Optical-fiber self-referred refractometer based on Fresnel reflection at the fiber tip

Abstract

An optical fiber refractometer based on the Fresnel reflection in the fiber tip is reported. For light intensity referencing, the refractometer head uses two equal low-reflective Fiber Bragg Gratings located near the fiber tip. A thermally wavelength-tunable Distributed Feedback diode (DFB) laser was used for the sensor interrogation. Method of Coherent Optical Frequency Domain Reflectometry was implemented for interrogating the sensor head. It assists to distinguish signals that originated from the different reflectors. Signal processing in the Fourier domain and phase shift monitoring enables the refractive index (RI) of the medium surrounding the fiber tip to be measured for values between 1.0 and 2.0RI units. An accuracy of RI measurements of 5×10−5 was demonstrated experimentally using different liquids including fused silica index-matching liquid. A simple sensor construction and the use of low-cost components make it useful for application in the chem/bio sensing field.