Spectrograph solutions for portable hyperspectral LIDAR
Vuin, Maria (2022)
Vuin, Maria
2022
Teknis-luonnontieteellinen DI-ohjelma - Master's Programme in Science and Engineering
Tekniikan ja luonnontieteiden tiedekunta - Faculty of Engineering and Natural Sciences
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Hyväksymispäivämäärä
2022-07-19
Julkaisun pysyvä osoite on
https://urn.fi/URN:NBN:fi:tuni-202207015948
https://urn.fi/URN:NBN:fi:tuni-202207015948
Tiivistelmä
Simultaneously sampling prism and grating-based spectrograph solutions for portable hyper-spectral LIDAR (Light Detection and Ranging) applications are compared in this thesis. Hyper-spectral LIDARs (HSL) are laser-based devices that are used in remote sensing and detection, as they obtain spectral and spatial information simultaneously. HSLs operate in a wide spectral range and are often used with supercontinuum laser sources as they produce a very broad spectral band of light. In this thesis, a HSL designed and provided by the Finnish Geospatial Research Institute (FGI) is used and studied. The spectrograph solutions are compared to each other, and some software-based solutions, such as Fraunhofer approximation (FA) and Rigorous Coupled Wave Analysis (RCWA) along with the data provided by manufacturers. The main three aspects that are studied are efficiency, resolution, and size of the overall solution.
The HSL instrument obtains spectral and spatial information simultaneously from the environment. The spectrum is measured using a diffractive optical element (DOE) that separates the different wavelengths spatially. Before the spectrum is guided into the DOE it enters a slit and is collimated either with a lens or a mirror. After the DOE has spread the spectrum, the light is focused with a lens or mirror element. Subsequently, the radiation is detected by either a flux or an image detector. The measurement setup for this project was executed to study how the DOE operates. The key point of this thesis is to compare these solutions to find out which would be the best possible element for the final HSL application.
The software solutions should be investigated more to achieve realistic results. Some errors were detected due to the material properties and unfamiliar geometry of the optical element. The data provided by the manufacturers corresponded well to the measurement result, unlike the software solutions that evaluated well only the efficiency peaks of the gratings. Comparing different optical elements’, the prism had the best spectral efficiency as compared to gratings. However, the prism’s disadvantage is its dispersion, which has a considerable impact on the overall response. When comparing different grating solutions, no major differences were seen. The only disadvantage compared to the prism was the overlapping that gratings have. An additional spectrograph solution was introduced in this thesis, which was a combination of optical filter, prism, and grating. This lays the groundwork for future solutions.
In the application, it was noticed that the sensor’s number of channels is the limiting factor of resolution. The light source and the detector are components that notably affect the response of the application. Therefore, the DOE should be selected to have the efficiency peak where the sensor or light source is not that sensitive.
The HSL instrument obtains spectral and spatial information simultaneously from the environment. The spectrum is measured using a diffractive optical element (DOE) that separates the different wavelengths spatially. Before the spectrum is guided into the DOE it enters a slit and is collimated either with a lens or a mirror. After the DOE has spread the spectrum, the light is focused with a lens or mirror element. Subsequently, the radiation is detected by either a flux or an image detector. The measurement setup for this project was executed to study how the DOE operates. The key point of this thesis is to compare these solutions to find out which would be the best possible element for the final HSL application.
The software solutions should be investigated more to achieve realistic results. Some errors were detected due to the material properties and unfamiliar geometry of the optical element. The data provided by the manufacturers corresponded well to the measurement result, unlike the software solutions that evaluated well only the efficiency peaks of the gratings. Comparing different optical elements’, the prism had the best spectral efficiency as compared to gratings. However, the prism’s disadvantage is its dispersion, which has a considerable impact on the overall response. When comparing different grating solutions, no major differences were seen. The only disadvantage compared to the prism was the overlapping that gratings have. An additional spectrograph solution was introduced in this thesis, which was a combination of optical filter, prism, and grating. This lays the groundwork for future solutions.
In the application, it was noticed that the sensor’s number of channels is the limiting factor of resolution. The light source and the detector are components that notably affect the response of the application. Therefore, the DOE should be selected to have the efficiency peak where the sensor or light source is not that sensitive.