CHAPTER 7

SYSTEM DESIGN

Transmission Types

• Two types of transmissions:- Link (point to point)

- Network-point to multipoint-Mesh-Ring

Elements of Link/ Network Design• Tx : Operating wavelength (), Linewidth (),

Rise time, Bit-rate, Line format, Power level

• Fiber : SMF/MMF, Fiber type – SMF28, DSF, etc,

Cable loss, Spool length

• Connection: No. of splice, Splice loss

No. of connectors, Connector Loss

• In Line Devices: Splitter, Filter, Attenuator, Amplifier

Insertion loss, Gain

• Rx : PSEN, PSAT, Rise time

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

System Factor Considerations Type of Fiber Single-mode or Multimode Operating Wavelength 780, 850, 1310 and 1550 nm

typical Transmitter Power Typically expressed in dBm Source Type Laser Receiver Sensitivity and Overload Characteristics

Typically expressed in dBm

Detector Type PIN Diode, APD or IDP

Factors for Evaluating Fiber Optic System Design

System Factor Considerations Modulation Code AM, FM, PCM or Digital Bit Error Rate (BER) (Digital Systems Only)

10-9 ,10-12 Typical

Signal to Noise Ratio Specified in decibels (dB) Number of Connectors Loss increases with the number of

connectors Number of Splices Loss is Loss increases with the

number of splices Environmental Requirements

Humidity, Temperature, Exposure to sunlight

Mechanical Requirements Flammability, Indoor/Outdoor Application

Factors for Evaluating Fiber Optic System Design

Source

LEDs• Output Power• Modulation

Bandwidth• Center Wavelength• Spectral Width• Source Size

Laser Diodes• Output Power• Modulation

Bandwidth• Center Wavelength,

Number of Modes• Linewidth

Fiber

Multimode Fiber• Attenuation• Multimode

Dispersion• Chromatic

Dispertion• Numerical Aperture• Core Diameter

Single-Mode Fiber• Attenuation• Chromatic

Dispersion• Cutoff Wavelength• Spot Size

Receiver/Photodiode

• Risetime/Bandwidth

• Response Wavelength Range

• Saturation Level

• Minimum Detection Level

Sample Link

OATX RXOA

Medium and Devices

Link Budget Considerations

Three budgets:

(1) Power Budget

(2) Bandwidth or Rise Time Budget

(3) Financial Budgets

Power Budget Requirements:

PTX - PRX = 2l C+ L + SYSTEM MARGIN l C = connector loss

= fiber attenuation PRX > PMIN

PRX = Received PowerPMIN = Minimum Power at a certain BER

PRX = PTX – Total Losses + Total Gain - PMARGIN

PTX = Transmitted Power

PMARGIN ≈ 6 dB

Requirements Cont’d:

• Loss,L = LIL + Lfiber + Lconn. + Lnon-linear + LD

LIL = Insertion Loss

Lfiber = Fiber Loss

Lconn. = Connector Loss

Lnon-linear = Non-linear Loss

LD = Dispersion-equalization penalty

LD = 128 (τ * BR)4

τ = Total delay or dispersion

BR= Transmission bit-rate

Requirements Cont’d:

• Gain,G = Gainamp + Gnon-linear

Gainamp = Amplifier Gain

Gnon-linear = Non-linear Gain

dB, dBm, mW

dB = 10 log (P1/P2)dBm Value % of 1 mW Power Application

0.0 100% 1.0 mW Typical laser Peak Output

-13.0 5% 50.0W Typical PIN Receiver Sensitivity

-30.0 0.1% 1.0W Typical APD Receiver Sensitivity

-40.0 0.01% 100.0W Typical LED Peak Output

dB Power Out as a % of Power In

% of Power Lost

Remarks

1 79% 21% - 2 63% 37% - 3 50% 50% ½ the power 4 40% 60% - 5 32% 68% 6 25% 75% ¼ the power

7 20% 80% 1/5 the power

8 16% 84% 1/6 the power

9 12% 88% 1/8 the power

10 10% 90% 1/10 the power

Decibel to Power Conversion

dB Power Out as a % of Power

In

% of Power Lost

Remarks

11 8.0% 92% 1/12 the power

12 6.3% 93.7% 1/16 the power

13 5.0% 95% 1/20 the power

14 4.0% 96.0% 1/25 the power

15 3.2% 96.8% 1/30 the power

16 2.5% 97.5% 1/40 the power

17 2.0% 98.0% 1/50 the power

18 1.6% 98.4% 1/60 the power

19 1.3% 98.7% 1/80 the power

20 1.0% 99.0% 1/100 the power

Decibel to Power Conversion

dB Power Out as a % of Power In

% of Power Lost

Remarks

25 0.3% 99.7% 1/300 the power

30 0.1% 99.9% 1/1000 the power

40 0.01% 99.99% 1/10,000 the power

50 0.001% 99.999% 1/100,000 the power

Decibel to Power Conversion

IS THIS SYSTEM GOOD?

Example: Power Budget Measurement

PTx = 0 dBm

185 km

PSEN = -28 dBm

Splice

Attenuation Coefficient, = 0.25 dB/km

Dispersion Coefficient, D = 18 ps/nm-km

Number of Splice = 46

Splice Loss = 0.1 dB

PMargin = 6 dBLD = ?

Connector Loss = 0.2 dB

Connector

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

PMargin = 6 dB

Total Losses = 46.3 + 4.6 + 0.4 = 51.3 dB

Power Budget, PRX < PSEN !!

PRX = -57.3 dB

PRX = PTX – Total Losses – PMargin

= 0 – 51.3 – 6

Connector Loss = 0.2 dB X 2 = 0.4 dB

How To Solve?Answer…Place an

amplifierBut… What is the gain value??

