EE 230: Optical Fiber Communication Lecture 17: System Considerations

EE 230: Optical Fiber Communication Lecture 17
System Considerations
From the movie

Warriors of the Net
Basic Network Topologies
Bitrate Distance Graph for various

point to point link technologies
System Design
Determine wavelength, link distance,
and bit-error rate
Work out power budget
Work out risetime budget
Work out cost budget
Power Budget Steps

Start with BER and bit rate, determine B
based on coding method
B = 1/2RC gives the maximum load
resistance R based on B and C
Based on R and M, determine detector
sensitivity (NEP), multiply by B1/2
Add system margin, typically 6 dB, to
determine necessary power at receiver
Power budget steps, continued

Add power penalties, if necessary, for
extinction ratio, intensity noise (includes S/N
degradation by amplifiers), timing jitter
Add loss of fiber based on link distance
Include loss contributions from connections
and splices
End up with required power of transmitter, or
maximum length of fiber for a given
transmitter power
Power budget example

Imagine we want to set up a link
operating at 1550 nm with a bit rate of 1
Gb/s using the RZ format and a BER of
10-9. We want to use a PIN photodiode,
which at this wavelength should be
InGaAs. The R0 for the diode is 0.9
A/W.
Bandwidth required for bit rate

For NRZ format, B=0.5 times bit rate
For RZ format, B=bit rate
For this example, the bandwidth B is
equal to the bit rate, 109 /s.
Bandwidth limit
C=2 pF for this photodiode.
B = 1/2RC, so the load resistance R
must be (2BC)-1 = 79.6
Noise Equivalent Power (NEP)

Signal power where S/N=1
Units are W/Hz1/2
h
NEP
e
4kT
2eI D M 2
M RL
x
In this case, M=1 and the dark current = 4 nA.

The factor outside the radical is 1/R0. We can
thus determine the NEP, which is 5.1x10-7
W, which equals -33.0 dBm.
Q Factor and BER

Q
Vth Voff
off
Von Vth
on
1
Q
BER 1 erf

2
2
For our BER of 10-9, Q=6 and S/N=12
Extinction ratio penalty

Extinction ratio rex=P0/P1
1 rex
2 RP
Q
1 rex on off
1 rex
ex 10 log
1 rex
If our extinction ratio is 0.1, the penalty is 0.87 dB.
Intensity noise penalty

rI=inverse of SNR of transmitted light
I R PrI
I 10 log1 r Q
2
I
Since our S/N is 12, rI=0.83, which leads to a

power penalty of 1.25 dB
Timing jitter penalty

Parameter B=fraction of bit period over
which apparent clock time varies
4 2
b
8 B
3
J 10 log
1 b / 2
2
2 2
1 b / 2 b Q / 2
If our jitter represents 10% of the bit period,

the power penalty is 0.34 dB
Fiber attenuation
If the attenuation in the fiber is 0.2 dB/km
and the link is 80 km long, the total loss
in the fiber will be 16.0 dB
Example results
Minimum power required for receiver:
-33.0 dBm
Safety margin: 6.0 dB
Extinction ratio power penalty: 0.87 dB
S/N power penalty: 1.25 dB
Timing jitter power penalty: 0.34 dB
Fiber loss over 80 km: 16.0 dB
Total= minimum transmitter power=
-8.54 dBm=0.14 mW=140 W
Further steps
Alternatively, previous data could be used
with a fixed transmitter power to
determine maximum length of a fiber
link
If power budget does not add up, one can
replace PIN photodiode with APD
add an EDFA to the link
Power Budget Example
Risetime Budget
Rise time budget components

bit rate and coding format determine upper
limit of rise time
rise time of transmitter (from manufacturer;
laser faster than LED)
pulse spread due to dispersion
rise time of receiver (from manufacturer; PIN
faster than APD)
Rise time components are combined by taking
the square root of sums of squares
Upper limit for rise time

For NRZ format, Tr=0.70/B
For RZ format, Tr=0.35/B
In this case, choose RZ format. Tr must
thus be less than or equal to 0.35/10 9 =
350 ps
Group Velocity Dispersion-based rise time
Calculate from laser optical bandwidth if

known, or from modulation rate:
c
c
2
c c B
In this case, D=17 ps/nm-km, L=80 km, and
=0.016 nm, so tf=21.8 ps.
Modal dispersion rise time

For multimode fiber, time spread due to modal
dispersion is based on core index and fiber
length L.
2
L
NA
For step-index fiber:
t
2cn1
For graded-index fiber:
L NA
t
8cn13
Total rise time
tr t t
2
TR
2
MD
2
GVD
2
RC
For this example, tMD=0, tTR=100 ps, tRC=0.5

ns, and tGVD= 21.8 ps as before. tr is
therefore 510 ps, and the rise time budget
does not meet the limit.
Can use NRZ format
Use faster detector or transmitter
Use graded-index fiber for less dispersion
Computer Based Link Simulation

Computer Simulation is often
used to model opticla links to
account for the complex
interaction between
components and nonlinear
effects
Commercial simulation tools are
now available such as:
Linksim from RSoft
and the tools from VPI Systems
Fiber-Optic Communication Systems-G. Agrawal

EE 230: Optical Fiber Communication Lecture 17: System Considerations

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EE 230: Optical Fiber Communication Lecture 17

From the movie

Basic Network Topologies

Bitrate Distance Graph for various

Power Budget Steps

Power budget steps, continued

Power budget example

Bandwidth required for bit rate

Noise Equivalent Power (NEP)

In this case, M=1 and the dark current = 4 nA.

Q Factor and BER

For our BER of 10-9, Q=6 and S/N=12

Extinction ratio penalty

Intensity noise penalty

Since our S/N is 12, rI=0.83, which leads to a

Timing jitter penalty

If our jitter represents 10% of the bit period,

Power Budget Example

Rise time budget components

Upper limit for rise time

Group Velocity Dispersion-based rise time

Calculate from laser optical bandwidth if

Modal dispersion rise time

Total rise time

For this example, tMD=0, tTR=100 ps, tRC=0.5

Computer Based Link Simulation

Fiber-Optic Communication Systems-G. Agrawal

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