Coherent Optical Modems for Full-Wavefield Lidar

TL;DR

This work introduces full-wavefield lidar (FWL) by repurposing off-the-shelf coherent optical modems to enable coherent free-space imaging with joint depth, velocity, and polarization estimation. FW Lidar relies on randomized, dual-polarization modulation and a time-resolved, likelihood-based reconstruction to recover per-pixel depth, radial velocity, and polarization changes, leveraging a measurement model that accounts for propagation delay, Doppler shifts, and polarization scrambling. The authors develop a two-stage, regularized optimization and demonstrate a hardware prototype at 1550 nm with a 74 GHz sampling rate, achieving mm-scale depth and sub-meter-per-second velocity estimates under challenging lighting and materials, while showing robustness to ambient light and translucent barriers. The approach broadens access to coherent lidar by using telecom-grade hardware, enabling flexible waveform control, mm-scale ranging, reliable velocimetry, and improved performance in interference-prone or scattering-rich environments, with potential for real-time imaging workflows in practice.

Abstract

The advent of the digital age has driven the development of coherent optical modems--devices that modulate the amplitude and phase of light in multiple polarization states. These modems transmit data through fiber optic cables that are thousands of kilometers in length at data rates exceeding one terabit per second. This remarkable technology is made possible through near-THz-rate programmable control and sensing of the full optical wavefield. While coherent optical modems form the backbone of telecommunications networks around the world, their extraordinary capabilities also provide unique opportunities for imaging. Here, we repurpose off-the-shelf coherent optical modems to introduce full-wavefield lidar: a type of random modulation continuous wave lidar that simultaneously measures depth, axial velocity, and polarization. We demonstrate this modality by combining a 74 GHz-bandwidth coherent optical modem with free-space coupling optics and scanning mirrors. We develop a time-resolved image formation model for this system and formulate a maximum-likelihood reconstruction algorithm to recover depth, velocity, and polarization information at each scene point from the modem's raw transmitted and received symbols. Compared to existing lidars, full-wavefield lidar promises improved mm-scale ranging accuracy from brief, microsecond exposure times, reliable velocimetry, and robustness to interference from ambient light or other lidar signals.

Figures (16)

• Figure 1: Qualitative comparison of FWL to the Kinect Azure bamji2018impixel and single-photon lidar. FWL recovers accurate depth and velocity with only 1 $\mu$s exposure times per pixel and an eye-safe 2 mW laser. The Kinect fails at light levels corresponding to 10 $\mu$s exposure times, which we emulate using neutral density filters. For the single-photon lidar system (see the supplement for a description), we emulate a 10 $\mu$s exposure time by thinning the detected photon counts lewis1979simulation.
• Figure 2: Illustration of coherent and incoherent detection schemes.
• Figure 3: Overview of data transmission in coherent optical modems. (a,b) Binary data are collected and encoded into a discrete sequence of symbols $\mathbf{x}_{n}$, each paired with a certain amplitude, phase, and polarization state of light. (c) This sequence of symbols is used to create a piecewise constant waveform with segments of duration $T$ (shown for a single polarization). In practice, a band-limited version of this waveform modulates laser light with amplitude $E_0$ and wavelength $\lambda = c \frac{2\pi}{\omega}$, where $\omega / 2\pi$ is the optical frequency and $c$ is the speed of light. (d) The modulated light is emitted, collected by a receiver, and interfered with an unmodulated receiver-side laser to remove the optical frequency shift. The resulting waveform is sampled to recover the demodulated symbols $\mathbf{Y}_{n}$.
• Figure 4: Illustration of imaging with coherent optical modems. (a) The modulated wavefield $\mathbf{e}_\text{TX}$ is transmitted to the target through a fiber optic cable, circulator, and collimator. The received wavefield $\mathbf{e}_\text{RX}$ is demodulated and detected by the optical modem. (b) The transmitted wavefield is distorted by multiple effects: the propagation delay induces a shift in the measurements, shown in the bottom plots of $\mathbf{e}_\text{RX}$; scattering off of a moving surface scrambles the two transmitted polarization channels (modeled by multiplication with a Jones matrix $\mathbf{r}$) and induces a Doppler shift of frequency $\nu$; the wavefield is attenuated as it propagates back to the collimator; last, the measurements are corrupted by noise $\eta$ from the optical modem or optical amplifiers (not shown).
• Figure 5: Our FWL prototype and coaxial lidar systems in general have non-idealities such as reflections from interfaces in the optical path.