Optical Transmission Systems

GOAL of the presentation• Overview of Optical Component Technologies• Basic understanding of certain key issues in Component TechnologiesOutline• Set the Context• Describe existing technologies• Explain fundamental limits• Describe practical considerations/tradeoffs

Light’s Dual NatureRaysWavesParticles

AbsorptionEmission

Interference RefractionReflection

Bandgap

Conduction band

Valence band

n0

n1

n0

Why Optical ?

• High Bandwidth ( 500 Tbps)• Low Attenuation (.25 dB/km)• Low BER (10 -13) • Light and occupies lesser space• Flexible and Reliable• Less crosstalk due to neutral photons• Hard to eavesdrop• Environmentally sound.

Bandwidth Comparison

All-Optical Network(Terabits Petabits)

TDM DWDM

0

5

10

15

20

25

30

35

40

Ban

dw

idth

8@OC-484@OC-192

4@OC-48

2@OC-48

2@1.2Gb/s(1310 nm, 1550 nm)

10 Gb/s

2.4 Gb/s1.2 Gb/s565 Mb/s

1.8 Gb/s810 Mb/s405 Mb/s

EnablersEDFA + Raman AmplifierDense WDM/FilterHigh Speed Opto-electronicsAdvanced Fiber

1982

1984

1988

1994

1996

1998

2000

2002

1990

1986

1992

16@OC-192

40 Gb/s

32@OC-192

176@OC-192

2004

2006

TDM (Gb/s)

EDFA

EDFA +Raman Amplifier

80@ 40Gb/s

Bandwidth Evolutionary LandmarksBandwidth Evolutionary Landmarks

Shannon’s Limit

Fiber Optic Transmission Bands

Near Infrared

Frequency

Wavelength1.6

229

1.0 0.8 µm0.6 0.41.8 1.4

UV

(vacuum) 1.2

THz193 461

0.2

353

Longhaul Telecom

Regional Telecom

Local Area Networks850 nm

1550 nm

1310 nmCD Players780 nm

HeNe Lasers633 nm

Optical Components

1. The Fiber A. Total Internal Reflection B. Multi Mode Fiber C. Single Mode Fiber D. Transmission Impairments Loss a. Rayleigh scattering b. Reflection c. Absorption Dispersion a. Chromatic Dispersion 1. Material Dispersion 2. Waveguide Dispersion b. Polarization Mode Dispersion Non-Linearities a. Stimulated Raman Scattering b. Stimulated Brillouin Scattering c. Four Wave Mixing d. Self Phase Modulation e. Cross Phase Modulation

Optical Components (Contd)

2. Couplers3. Isolators 4. Circulators5. Filters A. Diffraction Grating B. Reflection Grating C. Fabry Perot Filter D. Thin Film Dielectric Filter. E. Bragg Grating a. Short Period fiber bragg grating b. Long Period fiber bragg grating F. Mac Zhender Interferometer G. Arrayed Waveguide Grating H. Acoustoptic Tunable Filter

Optical Components (Contd)

6. Optical Amplifers A. Erbium Doped Fiber Amplifiers (EDFA) B. Raman Amplifiers (RA) C. Semiconductor Optical Amplifier (SOA)7. Lasers A. MLM Fabry Perot B. Single Mode Lasers a. Distributed Bragg Reflector Laser (DBR) b. Distributed Feedback Laser (DFB) c. External Cavity Lasers8. Modulators9. Detectors A. PIN B. APD

Optical Components (Contd)

10. Optical Switches Photonic Switches A. Mechanical switches B. Electrooptic switches C. Thermooptic switches D. Semiconductor optical Amplifiers

Lambda Switches A. Optoelectronic Method B. Semiconductor Optical Amplifier a. Cross Gain Modulation b. Cross Phase Modulation C. Four Wave Mixing

An Optical Network

Dispersion Managed FiberMedium Dispersion Fiber

Tx

Tx

TxWDM

OA

Tunable DCM

OA

DCFTunable DCM

DWDM

DynamicPMD Compensation

New Modulation Formats Forward Error Correction

ImpairmentTolerantReceiver

C+L Band OAS Band OA

Raman AmplifierDynamic OADM

. . . .

.

3000Km - 5000Km

Rx

Rx

Rx

PMDC

PMDC

PMDCBroadband

PMD Compensation

1. The Fiber

Glass n = 1.5

Air n = 1.0

Light at thisangle is refracted

Light at this angle isreflected back into glass1 2

1 = 2 For Total Internal Reflection

The Acceptance Cone

Multimode fiber

Pulse broadening due to multi-path transmission.

