Fiber-Optic Multiplexer Demultiplexer Systems 1gallep/TT702/A0 B.pdf · Multiplexer Demultiplexer 1 N channels 1 N MUX DEMUX Tx Rx ... F r - SBMO-95.ppt 36 Frag FFF –– UNICAMPUNICAMP

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Lecture 1 of 2 – For graduate level students of physics or electrical engineering

www.kyatera.fapesp.br

Fundamentals of Fiber-Optic Communication Systems

Fundamentals of Fiber-Optic Communication Systems

Prof. Hugo L. FragnitoOptics and Photonics Research Center at Unicamp

Tel. (xx55-19) [email protected]

UNICAMP-IFGWInstituto de Física Gleb Wataghin, Campinas, 13083-970, SP, Brazil

555 H. Fragnito H. Fragnito H. Fragnito CePOFCePOFCePOF ––– UNICAMPUNICAMPUNICAMP

Elements of a Communication SystemElements of a Communication System

ReceiverTransmitter

InputTx Transmission Line Rx

Output

Optical fiberLaser +

modulatorPhotodiode +Filter + clock recovery + ...


MultiplexingMultiplexing

Multiplexer DemultiplexerChannel 1

Channel N

Voice, video, or data channels

Channel 1

Channel N

MUX DEMUX

Tx Rx

Multiplexing can be in time domain (TDM), frequency domain (FDM), Polarization (PDM), Wavelength (WDM), ...

TDM = Time Division Multiplexing, FDM = Frequency Division Multiplexing,


TDM StandardsTDM Standards

SONET - Synchronous Optical Network SDH - Synchronous Digital Hierarchy

1 Digital voice channel (or voice circuit) = 64 kb/s (4kHz analog signal sampled at 8 kHz and 8 bits quantization)

SONET SDH B (Mb/s) ChannelsOC-1 51.84 672OC-3 STM-1 155.52 2,016OC-12 STM-4 622.08 8,064OC-48 STM-16 2,488.32 32,256OC-192 STM-64 9,953.28 129,024OC-768 STM-256 39,814.32 529,024

OC = Optical Carrier STM = Synchronous Transport Module


Digital Modulation FormatsDigital Modulation Formats

Binary Word 1 0 1 0 1 1 01 0 0 1

time(bit slot)T = 1/B

Non-Return to Zero

Return to Zero

NRZ

RZ

B/2

B

Better for clock recovery


Digital Modulation KeyingDigital Modulation Keying

E(t) = A(t) cos[ω(t)t + φ(t)]Optical field:

ASK - Amplitude Shifting KeyingA(t) = 0 or 1

PSK - Phase Shifting Keyingφ(t) = −π/2 or +π/2

FSK - Frequency Shifting Keyingω(t) = ω0 + δω/2 or ω0 −δω/2

1 0 1 0 1 0

Optical modulated signals:

Electrical binary data

Other names for ASK: OOK (On-Off Keying), ISK (Intensity Shifting Keying)


Traditional Optical Communication SystemTraditional Optical Communication System

Loss compensation: Repeaters at every 20-50 km

Tx

Timing & Shapingcircuits

Transmitter Repeater

Rx

Receiver

Laser

Optoelectronic repeater

Photodiode

Optical fiber (20-50 km)

System capacity limited by speed of electronic circuits

3R

3R = Retiming, Reshaping &Regeneration


Optically Amplified SystemsOptically Amplified Systems

EDFA = Erbium Doped Fiber Amplifier

Photodiode

EDFA

Laser

EDFAEDFA EDFA

Signal (1.55 µm)

WDM

Erbium Doped Fiber (10-50 m)Amplified

Signal

Pump Lasers (1.48 µm or 980 nm)

isolatorWDM

Booster Pre-AmpFiber (20-100 km)

Bits continue in photonic format


Characteristics of EDFA based SystemsCharacteristics of EDFA based Systems

Bandwidth > 4 THzdepends on glass host(100 nm in Telurite glass)

Low noiseNF ~ 4 – 7 dB

Optical power > 10 mWlarger distance between EDFAs

Transparent to bit rate, modulation format, and bit coding

Ideal for WDM (Wavelength Division Multiplexing)

15

20

30

Gai

n (d

B)

Wavelength (nm)1500 1520 1540 1560 1580

35 nm


DWDM SystemsDWDM SystemsDWDM = Dense Wavelength Division MultiplexingDWDM = Dense Wavelength Division Multiplexing

Transmitters (Bit rate = B)Optical Amplifiers

λ1

λ2

λN

Receivers

λ1

λ2

λN

MUX

DeM

UX

Pow

erde

nsity

Frequency, ν (THz)192.8 193.0 193.1192.9

∆ν ∆ν = 100 GHz, 50 GHz, (up to 40 Gb/s) DWDM

25 GHz, or 12.5 GHz(up to 10 Gb/s) UDWDM (Ultra Dense)

ITU gridνn = ν0 + n∆ν

ν0 = 193.1 THz(1552.52 nm)

(n = 0, ±1, ±2, ...)


