Introduction
The exponential growth in Internet usage, driven by the proliferation of bandwidth-intensive applications and the deployment of 5G and upcoming 6G mobile networks, has created an unprecedented demand for high-speed, low-latency, and massive-capacity optical access networks. To meet these requirements, the International Telecommunication Union (ITU-T) and the scientific community are actively working on specifications for next-generation optical access systems, including 100 Gb/s single-wavelength point-to-point (PtP) solutions and the Very High-Speed Passive Optical Network (VHSP), the successor to the 50 Gigabit-capable Passive Optical Network (50G-PON).
One of the key challenges in deploying these next-generation systems is the need to coexist with existing deployed infrastructure, particularly optical fibers, which represent the majority of the cost in optical access networks. This coexistence requires not only meeting stringent optical path loss (OPL) requirements but also ensuring compatibility with other technologies through careful wavelength planning.
This tutorial article presents a groundbreaking demonstration of the coexistence between a prototype 200 Gb/s symmetrical coherent point-to-multipoint (PtMP) system and a 50G-PON prototype over 20 km of standard single-mode fiber (SSMF). We will explore the design and performance of the coherent PtMP system, its ability to meet the demanding OPL requirements of optical access networks, and its successful coexistence with the 50G-PON prototype.
Coherent Point-to-Multipoint System Design
The coherent PtMP system consists of a "Hub" at the central office side and two "Leaves" at the client side, exploiting Frequency Division Multiplexing (FDM). Sixteen Sub-Carriers (SCs) transport 4 GBaud signals using 16-Quadrature Amplitude Modulation (QAM) on both polarization states, enabling a gross rate of 32 Gb/s per SC and a net rate of 25 Gb/s per SC after forward error correction.
The 16 SCs can be dynamically allocated between the Hub and Leaves, allowing for flexible bandwidth allocation. In this demonstration, four SCs per Leaf were allocated in both downstream (DS) and upstream (US), providing a global bitrate of 200 Gb/s in both directions (100 Gb/s per Leaf).
The Hub acts as a wavelength reference, initially set to 1544 nm, and the Leaves follow any wavelength drift, relaxing the constraints when demodulating the signal. This system can operate over the entire C-band, enabling coexistence with other technologies operating in different wavelength bands.
Experimental Setup
The experimental setup, depicted in Figure 1, consists of the 50G-PON Optical Line Termination (OLT) prototype, the coherent PtMP system, and a coexistence element (CEx) that combines the signals from both systems before propagating them through 20 km of SSMF and a variable optical attenuator (VOA).
The 50G-PON OLT operates at 50 Gb/s and 1342.7 nm in the downstream (DS), while the upstream (US) bitrate is 25 Gb/s at 1298.7 nm. A semiconductor optical amplifier (SOA) and fixed attenuators are used to optimize the signal levels and meet the OPL requirements.
The coherent PtMP system's Hub and Leaves are connected through an erbium-doped fiber amplifier (EDFA) in each direction, with a circulator separating and recombining the US and DS signals.
Results and Discussion
The performance of the coherent PtMP system was first assessed without coexisting with the 50G-PON. Figure 2 shows the percentage of received frames versus received power for a 20 km reach in both directions. The DS sensitivity is -25.3 dBm, while the US sensitivities of Leaves 1 and 2 are -23.1 dBm and -24.2 dBm, respectively.
Table I summarizes the measured OPLs for the coherent PtMP system in different configurations. In the DS, the OPL exceeds 39.1 dB in all three configurations, surpassing the upper limit of the PON class E2 ODN (35 dB). In the US, Leaf 2 meets the N1 class (29 dB) upper limit in back-to-back (BtB) operation.
