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 Time Division Multiplexing an overview

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

OTDM Broadcastand

Switched Based Networks

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

Packet OTDM- De-multiplexer

Splitter

Compressedpacket

&

&

&

&

&

Controlsignals

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]

Issues Associated with OTDM

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

OTDM Demultiplexing Techniques

OTDM - Demultiplexing Schemes

Electro-optic

All optical

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

OTDM- Demultiplexers

Performance Issues

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

Residual Crosstalk -TOAD

Depends on: - control pulse energy, - asymmetry, - SLA gain recovery time

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

Timing Jitter Noise

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

NOLM - BER

baseline

walk-off time

9ps 18ps

1ps

NOLMDEMUX

BPF

EDFA

Error Detection

Optical Receiver

OTDM Applications

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

8x8 Banyan Network Architecture

110

Input lines Output lines

000

001

010

011

100

101

110

111

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

High Speed Optical Transport Layer[D M Spirit, et al, 1994]

OTDM Add/Drop Demultiplexer

ADM

Drop Add

Input Output

OTDM Cross Connect

ADM1

42 31 4321

AddDrop

ADM2

4321Add Drop

42 31

OTDM-λ1

OTDM-out

OTDM-λ2

LAN Application

Principle of 3R Regenerator

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

Packet Switched Optical Newtork- Internal nodestructure

•Singlewavelengthpacket•bit rate of100 Gbps