Optical Transmission Systems
GOAL of the presentation
• Overview of Optical Component Technologies
• Basic understanding of certain key issues in
Component Technologies
Outline
• Set the Context
• Describe existing technologies
• Explain fundamental limits
• Describe practical considerations/tradeoffs
Light’s Dual Nature
Particles
Waves
Conduction band
Rays
n0
Bandgap
n1
n0
Valence band
Absorption
Emission
Interference
Refraction
Reflection
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
Bandwidth Evolutionary Landmarks
All-Optical Network
(Terabits  Petabits)
TDM (Gb/s)
80l @ 40Gb/s
40
176l @OC-192
35
25
20
15
32l @OC-192
EDFA +
Raman Amplifier
16l @OC-192
8l @OC-48
4l @OC-192
EDFA
10 Gb/s
10
2l @1.2Gb/s
(1310 nm, 1550 nm)
5
4l @OC-48
2l @OC-48
2.4 Gb/s
565 Mb/s 1.2 Gb/s
0
TDM
DWDM
2006
2004
2002
2000
1998
1996
1994
1992
1990
1988
810 Mb/s 1.8 Gb/s
1986
1984
405 Mb/s
1982
Bandwidth
30
Enablers
EDFA + Raman Amplifier
Dense WDM/Filter
High Speed Opto-electronics
Advanced Fiber
40 Gb/s
Shannon’s Limit
Fiber Optic Transmission Bands
193
Frequency
Wavelength
(vacuum)
Longhaul Telecom
Regional Telecom
Local Area Networks
229
353
461
THz
Near Infrared
1.8
1.6
1.4
1.2
UV
1.0
0.8
0.6
0.4
HeNe Lasers
633 nm
1550 nm
CD Players
780 nm
1310 nm
850 nm
0.2
µm
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. Couplers
3. Isolators
4. Circulators
5. 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 Lasers
8. Modulators
9. 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
3000Km - 5000Km
Tunable DCM
DCF
Tunable DCM
Dynamic
PMD Compensation
Tx
Tx
Tx
W
D
M
OA
Dispersion Managed Fiber
Medium Dispersion Fiber
New Modulation Formats
Forward Error Correction
OA
C+L Band OA
S Band OA
Raman Amplifier
Dynamic OADM
....
.
D
W
D
M
Broadband
PMD Compensation
PMDC
Rx
PMDC
Rx
PMDC
Rx
Impairment
Tolerant
Receiver
1. The Fiber
Light at this
angle is refracted
Glass n = 1.5
Light at this angle is
reflected back into glass
Air n = 1.0
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!
Gradient-Index (GI) Fiber
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 ~ 0.2
Bitrate x Distance product: ~ 1 Gb/s • km
r
Single-Mode Fiber (SMF)
Step-Index type with very small core
n
1.465
1.460
r
Transmission Impairments
Rayleigh Scattering
Reflection
Absorption
Transmission Impairments
Attenuation (dB/km)
2.5
“Optical
Windows”
2.0
3 4
5
2
OH
Absorption
1
1.5
AllWaveTM
eliminates
the 1385nm
water peak
1.0
0.5
900
1100
1300
1500
1700
Wavelength (nm)
Main cause of attenuation: Rayleigh scattering in the fiber core
Loss -
st
1
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 Dispersion
Power 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.
2.
3.
4.
5.
6.
3 dB couplers – for Broadcast and Select networks
Taps for monitoring purposes
Optical Switches
Mac Zhender Interferometers
Lambda selective multiplexers and demultiplexers
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
3000Km - 5000Km
Tunable DCM
DCF
Tunable DCM
Dynamic
PMD Compensation
Tx
Tx
Tx
W
D
M
OA
Dispersion Managed Fiber
Medium Dispersion Fiber
New Modulation Formats
Forward Error Correction
OA
C+L Band OA
S Band OA
Raman Amplifier
Dynamic OADM
....
.
D
W
D
M
Broadband
PMD Compensation
PMDC
Rx
PMDC
Rx
PMDC
Rx
Impairment
Tolerant
Receiver
Conclusion