| Hub to Leaf | Leafl to Hub | Lean to Hub |
TX Power at 1544 nm (dBm) | 13.8 | 1.2 | 2 |
OPL BtB at 1544 nm (dB) | 40.7 | 30.6 | 31.4 |
OPL 20 km at 1544 nm (dB) | 39.1 | 26.1 | 28 |
OPL 40 km at 1544 nm (dB) | 39.6 | 25.6 | 28 |
OPL 20 km at 1544 nm w/ 50G-PON (dB) | 39.1 | 26.1 | 28 |
OPL BtB at 1567 nm (dB) | 37.1 | 31.4 | / |
OPL BtB at 1528 nm (dB) | 41.6 | / | 29.3 |
Next, the coexistence of the 50G-PON system with the coherent PtMP system was evaluated. Figure 3 shows the ability of the 50G-PON ONU to remain connected to the OLT while the OPLs increase, using the Loss Of Signal alarm. No significant difference was observed in the 50G-PON performance when coexisting with the coherent PtMP system, demonstrating their compatibility.
Furthermore, the tunability of the coherent PtMP system over the entire C-band, from 1567 nm to 1528 nm, was assessed. Table I shows that the OPL variation remains within 4.5 dB from Hub to Leaf and 2.1 dB from Leaves to Hub in BtB operation, demonstrating the system's ability to ease coexistence with already deployed technologies by tuning its carrier over 39 nm while meeting at least 37.1 dB OPL in the DS and 29.3 dB in the US. Figure 4 illustrates the ITU-T wavelength plan for various PtMP optical systems, highlighting the wavelength separation between the coherent PtMP system operating in the C-band and the 50G-PON operating in the O-band.
In addition to meeting high OPLs, optical access networks require support for a dynamic range of up to 15 dB between the highest and lowest OPLs on the same Optical Distribution Network (ODN). Figure 5 shows that the coherent PtMP system can sustain a dynamic range of up to 10.0 dB by unbalancing the OPLs between Leaves 1 and 2.
Finally, to showcase the full performance capabilities of the coherent PtMP Hub, the Hub's emitter was connected to its own receiver without optical amplification, transmitting 400 Gb/s using all 16 SCs in the same direction. Table II summarizes the results, including the transmitted and received powers, OPL, latency, and jitter for both BtB and 20 km transmission reaches.
Table II. ”HUB-TO-HUB” 400GB/S PERFORMANCES
| 0 km | 20 km |
Transmitted power (dBm) | 3.1 | 3.1 |
Sensitivity (dBm) | -20.5 | -20.5 |
OPL (dB) | 23.6 | 23.6 |
Latency (µs) | 30.9 | 131 |
Jitter typ. / max. (µs) | 0.001 / 0.040 | 0.001 / 0.033 |
The Hub-to-Hub transmission achieved an OPL of 23.6 dB for both BtB and 20 km reaches, with a BtB latency of 30.9 μs (similar to commercially available products) and a maximum jitter of 0.040 μs in both cases.
Conclusion
This tutorial article has presented a groundbreaking demonstration of the coexistence between a prototype 200 Gb/s symmetrical coherent PtMP system and a 50G-PON prototype over 20 km of SSMF. The coherent PtMP system achieved OPLs exceeding 30.6 dB in both directions at 1544 nm, meeting the N1 OPL class requirements and demonstrating its ability to support the demanding requirements of optical access networks.
The successful coexistence between the two systems, enabled by careful wavelength planning, paves the way for the smooth migration and infrastructure sharing of next-generation optical access networks. The coherent PtMP system's flexibility was further showcased by its ability to tune its carrier over the entire C-band, its support for imbalanced OPLs of up to 10.0 dB, and its reconfigurability to transmit a maximum of 400 Gb/s in a single direction.
This demonstration represents a significant milestone in the evolution of optical access networks, addressing the challenges of meeting the ever-increasing bandwidth demands while leveraging existing infrastructure investments. The successful coexistence of the 200 Gb/s coherent PtMP system with 50G-PON highlights the potential of coherent technology to enable seamless upgrades and coexistence with legacy systems in next-generation optical access networks.
