Currently, most high-speed digital optical transport is carried over 10 Gb/s waves using a pair of fibers (TX/RX). Using 160 x 10G DWDM lambdas with double density spacing at 25 GHz, a fiber pair can transport 1.6 Tb/s or half that if 50 GHz spacing is used. Growth occurs in 10G increments by adding new transponders as more bandwidth is required.

This linear approach to bandwidth growth, while workable in the short term, does not fundamentally lower the cost per bit for transport over time, nor does it improve scalability by making more efficient use of existing resources (e.g., fiber) or reducing resource consumption (e.g., space requirements). As optical transport bandwidth continues to grow at a compounded 50 percent to 100 percent annually, network growth cannot continue to scale on a linear basis simply by adding more optical transponders. Aside from equipment costs, the required space, power, cooling and optical couplings rapidly become unmanageable.

The cable industry is beginning to migrate to 100G core optical transport waves that greatly improve fiber utilization while lowering transponder count for equivalent bandwidth. However, transporting 100G waves requires complex optical modulation to preserve performance and increase spectral efficiency. Complex modulation requires several additional discrete optical components per lambda, so the migration to 100G waves alone does not fully address the scalability issues of increasing cost, power, space and heat as bandwidth requirements continue to grow.

Optical networks rapidly are approaching the point where continual scaling of higher bit-rate optical lambdas using discrete optical components is reaching its limits. Photonic integration, which combines multiple optical components on a single IC, can efficiently support complex modulation schemes while decreasing component counts. By reducing hardware, power, space, cooling and reliability costs, photonic integration provides a scalable path for future growth.

Photonic Integration

Fortunately, technology is available to mitigate the increased complexity and component counts required for higher-order modulation. Modern photonic integration allows multiple optical subsystems to be manufactured monolithically on an Indium Phosphide (InP) chip using large-scale integration. It is possible today to put all the optical components necessary to support multiple lambdas, including mux/demux functions, on a single photonic integrated circuit (PIC) and address integration at the system level rather than at the component level.

Moore’s Law indicates significant improvements are yet possible in future generations of PICs, and 10 x 100G terabit PICs already have been produced and have been tested successfully in the lab. Current models predict PIC bandwidth will double about every three years, keeping pace with bandwidth growth while lowering the overall cost per bit.

In the future, it will make more sense to move beyond the current ITU channel plan and implement multi-carrier super-channels that eliminate the dead zones within the super-channel while preserving a guard band at the edges of each super-channel for filtering purposes. Multiple sub-rate carriers can be implemented readily within the super-channel using PIC technology, which also allows multiple modulation formats and flexible channel spacing to be supported and provisioned in software. Such DWDM systems should enable transport capacities of as much as 25 Tb/s per fiber, well-beyond the capacity of today’s systems.

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