We propose a novel absolute measurement method for precisely measuring the internal distance between two points, using a tandem low-coherence interferometer. In this method, a beam splitter is positioned between two plane mirrors, the locations of which are accurately determined at the nanometre level using tandem low-coherence interferometry. Using a He–Ne laser interferometer as a reference length standard, an internal distance of up to 100 mm was measured remotely, with a measurement uncertainty of 178 nm (k = 2).
A new laser tracker using an optical frequency comb with ball lenses as a reference and targets is proposed. In this system, high-accuracy absolute length measurement based on pulsed interferometry of an optical comb is utilized. By using a ball lens with a refractive index of 2.0 as a reference, it can avoid the error caused by reference point displacement. As targets, ball lenses have a wider acceptance angle than a conventional retroreflector. In this work, two new designs of the laser tracker are proposed. The simulation and experimental investigation of the refractive index characteristics of a ball lens were examined to obtain the optimal parameters for each design. Using these designs, the one-axis absolute length measurement of as much as 10 m was performed, and its combined uncertainty was evaluated to be [(0.145 µm) 2 + (3.05 × 10 −7 l) 2 ] 1/2 , where l is measured length.
A Fabry-Perot interferometer, also called an etalon, is used in various applications such as resonant structures for lasers, wavelength filters, length and frequency standards, and sensors. When the etalon is used as sensor, both the measurement accuracy and simplicity of the measurement mechanism are required. We propose a method that satisfies both using an optical comb and an etalon harmonically. Using this method, it is possible to achieve measurements with an accuracy of less than a few nanometers and improved sensitivity due to long measurement range of pulsed interferometry. This method can be implemented with a simple system, and traceability can be easily achieved from the characteristics of the optical comb without the need for a high-precision wavelength meter. In this study, in addition to proposing a method, we implemented measurement system that included a repetition frequency scanning optical comb and verified it experimentally. The repeatability of the measurement was approximately 3.4 nm (σ=2) when 175 mm etalon was used. The measured absolute value of the etalon length was compared to the CMM measurement results and difference between the two values was <1 m that indicated the absolute values agreed well with the MPE range of the CMM. The etalon displacement was measured and the result was compared to that of the conventional He-Ne interferometer. The difference between the two was less than 1 nm, indicating the validity of the measurement and its applicability as sensor. The accuracy was evaluated using the values obtained in the experiments. These results demonstrated that the proposed measurement method is feasible and functional for etalon sensor.
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