Razali Ngah, and Zabih Ghassemlooy Optical Communication Research Group

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Razali Ngah, and Zabih Ghassemlooy

Optical Communication Research Group

School of Engineering & Technology

Northumbria University, United Kingdom

http: soe.unn.ac.uk/ocr/

Bit Error Rate Performance of All Optical Router Based on SMZ

Switches

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Contents

Introduction OTDM All optical switches Symmetric Mach-Zehnder (SMZ) switch All OTDM Router Simulations and Results Conclusion

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Introduction

Solution: All optical transmission, multiplexing, switching, processing, etc.

Multiplexing:• Electrical

• Optical

Drawbacks with Electrical: Speed limitation beyond 40 Gb/s (80 Gb/s future) of:

Electo-optics/opto-electronics devices High power and low noise amplifiers

Router congestion and reduced throughputs: Due to optical-electronic-optical conversion

Limited modulation bandwidth of light sources, and modulators

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Multiplexing - Optical

Wavelength division multiplexing (WDM)

Optical time division multiplexing (OTDM) Hybrid WDM-OTDM

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Flexible bandwidth on demand at burst rates of 100 Gb/s/ The total capacity of single-channel OTDM network = DWDM Overcomes non-linear effects associated with WDM:

(i) Self Phase Modulation (SPM) – The signal intensity of a given channel modulates its own refractive index, and therefore its phase

(ii) Cross Phase Modulation (XPM) – In multi-channel systems, other interfering channels also modulate the refractive index of the desired channel and therefore its phase

(iii) Four Wave Mixing (FWM) – Intermodulation products between the WDM channels, as the nonlinearity is quadratic with electric field

Less complex end node equipment (single-channel Vs. multi-channels)

Can operate at both: 1500 nm (like WDM) due to EDFA 1300

OTDM

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OTDM - Principle of Operation

Multiplexing is sequential, and could be carried out in: A bit-by-bit basis (bit interleaving) A packet-by-packet basis (packet interleaving)

Clock

ReceiverTransmitter

Clockrecovery

LightsourceLight

source

Data (10 Gb/s)

N

Networknode

Networknode

Drop Add

Rx

Rx

Rx

10 GHzN*10 Gb/s

Data (10 Gb/s)

OTDM DEMUXOTDM MUXAmplifierModulatorsFibre delay line

Fibre

Span

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All Optical Switches

Control pulse

Data in Data out

Coupler

CW CCW

Long fibre loop

Port 1 Port 2

Control coupler

PC

x

Data In s

Data out

Coupler

SLA

CW CCW

Fibre loop

Control Pulse c

PC

Non-linear OpticalLoop Mirror (NOLM)

Terahertz Optical AsymmetricDemultiplexer (TOAD)

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All Optical Switches – contd.

Mach-Zehnder Interferometer (MZI)

Colliding pulse Mach-Zehnder (CPMZ)

Symmetric Mach Zehnder (SMZ)

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3 dBCoupler

Tdelay

OTDM Signal Pulses

Control Pulse (switch-on)

Optical filter

Control Pulse (switch-off)

SOA1

SOA2

Output Port 1

SMZ Switch: Principle

3 dBCoupler

OTDM Signal Pulses

Control Pulse Input Port 1

Control Pulse Input Port 2

SOA1

SOA2

Output Port 2

(i) No control pulses

(ii) With control pulses

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SMZ : Switching Window

)(cos.)()(2)()(4

1)( 2121 ttGtGtGtGtW

40 45 50 55 60 65 70 75 80 85 902

4

6

8

10

12

14

16

18

20

Gain Profile of Gc1(__) and Gc2(--)

Time (ps)

Gai

n

40 45 50 55 60 65 70 750

5

10

15

20

25SMZ switching window

Time (ps)

SM

Z g

ain

G1 and G2 are the gains profile of the data signal at the output of the SOA1

and SOA2 and ΔФ is the phase difference between the data signals

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1x2 All OTDM Router

Port 1

Port2

SMZ1 (clock

extract)

SMZ2 (read

address)

SMZ3 (route

payload )

( a)

( b)

( c) (e)

(d)

(f)

(a) OTDM Signal

(b) Extracted Clock

(c) Address + Payload

(d) Address

(e) Payload

(f) Payload

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Performance Issues

(1) Relative Intensity Noise (RIN)

Relative timing jitter between the control and the signal pulses induces intensity fluctuations of the demultiplexed signals

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Relative Intensity Noise (RIN)

The output signal can be described by:

dttptTtw x )()()(

dttptwE t )()()(

where Tx(t) is the switching window profile and p(t) is the input data profile

The expected of the output signal energy is given as:

pt(t) probability density function of the relative signal pulse arrival time:2

2

1

2

1)(

RMSt

t

RMS

t et

tp

where tRMS is the root mean square jitter

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Assuming that the mean arrival time of the target channel is at the centre of the switching window, RIN induced by the timing jitter of the output signal can be expressed as:

)(

)()(

2

E

VarRIN

The variance of the output signal, depending on the relative arrive time is:

)()()()( 22 EdttptwVar t

Relative Intensity Noise (RIN) – contd.