Bandwidth of WDM systems with Bandwidth of WDM systems with EDFAsEDFAs

• 4 THz Optical Bandwidth

• 80 WDM channels x 40 Gb/s(50 GHz channel spacing)

• Total capacity: 3.2 Tb/s

• Important issues:– Bandwidth– Gain flatness– Output power

15

20

30

Gai

n (d

B)

Wavelength (nm)1500 1520 1540 1560 1580

35 nm

EDFA = Erbium Doped Fiber AmplifierEDFA = Erbium Doped Fiber AmplifierC band (1530C band (1530--1560 nm)1560 nm)

Capacity of DWDM systems determined by bandwidth of optical amplifiers


Chirped Fiber Bragg GratingsChirped Fiber Bragg GratingsL12

λ1 λ2

• Dispersion compensationOptical Delay τ12 = nL12/c

• Gain flattening filter (GFF)(varying index modulation)

M. Guy and F. Trépanier, WDM, March 2001


Systems Concepts: REVIEWSystems Concepts: REVIEW

• Optical Communication system use several Multiplexing techniques; all include TDM (Time Division Multiplexing)– TDM standards (SONET, SDH): ex. OC-192 = STM-64 = 10 Gb/s– Modulation is ISK – Intensity Shifted Keying and NRZ format

• DWDM systems use Gb/s lasers at different lambdas– Aggregated bit rate = NxB– Channel spacing 50-100 GHz; ITU grid– Total optical bandwidth limited by bandwidth of optical amplifiers– Total power NxP (nonlinear optical effects are important)

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Some system componentsSome system components

LasersDetectors

ModulatorsIntegrated optics: Mux

Amplifiers


Laser diodeLaser diode

p-type

Low gap semiconductor

n-type

current

0.3 mm 0.2 m

m

0.1 mm

cooler

Fabry-Perot (FP) laser cavity


Spontaneous emission spectrumSpontaneous emission spectrum

1

0

0.5

0.90 0.95 1.00 1.05Energy (eV)

Wavelength (nm)1350 1300 1250 1200

InGaAsPT = 280 K

• Emission spectrum of LEDs(Light Emission Diode)

• ~ Noise spectrum or ASE of Lasers

(ASE = Amplified Spontaneous Emission) Po

wer

(a.u

.)


Cavity modesCavity modesL

Emission spectrumMode frequencies

Optical frequency, ν∆ν = c/2nL

Fabry-Perot lasers ~ 30 modes (2-10 nm linewidth) Mode jumping – Mode partition noise

DFB and DBR lasers: ~ 50 MHz linewidth

DFB: Distributed Feedback laser

DBR: Distributed Bragg Reflector

G.P. Agrawal, Fiber-Optic Communication Systems, Wiley, New York (1997)


Modulation bandwidth and chirpModulation bandwidth and chirp

Direct ModulationBW limited by• carrier diffusion• RC of wiring, contacts, packaging• Relaxation oscillationsModulation BW can be expanded by increasing injection current

Instantaneous frequency varies (chirp)• Dependence of refractive index on injection current (changes effective length of optical cavity)• Positive chirpChirp accelerates pulse distortion by fiber dispersion


23

PhotodiodesPhotodiodes


PhotodiodesPhotodiodes

p-type n-typep-n junction

p-i-n

APD avalanche photodiode

+ (reverse bias)-

i (intrinsic)p n

p+ i p n+

absorption gain(10X)


Materials for Photodiodes and lasersMaterials for Photodiodes and lasers

First communications window (830 nm)

Second window (1300 nm)

Third window (1550 nm)



Photodiode CharacteristicsPhotodiode Characteristics

parameter symbol unit Si Ge InGaAs

wavelength λ nm 400-1100 800-1800 1000-1700

Responsivity R A/W 0.5 0.6 0.8

Quantum eff. η % 85 50 65

Dark current Id nA 1-10 50-500 1-20

Rise time tr ps 20 100 20

Bandwidth ∆f GHz 50 30 40

Bias (p-I-n) Vb V 50 10 6

Bias (APD) Vb V 250 40 30

Gain (APD) M - 500 200 10


WDM MultiplexersWDM Multiplexers

Free Space Grating MUXLens Grating

AWG: Arrayed Waveguide Grating

Fibers

λ1 + λ2 + λ3

λ3

λ2

λ1


AWGAWGAWG: Arrayed Waveguide Grating

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ModulatorsModulators


ElectroElectro--Optic SwitchOptic Switch

Control Voltage

Pin

l

Pout = Pin (1+cos∆φ)/2V

d

r = EO - coefficient

∆φ = π πV V/

∆n rn V d= − 12

3( / )Refractive index change:

