All Optical Time Division Multiplexing - An alternative to WDMsoe.northumbria.ac.uk/ocr/downloads/otdm/otdmgv1-02.pdf · 2002-12-14 · OCRG - Current Research §All Optical Time
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All Optical Time Division Multiplexing -An alternative to WDM
Professor Z. GHASSEMLOOY
Optical Communications Research GroupSchool of Engineering,
Sheffield Hallam UniversityUK
Tel: +44 114 225 3274Fax: +44 114 225 3433
email: z.f.ghassemlooy@shu.ac.ukWed add.: http//www.shu.ac.uk/ocr
Optical Communications Research Group
People
§ Academics 4ú Engineers, and Mathematicians
§ Research Students 7§ MSc Students 2
OCRG - Current Research
§ All Optical Time Division Multiplexingú Terahertz Optical Asymmetric Demultiplexersú Optical Bufferingú Optical Packet Networkingú Optical Routing
§ Optical Wireless Communication System- Modulation Schemes - CDMA- Intelligent Optical Receiver
§ Optical Sensors
Our Work
§ System approachú Theoretical Investigationú Simulation
• Analytical• Computer
ú Designú Implementation
All Optical Networks Layered Structure
Optical transmission layer
Optical multiplex layer
Optical channel layer
To overcome the bandwidth bottleneck due to opto-electronicor electro-optic conversion in existing network based on
optical transmission and electronic switching
Network Technologies
E/OMUX O/E DEMUX
Channels Channels
ElectricalBottlenecks
Fibre
E/O O/E
E/O
E/O
MUX
Channels
DEMUX O/E
O/E
Channels
Control signalOptical
Fibre
All Optical Multiplexing
Is the key in meeting the explosive bandwidth requirementof future communication networks!
§ Wavelength division multiplexing (WDM)
§ Optical time division multiplexing (OTDM) Π
§ Hybrid WDM-OTDM
E/O O/E
E/O
E/O
MUX
Channels
DEMUX O/E
O/E
Channels
OA
λ1 λ2… λM
WDM
§ Up to 90 wavelengths§ > 200 Gbps§ Transparent to data format and rate
Fibre
Problems with WDM
§ Nonlinearity associated with fibre, eg. Stimulated RamanScattering results in SNR degradation as the number ofchannel increases
§ Four wave mixing: limits the channel spacing§ Cross phase modulation: limits the number of channels§ High gain flat amplifiers§ Packet switched service by means of light paths: an
extremely inefficient way of utilizing network resources
Solution§ Optical Time Division Multiplexing (OTDM)
(introduced early 90’s)
OTDM – What does it offer?
§ Flexible bandwidth on demand at burst rates of 100 Gb/s perwavelength (in the longer term).
§ The total capacity of single-channel network = DWDM , but OTDM provide:ú potential improvements in network performance in terms of user
access time, delay and throughput, depending on the user rates andstatistics.
§ Less complex end node equipment (single-channel Vs. multi-channels)
§ Can operate at both:ú 1500 nm (like WDM) due to EDFAú 1300
§ Offers both broadcast and switched based networks
OTDM - History
§ 1968 T S Kinsel & R T Denton, IEEE Proc. 56,§ 1970 S J Buchsburn & R Kompfiner, ‘TDM Optical transmission systems’,
§ 1981 M Thewalt, - ‘OTDM using mode locked laser’, Elec. Lett.
