Design of Pulsed-coherent Lidar
- Author(s): Chen, Li-Yang
- Advisor(s): Yang, Chih-Kong Ken
- et al.
Lidars are becoming common components for remote sensing in many emerging applications such as autonomous vehicle and facial recognition. To accurately visualize the point cloud for further data processing, metrics such as precision, sampling rate, and linearity are important. Phase detection has been widely used for high precision, however, resolving the ranging ambiguity beyond one carrier period lowers the system acquisition rate. Pulsed direct time-of-flight (ToF) detection, on the other hand, provides high sampling rate with single-shot measurements, but the precision is limited by the walk error. In this work, we present a pulsed-coherent detection to combine the advantages from both the pulsed detection with high sampling rate and the phase detection with high precision. The CW laser source is amplitude modulated by a high RF carrier and a low frequency pulsed envelope modulation. At the receiving end, the segmented time-of-flight detection algorithm measures the phase shift of the RF carrier as fine ToF and counts the arrival time of the low-frequency mask's envelope as intermediate and coarse ToF. This pulsed-coherent lidar can simplify the optical setup while achieving high precision and high sampling rate. Three different receiver prototypes are demonstrated with two test chips: (1) analog-based receiver which has separate paths for coarse and fine detection. The coarse ToF is recorded by detecting the pulse’s edge whereas the fine measurement is measured by phase detection. The proposed post-edge detection with automatic gain control loop suppresses the walk error; (2) DSP-based homodyne receiver which we reuse the same chip of analog-based receiver and collect the data by sub-sampling ADCs. Both the coarse detection and fine detection are calculated in the digital signal processor (DSP). This receiver improves the immunity to PVT variation and inherently aligns the segmented measurements by using the sampling clock as the reference hence achieving better precision at higher sampling rates; (3) DSP-based heterodyne receiver which we take a lower frequency clock as the reference input and use a local phase-locked loop (PLL) to generate the clock for the down-conversion mixer. This LO generation reduces the coupling between the clock and the incoming signal. Also, the two-step down-conversion with digital down-mixing improves the power efficiency and immunity to PVT variation without additional calibration. In addition to the innovation of the architecture, this work also presents some novelties of the circuit implementations: (1) we introduce a narrowband input-matching to improve the noise performance which uses direct wire-bonding between the photodiode bare die to the low noise amplifier; (2) we implement phase-invariant amplifiers to operate the lidar system in wide dynamic range; (3) we build a low phase noise LO generator based on a ring-VCO-based phase-locked loop. Two test chips are fabricated in TSMC 28nm CMOS technology. We successfully demonstrate the lidar system with both 1-D scanning and 2-D scanning. The system achieves <10�m precision with 5-MHz integration bandwidth and 30-�m INL at 2.5-m distance.