Centimeter-Wave Free-Space Time-of-Flight Imaging

TL;DR

This paper introduces a novel optical ToF imaging method that achieves micron-scale depth resolution in free-space, overcoming previous limitations by combining GHz modulation, dual-pass frequency doubling, and neural phase unwrapping.

Contribution

It presents a new all-optical modulation and computational phase unwrapping approach that significantly improves depth precision and robustness over existing ToF methods.

Findings

Achieves micron-scale depth resolution in free-space imaging.

Outperforms existing analog demodulation methods across various scenarios.

Demonstrates robustness to surface reflectance and ambient light.

Abstract

Depth cameras are emerging as a cornerstone modality with diverse applications that directly or indirectly rely on measured depth, including personal devices, robotics, and self-driving vehicles. Although time-of-flight (ToF) methods have fueled these applications, the precision and robustness of ToF methods is limited by relying on photon time-tagging or modulation after photo-conversion. Successful optical modulation approaches have been restricted fiber-coupled modulation with large coupling losses or interferometric modulation with sub-cm range, and the precision gap between interferometric methods and ToF methods is more than three orders of magnitudes. In this work, we close this gap and propose a computational imaging method for all-optical free-space correlation before photo-conversion that achieves micron-scale depth resolution with robustness to surface reflectance and ambient light with conventional silicon intensity sensors. To this end, we solve two technical challenges: modulating at GHz rates and computational phase unwrapping. We propose an imaging approach with resonant polarization modulators and devise a novel optical dual-pass frequency-doubling which achieves high modulation contrast at more than 10GHz. At the same time, centimeter-wave modulation together with a small modulation bandwidth render existing phase unwrapping methods ineffective. We tackle this problem with a neural phase unwrapping method that exploits that adjacent wraps are often highly correlated. We validate the proposed method in simulation and experimentally, where it achieves micron-scale depth precision. We demonstrate precise depth sensing independently of surface texture and ambient light and compare against existing analog demodulation methods, which we outperform across all tested scenarios.