The researchers say that 99% of links in datacentres have less than 10% utilisation. This means only 10% of the links’ work time is actually used for transmitting user data. The rest of the time is wasted by sending idle data packets. The rapid power-on/off feature is said to allow the links to be powered off during idle time and powered back on when the data is ready to be transmitted, enhancing link utilisation and reduce energy consumption on a chip or in an optical interconnect system. The researchers explain that, unlike many commercial optical transceivers that are always powered on regardless of transmission activity, power would only be used when data packets are transmitted through the optical link.
The design is packaged with an 850nm photodiode array and is said to target low-cost VCSEL-based optical links for datacentre interconnects.
"This is the first optical an receiver that combines high-speed data transmission rate and rapid power-on and off functionality while being extremely low lower in the 'power-on' state (about 88mW)," says Alessandro Cevrero from IBM.
"Our design, for the first time, allows for the on/off switching of and optical link on a per-packet basis," Cevrero continues. "There were previous scientific attempts to turn off the links when there is no data, however the timescale to switch on and off the link was orders of magnitude longer than that of an individual data packet. To achieve shorter power-on, time at a very high data transmission speed is the key challenge."
To address this, Cevrero's team designed an optical receiver with four identical channels associated with a proposed link protocol. The link protocol is said to be equipped with self-developed smart analog circuits that can rapidly align the receiver's clock with the arrival of the incoming data and detects the optical signal sequences to rapidly turn the link system on and off.
The researchers then tested the receiver at 40Gbit/s with a reference transmitter consisting of an 850nm Mach-Zehnder modulator, followed by a variable optical attenuator. They also performed power on/off experiments by generating an optical signal implementing the proposed link protocol.
They claim they observed correct power cycling across a 109 power cycle and that the receiver operated error-free at 40Gbit/s yielding an aggregate bandwidth of 160Gbit/s over multi-mode fibres. The experimental data also apparently showed that 10% link utilisation corresponds to 85% power saving on the receiver.
Cevrero notes that because one can ‘cram’ higher bandwidth in the same thermal budget of the package, the power efficiency of optical links can be improved, enabling scientists to build faster, higher performing computer systems.
He adds this also helps reduce the carbon dioxide emission from the optical network, leading to greener optical communication systems.
The next step for the team is to validate a complete optical interconnect system by measuring the optical transmitter and attempt to increase the data transmission speed on the receiver side to 56Gbit/s per channel.
