1. L5 Optical Fiber Link and LAN Design

2. Table of content Transmission Type Elements in Network Design
Factors for Evaluating Fiber Optic System Design
Link Budget Considerations
Power Budget
Power Budget Requirement
Example : Long-haul Transmission System
Example : LAN

3. Table of content (cont.)
Bandwidth Budget
System Rise Time
Example on STM-4, STM-16 and STM-64
Budget Summary
Sensitivity Analysis
Eye Diagrams
Signal to Noise Ratio (SNR)
Cost/ Performance Considerations
Summary

4. Transmission Types Two types of transmissions:
1. Link (point to point)
2. Network
a. point to multipoint
b. Mesh
c. Ring

5. Elements of Link/ Network Design
Transmitter :
Operating wavelength (), Linewidth (),
Rise time, Bit-rate, Line format, Power level
Fiber :
SMF/MMF, Fiber type – SMF28, DSF, etc,
Cable loss, Spool length
Rx :
PSEN, PSAT, Rise time

6. Elements of Link/ Network Design (cont.)
Connection:
No. of splice, Splice loss
No. of connectors, Connector Loss
In Line Devices:
Splitter, Filter, Attenuator, Amplifier Insertion loss, Gain

7. The Main Question
In Digital System
- Data Rate
- Bit Error Rate
In Analog System
- Bandwidth
- Signal to Noise
Ratios
The Main Problems
Attenuation and Loss
Dispersion

8. Factors for Evaluating Fiber Optic System Design

9. Factors for Evaluating Fiber Optic System Design
(cont.)

10. Optical Transmitter/ Sources
LEDs
Output Power
Modulation Bandwidth
Center Wavelength
Spectral Width
Source Size
Far-Field Pattern
Laser Diodes
Output Power
Modulation Bandwidth
Center Wavelength, Number of Modes
Chirp, Linewidth
Mode Field of the Gaussian beam

11. Optical Fiber Multimode Fiber Attenuation Multimode Dispersion
Chromatic Dispertion
Numerical Aperture
Core Diameter
Single-Mode Fiber
Attenuation
Chromatic Dispersion
Cutoff Wavelength
Spot Size

12. Optical Receiver/ Photodiode
Risetime/Bandwidth
Response Wavelength Range
Saturation Level
Minimum Detection Level

13. Simple Link TX RX
Medium and Devices OA

14. Link Budget Considerations
Three types of budgets:
(1) Power Budget
Bandwidth or Rise Time Budget ?

15. POWER BUDGET

16. Power Budget Requirements:
PB : PRX > PMIN
PRX = Received Power
PMIN = Minimum Power at a certain BER
PRX = PTX – Total Losses + Total Gain - PMARGIN
PTX = Transmitted Power
PMARGIN ≈ 6 dB

17. Requirements Cont’d: Loss,L = LIL + Lfiber + Lconn. + Lnon-linear
LIL = Insertion Loss
Lfiber = Fiber Loss
Lconn.= Connector Loss
Lnon-linear= Non-linear Loss
Gain,G = Gainamp + Gnon-linear
Gainamp = Amplifier Gain
Gnon-linear = Non-linear Gain

18. dB, dBm, mW
dB = 10 log (P1/P2)

19. Decibel to Power Conversion

20. Decibel to Power Conversion

21. Power Budget Measurement for Long Haul Transmission
Example:
Power Budget Measurement for Long Haul Transmission
185 km
Connector
Splice
PSEN = -28 dBm
PTx = 0 dBm
IS THIS SYSTEM GOOD?
Attenuation Coefficient,  = 0.25 dB/km
Dispersion Coefficient, D = 18 ps/nm-km
Number of Splice = 46
Splice Loss = 0.1 dB
Connector Loss = 0.2 dB
PMargin = 6 dB

22. CONCLUSION: BAD SYSTEM!!
Simple Calculation….
Fiber Loss = 0.25 dB/km X 185 km
= 46.3 dB
Splice Loss = 0.1 dB X 46
= 4.6 dB
CONCLUSION: BAD SYSTEM!!
Connector Loss = 0.2 dB X 2
= 0.4 dB
Total Losses =
= 51.3 dB
PMargin = 6 dB
PRX = PTX – Total Losses – PMargin
= 0 – 51.3 – 6
PRX = dB
Power Budget, PRX < PSEN !!

