An 8-Tap CMOS Lock-In Pixel Image Sensor for Short-Pulse Time-of-Flight Measurements

Abstract

An 8-tap CMOS lock-in pixel image sensor that has seven carrier-capturing and a draining time window was developed for short-pulse time-of-flight (TOF) measurements. The proposed pixel for the short-pulse TOF measurements has seven consecutive time-gating windows, each of which has the width of 6 ns, which is advantageous for high-resolution range imaging, particularly for relatively longer distances (>5 m) and under high ambient light operations. In order to enhance the depth resolution, a technique for the depth-adaptive time-gating-number assignment (DATA) for the short-pulse TOF measurement is proposed. A prototype of the 8-tap CMOS lock-in pixel image sensor is implemented with a 1POLY 4METAL 0.11-μm CIS process. The maximum non-linearity error of 1.56%FS for the range of 1–6.4 m and the depth resolution of 6.4 mm was obtained at 6.2 m using the DATA technique.

Hybrid Time-of-Flight Image Sensors for Middle-Range Outdoor Applications

“…The detailed procedure is similar to the method described in ref. [28]. The coarse TOF measurements can be done by using the same function used for the fine TOF, i. e., Eqs.…”

“…Additionally, the DATA method can be applied to the gating signals within each subframe, particularly for the subframe assigned to the closest range (SF1 in Fig. 6) [28]. An example of the assignment of four different time-gating numbers to 1…”

“…This hybrid TOF imaging technique with multipletap demodulation pixels and multiple sub-frames has a distinct advantage over iTOF counterparts, mainly because it avoids the ambient light charge from being captured in signal charge storage nodes in each pixel, resulting in a significant difference in TOF measurement precision under strong sunlight. Another key technique for mitigating signal saturation to very close objects is depth-adaptive time-gating-number assignment (DATA) based on the hTOF imaging technique, and a TOF imaging with a very wide working range is realized [28]. In this paper, based on the multi-tap demodulation pixels and introducing the DATA technique in multiple-subframe signal capturing, the concept of hTOF image sensors is fully established and their potentials in middle-range outdoor (under direct sunlight) high-image-resolution 3D imaging are discussed with the modeling and analysis of the actual performance of range precision.…”

“…The goal of this study is to demonstrate the potential of the proposed hybrid TOF (hTOF) image sensors as candidates for high-resolution TOF cameras for middle-range outdoor use under direct sunlight. The hTOF image sensors are based on the same pixel operation principle as the iTOF image sensors using a pulse modulation [22]- [25], but they are optimized for small duty-cycle modulated light pulses by implementing a highspeed charge draining gate and having as many signals taps as possible [22]- [28]. The time-gated signals in multiple taps and their range-shifted versions in subframes are first used to directly measure the coarse TOF based on the time-stamping concept used in dTOF image sensors and then used for fine TOF measurement based on the demodulation pixels principle employed in iTOF image sensors, leading to a coarse (direct) to fine (indirect) TOF measurement principle.…”

High-Linearity High-Resolution Time-of-Flight Linear-Array Digital Image Sensor Using Time-Domain Feedback

“…Two types of indirect ToF imaging methods exist, depending on the waveform of the modulated light. One is continuous wave (CW)-based [ 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 ], and the other is short pulse (SP)-based [ 16 , 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 ]. The SP-based indirect ToF has a better tolerance to ambient light because the light power is concentrated on the short pulse, and the charge-draining function of the ToF pixel reduces the influence of the ambient light.…”