This document discusses the system design verification of an optical communication link through power and risetime budgets. It explains that the power budget involves calculating power levels from the transmitter to receiver, accounting for factors like attenuation, coupling power, losses, equalization penalty, SNR requirements, and safety margin. The risetime budget involves calculating the total system risetime based on the risetimes of the source, fiber, amplifier, and detector. Several examples are provided to demonstrate calculating power budgets and risetime budgets to determine the maximum bit rate and link length possible for different optical system component specifications.

1. Taiz University, YEMEN
4 October 2018

2. Taiz University, YEMEN
• The basic system design verification can be done through:
1. Power budget
2. Risetime budget.
• The power budget involves the power level calculations from the
transmitter to the receiver.
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1. Attenuation
2. Coupled power
3. Other losses
4. Equalization penalty
(DL)
5. SNR requirements
6. Minimum power at
detector
7. BER
8. Safety margin (Ma)
  dB
B
D T
T
L
4
2
2
2 


3. Taiz University, YEMEN
• The optical power budget is then assembled taking into
account ALL these parameters.
𝑃𝑖 = 𝑃𝑜 + 𝐶𝐿 + 𝑀𝑎 + 𝐷𝐿 𝑑𝐵
where Pi = mean input power launched in the fiber
Po = mean optical power required at the receiver
CL = total channel loss
• The sensitivity of the detector is the minimum detectable
power.
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4. Taiz University, YEMEN
• Safety margin 𝑀𝑎 takes into account possible source
and modal noise, together with receiver impairments
such as equalization error, noise degradations and eye-
opening impairments.
• The safety margin depends to a large extent on the
system components as well as the system design
procedure and is typically in the range 5 to 10 dB.
• Systems using an injection laser transmitter generally
require a larger safety margin (e.g. 8 dB) than those
using an LED source (e.g. 6 dB) because the
temperature variation and aging of the LED are less
pronounced.
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5. Taiz University, YEMEN
• For probability of error calculations, P(e) or BER
is given by:
where erfc = complimentary error function.
• This value can be taken from the graph of P(e)
against SNR as shown in the figure below.
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 







2
2
2
1
)
(
2
/
1
SNR
erfc
e
P

6. Taiz University, YEMEN
• BER versus SNR
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7. Taiz University, YEMEN
• The finite bandwidth of the optical system may result
in overlapping of the received pulses or ISI, giving a
reduction in sensitivity at the optical receiver.
• Therefore, either a worse BER must be tolerated or the
ISI must be compensated by equalization within the
receiver.
• Equalization requires an increase in optical power at
the receiver which may be considered as an additional
loss penalty.
• This additional loss contribution is usually called
dispersion-equalization or ISI penalty, DL (dB).
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8. Taiz University, YEMEN
• For the risetime budget, the calculations will
involve the bandwidth.
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1. Risetime of the source, TS
2. Risetime of the fiber (dispersion), TF
3. Risetime of the amplifier, TA
4. Risetime of the detector, TD

9. Taiz University, YEMEN
The risetime budget is assembled as:
Tsyst = 1.1(TS
2 + TF
2 + TD
2 + TA
2)1/2
For non-return-to-zero (NRZ) data
For return-to zero (RZ) data
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T
syst
B
T
7
.
0

T
syst
B
T
35
.
0


10. Taiz University, YEMEN
We need to design a digital link to connect two points 10-km apart. The bit
rate needed is 30 Mb/s with BER = 10-12.
Determine whether the components listed are suitable for the link.
Source: LED 820nm GaAsAl; couples 12µW into 50µm
fiber; risetime 11ns
Fiber: Step Index fiber; 50µm core; NA = 0.24; 5.0
dB/km loss; dispersion 1ns/km; 4 connectors with
1.0dB loss per connector
Detector: PIN photodiode; R = 0.38A/W; Cj = 1.5pF,
Id = 10pA; risetime = 3.5ns; minimum mean optical
power = - 86dBm
Calculate also the SNR of the link if RL given is 5.3kΩ
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Example (1)

