Multi-Tone Continuous-Wave Lidar
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Multi-Tone Continuous-Wave Lidar

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Over the past years, light detection and ranging (lidar) systems have been vastly investigated and commercialized for various platforms such as self-driving cars, unmanned aerial vehicles, and spacecraft. A wide range of lidar applications such as topographical imaging, remote object sensing, oceanographic and atmospheric surveillance, navigation, and driver assistance mostly relies on range measurements. The majority of the lidar instrumentation used in these applications operates in a pulsed time-of-flight (PToF) mode to acquire the range information by comparing the delay between the ejection time of an optical impulse and the detection time of the pulse in flight. Inherently, this conventional approach lacks the capability of simultaneous velocimetry and fine-ranging resolution requires high-speed detection electronics. Moreover, continuous-wave (CW) laser-based lidar systems are being investigated. CW lidars are desired because they use coherent detection techniques that provide a higher signal-to-noise ratio (SNR) than the direct detection method used in PToF. Up to date, various CW lidar techniques such as amplitude-modulated continuous wave (AMCW) lidars, frequency modulated continuous wave (FMCW) lidars, and phase-based ranging have been demonstrated. Hence, to perform the measurements and to extract the precise distance of the target, the AMCW method requires sweeping of amplitude and phase, while the FMCW technique requires frequency sweeping. As an alternative to the existing lidar technologies, Multi-Tone Continuous-Wave (MTCW) lidar is developed, which eliminates the necessity of any form of phase, frequency, or amplitude sweeping in a CW lidar configuration. In the MTCW approach, multiple fixed phase-locked radio frequencies (RF) are utilized to modulate a CW laser by using Mach-Zehnder modulators. A portion of the light is kept as a local oscillator to realize coherent detection to enhance the SNR. The modulated beam is transmitted to free space and the echo signal is collected by the receiver of the lidar. Each sideband accumulates a different phase based on the frequency and target distance. It is possible to compute the target distance by either converting the phase differences into RF tone power variations or by exploiting the phase and frequency differences between RF tones. Three versions of the MTCW lidar are constructed based on the utilization of tone amplitudes or phases, namely amplitude-base MTCW (AB-MTCW), phase-enhanced amplitude-based MTCW (PE-MTCW), and phase-based MTCW (PB-MTCW) lidars. Theoretical modeling, numerical simulations, and experimental verifications are presented for each technique. For AB-MTCW lidar, <1cm ranging precision with 0.8cm/s velocimetry resolution is demonstrated. Furthermore, the PE-MTCW approach is matured to provide a solution to fast targets ranging via AB-MTCW lidar. Finally, the PB-MTCW methodology is developed that can perform the ranging of dynamic or static targets at a range that is ×500 larger than the coherence length of the CW laser with a <1cm ranging resolution and precision. It is also shown that, by integrating the modulated beam with a quasi-CW pulsation, it is possible to perform ranging beyond the unambiguous range of the CW lidar, as well. Therefore, the PB-MTCW lidar can be considered an alternative long-distance remote sensing device for aerial or space-based altimetry applications.

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