As global networks enter a new phase driven by cloud computing, AI workloads, and digital transformation, optical transport networks are facing unprecedented pressure on capacity, speed, and scalability.

At the China Optical Network Symposium, Wei Leping—Executive Deputy Director of the MIIT Communications Science and Technology Committee—outlined six major upgrade paths for Optical Transport Networks (OTN). These paths provide a structured framework for how backbone and metro networks can evolve over the next 10–20 years.

1. Bandwidth Expansion: Unlocking More Capacity per Fiber

Traditional optical systems are primarily confined to the C-band (≈4 THz), supporting roughly 80 wavelengths at 100G, or about 8 Tb/s per fiber. Recent advances have pushed spectrum utilization further:

• C+ band (4.8 THz):~96 × 100G → ~20% capacity increase.
• Extended C+ (6 THz):~120 × 100G → ~12 Tb/s (+50%).
• C++L band (≈11–12 THz):Up to 240 wavelengths → ~16 Tb/s (~200% increase).
• Long term:Further extension into S and U bands could theoretically increase capacity by nearly 5×, though this requires new optical amplifiers (TDFAs) and advanced power balancing.

2. Higher Line Rates: The Road to 1.6T

While 100G remains the dominant long-haul rate, the industry is firmly entering the 400G and 800G era, with clear milestones for the next generation.

• 400G/800G:Standardized for DCI (Data Center Interconnect) and long-haul backbones.
• The 1.2T Sweet Spot:Currently, 1.2T serves as a critical “performance ceiling” for high-end coherent engines (like Nokia’s PSE-6s or Ciena’s WaveLogic 6). It is the highest stable rate for regional and long-haul distances (up to 1,000km+) before reaching the physical limits of 800G/1.6T systems.
• 6T and Beyond:1.6T is the target for 2026+ deployments, primarily for point-to-point metro-DCI, enabled by symbol rates exceeding 200 GBaud.

3. Spectral Efficiency: Approaching Physical Limits

Beyond 100G, gains in spectral efficiency are increasingly constrained by the Shannon Limit.

• Baud Rate over Modulation:The industry is shifting toward higher baud rates rather than higher-order QAM (like 256QAM), which drastically reduces reach.
• Enabling Tech:Key enablers include advanced optical DSPs (oDSP), Probabilistic Constellation Shaping (PCS), and next-generation Forward Error Correction (FA-FEC).

4. Extended Reach: Media and Margin Optimization

Transmission distance is governed by the Signal-to-Noise Ratio (SNR).

• 654.E Fiber:Deploying ultra-low-loss, large-effective-area fiber (ITU-T G.654.E) can extend reach by up to 80% compared to legacy G.652.D fiber.
• AI-Assisted Margin:Real-time AI optimization is being used to reclaim “hidden” system margins, allowing for higher data rates over existing distances.

5. Multi-Core Fiber (MCF): High Potential, High Complexity

Multi-core fiber can theoretically increase capacity by 3–37×. However, it faces major real-world obstacles:

• Technical Barriers:Core-to-core crosstalk and the need for a completely new ecosystem of amplifiers, transceivers, and connectors.
• Primary Application:Currently suitable only for space-constrained submarine systems or specialized AI cluster fabrics.

6. High-Count Fiber Cables: The Most Practical SDM Solution

Space Division Multiplexing (SDM) via high-count cables is widely regarded as the most realistic long-term strategy.

• Scalability:High-count cables (864 to 1728+ fibers) paired with integrated amplifier arrays can support decades of growth.
• Economics:Fiber itself accounts for <5% of total system cost in modern networks; leveraging “more fiber” is often more cost-effective than deploying “more complex” modulation.

References & Industry Standards

• ITU-T G.694.1:Spectral grids for WDM applications (C-band and L-band definitions).
• ITU-T G.654:Characteristics of cut-off shifted single-mode optical fiber and cable (Category E for terrestrial long-haul).
• OIF-400ZR/800ZR:Implementation Agreements for Coherent Pluggable Optics.
• IEEE 802.3dj:Ongoing standard development for 200 Gbps, 400 Gbps, 800 Gbps, and 1.6 Tbps Ethernet.
• IOWN Global Forum:White Paper on Ultra Wideband Optical Transmission (UWOT) for C+L+S band evolution.