Novel absolute length measurement method for etalon sensor using optical comb pulsed interference and harmonic etalon reflections

Abstract

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 <1 μm that indicated the absolute values agreed well with the maximum permissible length error 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. These results demonstrated that the proposed measurement method is feasible and functional for etalon sensor.