23. How To Solve? Answer… But… And… Place an amplifier
What is the gain value? ?
And…
Where is the location?

24. Now…Where to put the amplifier?
First we calculate the amplifier’s gain..
Gain  PSEN - PRX
Gain  -28 – (-57.3)
Gain  29.3 dB
Gain  30 dB
To make it easy,
Now…Where to put the amplifier?

25. Three choices available
for the location
Power Amplifier – At the transmitter
Preamplifier – At the receiver
In Line – Any point along fiber

26. Let us check one by one… Power Amplifier: PTX + Gain = POUT
= 30 dBm
But is there any power amplifier with 30 dBm POUT?
NO, THERE ISN’T
Hence …

27. What about Preamplifier?
Remember…
POUT received = -57 dBm
Preamplifier with 30 dB available?
Yes
But, can it take –57 dBm?
Typically, NO
Hence …

28. Let us check In Line Amplifiers
30 dB gain amplifier available here…
But,
What value can it take?
Typically –30 dBm
So…
Now, we can find the location…

29. Where is the –30 dBm point? PTX – Loss At That Point = 0 dBm – 30 dB
Assume Other Loss = 0, Loss At That Point = Fiber Loss,
30 =  x Length of That Point
Remember  = 0.25,
Point Length = 30/0.25
= 120 km
But 120 km from Tx,
No. of splice = 120/4
= 30
Splice Loss = 0.1 dB x 30 = 3 dB

30. Also remember connector loss at amplifier and Tx…
+ 1 connector at Tx
2 connectors
Connector Loss = 0.2 dB x 3 = 0.6 dB
Actually, at 120 km,
Total Losses = Fiber Loss + Splice Loss + Connector Loss
= = 33.6 dB
33.6 dB > 30 dB!! NOT GOOD!
Now, We have excess of 3.6 dB…Find the distance,
Fiber Loss Length = 3.6/0.25 = 14.4 km
Good Location = 120 km – 14.4 km = km

31. CONFIRM…105.6 KM IS A GOOD LOCATION!!
Let us confirm the answer…
At km from Tx,
Fiber Loss = 0.25 x = 26.4 dB
No. of Splice at km = 105.6/4 =26.4 = 27
Splice Loss = 0.1 x 27 = 2.7 dB
Total Losses = = 29.1 dB
29.1 dB < 30 dB !!
CONFIRM…105.6 KM IS A GOOD LOCATION!!
PTx = 0 dBm
185 km
PSEN = -28 dBm
Splice
Connector
105.6 KM

32. Power Budget Measurement for LAN
Example:
Power Budget Measurement for LAN
Server A
Server B
500 m
Using 850nm
PSEN = -25 dBm
PTx = -15 dBm
IS THIS SYSTEM GOOD?
Attenuation Coefficient,  = 4.5 dB/km
Dispersion Coefficient, D = 18 ps/nm-km
Number of Splice = 0
Splice Loss = 0 dB
Connector Loss = 0.5 dB
PMargin = 2 dB

33. BANDWIDTH BUDGET

34. System Rise Time Calculate the total rise times Tx, Fiber, Rx
Calculate Fiber rise time, TFiber
Tfiber = D x  x L
D = Dispersion Coefficient
 = Linewidth
L = Fiber Length
Tx Rise Time, TTX = normally given by manufacturer
Rx Rise Time, TRX = normally given by manufacturer

35. Tsys=1.1(TTX2+TRX2+Tfiber2)1/2
Total Rise time, Tsys:
Tsys=1.1(TTX2+TRX2+Tfiber2)1/2

36. Bandwidth Budget T T’ RX TX OA OA
Δτ = T’ - T
Medium and Devices

37. What is a good Rise time? PW = 1/BitRate for NRZ 1/2BitRate for RZ
For a good reception of signal Tsys < 0.7 x Pulse Width (PW)
PW = 1/BitRate for NRZ
1/2BitRate for RZ

38. Rise Time Budget Measurement for Long Haul Application
Example:
Rise Time Budget Measurement for Long Haul Application
Tx rise time, TTX = 0.1 ns
Rx rise time, TRX= 0.5 ns
Linewidth() = 0.15 nm
Dispersion Coefficient, D = 18 ps/nm-km
Fiber length = 150km
Bit Rate = 622Mbps
Format = RZ

