Single-shot ranging and velocimetry with a CW lidar far beyond the coherence length of the CW laser

Abstract

The spectral linewidth of the continuous-wave (CW) lasers is one of the key limitations on the coherent lidar systems, which defines the maximum detection range. Furthermore, precise phase or frequency sweeping requirements are a deterrent in many applications. Here, we present the Phase-Based Multi-Tone Continuous Wave (PB-MTCW) lidar measurement technique that eliminates the necessity of using high coherence laser sources as well as any form of phase or frequency sweeping while employing coherent detection. In particular, we modulate a CW laser source with multiple radio-frequency (RF) tones to generate optical sidebands. Then, we utilize the relative phase variations between the sidebands that are free from laser phase noise to calculate the target distance via post-processing and triangulation algorithms. We prove that the PB-MTCW technique is capable of performing single-shot ranging and velocimetry measurements at more than 500× the coherence length of a CW laser in a benchtop experimental demonstration. Overall, precise phase or frequency sweeping requirements and the spectral linewidth of CW lasers, which defines the maximum detection range, are the key limitations of long-distance coherent lidar systems. The proposed approach overcomes these limitations and enables single-shot ranging and velocimetry measurements, especially for long-range applications such as spacecraft and airborne coherent lidars.

Machine-learning-enhanced, phase-based multi-tone continuous-wave lidar

“…For a specific n, the Lm solution for each ∆ωi,j should converge to a single value. We created a triangulation algorithm to find the common solution, which generates all possible solutions for each IF by sweeping the integer values of n. Then, by obtaining the common solution for all IFs, we estimate the target range 29,30 . A static target's position is determined using the experimental setup depicted in Figure 2.…”

“…We previously introduced and demonstrated the phase-based multi-tone continuous-wave (PB-MTCW) lidar, which can perform ranging well beyond the coherence length of a CW laser, to address the aforementioned constraints on coherent lidars 29 . In this method, the output of a CW laser is divided, one arm is amplitude modulated with a number of phase-locked radio frequencies (RF), and the other branch is left unmodulated to serve as a local oscillator (LO) for shot noise-limited heterodyne detection.…”

“…This method has the potential to help precise navigation and localization applications 35 and to address multi-path interference, one of the main problems with current lidar techniques. A continuous wave laser is modulated using a Mach Zehnder modulator in the previously demonstrated Phase-Based Multi-Tone Continuous Wave lidar (PB-MTCW) method 29 . Each tone has a frequency of fi.…”

Optical ranging and localization at beyond the coherence length of lasers

“…To remedy the aforementioned limitations to coherent lidars, we previously developed and demonstrated the phasebased multi-tone continuous-wave (PB-MTCW) lidar, which can perform ranges far beyond the coherence length limitations 27 . In this technique, we split the output of a CW laser and amplitude modulate one arm with multiple phaselocked radio-frequencies (RF), while the unmodulated branch is kept as a local oscillator (LO) for shot noise-limited heterodyne detection.…”

“…This approach has the potential to resolve one of the main challenges of existing lidar techniques, which is multi-path interference and to facilitate precise navigation and localization applications 32 . The previously demonstrated Phase-Based Multi-Tone Continuous Wave lidar (PB-MTCW) technique involves modulating a continuous wave laser with multiple phase-locked radio-frequency tones each having a frequency of fi using a Mach-Zehnder modulator 27 . The accumulated phases of these tones are encoded in the echo signal after light propagation.…”