A true fiber optic refractometer

The best instrument to measure the refractive index of liquids is the Abbe refractometer which can only provide accuracies of the order of 10(-5) at visible wavelengths and 10(-4) in the near infrared. Here we present a technique by which the exact wavelength positions in the near infrared frequency comb of a tilted grating inscribed in the core of an optical fiber can be used to measure the absolute value of the refractive index of a liquid in which the fiber is inserted, with an accuracy of +/- 5x10(-5). This is in contrast to typical fiber optic-based refractometry where only refractive index variations can be measured accurately, hence the appellation of true fiber optic refractometer here. In addition to the increased accuracy, the fiber refractometer proposed here offers the additional advantages associated with in situ measurements. The performance of this refractometer is demonstrated by measurements in water from room temperature down to near freezing at wavelengths in the 1550 nm window.