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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.