This paper introduces a transformative approach to 3D lidar imaging through the Multi-Tone Continuous Wave (MTCW) coherent lidar system. Addressing coherence length constraints in coherent continuous wave (CW) lidar systems. We present a method utilizing static RF modulation frequencies to achieve 3D imaging. Our system demonstrates the capability to measure distances up to 11km, surpassing the 950m coherence length of the laser. This approach has far-reaching implications for applications requiring extended ranging capabilities, marking a significant evolution in coherent lidar technology. The study concludes by highlighting the potential impact on various fields, including autonomous navigation and remote sensing, thereby paving the way for enhanced spatial awareness in diverse applications.
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.
Plasmonic structures have a wide variety of sensing applications because of their high field localization effect that leads to high sensitivity at lower powers. Specifically, plasmonic nanohole arrays are attractive platforms for sensing because of their easy alignment and measurement. In terms of fabricating these sensors, usually an adhesion layer is needed to ensure firm contact between the plasmonic metal layer and the substrate. Most fabrication efforts rely on titanium or chromium based metallic adhesion layers. However, the presence of the adhesion layer may hinder the plasmonic resonance by broadening the resonance and reducing the plasmonic field enhancement. This leads to degradation of sensing capabilities. We investigate the effect of tantalum, chromium, and titanium adhesion layers on plasmonic sensors made of nanohole arrays. Using the bulk refractive index data for metallic adhesion layers, we show that tantalum has the potential to show less damping effect compared to commonly used chromium and titanium. However, it still causes significant damping because of its high absorption, which becomes even larger for tantalum thin film according to our ellipsometry measurement results. We also propose here to use MgO dielectric adhesion layers to avoid the damping effect. Our investigation on MgO adhesion layers shows strong adhesion properties without scarifying sensor performance. Moreover, we will present an alternate sensor geometry that is less prone to damping by the adhesion layer and that can enhance the plasmonic resonance even if there is a metallic adhesion layer.
Because of its high range and resolution, light detection and ranging (LIDAR) is a significant technology for numerous applications, such as autonomous vehicles, robotics, aerial or terrestrial mapping, and atmospheric research. Current lidar market is mainly occupied by conventional pulsed time of flight lidars. However, recently emerging companies are utilizing frequency modulated continuous wave lidars for improved and robust range resolution, dynamic range, sensitivity and simultaneous velocity measurement. Here, we propose and demonstrate multi-tone modulated continuous wave (MTCW) lidar system made of a CW laser with multiple fixed RF tones for a high precision range finding and velocimetry. In the proposed approach, the interference of the scattered light with the reference is detected by a PIN photodiode to extract the modulation information. Since, the acquired light is traveled all the way to the target and back to the beam splitter, it carries the range and velocity information about the target as phase and frequency shift, respectively, on the RF modulation tones. We use 1550nm light source and multiple RF tone modulations ranging from 50 MHz to 6 GHz to demonstrate proof of principle for range finding. We also provide sine fitting algorithms on the measured RF tones to extract the range and velocity information in a single shot RF measurement. We show that the precision and range information are scaled by the selection of RF tones. By an engineered selection of RF tones and a laser source, the measurement precision can be increased without compromising the range.
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