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 Data Tx Rx Attenuation and dispersion limit BL product (as a benchmark for system’s performance) Ideal communication system: Unlimited transmission bit rate (B) Unlimited transmission distance (L) 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 x x x x x x Source: Agrawal

Long-haul Communication System 3 Optical Communication systems Generation Year Operation wavelength Attenuation Bit rate bit/s Repeat length Fibre type Special system 1st 1970s 0.8 μm 1 dB/km 45 M 10 km MMF Electronic repeater 2nd 1980s 1.3 μm 0.5 dB/km 100 M 50 km MMF Electronic repeater 3rd 1990 1.55 μm 0.2 dB/km 2.5 G 70 km SMF Electronic repeater 4th 1996 1.55 μm 0.2 dB/km 2.5 G 100 km SMF EDFA WDM 5th 1999 1.55 μm 0.2 dB/km 10 G 100 km SMF CD compen-sation

Problem Statement Single Mode Fibre (SMF) Previous generation of optical long-haul communication systems operate at 1.31 µm where chromatic dispersion is zero, but attenuation is 0.5 dB/km. Wavelength division multiplexing (WDM) system was introduced to increase the fibre bandwidth, the operating wavelength moved to 1.55 µm due to the low attenuation region of SMF and the operating wavelength window of EDFA, but the chromatic dispersion is occurred 17 ps/nm-km. ?  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 Fibre Transmitted pulse Dispersed pulse Previous generation of optical long-haul communication systems operate at 1.31 µm where chromatic dispersion is zero, but attenuation is 0.5 dB/km. Wavelength division multiplexing (WDM) system was introduced to increase the fibre bandwidth, the operating wavelength moved to 1.55 µm due to the low attenuation region of SMF and the operating wavelength window of EDFA, but the chromatic dispersion is occurred 17 ps/nm-km. Transmitted pulse Dispersed pulse restored pulse 3 bits pattern of dispersed pulse 3 bits pattern of restored pulse

Chromatic Dispersion 2 Core of fibre Previous generation of optical long-haul communication systems operate at 1.31 µm where chromatic dispersion is zero, but attenuation is 0.5 dB/km. Wavelength division multiplexing (WDM) system was introduced to increase the fibre bandwidth, the operating wavelength moved to 1.55 µm due to the low attenuation region of SMF and the operating wavelength window of EDFA, but the chromatic dispersion is occurred 17 ps/nm-km. Core of fibre

Chromatic Dispersion 3 Output Pulses of Different Lengths of SMF The dispersion will result in reduced pulse’s amplitude and increased the pulse’s width. At 160 km of SMF, the output pulse’s amplitude reduces by half but the pulse width doubled at full width at half maximum (FWHM) compare to the original pulse. The reduced amplitude is due to the dispersion and not because of the attenuation of the fibre.

Chromatic Dispersion 4 Chromatic Dispersion Effect Summed signal Chromatic Dispersion 4 Chromatic Dispersion Effect Transmitted pulse Summed signal Dispersed pulse at 111 km We should use the 7 bit pattern in this page and presentation. Diagram should be enlarged, with text at the bottom. Can the fonts in the diagram be increased in size?

Chromatic Dispersion 5 The Bit Rate-length Product Doubling the bit rate (B) would reduce the length (L) of optical link by a factor of 4. At the operating wavelength of 1.55 µm the dispersion is the limiting factor to the repeater-less span length of an optical communication system. 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 Dispersion shifted fibre DCF Dispersion compensating fibre FBG Fibre Bragg grating MZI Mach-Zehnder interferometer OPC Optical phase conjugation OAPF Optical all pass filter Optical Bandwidth Wide Narrow Insertion Loss Accept High Installation Difficult Dispersion Ripple No Ripple Rippled Temperature Stable Unstable Dispersion Tunable No Possible Cost

Parallel optical all pass filter equaliser (p-OAPF)

p-OAPF Equaliser 1 Compensated System by Using OAPF Please include all the information in the diagram, i.e. NRZ etc. Remove the text below the diagram and the diagram should occupy the whole page.

p-OAPF Equaliser 2 OAPF is Implemented With IIR Structure The passive OAPF equaliser can be implemented by using an IIR digital filter structure with optical components such as delays, attenuators and splitters.

p-OAPF Equaliser 3 Compensated System by Using p-OAPF A 100 ps optical pulse which propagated through 160 km of SMF was equalised back to 100 ps at FWHM . The larger pulse width in the low amplitude region would affect the pulse in the next time slot and thus causing ISI.

Conclusion CD limits 10 Gb/s system at 30 km p-OAPF Adjust the phase of the optical pulse back to the phase of transmitted optical pulse CD limits 10 Gb/s system at 30 km p-OAPF OAPFs can be used to compensate chromatic dispersion in SMF. OAPF based on the phase optimisation gives BER improvement of 10 to 20 dB for SMF length greater than 100 km. However further BER improvement is required due to the imperfect phase compensation for higher frequencies. Further work in the research will be to improve the OAPF design and the passive IIR structure implementation. 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

Publications Papers Posters 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. OAPFs can be used to compensate chromatic dispersion in SMF. OAPF based on the phase optimisation gives BER improvement of 10 to 20 dB for SMF length greater than 100 km. However further BER improvement is required due to the imperfect phase compensation for higher frequencies. Further work in the research will be to improve the OAPF design and the passive IIR structure implementation. 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. OAPFs can be used to compensate chromatic dispersion in SMF. OAPF based on the phase optimisation gives BER improvement of 10 to 20 dB for SMF length greater than 100 km. However further BER improvement is required due to the imperfect phase compensation for higher frequencies. Further work in the research will be to improve the OAPF design and the passive IIR structure implementation.

Thank you Discussion Question &

Optical All Pass Filter Equaliser 1 Phases of SMF, Rectangular and Dispersed Pulse Increase the diagram size and text should be point form, just highlighting the important point. Put the full text in here for example: The phase of the H(f) of SMF is parabolic. The phase of a 10 Gb/s rectangular pulse is zero at 193.5 THz ± 10 GHz and toggles between  and zero for every 10 GHz. In the region of interest (193.5 THz ± 10 GHz) the phase of the rectangular pulse after propagating 160 km in a SMF is the same as the phase of the SMF. 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 Bigger diagram 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 Please include all the information in the diagram, i.e. NRZ etc. Remove the text below the diagram and the diagram should occupy the whole page.

Optical All Pass Filter Equaliser 6 Output Pulses A 100 ps optical pulse which propagated through 160 km of SMF was equalised back to 100 ps at FWHM . The larger pulse width in the low amplitude region would affect the pulse in the next time slot and thus causing ISI. 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 The phase response of the compensated optical pulse is close to zero at lower frequency. The frequency range higher than 193.500 THz the phase response of the pulse is not properly compensated and results in larger pulse width. 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 A 100 ps optical pulse which propagated through 160 km of SMF was equalised back to 100 ps at FWHM . The larger pulse width in the low amplitude region would affect the pulse in the next time slot and thus causing ISI.

Results 2 Compensated Pulse by p-OAPF A 100 ps optical pulse which propagated through 160 km of SMF was equalised back to 100 ps at FWHM . The larger pulse width in the low amplitude region would affect the pulse in the next time slot and thus causing ISI.