Data center operators believe that CPO technology holds great promise

To keep up with ever-increasing speed and capacity demands, datacenter operators want to introduce Co-Packaged Optics (CPO).

What fiber and connectivity innovations are required to bring CPO to market with optics in the switches?

The arrival of co-packaged optics (CPO) has long been anticipated, but it is becoming increasingly desirable as data demands increase for the past years. Data centers face significant challenges as they scale and upgrade, particularly in terms of reducing power dissipation and cost per bit. CPO will play an important role in addressing these challenges.

Companies and institutions working on CPO have made significant progress in designing appropriate electrical fiber optic components components. However, for the first time, hundreds of meters of fiber will be jammed into the switch box, and patch panel connections density will sharply increase. As a result, the development of optical system solutions can be vital to CPO's success.

Active optical components tailored to the CPO application, as well as fiber management solutions in the switches, are critical in optimizing the entire optical system. Three aspects of CPO deployment are particularly dependent on fiber and optical interface properties: optical power loss, the balance between minimizing bend loss and controlling for MPI, and maintaining polarization state.

Fiber Layout – Bend Loss, MPI, and Reliability

With limited space inside the switch box, mechanical interference with other components should be avoided to the greatest extent feasible. When designing switch boxes with hundreds of fibers, it is critical that they be deployed consistently while limiting difficulty locations like crossings and avoiding difficulties like cable buckling.

Using tightly bent fiber to follow small pathways between the faceplate and chip will substantially aid in this management aim. Too much light may be lost at these bends with standard telecommunications-grade single-mode fiber, but we may alleviate this by adopting bend-insensitive fiber designs.

However, similar systems must be used with caution to avoid multipath interference (MPI).

At each optical interface in the switch box, power can be coupled and transmitted in more than one fiber mode. Because of the short fiber lengths commonly utilized in CPO, power in higher-order modes will not be extinguished before the next interface, when many modes will interfere with each other - a process called as MPI - resulting in wavelength-dependent power reaching the detector. Because of the individual losses at each interface, the power drop at some wavelengths might be up to twice as much in dB.

As a result, unabated MPI may complicate some of the advantages of employing low-bend-loss fiber. A bend-insensitive fiber that also reduces MPI in extremely small fiber lengths would need to be devised for these systems. One possible solution is to shorten the fiber cut-off wavelength in order to significantly increase HOM loss.

Even if MPI is rendered insignificant, the coupling losses at certain interfaces are significant. The revised bend-insensitive fiber must have a low coupling loss to Corning® SMF-28® Ultra or other fiber used to link switches in the data center. The mode-field diameter of the CPO signal cable is therefore constrained.

The fiber management strategy must include some way to handle length differences produced by the cable manufacturing process in order to allow for effective, low-cost provisioning of switch box optical connections. One approach is to tie down the cable at various locations along its course and allow it to travel in an unrestricted manner between these tie-down sites. For a given length variation, the lower the radius of curvature of that path, the less a bundle of wires will spread out.

As an alternative, dedicated accumulator structures can be provided to hold extra wire length. To keep such systems as inconspicuous as possible, the fiber should be able to be deployed in very tight loops as tiny as 10-mm in diameter while maintaining its low bend loss and good dependability. These characteristics are required of fiber that allows for "shortest path" and "constant length" routing.

Switch integrators may achieve a dependable, clean construction by utilizing optical cable management gear and layouts that adhere to the design standards offered by CPO-optimized fiber.

Polarization-maintaining Fiber Solution

External laser light is delivered to the PIC along the polarization-maintaining fiber (PMF) in some CPO implementations to maintain the polarization extinction ratio (PER - a measure of polarization state purity) and align it to the right polarization mode of the PIC waveguides.

Managing insertion loss (IL) from the laser source to the detector via PMF and signal fibers is required to meet the power budget for CPO connections. Because the PMF should ideally follow the same design principles as the signal fiber, a bend-insensitive PMF is also preferred.

In addition to the restrictions shared by signal fibers, the PMF must resist PER deterioration even when firmly bent. The PMF fiber should be ribbonized to retain its orientation throughout deployment in the switch box and control of the polarization state from the laser to the O/E chip.

The PMF will also transport large optical powers, maybe hundreds of milliwatts, therefore the reliability issues associated with leaking considerable power into connections or coatings must be considered.

Summary

CPO will soon be a reality, relying on a complicated, linked system that works well together. These components must be built with the unique needs of CPO in mind for maximum overall performance, which for the optical subsystem include efficient and unobtrusive deployment within a congested switch box, minimal power losses, lack of MPI impairments, and excellent reliability. Some CPO realizations require optical polarization state control as well.

While traditional fiber and connection technologies have excellent qualities, they are not optimal for the CPO application, and there is significant opportunity to improve optics performance by going beyond default solutions to ones specially intended for the role.

T&S offers fiber optic printed circuits to system integrators and equipment suppliers, helping them carry out complicate optical cross in CPO applications.

T&S fiber optic printed circuit features automatic fiber laying and automatic fiber path design based on a T&S patented software, which helps the circuit differentiate from its competitor in both optical performance and appearance. Visit www.china-tscom.com to learn more.