In-Line Fiber-Optic Fabry-Perot Refractive-Index Tip Sensors

We propose and demonstrate two novel in-line fiber-optic tip sensors for practical refractive-index (RI) measurement, which are based on an intrinsic Fabry-Perot interferometer (IFPI) formed by a section of conventional single-mode fiber or endlessly single-mode photonic crystal fiber (EPCF), respectively. Such an IFPI sensor has the advantages of easy fabrication, low joint and transmission losses, low-cost and good fringe visibility. Simultaneous measurement of RI and temperature can be realized by determination of the cavity length change and the fringe visibility of such an IFPI, respectively. In addition, it is found that the fringe visibility of the IFPI is insensitive to temperature change, providing a practical way to measure refractive index with self temperature compensation and offers a refractive-index resolution of similar to 10(-5) in its linear operating range. The experimental data agree well with the theoretical results.