Ultrasensitive fiber-optic Vernier salinometer based on an integrated dual-microcavity interferometer

In this Letter, we report what we believe to be a novel integrated optical fiber Vernier sensor for ultra-sensitive salinity measurement of seawater. Based on the cladding-share strategy, a no-core-hollow-core-no-core fiber structure (NCF-HCF-NCF) is sandwiched into a millimeter-length open-cavity with proper lateral offset. The obvious Vernier effect is then observed with high envelope visibility (>20 dB) by means of fine adjusting the equivalent optical path differences of interferometers. The systematic experimental tests are conducted, and the results indicate the magnification factor of 8.33 is obtained and the salinity sensitivity reaches 18.43 nm/‰ corresponding to a resolution of 0.0032‰, with high yield of fabrication. Moreover, the temperature sensitivity is -10.25 nm/℃, and the error of measurement can be constrained within 1.9%. With the merits of simple fabrication and low cost, the proposed sensor is very promising and potential for high-accuracy marine environment monitoring.