Avago shows multimode's still good for mega-DCs

Daryl Inniss/Ovum
17 Apr 2015

At OFC 2015, Avago Technologies, a leading supplier of optical components based in San Jose, California, and Singapore, demonstrated the use of vertical-cavity surface-emitting lasers (VCSELs) in links longer than 100m at 100G, bucking the trend of the demise of multimode fiber in data centers due to shortening distances as data rates increase.

Avago demonstrated that multimode links are far from dead, even as the data rates go to 100G and beyond. These links use fiber arrays with four transmitting and four receiving 28G. The mega-data center operators like Microsoft are calling for single-mode to be used in data centers to support their increasing size and bandwidth demands. This approach can work for new data centers but is expensive for most existing ones because multimode fiber is used. Increasing bandwidth reduces link lengths, suggesting that 100G and beyond would require new fiber.

Avago demonstrated extended-reach 100G in the QSFP28 form factor. It showed that the transceivers can communicate over 300m in multimode fiber. Moreover, Avago announced shipping over 1 million VCSELs that operate at 28G, suggesting a maturing manufacturing platform and strong market demand for the technology.

Avago also showcased optical links using PAM-4 (pulse-amplitude-modulation) technology. This demonstration is noteworthy because it is the modulation format being considered for 400G. It shows that VCSEL output is sufficiently linear that it can be used at this modulation format to build effective links.

Avago also showed two new transceivers in one demonstration: a QSFP28 transceiver at 128G Fiber Channel interoperating with four SFP+ transceivers at 32G.

Avago has been shipping a 40G QSFP+ transceiver that operates over duplex fiber to Cisco for over a year now. Each fiber has two wavelengths at 20G, one for transmit and one for receive. This solution saves fiber because the standard approach adopted by the IEEE (40G Base-SR4) uses eight fibers, four for transmit and four to receive each at 10G.

Given that the data center installed base of fiber is multimode, cost-effective solutions are needed to use this infrastructure. Avago presented numerous demonstrations illustrating the continued strength of the multimode.

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