Long-distance ranging in existing coherent lidar techniques suffer from the coherence length of lasers. Here we present a coherent multi-tone continuous-wave (MTCW) lidar technique that performs single-shot simultaneous ranging and velocimetry with a high resolution at distances far beyond the coherence length of a CW laser, without frequency/phase sweeping. The proposed technique utilizes relative phase accumulations at phase-locked RF sidebands and Doppler shifts to identify the range and velocity of the target after a heterodyne detection of the beating of the echo signal with an unmodulated CW optical local oscillator (LO). The predefined RF sidebands enable ultra-narrow-bandwidth RF filters in the analog or digital domain to suppress noise and achieve high SNR ranging and velocimetry. Up-to-date, we demonstrated that the MTCW-lidar could perform ranging ×500 beyond the coherence length of the laser with <1cm precision. In a quasi-CW configuration, >1km ranging is realized with <3cm precision. Moreover, we incorporate machine-learning algorithms into MTCW-lidar to identify the reflections from multiple targets and improve the range resolution. Since relative phases of RF-sidebands are utilized for ranging, and common phase noises can be suppressed in signal processing, we show that the LO in heterodyne detection does not have to be the same laser source. Hence a separate free-running laser can be used. This approach paves the way for novel optical localization. To prove the concept, we present that a receiver with a free-running CW LO can determine its relative distance to a remote transmitter at 1.5km away with a <5cm accuracy.
Lidar technologies have been investigated and commercialized for various applications such as autonomous driving and aerial vehicles. The pulsed time of flight and frequency-modulated continuous-wave lidars are the two common lidar technologies that dominate. As an alternative to the available lidars, we developed the phase-based multi-tone continuouswave (PB-MTCW) technology that can perform single-shot simultaneous ranging and velocimetry measurements with a high resolution at distances far beyond the coherence length of a CW laser, without employing any form of sweeping. The proposed technique utilizes relative phase accumulations at phase-locked RF sidebands to identify the range of the target after a heterodyne detection of the beating of the echo signal with an unmodulated CW optical local oscillator (LO). Upto-date, we demonstrated that the PB-MTCW lidar could perform ranging ×500 beyond the coherence length of the laser with <1cm precision. Here, we implement machine learning (ML) algorithms to the PB-MTCW architecture to improve the ranging resolution, as well as to provide a solution to multi-target reflections using tone-amplitude variations. We used four different training schemes by utilizing the acquired RF tones and phases from simulation results, experimental results, and their combinations in a convolutional neural network model. We demonstrate that the ML algorithm yields an average mean square error of ~0.3mm compared to the actual target distance, hence enhancing the ranging resolution of PB-MTCW lidar. It is also shown that the ML algorithm can distinguish multiple targets in the same line of sight with a 98%±0.7% success rate depending on the targets’ reflectance and distances.
Over the past years, light detection and ranging (lidar) technologies have been investigated and commercialized for various applications such as autonomous vehicles, terrestrial mappings, and precision measurements. Currently, the frequently used ranging methods are the pulsed time of flight (PToF) and frequency modulated continuous wave (FMCW) lidars that relies on frequency sweeping to capture range and velocity information. We have previously developed and demonstrated the multi-tone continuous wave (MTCW) that operates by employing amplitude modulation via multiple radio frequencies (RF) and coherent detection. Here, we present a theoretical and experimental study on phase-based MTCW lidar that can detect the range and velocity of objects with arbitrary velocities. The experiments demonstrate that the phase and frequency of the Doppler-shifted fixed RF tones can be used to extract the range and velocity information in a single shot measurement. We show that a <±1cm resolution in the ranging, limited by the temporal resolution of the detection system, and a 0.5cm/s speed resolution is limited by the frequency resolution of the detection system are achievable. Moreover, the proposed approach has the potential to mitigate the requirement for a narrow linewidth laser for coherent detection.
KEYWORDS: Satellites, Field programmable gate arrays, Ocean optics, Mirrors, LIDAR, Control systems, Receivers, Space operations, Solar cells, Laser components
In this work, we designed a 12U CubeSat Platform for a Multi-Tone Continuous Wave Lidar system, utilizing coherent detection, which is used as an optical altimetry and velocimetry measurement device. The spacecraft is designed to be operational for a period of 6 to 12 months, and the primary goals are to develop a standalone small spacecraft technology that enables an optical remote sensing. Here, we describe the mechanical design and the thermal analysis of the spacecraft. Due to the random vibration and shock response during launching, a vibration isolation was designed to protect the optical components and alignments. The necessity of high optical power creates thermally localized hot spots that need to be dissipated while remaining in the operational temperature range. We designed thermal dissipation systems, including radiators, heat pipes, thermo-electric coolers, and used space-grade exterior paint to sustain the operation of the MTCW Lidar in the 12U CS.
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