The document discusses the analysis of link power and rise time budget for point-to-point communication links, highlighting factors such as fiber losses, allowable loss, and performance constraints. It elaborates on the selection of components like fiber type, light source, and photodiode to meet desired performance specifications. Additionally, it covers important calculations related to link power budget and rise time analysis to ensure system efficiency and reliability over intended transmission distances.

1. 1
Link Power and Rise Time
Budget Analysis
MEC

2. 2
Contents
• Point to Point Links.
• Fiber Losses.
• Allowable Loss.
• Link Power Budget Analysis.
• Rise Time Budget Analysis.
• 3 - dB Bandwidth.
• Transmission Distance Limits.

3. 3
Point to Point Link
• Simplest transmission link has transmitter
and receiver – places least demand on
technology.
• Link design involves several variables,
source, fiber & detector characteristics &
several iterations.
User

4. 4
Point to Point Link
• Performance and Cost Constraints.
• Careful choice of components to ensure
desired performance over expected life
time.
• Key System Requirements:
- Desired/Possible Transmission Distance.
- Data Rate/Channel Bandwidth.
- Bit Error Rate.

5. 5
Point to Point Link –
Designer Choice
• Multimode or Single Mode Fiber.
- Core Size.
- Core Refractive Index Profile.
- Bandwidth or Dispersion.
- Attenuation.
- Numerical Aperture / Mode Field
Diameter.

6. 6
Point to Point Link –
Designer Choice
• LED or Laser Diode Source
- Emission Wavelength.
- Spectral Line Width.
- Output Power.
- Effective Radiating Area.
- Emission Pattern.
- Number of Emitting Modes.

7. 7
Point to Point Link –
Designer Choice
• pin or Avalanche Photodiode.
- Responsivity
- Operating Wavelength.
- Speed.
- Sensitivity.
• Analysis for desired system performance:
- Link Power Budget Analysis.
- Rise Time Budget Analysis.

8. 8
Why Link Power Budget Analysis?
• To determine:
- Power margin between optical transmitter
output and minimum receiver sensitivity to
establish specified BER.
- Margin allotted to connector, splice, fiber
loss, additional margins due to component
degradation, temperature effects.
- Component change/repeater insertion
requirements for desired transmission
distance.

9. 9
Link Power Budget Analysis
• Received optical power depends on the
amount of light coupled into the fiber, fiber
losses, losses at connectors and splice.
• Link loss budget derived from loss
contributions of each element (dB).
• If Pin and Pout are optical powers into and
out of the loss element, loss (dB) = 10log
• Link power margin for component ageing,
temperature fluctuations, components
added in future, 6 - 8dB if no future
additions.
out
in
P
P

10. 10
Link Power Budget Analysis
Two connectors, Loss = 2Ic
Sensitivity = PR
PS
PT = PS - PR

11. 11
Link Loss Budget
• Considers total optical power loss PT
allowed between source and detector, loss
due to cable attenuation, connector and
splice losses and system margin.
• PT = PS – PR = 2Ic + αfL + System Margin,
PS - optical power emerging from the end
of the fiber flylead attached to light source,
PR – receiver sensitivity, Ic – connector
loss, αf – fiber attenuation (dB/km), L- fiber
length, system margin taken as 6 dB.

12. 12
Receiver Sensitivity
BER = 10-11 for InGaAs APD
BER = 10-9 for pin and Si APD

13. 13
Link Loss Budget
800 nm LED/pin @ 20 Mb/s

14. 14
Why Rise Time Analysis?
• Ensure overall desired system
performance.
• Determine distance/dispersion limitations.
• Considers transmitter rise time, material
dispersion rise time, modal dispersion rise
time, receiver rise time.
• Total transition time degradation to be
within limits.

15. 15
Rise Time Budget
• Total transition time degradation not to
exceed 70% of an NRZ bit period, 35% bit
period of an RZ data (Bit Period = 1/Data
Rate).
• Transmitter and Receiver rise times known
to the designer.
• Transmitter rise time attributed to source
and the driving circuitry.

