At the time of writing this Ph.D. thesis, FSAN (Full Service Access Network) and ITU-T (International Telecommunication Union – Telecommunication Standardization Bureau) working groups are involved in the final definition of the Recommendation G.989, which has recently established that the Next Generation Passive Optical Network (NG-PON2) architecture will be based on Time- and Wavelength-Division Multiplexed PON (TWDM-PON), and it will consist in its first version on four wavelength-stacked XG-PONs using four wavelengths per direction, likely with a very narrow (for PON) spacing of 100 GHz or even 50 GHz. Furthermore, the Optical Distribution Network (ODN) will be splitter-based and passive, like all previous PON standards, avoiding the use of any wavelength selection/filtering and any optical amplification inside the ODN. Moreover, since complete ODN backward compatibility is required, the TWDM-PON must be compliant with the same previous ODN loss classes, i.e. 33 and 35 dB for the extended E1 and E2 classes, respectively, and up to 15 dB of Differential Optical Path Loss (DOPL). One of the few physical layer problems over which FSAN is still working before the final NG-PON2 Recommendation release, is related to the lack of available low-cost tunable lasers with sufficient tuning range and launch power, to be installed at the Optical Network Unit (ONU) side for the upstream signal generation. Indeed, they should have a precision compatible with a 100 GHz wavelength grid, and be able to tune on (at least) four wavelengths, adopting some protection systems during their switch-on time, in order to avoid interferences with other channels (due to the uncontrolled wavelength transmission at the laser power on); moreover, they should operate on a very wide temperature range and they should have a target price compatible with equipment to be installed at the customer premises. Furthermore, in the longer term, if more than four wavelengths will be used (for instance for WDM overly), this issue will be particularly critical. In order to avoid the use of expensive tunable lasers at the ONU, several WDM reflective PON architectures have been proposed in the last ten years, whose key idea is to generate the unmodulated upstream wavelengths at the Central Office, and modulate them in reflection at the ONU side. Unfortunately, most of the reflective WDM-PON architectures proposed in literature seem to be completely incompatible with TWDM-PON for a set of different reasons such as: - most of them require the use of Arrayed Waveguide Gratings (AWG) in the ODN, while most telecom operators believe that backward compatibility in PON is a must, and thus the splitter-based architecture should be maintained also in NG-PON2; - the achievable ODN loss is limited to typically 20-25 dB, due to several spurious effects such as the Rayleigh back-scattering and the low received optical power; - most of them use a dedicated wavelength for each ONU, and do not support burst-mode TDMA in the upstream. The main objective of this Ph.D. project is to try to solve all these issues, proposing a self-coherent, burst-mode, reflective PON architecture that completely satisfies the main requirements defined in the TWDM-PON standard. In order to reach this goal, the activity started in early 2012 in the Photonlab of Istituto Superiore Mario Boella (Turin, Italy), and was supported by the joint group composed by Dipartimento di Elettronica of Politecnico di Torino (Turin, Italy) and Cisco Photonics (Monza, Italy) throughout the entire Ph.D. project. At first, a full characterization of the Reflective Semiconductor Optical Amplifier (RSOA) was performed in order to find the best working parameters that provide the best performance if this device is used as reflective optical modulator at the ONU side. Thanks to this optimization, very good results were obtained using the RSOA in a full upstream self-coherent reflective PON setup, working in continuous mode at 1.25 Gbps. In order to overcome the limit of one dedicated wavelength for each user, the emulated ONU was forced to work in burst-mode, replacing the RSOA with the cascade of a Semiconductor Optical Amplifier (SOA) and a Reflective Electro-Absorption Modulator (REAM). The obtained results demonstrate that this solution, combined with a burst-mode self-coherent detection at the Central Office, supports a 2.5 Gbps TDMA upstream transmission, as required by TWDM-PON standard. The proposed architecture was then finalized by adding two important new features, i.e. the upstream bit rate upgrade to 10 Gbps per wavelength and the use of commercial distributed feedback (DFB) lasers rather than External Cavity Lasers (ECL), showing very good performance. To resume, the work performed during this Ph.D. activity demonstrated that, thanks to the burst mode reflective-ONUs and the burst mode, self-coherent detection at the OLT side, the proposed architecture supports the transmission of N simultaneous upstream signals on each wavelength, working at 2.5 Gbps, over up to 35 dB of ODN loss and more than 15 dB of DOPL. Therefore, most G.989 physical layer specifications are satisfied by this proposal, even without the need for a tunable laser at the ONU side, but only a tunable filter locked on OLT-generated optical wavelength combs. If properly integrated, the reflective ONU structure can be less expensive than the TWDM-PON solution in terms of CAPEX, and have a much simpler wavelength control. The only significant increase in cost in the proposed solution is due to the presence of one coherent receiver per each upstream wavelength, but it is shared among all the users with the same wavelength. The authors of the aforementioned writings hope that this work can contribute to the discussion currently taking place in FSAN and ITU-T on future PON generations, particularly for a future use of DWDM with significantly more than 4 wavelengths on a 50 GHz grid, which is a requirement that would make tunable lasers at ONU even more critical. While the need for more than 4 wavelengths seems really far-fetched for residential PONs, it could likely become more handy if mobile front-hauling for Long Term Evolution (LTE) and LTE-advanced will adopt bit rate hungry Common Public Radio Interface (CPRI) approaches, and it will be supported over PON.

Next generation optical access networks towards Gigabit/s per user / Straullu, Stefano. - (2015).