§ 1985 S K Korothy et al - ‘ High speed LiNbO3 as switch/modulator forOTDM’
§ 1988 S Fujita - 10 Gbps systems
§ 1993 A D Ellis - 40 Gbps§ 1998 M Nakazawa - 640 Gbps (60 km)§ 1999 P Tolire, et al - 100 Gbps Packet based§ 2000 K Yanenasu – 80 Gbps (168 km zero dispersion fibre)
Progress in Raw Speed
1980 1985 1990 1995 2000 20050.1
1.0
100
1000
10
Bit
time
(ps )
Year
2.5 Gb/s
40 Gb/s
1.24Tb/sSub-picosecond region
Source: W H Knox, 2000
• 2000 - 40 Gb/s commercial product•1.28 Tb/s uses 200 fs pulses, using optical loop mirrors for demultiplexing (not possible electrically)
Ultrafast Technology Impact on WDM andOTDM
1 10 100 1000 100001.0
100
1000
10
Num
ber o
f wav
elen
gth
Data rate (Gb/s) /wavelength
1 Tb/s
100Tb/s
10 Tb/s
Ultrafast OTDM limits
Ultr
afas
t bro
adba
ndso
urce
• Source: DFB laser• 10000 WDN is achieved using spectral slicing
Ultrafast Pulse Source
10 100 10001
100
1000
10
50 G
Hz
Ban
dwid
th
WD
M c
hann
els
Pulse width (fs)
Commercial WDM 80 channels or more
A- Broadcast OTDM Networks
§ Bit interleaving:ú Each node will have a pair of OTDM Tx/Rx.ú Just like broadcast WDM networks, it requires multi-channel
media access protocol.- Star: Offers better link budget- Bus: Offers natural ordering of nodes, easier
synchronisation and protocol design.
§ Packet interleavingú No need for multi-channel media access controlú Requires single-channel media access protocol
1- Bit Interleaved - Multiplexer
Mode-lockedlaser
Splitter
Fibre delay iτ
Modulators
M1
MiCombiner
orstar coupler
OTDM
Data (NRZ)
Frame pulse
M(N-1)Fibre delay (N-1) τ
Fibre delay τ
Node i
Frame 1 Frame 2Time
Framing pulsesTip= Tb/N+1
Tb
• Data rate: 100 Mb/s: Ethernet at 10 Mb/s,Token Ring at 10 Mb/s,or FDDI at 100 Mb/s.
Bit Interleaved - De-multiplexer
Splitteror
star couplerOTDM
Fibre delay jτ& Channel j
A
B C
A
Threshold level
B
C
Channel j
2- Packet Based
§ Data rate 100 Gbps/channel§ Packet duration = 10 ns – 100 ps
S-OT
DM
Slot1 Slot 2Time
Ch- 1
Ch- 2
Ch- N
Burst (packet) Guard band
•Signal processing, switching and routing arecarried out during guard band time
Header Data
Packet OTDM - Multiplexer
Mode-locked
laserModulator 1st stage 2nd stage ith stage
Compression
OTDM
Mod. data
1 1 1 10 0
Compressedpacket
T-τ
2 (T-τ)
ab
c d e
a)
b)
c)
d)
e)
OTDM
Pulse i locationat the output is:(2k - 1)(T - t) +(i - 1)τ
Packet Compression
∑∑−
=
−
=
−δ==1
0
1
0
)()()(N
ii
N
iiin AiTttItI Ii(t) is the ith bit in the packet
Ai = 0 or 1
§ For a single bit the composed output is: ∑−
=+ τ−−=
1
01
)]([21
)(N
jini TjtItO
∑∑∑−
=
−
=+
−
=
τ++−==1
0
1
01
1
0
])([2
1 N
jji
N
in
N
iiout AjTjitIOI§ The output signal for
N bits packet is:
(T-τ) 2(T-τ) 4(T-τ) 8(T-τ)
Input Output
T τ
Iin(t) Iout
OTDM Demultiplexer
Ch- 1
Clock
DemuxOTDM
Clocksyn.