The total RIN for the router is three times the value of single SMZ

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(2) Channel Crosstalk (CXT)

Due to demultiplexing of adjacent non-target channels to the output port when the switching profile overlaps into adjacent signal pulses

Performance Issues – contd.

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Channel crosstalk (CXT) – contd.

CXT is defined by the ratio of the transmitted power of one non-target channel to that of a target channel t

nt

E

ECXT log10

Et is the output signal energy due to the target channel

2/

2/

)()(Dc

Dc

Tt

Tt

cxt dtttptTE

Ent is the output signal energy due to the nontarget channel

2/

2/

)()(Dc

Dc

TTt

Tt

cxnt dtttptTE

The total crosstalk for the router 1)1( 3 CXTCXT

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BER Analysis

Assuming 100% energy switching ratio for SMZ and the probability of mark and space are equal, the mean photocurrents for mark Im and space Is are:

where R is the responsivity of the photodetector, ηin and ηout are the input and output coupling efficiencies of the optical amplifier, respectively; G is the optical amplifier internal gain, L is optical loss between amplifier and receiver, and Psig is the pre-amplified average signal power for a mark (excluding crosstalk)

The variance of receiver noise for mark and space:

eaL

keASExths Bi

R

KTBIIq

xrec

_2

___222 4

)(2,

]1[__

nsigm CXTII ][__

nsigs CXTII

sigoutinsig GLPRI ___

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The noise variance of optical amplifier

BER Analysis – cont.

2

22

,

)2(4

o

eoeASE

o

eASExxamp B

BBBI

B

BII

The average photo-current equivalent of ASE LqBGNI ooutspASE )1(

The expression for calculating BER is given as:

where 2

______

Total

sm IIQ

Q

QBER

)5.0exp(

2

1 2

The noise variance of RIN

ROUTERsigeTmmRIN RINIBRINI22

2, eTssRIN BRINI

22, and

2,

2,

2,

2,

2,

2,

2sRINmRINsampmampsrxmrxTotal The total variance

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Results

SMZ 1

Clock Address

SMZ 2

SMZ 3

Photo- detector

BER

1x2 Router Incoming OTDM Signal

Pin

Filter

t = ts

Pk

Receiver

Optical Amp.

Optical path Electrical path

Block diagram of a router with a receiver

System Parameters

Parameter in out out L R RL Tk Nsp RINT Bo Ia2 RINR

OUTER

RMSjitter

CXTn

Be

Value -2 dB

-2 dB

Gain (overall)25 dB

-2 dB

1 A/W

50

293 K

2 10-15 Hz-1

400

GHz

100

pA2

/Hz

-21 dB

1 ps -25 dB

0.7Rb

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Results – RIN and CXT

0 2 4 6 8 10 12 14 16 18 20-26

-24

-22

-20

-18

-16

-14

-12

-10

-8

Control signals separation (ps)

Rel

ativ

e in

tens

ity n

oise

(dB

)

OTDM router SMZ demultiplexerFWHM = 2ps

0 2 4 6 8 10 12 14 16 18 20-50

-45

-40

-35

-30

-25

-20

-15

-10

-5

0

Control signals separation (ps)

SM

Z cr

osst

alk

(dB

)

OTDM router SMZ demultiplexerFWHM = 2ps

RIN against control pulse separation for a single SMZ and a router

CXT against control pulse separation for a single SMZ and a router

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Results - BER

BER against average received power for baseline and with an optical router

-44 -42 -40 -38 -36 -34 -32 -30 -28 -26 -24 -22

10-12

10-10

10-8

10-6

10-4

10-2

Average received optical power (dBm)

Bit

erro

r ra

te

10Gb/s baseline 10Gb/s with router

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Conclusions

Relative intensity noise and channel crosstalk of 1x2 router is investigated

BER analysis has been performed. As expected the BER increases with the number of SMZ

stages due to the accumulation of ASE noise in the SOAs hence, resulting the RIN increases.

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THANK YOU