Hyperspectral long-distance metrology using a femtosecond laser supercontinuum
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Author
Date
2024Type
- Doctoral Thesis
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Abstract
Rapid and accurate distance measurements over ranges from tens of meters to kilometers are essential in various applications, such as large-scale industrial manufacturing, alignment of particle accelerators, surveying, and three-dimensional (3D) digitization of our environment. Conventionally, electro-optical distance measurement (EDM) techniques based on nearly monochromatic laser sources are widely used in such applications. The measurement principle relies on monitoring the round-trip propagation delay of optical signals to estimate the distance to a remote target. However, for distances exceeding several hundred meters, the measurement accuracy on cooperative targets is practically limited by the uncertainties in estimating the refractive index of air along the propagation path. It arises from spatial and temporal meteorological variations in the atmosphere and may lead to apparent distance errors of several millimeters or more. Nevertheless, these errors can be compensated by exploiting atmospheric dispersion through simultaneous distance measurements at multiple wavelengths, i.e. multiwavelength EDM. Apart from long-distance metrology, EDM also plays a pivotal role in LiDAR applications for capturing the 3D geometry of natural surfaces. Combining the range information with extended surface reflectance data over multiple (multispectral) or a continuous distribution of wavelengths (hyperspectral) has proved advantageous for automated material classification, point-cloud segmentation, and remote sensing applications.
With the advent of mode-locked femtosecond (fs) lasers, optical frequency combs have emerged as a promising alternative to conventional continuous-wave modulated lasers for high-precision distance metrology. While benefiting from the frequency comb technology, optical supercontinuum (SC) coherently broadened from a mode-locked fs laser is a viable solution for applications requiring measurements over a continuous broadband spectrum or on several spectral bands. This thesis introduces the use of SC for multiwavelength refractivity-compensated EDM and hyperspectral LiDAR for precision laser scanning. Corresponding experimental platforms are developed for the related investigations. Distance measurements are estimated from the differential phase delay of the intermode beats generated upon photodetection of the SC. Experimental analyses show the range precision, accuracy, factors influencing the measurement noise, and current limitations of the technique. The results obtained from the multiwavelength EDM demonstrate effective inline refractivity compensation, indicating the possibility of achieving precise distance measurements in practical conditions. Experiments using the developed hyperspectral LiDAR show favorable range precision compared to established solutions. They highlight its potential for joint high-precision scanning, spectrum-based material classification, and remote sensing of vegetation. The experimental investigations indicate that SC-based distance measurements using the intermode beating approach may lead to a promising technological basis for hyperspectral long-distance metrology. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000667524Publication status
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Contributors
Examiner: Wieser, Andreas
Examiner: Salido-Monzú, David
Examiner: Garcia-Asenjo Villamayor, Luis
Examiner: Lienhart, Werner
Publisher
ETH ZurichSubject
Optical distance metrology; Hyperspectral LiDAR; Supercontinuum; Precision laser scanningOrganisational unit
03964 - Wieser, Andreas / Wieser, Andreas
Funding
184988 - Augmented Capability EDM using Phase and Power Spectral Signatures (SNF)
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