Credit: Ecole Polytechnique Federale de Lausanne

A small lab-grown diamond measuring a few millimeters per side could one day enable civilian drones to be recharged in mid-flight through a laser. Thanks to the diamond, the laser beam can remain strong enough over a long distance to recharge photovoltaic cells on the drones' surface. This system, which poses no threat to human health, is being developed by EPFL spin-off LakeDiamond. It could also be used to transmit both power and data to satellites and has just been included in the ten projects supported for two years by of the Swiss Space Office.

Drones are being used for a growing number of purposes. Their designs are ever more efficient, and techniques for flying them are being further refined all the time. But drones still have the same weak point: their battery. This is particularly true of propeller drones, which are popular for information-gathering purposes in dangerous or hard-to-reach regions. These drones can fly for only around 15 minutes at a time because their engines quickly burn through their batteries. One way of addressing this limitation – without weighing the drones down – would be to recharge them while aloft using a power beaming system: an energy-rich laser beam that is guided by a tracking system and shines directly on photovoltaic cells on the drones' exterior.

Several labs around the world, including in the US, have been working on this idea in recent years. LakeDiamond, an EPFL spin-off based at Innovation Park, has now demonstrated the feasibility of using a high-power laser for this purpose. What's more, LakeDiamond's laser emits a wavelength that cannot damage human skin or eyes – the issue of safety is paramount, since the system is meant for use with civilian drones. LakeDiamond's technology is built around diamonds that are grown in the company's lab and subsequently etched at the atomic level.

World record for power

Despite appearances, standard laser beams are not as straight as they seem: as they travel, they expand ever so slightly, leading to a loss in density as they go. But LakeDiamond's system produces a laser beam with a wavelength of 1.5 µm that, in addition to being safe, can travel much farther without losing strength. "Systems developed by other companies and labs, often for military applications, employ lasers that are more powerful and thus more dangerous for humans," says Pascal Gallo, CEO of LakeDiamond. His company took the opposite tack: their technology transforms the rays emitted by a simple low-power diode into a high-quality laser beam. Their beam has a larger diameter, and its rays remain parallel over a longer distance – in this case up to several hundred meters.

In LakeDiamond's laser, the light produced by a diode is directed at a booster composed of reflective material, an optical component and a small metal plate to absorb the heat. The breakthrough lies not with this set-up, which already exists, but with the fact that the emitted beam is only a few dozen watts strong. The secret is using a small square lab-grown diamond as the optical component, as this delivers unparalleled performance. LakeDiamond's system holds the world record for continuous operation using a wavelength in the middle of the infrared range – it delivers more than 30 watts in its base configuration. "That's equivalent to around 10,000 laser pointers," adds Gallo.

The lab-grown diamonds' key properties include high transparency and thermal conductivity. Achieving those things – and mastering the nano-etching process – took the researchers over ten years of development. LakeDiamond grows its diamonds through a process of chemical vapor deposition, an approach that ensures their purity and reproducibility. The surfaces of the resulting tiny square diamonds are then sculpted at the nano level using expertise developed in Niels Quack's lab at EPFL. Thanks to their inherent properties and etched shapes, the diamonds are able to transfer heat to a small metal plate that dissipates it, while at the same time reflecting light in such a way as to create a laser beam.

"To achieve greater power – say to recharge a larger drone – these lasers could easily be operated in series," says Nicolas Malpiece, who is in charge of power beaming at LakeDiamond. The company's remote recharging system works in the lab but will require further development and refinement before it's ready for field use. What would happen if a drone flies behind an obstacle and is cut off from its laser energy source? Several approaches to this problem are currently being explored. A small back-up battery could take over temporarily, or, for information-gathering missions over rough terrain for example, the drone could simply return to within range of the laser in order to top up its battery.

This energy transmission system is also interesting for other areas of application. It can for example be used for charging and transmitting data to satellites. The development of the system is included in a support program of the Swiss Space Office, which began on 1 November and runs for two years.

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More information: Carving diamonds for optical components: Carving diamonds for optical components: actu.epfl.ch/news/carving-diam … -optical-components/