Integrating high-speed germanium modulators with silicon photonics and fast electronics

A major bottleneck in optical transmission is emerging, as silicon Mach-Zehnder modulators approach their limits. In our data-driven era, the industry urgently needs higher-bandwidth, energy-efficient modulators that are compatible with silicon photonics. Germanium electro-absorption modulators offer a promising route forwards.

By Daniel Steckler, Stefan Lischke, and Lars Zimmermann,IHP-Leibniz Institut für innovative Mikroelektronik

Optical connectivity plays a major role in all our daily lives, often without us even noticing it. Video calls, cloud storage, social media, and on-demand video and audio are all constantly at our fingertips. Artificial intelligence (AI) applications are also on the way to becoming ubiquitous everyday companions. But all of this is only possible thanks to modern datacentres. Fibre-optic communication, not only to and from datacentres, but also within them, is the key to coping with the enormous data flow these technologies generate.

According to the Ericsson Mobility Report from November 2023, global data traffic has more than doubled every two years over the past 10 years. By 2029, global mobile data traffic is projected to triple, reaching 403 exabytes per month [2]. Since every single bit transmitted consumes a certain amount of energy, the energy efficiency of optical and electrical devices is increasingly important. One hyperscale datacentre already consumes as much energy as 80,000 US households [1]. And that doesn’t even include the energy required for transport to the datacentres.

Figure 1: TEM images of germanium photodiode (a) and electro-absorption modulator (b). Cross sections cut perpendicular to the light-incidence direction. As indicated by the red boxes the Si-region has been reduced from 220 nm to 100 nm for the electro-absorption modulator.

Besides energy efficiency, other demands on future devices include high-volume and low-cost fabrication capabilities and high-speed performance. Thanks to sophistic–ated fabrication processes on 200 mm and 300 mm wafers, silicon photonics has seen rapid progress, becoming a major technology in fibre-optic communication applications. In fact, silicon photonic transceivers for datacentre interconnects enabling 400 G bitrates are already available on the market and 800 G transceivers have been demonstrated.

However, while the effective data rates of transceivers have increased from 10 G in 2007 to 800 G in 2024, the rate of transmitted light pulses (symbol or baud rate) has actually only scaled by a factor of five (from 10 GBaud, in 2007 to 50 GBaud today). For this reason, techniques such as higher-order modulation or multiple lanes are deployed on a large scale to cope with the data-traffic requirements [3].

Figure 2: Schematic longitudinal cut through the EAM to visualize the butt-coupling approach (not to scale).

On the receiver side, germanium photodetectors with a 3 dB bandwidth well beyond 200 GHz have already been demonstrated [4]. However, a major bottleneck is emerging on the transmitter side. With electro-optical 3 dB bandwidths of 50-70 GHz, silicon Mach-Zehnder modulators, the most established devices in silicon photonic transmitters, seem to have reached their performance limits already. Facing the data-traffic growth projections, it is unlikely that using the aforementioned modulation formats or even more parallelisation alone will be sufficient. Achieving symbol rates beyond 100 GBaud will require extensive equalisation and digital signal processing (DSP), but this is detrimental to the system’s power efficiency. To avoid this, the industry urgently needs modulators with 3 dB bandwidths above 100 GHz.