All That Matters

Data transmission schemes: In TDM different channels are assigned individual time slots in the transmitted pulse. WDM transmits data across different wavelength channels. In OFDM, these wavelength channels are closer together, although it means that the signal needs to be filtered out later on by frequency analysis. Reprinted by permission from Macmillan Publishers Ltd. Nature Photonics, advance online publication (2011)

Here in the UK, the fastest broadband download speeds on offer for fibre optic broadband are 40 Mbit per second, which is much better than the 8 Mbit/s or so offered via conventional copper cables. But to those for which 40Mbit/s is not enough, fear not: In a Nature Photonics paper, Juerg Leuthold and colleagues from the Karlsruhe Institute of Technology have now demonstrated 26 Tbit/s optical data transmission – some 650,000 times faster than those 40Mbit/s.

There are different techniques to send large amounts of data through an optical fibre. For example, in Time Division Multiplexing (TDM), the signal transmission is divided into a succession of time slots, and each channel gets one bit in every time slot. This makes sense if you want to combine a number of slower channels (e.g. from individual households) into one fast fibre. The disadvantage of this scheme is, however, that this needs ultrafast lasers. And because short laser pulses are spectrally broader than longer pulses, TDM systems needs to be capable of dealing with these short laser pulses across a broad range of frequencies, which is demanding to realize.

Another scheme is to use longer laser pulses, which are spectrally much narrower, but then to distribute these across several wavelengths, so that data is transmitted in channels each using a different colour. The drawback here again is that you can only squeeze a limited number of those wavelength into your optical setup. Also, great care has to be taken that the laser pulses don’t overlap in wavelength, as otherwise the signals are mixed up between the channels.

To send even more data than possible with these schemes, modern systems use techniques such as orthogonal frequency-division multiplexing (OFDM). OFDM is already widely used for wifi as well as for 4G wireless networks. In comparison to WDM, the different wavelength channels can be squeezed closer together, which means more channels, and therefore much faster data transmission. […]

Hydrogen sensing at the nanoscale. Hydrogen molecules (red) are absorbed by a palladium nanoparticle (silver) and the resulting changes in optical properties amplified by a gold antenna. (c) Mario Hentschel, Na Liu, Harald Giessen

Sensing the presence of molecules in gases and liquids is a billion dollar business. Just think about all the carbon monoxide detectors in private homes, or blood glucose sensors. In particular for many technical and scientific applications, ultrasmall and precise sensors are desired. This includes sensors to measure gases in catalytic nanoreactors and fuel cells, or the monitoring of biochemical processes.

Laura Na Liu and Ming Tang from the group of Paul Alivisatos, director of Lawrence Berkeley Lab in the USA, and Mario Hentschel from Harald Giessen‘s group at the University of Stuttgart in Germany have now developed a new class of optical nanoscale sensors that are able to measure specific molecular concentrations down to single particles. This, says Alivisatos, “should pave the road for the optical observation of chemical reactions and catalytic activities in nanoreactors, and for local biosensing.” Their paper is published this week in Nature Materials (declaration of interest: I was the handling editor of this paper, although I like to stress that I don’t benefit in my day job by blogging about this work). […]

Fibre optic cables transmit information so fast because they can make use of the unique properties of light and transmit many data channels at the same time. The digital 1s and 0s the light beams carry are imprinted onto the beams by semiconductors that in quick succession turn the light beam on and off. Unfortunately, that also puts a limit on the possible data rate, as materials switch slower than light. There are all-optical switches operating at the speed of light using special crystals, but what is needed are solutions that can be fabricated on a chip.

This is made possible now. Georgios Ctistis, Willem Vos, Jean-Michel Gérard and colleagues from the University of Twente and the FOM-Institute Amolf in the Netherlands, and the Institute for Nanoscience and Cryogenics in Grenoble in France have demonstrated that using a material to switch light is not a drawback anymore. They are able to switch a light beam within a semiconductor device at speeds of 0.3 picoseconds, where a picosecond is a millionth of a millionth second. That’s so fast that it approaches the limit set by the speed of light.

The principle of the ultimate optical switch. Top: a microcavity blocks the transmission of the red signal beam. Middle: in the presence of a control beam the cavity changes its properties and lets the beam pass. Bottom: as the control beam is off again, the switch also turns off. Figure provided by the authors.

In a conventional optical switch, a light beam (or an electrical voltage), is used to excite electrons in a semiconductor. These electrons then change the material’s optical properties in a way that switches the signal beam on or off. But this is a comparatively slow process. The idea here is to separate the optical effects from materials properties, which would only slow the device down. “The key advance is that both the switch-on and -off times of the semiconductor microcavity is completely determined by the properties of light itself,” says Vos.
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