Parallel Optical All Pass Filter
Equalisers and Implementation
by
Wisit Loedhammacakra
Supervision team
Dr Wai Pang Ng
Prof 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
Long-haul Communication System 1
Ideal Communication Systems
• Audio
• Video
Tx
Rx
• Data
Ideal communication system:
Attenuation and dispersion limit
• Unlimited transmission bit rate (B) BL product (as a benchmark for
• Unlimited transmission distance (L) 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 2
Evolution of Long-haul communication systems
λ
λ
λ
λ
15
1015
14
13
101212
1
2
3
n
WDM Tech.
10
1099
Lightwave
8
7
Coaxial
cables
4
3
3
x
x
x
Telegraph
20
00
19
50
x
19
00
10
x
Telephone
10
2
1
x
Microwave
1066
5
18
50
BL[(bit/s)-km]
11
Year
Source: Agrawal
Long-haul Communication System 3
Optical Communication systems
Generation
Year
Operation
wavelength
Attenuation
1st
1970s
0.8 μm
1 dB/km
45 M
10 km
MMF
2nd
1980s
1.3 μm
0.5 dB/km
100 M
50 km
MMF
3rd
1990
1.55 μm
0.2 dB/km
2.5 G
70 km
SMF
4th
1996
1.55 μm
0.2 dB/km
2.5 G
100 km
SMF
EDFA
WDM
SMF
CD
compensation
5th
1999
1.55 μm
0.2 dB/km
Bit rate Repeat
bit/s
length
10 G
100 km
Fibre
type
Special
system
Electronic
repeater
Electronic
repeater
Electronic
repeater
Problem Statement
Fibre attenuation (dB/km)
Dispersion (ps/nm-km)
(ps/nm-km)
Dispersion
Single Mode Fibre (SMF)
Wavelength (µm)

35
30
25
20
15
10
5
0
5
10
15
20
25
30
35
1.2
1.3
1.4
1.5
1.6
Wavelength
(µm)
Wavelength
Material dispersion
Waveguide dispersion
Chromatic dispersion
?
Attenuation
Dispersion
• 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).
Chromatic Dispersion 1
Magic
refresh room
OAPF
Fibre
0
1
0
0
Transmitted
pulse
1
0
Dispersion
compensator
Dispersed
pulse
1
0
0? or 1?
1
Pattern 100
Pattern 001
Pattern 101
3 bits pattern of dispersed pulse
1
0
0
1
0
restored
pulse
1
3 bits pattern of restored pulse
Chromatic Dispersion 2
I’m not better, but my
path has less resistance!!!
Why I’m so slow???
λ3
λ2
λ1
Core of fibre
Chromatic Dispersion 3
Output Pulses of Different Lengths of SMF
1
Amplitude
0.75
0.5
0.25
0
200
150
100
50
0
50
Time (ps)
Original pulse
SMF 80 km
SMF 100 km
SMF 160 km
100
150
200
250
300
Summed signal
Chromatic Dispersion 4
Chromatic Dispersion Effect
1.1
1
Transmitted
pulse
Summed
signal
0.9
0.8
Amplitude
0.7
0.6
Dispersed
pulse at 111
km
0.5
0.4
0.3
0.2
0.1
0
100
0
100
200
300
400
Time (ps)
500
600
700
800
Chromatic Dispersion 5
The Bit Rate-length Product
B2 L 
c
4Dλ 02
Repeater-less length of optical communication systems
Bit rate
SMF 1.3 μm
D ~ 1 ps/nm-km
SMF 1.55 μm
D ~ 17 ps/nm-km
DSF 1.55 μm
D ~ 1 ps/nm-km
2.5 Gb/s
L = 6,993 km
L = 294 km
L = 4,995 km
10 Gb/s
L = 437 km
L = 18 km
L = 312 km
• 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.
Chromatic Dispersion 6
Dispersion Compensation Techniques
DSF
DCF
FBG
MZI
OPC
OAPF
Dispersion shifted
fibre
Dispersion
compensating fibre
Fibre Bragg grating
Mach-Zehnder
interferometer
Optical phase
conjugation
Optical 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
Rippled
No
Ripple
No
Ripple
No
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 equaliser
(p-OAPF)
p-OAPF Equaliser 1
π
Compensated System by Using OAPF
1
Phase
Amplitude
0.75
OOK NRZ Format
Frequency (THz)
0
phase of fibre
phase of rectangular pulse
phase of out put pulse
Input Data
(a)
0 = 1.55 μm
200
100
0
100
200
300
Time (ps)
(a)
(b)
(c)
2


