Parallel Optical All Pass Filter Equalisers and Implementation

by

Wisit Loedhammacakra

Supervision team

Dr Wai Pang NgProf R. Cryan

Prof. Z. Ghassemlooy

Northumbria Communication Research Laboratories (NCRL)Northumbria University

13th June 2007

Overview

• Long-haul communication systems• Problem Statement• Chromatic Dispersion• Parallel Optical All Pass Filter Equaliser• Conclusion

Ideal communication system:

• Unlimited transmission bit rate (B)

• Unlimited transmission distance (L)

Long-haul Communication System 1Ideal Communication Systems

Tx Rx

• Audio

• Video

• Data

AttenuationAttenuation and dispersiondispersion limit BL product (as a benchmark for system’s performance)

Quality of digital communication systems can be

monitored from bit error rate (BER) of system.

Error-free detection, BER less than 10e-9

(Single error in one billion transmitted bits)

Long-haul Communication System 2Evolution of Long-haul communication systems

0

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

1850

1900

1950

2000

Year

BL

[(b

it/s

)-km

]

1015

103

106

109

1012

Lightwave

1

Telegraph

Coaxial cables

Telephone

Microwave

WDM Tech.

λ 1

λ 2

λ 3

λ n

Source: Agrawal

xx

x

x

x

x

Long-haul Communication System 3Optical Communication systems

1st 1970s 0.8 μm 1 dB/km 45 M 10 km MMFElectronic repeater

2nd 1980s 1.3 μm 0.5 dB/km 100 M 50 km MMFElectronic repeater

3rd 1990 1.55 μm 0.2 dB/km 2.5 G 70 km SMFElectronic repeater

4th 1996 1.55 μm 0.2 dB/km 2.5 G 100 km SMFEDFA WDM

5th 1999 1.55 μm 0.2 dB/km 10 G 100 km SMFCD

compen-sation

Generation YearOperation

wavelengthAttenuation

Bit rate bit/s

Repeat length

Fibre type

Special system

Problem StatementSingle Mode Fibre (SMF)

Wavelength (µm)

Fib

re a

tten

uati

on (

dB/k

m)

1.2 1.3 1.4 1.5 1.635

3025201510505

101520253035

Material dispersionWaveguide dispersionChromatic dispersion

Wavelength

Dis

pers

ion

(ps/

nm-k

m)

Dis

pers

ion

(ps/

nm-k

m)

Wavelength (µm)

• 1.31 μm chromatic dispersion (CD) is zero, but high attenuation is 0.5 dB/km.

• 1.55 μm has high CD of 17 ps/nm-km while attenuation is the lowest (0.2 dB/km).

Attenuation Dispersion ??

Chromatic Dispersion 1

Magic refresh room

Dispersion compensator

0 1 0 0 1 0 0 1 0

OAPF

1 0 1 Pattern 100 Pattern 001 Pattern 101 0? or 1?

1 0 1

Fibre

Transmitted pulse

restored pulse

Dispersed pulse

3 bits pattern of restored pulse3 bits pattern of dispersed pulse

Chromatic Dispersion 2

Why I’m so slow???

I’m not better, but my path has less resistance!!!

λ3

λ2

λ1

Core of fibre

Chromatic Dispersion 3Output Pulses of Different Lengths of SMF

200 150 100 50 0 50 100 150 200 250 3000

0.25

0.5

0.75

1

Original pulseSMF 80 kmSMF 100 kmSMF 160 km

Time (ps)

Am

pli

tude

Chromatic Dispersion 4Chromatic Dispersion Effect

100 0 100 200 300 400 500 600 700 8000

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

Time (ps)

Am

pli

tude

Summed signal

Summed signal

Dispersed pulse at 111

km

Transmitted pulse

Chromatic Dispersion 5 The Bit Rate-length Product

• Doubling the bit rate (B) would reduce the repeater-less length (L) of optical communication systems by a factor of 4.

• CD is the main limiting factor for repeater-less length.

220

c

4 λB L

D

L = 312 kmL = 18 kmL = 437 km10 Gb/s

L = 4,995 kmL = 294 kmL = 6,993 km2.5 Gb/s

DSF 1.55 μm

D ~ 1 ps/nm-km

SMF 1.55 μm

D ~ 17 ps/nm-km

SMF 1.3 μm

D ~ 1 ps/nm-km

Bit rate

Repeater-less length of optical communication systems

L = 312 kmL = 18 kmL = 437 km10 Gb/s

L = 4,995 kmL = 294 kmL = 6,993 km2.5 Gb/s

DSF 1.55 μm

D ~ 1 ps/nm-km

SMF 1.55 μm

D ~ 17 ps/nm-km

SMF 1.3 μm

D ~ 1 ps/nm-km

Bit rate

Repeater-less length of optical communication systems

Chromatic Dispersion 6Dispersion Compensation Techniques

DSFDispersion shifted

fibre

DCFDispersion

compensating fibre

FBGFibre Bragg grating

MZIMach-Zehnder interferometer

OPCOptical phase conjugation

OAPFOptical all pass

filter

Optical Bandwidth

Wide Wide Narrow Narrow Wide Wide

Insertion Loss

Accept High Accept Accept Accept Accept

Installation Difficult Difficult Accept Difficult Difficult Difficult

Dispersion Ripple

No Ripple

No Ripple

RippledNo

RippleNo

RippleNo

Ripple

Temperature Stable Stable Unstable Unstable Stable Stable

Dispersion Tunable

No No Possible Possible No Possible

Cost High High Accept High High Accept

Parallel optical all pass filter equaliserParallel optical all pass filter equaliser

