The document presents a comprehensive overview of digital fiber optic communication systems, focusing on point-to-point links, system design considerations, link power budgets, and rise time budgets. It discusses optical components, their characteristics, and how they influence performance metrics like bit error rate and dispersion. Additionally, it covers advanced topics such as wavelength division multiplexing (WDM) and noise penalties associated with optical power, providing insights into system flexibility and capacity upgrades.

1. TPCT’s College of Engineering,
Osmanabad
Department of Electronics and Telecommunication
Presentation
on
Unit-4 Digital FOC System
Subject- FOC Ele-II Class- BE(ECT)
AY-2019-20 Sem-II
Presented by- Prof A P Mane
1

2. Contents
• Introduction
• Point-To-Point Links
• System Considerations
• Link Power Budget
• Rise Time Budget
• Noise Penalties
• WDM
• WDM Network
2

3. Unit-4 Digital FOC System
Introduction
Already we have studied fundamental characteristics/operation of
each individual devices/components/elements/parts of an OFC
system. These includes optical source, photo detector, optical fiber
connectors, couplers, splices and their associated circuits.
Now , we will examine how these
devices/components/elements/parts can put together to form a
complete OFC system/link.
This requires the simplest case of point-to-point link which includes
examining the components that are available for a particular
application and seeing how these components relate to the system
performance criteria (Such as dispersion, bit error rate, etc.)
3

4. Digital FOC System
Point-To-Point Links
The simplest transmission link is a point-to-point line that has a
transmitter on one end and a receiver on the other as shown in Fig.
This type of link places the least demand on optical fibre technology and
thus sets the basis for examining more complex system architecture.
The following key system requirements are needed in analysing a link:
– The desired (or possible) transmission distance
– The data rate or channel bandwidth
– The bit error rate (BER)
4

5. Digital FOC System
To fulfil these requirements the designer has a choice of the
following components and their associated characteristics:
a) Optical Fiber-i) SMF or MMF
ii) Core size
iii) Core refractive-index profile(SIF/GIF)
iv) Bandwidth or dispersion
v) Attenuation or loss
vi) NA or mode-field diameter
b) Optical Source-i) LED or LASER diode
ii) Emission wavelength
iii) Spectral line width
iv) Output power
v) Effective radiating area
vi) Emission pattern
vii) Number of emitting modes
5

6. Digital FOC System
c) Optical Detector- i) p-i-n diode/Avalanche photodiode/PT
ii) Responsivity
iii) Operating wavelength
iv) Speed
To ensure and to meet desired system performance two analyses are
usually carried out and that are Link Power Budget(LPB) and System
Rise Time Budget(RTB).
In LPB-Determination of power margin between op. transmitter o/p
and min. receiver sensitivity needed to establish a specified BER.
This margin can be allocated to connectors, couplers, splices,& fiber
losses + any additional margins for possible component degradation,
T-line impairments, temperature effects etc.
Once LPB established, then RTB can be performed to ensure/verify
and to meet desired overall performance.
6

7. Digital FOC System
System considerations/System Design considerations
Step-1 To start with link design/link power budget- we first decide
at which wavelength data/signal to be transmitted over over link.
-If transmission distance is not too far, then we may decide to
operate in the 800-900nm region where attn. and disp. is more.
-If transmission distance is relatively long, then we may decide to
operate in 1300-1500nm region where attn. and disp. is lower.
Step-2 After deciding wavelength, next step to interrelate the
system performances of the 3 major blocks of link i.e. op.
receiver, op.transmitter, and optical fibre.
Normally designer chooses charactristics of 2 blocks i.e.
selecting charactristics of op receiver and op transmitter and then
selecting charactristics of optical fiber to get desired performance
(If optical fiber is over or under specified a design iterations may
be needed).
7

8. Digital FOC System
Optical Receiver/Detector- Choice of component as pin
diode/APD/PT as per their charactristics.
Mainly required min. optical power fall on detector to satisfy the BER
requirement at a specified data rate.
Additionally designer also need circuit design complexity and cost.
Optical transmitter/Source- Choice of component as LED/LD as per
their charactristics.
Mainly signal dispersion, data rates , transmission distance & cost.
Optical Fiber- We have a choice between SMF & MMF (either
SIF/GIF). This choice depends on the type of light source used and
on the amount of dispersion that can be tolerated by fiber.
Again power handling capability of fiber i.e. NA or Index difference.
Also consider attenuation charactristics of fiber to be selected. i.e.
losses due to absorption, scattering, radiation and coupling between
components.
8

