Round the bend

Efforts to see around corners get a boost from better detectors. Article from The Economist.

Efforts to see around corners get a boost from better detectors.

Using a cameraphone to catch the indivisible packets of light energy known as photons is easy: around a trillion of them impinge on its sensor every second. Catching just one photon, however—and knowing exactly when it arrived—is far trickier. Many branches of research in the physical sciences depend on specialised kit that can do this, but quotidian equipment is not up to the job. “So what?” you might think. But there is at least one application outside the laboratory that it might be useful for. This is seeing what is around a corner, something both soldiers and drivers would like to be able to do. And, as they describe in Nature Photonics, a group of researchers led by Daniele Faccio of Heriot-Watt University, in Edinburgh, have indeed worked out a way to capture simple movies of what is around the bend.

Dr Faccio’s group is one of several trying to do this. All use a method similar to that employed in radar, except that it substitutes light for radio waves and is thus known as LIDAR. Unlike conventional LIDAR, though, the pulses sent out from looking-around-the-corner equipment have to bounce off three things before they make it back to the detector. First, they must be reflected from something in the line of sight of their source. This pushes some of them in the direction of the hidden target. Then, they have to be reflected from that target back in the direction of the first reflector so that they can undergo a third reflection, towards the device that sent them out.

Since each step of this journey scatters or absorbs much of the incident light, what returns is a tiny fraction of what was sent out—the trillions of photons are reduced to just a few thousand. But even that handful is, in principle, enough to construct an image of what cannot be seen directly. To do so, however, it helps to have expensive detectors shielded by filters that let through only light of the frequency used by the LIDAR; a set of complex algorithms; and minutes or hours of scanning to provide the data for said algorithms to chew on.

Dr Faccio has managed to speed this process up. Partly, he does so by sacrificing detail. But his main advance is to use an array of devices called single-photon avalanche diodes (SPADs), deposited on a semiconductor chip, to create something rather like a camera sensor. As the name suggests, each SPAD can register the arrival of a single photon. Such a packet of light triggers an avalanche of electric charge through the diode’s semiconducting material, creating a signal.

This arrangement can measure a photon’s arrival time to within 45 trillionths of a second, a period during which light travels 1.35cm. Like conventional LIDAR, the round-the-corner sort works by measuring the time it takes for a pulse, or part of it, to return. It can then work out how far the photons it has seen are from the bit of the target they have reflected from.

And it works. The system can now see someone (usually one of Dr Faccio’s students) hidden four metres away around a corner. It can also produce new images sufficiently quickly (once every three seconds) to shoot a jerky movie of her.

That, of course, falls far short of something that might show a driver what lies around the bend. But Dr Faccio hopes for rapid improvements. A tenfold increase in sensitivity, for example, could be had just by using a more powerful laser. The SPADs themselves are improving swiftly, too. The ability to see around corners, then, could be just around the corner.

From the print edition