(p-OAPF)(p-OAPF)

p-OAPF Equaliser 1Compensated System by Using OAPF

B = 10 Gb/s

OOK NRZ Format

0 = 1.55 μm

Input Data Output Data

Photo Detector

OAPF Equaliser

Optical Source D = 17 ps/nm-km, L = 160 km

2220λ

j πc

( ) e

fD LB

BH f

[ jφ(ω)](ω) eOAPFH

193.47 193.48 193.49 193.5 193.51 193.52 193.531

0

1

phase of fibrephase of rectangular pulsephase of out put pulse

Frequency (THz)

Pha

se

π

-π

193.485193.49 193.495 193.5 193.505 193.51 193.5151

0

1

Ideal phase of equaliser for 160 km SMFPhase of OAPF equaliser

Frequency (THz)

Phase

π

-π

Fig. 5. Phases of ideal equaliser for 160 km and OAPF.

200 100 0 100 200 3000

0.25

0.5

0.75

1

Original pulse160 km fibre without equaliser160 km fibre with equaliser

Time (ps)

Am

plit

ude

100 ps

200 ps

(a)(b)(c)

(a)

(b) (c)

193.485 193.49 193.495 193.5 193.505 193.511

0.5

0

0.5

1

Phase of un-compensated optical pulse at 160 km SMFPhase of compensated optical pulse at 160 km SMF

Frequency (THz)

Pha

se

π

-π

p-OAPF Equaliser 2OAPF is Implemented With IIR Structure

x(n)

r2y(n)

r4

r2

2rcos(ω0)

2r3cos(ω0)

Splitter

Splitter

Splitter

Splitter

Attenuator

Attenuator

Attenuator

Attenuator

180◦

Phase shifter

180◦

Phase shifter

Time delay τ

Time delay τ

Time delay τ

Time delay τ

Combiner

p-OAPF Equaliser 3Compensated System by Using p-OAPF

B = 10 Gb/s

OOK NRZ Format

0 = 1.55 μm, Pi = 1 mW

Input Data Output Data

Optical Source

D = 17 ps/nm-km, α = 0.2 dB/km, L = 90 km

EDFA OBPF p-OAPF Equaliser

G = 30 dB BW = 20 GHz

Eq1

Eq2 Photo Detector

Photo Detector

Splitter

Electrical signal

Optical signal

0

α

10

22λ 2j πc

( ) 10 eL

f

fD LB

BH f

193.40935 193.41435 193.41935 193.42435 193.429351

0

1

Phase response of rectangular pulse.Phase response of SMF at 90 km.Compensated phase response by p-OAPF.

Frequency (GHz)

Phas

e (p

i)

-π

π

Phas

e

150 200 250 300 350 400 450 500 5500

0.2

0.4

0.6

0.8

1

1.2

Original pulseDispersive pulse at 90 km of SMFCompensated pulse by p-OAPF

Time (ps)

Nor

mal

ised

Am

plit

ude

350 300 400 250

Time (ps) N

orm

alis

ed A

mpl

itude

0

0.2

0.4

0.6

0.8

1

1.2

2.50E-10 3.00E-10 3.50E-10 4.00E-10 4.50E-10

450

Conclusion

p-OAPFp-OAPF

Be implemented in optical domain by

using IIR structure and optical components

Adjust the phase of the optical pulse

back to the phase of transmitted optical

pulse

Capable of extending the

length to 90 km in 10 Gb/s systems

CD limits 10 Gb/s system at

30 km

Publications1. W. Loedhammacakra, W. P. Ng, and R. A. Cryan, "Investigation of an Optical All Pass Filter for a 10 Gb/s Optical Communication System," presented at PG-NET 2005 Proceeding, Liverpool John Moores University, UK, pp. 170-175, 27-28 June 2005.

2. W. Loedhammacakra, W. P. Ng, and R. A. Cryan, "An Improved Chromatic Dispersion Compensation Technique Employing an Optical All Pass Filter Equaliser in a 10Gb/s Optical System," presented at The Tenth High Frequency Postgraduate Student Colloquium, University of Leeds, UK, pp. 105-108, 5-6 September 2005.

3. W. Loedhammacakra, W. P. Ng, and R. A. Cryan, "Chromatic Dispersion Compensation Using an Optical All Pass Filter for a 10 Gb/s Optical Communication System at 160 km," presented at London Communication Symposium 2005, University College London, UK, pp. 255-258, 8-9 September 2005.

