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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 high resolution and simplicity of the measurement mechanism are required. We propose a method using an optical comb and an etalon harmonically. Using this method, it is possible to achieve measurements with resolution of less than a few nanometers and improved sensitivity. 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 including a repetition frequency scanning optical comb and verified it experimentally. The repeatability of the measurement was approximately 3.0 nm (σ = 2) when 175 mm etalon was used. The measured absolute value of the etalon length was compared to the coordinate measuring machine (CMM) measurement results and difference between the two values was

Water-guided laser processing have been widely used because of low heat affected zone and high aspect ratio processing. However, its processing point is not determined and must be measured with several μm accuracy. For this purpose, in-process measurement method was proposed. Measurement beam of optical comb is coaxially introduced to processing beam so that absolute distance of optical path in water-guide is measured with using optical comb tandem pulsed interferometer in water. To verify the principle of proposed method, in-water absolute distance measurement was evaluated. Repeatability of in-water measurement was under 100 nm and 1.0 mm displacement of target mirror in water was properly measured with under 1 μm error from reference displacement sensor. From these results, it is confirmed that proposed method properly works in-water environment.