Refractive index measurement using OTDR-based ring-down technique with S fiber taper

Abstract We proposed and demonstrated a fiber loop ring-down (FLRD) sensor based on S fiber taper (SFT) cascaded with a Faraday rotator mirror (FRM) for refractive index (RI) measurement. The FLRD cavity is established conveniently by an optical time domain reflectometry (OTDR). The net loss of the cavity is modulated by the RI-sensitive SFT. The surrounding refractive index (SRI) measurement can be realized by monitoring the slope of the ring-down pulses envelope. Owing to the compensation provided by the erbium-doped fiber amplifier (EDFA), the ring-down pulses signal of the OTDR-based FLRD cavity is improved effectively. Within the measurement range of 1.3330 ∼ 1.3518, the proposed sensor has a linear response to the SRI with a sensitivity of 2646.307 dB/(km RIU) at the interrogating wavelength of 1550 nm. Compared with the conventional FLRD sensors, the proposed OTDR-based RI sensor has the advantages of simple design and reflective sensing structure, which is easily implementable and cost-effective.