4. W. Loedhammacakra, W. P. Ng, and R. A. Cryan, “Chromatic Dispersion Compensation Employing Optical All Pass Filter by Using IIR Structure for 10 Gb/s Optical Communication System,” presented at the IEE Photonics Professional Network Seminar on Optical Fibre Communications and Electronic Signal Processing, The IEE Savoy place, London, UK, pp 17/1-17/6, 15 December 2005.

5. W. Loedhammacakra, W. P. Ng, R. A. Cryan, and Z. Ghassemlooy, “Investigation of Optical All Pass Filter to Compensate Chromatic Dispersion in a 10 Gb/s Optical Communication System at 160 km,” CSNDSP 2006, Patras, Greece, pp. 454 – 458, 19 – 21 July 2006.

6. W. P. Ng, W. Loedhammacakra, R. A. Cryan, and Z. Ghassemlooy, “Performance Analysis of the Parallel Optical All-pass Filter Equalizer for Chromatic Dispersion Compensation at 10 Gb/s,” under-review by Globecom 2007.

7. W. P. Ng, W. Loedhammacakra, R. A. Cryan, and Z. Ghassemlooy, “Characterisation of a Parallel Optical All Pass Filter for Chromatic Dispersion Equalisation in 10 Gb/s System ,” under-review by IET processing on signal processing.

Posters1. Chromatic Dispersion Compensation Technique Employing OAPF in Optical Communication Systems, presented at UK Grad Poster Competitive 2006, Northumbria University, Newcastle, Aril 2006.

2. High Speed Optical Network Need Low Dispersion, presented at Britain’s Early-State Engineers on UK Engineering research and R&D, House of Commons, London, December 2006.

Papers

Acknowledgements

My supervision team (Dr. Wai Pang Ng, Prof. R. Cryan and Prof. Z. Ghassemlooy)

OCR Group leader (Prof. Z. Ghassemlooy) for all of his support

Dr Krishna Busawon and Dr Mark Leach for all of the useful discussions we had

My colleague in Room E405 and E409 Especially, Hoa, Popoola, Sujan and Ming Feng for discussion and helpful.

I would like to thank:

Thank you

Discussion

Question&

Optical All Pass Filter Equaliser 1 Phases of SMF, Rectangular and Dispersed Pulse

193.47 193.48 193.49 193.5 193.51 193.52 193.531

0

1

phase of fibrephase of rectangular pulsephase of out put pulse

Frequency (THz)

Pha

se

π

-π

• The interested bandwidth is between 193.49 – 193.51 THz, which phase response of dispersed pulse is same as phase response of SMF.

Optical All Pass Filter Equaliser 2Phase Response of Ideal Equaliser and OAPF

193.485193.49 193.495 193.5 193.505 193.51 193.5151

0

1

Ideal phase of equaliser for 160 km SMFPhase of OAPF equaliser

Frequency (THz)

Phase

π

-π

Fig. 5. Phases of ideal equaliser for 160 km and OAPF.

• The phase response of the ideal equaliser is used as the optimisation criterion.

• The phase response of OAPF at upper frequency does not equalise properly.

Optical All Pass Filter Equaliser 3Optical Communication System

B = 10 Gb/s

OOK NRZ Format

0 = 1.55 μm

Input Data Output Data

Photo Detector

OAPF Equaliser

Optical Source D = 17 ps/nm-km, L = 160 km

2220λ

j πc

( ) e

fD LB

BH f

[ jφ(ω)](ω) eOAPFH

Optical All Pass Filter Equaliser 6Output Pulses

200 100 0 100 200 3000

0.25

0.5

0.75

1

Original pulse160 km fibre without equaliser160 km fibre with equaliser

Time (ps)

Am

plit

ude

100 ps

200 ps

• A dispersed pulse was equalised back to 100 ps at FWHM.

• The larger pulse width on the right hand side of compensated pulse is not properly compensated and resulted in higher ISI and BER.

Optical All Pass Filter Equaliser 5Phase response

193.485 193.49 193.495 193.5 193.505 193.511

0.5

0

0.5

1

Phase of un-compensated optical pulse at 160 km SMFPhase of compensated optical pulse at 160 km SMF

Frequency (THz)

Pha

se

π

-π

• The compensated phase is close to zero at lower frequency.

• At the higher frequency, the phase response is not properly compensated.

Results 1Compensated Phase Response by p-OAPF

193.40935 193.41435 193.41935 193.42435 193.429351

0

1

Phase response of rectangular pulse.Phase response of SMF at 90 km.Compensated phase response by p-OAPF.

Frequency (GHz)

Phas

e (p

i)

-π

π Ph

ase

Results 2Compensated Pulse by p-OAPF

150 200 250 300 350 400 450 500 5500

0.2

0.4

0.6

0.8

1

1.2

Original pulseDispersive pulse at 90 km of SMFCompensated pulse by p-OAPF

Time (ps)

Nor

mal

ised

Am

plit

ude

350 300 400 250

Time (ps)

Nor

mal

ised

Am

plitu

de

0

0.2

0.4

0.6

0.8

1

1.2

2.50E-10 3.00E-10 3.50E-10 4.00E-10 4.50E-10

450