A novel long-period fiber grating (LPG) refractive index sensor is presented for the measurements of ambient refractive indices which are higher than that of the fiber cladding. As the measurement parameter, the transmission power of the core mode is interrogated at a single wavelength where the core mode and a leaky mode are phase-matched. Firstly, the couplings of the core mode to leaky modes in the novel structure are analyzed by using complex coupled-mode theory, and then classified into three cases analogous to those in the damped oscillations. The power evolutions of the core mode in the couplings are thus intuitively understood. Based on these, for the first time, we demonstrate, with optimized design parameters the transmission power of the core mode is rather sensitive to the change of a higher ambient refractive index at resonant wavelengths. Then we focus on two optimization objectives. One is to enlarge the operational range while keeping given sensitivity, the other is to enhance the sensitivity within a given operational range. Finally, we demonstrate the operational range of the LPG refractive index sensor can be from 1.46 to 1.7 with a sensitivity of ~10^-4, while for a given operational range from 1.455 to 1.465, the sensitivity can be ~10^-6, if assuming the dynamic range of the power is less than 30dB and the measurement resolution is 0.01dB for both cases.

Date Published: 9 November 2010
PDF: 9 pages
Proc. SPIE 7853, Advanced Sensor Systems and Applications IV, 78530F (9 November 2010); doi: 10.1117/12.882340

Published in SPIE Proceedings Vol. 7853:
Advanced Sensor Systems and Applications IV
Brian Culshaw; Yanbiao Liao; Anbo Wang; Xiaoyi Bao; Xudong Fan; Lin Zhang, Editor(s)