technology VCSELs
enabled data transmission rates of 49 Gbit/s at -14 °C and 47 Gbit/s at 0 °C. Larger apertures were needed at higher temperatures to deliver more power to the detector. Transmission at rates of 12.5 Gbit/s, 17 Gbit/s and 25 Gbit/s were possible at 155 °C, 145 °C and 120 °C, respectively. These results illustrate that a laser – especially a VCSEL – is a highly nonlinear, coupled system. Gains in operating temperature come at the expense of transmission speeds, and VCSEL performance can be improved substantially with chip cooling.
Efforts are already underway at bringing these advanced VCSEL devices to market. TU-Berlin´s spin- off company, VI-Systems GmbH is working on providing detectors and devices for the commercial 850-nm waveband, and we are working with them to develop a VCSEL operating in this spectral range. We have found that the devices, which are closer to those serving the existing multi-mode VCSEL market than our 980 nm designs, require just one-tenth of the energy of a regular VCSEL to send a bit (see Table 1 for details).
The energy consumption by these VCSELs is so frugal that they can meet the projections of the International Technology Roadmap for Semiconductors for energy efficient interconnects of 2015. Hitting this benchmark today proves that VCSELs are a technology that can already deliver the low-cost, energy-efficient, high-speed interconnects needed to overcome the bottleneck brought about by copper.
Future requirements on VCSELs Even if these novel devices can accommodate the demand for the next few years, it would be naive to believe that no further progress is needed. Fortunately, there is the potential for far higher speeds from monolithic electro-optical modulated VCSELs – according to our studies, they could hit serial bandwidths of around 100 Gbit/s. And advanced coding schemes should lead to further gains in serial bandwidths and link robustness.
Another trend that we can expect to see is the development of high-contrast meta-structures, which should open up degrees of freedom in device design, such as multiple wavelengths on a single VCSEL array. Meanwhile, if there are moves to longer wavelengths, like 1.3-1.6 µm, this will enable compatibility with silicon waveguides that form part of optical silicon-based chips.
The spectral range is accessible with different classes of laser: InP-based VCSELs, which are well understood and commercially available from the likes of Vertilas and Beam Express and quantum-dot devices, grown on cheaper GaAs substrates.
Further reading P. Wolf et al. Electronics Letters, in press (2012) P. Moser et al. Appl. Phys. Lett. 98 231106 (2011) V. Karagodsky et al. Optics Express 18 694 (2010) T. Germann et al. Optics Express 20 5099 (2012)
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www.compoundsemiconductor.net June 2012
Table 1.A comparison of the best results in energy-efficient data transmission,including values for the energy-to-data-ratio (EDR) and the heat-to-bit rate-ratio (HBR).The recently achieved results in Berlin won the Green Photonics Award 2012 at Photonics West in San Francisco
Fig
2..The gain in energy-efficiency resulting from the transition from copper to optical interconnects are substantial.However,a VCSEL is another two orders of magnitudes more energy efficient that optical interconnects derived from long-haul data-com
modules.The VCSELs demonstrated recently in Berlin gain yet another order of magnitude
Looking further into the future, when data traffic hits levels that are unthinkable today, even the footprint of a VCSEL might be too big. To address this, we are working in partnership with researchers at the University of Illinois at Urbana Champaign to develop nano-cavity lasers with metal cladding. This will shrink the output of optical sources for interconnects by a further two orders of magnitude, taking us into a realm where interconnects based on copper are never, ever considered.
© 2012 Angel Business Communications. Permission required.
W. Hofmann et al. IEEE Photonics Journal “Breakthrough in Photonics”, in press (2012) W. Hofmann et al. Semicond. Sci. Technol. 26 014011 (2010) C-Y. Lu et al. Applied Physics Letters 96 251101 (2010)
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