The attached narrated power point presentation attempts to explain the methods of computation of total power loss and system rise time in a fiber optic link. The material will be useful for KTU final year B Tech students who prepare for the subject EC 405, Optical Communications.
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.
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).
