Electrical & Computer Engineering, Department of


Date of this Version



Sensors 2013, 13, 8827-8834; doi:10.3390/s130708827


Copyright © 2013, the authors. Licensee MDPI, Basel, Switzerland. Open access, Creative Commons Attribution license 4.0.


We present a compact and highly-sensitive fiber-optic refractometer based on a high-Q π-phase-shifted fiber-Bragg-grating (πFBG) that is chemically etched to the core of the fiber. Due to the π phase-shift, a strong πFBG forms a high-Q optical resonator and the reflection spectrum features an extremely narrow notch that can be used for highly sensitivity refractive index measurement. The etched πFBG demonstrated here has a diameter of ~9.3 μm and a length of only 7 mm, leading to a refractive index responsivity of 2.9 nm/RIU (RIU: refractive index unit) at an ambient refractive index of 1.318. The reflection spectrum of the etched πFBG features an extremely narrow notch with a linewidth of only 2.1 pm in water centered at ~1,550 nm, corresponding to a Q-factor of 7.4 x 105, which allows for potentially significantly improved sensitivity over refractometers based on regular fiber Bragg gratings.