We report an easy-to-build, compact, and low-cost optical fiber refractive index sensor. It consists of a single fiber loop whose transmission spectra exhibit a series of notches produced by the resonant coupling between the fundamental mode and the cladding modes in a uniformly bent fiber. The wavelength of the notches, distributed in a wavelength span from 1,400 to 1,700 nm, can be tuned by adjusting the diameter of the fiber loop and are sensitive to refractive index changes of the external medium. Sensitivities of 170 and 800 nm per refractive index unit for water solutions and for the refractive index interval 1.40–1.442, respectively, are demonstrated. We estimate a long range resolution of 3 × 10 ？4 and a short range resolution of 2 × 10 ？5 for water solutions.
References
[1]
Zhu, T.; Rao, Y.-J.; Wang, J.-L.; Song, Y. A highly sensitive fiber-optic refractive index sensor based on an edge-written long-period fiber grating. IEEE Photon. Technol. Lett. 2007, 19, 1946–1948.
[2]
Iadiccico, A.; Campopiano, S.; Cutolo, A.; Giordono, M.; Cusano, A. Nonuniform thinned fiber Bragg gratings for simultaneous refractive index and temperature measurements. IEEE Photon. Technol. Lett. 2005, 17, 1495–1497.
[3]
Liang, W.; Huang, Y.; Xu, Y.; Lee, R.K.; Yariv, A. Highly sensitive fiber Bragg grating refractive index sensors. Appl. Phys. Lett. 2005, 86, 151122:1–151122:3.
[4]
Salceda-Delgado, G.; Monzón-Hernández, D.; Martínez-Ríos, A.; Cárdenas-Sevilla, G.A.; Villatoro, J. Optical microfiber mode interferometer for temperature-independent refractometric sensing. Opt. Lett. 2012, 37, 1974–1976.
[5]
Monzón-Hernández, D.; Villatoro, J. High-resolution refractive index sensing by means of a multiple-peak surface plasmon resonance optical fiber sensor. Sens. Actuators B Chem. 2005, 115, 227–231.
Smith, A.M. Birefringence induced by bends and twists in single mode optical fiber. Appl. Opt. 1980, 19, 2606–2611.
[13]
Marcuse, A. Influence of curvature on the losses of doubly clad fibers. Appl. Opt. 1982, 21, 4208–4213.
[14]
Block, U.L.; Dangui, V.; Digonnet, M.F.; Fejer, M.M. Origin of apparent resonance mode splitting in bent long-period fiber gratings. J. Lightw. Technol. 2006, 24, 1027–1034.
[15]
Wang, Q.; Farrell, G.; Freir, T. Theoretical and experimental investigations of maxcro-bend losses for standard single mode fibers. Opt. Express 2005, 13, 4476–4484.
[16]
Renner, H. Bending losses of coated single-mode fibers: A simple approach. J. Lightw. Technol. 1992, 10, 544–551.
[17]
Fasutini, L.; Martini, G. Bend loss in single-mode fibers. J. Lightw. Technol. 1997, 15, 671–679.
[18]
Silvestre, E.; Pinheiro-Ortega, T.; Andrés, P.; Miret, J.J.; Ortigosa-Blanch, A. Analytical evaluation of chromatic dispersion in photonic crystal fibers. Opt. Lett. 2005, 30, 453–455.
[19]
Yariv, A. Propagation and Coupling of Modes in Optical Dielectric Waveguides-Periodic Waveguides. In Optical Electronics in Modern Communications, 5th ed. ed.; Oxford University Press: New York, NY, USA, 1997; pp. 491–539.
[20]
Chamorro Enríquez, D.A.; da Cruz, A.R.; Rocco Giraldi, M.T.M. Hybrid FBG–LPG sensor for surrounding refractive index and temperature simultaneous discrimination. Opt. Las. Technol. 2012, 44, 981–986.
[21]
Yan, J.; Zhang, A.P.; Shao, L.-Y.; Ding, J.-F.; He, S. Simultaneous measurement of refractive index and temperature by using dual long-period gratings with an etching process. IEEE Sens. J. 2007, 7, 1360–1361.