9. Digital FOC System
Link Power Budget
To determine power margin between op. transmitter o/p and min.
sensitivity of receiver needed to establish specified BER. OR to
determine whether the OFC link meets the attenuation
requirements (to decide whether amplifiers are needed or not to
boost the signal.
This can be carried out by using an optical power loss model for
a point-to-point link as shown in Fig.
In this optical power received at detector depends on the amount of
power coupled in to fiber and the losses occurring in the fiber and
at the connectors & splices.
9

10. Digital FOC System
The link loss budget is derived from the sequential loss
contributions of each element in the link. Each of these loss
elements is expressed in decibels (dB) as
loss 10log(Pout / Pin )
where, Pin is the power emanating into the loss element and Pout is the
power emanating out of the loss element.
The link loss budget simply considers the total optical power loss PT
that is allowed between the light source and the photodetector, and
allocates this loss to fiber attenuation, connector loss, splice loss,
and system margin. Thus, if PS is the optical power emerging from
the end of a fibre flylead attached to the light source, and if PR is
the receiver sensitivity, then
PT = PS – PR = 2lc + f L + system margin
Where, lc is the connectorloss, f is the fibre attenuation(dB/Km)
L is the transmission distance and system margin is nominally
taken as 6 dB
10

11. Digital FOC System
Rise Time Budget
A rise-time budget analysis is a convenient method for
determining the dispersion limitation of an optical fibre link. This is
particularly useful for digital systems. In this approach, the total
rise time tsys of the link is the root sum square of the rise times
from each contributor ti to the pulse rise-time degradation
N
sys  itt 
The 4 basic elements that may significantly limit system speed/time are-
• The transmitter rise time ttx
• The group velocity dispersion (GVD) rise time tGVD of the fibre
(Intramodal Dispersion)
• The modal dispersion rise time tmod of the fibre (Intermodal
Dispersion)
• The receiver rise time trx
2
11
i=1

12. The transmitter rise time, ttx - Generally it is known to the designer. It
attributable primarily to the light source and i.ts drive circuitry.
The group velocity dispersion (GVD) rise time, tGVD – Resulting from GVD
over a length L can be approximated using equation:
tGVD = l D l * L *𝜎
The modal dispersion rise time - tmod = 440/BM(L), where BM(L) is BW of
fiber link of length L, which can be expressed as
BM(L)=B0/L q , where B0 is BW of 1 Km length fiber and parameter q
ranges between 0.5 to 1.0 with reasonable estimate 0.7
Therefore, tmod =440 L q / B0
The receiver rise time, trx – This results from photodetector response
and 3db electrical BW, Brx of receiver front end.
trx =350 / Brx
Therefore, tsyst = (t2
𝑡𝑥 + t2
𝑚𝑜𝑑 + t2
𝑔𝑣𝑑 + t2
𝑟𝑥 )1/2
= (t2
𝑡𝑥 + (440 L q / Bo)2 + ( D L 𝜎ƛ)2 + t2
𝑟𝑥 )1/2
Digital FOC System
ƛ
12

13. Digital FOC System
Noise Penalties / Optical Power Penalties
/Noise effects on system performance
We have assumed that the optical power falling on the photo
detector is a clearly defined function of time within the statistical nature
of the quantum detection process. And various interactions between
spectral imperfections in the propagating optical power and the
dispersive waveguide give rise to variations in the optical power level
falling on the photo detector. These variations create receiver output
noises and hence give rise to optical power penalties.
The main penalties are due to
• Modal noise
• Wavelength chirp/chirping
• Reflection Noise -Spectral broadening induced by optical
reflections back into the laser
• Mode partition noise
13

14. Modal Noise-
Modal noise is not present in SMF, however, mode partition noise,
chirping, and reflection noise are critical.
Modal noise arises in when the light from LASER is coupled into a
MMF. The following factor can produce modal noise in the fibre link:
- Mechanical disturbances along the link.- such as vibrations at
connectors, Splices, microbends and source & detector couplings.
Which results in differential mode delay and produces temporal
fluctuations at the receiver end which generates modal noise.
- Fluctuations in the frequency of an optical source.-This also gives
rise to intermodal delays.
To avoid this -Use SMF, LED
-Use fiber with large NA
-Use LASER which has large no. of longitudinal modes
(10 or more)
14 Digital FOC System

15. Digital FOC System15
Wavelength Chirp/ Chirping
This is a dynamic line broadening/spectral width broadening in case
of LASER under CW operation and when the injection current is
directly modulated. Also called as frequency chirping associated with
modulation induced changes in the carrier density. It leads the
significant dispersion for intensity modulated signals. To minimize
chirp- i) Increase bias of LASER so that modulation current does not
drive it below threshold.
ii)Choose LASER emission wavelength close to zero
dispersion wavelength of fiber.
Reflection Noise
It is a optical power reflections fromt refractive-index discontinuities
such as in splices, couplers, connectors, etc. and produces o/p
power fluctuations, pulse distortion, phase distortion, etc. which
degrade performance of both transmitter and receiver.
To reduce this noise-Use well prepared end fibers
-Use index matching gel at connectors
-Make well physical contact with source
-Use op. isolators when LASER is used.