Bitrate x Distance product is severely limited!

Doping profile designed to minimize “race” conditions(“outer” modes travel faster due to lower refractive index!)

Most common designs: 62.5/125 or 50/125 m, NA ~ 0.2Bitrate x Distance product: ~ 1 Gb/s • km

r

Gradient-Index (GI) Fiber

Step-Index type with very small core

Single-Mode Fiber (SMF)

n

r

1.465

1.460

Transmission Impairments

Rayleigh Scattering

Reflection

Absorption

Transmission Impairments

900 1100 1300 1500 1700

0.5

1.0

1.5

2.0

2.5

OH Absorption

Att

en

uati

on

(d

B/k

m)

Wavelength (nm)

“Optical Windows” 2 3

1

Main cause of attenuation: Rayleigh scattering in the fiber core

45

AllWaveTM eliminatesthe 1385nm water peak

Loss - 1st Order

Chromatic Dispersion

Cause of Chromatic Dispersion

Material Dispersion

• β = nω/c

• n2 (ω ) = 1 + χ(ω)

• P(r, ω) = έ χ(r,ω)E (r,ω)

Waveguide DispersionPower distribution between core and cladding

Dispersion Limits

Dispersion Compensation

• Dispersion Compensation Fiber

• Fiber Bragg Gratings

Polarization Mode Dispersion

PMD – The Networking Killer

• 20 % of installed fiber useless => 10 G• 75 % of installed fiber useless => 40 G• Leads to increased PDL• Limits transmission to 25 km!!

Two PMD Solutions

1. Optical mitigation

2. Electrical mitigation

PMD Compensation

Non-Linearities - SBS and SRS

Cause of Non-Linearity

P = έ [ χ1. E + χ 2. E.E + χ3.E.E.E +…]

For high values of E, the third order term

becomes significant.

Scattering Compensation

Ways to reduce SBS penalty

• Power below threshold

• Increase line width of the source

• Use Phase modulation schemes

Ways to reduce SRS penalty

• Keep the channels densely packed

• Keep power below threshold.

Four Wave Mixing

FWM Compensation

• Unequal channel spacing

• Increased channel spacing

• Reduced power below threshold

• Use spatial walk off – introduce time delay

Non-Linearties (Contd)

• Self Phase Modulation

Refractive index dependance on the power

of a signal

• Cross Phase Modulation

Refractive index dependance on the power

of another signal

Effects of Nonlinearites

Multiple Channels vs Single Channel

Tools to combat Impairments

• Power / Channel

• Dispersion Compensation

• Channel Spacing

• Wavelength / Frequency Choice

2. Couplers

Couplers (Contd.)

Applications:

1. 3 dB couplers – for Broadcast and Select networks2. Taps for monitoring purposes3. Optical Switches 4. Mac Zhender Interferometers5. Lambda selective multiplexers and demultiplexers6. Combine pump and signals in EDFAs

3. Isolators

4. Circulators

5. Filters

Fabry Perot Filter

Diffraction Grating

Bragg Grating

Mac Zhender Grating

Thin Film Dielectric Grating

Arrayed Waveguide Grating

Acuostooptic Tunable Filter

6. Optical Amplifiers

EDFA

EDFA (Contd.)

EDFA (Contd.)

SNR

All Optical System Capacity

Raman Amplification

Raman Amplifiers

Used as Pre-amplifier in the reverse direction

• Same pump laser as EDFA

• Higher gains

• Larger Bandwidth

• Distributed and hence lower noise figure’

• Lower Plaunched – reduced nonlinearities

Benefits of Raman

Semiconductor Optical Amplifier

7. Lasers

MLM Fabry Perot Laser

SLM Lasers

External Cavity Lasers

External Cavity Lasers (Contd. )

8. External Modulators

9. Photodetectors

A Photodiode

10. Optical Switches

• Photonic Switches (MEMS)

Electrooptic Switch

Lamda Switch

An Optical Network - Revisited

Dispersion Managed FiberMedium Dispersion Fiber

Tx

Tx

TxWDM

OA

Tunable DCM

OA

DCFTunable DCM

DWDM

DynamicPMD Compensation

New Modulation Formats Forward Error Correction

ImpairmentTolerantReceiver

C+L Band OAS Band OA

Raman AmplifierDynamic OADM

. . . .

.

3000Km - 5000Km

Rx

Rx

Rx

PMDC

PMDC

PMDCBroadband

PMD Compensation

Conclusion