Minimizing the optics footprint could mean routing fiber on the shortest path – consistent with the fiber properties – between the optical tile and its associated faceplate connector, but this would lead to at least eight different cable lengths for a 51.2 Tbps switch with 16 optical tiles and mirror symmetry. This proliferation of parts might be undesirable from a manufacturing point of view. If a reduced set of cable lengths were to be used, then the “constant length” routing would have to accommodate excess cable in some paths.

Image Courtesy of LightWave 

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With space inside the switch box at a premium, the risk of mechanical interference with other components should be reduced as much as possible. When building switch boxes containing hundreds of fibers, it will be essential to have them deployed predictably while minimizing trouble spots like crossings and avoiding issues such as cable buckling.

This management goal will be greatly facilitated by using tightly bent fiber to follow short paths between the faceplate and chip. With typical telecommunications-grade single-mode fiber, too much light may be lost at these bends, but we can mitigate this by using bend-insensitive fiber designs.  However, in using such designs, care will be required to control multipath interference (MPI).

Power can be coupled and propagated in more than one fiber mode at each optical interface in the switch box (e.g., connectors, FAUs). Given the short fiber lengths likely to be used in CPO, power in higher-order modes (HOMs) will not be extinguished before the following interface, where the multiple modes will interfere with each other – the phenomenon known as MPI – ultimately causing wavelength-dependent power at the detector. At some wavelengths, that power reduction could be up to twice in decibels, arising from the independent losses at each interface.

Thus, unmitigated MPI could complicate some benefits of using fiber with low bend-loss. For these systems, a bend-insensitive fiber that also suppresses MPI in very short fiber lengths would need to be designed. One potential approach is to reduce the fiber cut-off wavelength to increase HOM loss substantially.

Even if MPI is reduced to insignificance, the coupling losses at those interfaces matter, too. The redesigned bend-insensitive fiber must maintain low coupling loss to Corning® SMF-28® Ultra or other fiber used in the data center to connect switches. This imposes constraints on the mode-field diameter of the CPO signal fiber.

To permit practical, low-cost provisioning of the switch box optical cables, the fiber management approach must include some means to accommodate length variations introduced by the cable manufacturing process. One strategy is to tie down the cable at points along its path and allow it to take a relatively unconstrained path between these tie-down points. The smaller the radius of curvature of that path, the less a bundle of cables will spread out for a given length variation.

An alternative is to provide specific accumulator structures to contain excess cable length. To keep such structures unobtrusive, the fiber should tolerate deployment in very tight loops, as small as 10-mm in diameter, retaining its low-bend loss and high reliability. These attributes are required of fiber that lends itself to “shortest path” and “constant length” routing.

Conclusions:

CPO will soon be a reality that relies on a system of complex, interconnected components working well together. For optimum overall performance, these components must be designed with the specific requirements of CPO in mind, which for the optical subsystem include efficient and unobtrusive deployment within a crowded switch box, low power losses, absence of MPI impairments, and good reliability. Some CPO realizations also need optical polarization state control.

The familiar fiber and connectivity products, while having impressive attributes, are not optimum for the CPO application, and there is great scope for enhancing the performance of the optics by moving beyond default solutions to those specifically designed for the role.