IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 16, NO. 4, APRIL 2004 1149 Thinned Fiber Bragg Gratings as High Sensitivity Refractive Index Sensor A. Iadicicco, A. Cusano, Member, IEEE, A. Cutolo, R. Bernini, Member, IEEE, and M. Giordano Abstract—In this work, the numerical and experimental anal- ysis on the use of thinned fiber Bragg gratings as refractive index sensors have been carried out. Wet chemical etching in a buffered hydrofluoric acid solution was used for sensor fabrication. Exper- imental characterization for an almost full etched cladding sensor is presented demonstrating good agreement with numerical results and resolutions of and for outer refractive index around 1.45 and 1.333, respectively. Index Terms—Optical fibers, refractive index measurements, thinned fiber Bragg grating (FBG) sensors. I. INTRODUCTION O PTICAL sensors are very attractive in chemical and bio- chemical applications due to some unique characteristics such as immunity to electromagnetic interference and aggres- sive environments, high sensitivity, and fast response. Examples of integrated optical sensors include those utilizing schemes like direct fiber optic reflectometry [1], Mach–Zehnder interferome- ters, grating couplers, bend loss waveguides or ARROW waveg- uides [2], and long-period gratings [3]. On the other hand, in the last years, fiber Bragg grating (FBG) sensors have been widely used in many sensing applications in- cluding temperature and strain measurements [4]. The advan- tages of grating-based sensors are well known. High sensitivity, intrinsic codify of the measured parameter in an absolute param- eter, multiplexing capabilities, and very low cost are only a few of them. The principle of operation relies on the dependence of the Bragg resonance on effective refractive index and the grating pitch. Since the effective refractive index is not influenced by the external one for standard optical fibers, no sensitivity to external refractive index is expected. However, if fiber cladding diameter is reduced along the grating region, the effective refractive index is significantly affected by external refractive index. As a con- sequence, shifts in the Bragg wavelength combined with a mod- ulation of the reflected amplitude are expected. This approach has been already used to create grating-based tuneable filters [5]. In this letter, numerical and experimental analysis on the use of thinned FBGs for refractive index measurements is pre- sented. In particular, the three-layer model for the thinned op- tical fiber has been used to identify the dependence of the effec- Manuscript received September 25, 2003; revised November 27, 2003. A. Iadicicco, A. Cusano, and A. Cutolo are with the Optoelectronic Division- Engineering Department, University of Sannio, Benevento 82100, Italy (e-mail: a.cusano@unisannio.it). R. Bernini is with Institute for Electromagnetic Monitoring of Environment (IREA), National Research Council (CNR), 328-80124 Napoli, Italy. M. Giordano is with Institute of Composite Materials Technology National Research Council (ITMC-CNR), 80-80125 Napoli, Italy. Digital Object Identifier 10.1109/LPT.2004.824972 tive refractive index from the external one. From these results, the dependence of the sensor sensitivity in terms of wavelength shift has been analyzed. Sensor fabrication has been carried out by using standard single-mode optical fibers (SMF-28) and wet chemical etching in a buffered hydrofluoric acid (HF) solution. Finally, experimental characterization of the sensor response to external refractive indexes varying in the range 1.333–1.450 has been carried out by using broad-band interrogation and a com- mercial optical spectrum analyzer. II. ANALYSIS AND SIMULATIONS As is well known, Bragg resonance condition can be ex- pressed as [4] (1) where, is the effective refractive index of the fiber, is the grating pitch and is the reflected Bragg wavelength. In common optical fibers, the effective refractive index of the fundamental mode is practically independent from the refrac- tive index of the medium surrounding the cladding. However, if the cladding diameter is reduced, shows a nonlinear de- pendence on the external refractive index, leading to a shift in the reflected wavelength. Different from the common use of this class of sensors for temperature and strain measurements, in this case, only the refractive index is affected by measurand changes while the grating pitch was practically unchanged. In order to outline the dependence of the sensor responsivity on cladding diameter and external refractive index, in a thinned optical fiber was evaluated by resolving numerically the disper- sion equation of a doubly cladding fiber model [6]. The pa- rameters of the optical fiber for the numerical analysis have been chosen according to the SMF-28 standard: core diameter of 8.3 m, original cladding diameter of 125 m, core refrac- tive index of 1.460, refractive index difference of 0.36%, and effective refractive index of 1.4588 nm at an operating wave- length of 1550 nm. Fig. 1 shows the nonlinear behavior of in a thinned optical fiber versus outer medium refractive index in the range 1.333–1.4547 at an operating wavelength of 1550 nm for different cladding diameters. Here, the full etching curve is referred to a completely removed cladding and Dclad is the cladding diameter. For external refractive indexes around 1.333, the guided mode is well confined in the core region, leading to a week dependence on the outer medium refractive index. As external refractive index increases, higher sensitivity is observed, since the fundamental mode is less confined in the core region leading to an increased interaction with the ex- ternal medium. In addition, such interaction and as a conse- 1041-1135/04$20.00 © 2004 IEEE