• Synchronisation of the control signal is essential to achieveaccurate demultiplexing. This requires the extraction of a clockcomponent from the received data using:
• electro-optical- PLL
• all optical at high speed > 40 Gbps- mode-locked fibre ring laser
Broadcast OTDM Networks - Problems
§ Large splitting loss§ No routing or switching (signals are sent to all nodes)
Solution:§ Switched based networksú Tune-ability: select any time slotú Routing and switchingú Much faster
B- Switched Based OTDM Networks
§ Broadcast topology & media access protocolú Ethernetsú Token ringsú FDDI
§ Store & forward topologyú Meshú ATMú IP
1 3
42
3
S/W & routing nodes
Source & sink nodes
Switched Based OTDM Networks Node –Routing & switching
§ Packet may specify:ú Route chosenú Destination node (the choice of route is left to the routing nodes)
Switch
Control Input
DeMUX Processing
DeMUX Processing
Header recognition
To select o/p ports (use look-up table)
Packets
I/P buffers
O/P buffers
Switched Based OTDM Networks - Header
§ Header recognition:ú Optically (more complex)ú Electronically
§ Techniques used:ú Transmit at a much slower rate than packet itself,ú Allocate different wavelengthú Transmit as a separate sub-carrier channel on the same
wavelength
Switched Based OTDM Networks - Buffering
§ Buffer size depends on:ú Loadú Packet lossú No. of input/outputs in a S/W
- Practical: 23 or more limited:- by the number of possible re-circulations within a loop- by the excessive hardware for larger buffer depth.
Solution:- Multi-stage switched large optical buffer (>4000 for 8
input/output and 4 stages [D K Hunter et al, 1998]
OTDM - Issues
§ High speed electronics:ú Current ICs supporting 40 Gb/s will be available by 2001ú 40 Gb/s receiver are already available
§ Source: short pulse (tens of fs) and spectrally pure-• Gain switched semiconductor laser (broad source)• DFB laser (most suitable)• Mode locked fibre ring laser (pulse width < a few ps)• Semiconductor mode locked ( higher pulse width and not as pure as
MLFL)
§ Multiplexingú Electro-optically
§ 3 R regeneration:ú using semiconductor optical amplifier ∗ or nonlinear devices
∗ Passively
OTDM - Issues – cont.
§ Chromatics dispersion at 1550 nm is high, thus limiting the linkspan to 50 km at 200 Gb/s
• use dispersion shifted fibre b: But– can’t use the existing fibre– can’t compensate for a number of different wavelengths
• use dispersion equalisation techniques– by concatenating different fibres of opposite dispersion signs– chirped fibre Bragg gratings
• soliton pulse
§ Polarisation mode dispersion - limits the bit rate§ Demultiplexing techniques and devices Π§ Buffering Π
Optical Demultiplexers – All optical
§ Two main technologies:ú Kerr effect in optical fibresú Fast nonlinearities observed in semiconductor amplifiers
Types:
§ Optical loop mirror Π§ Interferometers with SA Π§ Four wave mixing
Types of Optical Loop Mirrors DEMUX
§ Loop mirrors:ú Nonlinear optical loop mirror (NOLM)-
with/without external control pulseintensity-dependent phase used as the nonlinearity
ú Nonlinear amplifier loop mirror (NALM)EDFA provides the nonlinearity
§ Terahertz optical asymmetric demultiplexer (TOAD)SLA provides the nonlinearity
A- Optical Loop Mirrors - Principle
( ) ( )2cos1 2 φ∆−=tTx
If ∆φ = π, then Tx (t) = 1 (i.e.100% transmittance.
Transmittance at port 2:
CW (0)
CW (0)
CCW (π/2)
2
1 3
4
(π/2)
CW (0)+CCW(π)
Destructive interference
Constructive interference
CW (π /2)+CCW(π/2)
Fibre loop
3dB coupler
Intensity I I/2
I/22
20 Innn +=
1- NOLM Demultiplxer
Control pulse
Data in Data out
Coupler
CW CCW
Long fibre loop
Port 1 Port 2
Control coupler
PC
( ) ( )2cos1 2 φ∆−=tTx
•If ∆φ = π, then Tx (t) = 1(i.e.100% transmittance in port2.
