Vernier-Effect-Based Fiber Microcoupler for Salinity Measurement

A highly sensitive fiber optic multimode coupler for salinity measurement based on Vernier effect is proposed. The coupler is manufactured by tapering two single-mode optical fiber whose sides touch each other. By controlling the shape of the coupler in the waist region, the enveloped interference spectrum caused by the Vernier effect can be observed. Compared with the common interference spectrum of a coupler, this enveloped spectrum exhibits a higher refractive index sensitivity. The experimental results show that the sample of this coupler could achieve a salinity sensitivity of 1.742nm/%o in the measurement range from 0% o to 50% o, and the magnification factor is about 2.93. This high-sensitivity, low-cost coupler can be applied to salinity or refractive index related measurements in industrial and marine fields.