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L5 Optical Fiber Link and LAN Design. 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.

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table of content
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
table of content cont
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
transmission types
Transmission Types
  • Two types of transmissions:

1. Link (point to point)

2. Network

a. point to multipoint

b. Mesh

c. Ring

elements of link network design
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

elements of link network design cont
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

slide7
The Main Problems

Attenuation and Loss

Dispersion

The Main Question

  • In Digital System

- Data Rate

- Bit Error Rate

  • In Analog System

- Bandwidth

- Signal to Noise

Ratios

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

Optical Transmitter/ Sources
optical fiber
Multimode Fiber

Attenuation

Multimode Dispersion

Chromatic Dispertion

Numerical Aperture

Core Diameter

Single-Mode Fiber

Attenuation

Chromatic Dispersion

Cutoff Wavelength

Spot Size

Optical Fiber
optical receiver photodiode
Optical Receiver/ Photodiode
  • Risetime/Bandwidth
  • Response Wavelength Range
  • Saturation Level
  • Minimum Detection Level
simple link

RX

TX

OA

OA

Medium and Devices

Simple Link
link budget considerations
Link Budget Considerations

Three types of budgets:

(1) Power Budget

  • Bandwidth or Rise Time Budget
  • ?
slide16

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
requirements cont d
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

db dbm mw
dB, dBm, mW

dB = 10 log (P1/P2)

slide21

Connector

Splice

Example:

Power Budget Measurement for Long Haul Transmission

185 km

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

slide22

Total Losses = 46.3 + 4.6 + 0.4

= 51.3 dB

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

PMargin = 6 dB

PRX = PTX – Total Losses – PMargin

= 0 – 51.3 – 6

PRX = -57.3 dB

Power Budget, PRX< PSEN !!

slide23

?

How To Solve?

Answer…

Place an amplifier

But…

What is the gain value?

And…

Where is the location?

slide24

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?

slide25

Three choices available

for the location

Power Amplifier – At the transmitter

Preamplifier – At the receiver

In Line – Any point along fiber

slide26

Hence …

Let us check one by one…

Power Amplifier: PTX + Gain = POUT

0 + 30 = 30 dBm

But is there any power amplifier with 30 dBm POUT?

NO, THERE ISN’T

slide27

Hence …

What about Preamplifier?

Remember…

POUT received = -57 dBm

Preamplifier with 30 dB available?

Yes

But, can it take –57 dBm?

Typically, NO

slide28

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…

slide29

Where is the –30 dBm point?

PTX – Loss At That Point = 0 dBm – 30 dB

Loss At That Point = -30 dBm

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

slide30

+ 1 connector at Tx

2 connectors

Also remember connector loss at amplifier and Tx…

Connector Loss = 0.2 dB x 3 = 0.6 dB

Actually, at 120 km,

Total Losses = Fiber Loss + Splice Loss + Connector Loss

= 30 + 3 + 0.6 = 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 = 105.6 km

slide31

Connector

Splice

185 km

PSEN = -28 dBm

PTx = 0 dBm

105.6 KM

Let us confirm the answer…

At 105.6 km from Tx,

Fiber Loss = 0.25 x 105.6 = 26.4 dB

No. of Splice at 105.6 km = 105.6/4 =26.4 = 27

Splice Loss = 0.1 x 27 = 2.7 dB

Total Losses = 26.4 + 2.7 = 29.1 dB

29.1 dB < 30 dB !!

CONFIRM…105.6 KM IS A GOOD LOCATION!!

slide32

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

system rise time
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

slide35

Total Rise time, Tsys:

Tsys=1.1(TTX2+TRX2+Tfiber2)1/2

bandwidth budget1

T

T’

Bandwidth Budget

RX

TX

OA

OA

Δτ = T’ - T

Medium and Devices

slide37

What is a good Rise time?

  • For a good reception of signal Tsys < 0.7 x Pulse Width (PW)
  • PW = 1/BitRate for NRZ

1/2BitRate for RZ

slide38

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

slide39

Total Rise time, TSYS = 1.1 TLS2 + TPD2 + TF2

= 1.1 0.01 + 0.25 + 0.16

TSYS = 0.77 ns

Simple Calculation….

Fiber rise time, TF =Length x D x Linewidth()

= 150 km x 18 x 0.15 nm

= 0.4 ns

slide40

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!!

slide41

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!!

budget summary

Option

Power Budget

Bandwidth Budget

Financial

A

Source (LED vs. LD)

Δλ

850nm

Mediocre

Bad

Cheap

1310nm

Good

Good

Less expensive

1550nm

Very good

Very good

Expensive

Modulation Bandwidth

LED

NA

Bad

Cheap

LD

NA

Good

Expensive

Output Power

LED

Mediocre

NA

Cheap

LD

Good

NA

Expensive

Radiation pattern

LED (far-field pattern)

NA

Bad

Cheap

LD (Gaussian beam)

NA

Good

Expensive

Budget Summary
budget summary1

B

Fiber

Option

Power Budget

Bandwidth Budget

Financial

Attenuation

MM

Mediocre

Mediocre

Cheap

SM

Good

Good

Expensive

Dispersion

MM

Mediocre

Mediocre

Cheap

SM

Good

Good

Expensive

Numerical Aperture (NA)

MM

Mediocre

Mediocre

Cheap

SM

Good

Good

Expensive

Core Diameter

MM

Mediocre

Mediocre

Cheap

SM

Good

Good

Expensive

Budget Summary
budget summary2

C

Receiver (PIN vs. APD)

Option

Power Budget

Bandwidth Budget

Financial

Rise time/ Bandwidth

PIN

Mediocre

Mediocre

Cheap

APD

Good

Good

Expensive

Response wavelength range

PIN

Mediocre

Mediocre

Cheap

APD

Good

Good

Expensive

Saturation Level

PIN

Mediocre

Mediocre

Cheap

APD

Good

Good

Expensive

Minimum detection level

PIN

Mediocre

Mediocre

Cheap

APD

Good

Good

Expensive

Budget Summary
sensitivity analysis
Sensitivity Analysis
  • Minimum optical power that must be present at the receiver in order to achieve the performance level required for a given system.
factors will affect this analysis
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
slide47
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
slide48
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)

signal to noise ratio
Signal to Noise Ratio

SNR = 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)

cost performance considerations
Cost/Performance Considerations

Components considerations such as :

  • Light Emitter Type
  • Emitter Wavelength
  • Connector Type
  • Fiber Type
  • Detector Type
summary
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.
summary1
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.