λ 02
2  f 
 j  πD
LB   
c
 B  


H( f )  e
π
(b)
D = 17 ps/nm-km, L = 160 km
-π
Original pulse
160 km fibre without equaliser
160 km fibre with equaliser
π
OAPF
Equaliser
(c)
Output Data
Photo
Detector
Phase
Optical
Source
200 ps
0.25
B = 10 Gb/s
Phase
-π
100 ps
0.5
[ jφ(ω)]
H OAPF (ω)  e
Frequency
(THz)
-π
Ideal phase of equaliser for 160 km SMF
Frequency (THz)
Phase of OAPF equaliser
Phase of un-compensated optical pulse at 160 km SMF
Phase of compensated optical pulse at 160 km SMF
Fig. 5. Phases of ideal equaliser for 160 km and OAPF.
p-OAPF Equaliser 2
OAPF is Implemented With IIR Structure
x(n)
r2y(n)
Attenuator
Splitter
r2
Splitter
Combiner
τ
Time delay
τ
Time delay
Splitter
Splitter
180◦
Phase shifter
Attenuator
Attenuator
2rcos(ω0)
2r3cos(ω0)
τ
Time delay
τ
Time delay
180◦
Attenuator
r4
Phase shifter
p-OAPF Equaliser 3
Compensated System by Using p-OAPF
B = 10 Gb/s
Normalised Amplitude
1.2
Electrical signal
OOK NRZ Format
Optical signal
Input Data
Output Data
H f ( f )  10
αL

10
2

λ 2
 f  
 j  πD 0 LB 2   
c
B
  


e 
1
0.8
0.6
0.4
0.2
0
150 200
250
300
350
400
450
500
550
Time (ps)
EDFA
G = 30 dB
OBPF
BW = 20 GHz
π1
Eq1
0
Photo
Detector
Splitter
-π1
1.2
Normalised Amplitude
Phase
0 = 1.55 μm,
Pi = 1 mW
D = 17 ps/nm-km, α = 0.2 dB/km,
L = 90 km
Original pulse
Dispersive pulse at 90 km of SMF
Compensated pulse by p-OAPF
p-OAPF
Equaliser
Phase (pi)
Optical
Source
1
0.8
0.6
0.4
193.40935
Eq2
Photo
Detector
193.41435
0.2
193.41935
193.42435
193.42935
Frequency (GHz)
Phase response of rectangular pulse.
Phase response of SMF at 90 km.
0Compensated phase response by p-OAPF.
300
2.50E-10
3.00E-10
3.50E-10
350
250
Time (ps)
400
4.00E-10
4.50E-10
450
Conclusion
CD limits 10 Gb/s
system at 30 km
Adjust the phase of
the optical pulse
back to the phase of
transmitted optical
pulse
p-OAPF
Be implemented in
optical domain by
using IIR structure
and optical
components
Capable of
extending the
length to 90 km in
10 Gb/s systems
Papers
Publications
1. 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.
Posters
1. 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.
Acknowledgements
I would like to thank:
 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.
Thank you
Question
&
Discussion
Optical All Pass Filter Equaliser 1
Phases of SMF, Rectangular and Dispersed Pulse
Phase
π
-π
Frequency (THz)
phase of fibre
phase of rectangular pulse
phase of out put pulse
• 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 2
Phase Response of Ideal Equaliser and OAPF
Phase
π
-π
Frequency (THz)
Ideal phase of equaliser for 160 km SMF
Phase of OAPF equaliser
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 3
Optical Communication System
B = 10 Gb/s
OOK NRZ Format
Input Data
Optical
Source
0 = 1.55 μm
Output Data
2


λ 02
2  f 
 j  πD
LB   
c
 B  


H( f )  e
D = 17 ps/nm-km, L = 160 km
OAPF
Equaliser
[ jφ(ω)]
H OAPF (ω)  e
Photo
Detector
Optical All Pass Filter Equaliser 6
Output Pulses
1
Amplitude
0.75
100 ps
0.5
200 ps
0.25
0
200
100
0
100
200
300
Time (ps)
Original pulse
160 km fibre without equaliser
160 km fibre with equaliser
• 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 5
Phase response
Phase
π
-π
Frequency (THz)
Phase of un-compensated optical pulse at 160 km SMF
Phase of compensated optical pulse at 160 km SMF
• The compensated phase is close to zero at lower frequency.
• At the higher frequency, the phase response is not properly compensated.
Results 1
Compensated Phase Response by p-OAPF
Phase
Phase (pi)
π1
0
-π1
193.40935
193.41435
193.41935
193.42435
193.42935
Frequency (GHz)
Phase response of rectangular pulse.
Phase response of SMF at 90 km.
Compensated phase response by p-OAPF.
Results 2
1.2
1.2
1
1
0.8
0.6
0.4
0.2
0
150 200
250
300
350
400
450
500
550
Normalised Amplitude
Normalised Amplitude
Compensated Pulse by p-OAPF
0.8
0.6
0.4
0.2
Time (ps)
Original pulse
Dispersive pulse at 90 km of SMF
Compensated pulse by p-OAPF
0
2.50E-10
250
300
3.00E-10
3.50E-10
350
Time (ps)
400
4.00E-10
4.50E-10
450