$256\times8$ SPAD Array With 256 Column TDCs for a Line Profiling Laser Radar

Abstract

A receiver circuit for pulsed time-of-flight line profiling laser radar has been designed in 0.35µm CMOS technology. The receiver consists of a line detector array with 256×8 SPADs with a pitch of 41.6µm. The diameter of the SPAD's active area is 25.6µm, which gives a fill factor of 0.35. Each column has 8 SPADs and a TDC with a range and resolution of 640 ns and 20 ps, respectively. The TDCs are based on the Nutt interpolation method where cyclic converters are utilized as fine converters. A compact demonstrator system has been built with a horizontal FOV of about 37 • , which gives a column-wise angular resolution of about 0.15 •. This solid-state system uses a pulsed laser diode with cylindrical transmitter optics to illuminate the full FOV of the receiver with a single laser pulse. The system can measure distances up to 30 m @ 28 fps with an accuracy and precision of few millimeters in typical indoor lighting conditions. Outdoors, in direct sunlight conditions, the measurement range is decreased to 5 m. Outdoor range, however, can be increased if frame rate is lowered and more laser shots are accumulated per frame. Index Terms-Time-to-digital converter (TDC), single photon avalanche diode (SPAD), time-of-flight (ToF), light detection and ranging (LiDAR). I. INTRODUCTION P ULSED time-of-flight (TOF) laser radar operates on the principle of sending a high power (>10W) and short (<5ns) laser pulse towards the target, and then measuring the time-of-flight by measuring the time difference between the transmitted pulse and the reflected echo pulse [1]-[4]. Since the velocity of light is relatively constant, the distance to the target can be calculated based on the measured transit time. These techniques have important advantages compared to other optical time-of-light methods, e.g. to the measurement of the phase difference between the transmitted continuous-wave (CW) optical signal and the reflected echo. For a pulsed TOF system, the non-ambiguous measurement range is limited basically by the pulsing rate of the transmitter and can thus be very long (i.e. 100m with 1MHz) whereas in the CW phase comparison techniques there is a trade-of between the non-ambiguous

Single Photon Lidar in Mobile Laser Scanning: The Sampling Rate Problem and Initial Solutions via Spatial Correlations

“…The experimental single-photon solid-state lidar that we use here is presented in and has been assembled by the authors of (Keränen, Kostamovaara, 2019), see Figure 1 (a). It is an advanced design of the one in (Keränen, Kostamovaara, 2018).…”

“…However, in contrast to (Keränen, Kostamovaara, 2018), each channel has 8 receivers, i.e. single photon avalanche diodes (SPADs), stacked on top of one another (Keränen, Kostamovaara, 2019). The digital to time converter (TDC) outputs the signal from the SPAD that triggers first.…”

“…the background illumination declines exponentially as a function of the range, when only one photon detection per emitted pulse per channel (or pixel) is recorded, as in (Keränen, Kostamovaara, 2019). Hence, such single photon detection benefits from a priori knowledge about the detection range in normalizing this exponential distribution away.…”

“…All the SPAD gates are opened simultaneously for measurement (Keränen, Kostamovaara, 2019) and for a constant light flux of background illumination, for each SPAD, there is a constant probability per time unit that a background photon triggers that SPAD. Specifically, background photon arrivals are Poisson distributed.…”

“…Hence, we model the noise distribution directly from the sample, such as the one in Figure 2 (a), and pre-process the signal as in Figure 2 (b) but with a smaller sample size as explained in the following. These steps are done as in the baseline method used in (Keränen, Kostamovaara, 2019, Keränen, Kostamovaara, 2018.…”