16. Digital FOC System16
Mode-Partition Noise-
This is associated with intensity fluctuations in the longitudinal
modes of a LASER diode. This is dominant in SMF and can occur in
MMF because each of longitudinal modes associated with slightly
different wavelength and has different attenuation and time delay
which causes Intensity fluctuations/power fluctuations results in
variations in the signal level at receiver.
This can be reduced by setting the bias point of LASER above
threshold.

17. Digital FOC System17
Wavelength Division Multiplexing(WDM)
Is a powerful aspect of an OFC link, in this many different
wavelengths can be sent along a single fiber simultaneously in the
1300 to 1600nm spectrum. Te basis of this is to use multiple sources
operating at different wavelengths and to transmit several
independent information streams over the same fiber. i.e. The
technology of combining a number of wavelengths on to the same
fiber is known as wavelength division multiplexing or WDM.
Here N independent optically
formatted info signals of differ
W/Ls (and diff data rates) are
Combined in Op Mux and
sent over single fiber link.

18. Digital FOC System18
Key System Features of WDM
Capacity upgrade: Main application of WDM is to upgrade capacity of existing
P-to-P OFC link. If each wavelength support an independent network signal
capacity of few Gbps, then WDM can increase the capacity of fiber optic network
dramatically.
Transparency/Flexibility: Using WDM any transmission format using different
wavelengths, fast or slow, asynchronous and synchronous, digital data or analog
information can be sent simultaneously and independently over the same fiber.
Wavelength routing: The use of wavelength sensitive optical routing devices
makes it possible to use wavelength as another dimension, in addition to the time
and space in designing communication networks and switches. In wavelength
routed networks, use the actual wavelength as intermediate (or) final address.
Wavelength switching: Wavelength routed networks are based on rigid fiber
infrastructure & W/L switched architecture which allows reconfigurations of the
optical layer.

19. Digital FOC System
• WDM is Essentially FDM at optical frequencies.
• WDM Standards are Developed by the ITU – which Specifies channel
spacing in terms of frequencies.
• The ITU-T G.692 specifies channel from 193.100 THz with spacing 100
GHz.
19
WDM Network
Implementation requires a varity of passive and active optical devices to
Combine (add), Distribute (split), Isolate and Amplify optical power at
different wavelengths.
Passive op Devices- No external control for their operation and they are
limited in their applications in WDN networks. These devices are mainly
used to combine and split or tap-off optical signals. Ex.- Mux, DeMux,
couplers, etc.
Active op Devices- Performance & operation of these devices canbe
controlled electronically which provide large network flexibility. Ex.-
Tunable op sources, tunable op filters (in op receiver), op amplifier, etc.

20. Digital FOC System20
Fig. shows typical WDM network
Transmitter- Consisting several independently modulated light
sources each emitting signals at unique wavelength.(called as
Tunable sources)
WDM Mux.- Is needed to combine these optical output in to a serial
spectrum of closely spaced W/L signals and to couple them in to a
single fiber. Also it provides low loss path from each source to Mux
output.
(Tunable op sources) From other path Dropped signals (Tunable op filters)
OXC

21. Digital FOC System
Optical Amplifiers- This stage/s is used toamplify optical signal in path.
Consisting optical to electrical conversion stage, electrical amplification
stage (Boosting, retiming, pulse shaping, etc) and then electrical to
optical conversion stage. This process is complex in multi W/L high
speed system and simpler in single W/L modrate speed system.
Two different types used are-i) Semiconductor Optical amplifier(SOA) &
ii) Doped Fiber Amplifier(DFA) or Erbium Doped Fiber Amplifier (DFA).
Optical Cross Connect(OXC)- This uses space switching without
wavelength conversion constructed by cascading of electronically
controlled optical directional couplers or by using S/C optical amplifier
switching gates.
DeMux.-To separate/spilt the optical signals in to appropriate detection
channel/receiver.
Receiver- This stage consisting photodetector with tunable filters to
detect signal and signal processing circuitry.
21

22. Thank you
…..
22