On Pulsed Time-of-Flight Laser Ranging With Sub-ns Laser Pulses and an Off–Focal Plane Digital CMOS SPAD Receiver

Pulsed time-of-flight lidar techniques based on the use of a gain-switched laser diode as a transmitter and a digital CMOS SPAD array with 9 × 9 SPAD unit elements and 10 time-to-digital converters are presented. The laser transmitter produces laser pulses with pulse energy, pulse width and repetition rate of 350 pJ, 65 ps and 100 kHz, respectively. The receiver uses a small aperture as a field stop to match the transmitter and receiver fields-of-view. The CMOS detector with the 9 × 9 SPAD unit elements is placed behind the focal plane in order to lower the random background detection rate of a single SPAD unit element. Thus, the possibility for detector blocking is also reduced in high background illumination conditions. Measurements show that the presented setup allows a distance measurement range of up to ∼90 meters to a target of brushed gray concrete in bright sunlight conditions (60 – 70 klux) with 5600 laser pulses sent and only activating a sub-array of 3 × 3 SPADs and 10 TDCs in the receiver. The measurement precision (σ-value) is in the millimeter range.