Eye-safe non-line-of-sight localization using compact nanosecond laser diodes and single-photon-avalanche-diode arrays

Active optical non-line-of-sight (NLOS) imaging faces severe photon loss and stringent timing demand, often motivating bulky, non-eye-safe systems with femto- or picosecond lasers and spatial scanning schemes. We present an eye-safe, compact NLOS localization approach that uses a single-photon-avalanche-diode array with on-chip timing and inexpensive and compact nanosecond pulsed laser diodes. To circumvent first-photon induced saturation in soft-gated detectors, the scene is illuminated from two positions placed outside the detector's field of view. The detector observes the relay wall in parallel, yielding a photon-efficient, non-confocal measurement. Transient simulation of multiple scattered laser pulses, that returned to the relay wall after an object interaction, are used with back projection reconstruction to assess illumination schemes. It is shown that superimposing reconstructions from two non-central illumination positions reduces the pulse-width-induced uncertainty of the determined target position, which is validated by measurements. Further compensation of the extended pulse width is analyzed with temporal and spatial filters. Matched filtering in the temporal domain outperforms spatial edge detection in the reconstruction volume, so that localization resolution becomes primarily limited by the detector's temporal resolution and measurement geometries rather than temporal pulse width. We outline a hybrid LiDAR-NLOS system for direct relay-wall calibration, pointing toward practical, eye-safe NLOS localization, with compact solid-state hardware.