FEATURE SINGLE PHOTON COUNTING
Developing extra sensitive environmental detectors
Andy Extance finds out how the SPEXS project is looking to extend single photon counting further into the infrared
I
n a world where it’s getting more common to use lasers to study our environment, researchers are seeking to reveal even more insights invisible
to our naked eyes. Gerald Buller from Heriot-Watt University in Edinburgh, UK, gives the example of using light detection and ranging (lidar) technology to map foliage from above. ‘It’s not enough to take
12 Electro Optics June 2020
a camera and look at the top of the tree canopy because you don’t know what’s beneath it,’ said Buller. Lidar seems to scatter photons across
its surroundings with abandon, but Buller’s expertise is in detectors sensitive enough to count their return one by one. He highlights that such single photon detectors enable active lidar imaging that can use laser pulses to see further, or through obscuring layers like an upper leaf canopy. ‘For some applications, single photon takes it to the next level in terms of depth resolution and sensitivity,’ he said. ‘Sensitivity is important because it means that you can look at smaller optical signals. That directly corresponds to using the same laser power for going to longer distances. If you want to see through
smoke and fog the single photon lidar approach is almost certainly going to be better.’
But today such single photon detectors
only work for certain light wavelengths. In some applications, including defence and environmental monitoring, reaching up to 5µm would help. This is the aim of the Single Photons – Expanding the Spectrum (SPEXS) project, funded by the UK’s Engineering and Physical Sciences Research Council (EPSRC). SPEXS started in late 2019, and Buller is its principal investigator. Buller explained that lidar measures
the time of flight, during which a photon travels to a target and reflects or scatters back again. That provides the distance to an object, but relies on resolving a
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