Measuring Absolute Distance Using Frequency Scanning Interferometry
Haitham Mohamed Hussein Mohamed;
Abstract
Unlike the conventional displacement interferometers where only the incremental displacement of a target is measured, absolute distance interferometers measure distance to a fixed target. In this thesis, frequency scanning interferometry (FSI) is used to measure distances from 1 to 5 meters absolutely. The FSI system consists mainly of, an external cavity tunable diode laser, Michelson interferometer, Fabry-Perot cavity, and a simple beam collimation system.
The external cavity tunable diode laser (ECDL) has a wavelength scanning range of 10 nm that covers the wavelength range from 665 nm to 675 nm. As with most of the tunable lasers, the ECDL contains one mode-hop in the middle of the scanning range. This mode-hop is considered as a drawback when the laser is used for distance measurements. The Michelson interferometer is a typical one with two high reflective mirrors and a beam splitter and a photodetector to detect the fringes. The Fabry-Pérot cavity is made out of a spacer and two mirrors with reflectivity of 50:50. The cavity spacer is made from an ultra-low-expansion glass (ULE-glass) of a thermal expansion of 0 ± 30 ppb/°C (from 5°C to 35°C). The thermal expansion of ULE-glass is lower 100 times than the normal glass. Therefore, the FP cavity length is independent from the environmental temperature changes and can be used as a frequency reference to determine the laser scanning range.
The distances from 1 to 5 m are measured by scanning the ECDL while acquiring interference signals from both the Michelson interferometer and the FP cavity. A software program is developed to count the acquired fringes in such a way that it avoids counting the fractional fringes and removes the mode-hop defected parts of the fringes without significantly affecting the accuracy of the distance measurement. The achieved distance repeatability of the constructed system is ±3.9 L µm.
A high accurate positioning stage with 1 𝜇m accuracy over the 26 mm scanning range is used to estimate the setup accuracy. The combined uncertainty of the setup is found to ±59 L μm which is limited by the accuracy of the used calibration stage.
Key words: Frequency scanning, Interference, Absolute distance, Michelson interferometer, Fabry-Pérot interferometer.
The external cavity tunable diode laser (ECDL) has a wavelength scanning range of 10 nm that covers the wavelength range from 665 nm to 675 nm. As with most of the tunable lasers, the ECDL contains one mode-hop in the middle of the scanning range. This mode-hop is considered as a drawback when the laser is used for distance measurements. The Michelson interferometer is a typical one with two high reflective mirrors and a beam splitter and a photodetector to detect the fringes. The Fabry-Pérot cavity is made out of a spacer and two mirrors with reflectivity of 50:50. The cavity spacer is made from an ultra-low-expansion glass (ULE-glass) of a thermal expansion of 0 ± 30 ppb/°C (from 5°C to 35°C). The thermal expansion of ULE-glass is lower 100 times than the normal glass. Therefore, the FP cavity length is independent from the environmental temperature changes and can be used as a frequency reference to determine the laser scanning range.
The distances from 1 to 5 m are measured by scanning the ECDL while acquiring interference signals from both the Michelson interferometer and the FP cavity. A software program is developed to count the acquired fringes in such a way that it avoids counting the fractional fringes and removes the mode-hop defected parts of the fringes without significantly affecting the accuracy of the distance measurement. The achieved distance repeatability of the constructed system is ±3.9 L µm.
A high accurate positioning stage with 1 𝜇m accuracy over the 26 mm scanning range is used to estimate the setup accuracy. The combined uncertainty of the setup is found to ±59 L μm which is limited by the accuracy of the used calibration stage.
Key words: Frequency scanning, Interference, Absolute distance, Michelson interferometer, Fabry-Pérot interferometer.
Other data
| Title | Measuring Absolute Distance Using Frequency Scanning Interferometry | Other Titles | قياس المسافة المطلقة بإستخدام تقنية مسح ترددات التداخل الضوئي | Authors | Haitham Mohamed Hussein Mohamed | Issue Date | 2015 |
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