39. Simple Calculation…. TSYS = 0.77 ns
Fiber rise time, TF =Length x D x Linewidth()
= 150 km x 18 x 0.15 nm
= 0.4 ns
Total Rise time, TSYS = 1.1 TLS2 + TPD2 + TF2
= 1.1
TSYS = 0.77 ns

40. Good Rise Time Budget!! Let say, Bit Rate = STM 4 = 622 Mbps
Format = RZ
Tsys < 0.7 x Pulse Width (PW)
Pulse Width (PW) = 1/(622x106)
= 1.6 ns
0.77 ns < 0.7 x 1.6 ns
0.77 ns < 1.1 ns !!
Good Rise Time Budget!!

41. Bad Rise Time Budget!! Let say, Bit Rate = STM 16 = 2.5 Gbps
Format = RZ
Tsys < 0.7 x Pulse Width (PW)
Pulse Width (PW) = 1/(2.5x109)
= 0.4 ns
0.77 ns < 0.7 x 0.4 ns
0.77 ns ≥ 0.28 ns !!
Bad Rise Time Budget!!

42. Budget Summary Option Δλ Power Budget Financial A Source (LED vs. LD)
Option
Power Budget
Bandwidth Budget
Financial A
Source (LED vs. LD) Δλ
850nm
Mediocre
Bad
Cheap
1310nm
Good
Less expensive
1550nm
Very good
Expensive
Modulation Bandwidth
LED NA LD
Output Power
Radiation pattern
LED (far-field pattern)
LD (Gaussian beam)

43. Budget Summary B Fiber Option Power Budget Attenuation MM Cheap SM
Bandwidth Budget
Financial
Attenuation MM
Mediocre
Cheap SM
Good
Expensive
Dispersion
Numerical Aperture (NA)
Core Diameter

44. Budget Summary C Receiver (PIN vs. APD) Rise time/ Bandwidth PIN Cheap
Option
Power Budget
Bandwidth Budget
Financial
Rise time/ Bandwidth
PIN
Mediocre
Cheap
APD
Good
Expensive
Response wavelength range
Saturation Level
Minimum detection level

45. Sensitivity Analysis
Minimum optical power that must be present at the receiver in order to achieve the performance level required for a given system.

46. Factors will affect this analysis
1. Source Intensity Noise - Refers to noise generated by the LED or Laser
Phase Noise - the difference in the phases of two optical wavetrains separated by time, cut out of the optical wave
Amplitude Noise - caused by the laser emission process.
2. Fiber Noise
Relates to modal partition noise
3. Receiver Noise
Photodiode, conversion resistor

47. 4. Time Jitter and Intersymbol Interference
Time Jitter - short term variation or instability in the duration of a specified interval
Intersymbol Interference
result of other bits interfering with the bit of interest
inversely proportional to the bandwidth
Eye diagrams - to see the effects of time jitter and intersymbol interference

48. 5. Bit error rate - main quality criterion for a digital transmission system
BER = Q [IMIN2/ (4 . N0 . B) ]
where :
N0 = Noise power spectral density (A2/Hz)
IMIN = Minimum effective signal amplitude (Amps)
B = Bandwidth
Q(x) = Cumulative distribution function (Gaussian distribution)

49. Eye Diagrams

50. Signal to Noise Ratio SNR = S/N SNR (dB) = 10 log10 (S/N)
S - represents the information to be transmitted
N - integration of all noise factors over the full system bandwidth
SNR (dB) = 10 log10 (S/N)

51. Cost/Performance Considerations
Components considerations such as :
Light Emitter Type
Emitter Wavelength
Connector Type
Fiber Type
Detector Type

52. Summary
The key factors that determine how far one can transmit over fiber are transmitter optical output power, operating wavelength, fiber attenuation, fiber bandwidth and receiver optical sensitivity.
The decibel (dB) is a convenient means of comparing two power levels.
The optical link loss budget analyzes a link to ensure that sufficient power is available to meet the demands of a given application.

53. Summary
Rise and fall times determine the overall response time and the resulting bandwidth.
A sensitivity analysis determines the amount of optical power that must be received for a system to perform properly.
Bit errors may be caused by source intensity noise, fiber noise, receiver noise, time jitter and intersymbol interference.
The five characteristics of a pulse are rise time, period, fall time, width and amplitude.

54. TUTORIAL

57. Thank You