•Control pulse will introduce nonlinearity
2- Nonlinear Amplifier Loop Mirror
Data in Data
out
I/O coupler
CW CCW
Fibre loop
Port 1 Port 2
EDFA
• The amplified CW pulseunder-goes a larger phaseshift compared to the CCW.
NOLM Model
• Maximised transmittance
( )[ ] ( )owwo TLTLLTT 2/tanhexp14 2 α−−β=πα
• Switching profile
( ) ( ) ( ) ( )[ ]{ }oowo TtTthTTtW ξβ−= *seccos1 22
2
( )( )
⟨τ⟩τ
≤τ≤ατ−=τξ
00
0exp
orTLTfor
TLTforTT
ow
owwo
where To is the pulse (soliton) width of the control puls, Tw is the walk-off time per unit length
α is the fibre lossαL is the interactive length of the fibre loopβ2 is the first order dispersion parameter
NOLMs DEMUX - Limitations
§ Intensity dependent phase shift in Si fibre is a weaknonlinearity
§ Long length of fibre (1 km) is required to achievenonlinearity
§ Nonlinearity is not easily controllable (i.e. to control anAND gate)
§ Polarisations-maintaining is essential
Solution:§ Terahertz Optical Asymmetric Demultiplexer (TOAD)
B- Terahertz Optical Asymmetric Demultiplexer(TOAD)
∆x
Data In λs
Data out
Coupler
SLA
CW CCW
Fibre loop
Control Pulse λc
PC
( )[ ]ccwcwccwGcwGccwGcwGTOADG θ−θ−+= cos241
• CW through SLA will face phase shift• The SW window size = 2n∆x/c (~ a few ps)• Timing between control and data Pulses are critical• SLA recovery time is large ~300 500ps
All Optical Demultiplexing – Reported works
NOLM based§ 1994, S Kawanishi – 6.25 Gbps§ 1998, M Nakazawa - 640 Gbps§ 1999 , K S Lee – Terabit
TOAD based§ 1993, A D Ellis – 40 Gbps§ 1999 B Mekelson - 160 Gbps§ 1996, A J Poustie - All optical circulating shift register§ 1998, A J Poustie - Optical regenerative memory
Residual Crosstalk - NOLM
§ Depends on:ú control pulse rateú control pulse widthú walkoff time between control and signal pulses
cw control pulses ccw signal pulses
Coupler Loop mirror
Adjacent Channel Crosstalk
Depends on:• shape of the switching window• width of the signal pulse
Adj
acen
t XT
Tim
ing
jitte
r noi
seW
indow w
idth
NOLM – Noise and Crosstalk
-40.00
-20.00
0.00
0 5 10 15 20
walk-off time (ps)
cro
ssta
lk (d
B)
-26.00
-25.00
-24.00
-23.00
-22.00
-21.00
-20.00
-19.00
-18.00
NO
LM r
elat
ive
inte
nsity
noi
se
(dB
)
BXT
CXT
RINNOLM
Crosstalk Comparison
BTX
CTX
NOLM SLA L=0.3mm, Tasy=1ps
-23.00
-22.00
-21.00
-20.00
-19.00
-18.00
-17.00
0.01
0.31
0.61
0.91
1.21
1.51
1.81
Switching energy (pj)
Cro
ssta
lk (
dB
)
TOAD
** bit rate: 100 Gb/s, and no. of channels: 10
All Optical Router - One Node
• Crosstalk: Adjacent channel & Residual ?• Timing jitter ?• Bit error rate ?
TOAD3•(route packet)
TOAD2•(read address)
Buffer
Output 1
Output 2
OI SOA
TOAD1(Clock Reco.)
