A miniature in-line fiber-optic Fabry-Perot etalon is fabricated on a photonic crystal fiber (PCF) by using 157 nm laser micromachining for the first time to our knowledge. Experimental results show that such a PCF-based etalon has an excellent fringe visibility of up to approximately 26 dB due to the mirror-finish quality of the two cavity surfaces inside the PCF. This etalon can be used as an ideal sensor for precise strain measurement under high temperature of up to 800 degrees C. It can also offer some other outstanding advantages, such as fast and easy fabrication, high reproducibility, capacity of mass production, low cost, low temperature-strain cross-sensitivity, and high signal-to-noise ratio.
Optical fiber extrinsic Fabry–Perot interferometers (EFPIs) have been extensively demonstrated for the measurement of displacement and displacement-related physical quantities, e.g., acceleration, pressure, with high sensitivity and resolution. Despite its wide and successful applications, a conventional EFPI is limited to measuring only one-dimensional (out-of-plane) movement of its external reflector. In this Letter, a new strategy for optical fiber sensing, particularly for EFPI sensing, is proposed and demonstrated, allowing for three-dimensional (3-D) measurements based on a hybrid and compact EFPI device. A 3-D integrated optical waveguide array is aligned against a lead-in optical fiber with an air gap, where an EFPI is formed by the end facet of the optical fiber and the end facet of the waveguide array. As a proof of concept, we experimentally demonstrate that 3-D positioning can be achieved from the EFPI with sub-micron resolution by simultaneously measuring the reflection and transmission of the device. The proposed strategy of using an optical waveguide as an external reflector for an optical fiber EFPI, combined with machine learning-based analysis, opens new avenues in the development of compact yet multi-dimensional sensors.
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