SPAD ToF Imager Thesis

1University of Oulu, Finland, publishes PhD Thesis "Time-gating technique for a single-photon detection-based solid-state time-of-flight 3D range imager" by Henna Ruokamo.

"This thesis is concerned with the development of a solid-state 3D range imager based on use of the sliding time-gate technique in a SPAD array and short (~200 ps), intensive laser pulses. The area of the in-pixel electronics needed in time-gated imagers is small, which leads to a high fill factor and a possibility for implementing large arrays. The use of short laser pulses increases the precision and frame rate, since depth measurement can be limited to the range of interest, e.g. around the surface of the target. To increase the frame rate further, the array can be divided into subarrays with independently defined ranges. Tolerance of high background light is achieved by using sub-ns time-gate widths.

A SPAD array of 80 x 25 pixels is developed and realized here. The array is divided into 40 subarrays, the narrow (less than 0.8 ns) time-gating positions for which can be set independently. The time-gating for each of the subarrays is selected separately with an on-chip DLL block that has 240 outputs and a delay grid of ~100 ps. The fill factor of the sensor area is 32%. A 3D range image measurement at ~10 frames per second with centimetre-level precision is demonstrated for the case of passive targets within a range of ~4 metres and a field of view of 18 × 28 degrees, requiring an average active illumination power of only 0.1 mW. A frame rate of 70 range images per second was achieved with a higher laser average illumination power (~5 mW) and pulsing rate (700 kHz) when limiting the scanning range for each subarray to 30 cm around the surfaces of the targets.

An FPGA-based algorithm which controls the time-gating of the SPAD array and produces the range images in real time was also developed and realized.