Polarization-Modulated Optical Homodyne for Time-of-Flight Imaging with Standard CMOS Sensors

Indirect time-of-flight (iToF) imaging is a widely applied technique to obtain a depth image from the phase difference of amplitude-modulated signals between emitted light and reflected light. The phase difference is computed via electrical correlation on a conventional iToF sensor. However, iToF sensors face a trade-off between spatial resolution and light collection efficiency because it is hard to downsize the circuit of the electrical correlation in a pixel. Thus, we propose a novel iToF depth imaging system based on polarization-modulated optical homodyne detection with a standard CMOS sensor. A resonant photoelastic modulator is employed to modulate the polarization state, enabling optical correlation through interaction with an analyzer. The homodyne detection enhances noise resistance and sensitivity in the phase difference estimation. Furthermore, the use of a polarization camera allows to reduce the number of measurements. We first validate the successful estimation of the phase difference in both setups with an avalanche photodiode or a CMOS sensor. The experimental results show accurate depth estimation even in challenging factors such as a low signal-to-noise ratio, temporal intensity variations, and speckle noise. The proposed system enables high-resolution iToF depth imaging using readily available image sensors.