The commercialization of co-packaged optics (CPO) has been long anticipated but is becoming increasingly desirable as data needs accelerate. Co-Packaged Optics are an advanced heterogeneous integration of optics and silicon on a single packaged substrate aimed at addressing next generation bandwidth and power challenges.
As the bandwidth of data center switches increases, a disproportionate amount of power is becoming dedicated to the switch – optics interface. Reducing the physical separation between these two components by co-packaging enables system power savings which is essential to continued bandwidth scaling.
CPO brings together a wide range of expertise in fiber optics, digital signal processing (DSP), switch ASICs, and state-of-the-art packaging and test to provide disruptive system value for the data center and cloud infrastructure.
The companies and institutions working on CPO have made great strides in developing suitable electronic components. But hundreds of meters of fiber will be packed into the switch box for the first time, and faceplate connections will have unprecedented densities. As a result, the design and development of optical system solutions will also be critical elements in the success of CPO. Optical components with performance tailored to the CPO application and effective solutions for managing the fiber in the switch box are vital in optimizing the complete optical system. Three aspects of CPO deployment, in particular, hinge on the properties of the fiber and the optical interfaces: optical power loss, the trade-off between minimizing bend loss and controlling for MPI and maintaining the polarization state if external lasers are used.
Image Courtesy of Broadcom
Data centers face substantial challenges as they scale, particularly in reducing power dissipation and cost per bit. CPO will play a significant role in helping to meet those challenges. In today’s data center switches, external fiber optic connections that carry data terminate on pluggable transceivers on the housing faceplate. The optical data stream is coupled to the electrical signals at that interface.
With a CPO realization of a 51.2 Tbps switch, the substrate connects a central regulator ASIC to 16 optoelectronic (O/E) tiles on the substrate perimeter. These tiles are connected to optical fiber signal cables that run to the switch box faceplate and receive power from external lasers that they modulate to produce the outgoing optical signal stream.
They communicate between the transceiver and the switch application-specific integrated circuit (ASIC) via copper traces on printed circuit boards. Under the CPO paradigm, as the optoelectronic conversion is pushed back from the faceplate to the switch substrate, long electrical traces are replaced with virtually loss-free optical fiber.
With CPO, the fiber path continues past a connector at the faceplate and into the switch box, ending at photonic integrated circuits (PICs) on optical tiles attached to the switch substrate. This shift presents the novel challenge of routing and connecting hundreds of optical fibers within a compact and crowded space, creating a need to minimize the footprint of the optics while still achieving performance and reliability targets.
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.