Nonlinear crystal

PockelsPockels effect: effect: •• Refractive index change proportional to applied electrical fielRefractive index change proportional to applied electrical fieldd•• Induced birefringenceInduced birefringence•• Present only in piezoelectric crystalsPresent only in piezoelectric crystals

Phase change:

V drnπλ

= 3lSwitching voltage:Switching voltage:


Waveguide ElectroWaveguide Electro--Optic SwitchOptic Switch

Since Since VVππ = = λλdd//rnrn33ll, , then decreasingthen decreasing dd and increasingand increasing ll reduces the reduces the switching voltageswitching voltage

V

0

LiNbO3 substrate

Waveguide (Ti in-diffused)

• Mach-Zehnder• Switching voltage: 5 - 8 V• Up to 40 Gb/s• Integrated optics• Fiber pigtail


Electroabsorption modulatorElectroabsorption modulator

Franz-Keldysh effect: energy gap of semiconductors depends on applied electric field

Effect is enhanced in Multiple Quantum Wells

50 GHz, 2V

Integrated with laser in the same chip

absorbing MQW layers

p-con

tact

p-type

n-type

n-typ

e sub

strate


Direct versus External ModulationDirect versus External Modulation

Direct modulationI(t)

laser

External modulationV(t)

laser

modulator

Broad spectral source (chirp) Transform limited pulses (~no chirp)

Bandwidth limited pulsesσλ = 0.02 nm (2.5 Gb/s modulation)

Dσλ = 0.3 ps/km

σ σ λt LD=

Fabry-Perot Laserσλ = 4 nm: Dσλ = 68 ps/km

DFB Laser (2.5 Gb/s modulation, 12 GHz chirp)σλ = 0.1 nm: Dσλ = 1.7 ps/km

Examples ( D = 17 ps/nm/km; λ = 1550 nm )

Pulse broadening due to fiber dispersion:

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Optical AmplifiersOptical Amplifiers


ErEr+3+3 energy levelsenergy levels• Pump:

♦ 980 nm or 1.48 µm ♦ pump power > 5 mW

• Emission:♦ 1.52 - 1.57 µm♦ long living upper state (10 ms)♦ gain ≈ 30 dB

Absorption (dB/m)

4F9/2

0.0

0.5

1.0

1.5

2.0

1542

988

810

659

τ = 10 ms

Ener

gy (e

V)

0 2 4 6 8

λ (nm)

4I9/2

4I11/2

4I13/2

4I15/2

τ = 10 µs

1.45 1.50 1.55 1.60

absorption

emission

λ (µm)

H. Fragnito - SBMO-95.ppt


Gain Saturation and Noise of EDFAGain Saturation and Noise of EDFA

• Signal to Noise Ratio (SNR) degradationDue to Amplified Spontaneous Emission (ASE)

• Noise Figure: NF = SNRin/SNRout > 3 dB

1500 1520 1540 1560 1580-50

-40

-30

-20

-10

0

PASE

Pout + PASE = GPin + PASE

Pow

er (d

Bm

)

Wavelength (nm)-20 -10 0 10 20

0

10

20

30

40

Pump Power 10 mW

30 mW

Gai

n (d

B)

Output Signal Power (dBm)

20 mW


SOASOA

Semiconductor Optical AmplifierLaser diode with antireflection coatings

Wide optical bandwidth: Operation in 1300 nm or 1500 nm windows

Wide modulation bandwidth, ~ 20 Gb/s

Compact, robust, ~inexpensive

But: • Too fast response (recombination time) ~ ns• Very sensitive to light polarization

(PDG: polarization dependent gain)• Somewhat noisy


PDG compensation in PDG compensation in SOAsSOAs

(PDG: Polarization Dependent Gain)



Components: REVIEWComponents: REVIEW

• Lasers and photodiodes– Linewidth depends on laser type:

• Fabry-Perot: 0.2–1 THz (2-8 nm)• DFB: 20–50 MHz (2–5 x 10-6 nm)

– Modulation bandwidth to 20 GHz but chirp may be a problem – Avalanche photodiodes provide gain– p-i-n photodiodes respond to 50 GHz

• integrated, waveguide devices (ex. AWG MUX)– Compact, highly stable devices

• High speed modulators:• Electro-optic (Lithium Niobate)• Electroabsorption (Semiconductor)

• Optical Amplifiers:• EDFA: 35 nm bandwidth, NF = 4-6 dB, τ = 10 ms• SOAs: Compact and inexpensive, but polarization dependent and τ ~ ns

40

System performance

System performance

Power BudgetDetection

BER and Q factorEye diagram


Power budgetPower budget

System design(dB units)

PRX = PTX – αL – Nsplicesηsplice - System margin

2 – 5 dB(aging)

Average splicing insertion loss

Is that simple? . If system is linear, yes!Almost never is! Then use:

i Ez

E E E i E∂∂

β ∂∂τ

α~ ~| ~| ~ ~ ...