Next generation optical access networks towards Gigabit/s per user

STRAULLU, STEFANO
2015

Abstract

At the time of writing this Ph.D. thesis, FSAN (Full Service Access Network) and ITU-T (International Telecommunication Union – Telecommunication Standardization Bureau) working groups are involved in the final definition of the Recommendation G.989, which has recently established that the Next Generation Passive Optical Network (NG-PON2) architecture will be based on Time- and Wavelength-Division Multiplexed PON (TWDM-PON), and it will consist in its first version on four wavelength-stacked XG-PONs using four wavelengths per direction, likely with a very narrow (for PON) spacing of 100 GHz or even 50 GHz. Furthermore, the Optical Distribution Network (ODN) will be splitter-based and passive, like all previous PON standards, avoiding the use of any wavelength selection/filtering and any optical amplification inside the ODN. Moreover, since complete ODN backward compatibility is required, the TWDM-PON must be compliant with the same previous ODN loss classes, i.e. 33 and 35 dB for the extended E1 and E2 classes, respectively, and up to 15 dB of Differential Optical Path Loss (DOPL). One of the few physical layer problems over which FSAN is still working before the final NG-PON2 Recommendation release, is related to the lack of available low-cost tunable lasers with sufficient tuning range and launch power, to be installed at the Optical Network Unit (ONU) side for the upstream signal generation. Indeed, they should have a precision compatible with a 100 GHz wavelength grid, and be able to tune on (at least) four wavelengths, adopting some protection systems during their switch-on time, in order to avoid interferences with other channels (due to the uncontrolled wavelength transmission at the laser power on); moreover, they should operate on a very wide temperature range and they should have a target price compatible with equipment to be installed at the customer premises. Furthermore, in the longer term, if more than four wavelengths will be used (for instance for WDM overly), this issue will be particularly critical. In order to avoid the use of expensive tunable lasers at the ONU, several WDM reflective PON architectures have been proposed in the last ten years, whose key idea is to generate the unmodulated upstream wavelengths at the Central Office, and modulate them in reflection at the ONU side. Unfortunately, most of the reflective WDM-PON architectures proposed in literature seem to be completely incompatible with TWDM-PON for a set of different reasons such as: - most of them require the use of Arrayed Waveguide Gratings (AWG) in the ODN, while most telecom operators believe that backward compatibility in PON is a must, and thus the splitter-based architecture should be maintained also in NG-PON2; - the achievable ODN loss is limited to typically 20-25 dB, due to several spurious effects such as the Rayleigh back-scattering and the low received optical power; - most of them use a dedicated wavelength for each ONU, and do not support burst-mode TDMA in the upstream. The main objective of this Ph.D. project is to try to solve all these issues, proposing a self-coherent, burst-mode, reflective PON architecture that completely satisfies the main requirements defined in the TWDM-PON standard. In order to reach this goal, the activity started in early 2012 in the Photonlab of Istituto Superiore Mario Boella (Turin, Italy), and was supported by the joint group composed by Dipartimento di Elettronica of Politecnico di Torino (Turin, Italy) and Cisco Photonics (Monza, Italy) throughout the entire Ph.D. project. At first, a full characterization of the Reflective Semiconductor Optical Amplifier (RSOA) was performed in order to find the best working parameters that provide the best performance if this device is used as reflective optical modulator at the ONU side. Thanks to this optimization, very good results were obtained using the RSOA in a full upstream self-coherent reflective PON setup, working in continuous mode at 1.25 Gbps. In order to overcome the limit of one dedicated wavelength for each user, the emulated ONU was forced to work in burst-mode, replacing the RSOA with the cascade of a Semiconductor Optical Amplifier (SOA) and a Reflective Electro-Absorption Modulator (REAM). The obtained results demonstrate that this solution, combined with a burst-mode self-coherent detection at the Central Office, supports a 2.5 Gbps TDMA upstream transmission, as required by TWDM-PON standard. The proposed architecture was then finalized by adding two important new features, i.e. the upstream bit rate upgrade to 10 Gbps per wavelength and the use of commercial distributed feedback (DFB) lasers rather than External Cavity Lasers (ECL), showing very good performance. To resume, the work performed during this Ph.D. activity demonstrated that, thanks to the burst mode reflective-ONUs and the burst mode, self-coherent detection at the OLT side, the proposed architecture supports the transmission of N simultaneous upstream signals on each wavelength, working at 2.5 Gbps, over up to 35 dB of ODN loss and more than 15 dB of DOPL. Therefore, most G.989 physical layer specifications are satisfied by this proposal, even without the need for a tunable laser at the ONU side, but only a tunable filter locked on OLT-generated optical wavelength combs. If properly integrated, the reflective ONU structure can be less expensive than the TWDM-PON solution in terms of CAPEX, and have a much simpler wavelength control. The only significant increase in cost in the proposed solution is due to the presence of one coherent receiver per each upstream wavelength, but it is shared among all the users with the same wavelength. The authors of the aforementioned writings hope that this work can contribute to the discussion currently taking place in FSAN and ITU-T on future PON generations, particularly for a future use of DWDM with significantly more than 4 wavelengths on a 50 GHz grid, which is a requirement that would make tunable lasers at ONU even more critical. While the need for more than 4 wavelengths seems really far-fetched for residential PONs, it could likely become more handy if mobile front-hauling for Long Term Evolution (LTE) and LTE-advanced will adopt bit rate hungry Common Public Radio Interface (CPRI) approaches, and it will be supported over PON.
2015
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11583/2592360
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