Temporal and Spatial Focusing in SPAD-Based Solid-State Pulsed Time-of-Flight Laser Range Imaging

Abstract

The relation between signal and background noise strengths in single-photon avalanche diode (SPAD)-based pulsed time-of-flight 3-D range imaging is analyzed on the assumption that the SPAD detector is operating in the single photon detection mode. Several practical measurement cases using a 256-pixel solid-state pulsed time-of-flight (TOF) line profiler are presented and analyzed in the light of the resulting analysis. It is shown that in this case it is advantageous to concentrate the available optical average power in short, intensive pulses and to focus the optical energy in spatial terms. In 3-D range imaging, this could be achieved by using block-based illumination instead of the regularly used flood illumination. One modification of this approach could be a source that would illuminate the system FOV only in narrow laser stripes. It is shown that a 256-pixel SPAD-based pulsed TOF line profiler following these design principles can achieve a measurement range of 5–10 m to non-cooperative targets at a rate of ~10 lines/s under bright sunlight conditions using an average optical power of only 260 µW.

SPADs and SiPMs Arrays for Long-Range High-Speed Light Detection and Ranging (LiDAR)

“…This can be achieved for instance by means of diffractive optical elements (DOEs), which generate spot illuminations of the scene. As demonstrated in [ 74 ], scanning can be convenient in terms of signal-to-background ratio (SBR) in respect to flash-LiDAR. In fact, given a certain amount of energy per pulse and total measurement time, scanning across M positions reduces the exposure time by a factor M, but it is compensated by the fact that the laser energy is concentrated in smaller area thus the energy per area density increases by a factor M as well.…”

Solid-state block-based pulsed laser illuminator for single-photon avalanche diode detection-based time-of-flight 3D range imaging

“…The "noise" in the histogram is predominantly produced by random background hits, and the "signal" by the signal hits occurring at a time interval corresponding to the transit time of the photons to the target and back to the receiver. [9][10][11][12][13] It is well known that focusing of the available average optical illumination power into energetic ns or even sub-ns impulse-like laser pulses improves the precision of the system and its SNR, especially under high background illumination conditions. 13 Many recent developments within this field have focused on the development of high performance, versatile 2D SPAD receivers with on-chip TDCs and signal processing units supporting a flood illumination-based flash 3D LIDAR architecture (as in Fig.…”

“…[14][15][16][17][18] Recently, however, it has been proposed that better system performance, within the limits of the permitted average illumination power, could be achieved with solid-state block-based illumination. 13,[19][20][21] This is a strategy, in which only part of the system FOV is illuminated by each emitted laser pulse, e.g., a narrow block covering 1/16 th of the total FOV of the system (marked in yellow in Fig. 2).…”

“…This follows from the fact that the noise is proportional to the ratio of the length of the detection window (which is 16 times shorter than with flood illumination) to the mean time interval between random detections (caused by detector dark counts and the background illumination, which typically dominates in outdoor measurements). As a result, the SNR (which is proportional to the ratio of the number of signal detections to the square root of the noise detections), would then be four times [ p (number of blocks)] 13 better, which means that a lower number of laser pulses would be needed to achieve the same SNR as with flood illumination. This improved SNR can be used to speed up the measurements (by a factor related to the number of blocks) or to increase the maximum measurement range.…”

“…This improved SNR can be used to speed up the measurements (by a factor related to the number of blocks) or to increase the maximum measurement range. 13,19 An important additional advantage is that the number of TDCs needed would be much lower with block-based illumination than with flood illumination (by a factor of 16 in the above example). This is a significant advantage since it markedly lowers the complexity and costs of the receiver circuit.…”