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FEATURE SINGLE PHOTON COUNTING


Distributed reflection of a light pulse could provide a tree canopy’s density distribution, Lamb continued, which the European Space Agency (ESA) is interested in to survey biomass


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a photon’s arrival time with respect to a synchronisation signal, with picosecond temporal resolution. Its usage has expanded in many applications over the last decade, Lamb noted. Leonardo’s interest is in defence,


locating and identifying objects through their shape. Photon counting’s extreme sensitivity can capture very weak reflected signals. Its temporal precision helps enable this in 3D imaging lidar, with photon counting detector arrays. Where a photon returns in the array provides two- dimensional information in the horizontal xy plane. Highly sensitive detection of the time of flight gives accurate depth information along the z direction. ‘You’ve now got three dimensional information from which you’ve worked out a shape,’ he said.


Distributed reflection of a light pulse


could provide a tree canopy’s density distribution, Lamb continued, which the European Space Agency (ESA) is interested in to survey biomass. Returning photons can be detected spectroscopically by satellites to track compositions


14 Electro Optics June 2020


of objects detected, like chlorophyll concentrations in leaves. ‘You could measure its seasonal variation, the health of the tree and therefore the health of an entire forest,’ Lamb emphasised. The light wavelength used in lidar determines which substances can be detected, and is one motivation for expanding the spectrum available in SPEXS. Atmospheric transmission windows


provide another motivation for spectrum expansion. Only certain light wavelengths can transmit through the atmosphere. Visible light is one, but lidar using that part of the spectrum could damage people’s eyes were the lasers to enter them. ‘The next atmospheric transmission window is in the shortwave infrared, which extends from around about 1.5µm to about 2.5µm,’ Lamb explained. However, single photon counting is much less mature in that region, he noted. As such, SPEXS is looking at different semiconductor technologies and superconducting wire technologies to improve single photon detectors performance for such wavelengths.


Lamb also warned that a potential


weakness is the need to build up data to gain meaningful images and movies. ‘Often a single photon doesn’t tell you a great deal because it might not actually be a signal,’ he said. ‘It could be solar background or noise in the detector. To circumvent that you repeat the measurement and you build up the signal-to-noise ratio photon-by-photon. The general disadvantage is it takes time. Typical lidars might run at a few tens of hertz. In photon counting, it’s not unusual to be running at megahertz or tens of megahertz.’


Entering uncharted territory Buller noted that processing the large amount of data generated has also been a limitation. However, he argues that progress has been made recently. For seeing through cloudy water, his team has produced videos using a pulsed picosecond laser diode source operating at 670nm and single photon avalanche diodes (SPADs). Buller and colleagues have also used infrared light to produce


@electrooptics | www.electrooptics.com


ESA/AOES Medialab


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