Published online 5 November 2008 | Nature | doi:10.1038/news.2008.1210

News: Briefing

The invention of an ultrafast oscilloscope could yield advances in fields from telecoms to nuclear fusion.

The ultrafast optical oscilloscope can pick up detail within pulses of light only a trillionith of a second long Punchstock

The oscilloscope has had an upgrade since its heyday as an extra in 1960s sci-fi movies. A version of this instrument — originally developed to show changes in voltage with time — can now measure the fine detail contained within extremely rapid light pulses, just 220 femtoseconds long. That's less than one trillionth of a second.

The ultrafast optical oscilloscope was constructed by Alexander Gaeta at Cornell University in Ithaca, New York, and his colleagues. It's based on a clever piece of kit they developed previously, called a four-wave mixing chip, which acts as a time lens. Just as a conventional diverging lens can make light waves spread out, a time lens can spread out waves over time.

In the device, the short, fast light signal being measured travels along a fibre-optic cable, together with an input laser signal, to the chip. When they get there, the two signals are combined. The time-lens chip spreads out the combined signal and turns it into a spectrum using a mathematical trick known as a Fourier transform. This modified signal then passes along another length of fibre-optic cable to a spectrometer where it can be read out.

The 'scope can measure changes as short as 220 femtoseconds within a signal that's more than 100 picoseconds long. This is "at least a factor of five better" than previous machines, says Gaeta, and it allows a signal containing a lot of fine detail to be measured. The work is published in Nature1.

So what are the potential uses for an ultrafast optical oscilloscope?

Watching worlds collide

Nuclear fusion can be triggered with a laser beam, and several big projects are under way to develop laser fusion as a future energy source. One example is the recently launched HiPER (High Power laser Energy Research facility) project in Europe and its predecessor, PETAL (PETawatt Aquitaine Laser) near Bordeaux in France.

The laser fusion reaction happens very quickly and the new oscilloscope is the one instrument able to measure the light that is given off as the nuclei collide, says Gaeta. These events only happen once in a while, and there's just one chance to measure them. One of the key benefits of the 'scope is that it can measure a single event, in what's known as a 'single-shot' technique.

Clearing the phone lines

If futuristic energy sources aren't your thing, how about better phone communication? Telecommunications work by sending packets of information-carrying light signals along fibre-optic cables, which are then decoded at the other end. To push more data down the cable, and so get more people connected at the same time, these packets of light need to be squeezed so that the pulses that make the signals are shorter. But when a problem occurs in the line, the pulses are so short, and coming so fast, that it's impossible to find the glitch. This comes back to single shots. Modern oscilloscopes often work by taking an average of very short pulses to give the reading. "They take an average of repeated signals. We can see a single pulse and tell exactly what it looks like," says co-author Michal Lipson, also from Cornell University.

In theory, this could be exploited to code the telecoms signal in the first place, suggests Brian Kolner from the University of California, Davis. "Making coded sequences on a really short time frame is hard," says Kolner, but if the 'scope is run backwards, it could squash longer signals into really short ones that whizz through a fibre-optic cable in droves.

Super-sizing your lab instrument

Gaeta also claims that his device could be used as a simple add-on to a normal laboratory machine. This is because it's made from standard electrical kit — a silicon chip forms the basis of the oscilloscope, and at the moment conventional optical fibre is used to transfer the signals, although the plan is to further develop the system so that the fibre isn't needed in future. "We're giving a system that's extremely elegant and simple," says Gaeta.

Watching biology and chemistry in action

Many atomic and molecular processes can occur in less than a picosecond. "Biological and chemical reactions happen on time scales that are just out of reach," says Kolner. He sees the 'scope being used by researchers who want to peer into these biological reactions. Gaeta agrees; his instrument will be useful for researchers in chemistry who need to capture any short, exact, weak signals — for example, chemical fluorescence from a very fast chemical reaction, he says.

Extreme physics

The oscilloscope will also help scientists trying to recreate the extreme processes that happen inside stars, says Corey Bennett, from the Lawrence Livermore National Laboratories (LLNL) in California. "The National Ignition Facility at LLNL is a facility where single-shot extreme physics experiments will soon be a regular occurrence," Bennett says. "The facility will generate the pressures, densities and temperatures, in a controlled laboratory environment, that normally only exist inside a star, at the centre of planets or in a nuclear burn." If an experiment cannot be repeated quickly, or can't be assumed to repeat identically, then traditional measurements aren't going to pick out the signal they're looking for, Bennett adds, but he suggests that an ultrafast, single-shot optical oscilloscope could do exactly that.