Electro-photonic processor chip communicates directly by light

December 24, 2015 // By Graham Prophet
In a letter to Nature, a group of researchers from University of California, Berkeley; Massachusetts Institute of Technology; and University of Colorado, Boulder, report the development of a highly-integrated photonic system comprising an IC that contains a processor, memory, and a large number of optical transceivers on a single die.

The development, described in a preview here, addresses the barriers presented to increasing bandwidths of inter-chip data communications by the physics of electrical signalling. Electrical SerDes interconnects have been extended far beyond what might once have been thought possible, but their limits may be in sight.

Optical solutions have been proposed for decades, but fabricating fully-integrated (monolithic) combinations of optical transceivers and current-generation logic has been challenging. Commercial suppliers such as the FPGA vendor Altera have proposed system-in-package solutions in which optical transceivers might be integrated as a separate die on a silicon substrate, tightly-coupled to logic dice (a.k.a. “2.5D integration”) but no (open-market) product has yet emerged.

One of the issues that has limited such developments is the need to integrate optical emitters on the same process as the dense and fast logic; as well as the very different semiconductor process requirement, there are thermal and power issues to be resolved. The team reporting their results in Nature circumvent (some of) these issues by not having the light generated on the integrated substrate. Externally-generated laser light illuminates the die and data is transmitted by capturing some of that light, modulating it, and coupling the modulated light into a fibre.

The demonstration described verifies the capabilities of the optical link by using two identical chips; each contains a dual-core RISC-V processor core, 850 optical transceivers, and 1 MB of memory – over 70 million transistors in total. The tests described ran code on the core of one IC, but using the memory of the other, connected by optical links. A single laser source, via a power splitter, illuminated the ring-modulator transmitter on each chip, and the modulated light (via an optical amplifier) was routed to a receiver on the alternate chip. On each IC, the processor cores and the memory array each have their own, dedicated, set of electro-optic transceiver sites. The complete IC is 3 x 6 mm in size.