Hybrid FBG–LPG sensor for surrounding refractive index and temperature simultaneous discrimination Daniel Alfredo Chamorro Enrı ´quez n , Alberto R. da Cruz, Maria Thereza M. Rocco Giraldi Photonics Laboratory Electrical Engineering Department, Instituto Militar de Engenharia, Rio de Janeiro, Brazil article info Article history: Received 19 August 2011 Received in revised form 13 October 2011 Accepted 25 October 2011 Available online 12 November 2011 Keywords: Fiber Bragg grating (FBG) Surrounding refractive index (SRI) Temperature sensor abstract In this work, we propose a configuration of a hybrid fiber Bragg grating together with a long period grating sensor used for simultaneously detect surrounding refractive index and temperature to be applied in aqueous environment and to reveal pollution. We present the simulation of such sensor and analyze the reflected wavelength and amplitude variations of the fiber Bragg grating spectrum to obtain the temperature and the external refractive index variations, respectively. The results show that the fiber Bragg grating reflected amplitude change depends on the variation of the long period grating transmission spectrum with the surrounding refractive index modification and the reflected wave- length shift depends on the temperature of the aqueous solution. & 2011 Elsevier Ltd. All rights reserved. 1. Introduction Optical fiber sensors are found in a series of applications in civil, industrial and military fields [1]. There are several advan- tages to use optical fiber sensors [2]. Due to its low attenuation, the optical fiber is capable of transmitting information over long distances. That property provides the use of optical fiber sensors in remote sensing, where the sensor head can be located several kilometers away from the analysis unit and still offering relia- bility [3]. In addition, it is also possible to make a distributed measurement along the fiber, if the necessity relies on an extensive data acquisition. The information amount that can be transmitted in an optical fiber is much higher than the one from the electrical technologies, thus, several parameters of light, such as wavelength, intensity, polarization and phase can be used for a single measurement by increasing the sensitivity of the sensor [3]. On the other hand, multiplexing allows the use of a large number of sensors in the same fiber. In some cases, the number of fiber sensors used to support a structural monitoring system could be quite large, perhaps involving thousands of fiber Bragg grating sensors [4]. Compared to conventional sensors, optical fiber sensors have potentially greater sensitivity, dynamic range and resolution when detecting small parameter variations [5]. In conventional fiber Bragg gratings (FBGs), for refractive- index sensing, etching of the cladding is required for the evanes- cent field of the guided mode to be accessed [6]. This reduces the strength and durability of the sensor and makes it susceptible to damage under harsh environmental conditions. Long period grating (LPG) refractive index sensors retain their endurance, as the integrity of the fiber is not violated. Although long period gratings have larger temperature, strain and refractive index coefficients than fiber Bragg gratings, the former have a number of limitations. Long period gratings are highly bend sensitive, and strain and refractive index sensors can be expected to suffer from temperature cross sensitiv- ities. Moreover, for a demodulation system that detects a wave- length shift that is a fixed fraction of the grating bandwidth, fiber Bragg gratings can present a higher sensitivity [7]. When pollution is concerned, simultaneous measurement of the temperature and the refractive index of the environment is a good way to predict any degradation. There are some works in the literature which show the possibility of measuring two different parameters simultaneously. In [8], Patrick et al. presented a hybrid configuration to discriminate simultaneously strain and tempera- ture using a LPG along with two FBGs. The sensor [8] uses the difference in strain and temperature response of fiber Bragg gratings and a long period grating to discriminate between strain and temperature induced wavelength shifts. It is a simple and elegant configuration although it needs two FBGs to interrogate the LPG. Eggleton et al. [9] proposed, in the 1990s, a procedure to fabricate gratings with a periodically varying envelope to be used as sensors. The reflection spectrum of such superstructure gratings shows a series of regularly spaced peaks. The authors [9] demonstrated how a periodic superstructure can be fabricated in an optical fiber by translating an ultraviolet beam along a fiber and phase mask assembly while the intensity of the beam is modulated. Guan et al. [10] then used the superstructure fiber Bragg grating (SFBG) proposed by Eggleton et al. [9], to simultaneously discriminate Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/optlastec Optics & Laser Technology 0030-3992/$ - see front matter & 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2011.10.020 n Corresponding author. E-mail addresses: danielchemil@yahoo.com, danielchemil@hotmail.com (D.A. Chamorro Enrı ´quez), albertorc@ime.eb.br (A.R. da Cruz), mtmrocco@ime.eb.br (M.T.M. Rocco Giraldi). Optics & Laser Technology 44 (2012) 981–986