Rise Time Budget Analysis of an Optical Fiber Cable and Design of its various components

1. RISE-TIME BUDGET ANALYSIS & DESIGN OF
COMPONENTS
OPTICAL AND SATELLITE COMMUNICATION
GROUP- 13
Group Members
Ritwik Kashwap
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Rohit Kumar Singh
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Samsunnahar Khan
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Saptarshi Mazumdar

2. DEFINITION & EXPLANATION
A rise-time budget analysis is a convenient method for determining the dispersion limitation of an optical
fiber link, useful for digital systems.
The total rise time Tsys of the link is the root sum square of the rise times from each contribution Ti, to the
pulse rise-time degradation.
The four basic elements that limit system speed are:
1. Transmitter rise time Ttx
2. Group-velocity dispersion (GVD) rise time Tgvd of the fiber
3. Modal dispersion rise time Tmod of the fiber
4. Receiver rise time Ttx

3. TRANSMITTER RISE TIME [ Tts]
This type of rise time is contributed by the light source and the driving circuit. Light sources can be either
LED or LASER.
Rise time of LED is far greater than rise time of LASER. Therefore in most of the cases we use LASER
for a longer distance communication.
Usually designer only calculates the budget according to this facts.
Trx= 350/ Brx

4. GROUP VELOCITY DISPERSION RISE TIME
Group Velocity Dispersion (GVD) is the basic calculation for group delay of optical fiber.
For an Optical Cable of length L, it’s given by,
Tgvd= |D| L σλ
Where, Tgvd= Tmat.
D = dispersion
σλ = Half power band width of source

5. MODAL DISPERSION RISE TIME
As the name suggests, Modal Dispersion Rise Time depends on the mode of the optical fiber. Single
mode optical fiber cable experiences minimal or NULL modal dispersion, whereas in Multimode optical
fiber cable, modal dispersion will be far more.
Modal Dispersion can be given as,
Tmod= 440/ Bm
Where, Bm = Bandwidth of 1km length of fiber cable

6. RECEIVER RISE TIME ( Trx )
If photo detector response with 3 dB electrical BW (Brx), then receiver rise time can be calculated using,
Trx= 350/ Brx
We can use Photo Diode, Pin Photo Diode, Avalanche Photo Diode as the receiver of the system.

7. CALCULATION
Therefore, total rise time of the system can be given by,

8. DIFFERENT SYSTEMS
Return to Zero ( RZ )
Total maximum system bandwidth [ Mbps ] =
Bt= 0.35/ Tsys
Non Return to Zero ( NRZ )
Total maximum system bandwidth [ Mbps ] =
Bt= 0.70/ Tsys
Therefore, we can draw a conclusion,
BW  1/ Tsys

9. Thank You