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Next Generation Optical Broadband Architectures and Components

Optical Broadband Working Group

Center for Integrated Photonic Systems

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Next Generation Optical Broadband Architectures and Components

OBWG White Paper July, 2006

Working Group Members MIT Randy Kirchain Fred Leonberger Rajeev Ram Richard Roth Tom Rand-Nash

BT Ivan Boyd Russell Davey Dave Payne Dave Faulkner Ranulf Scarbrough

Motorola Dan Grossman Tim Burke Ruo Li

JDSU Ed Murphy

Nokia Markku Tahkokorpi

Telecom Italia Paolo Solina Marco De Bartoli

Independent Paul Green

Additional Contributors Corning Bob Whitman

Novera Byong Yoon Kim

Ovum Karen Lui

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Executive Summary

This white paper introduces the key challenges facing hardware for next-generation optical broadband access systems, an area also referred to as FTTx - fiber to the “x”, where x might mean “home”, “premises”, “curb”, “node” or, in general, any situation where there is at most one optical/electronic conversion between central office and subscriber premises. The emphasis is on advanced network architectures and the component performance required to support them. For each architecture, the status of key components is analyzed, and important areas for future investigation are recommended. Next-generation access networks must provide improved performance (ideally including the capability of delivering gigabit service rates) while minimizing the operational and capital costs associated with providing that service. They must also be coordinated with management, operations, and economic cost trends currently being experienced by communication providers. This white paper considers two related architectures, the Long-reach Passive Optical Network (LR-PON) and the WDM-PON, together with the associated hardware challenges. The LR-PON component of this study is being driven by the pressures on the world’s carriers to exploit the low attenuation and high capacity offered by optical fiber in order to mitigate the operational expenditures tied to backhaul and the maintenance of many central offices. Wavelength division passive optical networks (WDM-PONs), which are distinct from the purely time-domain multiplexing (TDM) and time-domain multiple access employed in today’s PON offer the prospect of not only providing headroom for growth (due to fiber’s enormous capacity if WDM is used), but also the avoidance of the system loss in TDM PON due to the required power splitters. The baseline architectures that these next-generation directions will be compared to are the GPON standard defined by FSAN, and the Ethernet-based EPON defined by IEEE 802.3, and possible extensions of those standards. LR-PON is designed to lower the cost per customer by focusing on the termination of the access traffic deeper into the metropolitan area transport network. This includes eliminating the capital cost associated with termination equipment as well as reducing the operating cost for central offices by replacing many of them with amplifier/splitter cabinets. In the proposed LRPON, the goal is to provide 10 Gbps over 100 km with up to a 1000-way multistage split. Clearly, large splits are required to serve the greater number of customers accessible by the longer reach. For a TDM LR-PON, the larger split also requires an increase in the bit rates. To date, there are no field trials of LR-PON networks, but there have been laboratory demonstrations of their technical feasibility. The most important component innovations that could empower LR-PON are:

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(1) Electronics and photonics to support the wider dynamic range (due to longer reach) at10 Gbps. This innovation could be development of 10 Gbps burst-mode electronics in the OLT receiver (for automatic gain control) oroptical power leveling (such as integrated variable optical attenuators). The entire burst-mode part of the PON architecture will need to be reexamined quantitatively, since the increases of both the travel time and number of subscribers to be addressed per PON will be much larger for LR-PON than is the case for today’s PONs.

(2) Amplification to compensate for the large splitting ratio. This could be low-cost, burst mode-compatible EDFA technology which, since it would utilize the 1550 nm band currently used for video overlay, would require execution of existing plans for transition from video overlay to IPTV. An alternative technology that would be compatible with analog video overlay is the development of lownoise, high-power semiconductor optical amplifier (SOA) technology at 1300 nm and 1490 nm.

WDM-PON is designed to lower cost by increasing the aggregate capacity of the fiber distribution plant with better overall link power budgets than traditional TDM at the same service rate. Today’s TDM-based PONs incur a link power- budget penalty due to the passive splitters that dominates other losses. WDM-PON allows, at least in the laboratory, sustained service rates as high as 1 Gb/s per user employing existing components. The key challenges are in managing the inventory of ONUs for the various receive wavelengths and the lack of bandwidth sharing, which could be a significant barrier to efficient utilization of the network given the bursty nature of access traffic. On the other hand, considering the enormous bandwidth available together with the possible need to service different protocols on different wavelengths, it is offers a potentially attractive alternative to the time-slicing approaches of today’s PONs. Good progress has been made in developing fieldable and affordable WDM-PON systems, and successful trials at 100 Mbps have been completed in Korea. The most important component innovations that could further empower WDM-PON are:

(1) Cost-effective colorless ONUs that would eliminate the inventory management problem. The existing approaches include injection locking and reflective SOAs. The SNR ratio of both approaches must be proven at 1 Gb/s service rates over distances greater than 20 km.

(2) Low-cost athermal arrayed waveguide gratings (AWG) in the distribution plant. The cost of AWGs today is approximately $50/port - this is three times the cost of a power splitter.

The number of users supported per feeder fiber must be dramatically increased in order for the per customer cost of transport and backhaul to be competitive.

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This can be either through dramatic increases in the number of wavelengths per fiber or through hybrid WDM/TDM. Based on the analyses of WDM and LR PON systems in this white paper, two areas for future investigation are recommended:

• As presently understood, the above architectures explore two orthogonal approaches to increasing the performance and reducing the cost of access networks. From the preceding, it is clear that hybrid systems employing a combination of WDM and advanced TDM have the potential for addressing all perceivable performance and cost goals, while mitigating some of the risk associated with unproven component technology. By focusing on the above two architectures we hope to initiate a discussion on the optimal mix of WDM and TDM. Hybrid architectures also exploit the enhanced power budget of WDM systems while addressing the network cost savings of LRPON systems. Related issues should be addressed such as realizable strategies to transition from GPON to next-generation PONs, and methods of comparing system approaches and estimating future system costs.

• The second area follows from the first and is component oriented: to

investigate in depth the key devices highlighted in this paper so as to develop a detailed understanding of their present limitations and possible approaches to overcome these limitations, and to accelerate their network deployment. For both WDM-PON and LR-PON the long-term potential of SOA technology might be critical. Unsurprisingly, LR-PON places greater pressure on electronics (e.g., 10 Gb/s burst-mode receivers) whereas WDM-PON demands low-cost colorless ONUs and higher performance passive optical components (low cost, high port count athermal AWGs).