A refractometer based on fiber-to-liquid planar waveguide coupler

A fiber-optic liquid refractometer based on fiber-to-planar waveguide coupler is proposed and demonstrated. The liquid plays a role of highly multimode planar waveguide (PWG), which is evanescently coupled to a side-polished single mode fiber. A simple method, which acquire the refractive index (RI) of a liquid using two adjacent resonance wavelengths of the coupler, is presented. Furthermore, it is experimentally shown that the measurement range of RI can be expanded by series connection of two different fiber-to-PWG couplers. The resolution of the proposed and fabricated refractometer is of the order of 10−5 to 10−4.