 2km)

So, there are (L/2 1) Splices in the link.

So, (L/2 1) X (0.2) + L X 2 = 21dB

From this L = 10km.

oc6thsem,power budget

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