16. 16
Rise Time Budget
• Receiver rise time (10% – 90%) attributed
to photodetector response, 3 dB electrical
bandwidth of receiver front end (Brx).
• Receiver front end response modelled as
a first order low pass filter
u(t) – unit step function.
• Receiver front end rise time

17. 17
Rise Time Budget
• Receiver rise time defined between g(t) = 0.1
to g(t) = 0.9.
• For multimode fibers, rise time depends on
modal and material dispersions.
• In 800 – 900 nm range, material dispersion
adds about 0.07 ns/nm.km to rise time.
• Material dispersion effects to be neglected for
lasers & for LEDs at longer wavelengths.

18. 18
Rise Time Budget
• Total rise time of the link is the root sum
square of the rise times of each contributor tj
to the pulse rise time degradation.
• ttx – transmitter rise time, tmat – material dispersion
rise time, tmod – fiber modal dispersion rise time, trx
– receiver rise time.

19. 19
Fiber Bandwidth
• Fiber Bandwidth resulting from modal
dispersion inversely proportional to cable
length.
• For long continuous fiber, no joints, fiber
bandwidth decreases linearly with
increasing distance for lengths L < modal
equilibrium length Lc.
• For L > Lc, steady state equilibrium
established, bandwidth decreases as L0.5.

20. 20
Fiber Bandwidth
• Practical Case : several fibers joined to form
link.
• Modal redistribution occurs at fiber to fiber
joints – misaligned joints, different core
indices & different degrees of mode mixing in
individual fibers.
• Value of index grading parameter α that
minimizes pulse dispersion depends on
wavelength, fibers optimized for different
wavelengths have different indices.

21. 21
Fiber Bandwidth
• Variations in α at same wavelength leads
to overcompensated & undercompensated
refractive index profiles.
• Total Route Bandwidth a function of order
in which fibers are joined.
• Alternate over & undercompensated
profiles to attain a more modal delay
equalization – time consuming & unwieldy.
• Initial fiber control final link characteristics.

22. 22
Fiber Bandwidth
• Bandwidth BM in a fiber of length L,
(0.5<q<1, B0 – bandwidth of 1 km length)
q = 0.5 if steady state equilibrium, q = 1 if
little mode mixing, typically q = 0.7.
Also,
Bn – Bandwidth of the nth fiber section

23. 23
Pulse Broadening
• If tn – pulse broadening of the nth section,
pulse broadening occuring over N cable
sections:
• Empirical expression for pulse broadening
(0<rpk <1 – correlation coefficient between pth
and kth fiber):

24. 24
Fiber Rise Time and 3 dB
Bandwidth
• Optical power emerging from a fiber has a
gaussian temporal response (σ – rms
pulse width)
• Time t½ required for the pulse to reach its
maximum value ie;

25. 25
Fiber Rise Time and 3 dB
Bandwidth
• Full width of the pulse at its half maximum
value:
• 3-dB optical bandwidth – modulation
frequency f3 dB at which received optical
power has fallen to 0.5 of zero frequency
value.

26. 26
Fiber Rise Time and 3 dB
Bandwidth
• Letting tFWHM be the rise time resulting
from modal dispersion,
• tmod is given by:
tmod in ns and Bm in MHz.

27. 27
Fiber Rise Time and 3 dB
Bandwidth
all times in ns, σλ – source spectral width,
Dmat – fiber material dispersion factor ( = 0.07
ns/nm.km @ 800 – 900 nm).

28. 28
Transmission Distance Limits
800 MHz-km fiber
800 nm source
BER = 10-9
3.5dB/km

29. 29
Transmission Distance Limits
Single Mode Links
1550 nm DFB LD,
D - 2.5 ps/nm.km
Attenuation - 0.3 dB/km

30. 30
References
• https://www.youtube.com/watch?v=NctDb6p
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• https://www.youtube.com/watch?v=mqKaVbo
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• https://www.youtube.com/watch?v=mmvyDS
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• https://www.youtube.com/watch?v=SWvWpl8
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• https://www.youtube.com/watch?v=QRABE0
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31. 31
Thank You