Where is the location?And…

First we calculate the amplifier’s gain..

Gain PSEN - PRX

Gain -28 – (-57.3)Gain 29.3 dB

To make it easy,Gain 30 dB

Now…Where to put the amplifier?

Three choices availablefor the location

Power Amplifier – At the transmitter

Preamplifier – At the receiver

In Line – Any point along fiber

Let us check one by one…

Power Amplifier: PTX + Gain = POUT 0 + 30 = 30

dBmBut is there any power amplifier with 30 dBm POUT? NO, IT ISN’T

Hence …

What about Preamplifier?

POUT received = -57 dBm

Remember…

Preamplifier with 30 dB available?Yes

But, can it take –57 dBm?

Typically, NO

Hence …

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…

Where is the –30 dBm point?

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

Loss At That Point = -30 dBm

30 = x Length of That PointRemember = 0.25,Point Length = 30/0.25

= 120 kmBut 120 km from Tx,

No. of splice = 120/4

= 30

Assume Other Loss = 0, Loss At That Point = Fiber Loss,

Splice Loss = 0.1 dB x 30 = 3 dB

Also remember connector loss at amplifier and Tx…

2 connectors

Connector Loss = 0.2 dB x 3 = 0.6 dB

Total Losses = Fiber Loss + Splice Loss + Connector Loss

Actually, at 120 km,

= 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

+ 1 connector at Tx

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!!

PTx = 0 dBm

185 km

PSEN = -28 dBmSplice Connector

105.6 KM

Bandwidth/Rise Time Budget• Calculate the total rise times

Tx, Fiber, Rx

• Total Rise time, Tsys: Tsys=1.1(TTX

2+TRX2+Tfiber

2)1/2

• Tx Rise Time, TTX = normally given by manufacturer

• Rx Rise Time, TRX = normally given by manufacturer

Bandwidth/Rise Time Budget

• Calculate fiber rise time,

Tfiber2 = TIM

2 + (TCD + TPMD-2)2 + TPMD-1

2

T CD = T mat. + Twg. = D * Δλ * L

DG.652 = 18 ps/nm-km

DG.652 = Dispersion Coefficient = LinewidthL = Fiber Length

Bandwidth/Rise Time Budget

PMD coefficient

Coefficient TPMD-1 , χ = χ ps/(km)1/2

Coefficient TPMD-2 = 1.1 * χ ps/nm-km λ in μm

λ 2

Bandwidth Budget

OATX RXOA

Medium and Devices

T’

Δτ = T’ - T

T

What is a good Rise time?

• For a good reception of signalTsys < 0.7 x Pulse Width (PW)

• PW = 1/BitRate for NRZ1/2BitRate for RZ

From formula derived before TIM = 0 TCD = 90 ps TPMD-1 = 2.0 ps TPMD-2 = 9 ps

Fiber rise time, Tfiber2 = TIM

2 + (TCD + TPMD-2)2 + TPMD-1

2

Tfiber2 = (0)2 + (90 + 9)2 + (2)2

Tfiber = 99.02 ps = 0.09902 ns

Simple Calculation….for fiber length = 100km

Example: Rise Time Budget Measurement

Tx rise time, TTX = 0.1 nsRx rise time, TRX= 0.1 nsLinewidth() = 0.05 nm

Dispersion Coefficient, D = 18 ps/nm-km

Assume, Coefficient TPMD-1 , χ = 0.2 ps/(km)1/2

Coefficient TPMD-2 = 0.22 λ = 1.55 μm

(1.55)2

= 0.09 ps/nm-km

Total Rise time, TSYS = 1.1 TLS2 + TPD

2 + TF

2 = 1.1 0.01 + 0.01 + 0.0001

Simple Calculation….

TSYS = 0.16 ns

Let say,Bit Rate = STM 4= 622 MbpsFormat = NRZ

Tsys < 0.7 x Pulse Width (PW)

Pulse Width (PW) = 1/(622x106)

= 1.6 ns

0.16 ns < 0.7 x 1.6 ns

0.16 ns < 1.1 ns !!

Good Rise time Budget!!

Let say,Bit Rate = STM 16 = 2.5 GbpsFormat = NRZ

Tsys < 0.7 x Pulse Width (PW)

Pulse Width (PW) = 1/(2.5x109)

= 0.4 ns

0.16 ns < 0.7 x 0.4 ns

0.16 ns < 0.28 ns !!

Good Rise time Budget!!

Let say,Bit Rate = STM 64 = 10 GbpsFormat = NRZ

Tsys < 0.7 x Pulse Width (PW)

Pulse Width (PW) = 1/(10x109)

= 1.6 ns

0.16 ns < 0.7 x 0.1 ns

0.16 ns > 0.07 ns !!

Bad Rise time Budget!!

Budget Summary  Option Power

BudgetBandwidth 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

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 SummaryC 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

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 :

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

Sensitivity Analysis-contd..

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

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 = BandwidthQ(x) = Cumulative distribution function (Gaussian

distribution)

Eye Diagrams

Signal to Noise Ratio

SNR = S/NS - represents the information to be transmitted

N - integration of all noise factors over the full system bandwidth

SNR (dB) = 10 log10 (S/N)

Modulation Schemes

Process of passing information over the communication link :

• Encoding

• Transmitting

• Decoding

Types of Modulation Used for Encoding

Cost/Performance Considerations

Components considerations such as :– Light Emitter Type

– Emitter Wavelength

– Connector Type

– Fiber Type

– Detector Type

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.• 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.