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Fujitsu optical transceiver delivers 100Gbit/s CPU communication

Posted: 24 Feb 2015 ?? ?Print Version ?Bookmark and Share

Keywords:optical fibres? optical transceiver circuit? re-timer circuits?

Fujitsu Laboratories Ltd has developed what it claims as the world's first optical transceiver circuit wherein multiple circuit lanes can be laid out in parallel, paving the way for higher-capacity data transmissions between CPUs in future servers and supercomputers.

Re-timer circuits handle the waveform and noise reduction needed for high-density transmitter circuit and high-speed transmission signals. Because individual re-timer circuits are susceptible to coupling from adjacent ones, it has been difficult to lay them out in close proximity to each other. While the transmitter/receiver circuit width can be adjusted down to 0.25mm, the typical pitch of laid out optical fibres, the re-timer circuit required a pitch of at least 0.5mm.

By developing a mathematical model and analytic techniques for the impact of mutual coupling between re-timer circuits, Fujitsu Laboratories succeeded in laying out, for the first time in the world, re-timer circuits with spacing of just 0.25mm, the same distance between optical fibres. This result enables optical transceiver circuits to integrate with high density.

In tests of this technology, transmission bandwidth of 100Gbit/s (25Gbit/s x 4) was confirmed by running four lanes of an optical transceiver circuit with an integrated re-timer circuit and an optical device. Using this optical transceiver circuit with an integrated re-timer circuit and optical device in a 16-lane configuration will enable 400Gbit/s next-generation interconnects, which can be utilised for data transmissions between future servers and the high-volume data transmissions of next-generation supercomputers.

As the performance of servers and supercomputers improves, and as virtual environments advance, the volume of data transmitted between servers and between CPUs is increasing. The capacity of optical interconnects between servers and between CPUs is also increasing, but because there are limits to the increases in speed that can be achieved using a single channel, there is a need for high density optical transceiver circuits that connect multiple channels.

Technological issues

To increase transmission capacities, both higher speeds and higher densities are needed, but at higher speeds, noise is produced at the connection between the CPU and optical interconnect, leading to waveform degradation. Because noise that inhibits high-speed operation generates a phenomenon known as "jitter," which represents variance in the waveform in the time domain, the timing will be off when processing the signals (Figure 1).

 Noise in the time domain

Figure 1: Noise due to jitter requires circuits that correct waveform degradation in the time domain.

To fix this, there is a need for overall high-density circuit technology that includes circuits that correct waveform degradation in the time domain. At present, while there are single integrated circuits that can drive multiple channels of optical devices, the re-timer circuits that correct the waveform in the time domain have their own high-speed oscillators, in which there are embedded coils. Because these produce mutual coupling, the re-timer circuits needed to be spaced at a distance of at least 0.5mm from each other. This coupling has proven to be a significant obstacle to reducing sizes when attempting to integrate circuits for multiple channels.

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