Picosecond resolution time-correlated mode has emerged as a candidate technology for a variety of depth imaging applications in the visible, near-infrared and short-wave infrared regions. This presentation will examine this approach in a range of challenging sensing scenarios including: imaging though highly scattering underwater conditions; free-space imaging through obscurants such as smoke or fog; and depth imaging of complex scenes containing multiple surfaces.
Sub-pixel micro-scanning is a relatively simple way of utilizing a low pixel count sensor to better realise the resolution capabilities of a given objective lens. This technique accomplishes this by shifting the sensor array in the image plane through distances less than the pixel dimensions, gathering multiple images from different viewpoints that can be combined into a single, more detailed image. Applying this technique to a single-photon counting light detection and ranging (LiDAR) system allows for improved depth and intensity image reconstruction. Time-correlated single-photon counting (TCSPC) allowed for time-of-flight data to be measured, and the high-sensitivity and picosecond timing resolution this provided enabled us to create high-resolution intensity images and depth maps from distant targets whilst maintaining low average optical output power levels. The LiDAR system operated at a wavelength of 1550 nm, and used a pulsed fiber laser source for flood-illumination of the target scene. The detector was a 32 × 32 InGaAs/InP single-photon avalanche diode detector array mounted on precision translation stages. Operating in the short-wave infrared meant that the system could work at long range in daylight conditions, as the effect of solar background is reduced compared to shorter wavelengths and atmospheric transmission was relatively high. This paper presents depth and intensity profiles taken at a target range of approximately 325 m from the system location. The transceiver system operated at eye-safe, low average optical output power levels, typically below 5 mW.
In digital holography, the field of view (FOV) and lateral resolution are limited by the pixel pitch and sensor dimensions, respectively. A large numerical aperture can be synthesized to increase the FOV and spatial resolution by coherently combining low resolution holograms obtained for different illumination and/or observation directions. This is known as Synthetic Aperture Interferometry (SAI) and in this work we describe the design, construction, calibration and testing of high numerical aperture compact coherent imagers (CI) which constitute the optical building block of a multi-sensor SAI array. The CIs consist of a photodetector array, a highly divergent reference beam close to it and an aperture that acts as a spatial filter to prevent aliasing of the digital holograms. We explore different optical designs to produce a highly divergent reference beam close to the sensor, including bulk optics, micro-optics, and ion beam milled optical fibres. An optimization approach is used to characterize the reference wavefront for accurate digital reconstructions of the scattered field first at the aperture plane and then at the object plane. The performance of a compact CI is demonstrated by reconstructing an object 76 mm wide at 80 mm from the sensor, which corresponds to a numerical aperture NA>0.5.
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