This paper highlights a significant advance in time-of-flight depth imaging: by using a scanning transceiver which incorporated a free-running, low noise superconducting nanowire single-photon detector, we were able to obtain centimeter resolution depth images of low-signature objects in daylight at stand-off distances of the order of one kilometer at the relatively eye-safe wavelength of 1560 nm. The detector used had an efficiency of 18% at 1 kHz dark count rate, and the overall system jitter was ~100 ps. The depth images were acquired by illuminating the scene with an optical output power level of less than 250 µW average, and using per-pixel dwell times in the millisecond regime.

Figures (8)

Fig. 1 Schematic of the photon-counting depth imaging system based on sub-picosecond 1560 nm wavelength illumination and a superconducting nanowire single-photon detector (SNSPD). CFD, constant fraction discriminator; APD, avalanche photodiode; TCSPC module, time correlated single-photon module. Download Full Size | PPT Slide | PDF

Fig. 2 Processed histogram by cross-correlation method. Example of the cross-correlation function C of a processed pixel showing the return peak (at approximately 9 ns) standing clearly above the background. Also present are the internal back reflections from the scanning system. The inset shows a normalized instrumental response of the system with 98 ps Full Width at Half Maximum (FWHM) timing jitter. Download Full Size | PPT Slide | PDF

Fig. 3 Depth profile measurements made in daylight on a life-size mannequin from a standoff distance of 325 meters. The images shown in column (a) are close-up photographs showing different viewpoints of the scene which consisted of the mannequin against a hardboard backplane. The depth scans were acquired in daylight and each scan covered an area of approximately 800 × 1000 mm using 60 × 75 pixels, resulting in a pixel-to-pixel spacing of approximately 13 mm in x and y. Plots of the depth data obtained for per-pixel acquisition times of 5, 2, 1, and 0.5 ms are shown in columns (b) through (e) – each of these columns show two different viewpoints of the surface plot constructed from the data obtained using the specified per-pixel dwell time. A per-pixel dwell time of 1 ms equated to a total scan time of 4.5 s for this scene. The face-on view shown in the top row gives an indication of the spatial resolution of the system, and the centimeter-scale depth resolution can be gauged from the oblique view in the bottom row. The color shading in the plots is used to map depth, and the images are surface plots of the raw depth data for all pixels i.e. no curve fitting or extrapolation has been applied to enhance the data returned by the system. Download Full Size | PPT Slide | PDF

Fig. 4 Comparison between the results obtained from scans, of a mannequin and a human, at a range of 325 meters using a per-pixel dwell time of 10 ms. A close-up photo of the mannequin’s head is shown in (a), and the surface plot of depth data acquired at a range of 325 meters with a 10 ms per-pixel dwell time is shown from a similar viewpoint in (b). The plot shown in (c) uses color to map the calculated number of detected return photons in the histogram peak for each individual pixel. At per-pixel dwell times of 10 ms and greater, almost all of the pixels in this scene had sufficient detected photons to provide a true depth reading at this range as is evident from the centimeter-scale features evident in (b). However, the detected photon returns from human skin were significantly lower at this wavelength. The scene shown in (d) of one of the co-authors was scanned with a 10 ms per-pixel dwell time and the surface plot in (e) was obtained – in this case, most of the facial pixels had insufficient detected photons for the determination of a depth. The low number of detected return photons from the facial skin is obvious on the corresponding color map shown in (f). The plots in (c) and (f) also confirm that there are lower returns from the areas of the scene that the illuminating beam strikes at glancing angles e.g. edges. Download Full Size | PPT Slide | PDF

Fig. 5 Depth profile measurements made in daylight of a life-size mannequin from a standoff distance of 910 meters. The close-up photographs in column (a) are different viewpoints of the scene that was scanned which consisted of the mannequin against a hardboard backplane. The depth scans were acquired in daylight and each scan covered an area of approximately 800 × 2000 mm using 30 × 80 pixels, resulting in a pixel-to-pixel spacing of approximately 25 mm in x and y. Surface plots of the raw depth data obtained for per-pixel acquisition times of 100, 20, 10, 5, and 2 ms are shown in columns (b) to (f) - each of these columns show three different views of the data obtained with the specified per-pixel dwell time. A per-pixel dwell time of 10 ms equated to a total scan time 24 s for this scene. The color is used to map depth. The face-on view shown in the top row gives an indication of the spatial resolution of the system at a kilometer, and the centimeter scale depth resolution can be gauged from the oblique and side-on views in the bottom two rows. Download Full Size | PPT Slide | PDF

Fig. 6 Depth profile measurements of humans, acquired in daylight at a standoff distance of 910 meters. The close-up photographs in columns (a) and (c) are different viewpoints of the scenes which were scanned – two of the authors posed in front of a sheet of hardboard. The depth scans were acquired in bright daylight and each scan covered an area of approximately 800 × 2000 mm using 40 × 80 pixels, corresponding to a maximum field of view of 2 mrad. Surface plots of the raw depth data obtained are shown in columns (b) and (d) - these columns show two different views of the data obtained with the specified per-pixel dwell time. It took 32 seconds to acquire the data shown in (d). The color shading in the surface plots is used to map depth. Download Full Size | PPT Slide | PDF

Fig. 7 Depth profile measurement made on a scene from a standoff distance of 4400 meters in broad daylight. The photographs (a), (b) and (c) are different viewpoints of the scene that was scanned with (a) being the view as seen from the approximate direction of the transceiver. The scene consisted of 1.2 meter tall boards with retro-reflective material (one plywood panel with a set of five red retro-reflecting triangles, and two adjoining panels covered in white retro-reflecting material), and a 400 mm tall red retro-reflective roadside warning triangle. The distance between the base of the white panels and the base of the red retro-reflective warning triangle was approximately 1 meter, as indicated in (c). The per-pixel acquisition time was 2 seconds. The surface plot of the 20 × 12 pixel depth data is shown from three different viewpoints in (d), (e) and (f), nominally corresponding to the viewpoints of the photographs. Download Full Size | PPT Slide | PDF