Fiber Optic Communications Lecture 9: Wavelength Division ...... · • WDM Principles • WDM...
Transcript of Fiber Optic Communications Lecture 9: Wavelength Division ...... · • WDM Principles • WDM...
Fiber Optic CommunicationsLecture 9: Wavelength Division Multiplexing
• WDM Principles
• WDM Hardware
• Hybrid packet/optical networks
Recap: Multiplexing schemes
• FDM – each channel is assigned to a different
frequency
• TDM – each channel is transmitted in a different time
interval
• CDM – each channel is encoded differently
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Wavelength Division Multiplexing
Also known as
frequency division
multiplexing (FDM).
Demultiplexing is
performed by
spectral filtering.
Total bandwidth is
sum of each
wavelength
contribution
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original extended short
conventional
long ultralong
Bandwidth
• Total bandwidth is sum of each wavelength’s
contribution:
�� =������
• Available channels depends on spacing Δ�• Spectral efficiency � = �/Δ�• Should be maximized for best performance
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Channel Spacing
• Spacing Δ� must be large enough to minimize
crosstalk
• Traditionally, this is taken to mean Δ� ≥ 2�• This implied Δ� = 100 GHz (WDM, now deprecated)
• Can now find Δ� = 50 GHz (original DWDM),
Δ� = 25 GHz (DWDM) or even 12.5 GHz (UDWDM)
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Amplification Bandwidth
• Total bandwidth for DWDM extends over EDFA
amplification range: 1525-1565 nm and/or 1570-
1610 nm (~5 THz)
• Ideally, could extend over the full low-loss range of
fibers, 1260-1610 nm (~50 THz)
• Approx. number of channels available listed below
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Wavelength range WDM (50 GHz) DWDM (25 GHz) UDWDM (12.5 GHz)
C-band (1525-1565 nm) 80 160 320
L-band (1570-1610 nm) 80 160 320
O-band (1260-1360 nm) 200 400 800
All (1260-1610 nm) 800 1600 3200
Fiber Optic CommunicationsLecture 9: Wavelength Division Multiplexing
• WDM Principles
• WDM Hardware
• Hybrid packet/optical networks
DWDM Hardware
Key components include:
• Tunable optical filters
• Multiplexers & demultiplexers
• Add-drop multiplexers
• Star couplers
• Wavelength routers
• Optical cross-connects
• Wavelength converters
• Transmitters
• Receivers
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Tunable optical filters
• Desired features: wide tuning, low crosstalk,
fast tuning, low-loss, environmental stability,
low cost
• Four common types:
– Fabry-Perot: � < ��– Mach-Zender: � � = ∏ cos� �����– Bragg grating: �� = 2� sin !– Acousto-optic filters: �� = Δ" · Λ%
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Multiplexers & demultiplexers
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Can use filter elements discussed above to combine and separate
multiple wavelengths – e.g., a hierarchical Mach-Zender filter
Allows one to create multiple point-to-point links through a single
channel, as illustrated below
�&
��
�'
�(
�&,��
�',�(�&,��, �',�(
Add-drop multiplexers
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• Most easily implemented as series of MZ interferometers with
equal delays (a resonant coupler)
• Alternates include gratings and grating/circulator strategies
Star couplers
• Aim to divide inputs among � outputs equally, regardless of wavelength
• Can be made from 2x2 couplers or fused biconicaltapers
• Used for hierarchical transmission of videos and similar
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)
�
Star
coupler
() + �)/�
() + �)/�
() + �)/�
() + �)/�
() + �)/�
Wavelength routers
• Combines star coupler with multiplexing
• Complex design; usually made from Si or
InGaAsP/InP
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Idris et al., Opt. Express 24, 672 (2016).
Optical cross-connects
• Allows for dynamic routing of wavelengths across a network
• Often implemented using MEMS mirrors or MZ interferometers
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Wavelength converters
• Converts between wavelengths while
preserving data
• Needed to extend cross-connect capabilities
• Four implementations:
– Optoelectronic regeneration
– Semiconductor laser amplifier (SLA)
– Phase modulation in SLA
– Four-wave mixing in SLA
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WDM Transmitters
• Few channels: DFB lasers with calibrated
frequencies
• Many channels: tunable or multimode lasers
(potentially using arrayed waveguide gratings),
or supercontinuum sources
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WDM Receivers
• Photodiode array with filters to provide
broadband detection + differentiation by
wavelength (a planar concave-grating
demultiplexer or a WGR)
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Fiber Optic CommunicationsLecture 9: Wavelength Division Multiplexing
• WDM Principles
• WDM Hardware
• Hybrid packet/optical networks
Optical Transport Network (OTN)
• Physical implementation of a series of DWDM-
connected nodes
• Topologies can be any of a number discussed
earlier: e.g., bus; ring; star; or hierarchy
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Synchronous optical networking (SONET)
• Standardized protocol for transporting multiple bit streams over fiber optics
• Significant advance over predecessor, Plesiochronous Digital Hierarchy (PDH)
• Uses atomic clocks for synchronization
• Agnostic regarding specific communications protocol
• As a result, SONET and its variants (SDH) now used worldwide
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Synchronous optical networking (SONET) topology
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Synchronous transport signal 1 (STS-1) frame
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OTNs vs. SONET
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OTN SONET/SDH
Timing Synchronous with
clients
Internally
synchronous
Multiplexing Fixed multiplexing Flexible multiplexing
Frame properties Frame size fixed Frame rate fixed
Bandwidth
efficiency
Can be bandwidth
inefficient
Very bandwidth
efficient
Error correction Forward error
correction
None
OTN+SONET Example: Internet2
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General Relation to TCP/IP
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Hybrid Packet/Optical Network Architecture
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Courtesy Dr. John S. Graham (Indiana University)