PC SOA
Clock
Clock Data packet
PBS
Clock
Add. bits
Payload
Inputport
Clock Recovery Using TOAD
�∆•x
Clock + data packet in
•Data packet out
• •SLA
•Fibre•loop
•Reflected clock pulse
•PC
•PBS •PBS•Clock
•out
•SOA
•50:50
Series Router Configuration
P0
P32
Router1
Router2
Router2
Router3
Router3
Router3
Router3
Router3
Router3
Router3
Router3
P11
P21
P31
P12
P22
Parallel Router Configuration
P0
P0
Input 1
Input 2
Buffer
Buffer4
Router A
Router B
Pa Output port 1
Pb Outputport 2
Port 1a
Port 1b
Port 2a
Port 2bBuffer
High Speed OTDM Routing Networks
•Output•ports
•(•a)
•Input•ports
•1
•2
•N
•1
•2
•Star•coupler
•broadcast•to all
• •outputs
•HRU
•TST
•Delay
•OTDM•Demux
•HRU
•TST•OTDM•Demux
•HRU
•TST
•Buffer
•OTDM•Demux•OTDM frames
•Header
•Payload•Header
•N
•CU
•T
OTDM Routing Networks - Waveforms
Multiple copies of input frames generated within
TSTs
•N
Output port 2
Selected slot (sub cell) at the output of TSTs
•1 •2 •1
•Sub cell
Input to OTDM Dem.
OTDMDemux SW
window
•T•Sub cells
TOAD Based Packet Header Recognition -Parallel
TOAD1
TOAD2
TOAD3
Thresholddevice
Thresholddevice
Thresholddevice
Thresholddevice
TOAD0
Data
Clock
§ Can be extended to an arbitary number of bits§ Only limited by the data and clock power available
§ SW window ∆t = τ
τ
2τ
4τ
∆t = τ
SDH
ATM
IP
SDH
ATM
IP
Open Optical Interface
SDH ATM IP Other
All Optical Networks
All Optical Networks / Existing Networks
OTDM and WDM - Comparison
§ 40 Gbps OTDM ≡ 16 Channels WDM 2.5 Gbps.§ For a 2 nm channel separation WDM would occupy the
whole of EDFA bandwidth. OTDM occupies only 1 nm ofwavelength space
§ OTDM does require an active demultiplexer and channelalignment systems. WDM may also require accuratecontrol of filter and source wavelength (DWDM).
§ OTDM uses the available optical spectrum moreefficiently.
§ OTDM is less well advanced, and more costly than WDM,BUT future research and progress may alter this.
Challenges Ahead
§ Packet Routingú Algorithmsú Bit error rate and Packet error rate Analysisú Dispersion control (use soliton)ú Crosstalk analysisú Bufferingú Intelligent based routing
§ OTDM Based Cross Connectsú Sizeú BER and Crosstalk analysis
Remarks
§ OTDM is a powerful technique for delivery of high capacity backbone,as an alternative (but not a substitute) to WDM
§ Nonlinear devices can be used as a an all optical demultiplexer
§ OTDM data rate can be increased and its performance improved byemploying soliton pulse
§ Commercial realisation depends on future advancements in integrationtechniques, and devices etc.
§ The development of the capacity is not the ONLY GOAL. The flexibleuse of this potential and advantages of optical routing are the KEYFACTORS in the development of optical network.
§ Technologies should develop, integrate with and enhance those alreadyexisting.
Acknowledgements
Professor M ADAMS (Surry University)Professor A K RAY (SHU)Professor P BALL (Fujitsu Telecomm. Research)Dr E. D. KALUARACHCHI - Lucent Technology, Bell Lab. US
Mr R. U. REYHER, - Network Designer, Siemens AGe, Germany
Dr U. SCHILLER - Researcher, Philips Semiconductors AG, Zurich
Dr R. WICKRAMASINGHE - Radio Network Planning Eng., Nokia Telecomm. Ltd,UKDr L CHAO - Associate Professor, Nanyang Technological University, Singapore
Dr L SEED - The University of Sheffield, UK
DR J M HOLDING, SHU, UK
Sheffield Hallam universityThe University of SheffieldMany undergraduate and postgraduate students
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