PAUL MOMTAHAN COMPACT PLUGGABLES
Figure 2 - Generic Coherent Engine Architecture.
INCREASED TERRESTRIAL FIBRE CAPACITY In addition to incremental spectral efficiency gains, for terrestrial applications next-generation high- performance engines are also evolving to make use of more spectrum. For example, while Infinera’s ICE6 can also leverage the L-band for a total of up to 80+ Tbps per fibre pair, Infinera’s ICE7 with Infinera’s widely tuneable super-C and super-L indium phosphide-based transmit receive optical sub-assemblies (TROSAs) are able to leverage the additional spectrum provided by the Infinera GX optical line system to enable up to 100+ Tbps on a single fibre pair. Longer term, additional spectrum might be provided by also leveraging the S-band. For submarine networks, space- division multiplexing (SDM), which offers more fibre pairs and lower capacity per individual fibre but greater capacity per cable within the same cable power constraints, is becoming the preferred approach to increase capacity. Infinera’s next-generation high-performance engines will build on the industry-leading SDM feature set of ICE6. REDUCED COST, POWER, AND FOOTPRINT Minimising the cost, watts, and rack units per Gbps for a given reach/path requirement is also a high priority for long-haul and submarine applications. To date, the primary way of achieving this has been with higher baud rates. Higher baud rates increase wavelength- capacity-reach by leveraging lower- order modulation to achieve the same data rate. Lower-order modulations benefit from greater Euclidean distance between constellation points, making them easier to distinguish in the presence of noise. Embedded baud rates have evolved from around 30 Gbaud (100 Gbps then 200 Gbps per wavelength) to 90-100 Gbaud (800 Gbps per wavelength). Leveraging 5nm CMOS technology and Infinera’s seventh generation indium phosphide photonic
integrated circuit (PIC) technology, ICE7, with up to 148 Gbaud, enables cost and watts per bit savings of up to 33% relative to ICE6. But how much further can the industry evolve in terms of baud rates? In terms of the DSP, the CMOS industry has a roadmap through 2034 that will enable increased DSP baud rates, with 2 nm DSP ASICs expected in 2026/2027 and a further reduction (1.5 nm or 1.4 nm) in the future. In terms of modulator materials, while silicon photonics is limited to approximately 140 Gbaud, indium phosphide has a path to baud rates well beyond 200 Gbaud. Alternative high baud rate modulators, including those based on thin film lithium niobate and plasmonics, are at various stages of research and development but are currently largely unproven at volume. At industry conferences such as OFC and ECOC, novel material including plasmonic modulators and graphene photodetectors offer a possible road to terabaud (1,000 Gbaud) coherent. And while these baud rates might or might not be technically feasible, the other question for the industry is whether they are economically viable. For example, is it more cost-effective to have fewer, more expensive components or a larger number of less expensive components? REDUCED COST, POWER, AND FOOTPRINT WITH INTEGRATION AND VOLUME Other levers for reducing cost, power, and footprint include photonic integration, digital integration, and volume. Photonic integration, putting more optical components on a single photonic chip, continues to be a valuable approach to reducing cost and footprint. Digital integration, putting more functions into the digital ASIC/DSP, is another approach. In the past, functions such as forward error correction, framing, multiplexing, and encryption have been integrated into the digital ASIC, with additional functional integration an additional option for
cost, power, and footprint reduction in next-generation high-performance engines. One additional approach to reducing cost is to share components with pluggable engines, thus leveraging the much higher pluggables volumes to further drive down cost. MINIMISED OPERATIONAL COSTS An additional priority for many operators is minimising lifecycle operational costs. Maximising wavelength capacity- reach with higher baud rates and other advanced features is one way to do this with fewer wavelengths to install, provision, and manage. Embedded engines are also evolving to provide better monitoring and automation. Along with monitoring the optical engines themselves, this advanced monitoring can extend to the wider optical network including the fibre plant, possibly eliminating the need for monitoring devices such as optical time-domain reflectometers. One current example of environmental monitoring with Infinera’s ICE6 is the ability to leverage state-of-polarisation data to give early earthquake/tsunami detection in subsea cables. A good automation example is self-calibration to maximise the transponder data rate based on performance measurements including the available margin. SUMMARY To summarise, high-performance embedded optical engines have a continuing role to play in long-haul and submarine networks where maximising spectral efficiency and fibre capacity are key priorities alongside minimising the cost, power, and footprint per Gbps per km, followed by reducing lifecycle operational costs. To address these goals, embedded optical engines are evolving, leveraging CMOS and photonic material improvements, with higher baud rates, the ability to leverage more spectrum, and advanced features related to FEC,
nonlinear compensation, PCS, monitoring, and automation.
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ISSUE 34 | Q3 2023
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