Key Advantages and Implications
The demonstrated 200 Gb/s coherent PtMP system offers several key advantages and implications for next-generation optical access networks:
High-Capacity and Symmetrical Bandwidth: The ability to provide 200 Gb/s of symmetrical bandwidth in both downstream and upstream directions addresses the growing demand for high-speed, low-latency connectivity driven by emerging applications and technologies such as 5G/6G, cloud computing, and virtual/augmented reality.
Flexible Bandwidth Allocation: The dynamic allocation of subcarriers between the Hub and Leaves enables flexible bandwidth allocation, allowing for efficient resource utilization and the ability to adapt to changing traffic patterns and user demands.
Coexistence and Smooth Migration: The successful coexistence with the 50G-PON prototype, enabled by careful wavelength planning, paves the way for a smooth migration path from existing infrastructure to next-generation optical access networks, minimizing the need for costly fiber deployment and maximizing the return on investment.
Extended Reach and High OPL Support: The demonstrated ability to meet and exceed the stringent OPL requirements of optical access networks, including the N1 OPL class, enables extended reach up to 20 km while supporting a high dynamic range between the highest and lowest OPLs on the same Optical Distribution Network (ODN).
Tunability and Wavelength Flexibility: The coherent PtMP system's ability to tune its carrier over the entire C-band and operate in different wavelength regions facilitates coexistence with other deployed technologies, enabling efficient use of the available spectrum and minimizing potential interference.
Scalability and Future-Proofing: The coherent PtMP system's architecture, based on digital subcarrier multiplexing and advanced digital signal processing, provides a scalable and future-proof platform that can accommodate increasing bandwidth demands and evolving modulation formats and coding techniques.
Challenges and Future Perspectives
While the demonstrated coexistence of the 200 Gb/s coherent PtMP system with 50G-PON represents a significant step forward, several challenges and future perspectives should be considered:
Cost-Effectiveness: To enable widespread deployment in optical access networks, the coherent PtMP system's cost must be optimized through continued innovation in component integration, manufacturing processes, and system-level design.
Power Consumption: As with any high-speed optical system, power consumption is a crucial factor, particularly in access networks where a large number of network elements are deployed. Ongoing efforts to improve energy efficiency through optimized architectures, low-power signal processing, and advanced cooling solutions are essential.
Standardization and Interoperability: While the demonstrated system is a prototype, ongoing standardization efforts within the ITU-T and other relevant bodies are critical to ensuring interoperability and fostering a healthy ecosystem of vendors and solutions.
Integration with Software-Defined Networking (SDN) and Network Virtualization: The integration of the coherent PtMP system with SDN and network virtualization technologies will enable efficient resource allocation, dynamic bandwidth provisioning, and support for emerging use cases such as network slicing for 5G and beyond.
Hybrid Access Networks: The demonstrated coexistence with 50G-PON represents a significant step, but future research should explore the coexistence and integration with other access technologies, such as wireless and free-space optical communication, to create truly converged and flexible hybrid access networks.
Advanced Modulation and Coding: While the current demonstration utilized 16-QAM modulation, the exploration of higher-order modulation formats, advanced coding techniques, and
multi-carrier transmission schemes can further increase the spectral efficiency and capacity of the coherent PtMP system.
Operational Considerations: As with any new technology deployment, operational aspects such as network planning, monitoring, and maintenance must be carefully considered to ensure seamless integration into existing infrastructure and operational workflows.
By addressing these challenges and continuing to push the boundaries of innovation, the coherent PtMP technology demonstrated in this tutorial article can play a pivotal role in enabling the next generation of high-speed, flexible, and future-proof optical access networks, meeting the ever-growing demands of our increasingly connected world.
Reference
[2] G. Simon, F. Saliou, A. Afonso, C. Castro, A. Napoli, M. Bouzayani, F. Bonnafous, J. Potet, and P. Chanclou, "200 Gb/s Coherent Point-to-Multipoint Coexistence with 50G-PON for Next-Generation Optical Access," IEEE Photonics Technology Letters, Jan. 2024.
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