oDAC: Boosting data centre speeds with less power

Academics have developed an optical digital-to-analogue converter (oDAC) that promises to rethink how high-speed optical transmission is done.

Professor Ioannis Tomkos

Conceived under the European Commission-funded Flex-Scale project for 6G front-haul, the oDAC also promises terabit links inside the data centre.

The oDAC is expected to deliver a 40 per cent power savings for a 1.6 terabit optical transmitter, the ‘send’ path of an optical module.

“It might not be not 50 or 60 per cent, but in this field, even a 25 per cent power saving turns heads,” says Ioannis Tomkos, a professor at the Department of Electrical and Computer Engineering at the University of Patras, Greece, one of the researchers leading the work.

The first proof-of-concept oDAC photonic integrated circuit (PIC) has sent 250 gigabits per second (Gbps) over a single wavelength as part of the European Proteus programme.

The goal is to bring the oDAC to market in 2026.

High-speed optical transmission challenges

An optical interface acts as a gateway between the electrical and optical domains.

The main two classes of optical interfaces—pluggable modules for the data centre and coherent designs for longer-distance links—continue to grow in data rate.

The upcoming rate today is 1.6 terabits per second (Tbps), with 3.2Tbps optical links are in development. But going faster adds design complexity and consumes extra power.

Faster electrical signalling must use encoding schemes such as 4-level pulse amplitude modulation (PAM-4). And in the optical domain, PAM-4 is used in the data centre while higher-order modulation schemes such as 16-ary quadrature amplitude modulation (16-QAM) are used for long-haul optical transmissions. Quadrature amplitude modulation uses amplitude and phase modulation thyat doubles transmission capacity.

Such schemes require fast analogue-to-digital converters (ADCs), digital-to-analogue converters (DACs), and digital signal processing (DSPs) to compensate for transmission impairments. But as speeds increase, so does the signalling complexity and sampling rates, driving up the overall cost and power consumption.

The trends are leading researchers to consider alternative approaches, such as signal processing in the optical domain, to lessen the demands placed on the DSP and its DACs and ADCs. Researchers are even wondering if such an approach could remove the DSP altogether.

“Step by step,” cautions Tomkos.

Tomkos is working with Professor Moshe Nazarathy, a founder of the oDAC work, at the Faculty of Electrical Engineering at Technion University, Israel.

And it is developing the oDAC where they have first focussed their efforts. 

Electronic DAC versus the oDAC 

One way to view the oDAC is as a high-speed optical modulator. Another is as a multiplexer of multiple optical amplitude data streams.

The oDAC is a fundamental building block that trades extra optical components to simplify the electrical drivers for the high-speed transmitter. This is how the 40 per cent power saving is achieved.

The oDAC’s architecture is similar to that of a coherent optical transmitter but with notable differences. 

Source: Nazarathy and Tomkos

In a coherent optical transmitter setup, the laser source is split evenly to feed the in-phase and quadrature Mach-Zehnder modulators (MZMs), with a 90-degree phase shifter in one of the modulator’s arms (see diagram above, left).

In contrast, the oDAC employs a variable splitter and a combiner at the input and output stages, paired with identical Mach-Zehnder modulators (no phase shifter is used in one of the modulator’s arms, see diagram, right).  The ODACs can be used in a nested arrangement, as part of in-phase and quadrature arms, for coherent optical transmission. 

Conventional electronic DACs (eDACs in the diagram) sample the data at least as high as the symbol rate and have a finite bit resolution, which limits the higher-order modulation schemes that can be used.

They are used to drive the optical Mach-Zehnder modulator, which has a non-linear sine-shaped response. The non-linearity forces the modulator to work only in the linear region of its transfer function. (See graph below.)

Source: Nazarathy and Tomkos

This curtailing of the driver saves power but results in ‘modulator loss’ – the full potential of the modulator is not being used, impacting signal recovery at the optical receiver. 

In contrast, the oDAC can drive fully the modulator, thereby avoiding the modulator backoff loss. 

Another key oDAC benefit is that each of its Mach-Zehnder modulators is driven using simpler PAM driver chips to produce higher-order PAM signals: two standard PAM-4 drivers can produce PAM-16 and using two oDAC PAM-16s can be used to generate PAM-256 (each symbol carrying 4- or 8-bits, respectively).  

No commercial electronic PAM-16 drivers exist, says Tomkos.

Scaling data rates using PAM-4 drivers

A PAM-4 driver for the oDAC’s Mach-Zehnder modulator arm produces a four-level “staircase” waveform. Adjusting the oDAC’s splitter ratio to 4:1 and summing the outputs yields 16 distinct levels (diagram, below)

Source: Nazarathy and Tomkos

In effect, two simple signals can be stacked in multiple combinations to mimic a complex one. For PAM-16, one Mach-Zehnder modulator handles levels 0, 1, 2, and 3, while the other one, scaled differently (e.g., 0, 4, 8, 12), ensures a sum from 0 to 15.

The catch? Achieving a smooth staircase signal requires precise in-phase combining and level controls so there are no differences between the two Mach-Zehnder arms, which requires careful circuit control.

“Every programmable photonic circuit in general, for whatever application, needs some parametric control of the actual circuit,” says Tomkos. “For our case, it is so that it will not deviate if you change the temperature if you have vibrations or any other environmental changes.”

David Moor

David Moor, a post-doctorate researcher at ETH-Zürich, part of the Flex-Scale project, and the director of photonic IC design at Emitera, the start-up tasked with bringing the oDAC to market, has been putting the prototype oDAC photonic integrated circuit through lab tests. 

To send 500 Gbps over a single wavelength, a two-arm oDAC is used, with each PAM4-driven arm operating at 120 gigabaud symbol rate, or 250Gbps. While using two oDACs feeding an in-phase and quadrature coherent structure, doubles the data rate to 1Tbps.

Then, using a pair of PAM-16 oDACs (each driven by a pair of PAM-4 signals, in-phase and quadrature-combined in a coherent transmitter structure, further doubles the data rate to 1.6Tbps. 

Source: Nazarathy and Tomkos Transmissions at 3.2 terabits would need the symbol rate at 240 GBd.

What next?

Professor Nazarathy, working with Professor Birbas and his team at the University of Patras, are developing an FPGA-based control system to ensure the device operates optimally in real-world conditions.

“In the lab, the device has been quite stable,” says Moor. But any environmental changes throw it off track. oDAC device needs robust control to be a commercial product. 

A second-generation oDAC photonic integrated circuit design and an FPGA-based control system are in the works and are expected to be up and running in six months.

Applications: Data centres and front-haul

“The higher-order the modulation format used, from 16-QAM to 256-QAM, the less the distance,” says Tomkos. “This is a law of information theory. You cannot do otherwise; nobody can.”

But the benefit of the design grows the higher the modulation order and the higher bit rate. Thus, the oDAC comes into its own when using 16-QAM and higher-order signalling schemes. 

Accordingly, the ODAC’s sweet spot is for links up to 20 or even 40km, where terabits of data can be pushed over an optical wavelength. This makes the oDAC concept ideal for “coherent-lite” spans between campuses and when used inside the data centre.