Interview

Abstract

Prof. Dieter Bimberg of Technische Universität Berlin in Germany talks about the work behind the Letter ‘Energy-efficient and temperature-stable oxide-confined 980-nm VCSELs operating error-free at 38 Gb/s at 85 °C’ on page 103. Prof. Dieter Bimberg My group's work was originally focussing on the growth and characterisation of low-dimensional semiconductor structures – quantum wells, wires and dots. Mastering growth, I thought more and more about using such structures for devices having properties superior to classical double/multiple heterostructure based ones, being of particular importance for optical communication systems, like Ethernet or Datacom. Important properties are, for example, increased bit rate at large signal operation or high temperature performance. VCSELs are the key enabling devices for optical interconnects in work station clusters and high performance supercomputers. They must simultaneously operate with little energy dissipated, at high bit rate and at high temperature. Generally there is a trade-off between these parameters. We show, for the first time, that high bit rate at high temperatures can be achieved with record energy efficiency. An array of 25 of our VCSELs can transmit a data stream of 1 Tbit/s at an operating temperature of 75°C, typical for the inside of a work station. The internet and its computers at its active end, together with supercomputers, consume an increasing amount of our net electricity production. In 2013 the consumption amounted, in the US, to 5% of production, equalling the photovoltaic power production. For 2014 the prediction is 6% and some predict a brake-even or cross-over in 2023, if the power consumption of the photonic devices is not radically reduced. At the same time the devices must become faster, with larger cut-off frequencies, thus increasing energy consumption. We show here that careful design of the devices significantly reduces the power consumption of VCSELS and allows increased cut-off frequency and resulting maximum bit rate, still at the high temperatures inside a computer. The rapidly increasing number of optical interconnects for short reach (around 300 m and more), very short reach (less than 1 m) and ultra-short reach (less than 1 mm) leads to rapidly increasing consumption of electrical power. We need radical reduction of power consumption to enable further growth of the internet and creation of novel supercomputer generations like Exa-Flop ones, predicted for 2020. Systematic further increase of maximum bit rate of VCSELs and their drivers at the lowest possible energy consumption and still larger operating temperatures. Present theoretical device concepts must be extended to full numerical 3D ones in order to predict the optimum combination of the multitude of parameters which presently influence the performance. Shift of the emission wavelength from 850 nm, 980 nm to longer ones between 1200 and 1300 nm, also enabling p-side down mounting will further increase temperature stability. Device lift-off with redisposition on Si substrates allows us to benefit from the advantageous heat conductivity of Si. The next generation of supercomputers are true optical computers, where optics is decisive and counts for about 50% of the cost and performance. The footprint. We need lasers still much smaller than the present VCSELS in order to deposit arrays of them on memories, logics, microprocessors, where space is limited. Combination of metals with semiconductors will allow shrinking of the feature size of surface emitters by one order of magnitude or more. Plasmonic effects might help. The high speed properties of metal-cavity nanolasers are, however, yet unexplored. In the same way quantum dots as active material might stabilise the active area, reduce losses and improve temperature stability. Putting these features together we can make it happen!