Increased wavelength VCSELs reach out

Abstract

Work from Sweden and the US is extending the reach of low-cost laser and fibre technology to meet the growing needs of optical interconnects in the massive data centres that underpin our Internet-enabled world. Team member Ewa Simpanen holding up a set of VCSEL chips in the cleanroom at Chalmers University of Technology. VCSELs (Vertical-cavity surface-emitting lasers) are power efficient semiconductor lasers that can be produced in high volumes at low cost. They can also be directly modulated at high speed when biased at low currents. All of this makes them the preferred light source for the short-reach optical interconnects used in data centres and high-performance computing systems. In these applications, VCSELs are used with high modal bandwidth multimode fibre (MMF) to relax the laser-to-fibre alignment tolerance, which also reduces costs associated with assembly and packaging. With the development of cloud-based applications and Internet-delivered services, data centres operated by service providers like Facebook and Google are developing into very large facilities. These ‘mega’ data centres need optical interconnects that can reach beyond 1 km. This cannot be supported by current VCSEL-MMF technology owing to the large chromatic dispersion and attenuation in MMF at the standard wavelength – 850 nm. At longer wavelengths, dispersion and attenuation are much lower, allowing greater reach. This is the need that the team, from Chalmers University of Technology in Sweden, and Hewlett Packard Enterprise (HPE) in the US, are working to meet. Team member Ewa Simpanen told us, “The purpose of this work is to develop high-speed, longer wavelength those while still using the GaAs-based material systems as it enables the fabrication of VCSELs which are superior to those based on other material systems in terms of speed, efficiency, manufacturability, and cost.” Currently, large-scale data centres use single-mode fibres and long-wavelength silicon photonic transceivers to cover the needed distances. With longer wavelength GaAs-based VCSELs, it would be possible to build transmitters for long-range interconnects that are as power and cost efficient as those used for shorter reach interconnects. In this issue of Electronics Letters, the Chalmers and HPE team present a VCSEL design operating at 1060 nm. This high-speed, single-mode, low-resistance VCSEL can support bit-rates up to 50 Gbit/s while dissipating no more than 100 fJ of energy per bit; making it very energy efficient – on par with the state-of-the-art at 850 nm. “We have limited the wavelength extension to 1060 nm since the long-term reliability of GaAs-based VCSELs may be compromised if extending the wavelength beyond ∼1100 nm,” explained Simpanen. At 1060 nm, the fibre chromatic dispersion is reduced by 70% and attenuation by 50% compared to 850 nm. Using a single-mode VCSEL the narrow spectral width of the laser further reduces the impact of chromatic dispersion. With the single-mode VCSEL properly aligned to the MMF, only the fundamental mode of the fibre, or the lowest order mode group, is excited, which also reduces the impact of modal dispersion. “In these conditions, the fibre ideally acts as a pure attenuator without degrading the signal quality. All this together should enable data transmission distances compatible with what is required in mega data centres,” said Simpanen. The results reported in this issue deal with the modulation capacity of the VCSEL itself. Since then, the team have started investigating the data transmission capacity of optical interconnects using the VCSEL together with MMF designed for high modal bandwidth at 1060 nm and so far the results are very promising, according to Simpanen. “This prototype fibre is fabricated at and provided by Corning. In a first preliminary experiment, we have transmitted data at 25 Gbit/s over 1 km with some margin. We believe that higher data rates and/or longer transmission distances are possible with further optimisations. We will also investigate the alignment tolerances under mode selective launch as they are of utmost importance for practical implementations.” A microscope image of an array of VCSEL devices on chip; newly fabricated and ready for characterisation. Looking further ahead, the team think that continued engineering of conventional VCSEL designs and the implementation of more exotic VCSEL concepts for further significant speed improvements will play a big part in developing much higher capacity interconnects. Combined with electronic compensation techniques, such as equalisation and digital signal processing, higher order modulation formats, and more advanced forms of spatial and wavelength division multiplexing, they believe such lasers could eventually achieve aggregate interconnect capacities approaching 100 Tbit/s.