11. Taiz University, YEMEN
For this example, 3 factors need to be considered:
a) Bandwidth
b) Power levels
c) Error rate (SNR)
Risetime Budget
We start with the risetime budget. Assume using NRZ
coding, the system risetime is given by:
𝑇𝑠𝑦𝑠𝑡 =
0.7
𝐵𝑇
=
0.7
30 × 106
= 23.3 𝑛𝑠
Also:
𝑇𝑠𝑦𝑠𝑡 = 1.1 𝑇𝑆
2 +
𝑇𝐹
2 +
𝑇𝐷
2 1/2
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Solution:

12. Taiz University, YEMEN
Now we can assemble the total system risetime:
Total system risetime = 23.3 ns
Risetime of the source, TS = 11.0ns
Risetime of the fiber (dispersion), TF 10 x 1.0ns = 10.0ns
Allowance for the detector risetime, TD
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ns
T
T
Tsys
T S
F
D 09
.
15
1
.
1
2
2
2











13. Taiz University, YEMEN
Power Budget
Total power launched into fiber = -19dBm
Losses: Fiber attenuation 5dB/km x 10 = 50dB
4 connectors 1dB x 4 = 4dB
Power available at detector =[( -19dBm – 50dB- 4dB)] = -73 dBm
Since power available at the detector is –73 dBm, the sensitivity
of the detector must be less than this.
The safety margin, Ma = -73-(-86) dB
= 13dB
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14. Taiz University, YEMEN
The choice of components are suitable because;
a) TD calculated is greater than TD given
b) Total power available at the detector
is greater than the minimum power required by
the detector i.e Ma is positive.
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15. Taiz University, YEMEN
An optical link is to be designed to operate over an
8-km length without repeater. The risetime of the
chosen components are:
Source: 8 ns
Fiber:Intermodal 5 ns/km
Intramodal 1 ns/km
Detector 6ns
From the system risetime considerations estimate
the maximum bit rate that may be achieved on the
link using NRZ code.
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Example (2)

16. Taiz University, YEMEN
Tsyst = 1.1(TS
2 + TF
2 + TD
2)
= 1.1 [82 + (8 x 5)2 + (8 x 1)2 + 62]1/2
= 46.2 ns
Max bit rate =
𝐵𝑇𝑚𝑎𝑥 =
0.7
𝑇𝑠𝑦𝑠𝑡
= 15.2𝑀𝑏𝑝𝑠
Maximum bit rate = 15.2Mbps
Or 3 dB optical BW = 7.6MHz
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Solution:

17. Taiz University, YEMEN
The following parameters were chosen for a long haul single
mode optical fiber system operating at 1.3µm.
Mean power launched from laser = -3dBm
Cabled fiber loss = 0.4 dB/km
Splice loss = 0.1 dB/km
Connector loss at transmitter and receiver = 1 dB each
Mean power required at the APD
When operating at 35Mbps (BER = 10-9) -55 dBm
When operating at 400Mbps(BER = 10-9) -44 dBm
Required safety margin = +7 dB
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Example (3)

18. Taiz University, YEMEN
Estimate:
a) maximum possible link length without repeaters
when operating at 35Mbps. It may be assumed
that there is no dispersion-equalization penalty at
this rate.
b) maximum possible link length without repeaters
when operating at 400Mbps.
c) the reduction in the maximum possible link length
without repeaters of (b) when there is dispersion-
equalization penalty of 1.5dB.
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19. Taiz University, YEMEN
a)35Mbps
Pi – Po = [(Fiber cable loss + Splice losses ) x L + Connector loss
+ Ma ]dB
[-3dBm – (-55 dBm)] = (0.4 + 0.1)L + 2 + 7
0.5L = 52 –2-7
L = 86km
b) 400 Mbps
Pi – Po = [(Fiber cable loss + Splice losses ) x L + Connector loss
+ Ma ]dB
[-3dBm – (-44 dBm)] = (0.4 + 0.1)L + 2 + 7
0.5L = 41 –2-7
L = 64km
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Solution:

20. Taiz University, YEMEN
c) Including dispersion-equalization penalty of 1.5dB
Pi – Po = [(Fiber cable loss + Splice losses ) x L +
Connector loss + DL + Ma]dB
[-3dBm – (-44 dBm)] = (0.4 + 0.1)L + 2 + 1.5 + 7
0.5L = 41 –2 -1.5 - 7
L = 61km
Note: a reduction of 3 km in the maximum length without
repeaters when DL is taken to account.
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21. Taiz University, YEMEN
An optical link was designed to transmit data at a rate of
20 Mbps using RZ coding. The length of the link is 7 km
and uses an LED at 0.85µm. The channel used is a GRIN
fiber with 50µm core and attenuation of 2.6dB/km.
The cable requires splicing every kilometer with a
loss of 0.5dB per splice. The connector used at the
receiver has a loss of 1.5dB. The power launched into the
fiber is 100µW. The minimum power required at the
receiver is –41dBm to give a BER of 10-10. It is also
predicted that a safety margin of 6dB will be required.
Show by suitable method that the choice of
components is suitable for the link.
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Example (4)

22. Taiz University, YEMEN
The power launched into the fiber 100µW = -10 dBm
Minimum power required at the receiver - 41dBm
Total system margin - 31 dBm
Fiber loss 7 x 2.6 18.2dB
Splice loss 6 x 0.5 3.0 dB
Connector loss 1 x 1.5 1.5
Safety margin 6.0 dB
Total system loss 28.7dB
Excess power margin = -31 dBm - 28.7 dB = 2.3 dBm
Based on the figure given, the system is stable and provides an excess of 2.3
dB power margin. The system is suitable for the link and has safety margin to
support future splices if needed.
4 October 2018 22
Solution:

23. Taiz University, YEMEN
An optical communication system is given with the following
specifications.
Laser:  = 1.55µm,  = 0.15nm, power = 5dBm, tr = 1.0ns
Detector: tr = 0.5ns, sensitivity = -40dBm
Pre-amp: t A = 1.3ns
Fiber: total dispersion (M+Mg) = 15.5 psnm-1km-1; length =
100km; = 0.25dB/km
Source coupling loss = 3dB
Connector (2) loss = 2dB
Splice (50) loss = 5dB
System: 400 Mbps, NRZ, 100km
4 October 2018 23
Example (5)

24. Taiz University, YEMEN
For risetime budget
system budget, tsystem = = 1.75ns
source ts = 1.0ns …(1)
fiber tF =
= 0.25ns …(2)
Detector tR = 0.5ns
pre-amp tA = 1.3ns
for receiver,
total =
= 1.39ns …(3)
System risetime from (1),(2) and (3)
= = 1.73ns
The calculated tsystem = 1.9ns
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100
5
.
15 

 
2
2
A
R t
t 
2
2
4
.
1
25
.
0
0
.
1 

6
10
400
7
.
0
7
.
0


T
B
Solution:

25. Taiz University, YEMEN
Since the calculated tystem is greater than the available tystem, the components will not
be suitable to support the 400 Mbps signal.
For the power budget,
Laser power output 5 dBm
Source coupling loss 3 dB
Connector loss 2 dB
Splice loss 5 dB
Attenuation in the fiber 25 dB
Total loss 35 dB
Power available at the receiver = (5 dBm -35 dB) = -30 dBm
The detector sensitivity is -40 dBm which is 10 dB less. Therefore the chosen
components will allow sufficient power to arrive at the detector. Safety margin is +10
dB.
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26. Taiz University, YEMEN 26
4 October 2018