′= − + − + + +2

22

2 22 Third order dispersion RamanΓ

Generalized Nonlinear Schrödinger Equation (NLSE)

What about noise?


Optical Field NoiseOptical Field Noise

Optical communications region (λ = 1.3 - 1.6 µm)

hν (vacuum field noise)

Microwaves region

Noi

se p

ower

den

sity

(W/H

z)

wavelength, λ (µm)1 10 102 103 104 10510-36

10-33

10-30

10-27

10-24

10-21

10-18

Thermal Noise(Bose-Einstein statistics)

T = 1 K

T = 300 K

T = 1000 K

21

heh kT

νν/ −

At optical telecom frequencies:

quantum fluctuations of vacuum field >> thermal noise (even at 1273 ºC)

In practice, thermal noise of electronic part of receiver dominates


PhotodetectionPhotodetection noisenoise

Total current DTS iiiPheI +++νη= )/(1

Shot noise

Dark currentResponsivity

P = (A/W)

optical power (W)

Thermal noiseShot noiseSignal photocurrent

Thermal noise σTL

NkTR

F f2 4= ∆

RL

Noise figure of electrical amplifier

G = 1

σ σT s>>

Low pass filter

∆f

In practice Beating of signal and noise optical fields

Thermal motion of electrons passing across a resistor

feIiSS ∆>==<σ 122 2


BERBER

Bit Error RateBER = p(1) P(0/1) + p(0) P(1/0)

Probability

Sign

al c

urre

nt Q parameterI1σ1

IDP(1/0)

P(0/1)

Q I ID=−1

1σ

σ0Q I I

=−+

1 0

1 0σ σI0

Q ~ Signal/Noise

time

ID = decision threshold optimumI I ID =

++

σ σσ σ1 1 0 0

1 0


BER for Gaussian random processesBER for Gaussian random processes

IF fluctuations around I1 and I0 follow Gaussian distributions

I I ID =

++

σ σσ σ1 1 0 0

1 0AND (optimum) decision threshold

THEN

π−

≅2

)2/exp(BER

2

QQ

2 3 4 5 6 7 810-15

10-10

10-5

Q

BE

R

BER < 10-9

for Q > 6

Q I I=

−+

1 0

1 0σ σQ parameter

Q2 ≈ SNR(Electrical signal-to-noise ratio)


Eye diagramEye diagram

10 Gb/s NRZ215-1 PRBS

Before transmission

After transmission(DSF 80 km)

25 ps/div


Using the eye diagramUsing the eye diagram

I1 – I0

σ0

σ1

Q = (I1 – I0)/(σ1 + σ0) ≈ 7.5/(1+0.5) = 5 ( ⇒ BER ≈ 10-7 )

Oscilloscopes for telecom compute histograms and Q factor automatically


System Performance REVIEWSystem Performance REVIEW

• Noise in optical field gives shot noise, but in photodetected, electrical signals, thermal noise from electronic circuit dominates

• Eye diagram is a powerful tool for system diagnostics– BER is determined by Q factor (Q2 ~ SNR)– BER < 10-9 for Q > 6

• What are the physical limits of fiber optic transmission systems?– Attenuation?– Dispersion?– Nonlinearities?


Where to get more informationWhere to get more information

• G.P. Agrawal, Fiber-Optic Communication Systems, 3rd ed., Wiley, New York, 2002.

• WDM Solutions, PenWeell, free subscription: www.optoelectronics.world.com

• R. Ramaswami and K. N. Sivarajan, Optical Networks, 2nd ed., Morgan Kaufmann, San Francisco, 2002.

• I.P. Kaminow and T. Li, Eds., Optical Fiber Telecommunications, Vols. III-A, III-B (1997), IV-A and IV-B (2002), Academic Press, San Diego.

Documents

Fiber-Optic Multiplexer Demultiplexer Systems 1gallep/TT702/A0 B.pdf · Multiplexer Demultiplexer 1 N channels 1 N MUX DEMUX Tx Rx ... F r - SBMO-95.ppt 36 Frag FFF –– UNICAMPUNICAMP